Anatexis of ultrahigh-pressure eclogite during exhumation in the North Qaidam ultrahigh-pressure terrane: Constraints from petrology, zircon U-Pb dating, and geochemistry Yu Shengyao1,2,†, Zhang Jianxin1, Sun Deyou2, Li Yunshuai1, and Gong Jianghua1 State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang, Beijing 100037, China 2 College of Earth Sciences, Jilin University, Changchun 130061, China 1
ABSTRACT Plagioclase-rich leucosomes and K-feldspar–rich veins are widely distributed within retrogressed eclogite in the Xitieshan area of the North Qaidam ultrahigh-pressure metamorphic terrane, western China. In this contribution, a combined study of petrology, zircon internal structure, zircon U-Pb age, zircon trace-element composition, whole-rock geochemistry, and Sr-Nd isotopes in plagioclase-rich leucosomes and K-feldspar–rich veins provides insight into the nature and timing of partial melting in these rocks. Petrological evidence for partial melting is provided by elongated, highly cuspate feldspars, quartz along grain boundaries (e.g., muscovite, garnet, and clinopyroxene), and felsic veinlets composed of quartz and feldspar along the boundaries of garnet, clinopyroxene, and clinopyroxene-plagioclase symplectite in the metabasite. Whole-rock geochemistry suggests that the plagioclase-rich leucosomes have lower contents of both rare earth elements (REEs) and high field strength elements (HFSEs) but higher large ion lithophile elements (LILEs; e.g., Rb, Ba, K, Sr, and Pb) than the metabasite hosts. The plagioclase-rich leucosomes may be divided into two subgroups according to their distinct REE patterns: (1) higher total REE content with or without weak negative Eu anomalies, and (2) lower total REE content with conspicuous positive Eu anomalies. The Eu-rich group also shows higher Sr content than those in the Eu-poor group. However, the K-feldspar–rich veins show higher REE, HFSE, and LILE concentrations than the plagioclase-rich leucosomes and metabasites. The inherited cores of K-feldspar–rich veins are of typical magmatic origin (oscillatory zoning with high Th/U ratios, enriched heavy [H] REEs, and negative Eu anomalies), yielding 206Pb/238U ages ranging from 904 ± 8 Ma to 915 ± 28 Ma. The zircon cores of plagioclase-rich leucosomes and mantles of K-feldspar–rich veins exhibit no zoning or weak zoning, with flat HREE patterns and no negative Eu anomalies, and they contain mineral inclusions of garnet and clinopyroxene, implying that these zircon domains were formed during an episode of eclogite-facies metamorphism with 206Pb/238U ages ranging from 444 ± 10 Ma to 452 ± 9 Ma. Internal textures and mineral inclusions (quartz, plagioclase, and K-feldspar) and trace-element systematics (steep HREE patterns with negative Eu anomalies and low Th/U ratios) of the zircon rims from plagioclase-rich leucosomes and K-feldspar–rich veins are similar to anatectic zircon. These anatectic zircon domains yielded 206Pb/238U ages ranging from 430 ± 10 Ma to 440 ± 12 Ma, and 406 ± 12 Ma to 430 ± 13 Ma, respectively. An inteE-mail:
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grated study of petrology, geochronology, and geochemistry demonstrated that the metabasites of the North Qaidam ultrahigh-pressure terrane likely experienced initial partial melting with plagioclase-rich leucosome formation under eclogite-facies conditions and partial melt crystallization during the granulite-facies stage, triggered by dehydration melting involving zoisite and rare muscovite. The K-feldspar–rich veins within the metabasites may be the result of injected melts that were derived from partial melting of nearby gneisses. INTRODUCTION In recent years, hydrous silicate melts by dehydration-driven in situ partial melting constrained from experiments and natural rocks have been increasingly recognized in ultrahigh-pressure rocks, indicating partial melting of ultrahigh-pressure slab. Partial melting of ultrahigh-pressure metamorphic rocks can dramatically affect the rheology of deeply subducted crust and thus play a crucial role in accelerating the exhumation of ultrahigh-pressure slabs (Hermann et al., 2001; Labrousse et al., 2002; Chopin, 2003). Understanding the extent, conditions, and timing of partial melting in ultrahigh-pressure rocks in as many individual terranes as possible will therefore provide important constraints on