Early Paleoproterozoic Magmatism in the Quanji Massif

Gondwana Research 21 (2012) 152–166

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Early Paleoproterozoic magmatism in the Quanji Massif, northeastern margin of the Qinghai–Tibet Plateau and its tectonic signiﬁcance: LA-ICPMS U–Pb zircon geochronology and geochemistry

Songlin Gong a,b, Nengsong Chen a,c,⁎, Qinyan Wang a, T.M. Kusky b,c, Lu Wang b,c, Lu Zhang a, Jin Ba a, Fanxi Liao a a Faculty of Earth Science, China University of Geosciences, Wuhan 430074, China b Three Gorges Research Center for Geo-hazards, Ministry of Education, China University of Geosciences, Wuhan 430074, China c State Key Lab of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China article info abstract

Article history: The Quanji Massif is located on the north side of the Qaidam Block and is interpreted as an ancient cratonic Received 31 January 2011 remnant that was detached from the Tarim Craton. There are regionally exposed granitic gneisses in the Received in revised form 2 July 2011 basement of the Quanji Massif whose protoliths were granitic intrusive rocks. Previous studies obtained Accepted 15 July 2011 intrusion ages for some of these granitic gneiss protoliths. The intrusion ages span a wide range from ~2.2 Ga Available online 22 July 2011 to ~2.47 Ga. This study has determined the U–Pb zircon age of four granitic gneiss samples from the eastern,

Keywords: central and western parts of the Quanji Massif. CL images and trace elements show that the zircons from these Quanji Massif four granitic gneisses have typical magmatic origins, and experienced different degrees of Pb loss due to Early Paleoproterozoic granitic gneisses strong metamorphism and deformation. LA-ICPMS zircon dating yields an upper intercept age of 2381 ±41 Zircon LA-ICPMS U–Pb geochronology (2σ) Ma from monzo-granitic gneiss in the Hudesheng area and 2392±25 (2σ) Ma from granodioritic gneiss Tectonic evolution in the Mohe area, eastern Quanji Massif, and 2367±12 (2σ) Ma from monzo-granitic gneiss in the Delingha Tarim and North China Cratons area, central Quanji Massif, and 2372 ±22 (2σ) Ma from monzo-granitic gneiss in the Quanjishan area, Northeastern Tibet Plateau western Quanji Massif. These results reveal that the intrusive age of the protoliths of the widespread granitic gneisses in the Quanji Massif basement was restricted between 2.37 and 2.39 Ga, indicating regional granitic magmatism in the early Paleoproterozoic, perhaps related to the fragmentation stage of the Kenorland supercontinent. Geochemical results from the granodioritic gneiss from the Mohe area indicate that the protolith of this gneiss is characterized by adakitic rocks derived from partial melting of garnet-amphibolite beneath a thickened lower crust in a rifting regime after continent–continent collision and crustal thickening, genetically similar to the TTG gneisses in the North China Craton. This suggests that the Quanji Massif had a tectonic history similar to the Archean Central Orogenic Belt of North China Craton during the early Paleoproterozoic. We tentatively suggest that the Quanji Massif and the parental Tarim Craton and the North China Craton experienced rifting in the early Paleoproterozoic, after amalgamation at the end of the Archean. The Tarim Craton and North China Craton might have had close interaction from the late Neoarchean to the early Paleoproterozoic. © 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction 2010; Wang et al., 2010), indicating a globally low period of activity of magmatism. Condie and Aster (2010) use a plate subduction Global magmatic activity is characterized by episodic pulses of shutdown or slowdown theory (O'Neill et al., 2007; Silver and Behn, enhanced activity (Kemp et al., 2006; Campbell and Allen, 2008; 2008) to interpret this global trend, considering that it is related with Condie et al., 2009a). Statistical calculations from orogenic granite and the rifting events of the Kenorland supercontinent at the end of the detrital zircons from ancient and modern sediments reveals an early Neoarchean (Williams et al., 1991; Bleeker, 2003; Rogers and Santosh, Proterozoic global age trough between 2450 and 2200 Ma (Rino et al., 2004). 2004, 2008; Condie et al., 2005, 2009a, 2009b; Campbell and Allen, Circa ~2200 to ~2500 Ma granitic gneisses have been reported 2008; Belousova et al., 2010; Condie and Aster, 2010; Safonova et al., from the North China Craton (NCC) (Sun et al., 1991; He et al., 2005; Geng et al., 2006; Diwu et al., 2007; Zhao et al., 2008), Tarim Craton (TC) (Lu, 1992; Guo et al., 2003; Zhang et al., 2003b; Lu et al., 2006, ⁎ Corresponding author at: Faculty of Earth Science, China University of Geosciences, Wuhan 430074, China. 2008) and the Quanji Massif (QM) adjacent to the TC (Lu, 2002; Hao, E-mail address: [email protected] (N. Chen). 2005; Lu et al., 2006, 2008; Li et al., 2007; Wang et al., 2008). Debate

1342-937X/$ – see front matter © 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2011.07.011 S. Gong et al. / Gondwana Research 21 (2012) 152–166 153 has focused on the tectonic settings for the ~2200 to ~2500 Ma using the LA-ICPMS technique, to constrain the intrusive crystalliza- granitic gneisses in the NCC. One school considered that they were tion age of these granitic gneisses. We also re-examine the existing formed in an arc environment linked to subduction of the Western geochemical data from typical gneisses in both the QM and the NCC, in Block eastwards beneath the Eastern Block during about 2.5–1.85 Ga order to provide further insights to the tectonic setting of the protolith (Sun et al., 1991; Diwu et al., 2007; Dong et al., 2007; Zhao et al., of the granitic gneisses of the QM, TC and NCC. 2008), whereas the other school suggests that they formed in relation to rifting associated with the breakup of the NCC following 2. Geological background amalgamation of the Western Block with the Eastern Block in the late Archean (Kusky and Li, 2003;Geng et al., 2006). The ~2200 to The Quanji Massif (QM) is located in northwest China; it is ~2400 Ma granitic gneisses in the TC and the QM have not been well separated from the NCC, Yangtze Craton (YC) and TC by the Qilian– studied. They have been suggested to be products of the rifting North Qinling blocks to the northeast, the Qaidam block–South process following amalgamation of the TC with other unknown Qinling blocks to the southeast and the Altyn–Tagh fault to the continental blocks. Considering that the protoliths of these rocks and northwest, respectively (see inset in Fig. 1). The Qilian–North Qinling the closely associated metamorphosed contemporaneous mafic blocks and the Qaidam–Middle Qinling blocks evolved into Early enclaves and mafic dykes are bimodal magmas, the basic chemistry Paleozoic orogenic belts with relicts of metamorphosed Mesoproter- supports a continental rifting environment for their origin (Lu et al., ozoic strata intruded by early Neoproterozoic granites enclosed 2006, 2008). within the early Paleozoic orogen. The NCC, YC and TC are continental In addition to the problem of lacking detailed geochemical data, blocks that are composed of highly recrystallized basement and stable there has been another important problem that the ages of the sedimentary cover, whereas the QM is a newly discovered micro- protoliths of these granitic gneisses have not been well constrained. continental block (Lu, 2002). First, the reported ages were partly measured by the traditional TIMS The NCC is dominated by Archean rocks, including 3.8–2.5 Ga U–Pb method (Lu, 2002; Hao, 2005; Lu et al., 2006, 2008), which is gneiss, TTG, granite, migmatite, amphibolite, ultramafite, mica schist unsuitable for in-situ dating, although still offers high precision. and dolomitic marble, graphitic and sillimanite gneiss (khondalites), Therefore it is not suitable for zircon U–Pb dating on strongly banded iron formation (BIF), and meta-arkose (Jahn and Zhang, metamorphosed and deformed granites, i.e. the granitic gneisses. 1984a, 1984b; Zhai et al., 1985, 2005, 2010; Jahn et al., 1987; He et al., Second, most research focused on the eastern QM, and more work on 1991; Bai et al., 1992; Shen et al., 1992; Zhao et al., 1993; Bai et al., granitic gneisses in the central and western part of the complex needs 1996; Wang et al., 1997; Wu et al., 1998; Kusky et al., 2007a; Liu et al., to be carried out. Third, the age estimates have a 100–200 Ma 2011; Zhai and Santosh, 2011; Zhang et al., 2011b, 2011c). The final difference from different publications for granitic gneisses in different cratonization occurred at ~1.85 Ga resulting in the 1.85–1.60 Ga regions (Lu, 2002; Hao, 2005; Lu et al., 2006, 2008; Li et al., 2007). To Mesoproterozoic Changcheng (Great Wall) Series unconformably date, it is not clear whether this age difference is due to the overlying the Archean–early Paleoproterozoic recrystallized deficiencies of the dating method on these regionally distributed basement (Li et al., 2000a, 2000b; Kusky and Li, 2003). The NCC is granitic gneisses, or if it reflects the true age range of the precursor the largest craton in China and has one of the most complex evolution intrusive rocks. In this study, we report our new zircon U–Pb ages histories among the cratons in the world (Kusky and Li, 2003; Kusky

