Garrido et al., 2006; Amalaoulaou Massif, Berger et al., 2008) coexist with residues dominated by clinopyroxene, garnet and rutile. Based on macroscopic and ...
Lithos 198–199 (2014) 58–76
Contents lists available at ScienceDirect
Lithos journal homepage: www.elsevier.com/locate/lithos
Paleozoic HP granulite-facies metamorphism and anatexis in the Dulan area of the North Qaidam UHP terrane, western China: Constraints from petrology, zircon U–Pb and amphibole Ar–Ar geochronology Shengyao Yu a,⁎, Jianxin Zhang a, C.G. Mattinson b, Pablo García del Real c, Yunshuai Li a, Jianghua Gong a a b c
State Key Laboratory for Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang, Beijing 100037, PR China Department of Geological Sciences, Central Washington University, 400 E. University Way, Ellensburg, WA 98926, USA Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA
a r t i c l e
i n f o
Article history: Received 31 October 2013 Accepted 16 March 2014 Available online 29 March 2014 Keywords: North Qaidam UHP terrane HP granulite Partial melting Geochronology Paired belt
a b s t r a c t HP mafic and felsic adakitic granulite bodies in the Dulan area of the North Qaidam ultrahigh-pressure (UHP) terrane record high-pressure (HP) granulite-facies metamorphism and anatexis and provide temporal and tectonic constraints on deep subduction of continental crust and its subsequent exhumation. Mafic HP granulite components dominate the main outcrop and preserve features diagnostic of anatexis and locally may be described as migmatite. The HP mafic granulites comprise garnet, clinopyroxene, plagioclase and quartz. The felsic granulite (leucosome) is mainly composed of K-feldspar + plagioclase + quartz + kyanite + garnet. Detailed zircon U–Pb and amphibole Ar–Ar geochronology, combined with trace element geochemistry, indicate peak metamorphism for the mafic HP granulite at 434 ± 3 to 435 ± 3 Ma and peak metamorphism and partial melting for the felsic HP granulite at 433 ± 5 to 438 ± 4 Ma, which overlaps the ages of UHP metamorphism for adjacent eclogite (430–446 Ma). 40Ar/39Ar amphibole ages of 423 to 432 Ma represent amphibolite-facies retrograde metamorphism and indicate rapid cooling during exhumation of the HP granulite bodies. Our geochronological data, combined with field relationships, petrology and geochemistry suggest that HP granulite-facies metamorphism and the partial melting that produced adakitic melts represent the same tectonic event. In this case, the felsic HP granulites (leucosome) formed from an adakitic melt derived from partial melting of mafic HP granulite in the overriding plate in a relatively higher geothermal gradient (15–18 °C/km), leaving garnet-cumulate and/or meta-ultramafic (mainly garnet pyroxenite) as the residual component. In contrast, the nearby UHP eclogite is thought to have formed in the subducted plate in a relatively lower geothermal gradient (6–10 °C/km). Penecontemporaneous metamorphic ages but different geothermal gradients between HP granulites and related UHP eclogite define a possible paired metamorphic belt generated in a subduction–collision setting associated with the North Qaidam continental collisional orogeny during the Late Ordovician–Early Silurian. © 2014 Elsevier B.V. All rights reserved.
1. Introduction High-pressure (HP) granulites are exposed in a number of continental collision belts ranging in age from Palaeoproterozoic (e.g., Anderson et al., 2012; Zhao et al., 2001) to Cenozoic (e.g. Himalayas; Liu and Zhong, 1997). In general, felsic and metapelitic HP granulites mainly consist of garnet + plagioclase + kyanite + K-feldspar + quartz, whereas mafic HP granulite is characterized by the stability of the assemblage garnet + clinopyroxene + plagioclase + quartz (Carswell and O'Brien, 1993; Indares, 2003; O'Brien and Rötzler, 2003). However, some mafic HP granulites also contain kyanite in their peak mineral assemblage, depending on the bulk composition (e.g. Klemd and Brocker, 1999). Although HP granulite-facies metamorphism is generally accepted as the product of regional collisional orogenesis, there is still ⁎ Corresponding author.
http://dx.doi.org/10.1016/j.lithos.2014.03.016 0024-4937/© 2014 Elsevier B.V. All rights reserved.
controversy concerning the genetic relations between the P–T–t paths of the HP granulites and associated rocks, their tectonic settings, and geodynamic models. O'Brien and Rötzler (2003) distinguished two varieties of HP granulite corresponding to distinct geodynamic scenarios: (1) high to ultrahigh-temperature granulite (Type I) equilibrated at conditions above 900 °C and 1.5 GPa. It formed by deep subduction of crustal rocks to mantle depths, followed by fast buoyancy-driven exhumation to normal crustal depths; and (2) HP granulite type representing overprinted eclogites (Type II). This type of HP granulite formed at conditions around 700–850 °C and 1.0–1.4 GPa. It was formerly subducted into the eclogite field but was not exhumed fast enough to overcome the effects of thermal relaxation and thus experienced heating during decompression. Although these two types of HP granulites show different PT paths, an early deep subduction is suggested for both types. As a third possibility, based on the HP granulites in the Bohemian Massif of the Variscan orogens, some authors suggest that HP granulites
S. Yu et al. / Lithos 198–199 (2014) 58–76
could form in the root of the overthickened crust during collisional orogenesis and subsequent exhumation that were achieved by virtue of homogeneous, fold-dominated deformation of hot crustal domains in an orogenic belt (Pitra et al., 2010; Schulmann et al., 2008; Štipska et al., 2004). In general, HP granulite-facies metamorphism occurs at relatively elevated P–T conditions with P N 1 GPa, and T N 750 °C. Where P–T conditions exceed the dehydration solidus of hydrous minerals such as amphibole, zoisite and mica, HP granulite-facies metamorphism often accompanies extensive anatexis (Indares and Dunning, 2001; Nahodilová et al., 2011; Puelles et al., 2005). Thus, it is crucial to comprehend HP granulite-facies metamorphism and related partial melting in orogens, which will not only play an important role in studying petrogenesis, but also could provide essential constraints on formation, fractionation and evolution of crust and deep dynamics (e.g., Sizova et al., 2012). In several collisional orogens, HP granulites are spatially and temporally associated with eclogites, such as in the European Variscan orogen (Carswell and O'Brien, 1993; O'Brien and Rötzler, 2003) and the Greenland Caledonian orogen (Elvevold et al., 2003; Gilotti and Elvevold, 2002). However, the paragenetic relationship between UHP eclogite and associated HP granulite remains controversial. Some workers argue that the HP granulite is associated with the effects of thermal relaxation after eclogite-facies metamorphism (e.g. Galan and Marcos, 2000; O'Brien, 1997), whereas others consider that the HP granulites and eclogites formed concurrently in different thermal environments (e.g. Konopasek and Schulmann, 2005; O'Brien and Rötzler, 2003; Puelles et al., 2005). A third important possibility is that (U)HP granulites and eclogites formed concurrently under the same peak metamorphic conditions but mineral assemblages differ due to bulk chemical differences (e.g. Klemd and Brocker, 1999). The mineral textures, whole rock chemistry, mineral chemistry and age of the HP-granulites enable us to assess whether they experienced a single metamorphic evolution distinct from the UHP eclogites, if they represent overprinted UHP rocks, or if they formed concurrently in the same thermal environment. Understanding the relationship between eclogite and HP granulite can provide crucial constraints on the mechanisms for continental collision orogens. In the past ten years, extensive structural, petrological, geochemical and geochronological investigations have been carried out in the North Qaidam Mountains, which preserve fresh exposures of eclogite, garnet peridotite and associated gneiss (e.g. Mattinson et al., 2006a,b, 2007, 2009; Song et al., 2003a,b, 2004, 2005, 2006, 2007, 2010; Yang and Deng, 1994; Yang and Powell, 2008; Yang et al., 1998, 2001, 2002, 2006; Yu et al., 2012, 2013; Zhang et al., 2001, 2005, 2006, 2008a,b, 2009a,b,c, 2010). A Paleozoic UHP metamorphic belt has been established based on coesite inclusions in garnet, omphacite and zircon in eclogites, and in zircon of paragneisses, and diamond inclusions in zircon of garnet peridotites (Song et al., 2003a, 2004, 2005; Zhang et al., 2009a,c, 2010). The North Qaidam UHP metamorphic belt is divided into four units, which include a garnet peridotite-bearing unit (Luliangshan) and three eclogite-bearing units (Yuka, Xitieshan and Dulan). Recent petrological and geochronological investigations on the Dulan unit have focused on the characterization of the UHP components (eclogite and paragneiss), but few studies have addressed the HP granulite members (Yu et al., 2009, 2011a; Zhang et al., 2009b). To date, the metamorphic history of HP granulite and its relationship with UHP eclogite remains controversial. Song et al. (2003b) proposed that kyanite-bearing eclogite experienced a post-peak high-pressure granulite-facies metamorphic overprint associated with the effects of thermal relaxation. In contrast, based on petrographic textures, mineral compositions and thermobarometric data for the Dulan HP granulite, Yu et al. (2009, 2011a,b) defined a single clockwise P–T path that does not overlap with UHP eclogite conditions, and suggested that the HP granulite probably formed at the base of thickened continental crust, and is not associated with the effects of thermal relaxation after eclogite-
59
facies metamorphism. The Dulan felsic HP granulite component shows a chemical resemblance to adakites, and probably resulted from partial melting of thickened mafic lower crust (N50 km) compatible with peak P–T estimates from mafic HP granulites (Yu et al., 2012). Up to now, geochronological investigations of the HP granulites in the Dulan area are limited; initial work by Yu et al. (2010) identified peak HP granulite metamorphism at about 447 Ma based on LA-ICP-MS zircon U–Pb dating of one HP granulite sample. Geochronological data on HP granulite greatly contribute to validating the paragenetic and temporal constraints on HP granulite-facies metamorphism and related partial melting, and help establish the relationship between HP granulite and UHP eclogite. In this contribution, we present combined SHRIMP and LA-ICP-MS U–Pb zircon geochronology, Ar/Ar amphibole geochronology, trace element geochemistry, and extensive field and petrographic relations to characterize the timing of HP granulite-facies metamorphism and associated anatexis recorded in both mafic and felsic granulites. Our results place important constraints on the relationship between HP granulites and UHP eclogites and help unravel the tectonic regimes of subduction and exhumation of crustal rocks in orogenic belts. 2. Geological setting and previous studies The northwest–southeast trending North Qaidam Mountains, located at the northern margin of the Qinghai–Tibet Plateau, extend over a distance of 350 km and are bounded by the Qaidam basin to the southwest, the Altyn Tagh fault to the northwest and the Qilian block to the northeast (Fig. 1a). The basement of the North Qaidam Mountains is composed mainly of paragneiss and orthogneiss, rare marble, granulite, amphibolite, locally eclogite and various ultramafic rocks. These rocks are in depositional contact (locally faulted) with the overlying lower Paleozoic volcanic and sedimentary rocks of the Tanjianshan Group, and are intruded by granite plutons. Eclogite occurrences span N 350 km near the localities of Yuka, Xitieshan and Dulan and record metamorphic ages between 420 to 490 Ma (Mattinson et al., 2006a, 2006b, 2007; Song et al., 2003b, 2006; Yang et al., 2002, 2006; Yu et al., 2012; Zhang et al., 2001, 2005, 2006, 2008b, 2009a, c, 2010). Garnet peridotite outcrops in the Luliangshan area (Yang and Deng, 1994), and appears to have been exhumed from mantle depths N 200 km (Song et al., 2004, 2005). Based on rock associations, petrological criteria, and field relationships, four UHP metamorphic units can be distinguished along the North Qaidam Mountains (NQD) from east to west (Zhang et al., 2008a): (1) the Dulan eclogite–gneiss unit (DLU), which consists of granitic gneiss, paragneiss, eclogite and ultramafic lenses enclosed within gneiss; (2) the Xitieshan eclogite–gneiss unit (XTU), dominated by kyanite-, sillimanite-bearing paragneiss (schist) and orthogneiss with rare marble and amphibolite and intruded by granite plutons dated at 428 Ma (Meng et al., 2005); (3) the Luliangshan garnet peridotite–gneiss unit (LLU), defined by sillimanite-bearing paragneiss and orthogneiss with ultramafic rocks (garnet peridotite and garnet pyroxenite) as lenses and intruded by Silurian granite plutons (Zhang et al., 2008a); and (4) the Yuka eclogite–gneiss unit (YKU), which comprises eclogite, metapelite, orthogneiss and rare marble. Located approximately 30 km to the northeast of Dulan town, our DLU study area contains granitic orthogneiss and paragneiss (schist), enclosing eclogite lenses and minor ultramafic rocks, and was intruded by ca. 400 Ma granite plutons (Fig. 1b; Wu et al., 2004; Yu et al., 2011b). An amphibolite-facies foliation in the gneiss strikes northwest, dips steeply northeast and is modified by tight to isoclinal folds. A locally developed subhorizontal to shallowly northwest plunging lineation trends northwest (Mattinson et al., 2007, 2009). On the basis of spatial relations, mineral assemblages and compositions of the eclogitic lithologies, Song et al. (2003b) subdivided the DLU into the South Dulan Belt (SDB) and the North Dulan Belt (NDB). The main distinction was based on post-peak HP granulite-facies overprinting of the SDB eclogite.