tectonic models for exhumation of ultrahigh-pressure metamorphic rocks (Lang and Gilotti, 2007). It is difficult, however, to determine when partial melting took place in these ultrahigh-pressure rocks due to extensive retrograde reaction and equilibration during exhumation (Zheng et al., 2011). Direct dating of crustal melting is therefore crucial to build an understanding of the relationships among partial melting, granite intrusion, and orogenic processes (Foster et al., 2001; Keay et al., 2001; Whitney et al., 2003). Zircon is a stable mineral and has very high closure temperature and low rates of Pb, Th, and U diffusion at temperatures below 900 °C (Cherniak and Watson, 2003). The extremely stable nature of zircon and its high closure temperature for U-Pb diffusion mean that its isotopic system is little disturbed by any subsequent metamorphism. As zircon can be efficiently dissolved in anatectic melts and grow from them, it can therefore potentially be used to date anatexis that occurred at various spatial scales (Hermann et al., 2001; Gregory et al., 2009; Hermann and Rubatto, 2009; Rubatto et al., 2009). In this regard, in situ U-Pb dating of zoned zircon can provide significant age information regarding the complex evolutionary history of its leucosome and host rocks (Foster et al., 2001; Keay et al., 2001; Buick et al., 2008). The mineral abbreviations listed in this article follow the convention of Whitney and Evans (2010). Experiments and natural investigations suggest that the breakdown of hydrous minerals (e.g., micas, amphiboles, epidotes) has played a princi-
GSA Bulletin; September/October 2015; v. 127; no. 9/10; p. 1290–1312; doi: 10.1130/B31162.1; 13 figures; 7 tables; published online 3 April 2015.
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Anatexis of ultrahigh-pressure eclogite during exhumation in the North Qaidam ultrahigh-pressure terrane pal role in triggering the partial melting of ultrahigh-pressure metamorphic rocks (Zheng et al., 2011). The temperature at which rocks begin to melt is governed by the presence or absence of feldspars and/or quartz and the amount of H2O present either as a free volatile phase or, more commonly, bound within hydrous minerals such as micas, amphiboles, and epidotes (Johnson et al., 2012). In the presence of aqueous fluid, metapelites, metagraywackes, and metabasite begin to melt at temperatures of 650–700 °C (Brown and Korhonen, 2009; Sawyer, 2010; Zheng et al., 2011; Brown, 2013). In the absence of any influx of fluid, metapelites and metagraywackes are more fertile than metabasite (Clemens, 2006). Metapelites and metagraywackes may produce significant melt by breakdown of hydrous minerals (e.g., mica) at temperatures above 750 °C under fluid-undersaturated conditions (e.g., Patiño Douce and Harris, 1998; White et al., 2001). Partial melting of metabasite rocks via reactions that consume hydrous minerals requires higher temperatures under fluid-undersaturated conditions (Clemens, 2006). The North Qaidam Mountains, a typical collisional orogenic belt, are widely considered to have resulted from collision between the Qilian and Qaidam continental blocks at 420–460 Ma. Ultrahigh-pressure metamorphic rocks are widespread in the North Qaidam ultrahigh-pressure terrane over a distance of 350 km, mainly consisting of paragneiss and orthogneiss, and locally enclosing eclogites. Many indications of ultrahigh-pressure metamorphism have been documented in the gneiss and mafic-ultramafic rocks, e.g., coesite inclusions in garnet and clinopyroxene in eclogites, and in zircon of paragneisses and eclogites in the Dulan area and Xitieshan area (Yang et al., 2001; Song et al., 2003a, 2004, 2005; J.X. Zhang et al., 2009a, 2010; G.B. Zhang et al., 2009; X.C. Liu et al., 2012). Diamond inclusions in zircons of garnet peridotites have also been found in the Luliangshan area, suggesting subduction to mantle depths in excess of 100 km (Song et al., 2005). The high-pressure granulite and paragneiss record pressure-temperature (P-T) conditions in excess of 1.0 GPa and 800 °C (Yu et al., 2011; J.X. Zhang et al., 2008), and some eclogites also record temperatures higher than 750 °C during peak (P = 2.71–3.17 GPa) and/ or retrograde stages (Zhang et al., 2011). These conditions are more than sufficient for partial melting to take place in a range of quartzofeldspathic