江南造山带西南段梵净山地区镁铁质-超镁铁质岩:形成时代、地球化学特征与构造环境

摘要

The Neoproterozoic mafic-ultramafic rocks are widely developed in the Fanjingshan region, southwestern margin of the Jiangnan orogen, the rocks are all belong to tholeiitic series and including pillow lava, mafic-ultramafic sills and hypabyssal intrusive, gabbro. The pillow lavas are characterized by rich in strong incompatible elements such as Rb, Ba, Th and U as well as light rare earth elements ( LREE) , deplete in high-field-strong-elements ( HFSE) such as Nb and Ta, and lower εNd(t) values, different to the basalt formed in ocean ridge setting obviously, guess them formed by partial melting of enriched mantle in the tectonic setting of small ocean basin back arc-island arc related with subduction. The mafic-ultramafic sills are mainly composed of diabase and calc-pyroxene peridotite, there are abundant original calcite crystals in the ultramafic sills, indicating a extrusion (rift) tectonic setting. The gabbro may be a latest magmatic rock in the region, their SHRIMP zircon U-Pb age is 821 ±4Ma. The degrees of deplete in high-field-strong-elements and rich in light rare earth elements are depressing from pillow lava, mafic-ultramafic sills to gabbro, and the εNd(t) values from negative change to positive, show that the contribution of depleted mantle increase with time. Authors deduce that the gradation of Neoproterozoic magmatism in the Fanjingshan region is approximately as follow; pillow lava ( ~ 840Ma)→S-type granite ( ~ 838Ma) →calc-ultramafic sills→mafic sills→gabbro, the tectonic setting changed from subduction-collision to extended-rifting.%梵净山地区位于江南造山带的西南缘,这里新元古代的镁铁质-超镁铁质岩浆岩广泛发育,岩性包括枕状熔岩、超镁铁质-镁铁质岩床群以及浅成侵入的辉长岩,成分属拉斑玄武岩系列.其中枕状熔岩以富集轻稀土元素和Rb、Ba、Th、U等强不相容元素,亏损高场强元素Nb和Ta,低的εNd(t)值为特征,明显不同于洋脊玄武岩,推测其成因可能与富集型地幔的部分熔融有关,形成于与俯冲有关的弧后小洋盆环境.超镁铁质-镁铁质岩床群主要由辉绿岩和碳酸辉橄岩组成,其中超镁铁质岩床群中出现大量的原生碳酸盐矿物,指示它们形成于拉张(甚至裂谷)的构造环境.辉长岩可能是区内最晚形成的岩浆岩,其SHRIMP锆石U-Pb年龄为821±4Ma.由枕状熔岩经超镁铁质-镁铁质岩床群到辉长岩,高场强元素Nb和Ta的亏损程度减弱、轻稀土元素的富集程度降低、εNd(t)值由负值变为正值,指示随时间的由早到晚,来自亏损地幔的物质不断增加.推测梵净山地区新元古代岩浆作用的顺序大致为:枕状熔岩( ～ 840Ma)→白云母花岗岩(～838Ma)→碳酸超镁铁质岩床群→镁铁质岩床群→辉长岩( ～ 821Ma),构造环境由俯冲-碰撞到拉张-裂谷.