地质学报2018，Vol.92Issue(4)：638-664,27.
新疆乌鲁木齐东部早二叠世枕状玄武岩火山岩系的发现及大地构造意义
The Discovery of Early Permian Pillow Basalts in Eastern Urumqi,Xinjiang, and Its Tectonic Significance
摘要
Abstract
One series of volcanic rocks,represented by pillow basalts,was found to occur in the lower Permian strata nearby the Wild Zoo Park,about 25 km east of Urumqi,Xinjiang.Two layers of the pillow basalts crop out along the road,with the thickness of 7～8m to the top layer and 2～3m to the bottom layer.The basalts occur as kidney,round,pillow,banded,wormlike or massive with the convex top and concave bottom.Long axis of basalt pillows are parallel and distributed along the strata surfaces.Some pillows trap a large amount of fossils,such as sea lily stems,coral and brachiopod.TAS diagram shows that most of the basalts belong to trachybasalt,while FAM diagram suggests that they belong to calcium alkaline basalt.K2O-SiO2 diagram indicates that basalts are the low-K tholeiite to calcium alkaline basalt characteristics.The distribution of major elements is similar to that of island arc basalt.The trace elements distribution diagram indicates their origin of comagma and is of a highly similar evolution history.The trace elements determination diagram also shows an island-arc to back-arc basin environment.Low total REEs amount,similar REE distribution,slight LREE enrichment and weak Eu positive anomaly reveal that the basaltic magma in eastern Urumqi experienced differentiation to some extent.REE distribution pattern of the basalt is similar to that of back-arc basin basalt.The 283±8Ma of U-Pb age,along with stratigraphic and paleontological data,suggests that the basalt formed in the early Early Permian.Below the pillow basalt-limestone is a series of basal conglomerates,which was determined to be the boundary of Carboniferous and Permian.The medium to thick Early Carboniferous limestone occurring under the boundary has apparent karstification and unconformable contact,indicating the epeirogeny in the Late carboniferous but no orogeny.Geological and geochemical features of pillow hasalts in eastern Urmuqi and their formation environment and time all indicate that the east Urmuqi region was an island-arc and back-arc basin.Our study further confirms that the back-arc basin formed from closure-orogeny of northern margin of North Tianshan Mountain during Carboniferous-Permian collision and subsequent relaxation expansion,accompanied by large-scale intrusion of sea water.Finally closure time of collisional orogeny occurred likely in Late Permian.Due to the contamination of biofossils and terrestrial clasts,along with the addition of crust during its intrusion,the pillow basalts in eastern Urumqi,after its eruption,have complex geochemical features and multiple explanations.The study of the pillow basalt in eastern Urumqi is of great importance not only for understanding the tectonic sedimentary evolution and orogen in Bogda Shan,and even middle Tianshan Mountain during Late Mesozoic,but also for understanding formation and distribution of oil-gas resources in Junggar basin,Turpan-Hami basin and Santanghu basin.关键词
枕状玄武岩/岛弧/弧后盆地/早二叠世/化石/造山作用/乌东Key words
Pillow basalts/Island Arc/283Ma/Earlier Permian/Fossils/Eastern Urumqi/Orogenesis引用本文复制引用
钟建华,葛毓柱,陈彬,罗可,黄雷桐,曲俊利,范莉红,王韶洁,王雅琳,倪良田,邵珠福,孙宁亮,李旭平,刘选,郝兵,刘闯,杨冠群..新疆乌鲁木齐东部早二叠世枕状玄武岩火山岩系的发现及大地构造意义[J].地质学报,2018,92(4):638-664,27.基金项目
本文为国家自然科学基金项目(编号41572088)、国家973(编号2011CB20001)、国家重大科技专项(编号2111XZ05014-002-002)联合资助成果. （编号41572088）