Fig. 1. Geological sketch map of the Quanji Massif and sample locality of granitic gneisses. YC — Yangtze Craton, WNCC — western block of North China Craton, TC — Tarim Craton. Modiﬁed from Xu et al. (2006) and Chen et al. (2009). 154

Table 1 Summary of the geological events occurred in the Quanji Massif and the Tarim Craton in the Proterozoic.

Period Quanji Massif Tarim Craton

Geological events Ages Dating methods (references) Geological events Ages Dating methods (references) (Ma) (Ma)

Neo-Pt. Deposits of the Nanhua system 740±38 SHRIMP zircon (1, 2) Kuluketage Group 755±15 SHRIMP zircon (17) Intrusion of the diabase dike swarm 821±11 SHRIMP zircon (2) Intrusion of A-type granite and K-feldspar 815±57 SHRIMP zircon (18) .Gn ta./Gnwn eerh2 21)152 (2012) 21 Research Gondwana / al. et Gong S. granite 844±15 SHRIMP zircon (5) Meso-Pt. Greenschist-facies metamorphism 1022±64 Rb–Sr isochron (3) Blueschist facies metamorphism in Aksu 962±12, 943±13 Rb–Sr isochron (19) Formation of the Wandonggou Group Bimodal volcanic rocks of Yixi Formation 1200±82 Sm–Nd isochron (20) Intrusion of the rapakivi granite 1776±33 ID-TIMS zircon (4) 1763±53 ID-TIMS zircon (5) Paleo-Pt. Lower amphibolite facies metamorphism 1791±37 Sm–Nd isochron (6) of the mafic dike swarm and retrograded 1834±23 LA-ICPMS zircon (7) overprint on the older rocks Intrusion of the mafic dike swarm 1852±15 SHRIMP zircon (2, 5) Intrusion of quartz–monzonite 1855±23 TIMS zircon (5) Metamorphism and anatexis of the early 1939±21 ID-TIMS zircon (2, 8) Metamorphism and anatexis of the upper 1916±4.4 SHRIMP zircon (21) Paleoproterozoic granitoid gneisses and 1946.8±7.8 LA-ICPMS zircon (9) Milan Subgroup and other early the mafic rock enclaves of the Delingha 1960±17 Paleoproterozoic rocks 1978±50 SHRIMP zircon (2) Complex and the Dakendaban Group 1924±15 LA-ICPMS zircon (10, 11) 1986±29 SHRIMP zircon (2) 1913±38 LA-ICPMS zircon (12) Formation of the Dakendaban Group 1960–2200 LA-ICPMS detrital zircon (13, 14) Formation of the upper Milan Subgroup 1.98–2.32 Ga LA-ICPMS detrital zircon e.g.(2) and Xinditage Group Intrusion of the protolith of the granitoid 2200–2370 ID-TIMS zircon (1, 2, 15) Intrusion of the Aketashitage mafic dike 2351±21,2374±10 TIMS zircon (22) – gneisses of the Delingha Complex swarm and trondhjemite 2396±36 SHRIMP zircon (2) 166 Formation of the Delingha Complex 2370–2390 LA-ICPMS zircon (16) Formation of the Tiekelike Heluositan complex 2426±23, 2358 ±10 SHRIMP zircon (21) 2400–2480 LA-ICPMS detrital zircon (14) including intrusion of the Akazi monzo-granite and Xugou syenogranite Neo-Ar. Formation of the lower Milan Subgroup and 2670±12, 2604 ±102 TIMS zircon (2) Aketashitage complex including intrusion of the TTG

1 Lu, 2002;2Lu et al., 2008;3Yu et al., 1994;4Xiao et al., 2004;5Lu et al., 2006;6Zhang et al., 2001; 7Liao et al., unpublished results; 8 Hao et al., 2004;9Chen et al., 2009;10Wang et al., 2008;11Wang 2009;12Zhang et al., 2011a;13Huang et al., 2011; 14 Chen et al., unpublished results; 15 Hao, 2005; 16 this study; 17 Xu et al., 2005;18Zhang et al., 2003c;19Gao et al., 1993;20Zhang et al., 2003a;21Zhang et al., 2003b;22Lu and Yuan (2003). S. Gong et al. / Gondwana Research 21 (2012) 152–166 155

