贺兰山地区中晚三叠世物源分析及其时空演化过程*

系统分析物源的时空演化过程,必将为判断当时的盆地构造背景和盆地属性提供佐证。文中运用重矿物分析方法,结合砾石岩性组成、地层厚度变化及沉积相分布规律,对贺兰山地区中晚三叠世沉积物源进行系统分析。结果表明: 贺兰山地区中晚三叠世沉积物具有多源、混源以及上下层段时空演化差异明显的特点。中三叠世纸坊期物源主要来自盆地西北部和西部;晚三叠世延长期T3y1、T3y2段沉积时期,物源主要来自盆地西北部、西部、南部;T3y3至T3y5段沉积时期,盆地东部物源开始出现,并分布于贺兰山汝箕沟沟口至鬼头沟一线,此时汝箕沟—香棒子沟一带是沉积混源中心。西部物源主要为阿拉善地块之上的沉积地层,但西部南段受南部物源控制,与西部北段的物源岩性有较明显不同;西北部物源主要是孔兹岩带、前寒武纪变质岩及古生代岩浆岩;东部地区物源可能为现今银川地堑及其东部的沉积地层。贺兰山地区中晚三叠世物源时空演化的转折期,主要在纸坊期至延长期T3y1段沉积时期之间和延长期T3y2段至T3y3段沉积时期之间,2次明显的物源演化反映了贺兰山盆地构造动力学机制的重大转变。

Provenances of the Middle-Late Triassic in the Helan Mountain and its temporal-spatial evolution

Combined with conglomerate composition,strata thickness and sedimentary facies,heavy mineral compositions were firstly used to study the provenances of the Middle-Late Triassic in the Helan Mountain. Results showed that the provenance of the Middle-Late Triassic sediments in the Helan Mountain were characterized by multiple and mixed sources,and were varied greatly. Sediments of the Middle Triassic Zhifang Formation were mainly from northwest and west. However,sediments of the Late Triassic Yanchang Formation changed. During deposition of the lower two members T3y1 and T3y2,sediments were mainly derived from the northwest,west and south. During deposition of the upper three members T3y3 to T3y5,eastern sources appeared,which were distributed in the Ruqigou to Xiangbangzigou. The western provenance was mainly in the sedimentary strata on the Alxa Block. Sediments of northwestern part were mainly from the Precambrian metamorphic rocks and Paleozoic magmatic rocks in the Paleoproterozoic Khondalite Belt,which was different from the southwestern part. Sediments in its eastern part were sourced from the sedimentary strata of Yinchuan graben and areas to its east. Provenance transformation of the Middle-Late Triassic strata in the Helan basin occurred twice: One from the Zhifang to Yanchang formations T3y1,and the other from the T3y2 to T3y3.These two prominent transitions suggested significant transformation of the dynamic mechanism in the Helan basin.