冀东孤山子超基性岩体矿物岩石地球化学及Sr-Nd-Pb同位素特征

Mineralogy,Petrology,Geochemistry and Sr-Nd-Pb Isotope Systematics Characteristics of Gushanzi Ultrabasic Rocks in Eastern Heibei Province

孤山子超基性岩体位于华北克拉通北缘,主要由角闪石岩、辉长岩及辉石岩组成。本文通过角闪石、辉石、尖晶石的探针分析,角闪石岩、辉长岩的岩石地球化学及Sr-Nd-Pb同位素测试分析,研究孤山子超基性岩体地质地球化学特征,探讨其形成的构造环境。电子探针结果表明,角闪石属韭闪石亚族,具幔源特性;辉石属透辉石亚族,具弧堆晶岩特点;尖晶石属富Al尖晶石亚族,大量存在于辉石岩中。岩石地球化学及Sr-Nd同位素特征显示,孤山子超基性岩原岩浆几乎不受壳源物质的影响。低Mg#(0.44~0.55)及较高LREE/HREE(6.54~8.34),表明原岩浆分异程度较高。同时,全岩富集LREE、Rb、Sr、Ba等大离子亲石元素,亏损HREE、Cr、Ni、Nb、Ta、Zr、Hf等高场强元素,Sr、Nd同位素比值小,(87Sr/86Sr)ⅰ=0.70398~0.70430,εNd(t)=-1.60^-2.72,指示源岩为富集型地幔。孤山子超基性岩在晚古生代侵位于陆弧以南的沉积盆地中,地球化学特征介于弧堆晶岩和大陆裂谷岩浆岩之间。结合其大地构造背景及地球化学特征来看,孤山子超基性岩体应形成于陆缘弧后方局部伸展环境。

Gushanzi ultralbasic rocks locate in the northern margin of North China Craton and predominantly comprise hornblendite,gabbro and pyroxenite.Hornblende,pyroxene and spinel were chosen for electron probe analyses,while hornblendite and gabbro were chosen for major element,trace element and Sr-Nd-Pb isotope studies.This study uncovers the geological and geochemical characteristics of the plutons,as well as the tectonic setting.Based on the data of electron probe,subdivisible mineral species and geochemical characteristics of these minerals were confirmed.The amphiboles should be named as pargasites,which display mantle-derived characteristics.For the clinopyroxenes,all the plots are dropped in diopsides＇ area and present arc magmatic affinity.The spinels belong to aluminous spinel subtribe and they only cumulate massively in pyroxenite.Petrogeochemistry and Sr-Nd isotopes studies of Gushanzi ultrabasic rocks suggest the primitive magma assimilated merely a small portion of crust materials.The low value of Mg#（0.44～0.55）and the relative high LREE/HREE ratios（6.54～8.34）suggest that there was a relative high degree of magmatic differentiation.According to the geochemical data,i.e.,a high content of LILE（LREE,Rb,Sr,Ba et al.）,a small quantity of HFSE（HREE,Cr,Ni,Nb,Ta,Zr,Hf et al.）,along with a low value of Sr-Nd isotope ratios[（87Sr/86Sr）ⅰ=0.70398～0.70430,εNd（t）=-1.60～-2.72],this study demonstrates that Gushanzi ultrabasic rocks originated from the enriched mantle.Gushanzi ultrabasic magma intruded in the late Paleozoic sedimentary basin,which was located in the south of continental marginal arc with geochemistry characteristics of both Arc cumulates and continental rift magmatic rocks found in the intrusions.According to the geotectonic background and geochemistry characteristics,we suggest Gushanzi ultrabasic rocks formed in a partial extensional environment in the back of continental marginal arc.