protoliths, particularly in the presence of free water (e.g., Huang and Wyllie, 1981). In the North Qaidam Mountains, felsic leucosomes or quartzofeldspathic veins have been recognized in the metabasite (e.g., eclogite, high-pressure granulite, and amphibolite). These were interpreted as partial melting of their host rocks (Chen et al., 2012; Yu et al., 2012, 2014; Song et al., 2014). However, field and petrographic evidence in support of in situ partial melting of the metabasite is rarely provided. Experimental results indicate that mafic rocks have higher solidus temperatures than felsic rocks under fluid-undersaturated conditions (Schmidt and Poli, 2003). Thus, it is still not clear if the granitic leucosome or quartzofeldspathic veins within metabasite are in situ or injected. In addition, previous zircon dating of the anatexis ranges in age from 410 to 450 Ma in the Dulan and Xitieshan units, overlapping both ultrahigh-pressure metamorphism and later retrograde stages. Therefore, the precise age at which partial melting occurred is still uncertain. Some workers suggest that partial melting occurred in the thickened lower crust during the ultrahigh-pressure metamorphic stage (Yu et al., 2012, 2014), whereas others have proposed that it was restricted to the retrograde stage (Chen et al., 2012; Song et al., 2014). In this contribution, we present field and microscopic evidence for partial melting and new geochemistry, zircon U-Pb, and trace-element data from leucosomes within retrogressed eclogite from the Xitieshan area in the North Qaidam ultrahigh-pressure metamorphic terrane. These new results provide precise age information of zircon crystallization from anatectic melt and have implications for petrogenesis and the dynamic mechanism of anatexis in the North Qaidam ultrahigh-pressure terrane. Moreover,
understanding the conditions and timing of partial melting in high-grade metamorphic rocks will be crucial to place constraints on the relationships between partial melting and metamorphism evolution, and provide insights into process and mechanisms that operated in deep subduction zones during the continental collision. Moreover, these results will finally provide essential constraints on the rheology and rate of exhumation of the subducted continental crust with partial melting. GEOLOGICAL SETTING The NW-SE–trending North Qaidam Mountains, located at the northern margin of the Qinghai-Tibet Plateau, extend over 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 mainly composed of paragneiss and orthogneiss but also contains rare marble, granulite, amphibolite, local eclogite, and various ultramafic rocks. These basement 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 over 350 km near the localities of Yuka, Xitieshan, and Dulan and record peak metamorphic ages between 430 and 450 Ma (Yang et al., 2002, 2006; Song et al., 2003b; J.X. Zhang et al., 2005, 2006, 2008, 2009, 2010; G.B. Zhang et al., 2008, 2009; Mattinson et al., 2006, 2007; F.L. Liu et al., 2012; Yu et al., 2013). Garnet peridotite outcrops in the Luliangshan area (Yang and Deng, 1994) and appears to have been exhumed from mantle depths of more than 100 km or even more than 200 km (Song et al., 2004, 2005). Based on rock associations, petrologic criteria, and field relationships, four high-pressure to ultrahighpressure metamorphic units can be distinguished along the North Qaidam Mountains from east to west (J.X. Zhang et al., 2008): (1) the Dulan eclogite-gneiss unit, which consists of granitic gneiss, paragneiss, eclogite, and ultramafic lenses enclosed within gneiss, and intruded by ca. 400 Ma granite plutons (Wu et al., 2004); (2) the Xitieshan eclogite-gneiss unit, dominated by kyanite- and sillimanite-bearing paragneiss and granitic orthogneiss with rare marble and amphibolite and intruded by granite plutons dated at 428 ± 1 Ma (Meng et al., 2005); (3) the Luliangshan garnet peridotite-gneiss unit, defined by sillimanite-bearing paragneiss and granitic gneiss with ultramafic rocks (garnet peridotite and garnet pyroxenite) as lenses and intruded by Silurian granite plutons; and (4) the Yuka eclogite-gneiss unit, which is composed of eclogite, high-pressure metapelite, granitic gneiss, and rare marble. The Xitieshan unit, the focus of this contribution, mainly consists of garnet–kyanite (±sillimanite)–biotite paragneiss and felsic (granitic) orthogneiss