Geochemistry and Geochronology of Ophiolitic Rocks from the Dongco and Lanong Areas, Tibet: Insights into the Evolution History of the Bangong-Nujiang Tethys Ocean

Abstract

The Bangong-Nujiang Suture Zone (BNSZ) in central Tibet hosts a series of dismembered Jurassic ophiolites that are widely considered as remnants of the vanished Meso-Tethys Ocean. In this study we present new compositional, isotopic, and geochronological data from anorthosites and gabbros of the Dongco and Lanong ophiolites in order to test several hypotheses about the nature of subduction in the Bangong-Nujiang Tethys Ocean (BNTO) during the Mesozoic era. Uranium–Pb dating of magmatic zircons separated from the Dongco anorthosites yielded an (average) age of 169.0 ± 3.7 Ma. Zircons separated from the Lanong anorthosites and gabbros yielded U–Pb ages of 166.8 ± 0.9 Ma and 167.3 ± 1.1 Ma, respectively. Zircons separated from the Dongco and Lanong anorthosites have positive εHf(t) values (5.62–15.94 and 10.37–14.95, respectively). The Dongco anorthosites have moderate initial 87Sr/86Sr (0.703477–0.704144) and high εNd(t) (+6.50 to +7.91). The Lanong anorthosites have high (87Sr/86Sr)i (0.706058–0.712952) and εNd(t) in the range of −1.56 to +2.02. Furthermore, the Lanong gabbros have high (87Sr/86Sr)i (0.705826–0.706613) and εNd(t) in the range of −0.79 to +4.20. Most gabbros from Dongco and a few gabbros from Lanong show normal mid-ocean ridge basalt (N-MORB)-like primitive mantle (PM)-normalized multi-element patterns. In contrast, most gabbros from Lanong show U-shaped chondrite-normalized rare earth element (REE) profiles. The investigated gabbros are characterized by wide ranges of δEu {(Eu)N/[(Sm)N*(Gd)N]1/2} values (0.83–2.53), indicating that some of them are cumulative rocks. The trace element contents of all anorthosite samples imply that their composition was controlled by cumulative processes. The geochemical and isotopic compositions of the non-cumulative gabbros from Dongco (δEu: 0.95–1.04) and Lanong (δEu: 0.83–1.03) indicate that their parental melts were derived from melting of heterogeneously depleted, juvenile mantle reservoirs. These rocks have arc-related affinities, indicating that their mantle sources were influenced by minor inputs of subducted lithospheric components. Our preferred hypothesis for the origin of the non-cumulative gabbros from Dongco is that they were formed in a transient back-arc basin (BAB) setting in the middle-western segment of the BNTO, whereas our preferred scenario about the origin of the non-cumulative gabbros from Lanong is that they were generated in a forearc setting in the middle part of the BNTO. We conclude that both geotectonic settings were developed in response to the northward subduction of the BNTO during the Middle Jurassic.

Forearc tectonic evolution in the middle of the Bangong–Nujiang Tethys Ocean: New geochemical evidence of the Lanong ophiolites from the Zangbei lakes region

“…The five Lanong basalt samples have variable concentrations of SiO 2 (46.97–52.86 wt%), MgO (10.70–15.70 wt%), Na 2 O (0.20–3.46 wt%), K 2 O (0.03–0.57 wt%), and Al 2 O 3 (9.88–13.31 wt%) and Mg# values of 75 to 80 (Table S2). The basalts plot in the field of sub‐alkaline basalt and andesite/basalt on a total alkalis (Na 2 O + K 2 O) vs. SiO 2 diagram (Figure ), consistent with other mafic rocks from the Zangbei lakes region, including the Dongqiao basalt and diabase (Zhang et al, 2014), Pengtso basalts and diabases (Wang et al, ), the Jiangtso gabbros and diabases (Huang, Li, Cai, et al, ; Huang, Li, Xia, et al, ), and the Lanong gabbros, gabbro‐diabases, and diabases (Xu, , Xu et al, ; Zhong, Liu, et al, ; Yang et al, , under review).…”

“…Existing data for ophiolites from the Zangbei lakes region indicate they are Jurassic in age, including those at Jiangtso (188.1 ± 4.1 and 189.8 ± 3.3 Ma; Huang, Li, Cai, et al, ; Huang, Li, Xia, et al, ), Pengtso (159.0 ± 2.1 Ma; Wang et al, ), and Lanong (166.8 ± 0.9, 167.3 ± 1.1, 149.1 ± 1.2, and 147.6 ± 2.3 Ma; Zhong, Liu, et al, ; Yang et al, , under review). However, the tectonic setting of the ophiolites is debated, with some researchers favouring a backarc basin setting (Chen et al, ; Huang et al, ; Huang, Li, Cai, et al, ; Huang, Li, Xia, et al, ; Tang & Wang, ; Wang et al, ; Wang, Bao, & Deng, ; Wu, ; Xu et al, ) and others arguing for a forearc basin setting (Pearce & Deng, ; Liu et al, ; Zeng et al, ; Zhong, Liu, et al, ; Yang et al, , under review).…”

Geochemistry and geochronology of Pengco subduction‐related ophiolites, Tibet: Implications for Dongkaco microcontinent in the Bangong–Nujiang suture zone

“…Similarly, the Dongco ophiolites in Gerze region of central BNSZ also show various geochemical characteristics, such as N-MORB (Wang et al, 2016;Yang et al, 2019), E-MORB and OIB (Fan, Li, Wang, Liu, & Xie, 2017;Wang et al, 2016). Several tectonic settings were also proposed for their origins, such as back-arc basin (Yang et al, 2019), island arc (Wang et al, 2016) and oceanic island (Fan et al, 2017). Compared to the Laguoco ophiolites formed by the southward subduction of BNTO under NLT (Yuan et al, 2015), and the Chaerkangco ophiolites resulted from the northward subduction under NQT (Zhang, 2007), the Dongco ophiolites are obviously distinct from them.…”

“…Compared to the Laguoco ophiolites formed by the southward subduction of BNTO under NLT (Yuan et al, 2015), and the Chaerkangco ophiolites resulted from the northward subduction under NQT (Zhang, 2007), the Dongco ophiolites are obviously distinct from them. Thus, some researchers proposed that Dongco ophiolites were formed by a subduction of a minor basin (Wang et al, 2016;Yang et al, 2019). Another example of this hypothesis is in the Rutog region, where the Shiquanhe ophiolites and Larelaxin pluton were formed due to the southward and northward subduction of BNTO, respectively (Li et al, 2014;Liu, Huang, & Gu, 2018), while, in contrast, the Bangongco ophiolites present various types of magmatic rocks including boninite (Shi, Yang, Xu, & Qi, 2004), N-MORB (Shi, Yang, Xu, & Qi, 2008;Wang et al, 2016) and E-MORB (Shi et al, 2008), which presumably resulted from the subduction of another minor basin.…”

Geochemical characteristics of mafic intrusive rocks from the Naga Hills Ophiolite, north‐east India: Constraints on petrogenesis and tectonic setting

“…The interpretation of the ophiolitic assemblages as, on‐land analogues of oceanic lithosphere, has brought considerable interest to their investigation in orogenic belts (Dilek & Furnes, 2011, 2014; Moores, 1982; Yang et al, 2019). The important components of ophiolitic assemblages include mantle rocks, ultramafic cumulates, mafic intrusives, and extrusive crustal rocks, commonly associated with deep‐sea pelagic sediments (Coleman, 1977; Dilek & Furnes, 2011; Robertson, 2002).…”