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Late Cenozoic Magmatic Inflation, Crustal Thickening, and >2 Km Of Late Cenozoic magmatic inflation, crustal thickening, and >2 km of surface uplift in central Tibet Jian-Lin Chen1,2,3*, An Yin3*, Ji-Feng Xu1,2, Yan-Hui Dong4, and Zhi-Qiang Kang5 1State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China 2CAS (Chinese Academy of Sciences) Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China 3Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California 90095, USA 4Second Institute of Oceanography, State Oceanic Administration, Hangzhou 310012, China 5Guilin University of Technology, Guilin 541004, China ABSTRACT (58–68 wt%), low MgO (≤2.65 wt%), Cr (≤ 92.2 The high values of La/Yb (>15–20) obtained from late Eocene to Pliocene volcanic rocks of ppm), and Ni (≤37.3 ppm) values, a lack of nega- central Tibet indicate that source regions were in a thickened (>35 km) crust. The higher Neogene tive correlation between 87Sr/86Sr and 1/Sr, the La/Yb values compared to lower Paleogene La/Yb values required continuous crustal thickening, positive correlation between La and La/Yb (or Sr as La/Yb values are proportional to crustal thickness. Although the Paleogene high La/Yb values and MgO), and the weak negative Eu anomalies can be explained by coeval thrusting, this mechanism is unable explain the Neogene higher La/ all indicate a basaltic source (Baker et al., 1995; Yb values because central Tibet is covered by undeformed flat-lying early Miocene strata. Here Rapp and Watson, 1995). This interpretation is we correlate the temporal variation of Cenozoic volcanic geochemistry with the regional geol- further supported by the differences in large ion ogy of central Tibet, which was part of a larger Eocene–Oligocene thrust belt and the related lithophile element concentrations (e.g., Ba and foreland basin. The Paleogene foreland basin was partitioned into the southern (Hoh Xil) and Sr) and incompatible element ratios (e.g., Ba/Rb northern (Qaidam) subbasins by the Kunlun fault ca. 25–20 Ma. The two subbasins were at or Rb/Sr) between our samples and the contem- similar elevations at the time of their formation, but the southern basin, at ~5 km, is >2 km higher porary mantle-derived high-Mg potassic volca- than the northern basin at ~2.8 km. Because the southern basin and the Paleogene thrust belt nic rocks (Fig. 2D; Fig. DR3). High CaO (≥1.78 have been associated with Neogene volcanism and high La/Yb values, we suggest that the eleva- wt%), V (≥44 ppm), and Cr (≥5.37 ppm) contents tion difference between the two basins was induced by >15 km of magmatic inflation sourced rule out metasediments or felsic granulite as the from mantle melting. The magnitude of magmatic inflation is comparable to that of tectonic sole melting sources (e.g., Skjerlie and Johnston, thickening in central Tibet, highlighting the important and often neglected role of syncollisional 1992; King et al., 1997), whereas the Sr-Nd val- mantle melting and its induced magmatic inflation in constructing the thickened Tibetan crust. ues between those of the Paleozoic ophiolites in Central Asia and the mafic granulite xenoliths in INTRODUCTION Fenghuo Shan thrust belt (Fig. 1A). Although the Neogene volcanic rocks of central Tibet (Fig. Although magmatic inflation plays an impor- Paleogene volcanism dominates the Qiangtang 2C) require a juvenile basaltic source. Because tant role in crustal thickening of continental arcs terrane, whereas Neogene–Quaternary volca- La/Yb values are positively correlated to crustal (Lee et al., 2015; Perkins et al., 2016), how it nism dominates the Songpan-Ganzi terrane on thickness (e.g., Profeta et al., 2015; Farner and affects collisional orogeny remains uncertain. This the two sides of the Paleogene Fenghuo Shan Lee, 2017), the higher Neogene La/Yb (or Sm/ is exemplified by the existing crustal-thickening thrust belt, our study area is unique in that it Yb and Gd/Yb) values than those of the Eocene models of the Tibetan Plateau involving (1) con- records nearly continuous volcanism from 35 to volcanic rocks (Figs. 2D and 2E) indicate fur- stant-volume lithospheric shortening without mass 4 Ma (Wei et al., 2004; Liu et al., 2004; Jin et al., ther crustal thickening after 25–20 Ma. Using exchange between crust and mantle (e.g., England 2010) (Fig. 1), allowing us to monitor the tempo- normative quartz, albite, and orthoclase (Win- and Houseman, 1986), (2) volume-reduction litho- ral evolution of the source regions. We obtained kler, 1979), we estimate >10–15 kbar pressure spheric shortening during which eclogitized mafic two new 40Ar/30Ar ages of ca. 6 Ma (Figs. DR1 for