Geochemistry and Sr-Nd isotope characteristics of tonalites in Zêtang, Tibet: New evidence for intra-Tethyan subduction

Abstract

Tonalites from the island arc rock assemblage in the Zêtang segment of the Yarlung Zangbo suture zone were analyzed for major, trace elements (including REE) and Sr-Nd isotope. The experimental data demonstrate that the tonalites have the adakite-like characteristics, including high SiO 2 (58%-63%), Al 2 O 3 (18.4%-22.4%), Sr (810×10 −6 -940×10 −6 ), Sr/Y (77-106), low HREE (Y=9×10 −6 -11×10 −6 , Yb= 1×10 −6 -1.3×10 −6 ), with LREE enrichment and faint Eu positive anomaly. I Sr (0.70421-0.70487) is relatively low whereas 143 Nd/ 144 Nd (0.512896-0.512929) and ε Nd (t ) values (+6.7-+7.3) are high. These features suggest that Zêtang tonalites were formed by the partial melting of subducted oceanic crust, with involvement of a small amount of oceanic sediments. The identification of Zêtang adakites, derived from slab melting, presents new evidence for the intra-Tethyan subduction and the previous suggestion about the existence of intra-oceanic island arc within Tethys.adakite, Zêtang tonalites, subduction, Yarlung Zangbo suture zone, Tibet Adakite (adakite-like rock) is a type of igneous rock with remarkable geochemical characteristics: high SiO 2 (＞56%), Al 2 O 3 (＞15%), Sr (＞400×10 −6 ), Sr/Y(＞20-40), La/Yb＞20, enriched in Na 2 O (Na 2 O＞K 2 O) and LREE, but depleted in Y and HREE (Y＜18×10 −6 , Yb＜ 1.9×10 −6 ). It is a traditional view that adakite is derived from the partial melting of young and hot subducted oceanic crust at a depth about 75-85 km (corresponding to amphibolite-eclogite transition zone) [1] . Recent studies show that the melting of underplated basalt and delaminated lower crust can also produce the adakite (adakite-like rock) [2][3][4][5] . Whichever genesis it is, adakite (adakite-like rock) records the deep dynamic process of plate movement (such as slab subducting, basalt underplating, delamination of lower crust) and it is of high importance in geodynamics.The Tibetan Plateau is the result of Tethyan subduction, extinction and Indian-Asian continent collision, and has attracted extensive attention of geologists all over the world. Zhang et al. [6] first suggested the development of Cenozoic adakites on the Tibetan Plateau. In addition, some researchers have identified adakites in northern Tibet and proposed that these rocks were generated by the partial melting of thickened lower crust, recording the thickening event of the Tibetan Plateau [7][8][9][10][11] . The intrusive rocks within the Miocene Gangdese Cu-bearing porphyry belt in southern Tibet also exhibit a geochemical affinity with adakite. The genesis

Petrogenesis of the Early Eocene adakitic rocks in the Napuri area, southern Lhasa: Partial melting of thickened lower crust during slab break-off and implications for crustal thickening in southern Tibet

“…The adakitic intrusive rocks within the Gangdese batholith are considered to have been generated in two main stages, the Jurassic-Cretaceous (160-77 Ma) and : (1) Jurassic-Cretaceous adakitic rocks generated in an arc setting were related to Neo-Tethyan subduction processes, by melting of either subducted oceanic slabs (e.g., Jiang et al, 2012;Ma et al, 2013a;Wei et al, 2007;Zhang et al, 2010b;Zhu et al, 2009) or thickened mafic continental lower crust (e.g., Guan et al, 2010;Wen et al, 2008b); (2) Cenozoic adakitic rocks occurring as small-volume plugs or dikes/sills, which intrude or crosscut the Gangdese batholith, the Linzizong volcanic successions and associated sedimentary formations, and extend~1300 km across nearly the entire southern Lhasa (Fig. 1b) (e.g., Chung et al, 2003Chung et al, , 2005Chung et al, , 2009Gao et al, 2007Gao et al, , 2010Guan et al, 2012;Guo et al, 2007;Hou et al, 2004Hou et al, , 2012Ji et al, 2012;Jiang et al, 2011Jiang et al, , 2014Pan et al, 2012;Xu et al, 2010;Zhang et al, 2010a).…”

Zircon U-Pb Age, Sr-Nd-Hf Isotopic Characteristics of Baiyinhushuo Adakite in Inner Mongolia:Implications for Tectonic Evolution

“…Generally, Sr-Nd isotopic signatures of magmas close to MORB are directly derived from the melting of mantle-derived oceanic crust (MORB), which is related to the island arc setting of the subduction zone. If ( 87 Sr/ 86 Sr)i > 0.704, typically between 0.706 and 0.710, it is considered that the magma derived from the partial melting of the basalt of juvenile lower crust caused by underplating, which is closely related to the intracontinental post-collision tectonic environment during the thickening lithosphere delamination and mantle magma underplating [48][49][50]. The Baiyinhushuo adakite has a low ( 87 Sr/ 86 Sr)i (0.703382), high 143 Nd/ 144 Nd (0.512721) and εNd(t) values (+7.72).…”

Geochronology, geochemistry, Sr–Nd–Hf isotope composition of the late Permian adakite in West Ujimqin, Inner Mongolia: petrogenesis and tectonic implications

“…8). The West Ujimqin adakitic granites generally coincide with the adakitic intrusives in the Aleutian Ialands and Patagonian and Zedang of Tibet (Yogodzinski et al, 1995;Wei et al, 2007), which are considered to derive from partial melting of subducted oceanic slab (Fig. 8).…”