Fig. 2. Photos of representative outcrops of granitic gneisses and sample collection sites (the left row), and photomicrographs showing changes of their minerals in response to metamorphism and deformation (a–b) the Hudesheng monzo-granitic gneiss showing deformed features with visible blastoporphyritic K-feldspar, the allanite (Aln) was replaced by rims of epidote, the biotite (Bi) and the plagioclase (Pl) was changed to chlorite (Chl) and sericite (Ser) respectively; (c–d) the Mohe granodioritic gneiss, which underwent strong deformation and shows clear gneissic structure, the hornblende was changed to actinolite, and the biotite to chlorite, and plagioclase was replaced by an albite+zoisite/epidote (Ab+Zoi/Ep) assemblage; (e–f) the Delingha monzo-granitic gneiss, exhibiting intensive gneissic structure and is intruded by massive or poorly oriented alkali-feldspar granite veins, the biotite was changed to chlorite, the plagioclase to sericite then resorption of albite (Ab) developed on the margins; (g–h) the Quanjishan monzo-granitic gneiss, which develops garnet with diablastic texture at the contact with the wall rock of the Dakendaban Group, the garnet porphyroblasts were broken down to form intergrowths of biotite and plagioclase. Note that a thin rim developed on the edges of the quartz (Qtz) inclusions. 156 S. Gong et al. / Gondwana Research 21 (2012) 152–166 et al., 2007a, 2007b; Zhai et al., 2007). There are controversies about in the Middle- to Late-Paleoproterozoic (2.24–1.95 Ga, Huang et al., the tectonic framework and tectonic evolution of the North China 2011; Chen et al., unpublished results); and the Wandonggou group is Craton during the Neoarchean to Paleoproterozoic. Three leading a set of low grade metamorphosed volcanic–sedimentary rocks. groups of view have been proposed to interpret the complex regional Characteristics of the basement rock association and the cover strata structure, geochronology, and geological relationships. One view as well as the magmatic–metamorphic history of the QM is similar to suggests that the Eastern and Western blocks of the NCC formed that of the TC (Table 1), suggesting that the QM might be a continental separately in the Archean, and an active Andean-style margin was fragment detached from the TC. developed on the eastern block between 2.5 and 1.85 Ga, when the The basic geology of the cratons and micro-continental blocks in two blocks collided after about ~700 My of east dipping subduction China suggests that the YC and TC shared a similar history in the (Zhao et al., 2001, 2005 and references therein). The second view middle Neoproterozoic and in the Mesoproterozoic, but rather presumes the Eastern block rifted from an adjacent continental different from the NCC during the same period. However, the relation fragment at ~2.7 Ga, and experienced a collision with an arc (perhaps of the NCC, YC and TC in the Neoarchean and the early Paleoproter- attached to the Western block) above a west-dipping subduction at ozoic is poorly understood. Unlike the YC, the TC and the NCC about 2.5 Ga, and the ~1.93–1.85 Ga metamorphism is related to a developed ca. 2.4–2.2 Ga granitic gneisses. These granitic gneisses collision along the northern margin of the craton when the North might provide information about the relation between the TC and the China Craton joined the Columbia supercontinent (Kusky and Li, NCC, and can shed light on their tectonic settings in this period. 2003; Kusky et al., 2007a, 2007b; Kusky and Santosh, 2009; Kusky, 2011). Zhai and other researchers suggest a third tectonic evolution 3. Description of the typical granitic gneisses in the QM and model for the NCC (see Zhai and Santosh, 2011 and references comment on the available geochronological data therein). They consider that the crustal growth and stabilization of the NCC is related to three major geological events in the Precambrian: (1) a The metamorphosed and deformed granites in the QM strictly major phase of continental growth at ca. 2.7 Ga; (2) the amalgamation occur within the Delingha complex and their precursors have been of micro-blocks and cratonization at ca. 2.5 Ga; and (3) Paleoproterozoic dated at 2.2 Ga–2.4 Ga (Lu, 2002; Hao, 2005; Lu et al., 2006, 2008; Li rifting–subduction–accretion–collision tectonics and subsequent high- et al., 2007; this study). The typical exposed granitic gneisses in the grade granulite facies metamorphism–granitic magmatism during ca. eastern, central and the central west parts of the Quanji Massif include 2.35–1.82 Ga. the Hudesheng monzo-granitic gneiss, Mohe granodioritic gneiss, The YC has exposed basement that includes Proterozoic rocks with Delingha and Quanjishan monzo-granitic gneisses (Fig. 1). Precursors a few sparse outcrops of Archean rocks in the Kongling Complex and of these granitic gneisses experienced at least three metamorphic in the North Dabie Complex. The Archean TTG gneisses and the events since their emplacement (e.g. Table 1). The first metamorphic Paleoproterozoic khondalites of the Kongling Group underwent high- event occurred at around 1.91–1.96 Ga in response to the global grade metamorphism at ~1.95 Ga (Qiu et al., 2000; Ling et al., 2001; Columbia supercontinent amalgamation (Rogers and Santosh, 2004), Chen et al., 2005, 2006b; Zhang et al., 2006; Gong et al., 2007; Liu et al., resulting in regional middle- to upper-amphibolite facies metamor- 2008a; Sun et al., 2008; Wu et al., 2008). Mesoproterozoic carbonate phism and locally lower-granulite facies metamorphism, together strata are developed dominantly in the northwest part of the Kongling with regional migmatization due to anatexis of the granitic gneiss area and clastic and volcanic rocks dominantly in the western margin precursors and the Dakendaban Group (Lu, 2002; Hao, 2005; Wang of the YC after a ~2.0 Ga sedimentary gap (e.g. Lu et al., 2006). Final et al., 2008; Wang, 2009; Chen et al., 2009; e.g., Kusky and Santosh, cratonization of the YC occurred in the Middle-Neoproterozoic with 2009; Zhang et al., 2011a). The second metamorphic event took place deposits of Nanhua–Sinian strata cover unconformably on the by the end of the Paleoproterozoic following intrusion of the mafic basement. dyke swarm (Lu et al., 2006, 2008; Liao et al., unpublished results), The TC has a history that is similar to the YC since the which led to lower amphibolite-facies metamorphism of the mafic Neoproterozoic but different in the Paleoproterozoic period. It dyke swarm and other older rocks at ~1.83 Ga (Liao et al., unpublished completed its final cratonization in the Middle-Neoproterozoic and results). The third metamorphic event occurred during the turn of developed a cover of the Nanhua–Sinian Series which are similar to Meso-Neoproterozoic (e.g. Yu et al., 1994), the granites and all the those in the YC (Lu, 1992; Gao et al., 1993). The basement is exposed Dakendaban Group and Wandonggou Group as well as other older as areas of metamorphic rocks sporadically along the craton margins. rocks responded to the Rodinia supercontinent amalgamation, and It contains Neoarchean paragneisses and TTG gneisses with granulite experienced greenschist-facies metamorphism and strong deforma- enclaves and intrusions of 2.4–2.3 Ga K-feldspar granites and mafic tion. These metamorphic and accompanying deformational events dykes that are interpreted as bimodal intrusive rocks that indicate strongly reworked the granites in the Delingha complex and magmatism in a rift-related setting (e.g. Lu et al., 2006, 2008). Strata transformed them into granitic gneisses. with features of the khondalite suit formed in the Late Paleoproter- ozoic. These strata and the rest of the other older rocks underwent 3.1. Hudesheng biotite monzo-granitic gneiss amphibolite facies metamorphism and anatexis at ~1.95 Ga. After a ~0.2 Ga sedimentary gap a suit of Mesoproterozoic carbonate to The Hudesheng gneiss is located on the northern Hudesheng Hill, clastic strata was deposited. eastern Wulan County (Fig. 1). It is distributed along a NW–SE The QM micro-continent is bounded by the South Qilian orogenic direction; lenticular amphibolite enclaves and structurally dismem- belt in the north and is adjacent to the Qaidam block with the North bered meta-mafic dikes are observable on the outcrop. The monzo- Qaidam early Paleozoic subduction zone in the south (Fig. 1). Final granitic gneiss displays a pink color and gneissic–augen structure cratonization of the QM occurred in the Middle-Neoproterozoic with (Fig. 2a) which is overprinted by a crenulation cleavage. Brick-red deposits of Nanhua–Sinian strata cover unconformably on the color is also observed along the fractures where late stage K- basement. The Precambrian metamorphic basement is composed of feldspathization occurred. Generally the gneiss presents medium- three units, including the Delingha complex, the Dakendaban group grained blastogranitic texture, and locally preserves blastoporphyritic and the Wandonggou group (Lu, 2002; Lu et al., 2003, 2006). The texture. The major minerals are K-feldspar, plagioclase and quartz, Delingha complex is composed of metamorphosed-deformed granites with some of the K-feldspars as blastophenocrysts. The minor with enclaves of amphibolite (Lu, 2002; Lu et al., 2006, 2008); the minerals are biotite and allanite. The plagioclase, biotite and allanite Dakendaban group is a set of khondalites, characteristic of metamor- are generally retrogressed to sericite, chlorite and epidote, respec- phosed pelitic crustal rocks (Wang, 2009; Wang et al., 2009), formed tively (Fig. 2b). S. Gong et al. / Gondwana Research 21 (2012) 152–166 157

