十杭带湘南—桂北段中生代A型花岗岩带成岩成矿特征及成因讨论

Characteristics and Genesis of Mesozoic A-Type Granites and Associated Mineral Deposits in the Southern Hunan and Northern Guangxi Provinces along the Shi-Hang Belt,South China

十杭带是华南内陆一条重要的北北东向、具有高εNd（t）值和低tDM值的花岗岩带,该带在湘南—桂北段的花岗质岩体（千里山、骑田岭、西山、金鸡岭、花山和姑婆山等）均形成于151-163Ma间。但从西南往东北方向,形成时代有逐渐变年轻的趋势。这些岩体在地球化学组成上显示出较为相似的特征,岩石均富碱、高钾,富含Rb,Th,U等大离子亲石元素（LILE）和REE,Nb,Ta,Zr,Hf等高场强元素（HFSE）。在地球化学图解上均落入A型花岗岩区域,因此该花岗岩带应属于一条A型花岗岩带。进一步划分,这些花岗岩应该属于A2亚类。这些花岗岩均具有较低的（^87Sr/^86Sr）i值、较高的εNd（t）值和相对低的Nd模式年龄值,但从西南往东北方向,εNd（t）值具有逐渐降低的趋势。在这些花岗质岩体中暗色包体非常发育,岩石学和地球化学,特别是锆石的Hf同位素组成,指示这些花岗质岩石是通过壳-幔岩浆混合作用形成的,幔源岩浆端元来自亏损地幔,可能是软流圈地幔物质的直接参与。该A型花岗岩带可能形成于古太平洋板块俯冲引起的弧后或弧内拉张构造环境,软流圈地幔上涌及诱发的幔源岩浆沿超壳深断裂底侵,导致了强烈的壳幔岩浆混合作用,形成了该花岗岩带。该拉张事件从西南往东北方向进行,拉张强度由强变弱,混入花岗岩中的地幔物质也由多变少。该花岗岩带也是我国一条重要的W-Sn多金属成矿带。研究表明,这些花岗岩均属于富Sn花岗岩,但Sn在这些花岗岩中的富集机制与传统的结晶分异富集的方式不同。该区锡矿化类型十分丰富,除了存在传统的岩浆热液演化成矿外,还存在新类型的绿泥石化花岗岩锡矿化,丰富了A型花岗岩的成矿理论。

The Shi-Hang belt is an important magmatic zone composed of granites with relative higher εNd（t） values and younger tDM model ages and trends NNE. The granites in Southern Hunan and Northern Guangxi include the Qianlishan, the Qitianling, the Xishan, the Jinjiling, the Huashan and the Guposhan granites. Recently a series of zircon SHRIMP U-Pb ages indicate that all these granites were emplaced in a narrow time interval of 151-163 Ma. However, ages of granites decrease from the southwest to the northeast. These granites have similar geochemical characters. All of them are enriched in alkalies and potassium, and have high contents of LILE and HFSE. In geochemical discriminating plots, these granites fall in A-type granite field. Thus, the Southern Hunan-Northern Guangxi granite belt belongs to an A-type granite belt. These granites are further classified to A2 type. These granites have relatively lower （^87Sr/^86Sr）i values, higher εNd（t） values and younger tDM model ages. From the southwest to the northeast, εNd（t） values gradually decrease. Abundant mafic microgranular enclaves were found in most granites. Petrogeology and geochemistry, especially Hf isotopes of zircon from the MMEs and host rocks indicated that these granites formed by crust-mantle magma mixing processes. The mantle-derived magma end-member came from depleted mantle, and might be directly related with the asthenospheric mantle. The A-type granite belt might form at back-arc or intra-arc extension environment triggered by subduction of palaeo-Pacific plate. The strong mantle-crust interaction caused by upwelling and underplating of the asthenospheric mantle across the trans-crustal faults is the major mechanism for generating the A-type granites in the belt. The extension and thinning of the continental crust began from the southwest to the northeast, and the intensity of extension decreased from the southwest to the northeast. The Shi-Hang South Belt is an important W-Sn mineralization belt. All these granites belong to Sn-rich granite. Mechanism of tin concentration in these granites is different with common tin granites, in which tin is concentrated by fractional crystallization. Tin mineralization models are abundant in the belt. Different with traditional tin mineralization model, tin mineralization in the Furong tin deposit was related with chloritization of granites by later meteoric water hydrothermal system. Studies on these granites and related tin mineralization may enrich mineralization theories of A-type granites.