关键词: 冈底斯, 花岗岩, 晚三叠世&mdash, 早侏罗世, 构造背景, 新特提斯洋

Abstract: In order to discuss the genesis of Late Triassic to Early Jurassic magmatic rocks and its relationship with the early-stage
evolution of the Neo-Tethys in the southern margin of the Gangdese, we report petrography, geochronology and geochemistry of the
porphyritic granites from Tangbai in southern margin of Gangdese, Tibet. Zircon U-Pb dating yields an age of (190.37 ± 0.87) Ma
(MSWD=0.58) for the porphyritic granites, indicating that they were formed in Early Jurassic. Geochemically, the rocks are
characterized by high contents of SiO2 (75.20%~75.97%), Na2O (3.39%~4.12%) and ratios of Na2O/K2O (1.40~2.00) and low contents of
MgO (0.32% ~0.38% ), thus can be grouped into calc-alkaline I-type granites. The trace elements are characterized by enrichment of
LILEs (such as Rb, U and K) and LREEs and depletion of HFSEs (such as Nb, Ta and Ti) and HREEs, indicating that they were formed
in magmatic arc setting related to the northward subduction of the Neo-Tethys oceanic slab. Furthermore, the granites have low Mg#values (26.71~41.34, Mean=35.26), and with Nb/Ta ratios similar to the lower crust, indicating that the granites were mainly generated
from partial melting of the lower crust. Combined with previous research results, it is shown that the Late Triassic to Early Jurassic
magmatic rocks in the southern Gangdese were formed in magmatic arc related to the northward subduction of the Neo-Tethys oceanic
slab, and the initial age of northward subduction of the Neo-Tethys oceanic crust should prior to the crystallization age (190.37±0.87 Ma)
of the porphyritic granites reported in this paper.

Key words: gangdese, granite, Late Triassic to Early Jurassic, tectonic setting, Neo-Tethys