Hao (2005) reported a TIMS zircon U–Pb age of 2202±26 Ma, 3.4. Quanjishan monzo-granitic gneiss which is an upper intercept age composed of 5 data points along discordia. However, this age is not well constrained because the gneiss This gneiss was recognized by the authors from the Dakendaban underwent strong and complex deformation and amphibolite facies Group in Quanjishan area (Fig. 1) according to preliminary field metamorphism. Furthermore, the result of Hao (2005) is about observations. The Quanjishan–Dakendaban Group is divided into 150 Ma younger than ages obtained from other granitic gneisses, and upper and lower sections: the lower section is a set of paragneisses it is unclear if this difference is attributed to the true ages of the rocks containing metamorphic minerals including garnet, cordierite, mica, or to a zircon dating problem. A CL image-controlled in situ feldspar and quartz; the upper section is mostly migmatite, geochronological analysis on zircon is needed for unraveling the amphibolite and felsic gneissic rocks (Qinghai Bureau of Geology, intrusion age for the protolith of this gneiss. 1980). After our new field investigation, we determined that the felsic migmatites are mostly derived from a metamorphosed and deformed monzo-granitic pluton, which we call the Quanji monzo-granitic 3.2. Mohe amphibole monzo-granodioritic gneiss gneiss in this study. However, the actual distribution and boundaries of the rocks need be determined after more geological investigation. This gneiss is located to the southwest of the Hudesheng monzo- The Quanjishan monzo-granitic gneiss has a light pink color, and also granitic gneiss, eastern Quanji Massif. It is a 16 km2 scale apophysis, shows brick-red color due to late stage K-feldspathization. Generally striking NW–SE in south Hudesheng Hill, eastern Wulan County the gneiss presents gneissic–augen structure, medium-grained blas- (Fig. 1). Lenticular amphibolite enclaves and structurally dismem- togranitic or blastoporphyritic textures. The rock-forming minerals bered meta-mafic dikes are also observable on the outcrop of this are biotite, K-feldspar, plagioclase and quartz. The contact area with orthogneiss. This gneiss has a gray color, banded-gneissic structure the paragneiss has developed diablastic garnets that are transformed (Fig. 2c), and a medium-grained blastogranitic texture. The major to plagioclase and biotite (Fig. 2g and h), suggesting strong minerals are plagioclase, K-feldspar, quartz and hornblende, with metamorphism and retrograde modification. So far, there is no accessory minerals including zircon and apatite. The plagioclase is intrusive age reported for the Quanjishan monzo-granitic gneiss. altered to zoisite+albite, and the hornblende is altered to blue-green colored actinolitic hornblende and actinolite (Fig. 2d). 4. Sample collection, treatment and analysis method Hao (2005) determined a zircon U–Pb age of 2348±43 Ma with the ID-TIMS technique; however Li et al. (2007) obtained a zircon U– Rock samples were collected from the Hudesheng gneiss (sample Pb age of 2470±19 Ma with LA-ICPMS. These different data show a 03WL-41, N 36°42′23″, E 98°52′0.5″), the Mohe gneiss (sample large difference of ca. 120 Ma. To better constrain the formation age of GSL10NR-7; N 36°51′34.75″, E 98°38′17.13″), the Delingha gneiss this gneiss precursor, we re-investigated the occurrence and distri- (sample DLH07-5, N 37°23′23.2″, E 97°21′55.0″) and the Quanjishan bution of this gneiss and collected new samples. We found some felsic gneiss (04QJH-13, N 37°22′5.1″, E 95°55′21.6″) (e.g. Fig. 1). enclaves with fuzzy boundaries with the country rock due to intensive Zircon grains were separated from approximately 2 kg rock for metamorphism and deformation. Recollection of samples was needed each sample by use of standard techniques. Zircon crystals with good to assess the existing geochronological data (Hao, 2005; Li et al., 2007) crystallization and transparency were selected using a binocular and for better constraints on the age of the magmatic protolith of the microscope, and then were mounted in epoxy resin and polished Mohe gneiss. down to expose the grain centers. Polished zircon grains were first examined by cathodoluminescence (CL) imaging on a JEOL JXA-8100 electron microprobe and a LEO1450VP scanning electronic micro- 3.3. Delingha biotite monzo-granitic gneiss scope at the Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing. Zircon U–Pb dating was carried out by means of This gneiss is located north of Delingha city, elongated to the east LA-ICPMS at the State Key Laboratory of Geological Processes and into Herbute Hill. Its precise contacts have not yet been clearly Mineral Resources, China University of Geosciences in Wuhan. A defined. After the field investigation along Herbute Hill, we found GeoLas 2005 M laser-ablation system equipped with a 193 nm ArF- that the homogeneous migmatite and augen-striped migmatite excimer laser and an imaging optical system was used in conjunction defined in the original 1:200,000 scale Delingha Geological with an Agilent 7500 ICPMS. Helium was used as a carrier gas to Map (Qinghai Bureau of Geology, 1978) all belong to the constituents enhance the transport efficiency of ablated material. The spot of the Delingha monzo-granitic gneiss. This gneiss has experienced at diameter was 24 μm with a laser pulse rate of 8 Hz at 75 mJ/cm2 least two stages of deformation and preserves two generations of energy density. The analytical spots were selected on domains in foliations, and late alkali-feldspar granite intrusive veins cross-cut zircons that show clear oscillatory zoning or where they have not the previous gneissosity (Fig. 2e). Mafic dikes and lenticular to been affected by later deformation and metamorphic recrystallization irregular mafic enclaves are also present within the gneiss. The or fluid flow. Raw data were processed by using the in-house software monzo-granitic gneiss has a light pink color, and mostly shows brick- ICPMSDataCal (Liu et al., 2008b, 2010b, 2010c). During the analysis, red color due to late stage K-feldspathization. Generally the zircon 91500 was used as external standard for U–Pb dating, and was gneiss preserves a medium-grained blastogranitic texture. analyzed twice every five analyses. Time-dependent drifts of U–Th–Pb The rock-forming minerals are K-feldspar, plagioclase, quartz and isotopic ratios were corrected using a linear interpolation (with time) biotite. The biotite mostly changed to chlorite, and the plagioclase for every five analyses according to the variations of 91500. The changed to sericite or was metasomatized by microcline or albite standard deviation on the average of the isotopic ratios for Zircon (Fig. 2f). Lu (2002) reported the zircon age of monzo-granitic gneiss standards GJ-1 (Jackson et al., 2004) were analyzed as unknowns. and amphibolite enclaves within the Delingha complex as Because the common Pb is low in these standards, no common Pb 2366 ±10 Ma and 2412±14 Ma respectively. The results are correction was made. The obtained weighted mean 206Pb/238U ages reported with high precision, however, they were not well for the zircon GJ-1 is 602.0±3.0 Ma (2σ, n=10), consistent with the constrained because each result was the upper intercept age reported or recommended value (GJ-1: 206Pb/238U age 599.8±1.7 Ma composed of only 4 data points from highly Pb-loss zircons along (2 s), Jackson et al., 2004) within analytical error, indicating our data discordia and the analyses were undertaken without CL-imaging are reliable. For more detailed equipment conditions and data control. Recollection of samples was needed for better constraints on calculation please refer to Liu et al. (2008b, 2010b, 2010c). The U– the age of the magmatic protolith of the Delingha gneiss. Pb concordant age diagram and average weight calculation of the age 158 S. Gong et al. / Gondwana Research 21 (2012) 152–166 were accomplished by Isoplot/Ex_ver3 (Ludwig, 2003). The U, Pb well developed magmatic oscillatory growth zoning. The zircon rims isotopic analytical results of four gneisses samples are shown in Tables are recrystallized and dark gray in color, with large variations 2–5 (supplementary items). between different crystals. They are the product of metamorphism, deformation or fluid modification (Fig. 3, grains (e) to (i)). The Th/U – 5. Results ratio of the 19 zircon data points is 0.44 0.71 (Table 3), which is characteristic of a magmatic origin. On the concordia–discordia σ 5.1. Hudesheng biotite monzo-granitic gneiss (sample 03WL-41) diagram, the upper intercept age is 2392±25 Ma (2 )with MSWD=0.5, determined by 19 data points that have different Zircon grains from this granitic gneiss are elongated, prismatic and degrees of Pb loss (Fig. 5). Lu (2002) and Hao (2005) applied the – euhedral in shape. Their CL images show very intensive oscillatory ID-TIMS technique and obtained a zircon U Pb age of 2348±43 Ma, zoning, and different degrees of fuzzification from the rim to the center, an upper intercept age with a discordia line also composed of only 3 indicating typical recrystallized zircons of magmatic origin (Fig. 3,grains discordant data points. Li et al. (2007) applied LA-ICPMS and obtained – (a) to (d)). The 20 measurement points of zircons show Th/U ratios the zircon U Pb age of 2470±19 Ma. In our study, the results support between 0.12 and 0.80 (Table 2). All these indicate the magmatic origin the result of Lu (2002) and Hao (2005). Therefore, we determined the σ and recrystallized character of these zircons. On the concordia–discordia magma intrusion and crystallization age as 2392±25 Ma (2 ) for the diagram, a few of the measurements fall on the concordia curve, Mohe amphibole monzo-granodioritic gneiss. The age that Li et al. (2007) obtained is consistent with that of the inherited magmatic showing a near concordant age of ~2.20 Ga, while most plot below the concordia. An upper intercept age of 2334+68/−62 Ma (2σ)isyielded detrital zircons from the adjacent paragneiss (Chen et al., unpublished results). Maybe the sample for Li et al. (2007) belongs to the early with large uncertainty (MSWD=7.8) for a total of 20 data (Fig. 4). When some scattered data points are not considered, an upper intercept enclaves that experienced metamorphism and deformation. age of 2381±41 Ma (2σ) with MSWD=1.2 is yielded for the remaining – 15 data (Fig. 4). This result is about 180 Ma older than the U Pb zircon 5.3. Delingha biotite monzo-granitic gneiss (sample DLH07-5) age of 2202±26 Ma reported by Hao (2005) using the ID-TIMS method, where the upper intercept age is only determined by 5 discordant data Zircons from this granitic gneiss are mostly prismatic and euhedral points. As described before, the Hudesheng biotite-bearing monzo- in shape. The CL images show core–rim textures. The cores occupy the granitic gneiss had already experienced intensive metamorphism and largest part of the zircons and exhibit very complete magmatic strong deformation, leading to different degrees of recrystallization of oscillatory growth zoning. The zircon rims are recrystallized and have the zircon (Fig. 3, grains (a) to (d)) and Pb loss (Table 2 and Fig. 4). a dark gray color, produced by metamorphism, deformation, or fluid – Therefore, the traditional ID-TIMS U Pb dating method is not suitable modification (Fig. 3, grains (j) to (n)). The Th/U ratio of the 30 zircon for dating this kind of zircon, due to no in-situ dating function. What's data points from the core is 0.22–0.81 (Table 4), which is more, because of multi-stage metamorphism and deformation and characteristic of a magmatic origin. Among these 30 data points, complex Pb loss, too few data points can also make the upper intercept 207 206 apart from one of them which has a Pb/ Pb age of 2647±10 Ma, age along the discordia line not representative (Fig. 4). We suggest that all the other 29 data points present different degrees of Pb loss. On the our new age is better constrained than the previous age estimate of Hao concordia–discordia diagram, a small part of these 29 data points fall σ (2005). Therefore, the upper intercept age of 2381±41 Ma (2 )is on the concordia, and most others are below the concordia and interpreted to represent the crystallization age of the Hudesheng biotite constitute a good linear discordant line, with an upper intercept age of monzo-granitic gneiss protolith magma intrusion. 2367±12 Ma (MSWD=1.4) (Fig. 6). This age is consistent with the ID-TIMS zircon U–Pb age of 2366±10 Ma reported by Lu (2002) and 5.2. Mohe amphibole monzo-granodioritic gneiss (sample GSL10NR-7) Lu et al. (2008). Since these zircon grains have characteristic magmatic origin, this age should represent the crystallization age of Zircons from this gneiss are mostly prismatic and euhedral in the zircon, and the magmatic intrusive age of the Delingha biotite shape. The CL images show core–rim textures. The zircon cores have monzo-granitic gneiss.