60
S. Yu et al. / Lithos 198–199 (2014) 58–76
Fig. 1. (a) Schematic map of the North Qaidam Mountains showing major tectonic units, and locations of eclogite and garnet peridotite (modified from Zhang et al., 2005). (b) Geological sketch map showing the geological setting of the Dulan area (Zhang et al., 2010). DLU—the Dulan eclogite–gneiss unit, XTU—the Xitieshan eclogite–gneiss unit, YLU—the Yuka eclogite– gneiss unit, LLU—the Luliangshan garnet peridotite–gneiss unit.
However, new data suggest that the eclogites from the two sub-belts experienced a similar metamorphic history without HP granulitefacies overprinting (Zhang et al., 2009a, c, 2010). Moreover, a small HP granulite unit, which is predominately composed of mafic HP granulite with minor felsic HP granulite, paragneiss, orthogneiss and amphibolite, was recognized in the western margin of the SDB (Fig. 2). The HP granulite unit is presumed to be in fault contact with the eclogite-bearing unit (Yu et al., 2009, 2011a; Zhang et al., 2009b). On the basis of microtextures, reaction relations between mineral phases, and the compositional profiles of minerals, four mineral assemblages are recognized in the Dulan HP mafic granulites, including pre-peak amphibolite-facies prograde assemblages (M1), peak HP granulite-facies assemblages (M2), a post-peak amphibolite-facies assemblage (M3), and a late greenschist-facies retrogressive assemblage (M4) (Yu et al., 2009,
2011a,b). The peak metamorphic P–T conditions of the HP granulite are at 1.4–1.8 GPa and 800–950 °C. 3. Field relationships The HP granulite occurs as lenses, blocks or layers 10 to 100 m in length within the garnet- and kyanite-bearing paragneiss/schist, with garnet-bearing granitic gneiss and amphibolite as the country rocks (Fig. 2). The contact between the HP-granulite and Grt-Ky paragneiss is sharp (Yu et al., 2011a,b). Mafic HP granulite dominates the main outcrop and preserves features diagnostic of anatexis (Fig. 3a, b) and locally may be described as migmatite (Fig. 3c; Sawyer, 2008). Outcrops comprise a mixture of pale plagioclase-rich quartzo-feldspathic layers, interpreted as the former sites of melt segregation and/or accumulation,
S. Yu et al. / Lithos 198–199 (2014) 58–76
61
Fig. 2. Simplified geological map of the HP granulite unit showing sample locations. Modified from Yu et al. (2012).
and dark garnet–clinopyroxene-rich melanosome domains, interpreted as the residuum from which melt has been extracted. The relative proportion of leucosome and melanosome varies widely. The felsic layers range in composition from tonalite to granite and many preserve HP
granulite-facies mineral assemblages of garnet + kyanite + K-feldspar + plagioclase + quartz (Yu et al., 2011a,b), so we call it “felsic HP granulite” although it could represent melt formed by anatexis. The felsic HP granulite generally appears as thin layers and small leucocratic
Fig. 3. Field relations of HP granulite in the Dulan area. (a), (b) The felsic HP granulite (leucosome) occurs as layers, veins and patches in the mafic–ultramafic granulite. (c) Migmatitic character of the HP granulite. (d) Euhedral–subhedral clinopyroxene and garnet grains within the felsic HP granulite. (e) and (f) Coarse-grained patches of plagioclase + quartz ± K-feldspar leucosome enclose garnet and clinopyroxene.
62
S. Yu et al. / Lithos 198–199 (2014) 58–76
patches oriented sub-parallel to the main foliation in the HP mafic granulite (Fig. 3a, b). The proportion of felsic granulite varies greatly from outcrop to outcrop. Larger sheets of felsic HP granulite are centimeters to decimeters thick and may be continuous on an outcrop scale. Euhedral–subhedral clinopyroxene and garnet grains within the felsic HP granulite are commonly larger than those within the melanosome and occasionally exhibit replacement at their margins by amphibole (Fig. 3d). Locally coarse-grained patches consist of plagioclase + quartz ± K-feldspar commonly enclosing garnet or occasionally clinopyroxene (Fig. 3e, f). The biggest garnet grains reach 2–3 cm in diameter. These patches are interpreted to be in situ neosome consisting of leucosome surrounding peritectic phases formed by partial melting. Additional macroscopic evidence for partial melting consists of ellipsoidal leucosomes surrounded by a neosome consisting of garnet + clinopyroxene + quartz + plagioclase (Fig. 4a). In more deformed rocks the morphology of the neosome is more complex. Some mafic granulite layers contain large isolated pink garnets surrounded by quartz + plagioclase + K-feldspar leucosome, interpreted as residuum that has lost much of its melt (Fig. 4b). A few large outcrops show arrays of interconnected shear band structures that contain leucosome and scattered garnet (Fig. 4c). These arrays are interpreted to be remnants of the network of channels through which melt escaped (e.g. Bons et al., 2001; Sawyer, 2001). In some outcrops, segregated and unsegregated neosomes were identified, where the leucosome
did not migrate far from its source layer (Fig. 4d). In this case, in situ leucosome, residuum and unsegregated neosome could be distinguished clearly. The felsic component locally contains lenses, blocks or layers of garnet-cumulate and/or meta-ultramafic, which are considered to be the segregated neosome consisting of leucosome and surrounding residuum (or melanosome) and are interpreted as principal evidence for partial melting (Fig. 4e). Some layers or patches of melanosome comprise N 90 vol.% garnet and clinopyroxene, which locally form parallel compositional banding, defined by garnet-rich layers and clinopyroxene-rich layers (Fig. 4f). 4. Petrography and sample descriptions The petrographic features and mineral assemblages will be described separately for the representative mafic and felsic HP granulites chosen for zircon U–Pb analyses and Ar–Ar dating. The typical microtextures showing anatexis will be also described below. The mineral abbreviations follow Whitney and Evans (2010). 4.1. Mafic HP granulite The peak mineral assemblage of sample Q06-14-2 (N36°31.556′ E98°23.132′) is garnet (35%) + clinopyroxene (30%) + plagioclase (15%) + quartz (5%) + rutile (3%) + zoisite (5%), with minor
Fig. 4. Field relations of HP granulite in the Dulan area (2). (a) Elliptical leucosome is surrounded by neosome consisting of garnet + clinopyroxene + plagioclase + quartz. (b) Mafic granulite layers contain large, isolated pink garnets that have a small amount of quartz + plagioclase + K-feldspar leucosome around them. (c) Arrays of interconnected shear band structures contain leucosome and scattered coarse-grained garnet. (d) Textural relations of leucosome, residuum, paleosome and unsegregated neosome. (e) Block of garnet-cumulate (residuum) surrounded by leucosome. (f) Garnet and clinopyroxene-rich layer in residuum.
S. Yu et al. / Lithos 198–199 (2014) 58–76
63
Fig. 5. Microphotographs showing textures and mineral assemblages of mafic HP granulite. (a), (b) Peak equilibrium assemblage of garnet, clinopyroxene, plagioclase and quartz (Q06-14-2, crossed polarized light). Subhedral–euhedral garnet porphyroblast cores contain inclusions of quartz, epidote, plagioclase and amphibole. (c) Euhedral garnet is surrounded by oriented kyanite, plagioclase and zoisite defining the foliation (Q06-14L, crossed polarized light). (d) Garnet is surrounded by later amphibole (Q06-14 g, crossed polarized light).
amphibole and accessory zircon (Fig. 5a, b). The sample has a foliation defined by Cpx + Zo + Grt, with Zo forming thick, oriented clusters throughout the sample. Subhedral–euhedral garnet porphyroblasts contain core inclusions of quartz, epidote, plagioclase and amphibole,
which are commonly embayed and display conspicuous coronas consisting of symplectic amphibole + plagioclase. Rare fine-grained anhedral to subhedral clusters of garnets are usually devoid of inclusions.
Fig. 6. Microphotographs showing textures and mineral assemblages of felsic HP granulite. (a), (b) Peak equilibrium assemblage of garnet, kyanite, plagioclase, K-feldspar and quartz (plane polarized light) (Q08-15-3.1). (c) Felsic granulite Q08-14-4.3 mainly consists of garnet, kyanite, K-feldspar, plagioclase and quartz (crossed polarized light). The kyanite is generally rimmed by a symplectite of muscovite + plagioclase ± zoisite. (d) Garnet corona growing around kyanite within moats of plagioclase (crossed polarized light).
64
S. Yu et al. / Lithos 198–199 (2014) 58–76
Fig. 7. Microtextures showing evidence for partial melting. (a), (b) and (c) Thin, irregularly-shaped films of plagioclase (a, b) and quartz (c) with cuspate outlines occur between rounded grains of garnet and/or rutile (a, b from sample Q09-21-1.18, c from sample Q10-5-8.1). (d), (e) Thin K-feldspar or/and plagioclase with cuspate outlines occur between rounded grains of reactant quartz and plagioclase (Q08-15-3.1). (f) Coarse-grained clinopyroxene and garnet in leucosome (Q09-21-1.13).
Fig. 8. Cathodoluminescence (CL) images showing internal textures of mafic HP granulite Q06-14-2.
Table 1 LA-ICP-MS U–Pb analytical data for zircons from mafic HP granulite Q06-14-2 in the Dulan area, North Qaidam Mountains. Spot
U ppm
71 38 31 99 76 21 62 46 59 67 28 21 48 111 23 52 36 48 43 178 40 47 260 87 73 81 41 143 66 86
253 145 141 367 156 99 261 156 222 174 105 155 186 812 147 226 139 178 146 436 125 151 546 277 260 233 164 465 270 273
Th/U
0.28 0.27 0.22 0.27 0.49 0.21 0.24 0.29 0.27 0.39 0.26 0.14 0.26 0.14 0.16 0.23 0.26 0.27 0.29 0.41 0.32 0.31 0.48 0.32 0.28 0.35 0.25 0.31 0.24 0.31
Pb
Measured ratios
ppm
207
±1σ
207
±1σ
206
±1σ
207
±1σ
207
±1σ
206
±1σ
13.10 8.89 9.21 21.33 13.91 4.72 13.42 7.05 13.56 8.89 5.23 6.36 10.33 38.52 6.84 15.69 7.55 8.70 7.38 27.87 10.70 12.87 33.95 17.12 12.35 11.76 9.02 33.82 18.15 18.10
0.05523 0.05387 0.05556 0.05521 0.05598 0.05469 0.05569 0.05374 0.05591 0.05578 0.05535 0.05624 0.05587 0.05732 0.05634 0.05531 0.05621 0.05529 0.05628 0.05578 0.05403 0.05412 0.05635 0.05520 0.05535 0.05528 0.05444 0.05881 0.05535 0.05496
0.00041 0.00044 0.00036 0.00027 0.00040 0.00052 0.00037 0.00068 0.00033 0.00048 0.00063 0.00035 0.00029 0.00020 0.00036 0.00035 0.00038 0.00053 0.00063 0.00033 0.00039 0.00025 0.00023 0.00025 0.00026 0.00034 0.00031 0.00034 0.00024 0.00028
0.52936 0.51602 0.53785 0.53182 0.54216 0.52525 0.52955 0.51526 0.53635 0.53943 0.53112 0.54398 0.54254 0.55168 0.55296 0.53395 0.54050 0.52961 0.54181 0.52680 0.52276 0.52109 0.53285 0.53118 0.53612 0.52376 0.52535 0.56736 0.53368 0.53099
0.0065 0.0058 0.0050 0.0042 0.0054 0.0073 0.0047 0.0079 0.0044 0.0069 0.0072 0.0056 0.0052 0.0039 0.0066 0.0051 0.0053 0.0091 0.0096 0.0045 0.0046 0.0039 0.0044 0.0056 0.0053 0.0062 0.0054 0.0055 0.0058 0.0073
0.06951 0.06955 0.07023 0.06986 0.07027 0.06973 0.06898 0.06953 0.06959 0.07021 0.06960 0.07017 0.07043 0.06980 0.07124 0.07003 0.06979 0.06959 0.06982 0.06851 0.07017 0.06982 0.06862 0.06981 0.07028 0.06876 0.07004 0.07005 0.07000 0.07011
0.00069 0.00062 0.00050 0.00044 0.00054 0.00080 0.00046 0.00066 0.00045 0.00072 0.00057 0.00060 0.00056 0.00043 0.00074 0.00054 0.00055 0.00111 0.00097 0.00054 0.00035 0.00041 0.00057 0.00068 0.00066 0.00079 0.00065 0.00058 0.00074 0.00093
420.4 364.9 435.2 420.4 450.0 398.2 438.9 361.2 450.0 442.6 427.8 461.2 455.6 505.6 464.9 433.4 461.2 433.4 464.9 442.6 372.3 376.0 464.9 420.4 427.8 433.4 390.8 561.1 433.4 409.3
16.7 18.5 14.8 11.1 21.3 15.7 10.2 27.8 13.0 13.9 8.3 10.2 11.1 12.0 14.8 14.8 10.2 5.6 24.1 13.0 10.2 11.1 9.3 11.1 9.3 14.8 45.4 13.0 9.3 11.1
431.4 422.5 437.0 433.0 439.8 428.7 431.5 422.0 436.0 438.1 432.6 441.0 440.1 446.1 446.9 434.4 438.8 431.6 439.6 429.7 427.0 425.9 433.7 432.6 435.9 427.7 428.7 456.3 434.2 432.5
4.3 3.9 3.3 2.8 3.5 4.9 3.1 5.3 2.9 4.5 4.8 3.7 3.4 2.5 4.3 3.4 3.5 6.0 6.3 3.0 3.1 2.6 2.9 3.7 3.5 4.2 3.6 3.6 3.8 4.8
433.2 433.4 437.5 435.3 437.8 434.5 430.0 433.3 433.7 437.4 433.7 437.2 438.7 434.9 443.6 436.3 434.9 433.7 435.1 427.2 437.2 435.1 427.8 435.0 437.9 428.7 436.4 436.5 436.1 436.8
4.2 3.8 3.0 2.6 3.3 4.8 2.8 4.0 2.7 4.3 3.4 3.6 3.4 2.6 4.4 3.3 3.3 6.7 5.9 3.2 2.1 2.5 3.4 4.1 4.0 4.8 3.9 3.5 4.5 5.6
Pb/206Pb
Ages (Ma) Pb/235U
Pb/238U
Pb/206Pb
Inclusion Pb/235U
Pb/238U
No Grt No No No Qtz No Cpx Grt + Pl No No No Cpx No No No Cpx + Rt No No No Pl + Qtz No No No Rt No No No Ap No
S. Yu et al. / Lithos 198–199 (2014) 58–76
Q06-14-2-1 Q06-14-2-2 Q06-14-2-3 Q06-14-2-4 Q06-14-2-5 Q06-14-2-6 Q06-14-2-7 Q06-14-2-8 Q06-14-2-9 Q06-14-2-10 Q06-14-2-11 Q06-14-2-12 Q06-14-2-13 Q06-14-2-14 Q06-14-2-15 Q06-14-2-16 Q06-14-2-17 Q06-14-2-18 Q06-14-2-19 Q06-14-2-20 Q06-14-2-21 Q06-14-2-22 Q06-14-2-23 Q06-14-2-24 Q06-14-2-25 Q06-14-2-26 Q06-14-2-27 Q06-14-2-28 Q06-14-2-29 Q06-14-2-30
Th ppm
65
66
S. Yu et al. / Lithos 198–199 (2014) 58–76
Fig. 9. Zircon LA-ICP-MS U–Pb results for mafic HP granulites from the Dulan area. Error ellipses are 1σ.
