Early Paleoproterozoic Magmatism in the Quanji Massif
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Gondwana Research 21 (2012) 152–166 Contents lists available at ScienceDirect Gondwana Research journal homepage: www.elsevier.com/locate/gr 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.