Fig. 3. Representative selection of cathodoluminescence (CL) zircon images from the granitic gneisses of the Quanji Massif. Zircon grains of the (a) to (d) are from the Hudesheng monzo-granitic gneiss (sample 03WL-41), the (e) to (i) are from the Mohe granodioritic gneiss (sample GSL10NR-7), the (j) to (n) are from the Delingha monzo-granitic gneiss (sample DLH07-5), and the (o) to (r) are from the Quanjishan monzo-granitic gneiss (sample 04QJH-13). Descriptions of zircons are included in the text. All scale bars are 100 μm. S. Gong et al. / Gondwana Research 21 (2012) 152–166 159

Fig. 6. Concordia plots for zircons from the Delingha monzo-granitic gneiss. Fig. 4. Concordia plots for zircons from the Hudesheng monzo-granitic gneiss.

6. Geochemistry of the Mohe Gneiss — a revisited study

Chen et al. (2007) previously reported four geochemical data for 5.4. Quanjishan monzo-granitic gness (sample 04QJH-13) rocks of the Mohe gneiss which suggested that the Mohe gneiss is a characteristic I-type granite protolith. Re-investigation of this study Zircons from this granitic gneiss are mostly prismatic and euhedral shows that the rocks of the Mohe gneiss are geochemically in shape, with a length–width ratio of 2:1–3:1. The rim becomes characterized by adakitic rocks (Defant and Drummond, 1990, darker due to different degrees of recrystallization. The CL images 1993) (e.g Chen et al., 2007 and Table 6). show oscillatory growth zoning, or wide-banded texture (Fig. 3, grains (o) to (r)). Twenty four data points were obtained, of which 20 points were measured from the cores and 4 points were from the rims. 6.1. Major and trace elements The core data points were selected from locations with clear oscillatory growth zoning. Table 5 shows the Th/U ratio of the zircons The rocks of the Mohe gneiss are trondhjemitic to granodioritic falling between 0.14 and 0.82, together with oscillatory zoning, with SiO2 contents between 63.23 wt.% and 67.90 wt.%. The high – – showing a magmatic origin. All 24 data show different levels of Pb loss contents of Al2O3 (14.51 15.62 wt.%), Na2O (3.06 3.76 wt.%) and – (Table 5). On the concordia–discordia diagram, apart from data point higher Na2O/K2O (0.9 1.8) are very similar to those of adakites. The 18, the remaining 23 data points show a linear distribution. The upper MgO contents (1.02–2.53 wt.%) are relatively low, but fall in the lower intercept age is 2354+60/−74 Ma (2σ) with a large uncertainty range of the adakite ﬁeld at a given silica content. The rocks have total – (MSWD=15) on the simulated discordant line (Fig. 7). When some alkali contents with K2O+Na2O 5.87 7.51 wt.%, and are meta- scattered data were not considered, a new intercept age 2372±22 Ma aluminous with A/CNK=0.79–0.92, displaying high-K calc-alkaline – (2σ) with small uncertainty (MSWD=1.7) was obtained (Fig. 7). This series on K2O SiO2 plot (Fig. 8). age is considered as the magmatic intrusive age of the Quanjishan The rocks of the Mohe gneiss are generally enriched in highly biotite granitic gneiss. incompatible trace elements such as the large-ion lithophile elements (LILE) and the light-rare earth elements (LREE) and depleted in high ﬁeld strength elements (HFSE) and heavy rare earth elements (HREE)

Fig. 5. Concordia plot for zircons from the Mohe granodioritic gneiss. Fig. 7. Concordia plots for zircons from the Quanjishan monzo-granitic gneiss. 160 S. Gong et al. / Gondwana Research 21 (2012) 152–166

Table 6 Some geochemical data of the Mohe gneiss and TTG gneiss in NCC.

