Received: 30 April 2017
Revised: 18 July 2017
Accepted: 30 August 2017
DOI: 10.1002/gj.3041
SPECIAL ISSUE ARTICLE
Early Paleozoic arc magmatism and metamorphism in the northern Qilian Block, western China: Petrological and geochronological constraints Yin‐Biao Peng1 Lai‐Xi Tong4
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Sheng‐Yao Yu2,3
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Jian‐Xin Zhang1
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San‐Zhong Li2,3
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De‐You Sun5
1
Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China
The Datong–Menyuan Complex, located in the northern margin of the Qilian Block, is composed
2
dominantly of high‐grade metamorphic rocks covered by a Paleozoic–Mesozoic sedimentary
Key Laboratory of Submarine Geosciences and Prospecting Technique, MOE and College of Marine Geosciences, Ocean University of China, Qingdao, China
3
Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao, China
4
State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China
5
Jilin University, Changchun, China
Correspondence Sheng‐Yao Yu, Key Laboratory of Submarine Geosciences and Prospecting Technique, MOE and College of Marine Geosciences, Ocean University of China, 5 Yushan Road, Shinan Qu, Qingdao Shi, Shandong Sheng, China. Email:
[email protected] Funding information National Key R&D Plan of China, Grant/Award Number: 2017YFC0601401; National Natural Science Foundation of China, Grant/Award Number: 41572053, 41630207 and 41572180; Geological Survey Project of China, Grant/Award Number: 1212011502700; State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Grant/Award Number: SKLabIG‐KF‐16‐02
sequence. In this study, we present petrographic observation, conventional thermobarometry and P–T pseudosection modelling, whole‐rock major and trace geochemistry, zircons U–Pb dating and Hf isotopic data together, to reveal the relationship between the magmatism and metamorphism along the northern margin of the Qilian Block. The anticlockwise P–T paths of gneiss and amphibolites are obtained using the garnet isopleths thermobaromety combined with phase equilibria modelling. The P–T pseudosection also shows that the gneiss and amphibolite underwent similar retrograde metamorphism with slightly different peak metamorphic conditions at ~720 °C and ~6.4 kbar for the former and ~700 °C and ~7.1 kbar for the latter. This indicates the Datong–Menyuan Complex recorded an amphibolite–granulite‐facies metamorphism. The zircon U–Pb analyses of 3 intrusive rocks including granitic dike, diorite, and granite are dated at ca. 499.8 ± 4.3, 495.9 ± 3.3, and 505.5 ± 3.2 Ma, and 3 representative metamorphic rocks underwent contemporaneous metamorphism at ca. 498.9 ± 4.1, 504.4 ± 3.9, and 499.3 ± 2.9 Ma. Zircon Hf isotopic analyses show that 505‐Ma zircons from the granitoid have positive εHf(t) values ranging from +8.5 to +12.8, indicating a depleted mantle source. Based on the penecontemporaneous magmatic and metamorphic event, we suggest that the southward subduction of the north Qilian Ocean triggered the activity of mantle‐derived magma and coeval metamorphic event. A subsequent compression is attributed to crustal thickening, indicating counter clockwise P–T paths. Combining these data with previous studies, it suggests paired metamorphic belts: the penecontemporaneous high‐temperature metamorphic belt related to an arc along the northern Qilian Block and high‐pressure/low‐temperature metamorphic belt existed in the north Qilian suture. KEY W ORDS
arc magmatism and metamorphism, north Qilian, paired metamorphic belts, P–T path
Handling Editor: R. Li
1
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I N T RO D U CT I O N
an outward low‐pressure (LP) metamorphic belt which probably formed beneath a volcanic chain in the adjacent island arc or continen-
Since first proposed by Miyashiro (1961) in Japan, the paired metamor-
tal margin (Miyashiro, 1961, 1973). From the spatial arrangement of
phic belts have been commonly recognized in various orogenic
the two contrasting belts at the surface, it is possible to detect not only
belts and have been considered as the hallmark of accretionary orog-
former subduction activity but also the polarity of the subduction zone
eny (e.g., Brown, 2008, 2009, 2010; Ernst, 2006; Zhang, Yu, &
(Frisch, 2014). Recently, the concept of paired metamorphic belts has
Mattinson, 2017). The classic paired metamorphic belts consist of
been extended to collision orogenic belts (Brown, 2007, 2008; Ernst,
two contrasting belts running parallel: an inboard high‐pressure (HP)
2006; Liou, Ernst, Zhang, Tsuijimori, & Jahn, 2009; Maruyama & Liou,
metamorphic belt which probably formed beneath a trench zone and
2005; Tsujimori, Sisson, Liou, Harlow, & Sorensen, 2006; Yu, Zhang,
Geological Journal. 2017;52(S1):339–364.
wileyonlinelibrary.com/journal/gj
Copyright © 2017 John Wiley & Sons, Ltd.
339
340
PENG ET
AL.
