大别山燕子河云斜煌岩的地质背景和岩石学基本特征

The Geological Setting and Petrological Features of the Kersantite Near Yanzihe in Dabie Mountains

根据野外地质特征,大别山燕子河云斜煌岩脉是与燕山期后造山中酸性岩浆活动有时空联系的。所以,云斜煌岩脉及与其有成因联系的以减压为标志的麻粒岩相变质也发生在燕山期。云斜煌岩在矿物构成、岩石结构、岩石化学(主元素、稀土模式和微量元素蜘蛛图解)的特点都归属钙碱性煌斑岩,但它可能不是金的矿源。其SiO2含量和镁质指数偏低、CaO含量较高、缺乏高压矿物捕虏晶以及黑云母成分富钛指示的低压结晶条件表明它与原生基性岩浆有直接成因联系的可能性不大;相反它可能代表后来中酸性岩的母岩浆。其黑云母成分富钛指示的高氧逸度条件、岩石化学准铝型、稀土模式强烈右倾、耦合的TNT负异常等特征指示成岩中有陆壳物质(泥质或泥灰质)掺入。云斜煌岩浆的形成可能与大别碰撞造山带后造山伸张时减薄地壳在挥发份作用下减压重熔有关。

Based on field evidence, it is inferred that the kersantite pipes near Yanzihe in Dabie mountains are temporally and spatially related to Yanshanian postorogenic intermediate to acidic magmatism. Therefore the kersantite pipes and their related, decompression-featured granulite-facies metamorphism are also Yanshanian. The modal proportion, textural and petrochemical features including main element concentrations, REE patterns and trace element spidergrams all belong to talc-alkaline lamprophyres （CAL）, but they might not constitute the source of any gold deposits. The low values of SiO2 content and mg [moleeular %Mg/（Mg＋Fe^2＋）], the high value of CaO content, the nonexistence of high pressure xenocrysts and the high titanian content of biotite representing a low pressure crystallization condition indicate that, the possibility that they might be directly related to any primary basaltic scarce; and conversely they might represent the parent magma of subsequent granitoids. Characteristics such as the high oxygen fugacity condition indicated by biotite content, the metaluminous petrochemitry, strongly right-inclined REE patterns and coupled negative Ta-Nb-Ti （TNT） anomaly suggest that continental matters （like argillaceous and marlaceous rocks） might be incorporated in their genetic process. The magma genesis of kersantite pipes might be related to the decompressional remelting of the thinned crust with volatile fluxes during the postorogenic extension of Dabie collision zone.