Adakite Mohe gneiss TTG gneisses from NCC

(Defant and Drummond, (Chen et al., (Diwu et al., 2007) 1990) 2007)

SiO2 N56 wt.% 63.23–67.90 wt.% 59.00–73.16 wt.%

Al2O3 N15 wt.% 14.51–15.62 wt.% 14.99–17.53 wt.%

Na2O N3.5 wt.% 3.06–3.76 wt.% 3.93–5.32 wt.% MgO b3 wt.% 1.02–2.53 wt.% 0.60–3.43 wt.% (most 0.60–2.14) Mg# 50–56 39.8–41.9 46.8–50.1

Na2O/K2O N1 0.9–1.8 1.04–2.16 Cr 15.6–28.9 ppm 11.7–19.5 ppm Ni 12.2–18.3 ppm 13.2–18.2 ppm Sc 9.44–16.2 ppm 1.96–16.7 ppm Y b18 ppm 9.00–15.7 ppm 13.3–15.3 ppm Fig. 9. Primitive mantle-normalized trace elemental spider diagram for rocks of the Yb b1.9 ppm 0.97–1.59 ppm 1.27–1.98 ppm Mohe gneiss in the Quanji Massif. Sr N400 ppm 518–619 ppm 507–607 ppm Primitive mantle normalizing factors are from Sun and McDonough (1989). Eu/Eu* N1 0.66–1.06 0.95–1.07 Sr/Y N40 37.5–55.3 34.2–40.0 La/Yb N20 19.4–39.8 13.3–34.5

(La/Yb)N 13.9–28.6 9.00–23.3 Y/Yb 9.7–10.7 7.6–11.9 6.2. Nd isotopes

(Ho/Yb)N 1.07–1.36 0.96–1.01 143 144 – ( Nd/ Nd)t 0.509660 The rocks of the Mohe gneiss show small variation of Nd with 0.509761 (143Nd/144Nd) =0.509660–0.509761, using the t=2392 Ma, sug- εNd(2392 Ma) 1.29–3.27 εHf(t)=−4.95– t 3.68 gesting that they could be derived from the same source. The positive – – ε – – TDM2 2.43 2.58 Ga Hf TDM2 =2.57 Nd (t) values (1.29 3.27) and TDM2 (2.43 2.58 Ga) of the rocks 3.01 Ga suggest that parts of their components are products derived from partial melting of juvenile crust.

7. Discussion (Fig. 9), similar to adakite and most calc-alkaline arc magmas. Compatible elements such Cr, Ni and Sc concentrations (15.6– 7.1. Early Paleoproterozoic magmatism in Quanji Massif 28.9 ppm, and 12.2–18.3 ppm and 9.44–16.2 ppm, respectively) are low in these rocks. Sr is strongly enriched (518.6–619.5 ppm), Y is In this study, four typical granitic gneiss samples were selected for strongly depleted (9.37–16.5 ppm) and the Sr/Y values range from dating from the eastern, central and western parts of the Quanji Massif. 37.5 to 55.3, and fall in the adakite and classical TTG ﬁelds (Fig. 10). CL images and in-situ analysis technology ensure the reliability and The rare earth elements are characterized by moderately negative to clarity of the age data. LA-ICPMS zircon dating yields a crystallization age slightly positive Eu anomalies (Eu/Eu*=0.66–1.06), strong depletion of 2381±41 Ma (2σ) and 2392±25 Ma (2σ) respectively from monzo- of heavy REE (Yb=0.97–1.59 ppm) and high La/Yb and (La/Yb)N granitic gneiss in Hudesheng and granodioritic gneiss in Mohe, eastern values (19.4–39.8 and 13.9–28.6, respectively), however, the distribution of the heavy REEs shows nearly ﬂat patterns.

Fig. 10. Sr/Y vs. Y diagram for rocks of the Mohe gneiss in the Quanji Massif. The ﬁelds of adakites and trondjhemite–tonalite–granodiorite (TTG) and classical island arc andesite–dacite–rhyolite (ADR) are after Atherton and Petford (1993). Partial

Fig. 8. K2O vs. SiO2 (wt.%) diagram for rocks of the Mohe gneiss in the Quanji Massif. melting curves for basalt, leaving a residue of 10% garnet amphibolite, are after Field boundaries are after Peccerillo and Taylor (1976). Drummond and Defant (1990). S. Gong et al. / Gondwana Research 21 (2012) 152–166 161

Quanji Massif, and 2367±12 Ma (2σ) from monzo-granitic gneiss in gneiss are derived from depleted mantle. The TDM2 of the rocks of the Delingha area. The zircon age of the Quanjishan monzo-granitic gneiss Mohe gneiss is in range of 2.43–2.58 Ga, indicating a short time span in the western Quanji massif is 2372±22 Ma (2σ). These four age data of about 0.04–0.19 Ga between inputting of the depleted mantle indicate the intrusive crystallization age of the protoliths of these materials and re-melting and emplacement of the magma of the Mohe gneissic rocks. This indicates that regional granite magmatism in the gneiss (2.39 Ga), thus implying that parts of these magma were basement of the Quanji Massif occurred in the early Paleoproterozoic derived from partial melting of the juvenile crust. Plots of data of the

(generally between 2.37 Ga and 2.39 Ga; in the following text this three samples on the (La/Yb)N vs. YbN diagram fall closely on the magmatic event will be briefly referred to as the 2.38-Ga intrusive partial melting residue trends of 10% garnet amphibolite, and the rest event). Recent study of a potassium feldspar leptynite from the are between the partial melting residue trends of 10% garnet Dakendaban Group revealed that the detrital zircons in the rock are of amphibolite and eclogite (Fig. 10), suggesting that the juvenile crust magmatic origin, with regard to their CL images and the Th/U had converted to garnet amphibolite along with or before the partial values, suggesting derivation through short distance transportation melting occurred. Garnet and amphibole minerals occurring as from a provenance characterized by granite plutons or granitic gneisses. residues in the melts implies that the partial melting occurred at The 207Pb/206Pb age range is in 2094–2280 Ma,the 207Pb/206Pb age pressure conditions up to ~1.2 GPa or even higher at 850–950 °C (e.g. relative probability density plot for those measurements (~90% of the Drummond et al., 1996), at a crustal depth of ca. 40 km or more, total grains) with concordant degree over 90% exhibits a monomodal beneath a thickened lower crust. curve with a peak age of ~2190 Ma (Huang et al., 2011). Although the Such high-temperature partial melting should require extra heat age can also be explained by those ages obtained from recrystallized in addition to the normal heat flow in the crust. This extra heat could zircons from the Hudesheng gneiss, such a high percentage of highly be supplied by mantle upwelling and underplating in an extensional concordant zircon grains preserved in a paragneiss indicates that there tectonic regime after collision and crustal thickening. Incessant should exist granite plutons or granitic gneisses with ~2.2 Ga in the underplating of basaltic magma generated from decompressional basement in the Quanji Massif. partial melting of the upwelling asthenospheric mantle at ca. 2.43– 2.58 Ga, caused a gradual thickening of the crust and significant 7.2. Tectonic settings for the 2.38-Ga granitic gneisses in the Quanji increase of heat flow, resulting in high temperature and high pressure Massif metamorphism of the former underplated basaltic materials and finally leading to remelting of the garnet amphibolite at the base of Petrogenesis of the rocks of the Mohe gneiss can provide useful thickened crust to generate the adakitic melts, which ascended and constraints for revealing the tectonic settings of the 2.38-Ga granitic intruded in the lower and middle crust as the precursor of the Mohe gneisses in the QM. Moyen (2009) proposed four ways to form high gneiss in the QM. Therefore, the geochemistry of the one of the

