马达加斯加Bemavo铬铁矿区基性岩中锆石的U-Pb年龄、Hf同位素特征及其地质意义

摘要

There are more than one hundred metal deposits (chromium,nickle,iron,gold,etc.) distributed in the three greenstone belts (Maevatanana,Andriamena and Beforona) of central Madagascar.In this paper,we report LA-MC-ICP-MS zircon U-Pb ages and Lu-Hf isotopic compositions of the mafic igneous rocks from the Bemavo chromite deposit in Andriamena greenstone belt of north-center Madagascar.The age of the strong weathered chromite basalt (BM-Pbl) is 786±11 Ma,while the age of the chromite websterite (BM-2) is 818±11 Ma.The εHf(t) values of the newly produced zircons show that their magma sources were probably from the ancient enriched lithospheric mantle.And the εHf (t) values of xenocrystic zircons suggest that 3291±409 Ma is a major period of crustal growth in Madagascar,which may correspond to the Neoarchean "super accretion event" of Dharwar Craton.%马达加斯加中部Maevatanana、Andriamena和Beforona三条绿岩带共有上百处铬、镍、铁、金等金属矿点.本文对位于马达加斯加中北部Andriamena绿岩带的Bemavo铬铁矿区基性岩进行了锆石LA-MC-ICP-MS U-Pb定年及Lu-Hf同位素分析.强风化铬铁矿化基性岩(BM-Pb1)形成于(786±11) Ma,含铬铁矿二辉岩(BM-2)形成于(818±11) Ma.样品中新生锆石的εHf (t)值暗示母岩浆很可能起源于古老的富集岩石圈地幔,而捕获锆石的二阶段Hf模式年龄暗示马达加斯加在(3 291±409) Ma经历了重要的地壳生长事件,与印度Dharwar克拉通的地壳演化历史非常类似.