蛇绿岩铬矿床的分布与成因及中国铬矿床的类型

Distribution and Genesis of Podiform Chromite Deposits in Ophiolites in the World and the Types of the Chromite Deposits in China

全球许多条蛇绿岩带按其构造背景和形成时期可以归属为４个蛇绿岩体系（匈奴、泛地中海、环太平洋和古冈瓦纳）和一条加里东巨蛇绿岩带。全球大多数豆荚状铬矿床表现为纬向分布规律。最有利于生成豆荚状铬矿床的构造部位是板块边缘俯冲条件下既有张裂又有挤压作用的地幔底辟体里。在消减带沿张裂部位底辟上侵、已经遭受部分熔融被提取了玄武岩浆的地幔岩难熔残留物，因压力骤减而重熔，便产生富铬的熔体。而后因两侧一阵阵挤压作用而使熔体更富铬，再通过熔离作用而生成铬矿浆与共轭的苦橄岩熔体。在适当构造部位，铬矿浆聚集和冷凝，便在超镁铁构造岩体中形成具瘤状构造特色的豆荚状铬铁矿脉。中国蛇绿岩带分布较广，但很少有发育完全的蛇绿岩剖面．在新疆、内蒙、西藏各地蛇绿岩带的超镁铁构造岩建造里，常见富铬型豆荚状铬矿作伴有纯橄岩脉和辉石岩脉，而富铝型豆荚状铬矿床常伴有橄长岩及辉长岩脉。后一种情况和新疆洪古勒楞蛇绿岩带的超镁铁堆积岩逢造中产出富铝型铬矿体，表明豆荚状铬矿床与层状铬矿床之间有过渡性成矿机制。在新疆北部有三条蛇绿岩带各自紧密伴随暗色岩的堆积岩建造；而宁夏小松山及四川新街暗色岩体产出堆积岩型铬铁矿体，显示出蛇绿岩与暗色岩之间有过渡性的成岩成矿条件。

The ophiolite belts in the world are assembled into four ophiolite systems and one megabelt:Hun, Pan-Mediterranean, Circum-pacific and palaeo-Gondwana system and the Caledonean. Most of podiform setting of generating podiform chromite deposit is the subduction zone of plates of of compressional as well astensional plate movement The refractory mantle diapirs with partial re-melting took place when the pressure reducedalong the tension faults in subduction zone and in produced chrome-riched melt in the process of thedlaplr ascending into lower crust.The ultrabaslc-basic melt could b substantially rich in chrome during thepulses of compression to the mantle in subduction zone.Then,the re-melted mantele diaplr produced a pair ofmelts: chromite ore melt and picritic melt, by immiscible liquation. Hence,it formed the Podiform chromite orebodies characterized with tubercule texture in ophiolitic ultramafic tectonite massifs.There are a lot of ophiolite belts in China. However, few of them display any complete ophiolite section.The Al-riched type podiform chromite ore bodies always accompany troctolite veins in ultramafic tectonics inXinjiang and inner Mongolia areas. The chromite ore viens of Al-riched type occur in the layering ultramaficcumulate formation of the Hongguleleng ophiolite belt in Xinjiang, also. Thus,it is suggested that there are somesorts of interim chromite ore between podiform chromite ore and the chromite ore of layering chromite deposit.The chromite ore babies of cumulate type occur in the Xinjie trap intrusion in Sichuan Province as well as in theXiaosongshan intrusion of trap suite in Ningxia Autonomous Region. And there are three tectonic belts with anophiolite belt accompanied closely with Cu-Ni sulfide ore-bearing cumulate intrusions of trap suite in NorthernXinjiang. Thus, it may be sorts of interim rock formation in between ophiolite and trap.