Sr/Y and (La/Yb)N and low Y and Yb magmatic rocks: melting of precursors of the 2.38-Ga granitic gneisses in the QM reveals they ascending melts assimilated by high Sr/Y and (La/Yb)N sources, partial were formed in an extensional tectonic regime after continent– melting at depth where garnet exists as a residual phase, differenti- continent collision and crustal thickening. ation of maﬁc minerals of maﬁc melts, and moderate-acid melts reacted with mantle rocks. 7.3. Comparison to the TTG gneisses in the North China Craton

The relatively high SiO2 (from 63.23 wt.% to 67.90 wt.%) and low MgO, Cr, Ni and Sc concentrations and Mg# of the rocks of the Mohe TTG gneisses in the NCC and their geological data were reported by gneiss do not support their direct derivation from the mantle. It also Diwu et al. (2007). The gneisses are exposed in the Yiyang area of the precludes that the magma of these rocks was generated by Taihua region, south margin of the NCC within the southern segment fractionation crystallization of a basalt parent, because 90% crystal- of the Trans-North China Orogen (Zhao et al., 2001) or the Archean lization of a basalt parent would be required to produce trondhjemitic Central Orogenic belt (Kusky and Li, 2003). LA-ICPMS U–Pb dating on residual liquids (Spulber and Rutherford, 1983). If it is the case for the zircons yields ages in range from 2316±16 Ma to 2336±13 Ma for Mohe gneiss, there should be a huge volume of gabbro to diorite the TTG gneisses. Diwu et al. (2007) considered that magma of the gneisses formed in the region. However, only a small volume of precursors of these TTG gneisses formed in an arc setting. However, metamorphosed maﬁc rocks sporadically occurs as lenses in the lower we suggest a rifting setting alternatively for these rocks because they basement of the QM. show major and trace element geochemistry similar to the adakitic It has come to a consensus that the typical adakite is derived from rocks of the Mohe gneiss in the QM (e.g. Table 6). partial melting of garnet amphibolite or hydrous eclogite that The rocks of the Yiyang TTG gneisses have high SiO2 contents transformed from a subducted oceanic slab at depth of 75–85 km ranging from 59.00 wt.% to 73.16 wt.%, and high contents of Al2O3 with garnet and/or amphibole as residues, which produce melts of (14.99–17.53 wt.%) and Na2O (3.93–5.32 wt.%) and high values of high Sr and low Y contents because Sr is strongly enriched in the Na2O/K2O (1.04–2.16) but low MgO (most in the range of 0.60– molten plagioclase and the Y and HREE is strongly fractionated into 2.14 wt.% except for one having 3.43 wt.%), similar to the major the residue of garnet phases (Defant and Drummond, 1990, 1993). elemental geochemistry of adakite (Defant and Drummond, 1990). In The rocks of the Mohe gneiss have major and trace elements trace elements, the rocks of the Yiyang TTG gneisses are strongly geochemically similar to adakite, however, low MgO, Cr, Ni and Sc enriched in Sr (507–607 ppm) but strongly depleted in HREE contents and Mg# indicate that the magma could not be derived from (Y=13.3–15.3 ppm, Yb=1.27–1.98 ppm), thus have high values of the subducted oceanic slab, because in this case the ascending magma Sr/Y and La/Yb (34.2–40.0 and 13.3–34.5, respectively), all indicating should have reacted with the mantle wedge, resulting in high MgO, Cr, signatures of adakitic rock. The rocks have low concentrations of Cr Ni and Sc concentrations and Mg#. In other words, the magma of the (11.7–19.5 ppm), Ni (13.2–18.2 ppm) and Sc (1.96–16.7 ppm), rocks of the Mohe gneiss could not have been generated in a implying that they did not obviously interact with mantle materials subduction environment and thus are not related to an arc setting. and form in an arc environment. The arc dacite is characterized by

The rocks of the Mohe gneiss are depleted in HREE with (La/Yb)N high Y (average of 47 ppm) and Yb (average of 4.4 ppm) but low Sr/Y values of 13.9–28.6 and Y/Yb =9.7–10.7, indicating garnet was (average of 8.1 ppm) (e.g. Martin, 1999). The low concentrations of Y present as a residue in the source, the (Ho/Yb)N =1.07–1.36, and Yb and the Sr/Y value for the rocks do not support the suggesting a small volume of amphibole may also have been present interpretation that they crystallized from magma of arc-ADRs. as a residue in the source during the partial melting. Positive εNd(t) of We should keep in mind that magma for TTG is derived from the rocks indicates that the source rocks for magma of the Mohe partial melting of maﬁc rocks and not from totally melting the lower 162 S. Gong et al. / Gondwana Research 21 (2012) 152–166

crust. A wide range of TDM2 (2.57–3.01 Ga) of the zircons from the developed contemporaneous TTG gneisses (Diwu et al., 2007), their rocks of the Yiyang TTG gneisses suggests that magma of their source granite protolith also formed after continent–continent collision, rocks had a long crustal residence time before the partial melting indicating an extensional tectonic regime, supporting the evolution occurred. Large part of the zircon εHf(t) data have negative values, modelfortheArcheanCentralOrogenicBeltintheNCC(Kusky and Li, indicating that the magma were mainly derived from partial melting 2003; Kusky et al., 2007a, 2007b) and evolution model for the north of these ancient maﬁc rocks. Strong depletion in HREE with (La/Yb)N central zone and Jiao–Liao–Ji belt of the NCC (e.g. Zhai and Santosh, values of 9.00–23.3 and Y/Yb=7.6–11.9 indicates that there was a 2011 and references therein). Development of these 2.38-Ga granitic garnet residue in the source, and the (Ho/Yb)N of 0.96–1.01 suggests gneisses is consistent with a world-wide magmatic stagnation or obvious volumes of amphibole also were present as residue in the fragmentation stage after the formation of the Kenorland supercon- source during the partial melting. On the Sr/Y vs. Y diagram all the tinent (Williams et al., 1991; Bleeker, 2003; Rogers and Santosh, data of the Yiyang TTG gneisses also plots closely on the partial 2004) during 2.4 Ga to 2.2 Ga (e.g. Condie and Aster, 2010), melting residue trends of 10% garnet amphibolite (not shown). Thus, representing plate subduction shutdown or slowdown (O'Neill et the partial melting for generation of magma of the Yiyang TTG al., 2007; Silver and Behn, 2008). gneisses should have occurred under high temperature conditions at Amalgamation of the micro-blocks into a unique NCC is thought to ca. 40 km depth (Drummond et al., 1996) and extra heat was required have occurred at the end of the Archean (Kusky and Li, 2003; Kusky et for the partial melting to occur. Therefore they should also have al., 2007b; Kusky and Santosh, 2009; Kusky, 2011; e.g. Zhai and formed in an extensional tectonic regime after continent–continent Santosh, 2011 and the references therein). We tentatively consider collision and crustal thickening, similar to the rocks of the Mohe that the TC had an interaction with the NCC around the end of the gneiss in the QM. Archean to the early Paleoproterozoic. In detail, The TC and the NCC were amalgamated ﬁrst at around the end of Neoarchean and then experienced crustal extension at the Paleoproterozoic after collision 7.4. Implication for tectonic relation among the TC and the NCC at (Fig. 11). There are several lines supporting this model. around 2.5 Ga (1) The TC and the NCC had a similar crustal growth history during The QM is a micro-continent detached from the TC, and thus can the late Neoarchean and around ~2.5 Ga (the boundary of the provide further information about the early evolution of the TC. The Archean and Proterozoic), which suggest they shared a same QM and its parent TC developed 2.38-Ga granitic gneisses together; mantle–crust interaction before ~2.5 Ga. our geological data indicate that their protoliths were formed in an The Eastern NCC shows two Nd model age peaks at 3.6–3.2 and extensional tectonic regime after collision and crustal thickening. Lu 3.0–2.6, and ~2.8 Ga was considered to be the best estimation et al. (2006, 2008) also suggested that these 2.38-Ga granitic gneisses for the timing of the major crustal growth of the Eastern NCC were considered as a partner of the basalt protolith of the (e.g. Wu et al., 2005), however, crustal growth at ~2.5 Ga was amphibolite inclusions in the gneisses, both being the products of also important, also based on Nd isotopes (e.g. Kusky et al., the bimodal magmatism, suggesting that the protolith of the granitic 2007b). Recently, U–Pb age spectra and Hf compositions on gneisses were formed in a continental rift setting. The NCC also zircons have been increasingly obtained for the NCC, and the

