峨眉山大火成岩省岩石成因与空间差异性研究——基于全区高Ti玄武岩地球化学数据分析与模拟

A study of the petrogenesis and spatial difference of the Emeishan large igneous province:Based on geochemical analysis and simulation of the high Ti basalts in the whole region

高Ti玄武岩成因是峨眉山大火成岩省(ELIP)研究的热点问题。由于高Ti玄武岩地球化学特征在空间上存在差异,其岩石成因尚未达成共识。本文系统收集了峨眉山大火成岩省中高Ti玄武岩地球化学数据以及锆石ID-TIMS U-Pb测年结果,并进行统一处理分析与模拟。研究结果显示,峨眉山大火成岩省形成于约259-258 Ma,高Ti玄武岩在大火成岩省全区均有出露。自西向东,岩石年龄无明显变化规律,厚度逐渐变薄。高Ti玄武岩起源于具有富集地幔特征的地幔柱源区,几乎没有遭受地壳混染,经历了低程度部分熔融作用并可能混入了少量岩石圈地幔物质,发生了以单斜辉石为主的分离结晶作用。峨眉山大火成岩省深部存在一个非对称式的地幔柱,自西向东,高Ti玄武质岩浆起源深度变浅、温度降低,熔融深度和压力随之降低,熔融程度相对增大。模拟表明,源区石榴石相和尖晶石相的熔融程度分别为0.5%-2%和5%,石榴石相熔融比例自西向东由90%减小至40%,而尖晶石相熔融比例由10%增大至60%。

The petrogenesis of high Ti basalts is a hot issue in the study of the Emeishan large igneous province( ELIP). Due to the spatial differences in geochemical characteristics of high Ti basalts, there is no consensus on the petrogenesis. In this paper, the authors systematically collected geochemical data of high Ti basalts and CA-IDTIMS U-Pb dating results of zircons in the ELIP, respectively, and subsequently carried out integrated data processing, analysis and simulation. The research results show that the ELIP was formed in 259 ~ 258 Ma and the Emeishan high Ti basalts are distributed all over the ELIP. From west to east, the age of rocks shows no remarkable change, and the distribution of rocks changes from thick to thin. The high Ti basalts with little crustal contamination originated from mantle plume with the characteristics of enriched mantle. They experienced a low degree of partial melting with the possibility of the mixture of some lithospheric mantle components, and fractional crystallization was dominated by clinopyroxene. There was an asymmetric mantle plume at the depth of the ELIP. From west to east,the original depth and temperature of high Ti basaltic magma decreased, the melting depth and pressure decreased,resulting in the increase in melting degree. Simulation shows that the melting degree of garnet phase and spinel phase in the source area is 0.5% ~ 2% and 5% respectively, the melting ratio of garnet phase decreases from 90%to 40% and that of spinel phase increases from 10% to 60% from west to east.