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

新疆乌鲁木齐东部野生动物园附近的下二叠统下部发育了一套以枕状玄武岩为代表的火山岩系。该玄武岩在公路上发育有两层,上部一层较厚,可达7～8m,下部一层2～3m。岩枕近圆形、肾状、枕状、条带状或蠕虫状等,多呈顶突底凹。岩枕长轴平行排列,长轴基本顺层面分布;有的岩枕中含大量海百合茎、珊瑚及腕足等生物化石。在TAS图中大部分样品位于玄武粗安岩,而在K_2O-SiO_2图上主要落在低K拉斑玄武岩区间。MnOTiO_2-P_2O_5图解显示以岛弧为主,常量元素的特征总体上更接近岛弧玄武岩。在Hf/3-Th-Ta图解上也以岛弧为主;在Zr/4-2Nb-Y图解显示以火山弧为主,常量元素的特点总体上更接近岛弧玄武岩。微量元素原始地幔标准化蛛网图表明为同源岩浆产物,具高度相似的演化过程,多种微量元素判别图揭示岛弧-弧后盆地环境;稀土总量明显较低,在稀土元素球粒陨石标准化图解上,其稀土分布曲线一致性较好,呈轻稀土富集右倾型,Eu为负异常,揭示了乌东玄武岩岩浆有一定分异。稀土元素配分曲线与弧后盆地玄武岩(BABB)具有很好的一致性。U-Pb和谐年龄为283±8Ma,结合地层及古生物资料推测乌东玄武岩喷发的时间为早二叠世早期。乌东枕状玄武岩-灰岩之下有一套(磨拉石)底砾岩,初定为石炭-二叠系的界限。底砾岩之下为下石炭统的中厚层灰岩,具有明显的喀斯特化,为不整合接触,揭示在两者之间发生了造山作用。通过对新疆乌东早二叠世早期的枕状玄武岩的地质特征、地球化学特征、形成环境和时代的研究,表明乌东一带早二叠世主要为一个岛弧和弧后盆地环境,进一步揭示了北天山北缘石炭-二叠世碰撞闭合造山之后又发生了松弛扩张形成了弧后盆地,海水再度大规模入侵。最终的闭合碰撞造山的时间最早可能在晚二叠世。由于乌东枕状玄武岩在喷出地表后受到了生物化石和陆源碎屑的污染,再加上侵入过程中地壳的污染,使其具有非常复杂的地球化学特点和多解性。乌东玄武岩的研究对于了解博格达山、甚至天山中段在晚古生代的构造沉积演化及造山作用具有重要意义,同时对准噶尔盆地、吐哈盆地及三塘湖盆地油气资源的形成与分布具有重要意义。

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±8 Ma 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 basalts 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.