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Paleomagnetic Constraints on the Mesozoic Drift of the Lhasa Terrane (Tibet) from Gondwana to Eurasia

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Paleomagnetic Constraints on the Mesozoic Drift of the Lhasa Terrane (Tibet) from Gondwana to Eurasia Paleomagnetic constraints on the Mesozoic drift of the Lhasa terrane (Tibet) from Gondwana to Eurasia Zhenyu Li1, Lin Ding1,2*, Peter C. Lippert3, Peiping Song1, Yahui Yue1, and Douwe J.J. van Hinsbergen4 1Key Laboratory of Continental Collision and Plateau Uplift (LCPU), Institute of Tibetan Plateau Research, Chinese Academy of Sciences (ITPCAS), Beijing 100101, China 2Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, Beijing 100101, China 3Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112-9057, USA 4Department of Earth Sciences, Utrecht University, Heidelberglaan 2, 3584 CS Utrecht, Netherlands ABSTRACT Himalaya (the northernmost continental rocks The Mesozoic plate tectonic history of Gondwana-derived crustal blocks of the Tibetan derived from the Indian plate) that collided with Plateau is hotly debated, but so far, paleomagnetic constraints quantifying their paleolati- Lhasa in the Eocene along the Indus-Yarlung tude drift history remain sparse. Here, we compile existing data published mainly in Chinese suture zone (Yin and Harrison, 2000; Hu et al., literature and provide a new, high-quality, well-dated paleomagnetic pole from the ca. 180 2015; Huang et al., 2015). Ma Sangri Group volcanic rocks of the Lhasa terrane that yields a paleolatitude of 3.7°S Most authors describe an ideal Wilson-cycle ± 3.4°. This new pole confirms a trend in the data that suggests that Lhasa drifted away scenario, wherein the blocks of the Tibetan Pla- from Gondwana in Late Triassic time, instead of Permian time as widely perceived. A total teau all drifted from India in Paleozoic to Meso- northward drift of ~4500 km between ca. 220 and ca. 130 Ma yields an average south-north zoic time and were reunited with India in the plate motion rate of 5 cm/yr. Our results are consistent with either an Indian or an Austra- Cenozoic after closure of the Neotethyan and lian provenance of Lhasa. older oceans. These conclusions are commonly based on Paleozoic fossils and detrital zircon INTRODUCTION between India, South China, and North China, age spectra typical for the northern Gondwana The Tethyan oceans surrounded by the super- contains several such continental fragments margin in their stratigraphy, volcanism, and continent Pangea opened and closed as conti- (Fig. 1). From north to south, these include the ophiolite geology (e.g., Metcalfe, 1996; Gehrels nental fragments rifted and drifted away from Qiangtang terrane(s) that collided with north- et al., 2011; Torsvik and Cocks, 2013; Zhu et Gondwana in the south, opened the Meso- and eastern Tibet in the Late Triassic (Yin and Har- al., 2011, 2013; Cai et al., 2016). Neotethyan Oceans in their wake, and closed rison, 2000; Song et al., 2015); Lhasa, which Quantifying the rift and drift history of these Paleotethyan Ocean floor upon their approach collided with Qiangtang in the Early Cretaceous terranes allows us to reconstruct the opening and toward Eurasia (Şengör, 1992; Metcalfe, 1996; along the Bangong-Nujiang suture zone (e.g., closure of the Tethyan oceans. Assigning ages Stampfli and Borel, 2002; Domeier and Tors- Yin and Harrison, 2000; Kapp et al., 2007; Fan to continental breakup events commonly relies vik, 2014). The Tibetan Plateau, sandwiched et al., 2015; Zhu et al., 2016); and the Tibetan on the interpretation of stratigraphic records of Sangri Group lavas B 0° Tarim North China NE Tibetan terranes D=339.8±3.4° Himalaya I=-7.4±6.7° Song 30°N Figure 1. A: Schematic Qiangtang pan Garz n=60, K=30.1 tectonic map of Tibet Study area i A95=3.4 showing study area of Lhasa Sangri Group lavas and C TH outlines of Lhasa block and Tibetan Himalaya (TH). B: Lava site average India paleomagnetic directions of Sangri Group lavas (D— declination; I—inclination; K—Fisher precision parameter). C: Zircon U-Pb ages of two sam- ples of Sangri Group lavas 10°N (MSWD—mean square of weighted deviates). 90°E 110°E A *E-mail: [email protected] GEOLOGY, September 2016; v. 44; no. 9; p. 727–730 | Data Repository item 2016234 | doi:10.1130/G38030.1 | Published online 1 August 2016 GEOLOGY© 2016 Geological | Volume Society 44 | ofNumber America. 9 For| www.gsapubs.org permission to copy, contact [email protected]. 