松潘-甘孜地块三叠系砂岩的地球化学特征及其意义

Geochemical characteristics and significance of Triassic sandstones of Songpan-Ganze block.

松潘-甘孜地块位居中国西南部,北邻华北地块,西与青藏高原毗邻,东南缘与扬子地块相连。该区出露的巨厚层砂岩SiO_2含量变化范围大为48．64%～71．77%,稳定元素(Al_2O_3,Fe_2O_3,MnO)与不稳定元素(MgO,K_2O,Na_2O)基本持平,CIW值较低;La/Co值集中于3．0～4．5,Th/Co集中于1．0～1．5,La/Th值集中于2．7～3．1,Th/U值均大于4．0,轻稀土元素含量大于重稀土元素含量,具Eu负异常,稀土元素分布形态与上地壳一致;锆石的U-Pb年龄集中分布于1500～1900Ma、700～900Ma、200～400Ma,与扬子地块、南秦岭的岩浆活动相一致。分析表明松潘-甘孜地块具有稳定的物源区,主要以扬子地块为主;其碎屑母岩应主要源自上地壳,以长英质成分为主。松潘-甘孜地块在三叠纪时期处于大陆岛弧环境,周边地区基本处于稳定状态,没有大规模构造运动和岩浆活动,扬子地块向华北地块的俯冲明显减弱或可能已经停止,秦岭造山运动基本已经完成。

The Songpan-Ganze block is located in southwest of China. It is joined with the North China block, Tibet Plateau and Yangtze block in north, west and southeast. For the outcrop in the area, the content of SiO2 varies largely from 48.64% to 71.77%, the content of steady elements（Al2O3, Fe2O3, MnO） is equal to that of unsteady elements （MgO, K2O, Na2O） ,the value of CIW is very low; the value of La/Co concentrates in 3.0 - 4.5, Th/Co 1.0 - 1.5, La/Th 2.7 - 3.1, Th/U 〉 4.0, ∑ LREE 〉 ∑ HREE, Eu has negative abnormity, REE pattern is similar to upper crust. The U-Pb ages of zircon concentrate in 1500 - 1900Ma, 700 -900Ma, 200 400Ma, consistent with magmatism activities in Yangtze block and southern Qinling. Our study demonstrates the Songpan-Ganze block had a steady origin mainly Yangtze block. Its composition of detrital mineral was felsic mainly from upper crust. The block had stayed in continent island arc setting in Triassic. Its adjacent blocks were steady no tectonic movements, no magma events. The subduction of Yangtze block towards Northeastern China block became weak or stopped possibly, the movement of Qinling orogen had completed.