Subduction of the Bangong-Nujiang Ocean: constraints from granites in the Bangong Co area, Tibet

Abstract

We report geochronologic, whole‐rock geochemical and Sr–Nd isotopic analyses of the granites that are exposed to both the north and the inside of the Bangong–Nujiang Suture (BNS) zone as well as the implications for the Mesozoic history of Tibet. To the north of the BNS, the Larelaxin pluton consists of I‐type quartz diorite and highly fractionated I‐type biotite granite. The Larelaxin pluton is enriched in large‐ion lithophile elements (LILE) but depleted in high‐field‐strength elements (HFSE); therefore, it exhibits the features of volcanic arc rocks. The initial Sr (0.7102 to 0.7215) and negative εNd (t) (−2.91 to −5.20) values imply a mixture of depleted mantle and continental crust. The mean 206Pb/238U zircon age is 168 Ma; we therefore propose that the Bangong–Nujiang Ocean (BNO) had already been subducted beneath the Qiangtang terrain by the middle Jurassic. Inside the BNS, the Rutog granites intruded into the Lagongtang and Duoren formations, which show a continental margin and a forearc basin sedimentary facies, respectively. The mean 206Pb/238U zircon age is 101 Ma. The Rutog granites are monzogranites with a high Na/K ratio (Na2O/K2O > 1) and a high LILE/HFSE ratio, and A/CNK < 1.1. The high Sr/Y ratio (22 to 56) implies that these granites are adakitic. The low initial Sr (0.7044 to 0.7055) and positive εNd (t) (+1.46 to +2.70) values indicate that the protolith of the Rutog granites originated mainly from a depleted source. We attribute the Rutog plutonism to the development of an oceanic arc during the continuing northward subduction of the BNO and propose that the Rutog adakitic granites were formed by melting of the subducted BNO crust with limited crustal contamination. Copyright © 2013 John Wiley & Sons, Ltd.

Middle Jurassic MORB-type gabbro, high-Mg diorite, calc-alkaline diorite and granodiorite in the Ando area, central Tibet: Evidence for a slab roll-back of the Bangong-Nujiang Ocean

“…This suggests that the lithospheric metasomatism Irvine and Baragar, 1971), lava composition and trend lines for a typical sequence from Tingmuli volcano and a typical calc-alkaline trend from the Cascades lavas (after Carmichael, 1964). Literature data of Jurassic intrusive rocks are from Liu et al (2014Liu et al ( , 2015 and S.M. , J.X.…”

“…Li et al, 2014;J.X. Li et al, 2014;Liu et al, 2014Liu et al, , 2015. These magmatic rocks were considered to be produced in a magmatic arc setting (Guynn et al, 2006;Zhang et al, 2012).…”

“…Date sources are as follows: Ando orthogneiss from Harris et al (1988), Global marine sediments from Plank and Langmuir (1998), Bangong-Nujiang Ocean ophiolites from K.J. Zhang et al (2014) and X.Z. Zhang et al (2014), Granitoids in Larelaxin from Liu et al (2014), Granitoids in Central Qiangtang from Zhai et al (2013) and Li et al (2015), Granitoids in Rena Co from Hao et al (2015) and Granitoids in Kangqiong from Li et al (2015).…”

“…Zhang et al (2014) and X.Z. Zhang et al (2014), Granitoids in Larelaxin from Liu et al (2014), Granitoids in Central Qiangtang from Zhai et al (2013) and Li et al (2015), Granitoids in Rena Co from Hao et al (2015) and Granitoids in Kangqiong from Li et al (2015). high-Mg diorites.…”

“…Recently, more and more evidence indicates that the preCenozoic tectonics and magmatism probably contributed to the accretion and formation of the central and northern Tibetan Plateau before the final India and Eurasian collision (Guynn et al, 2012;Kapp et al, 2003Kapp et al, , 2007Long et al, 2015;Murphy et al, 1997;Pullen et al, 2011). For example, an active continental margin is proposed to have developed in the Southern Qiangtang caused by the subduction of the Bangong-Nujiang Ocean (BNO), which is predated to the collision between the Lhasa and Qiangtang terranes Liu et al, 2014). Late Mesozoic intermediate-felsic intrusions in the Southern Qiangtang terrane are considered to have been produced by the northward subduction of the Bangong-Nujiang Ocean (S.M.…”