华北克拉通木吉村斑岩铜矿含矿斑岩成因及其地质意义

Petrogenesis of ore-bearing porphyry of a porphyry Cu deposit at Mujicun in the North China Craton and its geological implications

国内外学者对发育于弧环境和碰撞造山环境的斑岩铜矿成岩机制已取得了深刻的理解,但对产出在克拉通内部的斑岩铜矿成岩机制在认识上还有所欠缺。本文报道了侵位于144Ma木吉村斑岩铜矿内的含矿闪长玢岩的岩石地球化学资料,以揭示木吉村斑岩铜矿的含矿斑岩成因。岩石地球化学资料显示:含矿闪长玢岩具有较低的(^(87)Sr/^(86)Sr)i值(0.70599~70690)和εNd(t)值(-19.4~-14.9),富集轻稀土元素(LREEs)和大离子亲石元素(LILEs),相对亏损元素Y和Yb。木吉村含矿斑岩与王安镇杂岩体镁铁质岩(辉石岩、角闪石岩、辉长岩、辉长闪长岩、安山岩)具有相似的微量元素配分模式、Sr-Nd同位素组成及亲EMⅠ地幔特征,这些岩浆岩呈现持续性的成份演化趋势,反映出它们的同源性。木吉村含矿斑岩及相关的镁铁质岩石具有相对较低的(^(87)Sr/^(86)Sr)i值(0.70564~706897)和εNd(t)值(-19.4~-13.8),显著区别于中上地壳同位素区域。含矿斑岩起源于伸展背景下富集地幔部分熔融形成的玄武质岩浆,其上升过程中遭受了一定程度的下地壳混染。玄武质岩浆在深部岩浆房发生堆晶作用形成具典型堆晶结构的角闪石岩等基性岩,随后在浅部岩浆房经历两阶段辉长岩-辉长闪长岩、安山岩-闪长玢岩系列分离结晶作用形成含矿斑岩。此过程可能是形成华北克拉通木吉村斑岩铜矿含矿斑岩的机制和动力诱因。

Although,scholars have obtained a deep understanding of the petrogenesis of porphyry copper deposits developed in arc orogenic and collision orogenic environments,their origins in intracontinental environments remain unclear.This paper reports the geochemical data of ore-bearing dioritic porphyry,which was intruded at 144Ma in the Wanganzhen complex from the northern Taihang Mountains in the North China Craton,with the aim of revealing the petrogenesis of ore-bearing porphyry.The geochemical data shows that the porphyries have moderately low initial(^(87)Sr/^(86)Sr)i(0.70599~0.70690),relatively lowεNd(t)values(-19.4~-14.9).They are enriched in light rare earth elements(LREEs)and large ion lithophile elements(LILEs),and are relatively depleted in Y and Yb.The Mujicun ore-bearing porphyry and the mafic rocks(pyroxenite,hornblende,gabbro,gabbro diorite,andesite)of the Wanganzhen complex have similar trace element distribution patterns,Sr-Nd isotope composition and EMⅠ-like mantle characteristics,those magmatic rocks present a continuous compositional evolution trend,reflecting the consanguinity of these rocks.Mujicun ore-bearing porphyry and related mafic rocks have relatively low(^(87)Sr/^(86)Sr)i(0.70564~70690)andεNd(t)value(-19.4~-13.8),which is significantly different from the isotopic region of the middle and upper crust.It originated from basaltic magma formed by partial melting of the enriched mantle under the extensional background,and suffered a certain degree of lower crust contamination during the rising process.The mafic rocks such as hornblendite with typical cumulate structure were formed by cumulation in the deep magma chamber,and then the ore-bearing porphyry was formed by two-stage fractional crystallization of the gabbro-gabbro diorite and andesite-dioritic porphyrite series in the shallow chamber,which may be the mechanism and dynamic inducement for the formation of the ore-bearing porphyry of the Mujicun porphyry copper deposit in the North China Craton.