湖北枣阳大阜山镁铁/超镁铁杂岩体与金红石矿床成因

The Dafushan mafic/ultramafic complex and genesis of the related rutile ore deposit at Zaoyang,Hubei

湖北省北部枣阳市境内的大阜山镁铁/超镁铁杂岩体主要由未变质的纯橄岩和橄长岩组成，伴生变质的石榴石角闪石岩、石榴石钠黝帘石角闪石岩和角闪石钠黝帘石岩。杂岩体的围岩为大理岩。该岩体中的金红石矿床产于变质的石榴石角闪石岩或蚀变的石榴石角闪石岩中。橄长岩中锆石SHRIMPU-Pb同位素定年结果表明镁铁/超镁铁质岩体的成岩年龄约为600Ma。变质的岩石和未变质的岩石几乎具有完全相同的地球化学特征，表明前者的原岩应为橄长岩，整个岩体是一个分异的镁铁/超镁铁侵入体。金红石赋存于角闪石解理中等岩相特征表明矿床的含矿岩石就是石榴石角闪石岩，不是由石榴石辉石岩退变质形成的；且其中的石榴石为钙铁榴石，不是富镁、富铝的与高压/超高压作用有关的石榴石。金红石矿床的形成和岩体的局部变质应该与由北向南的近水平推覆构造有关，动力变质作用提供的热和流体，结合推覆过程中的变形作用，导致围岩大理岩和镁铁质岩体发生交代作用，形成石榴石角闪石岩和金红石矿床。

The Dafushan mafic/ultramafic complex at Zaoyang, northern Hubei, located to the south of the Qinling-Tongbai-Dabie orogen. The complex consisted dominantly of not only peridotite and troctolite,hut also garnet amphibolite,garnet saussurite amphibolite and amphibole saussuritite,and surounded by the marble. The hosting rutile ore doposit occurred within metamorphic garnet amphibolite or altered garnet amphibolites. The zircon U-Pb SHRIMP data from the troctolite showed the emplacement age about 600 Ma for the mafie/nltramafic intrusions. All these rocks shared almost the same geochemical characteristics for the unaltered mafic/uhrarrlafic intrusive rocks and the metamorphic rocks,which suggested the protoliths of the metamorphic garnet amphibolite being probably troctolite, and all the complex being a differentiated mafic/ nhramafic intrusive body. The rutile occurring within the intersections of two cleavages of am- phibole suggested that the primary ore-host rock should be garnet amphibolite rather than garnet pyroxenite, which was thought to be retrograded to later garnet amphibolite. The garnet was andradite rather than Mg and Al-rich one related to high-pressure metamorphism,suggesting none of involvement into the Indosinian ultrahigh pressure metamorphism. The formation of futile ore deposit and localized metamorphism for the mafic/ultramafic complex might be resulted from the horizontal thrusting southwards. The energy and fluid by the thrusting combined with the deformation of the mafic/ultramafic intrusive rocks caused the metaso,natic metamorphism between the intrusive body and its wall rock carbonate one, and formed garnet amphibolites and rutile ore deposit.