& Del Real, 2011; Zhang, Wang, & Song, 2009; Zhang et al., 2017). In a
2013). In contrast, the low‐pressure/high‐temperature (LP/HT) meta-
broader
are
morphic rocks related to arc magmatism have not been reported.
penecontemporaneous belts of contrasting type of metamorphism
Recently, several Alaska‐type zoned mafic complexes and “I” type
that record different apparent thermal gradient, one warmer and the
granites with an age of ca.500 Ma have been documented along the
other colder, juxtaposed by tectonic process (Brown, 2009, 2010).
northern margin of the Qilian Block (Huang et al., 2015; Wu et al.,
interpretation,
the
paired
metamorphic
belts
The north Qilian Orogen (NQL) in the northern Tibetan Plateau
2009, 2010; Zhang et al., 1997), and further studies on their geological
(Figure 1) consists of island arcs, accretionary wedges, ophiolitic
and geochemical features indicate a possible origin related to the
melange, and high‐pressure/low‐temperature (HP/LT) metamorphic
southward subduction of the north Qillian Ocean (Gehrels et al.,
rocks (Gehrels, Yin, & Wang, 2003a, 2003b; Li, 1979; Li et al., 2017;
2003a, 2003b; Xiao et al., 2009; Zhang et al., 2012). Recently, we
Smith, Lian, Chung, & Yang, 1997; Smith, 2006; Smith & Yang, 2006;
newly recognized the LP/HT metamorphic rocks along the northern
Song, 1997, Song et al., 2006, Song et al., 2009, Song, Niu, Su, & Xia,
margin of the Qilian Block. This LP/HT metamorphic belt forms a pos-
2013; Wang & Liu, 1976; Wu, Feng, & Song, 1993; Xiao, Chen, &
sible paired metamorphic belt with the NQL HP/LT metamorphic
Zhu, 1974, 1978; Xu et al., 1994; Zhang, Xu, Chen, & Xu, 1997; Zhang,
rocks. However, the metamorphic evolution, the time of these meta-
Xu, Xu, & Li, 1998; Zhang, Meng, & Wan, 2007, Zhang et al., 2017). It
morphic rocks and their relationship to the associated magmatic rocks
has been considered as a typical accretionary orogenic belt which is
have not been constrained. This hinders the further understanding of
resulted from the subduction of the north Qilian Ocean in the Early
paired metamorphic belts and tectonic evolution of the NQL. In this contribution, we present petrographic observations, pres-
Paleozoic (Li et al., 2016a). In the past two decades, most studies focused on the HP/LT meta-
sure–temperature
(P–T)
pseudosection
modelling,
conventional
morphic rocks (Ker et al., 2015; Liou, Wang, Coleman, Zhang, &
thermobarometry, geochemistry, zircons U–Pb dating, and Hf isotopic
Maruyama, 1989; Liu, Neubauer, & Genser, 2006; Song et al., 2004,
data for high‐grade metamorphic rocks and associated magmatic rocks.
2006, 2007; Song et al., 2013; Wu et al., 1993; Xu et al., 1994; Zhang
Our results place important constraints on arc metamorphism and
et al., 2007; Zhang & Meng, 2006; Zhang et al., 1997, 1998; Zhang
magmatism related to the oceanic subduction and further unravel the
et al., 2017) and widespread granitoids in the NQL (Chen, Song, Niu,
subduction polarity of the Qilian Ocean (Proto‐Tethys) during the Early
& Wei, 2014; Chen, Xia, & Song, 2013; Huang et al., 2015; Song
Paleozoic.
et al., 2013; Tseng et al., 2009; Wang et al., 2006; Wang, Zhang, Qian, & Zhou, 2005; Wu et al., 2009; Wu et al., 2010; Wu et al., 2004; Wu, Yao, & Zeng, 2006; Xiong, Zhang, & Zhang, 2012; Yu, Zhang, Qin, Sun,
2
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G E O L O G I C A L S ET T I N G
& Zhao, 2015). The peak pressure and temperature conditions of the HP/LT metamorphic rocks have been restricted to 19–24 kbar and
The northwest‐southeast‐trending Altyn Tagh–Qilian Orogen (AT‐QL),
420–580 °C (Wei & Song, 2008; Zhang et al., 2007; Zhang et al.,
is located on the northern margin of the Qinghai–Tibet Plateau,
2012). The HP/LT metamorphic age was dated at 440–489 Ma (Liou
bounded by the Alashan Block–Dunhuang to the north, the Tarim
et al., 1989; Liu et al., 2006; Song et al., 2004, 2006; Wu et al., 1993;
Basin to the northwest, and the Qaidam Block to the south. The AT‐
Zhang et al., 1997; Zhang et al., 2007). The granitoids have been
QL was separated into Altyn Tagh and Qilian by the left‐lateral Altyn
subdivided into volcanic arc granite (520–460 Ma), syn‐collisional
Tagh Fault with about 400 km offset (Ge & Liu, 1999; Xu et al.,
granite (440–420 Ma) and post‐collisional granite (≤420 Ma) based
1999; Yang et al., 2001; Zhang, Meng, & Yang, 2005; Zhang, Zhang,
on the forming ages and geochemical characteristics (Song et al.,
Xu, Yang, & Cui, 2001; Zhang et al., 2017). The Qilian Orogen consists
FIGURE 1 Schematic map showing major tectonic units of the Altyn Tagh–Qilian orogenic system in north Tibet. ALB = the Alxa Block; CAB = the central Altun Block; DHB = the Dunhuang Block; EKL = the eastern Kunlun Mountains; HMLY = the Himalaya Mountains; INP = the Indian Plate; NAT = the north Altun subduction–accretion Complex; NQD = the north Qaidam subduction–collision Complex; NQL = the north Qilian subduction–accretion Complex; QDB = the Qaidam Block; QL = the Qilian Mountains; QLB = the Qilian Block; SAT = the south Altun subduction– collision Complex; TRB = the Tarim Basin; WKL = the western Kunlun Mountains. Modified after Zhang et al. (2012)
PENG
341
ET AL.
of three tectonic units from south to north: the north Qaidam subduc-
relating to subduction of north Qilian Ocean in the early Paleozoic (Ker
tion–collision Complex (NQD), the Qilian Block (QLB), and the north
et al., 2015; Wang & Liu, 1976; Wu et al., 1993; Xia, Xia, & Xu, 2003;
Qilian subduction–accretion Complex (NQL; Figure 1).
