鄂东南铜山口、殷祖埃达克质(adakitic)侵入岩的地球化学特征对比:(拆沉)下地壳熔融与斑岩铜矿的成因

The geochemical comparison between the Tongshankou and Yinzu adakitic intrusive rocks in southeastern Hubei: (delaminated) lower crustal melting and the genesis of porphyry copper deposit.

鄂东南地区铜山口花岗闪长斑岩体是与斑岩铜钼矿床共生的岩体,但殷祖花岗闪长岩体是与金属成矿无关的岩体。铜山口和殷祖侵入岩的元素地球化学特征与埃达克岩的地球化学特征非常类似,如高Al2O3、Sr含量与La/Yb、Sr/Y比值,富Na2O(Na2O/K2O>1.0),亏损Y与Yb,极弱负Eu异常-正Eu异常以及正Sr异常等。但是铜山口和殷祖侵入岩也存在 明显的差别:前者比后者更偏酸性,但具有较高的K2O,MgO,Cr,Ni和Sr含量,较低的Y和Yb含量,轻重稀土元素分异更明显,并主要显示出正铕异常,区别于后者的极弱负Eu异常-不明显Eu异常。这表明铜山口埃达克质侵入岩的岩浆来源可能比殷祖埃达克质侵入岩的岩浆来源更深:前者可能由拆沉的下地壳熔融形成,残留物主要含石榴子石;而后者可能由增厚的下地壳熔融形成,残留物可能为石榴子石±斜长石±角闪石。另外,热的地幔上涌,底辟(diapir)进入下地壳,导致含角闪石的榴辉岩发生熔融也可形成铜山口埃达克质岩浆。铜山口埃达克质岩浆在穿过地幔的过程中,将会与地幔橄榄岩发生交换反应:一方面由于受橄榄岩的混染而使得岩浆的MgO,Cr和Ni增高;另一方面岩浆中的Fe2O3不断加入到地幔中,导致地幔的氧逸度(fo2)增高,地幔中金属硫化物被氧化并进入岩浆中。

In southeastern Hubei, the Tongshankou granodiorite porphyry pluton is closely associated with porphyry copper-molybdenum deposit, but the Yinzu granodiorite pluton is not related to metal mineralization. The Tongshankou and Yinzu intrusive rocks are geochemically similar to adakites, e. g, high Al2O3 and Sr contents and La/Yb and Sr/Y ratios, rich Na2O (Na2O/K2O > 1.0), depletion of Y and Yb, very weak negative Eu-positive Eu anomalies and positive Sr anomalies. However, they have some differences in geochemistry: the Tongshankou intrusive rocks are acider, and have higher K2O, MgO, Cr, Ni and Sr contents, lower Y and Yb contents, and more obvious differentiation degree between light and heavy rare earth elements than the Yinzu intrusive rocks. Moreover, the Tongshankou intrusive rocks mainly show positive Eu anomalies, but the Yinzu intrusive rocks have very weak negative Euno clear Eu anomalies. It is indicated that the magmas of the Tongshankou intrusive rocks were possibly derived from deeper source than that of the Yinzu intrusive rocks: the former likely originated from partial melting of delaminated lower crust, with main residual mineral garnet; but the latter likely resulted from partial melting of thickened lower crust, with residual garnet +/- plagioclase +/- hornblende. Alternatively, the Tongshankou adakitic magmas are also possibly originated from partial melting of the amphibole-bearing eclogitic lower crust owing to intruding diapirically of hot upwelling mantle into the lower crust. When the Tongshankou adakitic magmas went through the mantle, the interactions between the adakitic magmas and peridotites possibly took place: on one side, the MgO, Cr and Ni contents of the adakitic magmas were likely elevated due to the contamination of mantle peridotites; on the other side, the Fe2O3 from the adakitic magmas was likely added into the mantle and the oxygen fugacities (f(O2)) of the mantle were obviously elevated, which made metallic sulfide in the mantle oxidized and the chalcophile elements (e.g., Cu and Mo) enter into the adakitic magmas. The adakitic magmas enriched in Cu and Mo ascended and easily led to the formation of porphyry Cu-Mo deposit. But the Yinzu adakitic magmas derived by partial melting of thickened lower crust possibly had little metallogenetic materials as they had not interacted with mantle peridotite, and did impossibly favor metal mineralization.