Sample Q06-14L (N36°31.478′ E98°23.133′) mainly consists of garnet (40%), clinopyroxene (35%), plagioclase (15%), quartz (10%), kyanite (2%), zoisite (3%) and rutile (1%). Rare amphibole generally occurs as symplectite or as inclusions in garnet. A population of idioblastic coarse garnet grains are surrounded by oriented kyanite, zoisite, plagioclase and rare clinopyroxene, defining the foliation and lineation (Fig. 5c). The sample shows compositional layering defined by alternating bands of Grt + Cpx + Pl and Zo + Ky ± Cpx. Three retrogressed HP granulite samples Q06-14 g, Q06-14 and Q08-14-4.4-2 show similar mineral assemblages of garnet, amphibole, plagioclase, biotite and quartz, with rare residual clinopyroxene (Fig. 5d). The amphibole generally occurs as the following two textures: (1) as symplectite intergrown with plagioclase; (2) as coarse-grained porphyroblasts. 4.2. Felsic HP granulite (leucosome) The two felsic HP granulite samples chosen for zircon U–Pb dating occur as thick layers in the mafic granulite, and probably represent the melt crystallized after partial melting. Felsic granulite sample Q08-15-3.1 (N36°31.671′, E98°23.414′) is mainly composed of garnet (15%), kyanite (5%), K-feldspar (10%), plagioclase (25%), and quartz (40%), with rare zoisite, amphibole and biotite as secondary minerals (Fig. 6a, b). A weak foliation is defined by oriented kyanite, K-feldspar, plagioclase and quartz. Sample Q08-14-4.3 (N36°31.412′ E98°23.477′) contains a peak mineral assemblage of garnet (10%) + kyanite (5%) + K-feldspar (15%) + plagioclase (35%) + quartz (30%) (Fig. 6c). Garnet occurs
mostly as porphyroblasts in the matrix, and occasionally as corona around kyanite (Fig. 6d). Microtextures indicating anatexis are described in detail below. 4.3. Microtextures of partial melting Unequivocal microtextural evidence for partial melting in thin section is rare owing to extensive deformation-driven recrystallization and reworking during protracted cooling and retrograde evolution of the rocks from the HP granulite-facies peak (see Sawyer, 2008). For the Dulan HP granulites, the best thin section evidence for partial melting consists of thin cuspate interstitial films of quartz, plagioclase and/or K-feldspar within the garnet-cumulate and/or meta-ultramafic (garnet pyroxenite) residuum (Fig. 7a, b and c). These are interpreted as crystallized pockets of melt that remained trapped within the residuum following migration of most of the melt away from its source. Clear microtextures indicative of crystallized melt are generally not preserved within the leucosomes (felsic HP granulite) owing to extensive recrystallization, although thin irregularly shaped films of K-feldspar or/and plagioclase with cuspate outlines locally occur between rounded grains of reactant quartz and plagioclase (Fig. 7d, e), which are interpreted to represent areas dominated by segregated crystallized melt (Johnson et al., 2012, 2013). In addition, the leucosomes contain rare euhedral clinopyroxene and garnet (Fig. 7f), which are abundant in the mafic HP granulite. The spatial association of peritectic clinopyroxene and garnet (the solid product of incongruent melting) with the leucosome (crystallized from the melt) is critical to an interpretation of in situ melting, as opposed to an origin via injection of melt from a more distal source.
Fig. 10. Chondrite-normalized REE patterns of zircon from Dulan mafic HP granulite.
2.8 4.7 3.6 3.0 8.5 7.9 3.3 4.4 4.3 2.4 5.4 2.9 7.1 4.1 3.7 2.9 5.0 4.5 4.3 3.5 5.1 3.8 4.9 3.5 3.0 8.3 8.5 3.2 4.9 4.5 2.6 6.2 2.8 8.5 4.7 4.0 2.7 5.2 5.0 4.8 3.9 5.7 8.3 20.4 13.0 9.3 26.8 44.4 11.1 16.7 20.4 11.1 30.6 13.9 42.6 13.9 21.3 11.1 19.4 25.0 15.7 16.7 30.6 0.00047 0.00079 0.00060 0.00049 0.00141 0.00132 0.00055 0.00073 0.00072 0.00040 0.00090 0.00049 0.00118 0.00068 0.00062 0.00047 0.00083 0.00075 0.00071 0.00058 0.00084
Pb/ Pb/ Pb/ Pb/
0.06999 0.06884 0.06961 0.06838 0.07028 0.06964 0.06878 0.07000 0.06985 0.06992 0.07001 0.06914 0.06957 0.07000 0.06981 0.06993 0.07005 0.07001 0.06970 0.06970 0.06890 0.00036 0.00053 0.00029 0.00022 0.00086 0.00117 0.00024 0.00045 0.00050 0.00027 0.00066 0.00022 0.00111 0.00047 0.00045 0.00029 0.00060 0.00054 0.00051 0.00041 0.00062 0.05731 0.05620 0.05489 0.05513 0.05961 0.05791 0.05585 0.05703 0.05665 0.05624 0.05689 0.05507 0.05714 0.05693 0.05600 0.05632 0.05591 0.05592 0.05693 0.05636 0.05575
Pb/ Pb/ ppm ppm
255 59 261 40 29 30 411 70 66 285 46 80 41 96 119 228 53 96 89 101 225 212 38 163 18 19 14 302 37 46 177 26 20 18 33 71 137 22 47 79 81 81 Q06-14L-1 Q06-14L-2 Q06-14L-3 Q06-14L-4 Q06-14L-5 Q06-14L-6 Q06-14L-7 Q06-14L-8 Q06-14L-9 Q06-14L-10 Q06-14L-11 Q06-14L-12 Q06-14L-13 Q06-14L-14 Q06-14L-15 Q06-14L-16 Q06-14L-17 Q06-14L-18 Q06-14L-19 Q06-14L-20 Q06-14L-21
ppm
0.83 0.65 0.63 0.45 0.67 0.46 0.73 0.53 0.70 0.62 0.56 0.25 0.44 0.34 0.59 0.60 0.41 0.49 0.89 0.80 0.40
23.35 5.22 20.47 36.55 2.92 1.45 36.18 5.71 5.28 22.46 5.32 18.03 3.56 6.25 7.92 15.80 2.24 7.18 8.78 8.76 8.55
206
Pb
±1σ
0.55367 0.53212 0.52637 0.51943 0.57234 0.54932 0.52906 0.55030 0.54432 0.54196 0.54837 0.52512 0.54601 0.54965 0.53856 0.54261 0.53828 0.53884 0.54680 0.54147 0.52898
U
±1σ
206 235 207
Pb Th/U U Th Spot
Table 2 LA-ICP-MS U–Pb analytical data for zircons from mafic HP granulite Q06-14L in the Dulan area, North Qaidam Mountains.
0.0058 0.0073 0.0053 0.0044 0.0129 0.0130 0.0048 0.0074 0.0069 0.0040 0.0094 0.0042 0.0130 0.0071 0.0061 0.0041 0.0079 0.0075 0.0073 0.0059 0.0085
238
U
±1σ
501.9 461.2 409.3 416.7 590.8 527.8 455.6 500.0 476.0 461.2 487.1 416.7 498.2 487.1 453.8 464.9 450.0 450.0 487.1 464.9 442.6
206
Ages (Ma)
207
Measured ratios
207
Pb
±1σ
207
447.4 433.2 429.4 424.8 459.5 444.5 431.2 445.2 441.3 439.7 443.9 428.6 442.4 444.8 437.5 440.1 437.3 437.7 442.9 439.4 431.1
235
U
±1σ
206
436.1 429.2 433.8 426.4 437.8 434.0 428.8 436.1 435.3 435.7 436.2 431.0 433.5 436.2 435.0 435.7 436.5 436.2 434.4 434.4 429.5
238
U
±1σ
Inclusion
No No Grt No Cpx + Rt Grt + Pl No No Cpx No No Grt + Pl No Pl No No No No Ap Rt + Qtz No
S. Yu et al. / Lithos 198–199 (2014) 58–76
67
5. Geochronology 5.1. Analytical methods 5.1.1. Zircon U–Pb geochronology Four representative fresh mafic (Q06-14-2, Q06-14L) and felsic (Q08-14-4.3, Q08-15-3.1) HP granulite samples were chosen for zircon U–Pb analyses. Zircons were separated from approximately 2–5 kg of each sample by crushing and sieving, followed by standard magnetic and heavy liquid separation techniques. After selection under a binocular microscope, zircons were mounted in epoxy resin and polished to expose the center of the grains. Mineral inclusions in zircon were identified by Laser Raman spectrophotometry, and the compositions were determined at the Laboratory of Continental Dynamics, Institute of Geology, CAGS, Beijing, China, using a JEOL JSM-5610 LV scanning microscope equipped with an EDS Oxford ISIS analytical system at conditions of 20 keV accelerating voltage and 45 nA beam current. Transmitted- and reflected-light images were taken to document the shape of zircon grains and their position in the mount. Cathodoluminescence (CL) imaging of the zircons was performed in a FEI PHILIPS XL30 SFEG instrument with 2 min scanning time at conditions of 15 kV and 120 nA at the Beijing SHRIMP Center, CAGS. Zircon U–Pb analyses of mafic granulite samples Q06-14-2 and Q06-14L, were conducted by LA-MC-ICP-MS at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing. Operating conditions for the laser ablation system, the MC-ICP-MS, and data reduction procedures are the same as described by Hou et al. (2009). Zircon GJ1 was used as the external standard for U–Pb dating, and was analyzed twice every 5–10 analyses (i.e., 2 zircon GJ1 + 5–10 samples + 2 zircon GJ1). Time-dependent drifts of U–Th–Pb isotopic ratios were corrected using a linear interpolation (with time) according to the variations of GJ1 (Liu et al., 2010). Concordia diagrams and weighted mean calculations were made using Isoplot/Ex ver. 3 (Ludwig, 2003). The zircon Plesovice was dated as an unknown sample and yielded a weighted mean 206Pb/238U age of 337 ± 2 Ma (2SD, n = 12), which is in good agreement with the recommended 206Pb/238U age of 337.13 ± 0.37 Ma (2SD) (Sláma et al., 2008). Trace element concentrations in the same or nearest U–Pb dated zircon domains of mafic granulite samples Q06-14-2 and Q06-14L were determined using a Thermo-Finnigan Element II sector field ICP-MS coupled to a New Wave UP213 nm ultraviolet laser system at the National Research Center for Geoanalysis, CAGS. Isotopes of 25 elements were analyzed in peak jumping mode using a 30 μm spot size, a repetition rate of 10 Hz and a laser energy of 90 mJ. Data were reduced offline using GLITTER 4.0 with 29Si for internal standardization and the NIST612 standard glass as the external calibration. The detailed analyzed method is described by Hu et al. (2008). Zircon U–Pb dating of felsic HP granulite samples Q08-14-4.3 and Q08-15-4.3 was carried out on a SHRIMP-II instrument at the Beijing SHRIMP Center, Institute of Geology, CAGS, Beijing. The spot size of the ion beam was ~ 30 μm diameter, and analytical procedures were similar to those described by Williams (1998). Standards were SL13, with an age of 572 Ma and U content of 238 ppm, and TEM, with an age of 417 Ma (Black et al., 2003; Williams, 1998). Age calculations were performed using the Isoplot and Squid programs (Ludwig, 2001, 2003) and IUGS recommended decay constants (Steiger and Jaeger, 1977), with data and concordia plots reported at 1σ error and uncertainties in weighted mean ages analyzed at the 95% confidence level (2σ). 5.1.2. 40Ar/39Ar dating of amphibole As described above, the amphiboles in mafic HP granulite occur as porphyroblasts or symplectite in the matrix, and occasionally as inclusions in the core of garnets (Yu et al., 2011a). Only coarse-grained porphyroblastic amphiboles from three retrogressed HP granulite samples Q06-14 g, Q06-14 and Q08-14-4.4-2 were chosen for
