广西三叉冲钨矿有关岩体岩石成因:锆石U-Pb年代学、元素地球化学及Nd同位素制约

Geochronology, petrology, geochemistry and Nd isotopic constraints on the origin of the Sanchachong tungsten-related pluton, Guangxi, South China

华南西南部博白-岑溪多金属成矿带是华南地区重要成矿带。在该带中先后发现的米场、油麻坡、三叉冲等中大型钨钼矿与晚中生代白垩纪花岗岩关系密切。本文选择了该区典型岩体——三叉冲岩体进行研究,通过对其特征及成因的研究对该地区成矿作用提供指示意义。三叉冲岩体由斑状中粒黑云母花岗岩和细粒二云母花岗岩组成。LA-ICP-MS锆石U-Pb同位素定年表明,斑状中粒黑云母花岗岩形成于101-104 Ma,细粒二云母花岗岩形成于103-105 Ma。两种花岗岩都具有较大的化学成分变化,但具有相似的Nd同位素组成,εNd（t）值分别为–7.5-–6.5和–7.8-–6.7。我们认为中粒黑云母花岗岩为中元古界中-高钾玄武岩成分基底部分熔融,并与幔源基性岩浆发生岩浆混合作用形成的。在中粒黑云母花岗岩的演化过程中有角闪石＋少量斜长石的分离结晶作用。细粒二云母花岗岩具有明显不同于中粒黑云母花岗岩的Na2O和K2O演化趋势；且具有更高的 CaO 和 Sr 含量；同时,细粒二云母花岗岩具有高得多的 La/Lu 比值,这些证据都表明细粒二云母花岗岩并非中粒黑云母花岗岩通过分离结晶形成的。细粒二云母花岗岩为中元古界长英质麻粒岩＋少量拉斑玄武岩成分部分熔融产生中酸性岩浆,并与幔源岩浆发生混合作用形成的。幔源岩浆贡献的挥发分F使得细粒二云母花岗岩朝更富Na2O的方向演化。在细粒二云母花岗岩的演化过程中以黑云母＋钾长石＋斜长石的分离结晶为主。野外地质调查、岩石学及地球化学数据表明,细粒二云母花岗岩与三叉冲钨钼成矿作用的关系更为密切。晚中生代,受太平洋板块的俯冲作用的影响,桂东南地区处于拉张构造环境。软流圈上涌及玄武质岩浆底侵为基底的部分熔融提供了必要的热源,同时也为三叉冲岩体的形成提供了物质来源。

Bobai-Cenxi polymetallic belt is an important metallic belt in South China. Several medium- to large-size tungsten deposits associated with the Cretaceous granites, such as Michang, Youmapo and Sanchachong tungsten deposits, have been found in the belt. In this paper, we choose to study the ages, chemistry and Nd isotopes of the Sanchachong pluton from the region to shed light on the petrogenesis of the pluton and relationship with associated tungsten mineralization. The Sanchachong pluton is composed of porphyritic medium-grained biotite granite and fine-grained two-mica granite. LA-ICP-MS in situ zircon U-Pb isotope data show that the porphyritic medium-grained biotite granite was emplaced at 101-104 Ma, and the fine-grained two-mica granite at＆amp;nbsp;103-105 Ma. The two granites have large chemical variations, but show similar Nd isotopic compositions, withεNd（t） = –7.5- –6.5 and –7.8- –6.7, respectively. We suggest that the porphyritic medium-grained biotite granite originated from a magma mixing process between felsic magma derived from melting of the Meso-Proterozoic medium-to-high K basaltic basement rocks and mantle-derived mafic magma, followed by fractional crystallization of hornblende ＋ plagioclase. The fine-grained two-mica granite shows different Na2O and K2O evolution trends from the medium-grained biotite granite, and has higher CaO and Sr contents, and higher La/Lu ratios than those of the medium-grained biotite granite. These evidences indicate that the fine-grained two-mica granite was not the derivative of the medium-grained biotite granite by fractional crystallization, but being derived from melting of the Meso-Proterozoic felsic granulite with little tholeiitic basaltic basement rocks, followed by mixing with mantle-derived mafic magma. Biotite, alkali feldspar and plagioclase have played a role in the magma evolution of the fine-grained two-mica granite. Involvement of volatile F originated from the mantle resulted in the enrichment of Na2O in the evolved fine-grained two-mica granite. Field investigation, petrology and geochemistry suggest that the tungsten mineralization is genetically linked with the fine-grained two-mica granite. In late Mesozoic, SE Guangxi was in extensional tectonic settings related to the subduction of the Pacific slab. Upwelling of asthenosphere and formation of underplated basaltic magma triggered partial melting of the basement rocks, forming felsic melts. The Sanchachong pluton formed from the mixing between the mafic and felsic magmas.