Xiao et al., 1974, 1978; Xu et al., 1994; Zhang et al., 2012; Zhang et al.,
The NQD, located between the QLB and the Qaidam Block, is char-
1998; Zheng, Xiao, & Zhao, 2013). Three ophiolite belts have been rec-
acterized by the ultrahigh‐pressure (UHP) metamorphic rocks consisting
ognized in the NQL (Xia, Li, Yu, & Wang, 2016), including the 550‐Ma
of felsic gneiss, enclosing eclogite and garnet peridotites. Reliable zircon
Yushigou ophiolite belt in the south (Shi, Yang, & Wu, 2004), the 517‐
U–Pb ages indicate the HP‐UHP metamorphism occurred between 423
to 487‐Ma Dachadaban ophiolite belt in the middle (Meng, Zhang, Kar,
and 460 Ma (Song et al., 2003, 2005, 2006; Yang et al., 2001, 2002; Yang
& Li, 2010.), and the 490‐Ma Jiugequan ophiolite belt in the north (Xia
et al., 2003; Zhang et al., 2012; Zhang et al., 2005).
& Song, 2010). The HP/LT blueschist and eclogite‐facies rocks record a
The QLB, located between the NQL and NQD, is composed of
cold oceanic subduction zone with peak metamorphic ages of 463–
Precambrian basement overlain by a Neoproterozoic to Mesozoic sed-
489 Ma and protolith ages of 544–710 Ma (Song et al., 2007; Zhang
imentary sequence (Chen et al., 2009; Chen, Wang, & Zhang, 2007; Yu
et al., 2007; Zhang et al., 2009). The arc volcanic rocks and granite plu-
et al., 2013; Zhang et al., 2012). The QLB was previously suggested as
tons in the Early Paleozoic are also widespread in NQL.
a continental fragment rifted from the North China Craton, but recent
The Datong–Menyuan Complex, which is the focus of this contri-
geochemical and geochronological data indicate that the metamorphic
bution, is located in the northern margin of the Qilian Block (or the
basement has a close affinity to the South China Craton rather than
southern NQL; Figures 1 and 2). It consists mainly of high‐grade
the North China Craton (Li, Yang, Zhao, Li, Suo, et al., 2016b; Li
metamorphic rocks intruded by the Early Paleozoic plutons and
et al., 2016; Tung et al., 2007; Wan, Xu, Yan, & Zhang, 2001; Wan,
was traditionally considered as Precambrian basement of the Qilian
Zhang, Yang, & Xu, 2006).
Block (termed as the Huangyuan Group). The high‐grade metamor-
The NQL, located between the Alxa Block and Qilian Block, is
phic rocks are composed of paragneiss, granitic gneiss, amphibolites,
characterized by the ophiolite, HP/LT blueschist and eclogite, island‐
pelitic gneisses, and marble. Recent geochronologic results demon-
arc volcanic rocks with granite plutons, and Silurian flysch formations,
strate the granitic gneisses yield protolith ages of 950–870 Ma
FIGURE 2
Geological map of Datong–Menyuan Complex in the north Qilian Orogen with localities of representative samples
342
PENG ET
AL.
(Tung, Yang, Yang, Liu, Zhang, Wan, et al., 2007; Tung, Yang, Liu,
2.1.2
Zhang, Yang, Shau et al., 2012, 2013; Wan et al., 2001; Xu, Zhang,
The metamorphic rocks in the study area include metabasites and
& Liu, 2007; Yu et al., 2013). However, reliable metamorphic timing
metasedimentary rocks. The metabasites were surrounded by the
has not been obtained.
country rocks or intruded into the supracrustal rocks (Figure 3b,c).
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Metamorphic rocks
The paragneisses commonly occur as interlayers within other supracrustal rocks (Figure 3d,e).In the field, you can see clearly many
2.1
Sample descriptions
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small felsic leucosomes appear within the garnet–biotite gneiss, which
Studied samples including amphibolites, gneisses, schist, granite, and
are thin layers, veinlets or irregular without migrated far away and may
diorite were collected in the Datong–Menyuan Complex, about
represent an in situ leucosome (Figure 3f). The gneisses (QL11‐22‐3.3) display a gneissic structure defined by
100 km NE of Xi'ning City (Figure 2). The petrology, mineralogy, magmatic or metamorphic age, and sample locations are listed in Table 1.
the directional mica and elongated quartz and feldspar (Figure 4a). The gneiss (QL11‐22‐3.3) consists of garnet, plagioclase, K‐feld-
Mineral abbreviations are after Whitney and Evans (2010).
spar, biotite, muscovite, sillimanite, and quartz. The cores of the garnet porphyroblasts contain inclusions of biotite and quartz, whereas the
2.1.1
|
Intrusive rocks
rims have no inclusions (Figure 4c,d). The biotite occurs as small relicts.