68
S. Yu et al. / Lithos 198–199 (2014) 58–76
Table 3 LA-ICP-MS rare earth elements of zircons from Dulan mafic HP granulite (×10−6). Sample
La
Ce
Pr
Q06-14-2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
0.090 0.103 0.005 0.034 0.017 0.054 0.056 0.249 0.009 0.029 0.006 0.010 0.154 0.044 0.000 0.016 0.038 0.033 0.609
10.23 7.47 2.94 5.12 3.48 6.01 12.02 8.00 6.36 4.14 7.68 6.78 8.81 8.80 4.61 9.22 5.99 3.02 11.06
0.13 0.18 0.04 0.05 0.04 0.09 0.32 0.23 0.06 0.05 0.07 0.08 0.23 0.10 0.04 0.12 0.08 0.04 0.22
006-14L 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
0.035 0.724 0.168 0.046 0.038 0.236 1.872 0.051 0.044 0.215 0.152 0.034 0.091 0.039 0.014 0.613 0.053
8.77 14.16 7.36 6.15 8.77 8.94 19.40 7.42 8.40 14.74 10.03 11.50 7.74 11.76 9.02 14.33 8.14
0.17 1.15 0.13 0.10 0.13 0.36 1.77 0.14 0.15 0.44 0.11 0.16 0.17 0.17 0.10 0.62 0.13
40
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Eu/Eu*
1.73 2.17 0.50 0.50 0.72 1.21 4.09 2.47 1.01 0.75 1.65 1.36 2.93 1.60 1.02 2.33 1.22 0.72 1.73
3.25 3.22 1.72 1.33 1.93 2.31 6.66 3.45 2.21 1.62 3.29 2.96 4.86 3.53 3.01 4.63 2.65 1.13 2.73
1.17 1.23 0.97 0.62 1.00 1.00 2.51 1.28 1.08 0.75 1.33 1.26 1.59 1.31 1.08 1.92 1.11 0.71 1.08
8.62 9.43 7.25 4.76 9.17 8.19 17.06 9.06 7.62 6.19 9.63 10.18 15.98 9.67 10.00 14.24 8.55 4.93 8.07
1.48 1.78 1.67 0.97 2.02 1.70 2.85 1.85 1.52 1.36 1.84 2.14 4.22 1.76 2.01 2.69 1.67 1.19 1.37
10.41 12.88 15.04 7.10 17.58 12.66 17.67 13.41 11.28 10.92 12.87 14.36 45.68 12.01 15.01 20.04 12.79 11.11 9.15
2.27 3.05 3.75 1.77 4.86 3.02 3.62 3.04 2.74 2.77 2.92 3.34 15.24 2.42 3.61 4.60 3.02 2.80 1.83
7.18 10.29 12.86 5.87 17.50 10.08 10.40 10.53 8.89 9.71 9.75 10.46 68.19 7.35 12.89 15.46 10.23 11.24 5.20
1.24 1.82 2.39 1.01 3.42 1.78 1.65 1.70 1.54 1.75 1.62 1.79 14.20 1.30 2.22 2.62 1.73 2.00 0.82
8.83 13.61 19.77 7.83 26.44 13.68 12.55 13.22 11.65 13.85 12.24 13.86 122.38 9.50 17.14 20.02 13.49 15.96 5.80
1.48 2.41 3.73 1.34 4.88 2.34 2.02 2.30 1.95 2.41 1.97 2.24 21.78 1.56 2.99 3.42 2.23 2.94 0.89
0.64 0.64 0.72 0.68 0.60 0.63 0.68 0.66 0.72 0.63 0.67 0.63 0.50 0.64 0.54 0.67 0.65 0.78 0.65
2.21 11.03 1.19 1.57 2.11 3.78 15.11 2.12 1.78 4.71 1.99 2.70 1.81 2.49 1.56 5.91 1.97
3.75 8.33 1.85 2.11 3.48 3.77 9.38 2.91 2.82 6.04 3.32 5.61 2.80 4.99 2.46 6.49 3.23
1.48 2.95 0.86 0.83 1.40 1.70 3.11 1.40 1.27 2.58 1.53 2.80 1.19 2.56 1.15 2.84 1.56
10.38 22.50 5.21 6.47 9.76 9.75 21.74 9.14 8.91 15.74 11.12 22.61 8.78 18.98 8.01 22.08 11.24
2.14 4.44 1.03 1.32 2.03 2.04 4.47 1.72 1.71 2.95 2.25 5.15 1.86 4.43 1.56 4.84 2.49
15.78 38.72 7.64 10.29 16.06 13.61 38.60 13.39 12.72 19.78 16.18 40.89 14.67 36.03 11.66 39.30 22.18
3.72 9.99 1.75 2.33 3.80 3.31 10.53 2.98 3.20 4.37 3.71 10.22 3.73 9.05 2.46 9.57 5.93
12.02 36.60 5.14 8.22 11.01 8.84 39.74 9.02 10.46 11.75 10.83 33.51 14.09 30.08 7.78 31.61 23.82
1.93 6.72 0.93 1.40 1.85 1.41 7.91 1.52 1.85 1.93 1.82 5.85 2.76 5.46 1.29 5.40 4.87
14.97 56.22 7.17 12.02 14.79 11.91 65.51 12.48 15.66 14.01 13.36 44.58 24.21 41.98 9.62 40.87 42.85
2.64 10.44 1.22 2.38 2.56 1.82 12.28 2.09 3.02 2.18 2.18 8.01 4.79 7.04 1.59 7.13 8.33
0.68 0.62 0.79 0.63 0.69 0.81 0.64 0.76 0.71 0.77 0.70 0.66 0.67 0.71 0.72 0.65 0.71
Ar/39Ar dating. The amphiboles were purified using magnetic separation, and further cleaned in an ultrasonic bath with ethanol. Each amphibole sample was wrapped in aluminum foil and loaded into an aluminum tube, and then sealed into a quartz bottle (height: 40 mm; diameter: 50 mm). The bottle was irradiated for 65 h at the nuclear facility Swimming Pool Reactor, Chinese Institute of Atomic Energy, Beijing, China. The reactor delivered a neutron flux of ~6 × 1012 n/cm2/s and the integrated neutron flux is about 1.16 × 1018 n/cm2. Ar isotope analysis was carried out on a MM-1200B mass spectrometer at the Institute of Geology, CAGS, Beijing, China. The measured isotopic ratios
were corrected for mass discrimination, atmospheric Ar component, procedural blanks and mass interference induced by irradiation. The blanks at m/e of 40Ar, 39Ar, 37Ar and 36Ar are less than 6 × 10−15, 4 × 10−16, 8 × 10−17 and 2 × 10−17 mol, respectively. The correction factors of interfering isotopes produced during the irradiation were determined by the analysis of irradiated K2SO4 and CaF2 pure salts. Their values are (36Ar/37Aro)Ca = 0.0002389, (40Ar/39Ar)K = 0.004782 and (39Ar/37Aro)Ca = 0.000806. All 37Ar measurements were corrected for radiogenic decay (half-life 35.1 days). The 40K decay constant used is 5.543 × 10−10 a−1 (Steiger and Jaeger, 1977). The standard sample is
Fig. 11. Cathodoluminescence (CL) images showing internal textures of zircons from mafic HP granulite Q06-14L.
S. Yu et al. / Lithos 198–199 (2014) 58–76
69
Fig. 12. Cathodoluminescence (CL) images showing internal textures of zircons from felsic HP granulite Q08-15-3.1. DC: dark, irregular CL zoned zircon, BR: CL-bright, thin unzoned zircon rims; TB: weakly zoned to unzoned zircon with high luminescence.
ZBH-25, which has an age of 132.7 ± 1.2 Ma and a potassium content of 7.6% (Chen et al., 2002). The uncertainties on the apparent ages on each step are quoted at the 1σ level, but weighted mean plateau ages and isochron ages are given at the 2σ level (Ludwig, 2001). 5.2. Zircon U–Pb dating results 5.2.1. Mafic granulite sample Q06-14-2 Zircons from sample Q06-14-2 are translucent, spherical and multifaceted crystals 40–100 μm in diameter. CL images reveal well-developed sector zoning and oscillatory zoning (Fig. 8). Garnet, clinopyroxene, plagioclase, rutile and quartz were identified as inclusions in zircons, which is similar to the peak mineral assemblage of mafic HP granulite, and indicates that zircon is of HP granulite-facies metamorphic origin. The detailed zircon U–Pb data of sample Q06-14-2 are listed in Table 1. A total of 29 laser ablation spots on 29 zircon grains yielded 206Pb/238U ages between 427.3 ± 3.2 Ma and 443.6 ± 4.4 Ma, with a weighted mean of 435 ± 1 Ma (MSWD = 0.9) (Fig. 9a). All zircon grains show Th/U ratios of 0.14–0.49. Zircon REE
shows strong HREE depletion and moderate negative Eu anomalies (Fig. 10a; Table 3), consistent with growth under HP granulite conditions (Rubatto, 2002). 5.2.2. Mafic granulite sample Q06-14L Zircons from sample Q06-14L are oval to spherical crystals 50–100 μm in diameter. CL images reveal well-developed oscillatory zoning and sector zoning (Fig. 11). Similar to sample Q06-14-2, garnet, clinopyroxene, plagioclase, rutile and quartz were identified as inclusions in zircons of sample Q06-14L, indicating that zircon is of HP granulite-facies metamorphic origin. The detailed zircon U–Pb data of sample Q06-14L are listed in Table 2. Twenty-one U–Pb analyses on 21 zircon grains are highly clustered, yielding a weighted mean 206 Pb/238U age of 434 ± 3 Ma (MSWD = 0.71) (Fig. 9b). Th and U concentrations of zircons range from 14 to 302 ppm, and from 29 to 573 ppm, respectively, yielding Th/U ratios from 0.25 to 0.89. Zircon REE shows strong HREE depletion and moderate negative Eu anomalies (Fig. 10b; Table 3), consistent with growth under HP granulite conditions (Rubatto, 2002).
Fig. 13. Zircon SHRIMP U–Pb results for felsic HP granulites from the Dulan area. Error ellipses are 1σ.
70
S. Yu et al. / Lithos 198–199 (2014) 58–76
Fig. 14. Cathodoluminescence (CL) images showing internal textures of zircons from felsic HP granulite Q08-14-4.3 (the scale is 0.02 mm).
Rare bright zircon grains with weak zoning are also recognized, probably indicating metamorphic origin (Ayers et al., 2002; Corfu et al., 2003). Mineral inclusions in zircons comprise the assemblage of clinopyroxene, plagioclase, rutile, and quartz, which is indicative of HP granulite-facies metamorphism. Occasionally, muscovite is observed as an inclusion in zircon, likely representing a later metamorphic overprinting event. Twenty-one SHRIMP analyses on 21 zircon grains yielded 206Pb/238U ages between 372 ± 8 Ma and 482 ± 12 Ma with variable Th/U ratios of 0.01–0.82 (Table 5). Despite the apparent variance in morphologies and internal structures of zircons, no distinct zircon age populations can be readily distinguished. Of the 21 analyses, one of the zircons with mottled zones yielded a considerably older age (ca. 482 Ma) than the other analyses, and is not included in the age calculation for the sample. The 206Pb/238U ages of a few spots are statistically slightly younger, which may indicate Pb-loss or late metamorphic overprinting. Excluding these analyses, the remaining 14 spots gave a weighted mean 206 Pb/238U age of 438 ± 4 Ma (MSWD = 1.6) (Fig. 13b).
5.2.3. Felsic granulite sample Q08-15-3.1 Zircons from sample Q08-15-3.1 are colorless, elongate, subhedral crystals 100–300 μm long, with the main population around 200 μm (Fig. 12). Based on morphological and internal patterns, two types of zircons can be identified. Most grains (Type I) exhibit dark, irregular CL zoning (denoted as DC), with or without CL-bright, thin unzoned rims (denoted as BR); spot analysis of the BR zones is difficult due to their narrow width (b25 μm). The boundaries between DC and BR domains are irregular and sharp. Rare zircons (Type II) are weakly zoned to unzoned with high luminescence (denoted as TB), indicative of metamorphic origin (Corfu et al., 2003). Quartz and clinopyroxene inclusions are observed in both types of zircons from sample Q08-15-3.1. Twelve SHRIMP analyses on 12 grains including both types of zircons gave 206 Pb/ 238 U ages between 405 ± 11 Ma and 450 ± 7 Ma (Table 4). There is no correlation between age and zircon morphology or spot location. The 206Pb/238U ages of three analyses are slightly younger, which may indicate Pb-loss or late metamorphic overprinting. The remaining 9 spots gave a weighted mean 206 Pb/238U age of 433 ± 5 Ma (MSWD = 1.8) (Fig. 13a).
