扬子北缘毕机沟钒钛磁铁矿床形成机制及大地构造环境

Formation Mechanism and Geotectonic Environment of the Bijigou Vanadium-Titanium Magnetite Deposit in the Northern Margin of the Yangtze Block

摘要: 毕机沟大型钒钛磁铁矿床位于扬子陆块北缘，前人虽然对该杂岩带中典型岩体进行了大量研究，但对于扬子陆块北缘新元古代时期构造环境仍存在分歧。本文主要通过研究毕机沟堆晶杂岩体的野外产出特征及规模，岩相组合及分带特征和全岩主微量及稀土元素地球化学特征，查明了毕机沟岩矿体母岩浆性质和矿床成因，并为扬子北缘寻找类似矿床提供更多信息。毕机沟辉长岩具有拉斑玄武岩系列特征，贫Si，富Al、Ca，中等的Fe和Mg，低K、Na，在稀土元素球粒陨石标准化曲线显示出“右倾型”的平缓形式，Eu正异常，轻稀土略富集重稀土相对亏损；微量元素蛛网图中显示出富集大离子亲石元素(LILE)，亏损Nb、Ta、Zr、Hf等高场强元素(HFSE)，Th、U负异常以及Sr正异常特征。说明该岩体在岩浆演化过程中经历了明显的结晶分异作用，显示出了大陆边缘弧的地球化学特征。新元古代时期(~760 Ma)，洋壳向扬子板块之下俯冲，导致源于亏损地幔的拉斑玄武质岩浆上侵。早期的橄榄石和斜长石堆晶作用形成基性程度较高的下部超镁铁质岩相带，剩余岩浆经过稳定的结晶分异作用形成中部和上部岩相带，在结晶分异过程中富含Fe、Ti、V的残余岩浆逐步富集形成钒钛磁铁矿体，毕机沟层状基性杂岩体成矿过程受分离结晶作用控制。

Abstract: The Bijigou large vanadium-titanium magnetite deposit is located at the northern margin of the Yangtze block. Although a large number of studies have been carried out on the typical rock mass in the complex, there are still differences in the Neoproterozoic tectonic setting in the northern margin of the Yangtze block. In this paper, the field magma of the Bijigou rock ore body was identified by studying the field output characteristics and scale of the Bijigou pile-crystal complex, the combination of lithofacies and zoning, and the geochemical characteristics of the main trace and rare earth elements. Nature and mineral of the deposit provide more information for the search for similar deposits in the northern margin of the Yangtze. Bijigou gabbro has the characteristics of basalt series, silicon-depleted, aluminum-rich, calcium, medium iron and magnesium, low-potassium, sodium, and the normalized curve of rare earth element chondrite shows a gentle form of “right-dip”. The REE patterns show positive Eu anomaly and light rare earth elements (LREE) enrichment relative to heavy rare earth elements (HREE); trace element spider network shows enrichment of large ion lithophile element (LILE), loss of Nb, Ta, zirconium, lanthanum and other high field strength elements (HFSE), Th, uranium negative anomalies and Sr positive anomalies. It shows that the rock mass experienced obvious crystallization differentiation during the magma evolution, showing the geochemical characteristics like the continental marginal arc. During the Neoproterozoic (~760 Ma), the oceanic crust subducted beneath the Yangtze plate, resulting in the intrusion of the basaltic magma from the depleted mantle. The early olivine and plagioclase crystals form a lower ul-tra-mafic porphyry facies belt with higher basicity. The remaining magma formed a central and upper lithofacies zone through stable crystallization differentiation, and is rich in crystallization differentiation process. The residual magma containing iron, titanium and V is gradually enriched to form vanadium-titanium magnetite ore body. The ore-forming process of the Bijigou layered basic complex is controlled by separation and crystallization.