The intrusive rocks in the study area mainly consist of gabbro, diorite,
The K‐feldspar occurs as porphyroblasts and is partly replaced by pla-
tonalite, and granite. The granitoids consist mainly of plagioclase,
gioclase + muscovite, and the plagioclase is stable all the time. The
quartz, amphibole, and biotite. The granite dike (AQ15‐10‐2.1) and
muscovite occurs as big subhedral to euhedral porphyroblasts indicat-
diorite (AQ15‐10‐3.1) are symbiotic and the diorite is cut by the
ing they formed in the later stage (Figure 4c,d). This texture and the
granite dike (Figure 3a). However, the granite dike (AQ15‐10‐2.1)
compositional zoning of garnet described below indicate that the
and gabbroic diorite (AQ15‐10‐3.1) are nearly homochronous within
gneiss has two‐stage mineral assemblages. The garnet cores and the
the margin of error as discussed below. The granite dike (AQ15‐10‐2.1)
mineral
is light coloured with a slight pink colour in the field and composed
Grt + Bt + Sill + Pl + Kfs + Qz and the garnet rims and matrix minerals
mainly of euhedral to subhedral K‐feldspar, plagioclase, amphibole, and
form a late assemblage of Grt + Bt + Ms + Sill + Pl + Kfs + Qz.
inclusions
form
an
early
mineral
assemblage
of
anhedral quartz (Figure 4a). The large subhedral crystals or euhedral
The garnet‐bearing amphibolites (AQ14‐12‐2.2) consist of garnet,
laths of K‐feldspar and plagioclase form a granoblastic texture, with
amphibole, plagioclase, quartz, and minor opaque minerals (Figure 4e).
the small irregular crystals of K‐feldspar, plagioclase, and quartz fill
The garnet occurs as subhedral grains and contain inclusions of amphi-
the interstitial space. The gabbroic diorite (AQ15‐10‐3.1) consist of
bole, plagioclase, and quartz. The amphibole occurs as subhedral grains
plagioclase, amphibole, quartz, and minor biotite (Figure 4b). The
with inclusions of plagioclase and quartz. On the basis of petrographical
gabbroic diorite shows massive structure and gabbroic texture, in
observations, the garnet core and mineral inclusions may represent a
which amphibole and plagioclase are interspersed with each other.
peak mineral assemblage of Grt + Pl + Amp + Qz. Moreover, the mineral
TABLE 1
The major features of the studied rocks from the Datong–Menyuan Complex
Sample
Rock
Mineral assemblage
Magmatic age (Ma)
Metamorphic age (Ma)
Location
Intrusive rocks AQ15‐2‐4.2‐1
Granite
Pl, Kf, Bt, Qz
N37°29.798′,E101°17.330′
AQ14‐3‐5.2
Diorite
Amp, Pl, Qz
N37°27.199′,E101°10.419′ N37°27.945′,E101°12.188′
AQ14‐3‐8.1
Diorite
Amp, Pl, Qz
AQ14‐4‐7.1
Tonalite
Pl, Bt, Amp, Qz
AQ15‐10‐2.1
Granite dike
Pl, Kf, Qz, Amp
498.2 ± 3.6
N37°16.688′,E101°30.496′
N37°26.483′,E101°08.935′
AQ15‐10‐3.1
Diorite
Amph, Pl, Bt, Qz
495.9 ± 3.3
N37°16.688′,E101°30.496′
AQ14‐2‐4.2
Granite
Pl, Kf, Bt, Qz
505.5 ± 3.2
N37°25.813′,E101°03.933′
Metamorphic rocks AQ16‐25‐4B
Grt–Bi gneiss
Grt, Fsp, Bt, Ms, Ilm, Rt, Qz, Sill
N37°20.595′,E101°08.593′
AQ16‐25‐4C
Grt–Bt–Amp gneiss
Grt, Cpx, Amp, Fsp, Bt, Qz, Ttn, Ilm, Ap
N37°20.595′,E101°08.593′
AQ14‐12‐1.1
Schist
Grt, Fsp, Bt, Qz
499.3 ± 2.9
N37°18.395′,E101°15.560′
AQ14‐12‐2.2
Grt amphibolite
Grt, Fsp, Amp, Ilm, Qz
498.9 ± 4.1
N37°17.922′,E101°15.666′
504.4 ± 3.9
N37°26.751′,E101°01.753′
AQ14‐12‐2.1
Grt‐Bi schist
Grt, Fsp, Bt, Qz
QL11‐22‐3.3
Grt–Bi gneiss
Grt, Bt, Ms, Pl, Kf, Sill, Ilm, Qz
N37°17.922′,E101°15.666′
AQ14‐1‐1.2
Grt–Bi–Amp gneiss
Grt, Cpx, Amp, Fsp, Bt, Qz
N37°27.385′,E100°59.987′
AQ14‐1‐1.4
Grt–Bi gneiss
Grt, Fsp, Bt, Qz
N37°27.385′,E100°59.987′
AQ14‐2‐2.1
Grt amphibolite
Grt, Amp, Fsp, Bt, Ilm, Qz
N37°25.640′,E101°03.093′
AQ14‐2‐10.4
Grt–Bi–Amp gneiss
Grt, Amp, Fsp, Bt, Ilm, Qz
N37°26.063′,E101°07.737′
PENG
343
ET AL.
FIGURE 3
Photographs of outcrops of intrusive rocks and metamorphic rocks. (a) Field outcrop of the Early Paleozoic gabbroic plutons and granite dike. (b,c) Garnet‐bearing amphibolites enclave within the garnet–biotite gneiss; the hammer for scale is 30 cm long. (d,e) Garnet–biotite gneiss shows the bedding structure. The human for scale is 170 cm tall. (f) Garnet–biotite gneiss. The pen cap for scale is 4 cm long
assemblages are slightly different due to the distinct composition of protolith and metamorphic conditions. The garnet‐bearing schists (AQ14‐12‐2.1) shows a similar mineral assemblage and is composed of garnet, plagioclase, biotite, and quartz (Figure 4f).