5.3. 40Ar/39Ar dating results The results of 40Ar/39Ar dating are shown in Tables 6, 7 and 8. The amphibole samples (Q06-14 g, Q06-14 and Q08-14-4.4-2) yielded well-defined plateau ages of 424 ± 4 Ma, 423 ± 2 Ma and 432 ± 2 Ma, and inverse isochron ages of 422 ± 5 Ma, 426 ± 8 Ma and 434 ± 3 Ma for mafic HP granulite in the SDU (Fig. 15). The
5.2.4. Felsic granulite sample Q08-14-4.3 Zircons from sample Q08-14-4.3 are translucent, elongate, subhedral crystals 40–100 μm in diameter. CL images reveal that most zircon grains exhibit oscillatory or irregular zoning, with low luminescence (Fig. 14).
Table 4 SHRIMP U–Pb analytical data for zircon from Q08-15-3.1 in the Dulan area, North Qaidam Mountains. Spot
U ppm
Th ppm
Th/U
Com. Pb (%)
Pb (ppm)
207
Pb/235U
±%
206
Pb/238U
±%
206 Pb/238U Age (Ma)
1σ
Inclusions
Q08-15-3.1-1 Q08-15-3.1-2 Q08-15-3.1-3 Q08-15-3.1-4 Q08-15-3.1-5 Q08-15-3.1-6 Q08-15-3,1-7 Q08-15-3,1-8 Q08-15-3,1-9 Q08-15-3.1-10 Q08-15-3.1-11 Q08-15-3.1-12
617 168 4627 356 1128 104 1456 807 2015 59 455 316
35 34 5644 22 41 16 161 959 337 14 29 13
0.06 0.21 1.26 0.06 0.04 0.16 0.11 1.23 0.17 0.26 0.07 0.04
0.11 0.82 0.01 0.73 1.07 1.38 0.60 0.22 0.40 2.08 0.33 0.99
36.1 9.45 264 21.6 64.8 6.42 85.4 47.8 126 3.56 27.8 18.5
0.534 0.491 0.4973 0.501 0.504 0.501 0.517 0.527 0.557 0.439 0.528 0.487
2.8 7.0 1.8 4.1 3.0 12 2.7 2.8 2.6 16 2.5 6.0
0.0680 0.0648 0.0664 0.0700 0.0662 0.0708 0.0679 0.0688 0.0724 0.0693 0.0707 0.0675
1.7 2.9 1.6 1.8 1.7 2.1 1.7 1.9 1.6 2.4 1.7 1.8
424 405 415 436 413 441 423 429 450 432 441 421
7 11 7 8 7 9 7 8 7 10 7 7
No No Qtz No No Cpx Qtz No Cpx No No No
S. Yu et al. / Lithos 198–199 (2014) 58–76
71
Table 5 SHRIMP U–Pb analytical data for zircon from Q08-14-4.3 in the Dulan area, North Qaidam Mountains. Spot
U ppm
Th ppm
Th/U
Com. Pb (%)
Pb
207
Pb/235U
±%
206
±%
206 Pb/238U Age (Ma)
1σ
Inclusions
Q08-14-4.3-1 Q08-14-4.3-2 Q08-14-4.3-3 Q08-14-4.3-4 Q08-14-4.3-5 Q08-14-4.3-6 Q08-14-4.3-7 Q08-14-4.3-8 Q08-14-4.3-9 Q08-14-4.3-10 Q08-14-4.3-11 Q08-14-4.3-12 Q08-14-4.3-13 Q08-14-4.3-14 Q08-14-4.3-15 Q08-14-4.3-16 Q08-14-4.3-17 Q08-14-4.3-18 Q08-14-4.3-19 Q08-14-4.3-20 Q08-14-4.3-21
358 498 80 211 101 122 148 88 102 567 294 695 284 26 98 465 824 210 534 493 194
188 353 20 82 23 16 46 19 47 106 189 326 188 b1 34 348 539 54 426 194 111
0.54 0.73 0.26 0.40 0.24 0.14 0.32 0.22 0.48 0.19 0.67 0.49 0.69 0.01 0.36 0.77 0.68 0.27 0.82 0.41 0.59
0.47 0.08 0.97 0.03 1.57 1.62 0.05 2.58 3.11 – 0.52 0.40 0.61 8.26 2.99 0.49 0.25 0.58 0.35 0.23 0.62
22.2 30.4 4.87 13.3 6.48 6.67 9.87 5.22 5.40 34.1 17.9 41.8 17.5 1.46 5.70 25.6 48.7 12.9 31.4 29.5 11.4
0.517 0.555 0.509 0.598 0.460 0.386 0.645 0.36 0.268 0.542 0.528 0.504 0.508 – 0.365 0.448 0.509 0.522 0.497 0.525 0.480
3.4 2.3 8.8 3.4 12 17 6.8 29 25 2.4 3.6 2.8 4.4 – 19 3.8 2.4 4.9 3.3 2.8 10
0.0716 0.0709 0.0706 0.0733 0.0732 0.0625 0.0776 0.0673 0.0594 0.0700 0.0707 0.0698 0.0714 0.0597 0.0655 0.0638 0.0687 0.0713 0.0682 0.0695 0.0679
1.7 1.7 2.2 1.8 2.2 2.5 2.5 2.7 2.3 1.8 2.0 1.7 1.8 5.0 2.2 1.8 1.7 1.9 1.7 2.0 1.9
446 442 440 456 455 391 482 420 372 436 440 435 445 374 409 399 428 444 425 433 424
7 7 9 8 9 9 12 11 8 8 8 7 7 18 8 7 7 87 77 8 8
No No Cpx, Qtz Qtz No No No No Qtz Qtz Qtz No No No Ms, Qtz No No Qtz, Pl No No Rt, Qtz
agreement between 40Ar/36Ar intercept values and the atmosphere value (295.5) is within error for three samples, and together with the agreement between the plateau and reverse isochron ages, indicates the absence of an excess argon component in the amphibole. 6. Discussion 6.1. Age of HP metamorphism, partial melting and cooling Zircon U–Pb analyses yield average ages of 435 ± 1 Ma and 434 ± 3 Ma from mafic HP granulite samples Q06-14-2 and Q06-14L, respectively. The oval to spherical zircon morphology and weak sector and oscillatory CL zoning is similar to zircons growing during partial melting (Keay et al., 2001; Rubatto et al., 2001; Vavra et al., 1999), and REE patterns and the mineral inclusion assemblage of garnet, clinopyroxene, plagioclase and quartz demonstrate that the zircons grew under HP granulite-facies conditions. The relatively high Th/U ratios (0.14–0.89) of zircons may be related to dissolution of Th-rich mineral phases (e.g. monazite or zoisite) or may reflect protolith characteristics (e.g. Bröcker et al., 2010; Hermann, 2002; Schulz et al., 2006). Schaltegger et al. (1999) suggested that HP granulite-facies zircons experience a complex evolutionary history within a short time span, with highly variable internal zircon structures recording distinct stages: (1) subsolidus growth produces round anhedral morphologies and sector zoning; (2) growth in an intergranular fluid or melt phase during incipient dehydration melting produces planar or oscillatory growth zones in euhedral equant or prismatic zircons; and (3) advanced
Pb/238U
melting generates euhedral prismatic zircons with oscillatory zoning overgrowing the sector zones. In the SDU, although zircons in felsic HP granulite samples show two distinct internal CL textures, there is no correlation between age and zircon morphology, internal texture or spot location. We interpret these observations to reflect variations in the time of crystallization of individual zircons during the process of partial melting and HP granulite-facies metamorphism. The irregular and oscillatory-zoned zircon domains with low luminescence (DC) are similar to zircons crystallized from hydrous melts during anatexis (Chen et al., 2010; Li et al., 2013; Liati, 2005). The weakly zoned to unzoned zircon domains with high luminescence (BR and TB) resemble those precipitated from metamorphic aqueous fluid (Li et al., 2013; Rubatto and Hermann, 2003). Therefore, we interpret the zircon U–Pb ages of 433 ± 5 Ma and 438 ± 4 Ma from felsic HP granulite to represent the time of HP granulite-facies metamorphism and partial melting, which is consistent with the ages obtained from mafic HP granulite. Partial melting at this time is also consistent with 428–437 Ma zircon ages from leucosomes and tonalites in the Dulan HP granulite unit (Yu et al., 2012). In view of the 490–578 °C Ar closure temperature of amphibole (Harrison, 1981), we conclude that our amphibole 40Ar/39Ar ages of 423–432 Ma represent the time of cooling during the amphibolitefacies retrogression. Significantly, the amphibole 40Ar/39Ar ages (423–432 Ma) from the HP granulite are only slightly younger than the zircon ages (433–438 Ma), suggesting rapid cooling and exhumation of the rocks, which has been also reported in Bohemian Massif (Kotkova et al., 1996; Willner et al., 1997; Zulauf et al., 2002). Rapid
Table 6 Results of 40Ar/39Ar stepwise heating analysis for amphibole from mafic HP granulite Q06-14 g. T (°C)
(40Ar/39Ar)m
(36Ar/39Ar)m
(37Ar/39Ar)m
(38Ar/39Ar)m
40
Ar (%)
F
39
39 Ar (%)
Age (Ma)
±1σ (Ma)
500 600 700 800 900 1000 1100 1160 1220 1300 1400
43.5523 38.8337 37.3064 33.0026 33.3484 37.1113 47.8509 48.6858 48.8877 49.5704 51.1610
0.0249 0.0128 0.0146 0.0141 0.0103 0.0127 0.0062 0.0028 0.0047 0.0079 0.0123
2.4428 2.5590 1.9694 1.9843 1.2371 4.2818 4.0630 3.5932 4.5187 8.3722 11.2626
0.0379 0.0271 0.0286 0.0409 0.0324 0.0378 0.0200 0.0179 0.0196 0.0194 0.0208
83.49 90.69 88.79 87.83 91.14 90.70 96.74 98.82 97.81 96.50 94.43
36.4331 35.2904 33.1779 29.0321 30.4242 33.7756 46.4454 48.2507 47.9902 48.1613 48.7554
93.48 71.64 66.49 85.79 121.10 167.47 1086.68 1281.75 440.71 292.36 171.22
2.41 4.26 5.97 8.18 11.31 15.62 43.64 76.69 88.05 95.59 100.00
328.4 318.9 301.4 266.3 278.2 306.4 409.1 423.2 421.2 422.5 427.2
4.1 4.5 5.3 3.7 4.0 3.8 4.3 3.9 4.4 4.8 4.6
Ar
72
S. Yu et al. / Lithos 198–199 (2014) 58–76
Garrido et al., 2006; Amalaoulaou Massif, Berger et al., 2008) coexist with residues dominated by clinopyroxene, garnet and rutile. Based on macroscopic and microscopic observations of anatexis in the Dulan HP granulite unit, the residuum mainly consists of garnet, clinopyroxene and rare rutile and sphene; modal proportions of clinopyroxene and garnet are higher in the residuum than in the paleosome metabasite. In addition, euhedral–subhedral and coarse-grained clinopyroxene and garnet represent peritectic phases in leucosome. Thus, the generation of tonalitic and trondhjemitic felsic melts accompanied by the production of garnet and clinopyroxene in the Dulan area is consistent with partial melting of amphibole according to the reactions above. Furthermore, kyanite is also recognized in Dulan leucosomes (Fig. 6; Yu et al., 2011a), which indicates that the zoisite was also a reactant during partial melting. Dehydration melting of zoisite can form LREE-Sr-rich tonalitic to trondhjemitic melt (Skjerlie and Patiño Douce, 2002) geochemically similar to the Dulan felsic granulite (leucosome), which resembles adakite (Yu et al., 2012). In contrast, dehydration melting of mica in metabasite generates melt rich in K, Na and Al (Schmidt and Poli, 2003). The highSr-LREE, positive Eu anomalies, and low-K compositions of the Dulan felsic HP granulite (leucosome) are different from the melt produced by mica dehydration melting, but consistent with those produced by zoisite and amphibole dehydration melting. In experiments on the water– undersaturated metabasalt system, amphibole dehydration melting occurs at 650 °C between 1.5 and 2 GPa (Rapp and Watson, 1995), and fluid-absent dehydration melting of amphibole generally initiates at temperatures N 800 °C and at pressures N1.5 GPa (López and Castro, 2001). Zoisite dehydration melting in metabasites occurs at ~ 850 °C and 1.5 GPa (Skjerlie and Patiño Douce, 2002). The P–T path of Dulan HP granulite (Yu et al., 2011a) crosses the amphibole and zoisite stability curves constrained by these experiments. Thus, the Dulan felsic HP granulite (leucosome) likely formed from melt produced by both amphibole and zoisite dehydration melting.
