The Chronology And Tectonic Attribute Of Granodiorite Located In Riwanchaka Area, Central Qiangtang, Tibetan Plateau

Qiangtang area is located north of the Tibetan Plateau，at the transfer parts ofGondwana and Eurasia. Which is the key area to investigate subduction and collision ofthe Longmu Co–Shuanghu–Lancangjiang ocean. This study is about the late Triassicgranite were exposed in Riwanchaka area. Here we present the results of geochemistry andzircon U–Pb geochronological studies of granodiorite plutons. In combination with earlystudies of others, according to the geochronology and geochemical, to determine theformation tectonic environment of the granite. So we can provide restriction for theevolution of Longmu Co–Shuanghu–Lancangjiang ocean.Riwanchaka area was located in the central Qiangtang. There are many magmaticrocks, especially Triassic granite which characteristic is many times in short period. Themagmatism are important basis for the evolution of Longmu Co–Shuanghu–Lancangjiangocean. Four magmatism were identified in the study area which marked as R13T14,R12T1, R12T8and R12T9, yielded magmatic crystallization ages of226Ma,215Ma,215Ma and203Ma. The rocks are granodiorite and granite. According to the geochemical,the rocks can be devided into two groups. One is normal granite(R13T14, R12T1, R12T8),one is adakites(R12T9). The normal granite it belongs to aluminous-peraluminous, showthat it is characterized by high silicon and aluminum, geochemical characteristics showsthat they are I-type granite. and they are negative anomalies of Nb, Ta, Ti, P, and Negative Eu anomaly, instructed that the magma had fractional crystallization. The R13T14andR12T1has negative εHf(t) values-20.11~-8.06and-2.98~-0.14, suggested that these rockswere generated by melting of lower crust. The preliminary analysis that they weregenetated from partial melting of ancient crustal by underplating of mantle derivedmagma. And mix with mantle source,then experienced fractional crystallization thengenerated the ultima magma. Combining with results of previous studies, range226Ma to203Ma, Riwanchaka area was under a post-collisional extensional setting, and the Oceanlithosphere was subducting. The magmatism can be interpreted as having resulted fromthe slab breakoff of the subduction of the Longmu Co–Shuanghu–Lancangjiang Oceanlithosphere. And the adakites(203Ma) produced by partial melting of a juvenile thickenedlower mafic crust.Combining with regional research progress, our new geochemical data andgeochronological results demonstrate that the magmatism were formed in apost-collisional tectonic setting. In203Ma we proposed that some syn-orogenicmagmatism and the the extensive magmatism which caused by the slab break off mayhave led to lateral crust growth. This protected that the LongmuCo–Shuanghu–Lancangjiang ocean has been closed, and was accompanied by the slabbreak-off and the crustal growth.