3.2
|
Bulk‐rock geochemical analysis
Bulk‐rock major, trace and rare‐earth element (REE) concentrations were obtained by X‐ray fluorescence and inductively coupled plasma mass spectrometry (ICP‐MS) at the National Research Center for Geoanalysis, CAGS. The major elements were analysed by X‐ray fluorescence with analytical uncertainties 2.0 wt%; Table 3).
zircon
(176Hf/177Hf = 0.282772,
U–Pb
ages
and
chondritic
176
Albarede, 1997).
and fluctuant SiO2 (40.72–44.33 wt%; Table 4), belonging to the calcic amphiboles, and the amphiboles are mainly ferropargasite with Si of 6.2 to 6.5 and Mg/(Mg + Fe2+) of 0.38 to 0.62 (Leake et al., 1997).
4
RESULTS
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Plagioclase is common in all metamorphic rocks. The plagioclases in the garnet–amphibolites have the highest anorthite contents rang-
4.1
|
Mineral compositions
ing from 77.48 to 85.83 mol% (Table 5). Plagioclase from the gneiss have relatively lower anorthite contents of 22.85–73.09 mol%. The
Representative data are listed in Tables 2–5, and characteristic
schist
features are described below. Fe2O3 contents of minerals were
38.41–39.15 mol% (Table 5). The difference of anorthite and albite
calculated by stoichiometry.
(AQ14‐12‐2.1)
has
anorthite
contents
with
average
7.4
0
0.07
MgO
CaO
Na2O
2.671
0.138
1.694
0.007
0
1.456
0.015
0.863
0
0.011
0.982
7.837
Si
Ti
Al
Cr
Fe3
Fe2
Mn
Mg
Ca
Na
K
Cations
11
0.22
MnO
Oxygens
9.94
22.26
FeO
9.84
0
Fe2O3
94.77
0.12
K2O
0.16
18.37
Al2O3
Cr2O3
Totals
18.07
2.35
TiO2
7.822
0.994
0.024
0
0.827
0.012
1.443
0
0
1.669
0.151
2.702
11
94.5
0
7.08
0.18
22.02
0
0
2.56
34.14
SiO2
34.49
Gneiss QL11‐22‐3.3
7.842
0.991
0.009
0
0.836
0.014
1.494
0
0.007
1.669
0.149
2.672
11
94.15
9.83
0.06
0
7.1
0.21
22.6
0
0.12
17.92
2.5
33.81
7.726
0.911
0.017
0
0.88
0.014
1.471
0
0
1.387
0.237
2.808
11
95.26
9.21
0.11
0
7.61
0.22
22.68
0
0
15.17
4.06
36.2
Gneiss AQ14‐2‐10.4
7.719
0.89
0.017
0.011
0.841
0.011
1.502
0
0.001
1.422
0.221
2.803
11
95.24
8.99
0.11
0.13
7.27
0.16
23.14
0
0.02
15.54
3.78
36.1
7.769
0.926
0.016
0.004
1.004
0.01
1.42
0
0
1.373
0.188
2.828
11
95.98
9.46
0.11
0.05
8.78
0.15
22.13
0
0
15.18
3.26
36.86
Gneiss AQ14‐1‐1.2
The compositions of the representative biotites from the metamorphic rocks
Rock Sample
TABLE 3
7.698
0.897
0.034
0.003
0.907
0.006
1.367
0
0.001
1.427
0.239
2.815
11
94.73
9.1
0.23
0.04
7.88
0.09
21.15
0
0.02
15.67
4.12
36.43
7.8
0.89
0.047
0
1.747
0.003
0.752
0
0.006
1.375
0.167
2.811
11
95.23
9.48
0.33
0
15.94
0.05
12.23
0
0.1
15.86
3.02
38.22
Gneiss AQ16‐25‐4B
7.812
0.802
0.107
0.002
1.912
0.002
0.637
0.06
0.003
1.349
0.157
2.78
11
94.81
8.58
0.75
0.03
17.5
0.04
10.4
1.06
0.05
15.61
2.85
37.93
7.734
0.892
0.025
0.001
1.155
0.001
1.257
0
0.002
1.352
0.253
2.795
11
95.93
9.22
0.17
0.01
10.22
0.02
19.83
0
0.04
15.12
4.44
36.86
Gneiss AQ16‐25‐4C
7.806
0.925
0.018
0.002
1.149
0.014
1.265
0
0.002
1.531
0.129
2.77
11
95.37
9.5
0.12
0.03
10.1
0.22
19.81
0
0.04
17.01
2.25
36.29
7.797
0.967
0.013
0.002
1.096
0.019
1.278
0
0.007
1.447
0.198
2.769
11
95.54
9.89
0.09
0.03
9.6
0.29
19.94
0
0.11
16.02
3.43
36.14
Schist AQ14‐12‐2.1
7.74
0.934
0.013
0.002
1.209
0.006
1.143
0
0.004
1.392
0.203
2.833
11
95.52
9.69
0.09
0.03
10.74
0.09
18.1
0
0.07
15.63
3.57
37.51
Gneiss AQ14‐1‐1.4
PENG ET AL.
347
348
PENG ET
TABLE 4
AL.