cooling of HP granulite in the SDU is also supported by the preservation of chemical zoning in garnet related to early prograde metamorphism (Yu et al., 2011a), which defines a restricted residence time at peak metamorphic temperature and rapid cooling rate (Cooke et al., 2000; Klemd and Brocker, 1999; Medaris et al., 2006). 6.2. The relationship between HP granulite-facies metamorphism and partial melting Both macroscopic and microscopic observations provide strong evidence that felsic and mafic HP granulites record in situ partial melting in the Dulan unit, as described above. Field evidence for in situ partial melting is provided by injection patches, lenses, and layering of leucosome within the mafic HP granulite with variable migmatitic character (Figs. 3, 4). The best thin section evidence for in situ partial melting is thin irregularly shaped films of quartz, plagioclase, and/or K-feldspar within garnet-cumulate or garnet pyroxenite (Fig. 7). In addition, our new and previous geochronological data indicate that partial melting and HP granulite-facies metamorphism occurred contemporaneously between 433 and 438 Ma. The affinity between mafic and felsic granulites is also supported by similar Nd and Hf isotopic values (Yu et al., 2012). Furthermore, dehydration melting experiments confirm that the Dulan melt compositions (leucosome or felsic HP granulite) should form at P N 1.5 GPa and T N 900 °C (Sen and Dunn, 1994; Yu et al., 2012), which agrees well with the peak metamorphic condition of the adjacent mafic HP granulite (Yu et al., 2009, 2011a). Consequently, field relationships, age resemblance, geochemistry, and P–T conditions suggest that HP granulite-facies metamorphism and partial melting resulted from a single tectonic event. In this case, melt derived from partial melting of metabasite rocks would leave garnet-cumulate and/or meta-ultramafic rocks (garnet pyroxenite) as the residual component. Similar scenarios of synchronous HP granulite-facies metamorphism and in situ anatexis have also been reported in other HP granulite units. For example, Stowell et al. (2010) proposed that the anatexis accompanied HP granulite-facies metamorphism in Fiordland, New Zealand, with crystallization of adakitic melt b3–10 Ma later. Experiments and natural observations suggest that the breakdown of hydrous minerals (e.g. micas, amphibole and zoisite) plays a principal role in triggering the partial melting of HP/UHP metamorphic rocks (Zheng et al., 2011), following the reactions: amphibole + plagioclase → garnet + clinopyroxene ± sphene ± epidote + melt (López and Castro, 2001; Patiño Douce, 2005); zoisite + quartz → garnet + kyanite + melt; muscovite (phengite) + quartz → garnet + kyanite + melt (Patiño Douce, 2005). Tonalitic and trondhjemitic melts produced experimentally by dehydration melting of amphibole (López and Castro, 2001; Rapp and Watson, 1995; Sen and Dunn, 1994) and also observed in nature (e.g. Kohistan complex,
6.3. Tectonic implications In the SDU, Song et al. (2003a) proposed that HP granulites resulted from the high-temperature overprint of kyanite-bearing eclogite during slow exhumation associated with the effects of thermal relaxation. However, recent field, petrology, and mineral chemistry evidence together with our new data suggests that the HP granulite did not undergo eclogite-facies metamorphism, and instead experienced an independent metamorphic history, probably in thickened continental crust (Yu et al., 2009, 2011a). The main lines of evidence are: (1) no direct contact between the eclogite unit and HP granulite unit has been observed and intervening outcrops show strong amphibolite-facies deformation fabrics (Zhang and Yu, unpublished data), which suggests a tectonic relationship. (2) The inclusions of amphibole, epidote, plagioclase and quartz, together with typical prograde growth zoning in garnet
Table 7 Results of 40Ar/39Ar stepwise heating analysis for amphibole from mafic HP granulite Q06-14. T (°C)
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
40
Ar*/39Ark
40
Ar* (%)
39 Ark (%)
Age (Ma)
±2σ
850 °C 920 °C 970 °C 1010 °C 1020 °C 1030 °C 1040 °C 1050 °C 1060 °C 1070 °C 1080 °C 1120 °C 1200 °C 1400 °C
148.0791 154.6745 132.3625 108.0968 105.5874 101.6032 97.95459 95.68210 95.40985 96.32143 100.5268 105.4401 109.6832 99.50653
1.63092 2.27827 5.54514 4.42595 4.13872 3.78180 3.60261 3.51141 3.49599 3.51665 3.66689 4.20506 5.31578 4.41017
0.17362 0.20660 0.10674 0.02992 0.01490 0.00984 0.00631 0.00489 0.00378 0.00425 0.00383 0.01117 0.01676 0.00941
97.03849 93.98699 101.74302 99.98406 101.87247 99.31755 96.67365 94.79833 94.85428 95.63168 99.99986 102.84197 105.63297 97.44396
65.44 60.65 76.50 92.15 96.14 97.44 98.39 98.78 99.12 98.99 99.17 97.19 95.87 97.56
1.57 1.25 1.07 3.36 3.02 5.62 14.17 19.81 18.66 11.53 3.70 2.75 2.66 10.82
428.71 416.66 447.12 440.25 447.62 437.65 427.27 419.87 420.09 423.16 440.32 451.39 462.20 430.30
6.03 7.67 11.20 3.32 2.94 2.76 2.52 2.37 3.00 3.25 3.74 3.41 4.21 3.22
S. Yu et al. / Lithos 198–199 (2014) 58–76
suggest that the HP granulite evolved from amphibolite-facies to HP granulite-facies without experiencing eclogite-facies metamorphism. (3) Our new geochronological results show that the HP granulite recorded peak metamorphism between 433 and 438 Ma, which overlaps with the ages of UHP metamorphism for eclogite (430–446 Ma; Zhang et al., 2010). These identical ages are inconsistent with the possibility that the HP granulite was derived from overprinting of pre-existing eclogite during slow exhumation. Similar scenarios of closely juxtaposed HP granulite and UHP eclogite with different peak metamorphic conditions but identical metamorphic ages have been reported in many typical
73
collisional orogenic belts, such as in the Bohemian and Iberian massifs in the Variscan orogeny (Konopasek and Schulmann, 2005; Puelles et al., 2005), the Argentera Massif in the western Alps (Ferrando et al., 2008), and the Himalayas, Qinling and South Altyn in China (Zhang et al., 2009b, 2010). The contrasting metamorphic histories defined by distinct P–T conditions and geothermal gradients suggest that eclogite and HP granulite are not derived from one tectonic environment. The UHP eclogite in the SDU is thought to have formed in subducted crust in a relatively low geothermal gradient (6–10 °C/km) at depths N90 km, although the nature of the subduction is disputed (Song et al.,
Fig. 15. 40Ar/39Ar age plateaux and inverse isochrons for the amphibole from mafic HP granulite.
74
S. Yu et al. / Lithos 198–199 (2014) 58–76
Table 8 Results of 40Ar/39Ar stepwise heating analysis for amphibole from mafic HP granulite Q08-14-4.4-2. T (°C)
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
40
40 Ar* (%)
39 Ark (%)
Age (Ma)
±2σ
750 °C 850 °C 920 °C 970 °C 1000 °C 1020 °C 1030 °C 1040 °C 1050 °C 1060 °C 1080 °C 1200 °C 1400 °C
2655.94587 161.47184 124.28456 118.68555 105.14085 99.74564 99.29590 101.34033 100.49289 103.17165 128.83581 116.52324 110.06192
2.00346 1.14052 1.53139 4.76234 5.12770 4.75721 4.64751 4.65687 4.65160 4.80352 5.59662 5.65470 5.27356
8.69717 0.20441 0.09114 0.07622 0.02425 0.01089 0.00827 0.01262 0.00780 0.00608 0.04153 0.03827 0.03446
86.237313 101.257311 97.600794 96.934001 98.813301 97.298460 97.606888 98.372601 98.949108 102.176644 117.569014 106.174963 100.750673
3.24 62.65 78.43 81.34 93.57 97.15 97.91 96.69 98.07 98.63 90.82 90.68 91.13
1.56 2.24 2.83 2.99 5.86 14.54 16.14 14.23 14.88 8.84 2.50 3.22 10.16
386.72 446.37 432.03 429.40 436.80 430.84 432.05 435.06 437.33 449.96 508.99 465.48 444.39
±258.02 ±10.67 ±5.67 ±4.46 ±2.92 ±2.50 ±3.06 ±2.59 ±2.66 ±2.90 ±4.26 ±3.54 ±3.04
2003b; Yu et al., 2012; Zhang et al., 2008b, 2009a, 2010). In contrast, the peak P–T conditions of the HP granulites indicate burial to depths of approximately 50–60 km in a relatively higher geothermal gradient (15–18 °C/km), which is consistent with formation in the base of thickened crust of the overriding plate (Zhang et al., 2009b). In addition, the supracrustal metapelite enclosing the HP granulite in the Dulan area also experienced coeval HP granulite facies metamorphism, with a peak mineral assemblage of garnet, ternary-feldspar, kyanite, rutile and quartz (Christensen, 2011; Christensen and Mattinson, 2011; Yu et al., unpublished data), and likely originated in the upper and/or middle crust. This suggests that the supracrustal metapelite and enclosed mafic HP granulite may have occurred as part of the overriding plate that was dragged to the base of the lower crust at upper mantle depths by subduction erosion (Hacker et al., 2011). This model also agrees well with the clockwise P–T path of the mafic granulite (Yu et al., 2011a), but additional research is needed to test this model. Differences in thermal gradients for penecontemporaneous UHP metamorphism and HP–HT metamorphism can be explained by applying a paired metamorphic belt model in a subduction-tocollision orogenic system, analogous to the more common, classic paired metamorphic belt in accretionary orogens (Brown, 2009; Zhang et al., 2009a,b,c). Classic ‘paired’ metamorphic belts, proposed by Miyashiro, 1961, are idealized as an inboard high dT/dP metamorphic belt juxtaposed against an outboard low dT/dP metamorphic belt along a tectonic contact. Examples of these belts in accretionary orogens include the Ryoke and Sanbagawa belts in Japan. Following the recognition of ultrahigh-temperature (UHT) and UHP metamorphism (Chopin, 1984; Ellis, 1980; Hensen and Harley, 1990; Smith, 1984), the expanded P–T range has led to the subdivision of other P–T regimes, such as granulite-ultrahigh-temperature metamorphism (G-UHTM), medium-temperature eclogite–high pressure granulite metamorphism (E-HPGM) and high-pressure metamorphism–ultrahighpressure metamorphism (HPM–UHPM). These distinct systems occur in a variety of tectonic settings with contrasting thermal regimes at convergent plate boundary zones (Brown, 2006, 2007, 2008, 2009, 2010). Therefore, the paired metamorphic belt model has been applied to subduction-to-collision orogenic systems in addition to accretionary orogenic systems. Such a model represents penecontemporaneous belts of contrasting metamorphic type that record different apparent thermal gradients, one warmer and the other colder, juxtaposed by plate tectonic processes (Brown, 2010). The combination of HPM–UHPM and/or E-HPGM with G-UHTM in the geological record supports paired metamorphism, where the suture and lower plate materials experience the imprint of low dT/dP and the upper plate registers penecontemporaneous high dT/dP metamorphism (Brown, 2009, 2010). In the SDU, the HP granulite belongs to the E-HPGM region, whereas the UHP eclogite represents HPM–UHPM conditions. This suggests that the HP granulite unit and penecontemporaneous UHP eclogite in SDU may represent a paired metamorphic belt formed during the Silurian.
Ar*/39Ark
7. Conclusions (1) Macroscopic and microscopic observations provide strong evidence that felsic and mafic HP granulites record in situ partial melting and migmatization, and the felsic HP granulite represents the leucosome crystallized from the former sites of melt segregation and/or accumulation. (2) Zircon U–Pb ages suggest that the Dulan unit of the NQD experienced HP granulite-facies metamorphism and associated partial melting during the Silurian between 432 and 438 Ma. (3) Amphibolite-facies retrograde metamorphism of the HP granulite occurred at 423–432 Ma based on 40Ar/39Ar dating of amphibole, suggesting rapid cooling and exhumation of HP granulite in SDU. (4) The mafic HP granulite and felsic HP granulite (leucosome) with adakitic characteristics formed in one tectonic event, probably in the upper plate of the collision zone: the felsic HP granulites formed from an adakitic melt derived from partial melting of zoisite and amphibole in metabasite rocks, leaving behind residual garnet-cumulate and/or meta-ultramafic (garnet pyroxenite). (5) Penecontemporaneous metamorphic ages but different apparent geothermal gradients between HP granulite and UHP eclogite define a paired metamorphic belt generated in a subduction– collision setting associated with a major continental collisional orogeny. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant nos. 41202037, 41072151 and 41272210), the Ministry of Land and Resources of China (Nos. 201011034), the Geological Survey Project of China (No. 1212011120157), and the Basic Foundation of the Institute of Geology, CAGS (Grant no. J1202). The paper was substantially improved by the constructive reviews of two journal reviewers and Editor Marco Scambelluri. References Anderson, J.R., Payne, J.L., Kelsey, D.E., Hand, M., Collins, A.S., Santosh, M., 2012. High pressure granulites at the dawn of the Proterozoic. Geology 40, 431–434. Ayers, J.C., Dunkle, S., Gao, S., Miller, C.F., 2002. Constraints on timing of peak and retrograde metamorphism in the Dabie Shan ultrahigh-pressure metamorphic belt, east-central China, using U–Th–Pb dating of zircon and monazite. Chemical Geology 186, 315–331. Berger, J., Caby, R., Liegeois, J.P., Mercier, J.C., Demaiffe, D., 2008. Dehydration, melting and related garnet growth in the deep root of the Amalaoulaou Neoproterozoic magmatic arc. Geological Magazine 146, 1–14. Black, L.P., Kamo, S.L., Allen, C.M., et al., 2003. TEMORA 1: a new zircon standard for phanerozoic U–Pb geochronology. Chemical Geology 155–170. Bons, P.D., Dougherty-Page, J., Elburg, M.A., 2001. Stepwise accumulation and ascent of magmas. Journal of Metamorphic Geology 19, 627–633.