the Neogene source (Fig. DR4). The source crust foundered into the mantle (e.g., Le Pichon et and DR2 and Table DR1 in the GSA Data temperature is estimated to be >900 °C (Fig. 2A) al., 1992), and (3) volume-gain lithospheric short- Repository1), similar to existing K-Ar ages of the (Green and Pearson, 1986), consistent with the ening coeval with magmatic inflation and thus same rocks (Fig. 1B). We analyzed the geochem- equilibrium temperatures of 800–1100 °C for the further crustal thickening due to syncollisional istry of the dated rocks that show A-type granit- xenoliths in the Neogene volcanic rocks (Hacker mantle melting (e.g., Yin and Harrison, 2000). To oid composition, low Mg numbers from 22 to 51, et al., 2000; Lai et al., 2011) and required by the address this issue we use regional geology and new and high values of high field strength elements high HFSE concentrations (King et al., 1997). geochemical data from Neogene volcanic rocks to (HFSE) (Fig. 2; Table DR2). An HFSE-enriched The drier and hotter Neogene source region show that central Tibet has been uplifted >2 km magma can be generated by crustal anatexis of (Fig. 2A) led to lower Neogene than Paleogene by >15 km of basaltic inflation since 25–20 Ma. tonalitic, granitic, and metasedimentary rocks Sr/Y values, contrasting the typical positive cor- (Patiño Douce, 1997), or fractionation of basaltic relation between Sr/Y values and crustal thick- CENOZOIC VOLCANISM IN CENTRAL melt (e.g., Schmitt et al., 2000). The high SiO2 ness for continental arcs (e.g., Chiaradia, 2015). TIBET Our study area is located in the northern MAGMATIC INFLATION AND 1 GSA Data Repository item 2018004, analytical Qiangtang terrane within the Eocene–Oligocene methods and supplemental figures and tables, is avail- FORMATION OF THE TIBETAN PLATEAU able online at http://www.geosociety.org/datarepository The high La/Yb value (>20) Eocene vol- *E-mails: [email protected]; [email protected] /2018/ or on request from [email protected]. canism and implied thickened (> 35 km) crust GEOLOGY, January 2018; v. 46; no. 1; p. 19–22 | Data Repository item 2018004 | https://doi.org/10.1130/G39699.1 | Published online 31 October 2017 ©GEOLOGY 2017 Geological | Volume Society 46 | ofNumber America. 1 For| www.gsapubs.org permission to copy, contact [email protected]. 19 85oE90oE95oE Figure 1. A: Tectonic map of central Tibet (after (A) Yin et al., 2008); ages of Cenozoic volcanic Asia rocks are from Wang et al. (2008), Jiang et al. India Qaidam (2009), and this study. See inset map for loca- tion. KS—Kunlun suture; JS—Jinsha suture; KS Basin BNS—Bangong suture; YS—Yarlung-Zangbo Hoh Xil suture. B: Age distribution of volcanic rocks Songpang-Ganzi in the study area. Basin JS 35oN Kunlun fault system Fig. 1B BNS Fenghuo Shan Figure 2. A: TiO versus SiO plot based on Qiangtang 2 2 thrust belt Green and Pearson (1986). B: Age versus Nb YS plot. SG—Songpan-Ganzi terrane; NQT—north- v v 60 km ern Qiangtang terrane. C: 87Sr/86Sr versus ε 20-0 Ma Eocene volcanics 8.8 Ma (B) (0) Nd(0) v v plot. MORB—mid-oceanic ridge basalt; HFSE— Lhasa Neogene volcanics 7.5 Ma 25-10 Ma 5.8 Ma 4.2 Ma high field strength elements. D: La/Yb versus India 8.8 Ma K-Ar age 7.2 Ma Yb plot based on Kay and Mpodpzos (2001) 50-20 Ma (Wei et al., 2004; Jin 6.0 Ma and Haschke et al. (2002). E: Gd/Yb versus age et al., 2010) 4.6 Ma plots (inset in D). See Table DR2 (see footnote 1) 45-4 Ma for data source. Paleo-Asia MORB data are from 30oN 17.6 Ma Liu et al. (2014). NQT—northern Qiangtang 4.7 Ma o 5.5 Ma terrane; SQT—southern Qiangtang terrane; 80 E 60-45 Ma 5.8 Ma Ar/Ar age (this study) 19.6 Ma YS SG—Songpan-Ganzi terrane. 1050 High-Mg potassic ABഒ ഒ rocks in SG High-Mg potassic rocks of NQT ഒ 1000ഒ ഒ ഒ P W S Z S ഒ E 2 L 1 7 ഒ 7.5kbar ഒ 12kbar Mafic dykes in NQT 20kbar $JH 0D 6L2 ZW /D<E MORB 0LRFHQHUK\ROLWHLQ6* *G<E e CDof Paleo- &HQR]RLFUK\ROLWHLQ147 r u (RFHQHDQG2OLJRFHQH s Asian s DGDNLWLFURFNVLQ647 e Ocean r p (RFHQHDGDNLWLFURFNVLQ147 g n 1HRJHQHHQULFKPHQW+ )6( i e s r URFNVLQ147 a u e r 1HRJHQHHQULFKPHQW+)6( s c s URFNVLQ6* n I e r p G g 1 $JH 0D n Adakite ˢ i E s a e r c n I 0DILF[HQROLWKVLQ147 “Normal” andesite- dacite-rhyolite (ADR) )HOVLF[HQROLWKVLQ147 D 6U6U <E SSP 20 www.gsapubs.org | Volume 46 | Number 1 | GEOLOGY (Wang et al., 2008) in northern Qiangtang of This is because the estimated crustal thickness Chinese Academy of Sciences (XDB03010300), central Tibet can be attributed to coeval intra- of central Tibet varies drastically, from ~60 km Chinese National Key Research and Development Program (2016YFC0600305), Chinese Major State plateau thrusting (e.g., Yin, 2010).
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