Fig. 11. A tectonic model for the North China Craton and Tarim Craton in the early Paleoproterozoic period, location of the greenstone belts is after Zhai and Santosh (2011). Location of the 2.3–2.4 Ga granitic gneisses of the Hengshan–Lüliang complexes are from Geng et al. (2006) and Zhao et al. (2008), the Yiyang TTG gneisses from Diwu et al. (2007), the Longshan gneisses from He et al. (2005), the Bayanwulashan gneisses from Dong et al. (2007), NE Tarim and Quanji Massif adakitic granitic gneisses are from Lu et al. (2006, 2008) and this study. S. Gong et al. / Gondwana Research 21 (2012) 152–166 163

available data shows that two significant crustal growth events Ma, rather than 2202±26 Ma as reported before (Hao, 2005). They occurred at ~2.7 Ga and ~2.5 Ga (Xia et al., 2006; Jiang et al., do have some zircons with a nearly concordant age of ~2200 Ma, 2010; Wan et al., 2010, 2011; Peng et al., 2011; Yin et al., 2011). but they recrystallized from the 2380 Ma magmatic zircons. The The signal of ~2.5 Ga crustal growth is remarkably well granodioritic gneiss in Mohe area, eastern Quanji Massif, yields an recorded in zircons from the Archean Central Orogenic Belt of upper intercept age of 2392±25 (2σ) Ma, consistent with the the NCC (Liu et al., 2009; Liu et al., 2010a, 2011). A similar former age of 2348±43 Ma (Hao, 2005), not supporting another suggestion was proposed earlier by Windley (1995), Wilde et former age of 2470 ± 19 Ma (Li et al., 2007). The monzo-granitic al. (2005), Kröner et al. (2005) and Chen et al. (2006a) and the gneiss in Delingha area, central Quanji Massif, yields an upper crustal growth that occurred in the ENCC at ~2.5 Ga may be intercept age of 2367±12 (2σ) Ma, consistent with the former age different from other cratons worldwide (Windley, 1995). of 2366 ±10 Ma (Lu, 2002, Lu et al., 2006, 2008). The monzo- Lu (2002) suggested that crustal growth of the TC occurred at granitic gneiss in the Quanjishan area, western Quanji Massif, 3.5–3.6 Ga, 3.0–3.1 Ga and 2.6–2.7 Ga based on a combination yields an upper intercept age of 2372 ±22 (2σ) Ma. Our new data of zircon U–Pb ages and whole-rock Nd-isotopes from granitic indicate that the protoliths of these granitic gneisses intruded gneisses in the TC. Whole-rock Nd-isotopes of the Mohe gneiss during 2.37–2.39 Ga, however, the existence of the ~2.2 Ga granite in the QM also reveals crustal growth during 2.4–2.6 Ga (Chen plutons cannot be ruled out according to data available from et al., 2007), which is confirmed again by our recalculation detrital zircons. Our study reinforces regional granitic magmatism using the new age data in this study (Table 6). Long et al. and an important tectonic event in the QM and in its parental (2010) recently studied zircon U–Pb and Hf isotopes for a Tarim Craton during the early Paleoproterozoic. number of granitic gneisses in the TC further supporting crustal A revisited study on geochemical data of the rocks of the Mohe growth at 2.6–2.7 Ga. U–Pb ages and Hf isotopes of detrital gneiss in the Quanji Massif and of TTG gneiss in the North China zircons from paragneisses the Dakendaban Group record the Craton reveals that the magmas of these rocks are enriched in Sr crustal growth of the provenance, i.e. the lower basement of the and depleted in Y and have high values of Sr/Y with characteristics TC at ~2.7 Ga and ~2.5 Ga (Chen et al., unpublished results). of adakitic rocks, and that their magmas were generated in an (2) The lower basement of the TC and the NCC developed dominantly extensional tectonic regime after collision and crustal thickening. Neoarchean TTG gneisses with some volcanic–sedimentary rocks This indicates that the Tarim Craton and the North China Craton (Lu, 2002; Lu et al., 2006, 2008; Kusky and Li, 2003; e.g. Zhai and were in a rifting setting rather than in subduction or collision Santosh, 2011 and references therein). In addition to deposits of environment in the early Paleoproterozoic. Formation of the early early Paleoproterozoic volcanic–sedimentary strata on the Paleoproterozoic granites in the Tarim and North China Cratons is Neoarchean sequences, both cratons developed a suit of consistent with a world-wide magmatic stagnation or fragmenta- khondalites during middle-late Paleoproterozoic; they are the tion stage after the formation of the Kenorland supercontinent upper Milan Subgroup in the TC and the Dakendaban Group in during 2.4 Ga to 2.2 Ga, representing plate subduction shutdown or the QM (Lu, 2002; Lu et al., 2006, 2008; Wang, 2009)(see slowdown. Table 1), and those strata reported along the khondalite belt The Tarim Craton might have closely interacted with the North along northwestern margin of the Erdos terrane of the WNCC China Craton from the late Neoarchean to the early Paleoproterozoic, (e.g. Kusky and Li, 2003; Zhao et al., 2005 and references therein). including crustal growth during 2.6–2.8 Ga and around 2.5 Ga, late (3) The 2.2–2.4 Ga Paleoproterozoic granitic gneisses or TTG Neoarchean TTG gneisses as the dominant rocks in their lower gneisses, which are not known from the YC, are widespread basements, deposition of a khondalite suite during the middle to late in both the TC and NCC (e.g. Lu, 2002; Lu et al., 2006, 2008; He Paleoproterozoic and two collisional metamorphic events at 1.91– et al., 2005; Geng et al., 2006; Diwu et al., 2007; Zhao et al., 1.96 Ga and 1.80–1.85 Ga in response to the amalgamation of the 2008; this study), which record magmatism in an extensional Tarim Craton and North China Craton with the Columbia supercon- tectonic regime after collision following amalgamation of the tinent, in addition to regional intrusion of granites in both cratons TC and the NCC (Fig. 11). during 2.2–2.4 Ga. (4) In addition to metamorphic events occurring at the end of the Supplementary materials related to this article can be found online Neoarchean in the NCC and probably in the TC, both cratons at doi:10.1016/j.gr.2011.07.011. underwent two episodes of metamorphism in the late Paleoproterozoic. The first metamorphic event occurred be- Acknowledgments tween 1.91 and 1.96 Ga with conditions of amphibolite- to granulite facies in the TC and NCC (Lu et al., 2006, 2008; Hao, We thank Professor Zhengfu Guo and an anonymous reviewer of 2005; Chen et al., 2009; Wang et al., 2009; Zhang et al., 2011a), Gondwana Research who offered critical, helpful and insightful and locally with ultrahigh temperature and pressure conditions comments. This study was supported by National Natural Science in the NCC (Santosh et al., 2007a, 2007b). The second Foundation of China (Grants 40972042, and 40772041). 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