江南地背斜东南缘晚元古宙岛弧型火山岩及其成矿远景

AN ISLAND ARC VOLCANIC BELT OF LATE PROTEROZOIC ALONG THE SOUTHEASTERN MARGIN OF JIANGNAN GEOANTICLINE AND ITS METALLOGENETIC PROSPECT

江南地背斜东南缘分布了许多晚前寒武纪火山岩系,其时代为晚元古宙,在空间上构成了一个完整的火山岩带。形成于岛弧环境,与古洋壳的俯冲及部分熔融有关。成矿特征与现代岛弧类似,在早期喷发的细碧角斑岩中赋存着重要的层状铜矿,并有金矿化。

In the southeastern margin of Jiangnan geoanticline, occurs a lot of volcanic rocks of late Pre- cambrian, e. g., Shuangxiwu Group in north Zhejiang, Dengshan Group in northeast JiangXi andJingtan Group in south Anhui. It is demonstrated by up-to-date stratigraphic and chronological datathat this volcanic sequence is of late Proterozoic in age and is stratigraphically comparable with Qing-baiko Group in north China. The sequence distributes along the southeast margin of Jiangnan geoanti-cline continuously and forms an integrate volcanic belt. In the late Proterozoic volcanic-sedimentary sequence of this area, there is a variation tendencyof its petrological assemblage from the lower part towards the upper one. Take Dengshan Group as anexample. The lower part of the Group is composed mainly of turbidite volcaniclastic flyschgraywacke, associated with basalt, spilite and keratophyre, and changes into tuffite and volcaniclasticrocks of shallow-sea facies; the upper part of the Group is predominantly consisted of andesite anddacite of terrestrial facies. Similar Petrological assemblage has been found in Shuangxiwu Group innorth Zhejiang, but the different assemblage exposed in different districts. The petrological assemblagefeatures of this sequence are comparable with the volcanic-sedimentary formation found in presentplate collision boundary. The spilite-keratophvie and flysch formation in the lower part of the se-quence is usually found in underwater primary arc, but the intermediate-acid volcanic formation in theupper part shows that it is a mature island arc in the late stage of its evolution. The petrochemicalcharacter, trace element composition and the distribution pattern of REE of the volcanic rocks alsodemonstrate that the sequence is formed in island arc environment and is genetically related to the sub-duction and partial melting of ancient oceanic plate. The metallogenetic features of the ancient volcanic arc in Zhejiang, Anhui and Jiangxi boundaryarea is similar to present island arc. Important strata-bound copper deposits have been found in spilite-keratophyre sequence erupted in the early stage. This kind of deposits have great prospect in the frontof the volcanic arc, i. e., along Shangrao-Jiangshan-Shaoxin fault. There are two types gold mineral-ization with industrial significance. One of them is related to volcanic hydrothermal solution, usuallyfound near the eruption centre; another one is formed by dynamical metamorphism of regional faultand is controlled by mylonite belt. In the A-type subduction and relevant mapmatism of Mesozoic Era,inetallogenetic elements in the late Proterozoic tectono-stratum are remobilized and concentrated toform new deposits. The remobillzation of gold is very important. It is an associated element in Yan-shanian porphyry Cu and porphyry Ag, Pb, Zn deposits. In the boundary area of Zhejiang and Anhuiprovince, gold mineralization of Carlin type has been found.