Petrogenesis And Tectonic Significance Of The Late Triassic(sub-)Volcanic Rocks In The East Kunlun Orogenic Belt, Northern Tibet Plateau

Zircon U-Pb ages and geochemical data of the late Triassic mafic dikes(diabase) and felsic volcanic rocks(rhyolite and rhyolitic tuff) in the East Kunlun Orogenic Belt(EKOB) are presented in this thesis. The age data give a window of 228-218 Ma. The mafic dikes are evolved alkaline basaltic melts, which intruded volumetrically more abundant and ~ 8-9 Myrs older A-type granite batholith. Their multi-element and rare earth element(REE) patterns are similar to those of the present-day ocean island basalts(OIB). However, they also have a weak continental crustal signature(i.e., enriched Rb and Pb and weakly depleted Nb, Ta and Ti). Their trace element features along with the low εNd(t)(-2.18 to-3.46), low εHf(t)(-2.85 to-4.59), high 87Sr/86Sr(0.7076- 0.7104) and variable Pb isotopic ratios are consistent with melts derived from metasomatized subcontinental lithospheric mantle with crustal contamination. The felsic volcanic rocks possess high LREE/HREE(e.g., [La/Yb]N = 5.71-17.00) with a negative anomaly of Eu, Sr and P, like the model upper continental crust(UCC). Given less negative εNd(t)(-3.83 to-5.09) and εHf(t)(-3.06 to-3.83) as well as the high 87Sr/86Sr(0.7213-0.7550) than the UCC, together with the overlapping isotopes with the high Nb-Ta rhyolites and mafic dikes, the felsic volcanic rocks are likely resulted from melting-induced mixing with 45-50% crustal materials and 50-55% mantle-derived mafic melts which may be the parental magmas to the mafic dikes. Such mantle-derived melts underplated and intruded the lower crust as juvenile crustal materials. Partial melting of such juvenile crust generated felsic melts parental to the felsic volcanic rocks of the EKOB. We hypothesize that the late Triassic mafic dikes and felsic volcanic rocks are related to post-collisional extension as well as associated orogenic collapse. Such processes are important to causing asthenospheric upwelling, decompression melting, triggered melting of the prior metasomatized mantle lithosphere even the existing crust. This study is helpful to understand the origin of the juvenile crust and continental crustal accretion through magmatism in the broad context of orogenesis from seafloor subduction to continental collision and to post-collisional processes.