The compositions of the representative amphiboles from the metamorphic rocks Gneiss AQ16‐25‐4C
Amphibolite
Amphibolite AQ14‐2‐2.1
Gneiss AQ14‐2‐10.4
SiO2
40.72
40.90
41.22
41.56
42.41
41.39
43.69
42.15
42.39
42.91
42.10
44.04
43.97
TiO2
2.38
2.28
1.44
1.68
1.56
1.74
1.62
1.86
0.62
0.80
1.62
1.41
1.45
1.31
Al2O3
12.66
12.46
12.74
12.86
12.51
12.99
10.65
11.58
13.16
12.04
12.05
11.75
11.92
11.55
Cr2O3
0.02
0.00
0.00
0.00
0.03
0.04
0.00
0.05
0.02
0.01
0.00
0.02
0.00
0.03
Fe2O3
Gneiss AQ14‐1‐1.2
AQ14‐12‐2.2
Rock Sample
44.33
2.12
2.16
2.42
2.50
1.92
2.75
2.25
0.01
2.71
3.77
4.10
2.84
3.05
3.46
FeO
18.88
18.16
19.19
16.80
16.47
16.96
16.09
17.57
17.69
16.73
17.09
13.01
12.95
12.48
MnO
0.18
0.21
0.31
0.10
0.12
0.08
0.14
0.15
0.11
0.12
0.19
0.13
0.10
0.13
MgO
6.99
7.26
6.65
7.92
8.30
7.89
9.73
8.52
7.30
7.92
7.85
11.13
11.07
11.35
CaO
10.80
10.70
11.02
10.68
10.86
10.81
11.45
11.62
11.69
11.16
11.04
11.34
11.25
11.41
Na2O
1.48
1.32
1.15
1.22
1.22
1.18
1.24
1.32
1.10
1.07
1.21
1.28
1.27
1.16
K2O
1.73
1.78
1.72
1.69
1.49
1.67
1.16
1.68
0.79
0.68
0.94
1.00
1.00
0.87
Totals
97.96
97.24
97.86
97.01
96.89
97.5
98.02
96.51
97.58
97.21
98.19
97.94
98.03
98.08
Oxygens
23
23
23
23
23
23
23
23
23
23
23
23
23
23
Si
6.231
6.281
6.316
6.336
6.438
6.291
6.548
6.464
6.417
6.497
6.357
6.501
6.484
6.521
Ti
0.274
0.263
0.166
0.193
0.178
0.199
0.183
0.215
0.071
0.091
0.184
0.157
0.161
0.145
Al
2.284
2.256
2.301
2.312
2.239
2.328
1.882
2.094
2.349
2.149
2.145
2.045
2.072
2.003
Cr
0.002
0
0
0
0.004
0.005
0
0.006
0.002
0.001
0
0.002
0
0.003
Fe3
0.245
0.25
0.279
0.287
0.219
0.314
0.254
0.001
0.309
0.43
0.466
0.315
0.338
0.383
Fe2
2.416
2.333
2.459
2.142
2.091
2.156
2.016
2.253
2.24
2.118
2.158
1.606
1.597
1.535
Mn
0.023
0.027
0.04
0.013
0.015
0.01
0.018
0.019
0.014
0.015
0.024
0.016
0.012
0.016
Mg
1.594
1.662
1.518
1.8
1.878
1.787
2.173
1.947
1.647
1.787
1.766
2.449
2.433
2.488
Ca
1.771
1.761
1.809
1.745
1.766
1.761
1.839
1.91
1.896
1.811
1.786
1.794
1.778
1.798
Na
0.439
0.393
0.342
0.361
0.359
0.348
0.36
0.393
0.323
0.314
0.354
0.366
0.363
0.331
K
0.338
0.349
0.336
0.329
0.289
0.324
0.222
0.329
0.153
0.131
0.181
0.188
0.188
0.163
15.701
15.659
15.662
15.614
15.551
15.629
15.579
15.632
15.524
15.489
Cations
15.58
15.545
15.54
15.516
contents in plagioclases may have resulted from the different
(Figure 7a). The intrusive rocks are characterized with high Na2O
compositions
contents
contents (mostly>3.2%) and low A/CNK values (distinctly3.2%) and low A/CNK values (lower than
(Bohlen, 1991; Zhao & Zhai, 2013). So the granulite‐facies metamor-
1.1), and are all metaluminous (Table 6 and Figure 7b). The chondrite‐
phism was closely related to the mantle‐derived magmatism. Previous
normalized REE diagram shows a fractioned patterns with enrichment
studies show that the north Qilian belt may have undergone several
of LREE and depletion of HREE, without obvious Eu anomalies
periods of metamorphism in the Early Paleozoic and the metamorphic
(Figure 8a).The trace element diagram shows depletion of high field
age range from 440 to 512 Ma (Liu et al., 2006; Song et al., 2004,
strength elements (e.g., Nb, Ta, Zr, Hf, and Ti) and enrichment of LILE
2006; Zhang et al., 2007; Zhang et al., 1997). One of the most vital
(e.g., Rb, Ba, and Sr; Figure 8b). These features are consistent with
metamorphic event may have happened at ~500 Ma, as revealed in
most continental arc magmatism related to subduction (Gill, 1981;
this study by the metamorphic age of three samples. Taking the three
Ickert, Thorkelson, Marshall, & Ullrich, 2009; Pearce & Peate, 1995).