with Neoproterozoic protolith ages that were overthrusted by early Paleozoic volcanic-sedimentary rocks. The predominant deformation fabrics are SSE-NNW–trending foliations (S1) and subhorizontal stretching lineations that are defined by oriented sillimanite and biotite in paragneisses, suggesting high-temperature deformation. The deformation fabrics are overprinted by SSE-NNW–trending folds (J.X. Zhang et al., 2008). Although field relationships between the paragneiss and the orthogneiss are difficult to determine, the paragneiss is suggested to be intruded by the orthogneiss (Wan et al., 2006). The eclogite in this unit occurs as lenses and boudins within the garnet–kyanite (±sillimanite)–biotite paragneiss and granitic orthogneiss. In contrast to the Dulan and Yuka units, the eclogites in the Xitieshan unit were extensively retrogressed. Fresh eclogites are rare and are mainly preserved in the center of large boudins. From the center to rim of large mafic boudins, it is easy to recognize the transition from eclogite to (garnet) amphibolite via garnet granulite, suggesting successive retrogression from eclogite facies to amphibolite facies. Evidence of ultrahigh-pressure metamorphism, which is provided
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Figure 1. (A) Schematic map of the North Qaidam Mountains showing major tectonic units, and locations of eclogite and garnet peridotite. (B) Geologic sketch map showing the geological setting of the Xitieshan area (Zhang et al., 2010). DLU—Dulan eclogite-gneiss unit, XTU— Xitieshan eclogite-gneiss unit, YLU—Yuka eclogite-gneiss unit, LLU—Luliangshan garnet peridotite-gneiss unit.
Anatexis of ultrahigh-pressure eclogite during exhumation in the North Qaidam ultrahigh-pressure terrane by coesite as inclusions in zircons, is rarely found in the eclogites (X.C. Liu et al., 2012). Due to lack of phengite to constrain the peak pressure, Zhang et al. (2005) reported a minimum peak pressure of >1.4 GPa and temperature of 730–830 °C and granulite-facies overprinting of 1.0–1.5 GPa and 750–865 °C. Recently, peak conditions of 2.71–3.17 GPa and 751–791 °C were constrained by the relict peak ultrahigh-pressure mineral assemblage garnet + omphacite + phengite + rutile and rare coesite pseudomorphs in omphacite (Zhang et al., 2011). Zhang et al. (2005) first reported the peak metamorphic age of 480–486 Ma using zircon U-Pb dating. Recently, Zhang et al. (2011) and F.L. Liu et al. (2012) yielded younger metamorphic ages of eclogites, ranging from 440 to 460 Ma. Felsic veins, mainly consisting of feldspar and quartz, have been recognized in ultrahigh-pressure eclogites. The felsic veins were suggested to derive from partial melting of the host ultrahigh-pressure eclogite during exhumation (Chen et al., 2012). FIELD RELATIONSHIP AND MICROSCOPIC TEXTURE Field Relationship In the Xitieshan area, fresh eclogites are rare and are mainly preserved at the centers of large mafic boudins. From core to rim of the large mafic boudins, it is easy to recognize the transition from eclogite via garnet granulite to (garnet) amphibolite, suggesting successive retrogression from eclogite facies to amphibolite facies. In the large mafic boudins, quartzofeldspathic leucosomes (plagioclase-rich leucosome) generally occur as numerous thin layers, veinlets, or patches in the metabasite layers (Fig. 2A). The proportion of thin plagioclase-rich leucosomes varies greatly from outcrop to outcrop, ranging in thickness from millimeters to decimeters. At the regions of contact between eclogite and leucocratic veins, the eclogite is generally retrograded into high-pressure granulite. The plagioclase-rich (Pl-rich) leucosomes are generally parallel to the foliation in the metabasite (Fig. 2B). Euhedral-subhedral garnet grains within the leucosomes are commonly larger than those within the melanosome (Fig. 2C). Locally coarse-grained patches consist of plagioclase + quartz ± K-feldspar commonly enclosing large crystals of pink garnet or occasionally clinopyroxene (Fig. 2D). The largest 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. In some retrogressed eclogites, K-feldspar veins (K-feldspar–rich [Kfs-rich] vein) were also recognized, with thickness generally greater than 5 cm. The Kfs-rich veins generally cut across amphibolite-facies foliations in the retrogressed eclogites (Figs. 2E and 2F).
