贺兰山北段晚三叠世沉积物源分析

贺兰山北段晚三叠世地层分布较广泛，其物源问题是进行鄂尔多斯盆地西北部原盆边界恢复和其两侧盆地该时期面貌恢复的关键，而对此存在一定的争议。该研究以贺兰山北段的沉积-构造背景为约束，通过晚三叠世地层的砾石成分、古流向、稀土元素、锆石测年及沉积趋势等物源分析方法综合运用，指出贺兰山北段晚三叠世不存在东部物源，银川古隆起不存在；物源来自西北部的阿拉善地块和兴蒙造山带太古代-古元古代的变质岩（片麻岩和变粒岩等）和岩浆岩，部分为阿拉善地块古生代和早中三叠世的沉积岩及岩浆岩。贺兰山西北部具有近物源和快速堆积的特点。同时，贺兰山北段晚三叠世物源具有多源性，其经历了多期的构造改造，恢复物源区演化模式对周邻造山带和块体的演化具有深远意义。

Analysis of Late Triassic Sedimentary Provenance in the North of Helan Mountain

Based on the sedimentary and tectonic background of the north of Helan Mountains,the paper discussed the features of the provenance through comprehensive analysis of the conglomerate component and gravel features,paleo-current direction,rare earth element,zircon dating and sedimentary trending.The conglomerate at the bottom of Upper Triassic mainly distributed in four different areas of Taerlin,Beisi,Shuimogou and Rujigou in Helan Mountain shows that the gravels were composed of quartzite,quartz sandstone,carbonate and granite. Paleocurrent data from various researchers reveal that the main current direction was northwest:wards which meant the river flowed from the northwest to the southeast in Triassic.And there was a successive paleo-current direction throughout the Triassic.The result is also closely consistent to the paleocurrent in Shigouyi and Ciyaobao area in the west margin of Ordos Basin and they all indicate that there was a common provenance from the northwest.The rare earth element in sandstone can also give certain clues of the provenance.Nineteen samples were chosen to detect the quantity of rare elements from the Upper Triassic,Pre-Cambrian and Cambrian to Middle Triassic.The paper also collects other rare element data of the gneiss and granulite of Helanshan Group in the northwest margin of Helan Mountain and of the granite formed during Hercynian to Indosinian period to the west of Helan Mountains.This pattern is almost the same as that in strata from Cambrian to Mid-Triassic and Pre-Cambrian as well as the granite formed between Hercynian and Indosinian period,which supports that the provenance is from the corresponding rocks formed earlier than Late Triassic.The age peaks of the U-Pb dating of zircon show the provenance of Late Triassic is related to the metamorphic rock in Archean and Paleo-proterozoic and also the magnetic rock during Hercynian and Indosinian period. Meanwhile,sedimentary environment analysis indicates that the gravels become smaller and sedimentary facies turns from alluvial fan,fluvial to lake from the west to the east of Helan Mountains.This proves that the Yinchuan Paleo-uplift did not appear in Late Triassic.Therefore the eastward provenance does not exist in Late Triassic of Helan Mountains. The paleo-current data,heavy minerals and zircon U-Pb dating in Shigouyi area of the west of Ordos Basin suggest a similar provenance with Helan Mountains in direction and rock feature in Late Triassic.The fission track analysis shows that the uplift age of the whole Helan Mountains is later than Late Triassic.Two zircon fission track ages are 252Ma and 253Ma,older than the strata(Mid-Jurassic and Late Triassic) from which they were chosen,which might propose that the provenance area had gone through the structure and thermal event and had been uplift in the late of Permian to supply major sedimentary source for Mesozoic. It is concluded that there is no east provenance and main northwest provenance in the north part of Helan Mountains.The provenance is predominant the metamorphic rocks(such as Helan Group and Alxa Group) and magmatic rocks from Alxa block and Xing’anling Mongolian Orogenic Belt in Archean and Paleoproterozoic.Several sedimentary and magmatic rocks rolled into the old orogenic belt partly provide the Late Triassic provenance.Meanwhile there are multiple provenances with various tectonic reformations and restoration of the model of the provenance evolution is significant to the evolution of the periphery orogenic belt and blocks.