arc granite samples at ~500 Ma into account, it is reliable to predict
The relatively low Y, Nb, and Rb contents also restrict the granitoids to
that the metamorphism at 500 Ma was closely associated to the
the volcanic arc fields (Figure 7c,d), indicating being typical arc magmatic
homochronous magmatism resulting from the subbduction of the
rocks. Similar arc igneous rocks occurred as individual elongated bodies
north Qilian Ocean.
within the arc igneous belt, consisting Kekeli I‐type granodiorite intru-
Sarbadhikari and Bhowmik (2008) presented an interpretation of
sions, Chaidanuo S‐type monzonitic granite, Niuxinshan I‐type quartz‐
tectonic thickening of the hot midcrust for the counter clockwise
diorite intrusions, and Huanyuan plagiogranite‐like granite. Zircon
P–T path of granulite. The peak‐stage amphibolites–granulite‐facies
U–Pb dating revealed that they formed at 460–520 Ma and in an arc
metamorphism at relatively low pressures (6.4–7.1 kbar) at a normal
environment by related geochemistry features (Huang et al., 2015; Song
midcrust for the metamorphic rocks indicating the samples may have
et al., 2004; Song et al., 2013; Wu et al., 2004; Wu et al., 2006; Wu et al.,
not undergone large‐scale crustal thickening. In this study, the IBC is
2009; Wu et al., 2010). All these arc igneous rocks in other areas demon-
a typical of magma accretion rather than magma loading, as the phase
strated an extensive Early Paleozoic arc magmatic activity in NQL and
equilibria modelling shows no isothermal/heating increasing pressure
north QLB rather than only in the Datong–Menyuan Complex.
process, which may be related to magma loading. So it is likely that
As described above, the positive Hf(t) values (range from +8.5 to
the magma accretion resulted in slight crustal thickening.
+12.8) indicate a mantle source for the granitoid of Datong–Menyuan,
In summary, the arc magmatism is due to the mantle‐derived
which may have played an important role in crustal growth (Chu et al.,
magma accretion, which results from the upwelling of the astheno-
2006; Kinny & Maas, 2003). It is widely accepted that the modern con-
sphere due to the break‐off of the deeply subducted north Qilian
tinental crust is dominantly created at convergent margins (Rudnick,
oceanic slab. And arc magmatism may have resulted in slight crustal
1995) and yields an andesite composition on average (Jagoutz &
thickening and growth and associated granulite‐facies metamorphism
Schmidt, 2013).The classical models for continental crust growth holds
(Collins, 2002a, 2002b; Hollis, Clarke, Klepeis, Daczko, & Ireland,
that the arcs produce andesitic bulk composition of continental crust
2004; Stowell, Tulloch, Zuluaga, & Koenig, 2010; Yoshino, Yamamoto,
through accumulation and fraction (Castro et al., 2013; Taylor, 1967).
Okudaira, & Toriumi, 1998).
The Datong–Menyuan Complex comprise an andesitic composition with average SiO2 of 58.04 wt%, K2O of 1.57 wt%, and Mg# 45.38, which are in accordance with the andesitic rocks in subduction‐related
5.3
|
Tectonic implications
magmatic arcs, with SiO2 of 57–65 wt%, K2O of 0.96–2.89 wt%, Mg#
The subduction polarity of the Qilian Ocean during the early Paleozoic
of 45–54 wt% (Kelemen, Yogodzinski, & Scholl, 2003). As the Mg num-
remains controversial. Most researchers (Wu et al., 2009, 2010; Zhang
ber is strongly influenced by fractional crystallization rather than par-
et al., 2012; Zuo & Liu, 1987) suggested a double subduction model. In
tial melting, with Mg# of 68–75 wt% for the primitive magma (Best &
contrast, some authors argued for north‐dipping subduction based on
Christiansen, 2001). The wide range of SiO2 (range from 52.07 to
the characteristic trench–arc–basin system (Song et al., 2013; Xia,
71.79 wt%) and low Mg# (range from 36.96 to 57.22 wt%) indicate that
Xia, & Xu, 1995, 1996, 2003; Xu et al., 1994; Zhang et al., 1997,
the primary magma has undergone to some degree of fractional crys-
1998) or south‐dipping subduction (Gehrels et al., 2003a; Sobel &
tallization (Castro et al., 2013). Thus, the granitoids in Datong–
Arnaud, 1999; Song, 1997; Yin et al., 2007). Recently, more and more
Menyuan with andesitic composition that come from the mantle may
Early Ordovician (516–477 Ma) granitoids have been documented in
make a contribution to the continental crust growth.
the southern margin of the NQL (or northern QLB), indicating initial
Phase equilibrium modelling shows that the gneiss and amphibolites
south‐dipping subduction of the Paleo–Qilian Ocean. For example, sev-
experienced similar peak‐stage granulite‐facies metamorphism with
eral ~500 Ma Alaska basic Complex were reported in the southern part
anticlockwise P–T paths, characterized by a near IBC (Figures 13–15).
of the north Qilian orogenic belt (Huang et al., 2015; Wu et al., 2009,
The amphibolites–granulite‐facies metamorphic rocks in the Datong–
2010; Zhang et al., 1997). Coeval arc magmatism are widespread in
PENG
359
ET AL.
