北秦岭松树沟橄榄岩与铬铁矿矿床的成因关系

Genetic relationship between peridotites and chromite deposit from Songshugou area of North Qinling

松树沟橄榄岩体是秦岭造山带中规模最大的赋存铬铁矿床的超基性岩体。松树沟橄榄岩主要由细粒橄榄岩质糜棱岩和中粗粒橄榄岩组成。本文通过对松树沟橄榄岩的岩相学、主微量、稀土元素地球化学的系统研究,认为松树沟细粒方辉橄榄岩为洋脊扩张过程中地幔岩减压-近分离熔融产生的残留体,细粒纯橄岩主要由地幔橄榄岩熔融残留橄榄石、消耗辉石的减压熔融反应:aCpx+bOpx+cSpl=dOl+1Melt生成的橄榄石和少量的地幔方辉橄榄岩残留体组成,但均受到了后期渗滤熔体的再富集作用;中粗粒纯橄岩和方辉橄榄岩主要为上述反应产生的渗滤熔体被圈闭在迁移通道或减压扩容带内在热边界层(TBL)通过反应:MeltA=Ol+MeltB冷凝结晶而成,属堆晶橄榄岩。Pb-Sr-Nd同位素地球化学的证据显示,松树沟橄榄岩与基性岩具有共同的地幔源区,二者同为松树沟蛇绿岩的重要组成部分。通过矿床地质特征及铬铁矿电子探针测试研究,认为松树沟铬铁矿床是产于中粗粒堆晶纯橄岩中的层状铬铁矿床,形成于格林威尔期松树沟洋盆的扩张过程中,是中粗粒纯橄岩在热边界层(TBL)的冷凝结晶过程中岩浆分异作用的产物。

The Songshugou peridotite pluton hosting chromite deposite is the largest ultramafic massif occurred in Qinling Orogenic Belt.The Songshugou peridotites mainly comprise fine-grained peridotitic mylonite and medium-grained peridotite.According to combined researches of major element,trace element,and REE geochemistry characteristics,as well as petrography for the Songshugou peridotites,we propose that the fine-grained harzburgites is the residue produced by decompression-near fractional melting of mantle peridotite during ridge spreading,the fine-grained dunite is mainly composed of olivine residue left by mantle peridotite melting,olivine produced by the reaction:aCpx + bOpx + cSpl = dOl + 1Melt,and minor harzburgite residue,furthermore,both of the fine-grained dunite and harzburgite have undergone post-melting refertilization;medium-grained dunite and harzburgite are cumulate peridotite,producing by crystallization of porous melts trapped in the migration path or in decompression-dilatant zone of the thermal boundary layer(TBL) through the following reaction:Melt A = Ol + Melt B.Pb-Sr-Nd isotopic evidences suggest that,the Songshugou peridotites and mafic rocks share the common mantle source,and both of them are segments of the Songshugou ophiolite.On the basis of geological features and electron probe study on chromites,we conclude that,formed during spreading process of the Grenvillian Songshugou Ocean,the Songshugou chromite deposit is a stratiform chromite deposit hosted in the medium-grained cumulate dunite,and it is the product of magmatic crystallization and differentiation during the formation of medium-grained dunite in the TBL.