川北阿坝地区三叠系粘土矿物特征及地质意义

分析了川北阿坝地区三叠系泥质岩的岩石矿物组合、粘土矿物类型、伊利石结晶度和镜质体反射率特征,并以此划分成岩演化阶段及变质程度。以成岩作用—极低级变质作用的研究方法,明确了阿坝地区成岩作用及极低级变质作用的上、下界限及区域分布规律:以唐克为中心的地区处于成岩作用晚期阶段,成岩作用温度200～212℃,伊利石结晶度IC≥0.42;周围地区处于极低级变质作用阶段,成岩作用温度212～222℃,伊利石结晶度IC<0.42。镜质体反射率(Ro)也具有同样的演化规律。对该地区域上成岩演化的差异及原因作如下解释:由于构造活动的远近程效应存在差异,该地区的周缘明显受到构造作用的影响,因叠加动力变质而达到极低级变质作用阶段;唐克位于阿坝地块中心,受构造作用的影响相对较小,因而处于成岩作用晚期阶段。

CHARACTERISTICS OF THE TRIASSIC CLAY MINERALS IN ABA AREA, NORTHERN SICHUAN, AND ITS GEOLOGICAL IMPLICATIONS

The Triassic is extensively distributed in Aba area,northern Sichuan,where is located in the eastern part of the Qinghai-Xizang Plateau.The Triassic diagenesis and diagenesis evolution stages were critical for oil and gas exploration in the region.Based on analyses of the clay mineralogy,diagenetic-metamorphic mineral assemblages,illite crystallinity and vitrinite reflectance(Ro),with the indicatrixes comprehensively discussed for discriminating diagenesis and very low-grade metamorphism,different diagenesis stages were recognized,and the main conclusions were as follows.The Triassic at central part near Tangke was still at later diagenetic stage with diagenetic temperature being 200～212 ℃,illite crystallinity(IC)≥0.42(Δ2θ);The Triassic in the periphery areas was at very low-grade metamorphic stage with diagenetic temperature being 212～222 ℃,illite crystallinity(IC)<0.42(Δ2θ).The vitrinite reflectance(Ro) had same evolutional characters.The boundaries between different diagenetic stages were proposed.The differential diagenesis and its evolution in the northern Sichuan area could be reasonably explained by long-range effect of tectonism.The structural movement exerted lesser effect at the central part and stronger effect on the periphery of Aba area,so that the very low-grade metamorphism occurred only along the marginal areas.