S. Yu et al. / Lithos 198–199 (2014) 58–76 Bröcker, M., Klemd, R., Kooijman, E., Jasper, B., Larionov, A., 2010. Zircon geochronology and trace element characteristics of eclogites and granulites from the Orlica–Snieznik complex, Bohemian Massif. Geological Magazine 147, 339–362. Brown, M., 2006. A duality of thermal regimes is the distinctive characteristic of plate tectonics since the Neoarchean. Geology 34, 961–964. Brown, M., 2007. Metamorphic conditions in orogenic belts: a record of secular change. International Geology Review 49, 193–234. Brown, M., 2008. Characteristic thermal regimes of plate tectonics and their metamorphic imprint throughout Earth history. In: Condie, K., Pease, V. (Eds.), When Did Plate Tectonics Begin? . Geological Society of America, Special Papers, 440, pp. 97–128. Brown, M., 2009. Metamorphic patterns in orogenic systems and the geological record. In: Cawood, P.A., Kröner, A. (Eds.), Accretionary Orogens in Space and Time. Geological Society, London, Special Publications, 318, pp. 37–74. Brown, M., 2010. Paired metamorphic belts revisited. Gondwana Research 18, 46–59. Carswell, D.A., O'Brien, P.J., 1993. Thermobarometry and geotectonic significance of highpressure granulites: examples from the Moldanubian zone of the Bohemian Massif in lower Austria. Journal of Petrology 34, 427–459. Chen, W., Zhang, Y., Ji, Q., Wang, S.S., Zhang, J.X., 2002. Magmatism and deformation times of the Xidatan rock series, East Kunlun Mountains. Science in China, Series B: Chemistry 45, 20–27. Chen, R.X., Zheng, Y.F., Xie, L.W., 2010. Metamorphic growth and recrystallization of zircon: distinction by simultaneous in-situ analyses of trace elements, U–Th–Pb and Lu–Hf isotopes in zircon from eclogite-facies rocks in the Sulu orogen. Lithos 114, 132–154. Chopin, C., 1984. Coesite and pure pyrope in high grade blueschists of the Western Alps: a first record and some consequences. Contributions to Mineralogy and Petrology 86, 107–118. Christensen, B.D.J., 2011. Geochemistry, Geothermobarometry and Geochronology of High-Pressure Granulites and Implications for the Exhumation History of UltrahighPressure Terranes: Dulan, Western China. MS thesis Central Washington University (152 pp.). Christensen, B.D.J., Mattinson, C.G., 2011. Deep orogenic processes constrained by pressure and temperature estimates of high-pressure granulites. Geological Society of America Abstracts with Programs 43 (4), 77. Cooke, R.A., O'Brien, P.J., Carswell, D.A., 2000. Garnet zoning and the identification of equilibrium mineral compositions in high-pressure-temperature granulites from the Moldanubian Zone, Austria. Journal of Metamorphic Geology 18, 551–569. Corfu, F., Hanchar, J.M., Hoskin, P.W.O., Kinny, P., 2003. Atlas of zircon textures. Reviews in Mineralogy and Geochemistry 53, 469–500. Ellis, D.J., 1980. Osumilite–sapphirine–quartz granulites from Enderby Land, Antarctica: P–T conditions of metamorphism, implications for garnet-cordierite equilibria and the evolution of the deep crust. Contributions to Mineralogy and Petrology 74, 201–210. Elvevold, K., Thrane, Gilotti, J.A., 2003. Metamorphic history of high-pressure granulites in Payer Land, Greenland Caledonides. Journal of Metamorphic Geology 21, 49–63. Ferrando, S., Lombardo, B., Compagnoni, R., 2008. Metamorphic history of HP mafic granulites from the Gesso-Stura Terrain (Argentera Massif, Western Alps, Italy). European Journal of Mineralogy 20, 777–790. Galan, G., Marcos, A., 2000. The metamorphic evolution of the high-pressure mafic granulites of the Bacariza Formation (Cabo Ortegal Complex, Hercynian belt, NW Spain). Lithos 54, 139–171. Garrido, C.J., Bodinie, J.L., Burg, J.P., Zeilinger, G., Hussain, S.S., Dawood, H., Chaudhry, M.N., Gervilia, F., 2006. Petrogenesis of mafic garnet granulites in the lower crust of the Kohistan palaeoarc complex (Northern Pakistan): implications for intracrustal differentiation of island arcs and generation of continental crust. Journal of Petrology 47, 1873–1914. Gilotti, J.A., Elvevold, S., 2002. Extensional exhumation of a high-pressure granulite terrane in Payer Land, Greenland Caledonides: structural, petrologic, and geochronologic evidence from metapelites. Canadian Journal of Earth Sciences 39, 1169–1187. Hacker, B.R., Kelemen, P.B., Behn, M.D., 2011. Differentiation of the continental crust by relamination. Earth and Planetary Science Letters 307, 501–516. Harrison, T.M., 1981. Diffusion of Ar in hornblende. Contributions to Mineralogy and Petrology 728, 324–331. Hensen, B.J., Harley, S.L., 1990. Graphical analysis of P–T–X relations in granulite-facies metapelites. High Temperature Metamorphism and Crustal AnatexisUnwin Hyman, London pp. 19–56. Hermann, J., 2002. Allanite: thorium and light rare earth element carrier in subducted crust. Chemical Geology 192, 289–306. Hou, K.J., Li, Y.H., Tian, Y.Y., 2009. In situ U–Pb zircon dating using laser ablation-multi ion counting-ICP-MS. Mineral Deposits 28, 481–492 (in Chinese with English abstract). Hu, M.Y., He, H.L., Zhan, X.C., Fan, X.T., Wang, G., Jia, Z.R., 2008. Matrix normalization for in-situ multi-element quantitative analysis of zircon in laser ablation-inductively coupled plasma mass spectrometry. Chinese Journal of Analytical Chemistry 36, 947–953 (in Chinese with English abstract). Indares, A.D., 2003. Metamorphic textures and P–T evolution of high-P granulites from the Lelukuau terrane, NE Grenville Province. Journal of Metamorphic Geology 21, 35–48. Indares, A., Dunning, G., 2001. Partial melting of high-P–T metapelites from the Tshenukutish terrane (Grenville Province): petrography and U–Pb geochronology. Journal of Petrology 42, 1547–1565. Johnson, T.E., Fischer, S., White, R.W., Brown, M., Rollinson, H.R., 2012. Archean intracrustal differentiation from partial melting of metagabbro-field and geochemical evidence from the central region of the Lewisian complex, NW Scotland. Journal of Petrology 58, 2115–2138. Johnson, T.E., Fischer, S., White, R.W., 2013. Field and petrographic evidence for partial melting of TTG gneiss from the central region of the mainland Lewisian complex, NW Scotland. Journal of the Geological Society (London) 170, 319–326.
75
Keay, S., Lister, G., Buick, I., 2001. The timing of partial melting, Barrovian metamorphism and granite intrusion in the Naxos metamorphic core complex, Cyclades, Aegean Sea, Greece. Tectonophysics 342, 275–312. Klemd, R., Brocker, M., 1999. Fluid influence on mineral reactions in ultrahigh-pressure granulites: a case study in the Snieznik Mts. (West Sudetes, Poland). Contributions to Mineralogy and Petrology 136, 358–373. Konopasek, J., Schulmann, K., 2005. Contrasting Early Carboniferous field geotherms: evidence for accretion of a thickened orogenic root and subducted Saxothuringian crust (Central European Variscides). Journal of the Geological Society 162, 463–470. Kotkova, J., Kröner, A., Todt, W., Fiaia, J., 1996. Lower Carboniferous high-pressure event in the Bohemian Massif: evidence from North Bohemian granulites. Geologische Rundschau 85, 154–161. Li, W.C., Chen, R.X., Zheng, Y.F., Li, Q.L., Hu, Z.C., 2013. Zirconological tracing of transition between aqueous fluid and hydrous melt in the crust: constraints from pegmatite vein and host gneiss in the Sulu orogen. Lithos 162, 157–174. Liati, A., 2005. Identification of repeated Alpine (ultra) high-pressure metamorphic events by U–Pb SHRIMP geochronology and REE geochemistry of zircon: the Rhodope zone of Northern Greece. Contributions to Mineralogy and Petrology 150, 608–630. Liu, Y., Zhong, D., 1997. Petrology of high-pressure granulites from the eastern Himalayan syntaxis. Journal of Metamorphic Geology 15, 451–466. Liu, Y.S., Gao, S., Hu, Z.C., Gao, C.G., Zong, K.Q., Wang, D.B., 2010. Continental and oceanic crust recycling-induced melt–peridotite interactions in the Trans-North China Orogen: U–Pb dating, Hf isotopes and trace elements in zircons from mantle xenoliths. Journal of Petrology 51, 537–571. López, S., Castro, A., 2001. Determination of the fluid-absent solidus and supersolidus phase relationships of MORB-derived amphibolites in the range 4–14 kbar. American Mineralogist 86, 1396–1403. Ludwig, K.R., 2001. Squid 1.02: A User's Manual. Berkeley Geochronology Center Special Publication 2, Berkeley, USA p. 19. Ludwig, K.R., 2003. User's Manual for Isoplot 3.00: A Geochronologic Toolkit for Microsoft Excel. Berkeley Geochronology Center Special Publication 4, Berkeley, USA p. 70. Mattinson, C.G., Wooden, J.L., Liou, J.G., Bird, D.K., Wu, C.L., 2006a. Age and duration of eclogite-facies metamorphism, North Qaidam HP/UHP terrane, Western China. American Journal of Science 306, 683–711. Mattinson, C.G., Wooden, J.L., Liou, J.G., Bird, D.K., Wu, C.L., 2006b. Geochronology and tectonic significance of Middle Proterozoic granitic orthogneiss, North Qaidam HP/UHP terrane, Western China. Mineralogy and Petrology 88, 227–241. Mattinson, C.G., Menold, C.A., Zhang, J.X., Bird, D.K., 2007. High and ultrahigh-pressure metamorphism in the North Qaidam and South Altyn terranes, Western China. International Geology Review 49, 969–995. Mattinson, C.G., Menold, C.A., Zhang, J.X., Bird, D.K., 2009. Paragneiss zircon geochronology and trace element geochemistry, North Qaidam HP/UHP terrane, western China. Journal of Asian Earth Sciences 35, 298–309. Medaris, E.D., Ghent, H.F., Wang, J.H., Fournelle, Jelinek, 2006. The Spacice eclogite: constraints on the P–T–t history of the Gfohl granulite terrane, Moldanubian Zone, Bohemian Massif. Journal of Metamorphic Geology 86, 203–220. Meng, F.C., Zhang, J.X., Yang, J.S., 2005. Tectono-thermal event of post-HP/UHP metamorphism in Xitieshan area of the North Qaidam Mountains, western China: isotopic and geochemical evidence of granite and gneiss. Acta Petrologica Sinica 21 (1), 45–56 (In Chinese with English abstract). Miyashiro, A., 1961. Evolution of metamorphic belts. Journal of Petrology 2, 277–311. Nahodilová, R., Faryad, S.W., Dolejs, D., Tropper, P., Konzett, J., 2011. High-pressure partial melting and melt loss in felsic granulites in the Kutná Hora complex, Bohemian Massif (Czech Republic). Lithos 125, 641–658. O'Brien, P.J., Rötzler, J., 2003. High-Pressure granulites: formation, recovery of peak conditions and implication for tectonics. Journal of Metamorphic Geology 21, 3–20. O'Brien, P.J., 1997. Garnet zoning and reaction textures in overprinted eclogites, Bohemian Massif, European Variscides: a record of their thermal history during exhumation. Lithos 41, 119–133. Patiño Douce, A.E., 2005. Vapor-absent melting of tonalite at 15–32 kbar. Journal of Petrology 46, 275–290. Pitra, P., Kouamelan, A.N., Ballevre, M., Peucat, J.J., 2010. Palaeoproterozoic high-pressure granulite overprint of the Archean continental crust: evidence for homogeneous crustal thickening (Man Rise, Ivory Coast). Journal of Metamorphic Geology 28, 41–58. Puelles, B., Balos, A., Ibarguchi, J.G., 2005. Metamorphic evolution and thermobaric structure of the subduction related Bacariza high-pressure granulite formation (Cabo Ortegal Complex, NW Spain). Lithos 84, 125–149. Rapp, R.P., Watson, E.B., 1995. Dehydration melting of metabasalt at 8–32 kbar: implications for continental growth and crust–mantle recycling. Journal of Petrology 36, 891–931. Rubatto, D., 2002. Zircon trace element geochemistry: partitioning with garnet and the link between U–Pb ages and metamorphism. Chemical Geology 184, 123–138. Rubatto, D., Hermann, J., 2003. Zircon formation during fluid circulation in eclogites (Monviso, Western Alps): Implications for Zr and Hf budget in subduction zones. Geochimica et Cosmochimica Acta 67, 2173–2187. Rubatto, D., Williams, I.S., Buick, I.S., 2001. Zircon and monazite response to prograde metamorphism in the Reynolds Range, central Australia. Contributions to Mineralogy and Petrology 140, 458–468. Sawyer, E.W., 2001. Melt segregation in the continental crust: distribution and movement of melt in anatectic rocks. Journal of Metamorphic Geology 19, 291–309. Sawyer, E.W., 2008. Atlas of Migmatites. Canadian Mineralogist, Special Publication, 9 (371 pp.). Schaltegger, U., Fanning, C.M., Günther, D., Maurin, J.C., Schulmann, K., Gebauer, D., 1999. Growth, annealing and recrystallization of zircon and preservation of monazite in highgrade metamorphism: conventional and in-situ U–Pb isotope, cathodoluminescence and microchemical evidence. Contributions to Mineralogy and Petrology 134, 186–201.
