The Juhuashan-tanggula-leiwuqi Volcanic Arc Of Tibetan Plateau-record Of The Collision Between Gondwana And Yangtze Continents

The distribution of the Juhuashan–Tanggula–Leiwuqi volcanic arc is along theLongmu Co–Shuanghu–Lancangjiang suture zone, extending from Gangmacuo andJuhuashan areas of central Qiangtang, to the Tanggula, Leiwuqi, and Jitang areas ofeastern Tibet.In the Juhuashan area of Qiangtang, the Juhuashan–Tanggula–Leiwuqi volcanicarc consists of arc volcanic rocks, granites and diorites. The arc volcanic rocks anddiorites exhibit strong negative Nb, Ti, and Eu anomalies. These rocks fall in the‘‘volcanic arc granite’’(VAG) field in the Nb versus Y and Rb versus Y+Nb tectonicdiscrimination diagrams, which are comparable to those formed in a volcanic arctectonic setting. The arc volcanic rocks and diorites can be devided into two groups.The older group (225–219Ma) have high Sr (291–1367ppm, averaging799ppm) andlow Y (5.47–13.9ppm, averaging10.2ppm) contents with high Sr/Y ratios (28–116,averaging79). They all plot in the adakite field in the Sr/Y versus Y diagram. Thehigh SiO2contents (62.84–77.82wt.%) suggests that these rocks belong to high-SiO2adakites (HSA). In the Sr/Y versus La/Yb diagram, these rocks show features of themodern arc adakite formed by slab melting. They are significantly different from theTibetan and Dabie adakites formed by partial melting of the lower crust. Collectively,these rocks have the geochemical characters similar to the modern adakite, a rock typeoriginally defined as felsic magma produced by partial melting of subducted oceaniccrust. The negative εNd(t) value (-2.6) and positive εHf(t) values (+3.2to+5.4)suggested that these rocks were generated by the melting of subducted oceanic crustand associated sediments. The younger group (217–205Ma) have relatively low Sr contents (37–389ppm, averaging186ppm) and Sr/Y ratios (2–25, averaging8) withvarying SiO2contents (60.30–80.90wt.%). Moreover, these rocks display high initial87Sr/86Sr ratios (0.708–0.714), and distinguished negative εNd(t) values (-9.6to-7.9),indicating that these rocks were probably derived from anatexis of ancient continentcrust of the Northern Qiangtang–Qamdo terrane. A slab break-off model waspresented to interpret the compositional transition from ‘‘adakitic’’ to ‘‘non-adakitic’’magmatism at ca.219Ma.The studying degree of the predecessor is low to the Triassic granites in the theJuhuashan area of Qiangtang. These Triassic granites can also be devided into twogroups. The younger group (215–201Ma) are in the majority. Thereinto, the granitesof eastern Riwanchaka, eastern Guoganjianian, and Xiangtaohu are I-type, and thegranites of Bensongcuo and Gemuri are S-type. The granites of the younger groupwere related to the slab break-off of the Longmu Co–Shuanghu–Lancangjiang ocean.The older group (225Ma) includes only the Guoganjianian granodiorite. Consideringthe contemporaneous adakites (225–219Ma), this granodiorite was related to asubduction setting.In the Tanggula–Leiwuqi area of eastern Tibet, the Juhuashan–Tanggula–Leiwuqi volcanic arc mainly consists of deformated granites which were comparableto the Triassic granites in the the Juhuashan area of Qiangtang. These granites can besubdivided into S-and I-types. The S-type granites were generated by the melting ofthe Paleoproterozoic continent crust of Northern Qiangtang-Qamdo terrane; the I-typegranites were probably derived from a relatively young source of Neoproterozoicjuvenile crust.Collectively, the currently available high-quality geochronological data revealthat the Juhuashan–Tanggula–Leiwuqi volcanic arc was formed during Triassic(251–205Ma); the western section (225–205Ma) is younger than the eastern section(251–236Ma). Therefore, the closure of the Longmu Co–Shuanghu–Lancangjiangocean is probably diachronous. In central Qiangtang area, the Southern Qiangtang–Baoshan collided with Northern Qiangtang–Qamdo terranes at ca.237~230Ma andthe time of the oceanic subduction ranges from237to219Ma. The oceanic slab wassubsequently detached at ca.219Ma. In the meantime, the break-off induced the hotasthenosphere to underplate the North–Qiangtang subterrane and caused crustalanatexis in the ancient basement. Futhermore, asthenosphere upwelling also led to theuplifting and exhumation of the High-pressure metamorphic belt. In the Tanggula–Leiwuqi area of eastern Tibet, the Southern Qiangtang–Baoshan collidedwith Northern Qiangtang–Qamdo terranes at Late Permian and the related oceanicslab was probably detached at Early–Middle Triassic.Triassic granites are widespread in the Lincang area of the Changning–Mengliansuture zone in western Yunnan of China. We proposed that these Triassic granites areprobably the sourthern extension of the Juhuashan–Tanggula–Leiwuqi volcanic arc,though additional investigations and data will be required to test our arguments.