727 presupposed or demonstrated once-adjacent GEOLOGICAL SETTING AND origin of orthogonal vector plots. This character- blocks. For example, Lower Permian mafic SAMPLING istic remanent magnetization (ChRM), carried volcanics and middle Permian overlying pas- Lhasa is an east-west–trending continental by titanomagnetite, is either shallowly upward sive margin clastics and pelagic limestones in tectonostratigraphic belt separated from the or downward with a northerly or southerly dec- the Tibetan Himalaya are interpreted to reflect Qiangtang terrane along the Bangong-Nujiang lination. We calculated lava site averages using the departure of a continental fragment from suture zone, a >2000-km-long zone with ultra- Fisher statistics (Fisher, 1953) on directions and the Indian segment of Gondwana (Garzanti, mafic rocks and mélange along which the Ban- applied the quality criteria explained in Lippert 1999). Many authors have concluded that this gong-Nujiang (or Mesotethys) ocean subducted et al. (2014); a notable exception to this filter fragment was Lhasa (e.g., Stampfli and Borel, since the Late Triassic (Yin and Harrison, 2000; is that we accept site mean directions defined 2002; Torsvik and Cocks, 2013; Domeier and Kapp et al., 2007; Zhu et al., 2011, 2013; Zeng by three or more ChRM directions. Sixty-two Torsvik, 2014); others, however, have suggested et al., 2015). The southern margin of Lhasa is (62) lava sites meet our quality criteria, of which that Lhasa did not rift from Gondwana until the characterized by the widespread Gangdese mag- two fall outside a 45° cutoff angle around the Middle–Late Triassic (e.g., Metcalfe, 1996; Li et matic belt of dominantly calc-alkline granitoids mean direction; these data provide an average al., 2004). Some have argued that Lhasa rifted that were emplaced mainly from the Early Cre- direction (declination, D; inclination, I) of D ± from northwestern Australia rather than western taceous through the Eocene (e.g., Chung et al., DD = 339.8° ± 3.4°, I ± DI = -7.4° ± 6.7°, A95 Greater India in the Late Triassic (Zhu et al., 2005; Zhu et al., 2011). Older Mesozoic vol- = 3.4°, Fisher precision parameter K = 30.1, n 2011; 2013; Ran et al., 2012), consistent with canic rocks include the Jurassic–Lower Creta- = 60, corresponding to a paleolatitude of 3.7°S sediment provenance of Upper Triassic turbi- ceous adakite-like andesitic and basaltic rocks (uncertainty window between 7.1°S, 0.3°S). The dites interpreted to be part of the Lhasa block of the Sangri Group (Kang et al., 2014). A95 value falls within the n-dependent confidence (Cai et al., 2016). The Sangri Group is an ~500-m-thick suite envelope of Deenen et al. (2011) (A95min = 2.3°; The transfer of continental blocks from of widely distributed volcanic and sedimen- A95max = 6.2°), indicating that the data scatter can Gondwana to Eurasia involved large south-north tary rocks that forms a linear belt along the be straightforwardly explained by paleosecular (i.e., latitudinal) plate motions, which can be southern margin of Lhasa (Fig. 1; see the Data variation of the paleomagnetic field alone. The quantitatively constrained with paleomagne- Repository); it is typically characterized by data set passes the reversal test of McFadden and tism. In this paper, we quantify the paleolati- 1–3-m-thick calc-alkaline lavas and intercalated McElhinny (1990) (classification C), but fails tude history of Lhasa from the late Paleozoic reddish-beige sandstones, and limestones. Geo- the fold test of Tauxe and Watson (1994). Maxi- to present by reviewing existing paleomagnetic chemical and petrographic studies suggest that mum clustering of directions occurs between data and by providing a new, well-dated, high- the Sangri Group lavas are the product of partial 60% and 100% unfolding, but we note that the quality paleomagnetic pole from Lower Jurassic melting of subducting oceanic crust (Kang et optimal clustering at the 80% unfolding level is volcanic rocks of Lhasa. We compare these con- al., 2014). Kang et al. (2014) recently reported only marginally better than at 100% unfolding straints to the global apparent polar wander path zircon U-Pb ages of 195 ± 3.0 Ma and 189 ± (K = 30.9 versus K = 30.1), and the paleolati- in Eurasian and Indian/Gondwanan coordinates 3.0 Ma from the middle part of the section at tude obtained at 80% unfolding of 2.4°S [5.8°S, (Torsvik et al., 2012) and show the implications Kamadang village near Sangri County. Our 0.9°N] is statistically indistinguishable from for paleogeographic reconstructions of the own zircon U-Pb LA-ICP-MS analyses of the the one obtained at 100% unfolding. Therefore, Meso zoic Tethyan realm. upper and lower parts of the section at Sangri we interpret the HT ChRM direction at 100% County (Fig. 1) provide overlapping weighted unfolding as primary and use it for the tectonic PREVIOUS PALEOMAGNETIC mean ages of 181.7 ± 5.4 Ma (mean square of analyses discussed below. CONSTRAINTS weighted deviates [MSWD] = 2.1, n = 26) and Several high-quality paleomagnetic poles 179.9 ± 7.2 Ma (MSWD = 0.33, n = 13) (see DISCUSSION have been reported from Lower Cretaceous the Data Repository for details). The paleomagnetic data for Lhasa—albeit and Paleogene rocks of Lhasa (Lippert et al., For paleomagnetic studies, we collected 589 sparse—suggest that it drifted away from the 2014; Huang et al., 2015, and references therein; samples from 62 sites distributed across two loca- northern margin of Gondwana in Late Triassic see the GSA Data Repository1).
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