FIGURE 16 (a) P–T diagram to show the metamorphic facies location of selected samples. HPM‐UHPM includes the following: amphibole eclogite facies (AE), amphibole–epidote eclogite facies (AEE), amphibole–lawsonite eclogite facies (ALE), blueschist facies (BS), and lawsonite eclogite facies (LE). E‐HPG = medium temperature eclogite‐high‐pressure granulite metamorphism; G = granulite‐facies metamorphism; UHPM = ultrahigh pressure metamorphism; UHTM = the ultrahigh temperature metamorphic part of the granulite facies. (b) Metamorphic patterns show the thermal gradient of representative samples in NQL based on the peak metamorphic P–T conditions. The red and blue squares refer to the samples from the Qingshuigou–Baijingsi (central of NQL) and Datong‐Menyuan (south of the NQL)
the south of the north Qilian orogen, such as the Kekeli and Niuxingshan
earlier than 510 Ma (Song et al., 2013; Xia et al., 2016). Taking the
granitoids, indicating a continental arc resulted from the south subduc-
arc magmatism and metamorphism of Datong–Menyuan Complex at
tion of the north Qilian Ocean (Gehrels et al., 2003a; Gehrels et al.,
~500 Ma into consideration, we can infer that the north Qilian Ocean
2011; Xiao et al., 2009; Yin et al., 2007; Zhang et al., 2012).
initial subduction toward the south began earlier than 500 Ma, which
The ocean crust subduction may form an inboard HP/LT metamorphic belt beneath a trench zone and an HT/LP metamorphic belt
brought about the subsequent magmatism and metamorphism in the Datong–Menyuan Complex.
beneath a volcanic chain in the adjacent arc or continental margin (Miyashiro, 1961, 1973). In compression with HP/LT metamorphic rocks, we plot the peak P–T values of metamorphic rocks in Datong–
6
|
CO NC LUSIO NS
Menyuan Complex and Qingshuigou–Baijingsi on the P–T diagram and metamorphic patterns (Brown, 2010). As shown in Figures 15 and
This paper presents the results of an integrated petrological, phase
16, the peak metamorphic temperature (700–720 °C) and pressure
equilibria modelling, whole‐rock geochemistry, zircons U–Pb dating
(6.4–7.1 kbar) of the Datong–Menyuan Complex demonstrate that they
and Hf isotopic data of representative rocks in the Datong–Menyuan
have experienced granulite‐facies metamorphism and high thermal
Complex, north Tibetan Plateau. The main conclusions are summarized
gradient more than 750 °C/GPa. In contrast, the HP/LT metamorphic
as follows:
rocks in Qingshuigou–Baijingsi has undergone eclogite‐facies metamorphism with low gradient less than 350 °C/GPa. As demonstrated in this study, the HT/LP metamorphism of the metamorphic rocks in Datong– Menyuan Complex (south of NQL) occurred at about 500 Ma. And previous research also confirmed the HP/LT metamorphism of metamorphic rocks in Qingshuigou–Baijingsi (central part of NQL) occurred at nearly 500 Ma (Wei & Song, 2008; Zhang et al., 2007; Zhang
1. The U–Pb ages of three intrusive rocks granite dike, diorite, and granite are constrained at ca. 499.8 ± 4.3, 495.9 ± 3.3, and 505.5 ± 3.2 Ma, and three representative metamorphic rocks samples
underwent
contemporaneous
metamorphism
at
ca.
498.9 ± 4.1, 504.4.3 ± 3.9, and 499.3 ± 2.9 Ma.
et al., 2012). Difference in thermal gradient for penecontemporaneous
2. Phase equilibria modelling and garnet isopleths modelling has
HT/LP metamorphism and HP/LT metamorphism in NQL can be well
defined retrograde P–T paths, with the peak metamorphic P–T
explained by applying a typical paired metamorphic belts model
conditions for sample QL11‐22‐3.3 at ~720 °C and ~6.4 kbar,
(Brown, 2009, 2010). Based on the spatial arrangement of these
for sample AQ14‐12‐2.2 at ~700 °C and ~7.1 kbar, respectively.
two contrasting belts at the NQL, it is possible to detect the north
The Datong‐Menyuan Complex has experienced amphibolites–
Qilian Ocean subduction to the south with the concept of double
granulite‐facies metamorphism in the Early Paleozoic.
metamorphic belts of Frisch (2014). As accepted by a lot of scholars,
3. The observed assemblages and calculated P–T conditions in this
the north Qilian Ocean initial subduction toward the north began
paper define an anticlock P–T path that involve a near IBC and
360
PENG ET
subsequently compression of hot midcrust with or without slight crustal thickening. 4. The HT metamorphic rocks in Datong–Menyuan Complex and HP/LT metamorphic rocks in the central part of NQL formed a typical paired metamorphic belts, which indicated the south subduction of the north Qilian Ocean in the Early Paleozoic.
ACKNOWLEDGEMEN TS This study was financially supported by the National Key R&D Plan of China (Grant 2017YFC0601401), the National Natural Science Foundation of China (Grants 41572053, 41630207, and 41572180), the Geological Survey Project of China (Grant 1212011502700), and the research grant of State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (SKLabIG‐KF‐16‐02). We thank three anonymous reviewers for their critical reviews and constructive comments, which significantly improved the manuscript. ORCID Yin‐Biao Peng
http://orcid.org/0000-0002-3558-2528
Sheng‐Yao Yu
http://orcid.org/0000-0001-8623-6863
San‐Zhong Li
http://orcid.org/0000-0002-3436-2793
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How to cite this article: Peng Y‐B, Yu S‐Y, Zhang J‐X, Li S‐Z, Tong L‐X, Sun D‐Y. Early Paleozoic arc magmatism and metamorphism in the northern Qilian Block, western China: Petrological and geochronological constraints. Geological Journal. 2017;52(S1):339–364. https://doi.org/10.1002/gj.3041