76
S. Yu et al. / Lithos 198–199 (2014) 58–76
Schmidt, M.W., Poli, S., 2003. Generation of mobile components during subduction of oceanic crust. Treatise on Geochemistry 3, 567–591. Schulmann, K., Lexa, O., Stipska, P., Racek, M., Tajcmanova, L., Konopasek, J., Edel, J.B., Peschler, A., Lehmann, J., 2008. Vertical extrusion and horizontal channel flow of orogenic lower crust: key exhumation mechanisms in large hot orogens? Journal of Metamorphic Geology 26, 273–297. Schulz, K.G., Zollner, E., Bellerby, R., Middelburg, J., Riebesell, U., 2006. Carbon fluxes within a natural plankton community at elevated atmospheric CO2. Geochimica et Cosmochimica Acta 70, 679–710. Sen, C., Dunn, T., 1994. Dehydration melting of a basaltic composition amphibolite at 1.5 and 2.0 GPa: implications for the origin of adakites. Contributions to Mineralogy and Petrology 117, 394–409. Sizova, E., Gerya, T., Brown, M., 2012. Exhumation mechanisms of melt-bearing ultrahigh pressure crustal rocks during collision of spontaneously moving plates. Journal of Metamorphic Geology 30, 927–955. Skjerlie, K.P., Patiño Douce, A.E., 2002. The fluid-absent partial melting of a zoisite-bearing quartz eclogite from 1.0 to 3.2 GPa: implications for melting in thickened continental crust and for subduction-zone processes. Journal of Petrology 43, 291–314. Sláma, J., Kosler, J., Condon, D.J., Crowley, J.L., Gerdes, A., Hanchar, J.M., Horstwood, M.S.A., Morris, G.A., Nasdala, L., Norberg, N., Schaltegger, U., Schoene, B., Tubrett, M.N., Whitehouse, M.J., 2008. Plesovice zircon — a new natural reference material for U–Pb and Hf isotopic microanalysis. Chemical Geology 249, 1–35. Smith, D.C., 1984. Coesite in clinopyroxene in the Caledonides and its implications for geodynamics. Nature 310, 641–644. Song, S.G., Yang, J.S., Xu, Z.Q., Liou, J.G., Shi, R.D., 2003a. Metamorphic evolution of the coesite-bearing ultrahigh-pressure terrane in the north Qaidam, northern Tibet, NW China. Journal of Metamorphic Geology 21, 631–644. Song, S.G., Yang, J.S., Liou, J.G., Wu, C.L., Shi, R.D., Xu, Z.Q., 2003b. Petrology, geochemistry and isotopic ages of eclogites from the Dulan UHPM Terrane, the North Qaidam, NW China. Lithos 70, 195–211. Song, S.G., Zhang, L.F., Niu, Y.L., 2004. Ultra-deep origin of garnet peridotite from the North Qaidam ultrahigh-pressure belt, Northern Tibetan Plateau, NW China. American Mineralogist 89, 1330–1336. Song, S.G., Zhang, L.F., Niu, Y., Su, L., Jian, P., Liu, D.Y., 2005. Geochronology of diamondbearing zircons from garnet peridotite in the North Qaidam UHPM belt, Northern Tibetan Plateau: a record of complex histories from oceanic lithosphere subduction to continental collision. Earth and Planetary Science Letters 234, 99–118. Song, S.G., Zhang, L.F., Niu, Y.L., Su, L., Song, B., Liu, D.Y., 2006. Evolution from oceanic subduction to continental collision: a case study of the Northern Tibetan Plateau inferred from geochemical and geochronologic data. Journal of Petrology 47, 435–455. Song, S.G., Su, L., Niu, Y.L., et al., 2007. Petrological and geochemical constraints on the origin of garnet peridotite in the North Qaidam ultrahigh-pressure metamorphic belt, northwestern China. Lithos 96, 243–265. Song, S.G., Su, L., Li, X.H., Zhang, G.B., Niu, Y.L., Zhang, L.F., 2010. Tracing the 850 Ma continental flood basalts from a piece of subducted continental crust in the North Qaidam UHPM belt, NW China. Precambrian Research 183, 805–816. Steiger, R.H., Jaeger, E., 1977. Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters 36, 359–362. Štipska, P., Schulmann, K., Kroner, A., 2004. Vertical extrusion and middle crustal spreading of omphacite granulite: a model of syn-convergent exhumation (Bohemian Massif, Czech Republic). Journal of Metamorphic Geology 22, 179–198. Stowell, H., Tulloch, A., Zuluaga, C., Koenig, A., 2010. Timing and duration of garnet granulite metamorphism in magmatic arc crust, Fiordland, New Zealand. Chemical Geology 273, 91–110. Vavra, G., Schmid, R., Gebauer, D., 1999. Internal morphology, habit and U–Th–Pb microanalysis of amphibolite-to-granulite-facies zircons: geochronology of the Ivera Zone (Southern Alps). Contributions to Mineralogy and Petrology 134, 380–404. Whitney, D.L., Evans, B.W., 2010. Abbreviations for names of rock-forming minerals. American Mineralogist 95, 185–187. Williams, I.S., 1998. U–Th–Pb geochronology by ion microprobe. In: McKibben, M.A., Shanks, W.C., Ridley, W.I. (Eds.), Applications of Microanalytical Techniques to Understanding Mineralizing Processes. Reviews in Economic Geology, 7, pp. 1–35. Willner, A.P., Rötzler, K., Maresch, W.V., 1997. Pressure–temperature and fluid evolution of quartzo-feldspathic metamorphic rocks with a relic high-pressure, granulitefacies history from the Central Erzgebirge (Saxony, Germany). Journal of Petrology 38, 307–336. Wu, C.L., Yang, J.S., Wooden, J., Shi, R.D., Chen, S.Y., Meibom, A., Mattinson, C.G., 2004. Zircon U–Pb SHRIMP dating of the Yematan batholith in Dulan, north Qaidam, NW China. Chinese Science Bulletin 49, 1736–1740. Yang, J.J., Deng, J.F., 1994. Garnet peridotite and eclogites in the northern Qaidam Mountains. Tibetan Plateau: A First Record on UHP Metamorphism and Tectonics, II.P Task Group III-6, Stanford, A-20.
Yang, J.J., Powell, R., 2008. Ultrahigh-pressure garnet peridotites from the devolatilization of sea-floor hydrated ultramafic rocks. Journal of Metamorphic Geology 26, 695–716. Yang, J.S., Xu, Z.Q., Li, H.B., Wu, C.L., Cui, J.W., Zhang, J.X., Chen, W., 1998. Discovery of eclogite at northern margin of Qaidam basin, NW China. Chinese Science Bulletin 43, 1755–1760. Yang, J.S., Xu, Z.Q., Song, S.G., Zhang, J.X., Wu, C.L., Shi, R.D., Li, H.B., Brunel, M., 2001. Discovery of coesite in the north Qaidam early palaeozoic ultrahigh pressure (UHP) metamorphic belt, NW China. Sciences de la Terre et des Planets 333, 719–724. Yang, J.S., Xu, Z.Q., Zhang, J.X., Song, S.G., Wu, C.L., Shi, R.D., Li, H.B., Brunel, M., 2002. Early Paleozoic North Qaidam UHP metamorphic belt on the northeastern Tibetan plateau and a paired subduction model. Terra Nova 14, 397–404. Yang, J.S., Wu, C.L., Zhang, J.X., Shi, R.D., Meng, F.C., Wooden, J., Yang, H.Y., 2006. Protolith of eclogites in the North Qaidam and Altun UHP terrane, NW China: earlier oceanic crust? Journal of Asian Earth Sciences 28, 185–204. Yu, S.Y., Zhang, J.X., Li, J.P., 2009. Metamorphism history and dynamics of high-pressure granulites in Dulan area of the North Qaidam Mountains, northwest China. Acta Petrologica Sinica 25 (9), 2224–2234 (in Chinese with English abstract). Yu, S.Y., Zhang, J.X., Li, J.P., Meng, F.C., 2010. Zircon U–Pb geochronology of high-pressure granulite and its tectonic implications in the Dulan area, North Qaidam Mountains, western China. Acta Petrologica et Mineralogica 29, 139–150 (in Chinese with English abstract). Yu, S.Y., Zhang, J.X., Real, P.G.D., 2011a. Petrology and P–T path of high-pressure granulite from the Dulan area, North Qaidam Mountains, northwestern China. The Journal of Asian Earth Sciences 42, 641–660. Yu, S.Y., Zhang, J.X., Hou, K.J., 2011b. Two contrasting magmatic events in the Dulan UHP metamorphic terrane: implication for collisional orogeny. Acta Petrologica Sinica 27, 3335–3349 (In Chinese with English abstract). Yu, S.Y., Zhang, J.X., Real, P.G.D., 2012. Geochemistry and zircon U–Pb ages of adakitic rocks from the Dulan area of the North Qaidam UHP terrane, north Tibet: constraints on the timing and nature of regional tectonothermal events. Gondwana Research 21, 167–179. Yu, S.Y., Zhang, J.X., Li, H.K., Hou, K.J., Mattinson, C.G., Gong, J.H., 2013. Geochemistry, zircon U–Pb geochronology and Lu–Hf isotopic composition of eclogite and their host gneisses in the Dulan area, North Qaidam UHP terrane: new evidence for deep continental subduction. Gondwana Research 23, 901–919. Zhang, J.X., Zhang, Z.M., Xu, Z.Q., Yang, J.S., Cui, J.W., 2001. Petrology and geochronology of eclogites from the western segment of the Altyn Tagh, northwestern China. Lithos 56, 189–208. Zhang, J.X., Yang, J.S., Mattinson, C.G., Xu, Z.Q., Meng, F.C., Shi, R.D., 2005. Two contrasting eclogite cooling histories, North Qaidam HP/UHP terrane, western China: petrological and isotopic constraints. Lithos 84, 51–76. Zhang, J.X., Yang, J.S., Meng, F.C., Wan, Y.S., Li, H.B., Wu, C.L., 2006. U–Pb isotopic studies of eclogites and their host gneisses in the Xitieshan area of the North Qaidam Mountains, western China: new evidence for an early Paleozoic HP–UHP metamorphic belt. Journal of Asian Earth Sciences 28, 143–150. Zhang, J.X., Mattinson, C.G., Meng, F.C., Wan, Y.S., Tung, K., 2008a. Polyphase tectonothermal history recorded in granulitized gneisses from the North Qaidam HP/UHP metamorphic terrane, Western China: evidence from zircon U–Pb geochronology. Geological Society of America Bulletin 120, 732–749. Zhang, G.B., Song, S.G., Zhang, L.F., Niu, Y.L., 2008b. The subducted oceanic crust within continental-type UHP metamorphic belt in the North Qaidam, NW China: evidence from petrology, geochemistry and geochronology. Lithos 104, 99–118. Zhang, J.X., Meng, F.C., Li, J.P., Mattinson, C.G., 2009a. Coesite in eclogite from the North Qaidam and its implications. Chinese Science Bulletin 54, 1105–1110. Zhang, J.X., Yu, S.Y., Meng, F.C., Li, J.P., 2009b. Paired high-pressure granulite and eclogite in collision orogens and their geodynamic implications. Acta Petrologica Sinica 25, 2050–2066 (in Chinese with English abstract). Zhang, G.B., Zhang, L.F., Song, S.G., 2009c. UHP metamorphic evolution and SHRIMP geochronology of a meta-ophiolitic gabbro in the North Qaidam, NW China. Journal of Asian Earth Sciences 35, 310–322. Zhang, J.X., Mattinson, C.G., Yu, S.Y., Li, J.P., Meng, F.C., 2010. U–Pb zircon geochronology of coesite-bearing eclogites from the southern Dulan area of the North Qaidam UHP terrane, northwestern China: spatially and temporally extensive UHP metamorphism during continental subduction. Journal of Metamorphic Geology 28, 955–978. Zhao, G.C., Cawood, P.A., Wilde, S.A., Lu, L.Z., 2001. High-pressure granulites from the Hengshan Complex, North China Craton: petrology and tectonic implications. Journal of Petrology 42, 1141–1170. Zheng, Y.F., Xia, Q.X., Chen, R.X., Gao, X.Y., 2011. Partial melting, fluid supercriticality and element mobility in ultrahigh-pressure metamorphic rocks during continental collision. Earth Science Reviews 107, 342–374. Zulauf, G., Dorr, W., Fiala, J., Kotková, J., Maluski, H., Valverde, V.P., 2002. Evidence for high-temperature diffusional creep preserved by rapid cooling of lower crust (North Bohemian shear zone, Czech Republic). Terra Nova 14, 343–354.