and quartz grains in the leucosome layers are much coarser than those in the metabasite layers at the boundary between leucosome and metabasite (Fig. 3B). Microscopic texture related to partial melting could be recognized. Elongated quartz and/or feldspar grains occur like veinlets along the phengite-phengite and phengite-clinopyroxene (garnet) grain boundaries, showing low dihedral angles (Fig. 3C). In the core of the garnet, polymineral inclusions of muscovite + feldspar + quartz could represent crystallization of a leucogranitic magma droplet trapped by garnet during growth (Fig. 3D). In some retrogressed eclogite samples, felsic veinlets composed of quartz and feldspar were recognized across the boundaries of garnet, clinopyroxene, amphibole, and clinopyroxene + plagioclase symplectite, which may represent melt channels (Fig. 3E). Similar textures have been reported for mid-temperature, ultrahigh-pressure retrograde eclogite from the Sulu orogen, suggesting microscale transport of silicic melts relative to the rest of the rock and partial melts that did not escape from the rock (Zhao et al., 2007). Irregularly shaped films of K-feldspar were recognized between garnet and clinopyroxene (Fig. 3F). These microstructures are interpreted as trapped melt pockets that crystallized within the residual melanosome, and they provide strong evidence for in situ partial melting (Sawyer, 2008; Johnson et al., 2013). Plagioclase-Rich Leucosome and K-Feldspar–Rich Vein The Pl-rich leucosome within metabasite is mainly composed of plagioclase and quartz, accompanied by a variety of minor phases such as K-feldspar, antiperthitic feldspar, biotite, garnet, muscovite, apatite, rutile, and amphibole in different samples (Fig. 4A). The dominant plagioclase and quartz together constitute greater than 90 vol% of most samples, but the proportion of these two minerals varies considerably. The plagioclase generally occurs as subhedral to anhedral grains, with grain sizes of 1–2 mm, and locally may be larger than 5 mm. Similar to the plagioclase, the quartz also mainly occurs as subhedral to anhedral grains. However, elongated aggregation of quartz has also been recognized along the boundaries between quartz and plagioclase. In some cases, irregular cuspate K-feldspar (microcline) grains occur along the boundaries between plagioclase and quartz (Fig. 4B). The Kfs-rich veins show distinct mineral assemblages compared to the Pl-rich leucosomes. The Kfs-rich veins are composed mainly of K-feldspar, quartz, and plagioclase, with minor biotite, muscovite, titanite, and zircon (Figs. 4C and 4D) and locally rare garnet. The K-feldspar generally occurs as coarse-grained porphyroblasts, with grain sizes reaching 3 cm. ANALYTICAL METHODS Bulk-Rock Geochemical Analysis
Microscopic Texture More than 20 metabasite samples (retrogressed eclogite), Pl-rich leucosomes, and Kfs-rich veins within metabasites were chosen and observed in thin sections. However, fresh eclogite without obvious characteristics of partial melting are not described herein. Metabasite The host of the leucosome generally consists of garnet, clinopyroxene, plagioclase, amphibole, zoisite, phengite, and quartz (Fig. 3A), with accessory minerals such as zircon, apatite, and rutile. The garnet occurs as subhedral coarse-grained porphyroblasts, which are mostly surrounded by plagioclase or amphibole + plagioclase ± clinopyroxene coronas. Symplectite of clinopyroxene + plagioclase is widely scattered in the matrix, representing pseudomorphs of former omphacite, which suggest that the metabasite was retrogressed from eclogite. In thin sections, the plagioclase
Bulk-rock major-element concentrations were obtained by X-ray fluorescence (XRF), and trace-element and rare earth element (REE) concentrations were obtained via inductively coupled plasma–mass spectrometry (ICP-MS) at the National Research Center for Geoanalysis, Chinese Academy of Geological Sciences (CAGS). Major elements analyzed by XRF have analytical uncertainties of
