塔里木盆地库鲁克塔格地区二叠纪末-中三叠世基性岩床的发现及其地质意义

Late Permian-Middle Triassic mafic sills from Kuruktag,Tarim basin:Petrochemical characteristics and geological implication

新疆塔里木盆地东北缘库鲁克塔格地区的中寒武统莫合尔山组沉积地层中发现三条形成于二叠纪末到中三叠世的基性岩床。三条岩床沿沉积岩层面顺层侵入,按照从下到上的顺序,分别为辉绿岩床(厚3m)、辉长岩床(厚20m)和粗玄岩床(厚2.5m)。三条岩床的地球化学特征相似,稀土含量高(∑REE=210×10^(-6)～297×10^(-6)),轻稀土元素富集((La/Yb)_N=13.54～17.96),Eu负异常不明显(δEu=0.97～1.21),Ce稍具正异常(δCe=1.11～1.25)。大离子亲石元素(Rb、Ba、Th、Sr)富集,Sr具明显正异常;Ta、Nb和Hf具负异常。低氧化度(0.13～0.40)。三条岩床为同源岩浆活动的产物,岩浆源于上地幔或下地壳,但有下陆壳成分混染。三个样品的MgO含量在4%～6%之间,Mg~#值在37.6～55.6之间,固结指数SI值位于18.7～29.3之间,反映原始的岩浆经历了比较明显的分异作用。粗玄岩床K-Ar年龄为255.2±7.3Ma,辉绿岩床K-Ar年龄为242.8±5.7Ma,辉长岩床Ar-Ar年龄为229.7±3.7Ma。本次岩浆侵入事件发生在南天山洋闭合之后,构造背景为造山后碰撞伸展环境。

In Mohoershan Formation(ε_2m),Middle Cambrian,3 mafic sills formed during Late Permian to Middle Triassic have been detailed lithologically logged and sampled for geological dating and geochemical analysis in Kuruktag,Tarim basin,Xinjiang,NW China.The mafic sills were composed of lower diabase sill(with thickness of 3m),middle gabbro sill(with thickness of 20m) and upper dolerite sill(with thickness of 2.5m) which intruded into Middle Cambrian strata along the interface of neighboring sedimentary beds.The geochemical characteristics of these sills show great similarities,for instance,the high absolute concentration of total rare earth element(ΣREE = 210×10^(-6)-297×10^(-6)),the enrichment of LREE((La/Yb) _N = 13.54-17.96),weak Eu anomaly(δEu =0.97-1.21),weak positive Ce anomaly(δCe = 1.11-1.25),enrichment of large ion litbophile element(LILE),significant positive anomaly of Sr and negative anomaly for Ta,Nb and Hf.Taking into account the geochemical features as well as the geological settings,these sills derived from same magma source.The magma was formed within the upper mantle and partially contaminated by the lower continental crust according to the standard mineral forming pressure,the REE pattern and trace earth element spider plot.The low MgO content(4%-6%),Mg~#(37.6-55.6) and SI(18.7-29.3) hinted the primary magma had undergone differentiation. The K-Ar age of dolerite sill is about 255.2±7.3Ma,the K-Ar age of diabase sill is about 242.8±5.7Ma and the Ar-Ar age of gabbro sill is about 229.7±3.7Ma.The geological dating results indicated that the magma intrusion took place after the close of south Tianshan ocean,the paleo-teetonic environment is extensional settings of post-collision.