辽-蒙交界地区晚侏罗世高硅花岗岩: 岩石成因与地质意义

高硅(SiO₂>70%)花岗岩不仅是指示大陆地壳成熟度的重要标志，而且蕴含大陆地壳分异机制和稀有金属元素运移行为的关键信息。跨越辽宁-内蒙古两地分布的白音花岩基是沿华北克拉通与中亚造山带边界断裂带侵位的重要的中生代高硅花岗岩，但其形成时代和岩石地球化学特征一直缺乏系统刻画。本次离子探针(SIMS)锆石U-Pb测年指示该花岗岩岩基于晚侏罗世(162~161Ma)侵位。岩体SiO₂含量介于75.7%~77.3%；钙碱性、贫铁镁、弱过铝；富集Th与U，亏损Ba与Sr；稀土元素总量较低(∑REE=40.2×10^-6~117×10^-6)，Eu负异常明显；Zr/Hf和Nb/Ta分异显著；这些特征契合高分异Ⅰ型花岗岩的典型元素地球化学行为。同时，白音花花岗岩具有低负的ε_Nd(t)值(-3.5~-2.6)以及低正的锆石ε_Hf(t)值(+0.1~+5.9)。元素与同位素地球化学示踪指示白音花花岗岩可能源自由中亚造山带型新生安山质地壳与少量古老地壳组成的复合源区，其部分熔融生成的原始酸性岩浆经历结晶分异形成高硅花岗岩。作为记录华北克拉通/中亚造山带边界断裂带晚侏罗世走滑/伸展活动的非造山型钉合岩体，白音花高硅花岗岩见证了蒙古-华北板块有别于早白垩世大规模上地壳伸展构造的中晚侏罗世区域弥散状中下地壳伸展行为。中晚侏罗世和早白垩世两段式地壳伸展轨迹不仅契合蒙古-鄂霍茨克构造域造山后重力垮塌过程，而且是促使新晋蒙古-华北板块大陆地壳垂向分异-成熟的高效途径。

Late Jurassic high silica granites from the border area between Liaoning and Inner Mongolia: Petrogenesis and tectonic implication

High silica (>70% SiO2) granites not only present important hallmarks for the maturity of continental crust, but also encapsulate essential information for calibrating the mechanism of continental crustal differentiation and the mobility behavior of rare metal elements. This study documents one pivotal occurrence of Mesozoic high-silica granites from the border area between Liaoning and Inner Mongolia that coincides with the boundary between the North China Craton (NCC) and Central Asian Orogenic belt (CAOB), i.e., the Baiyinhua batholith. SIMS zircon U-Pb dating yields a well-constrained Late Jurassic emplacement age of ca. 162~161Ma for the batholith. The rocks range in SiO2 content from 75.7% to 77.3%, and exhibit calc-alkali and weakly peraluminous affinities. They are enriched in Th and U but depleted in Ba and Sr, with low rare earth element abundance (∑REE=40.2×10-6~117×10-6), strong negative Eu anomalies (δEu=0.10~0.47) as well as fractionated Zr/Hf and Na/Ta. These elemental features are typical of highly fractionated Ⅰ-type felsic magmas. With respect to isotopic composition, the granites are characterized with whole rock εNd(t) of -3.5~-2.6 and zircon εHf(t) of +0.1~+5.9. These elemental and isotopic characters suggest that the parental magma for the Baiyinhua granites might derive from the partial melting of a composite middle crustal protolith containing dominant relaminated juvenile intermediate plutonic components and minor ancient crustal ingredients, while its prolonged fractional crystallization culminated in the formation of high-silica granites. Acting as a kind of anorogenic stitching pluton indicative of Late Jurassic extensional/strike slip activities along the boundary faults between the NCC and the CAOB, the Baiyinhua granites might witness the prevalent diffusive middle-lower crustal extension in the newly-amalgamated North China-Mongolia plate during Middle-Late Jurassic, which is distinct from large-scale Early Cretaceous upper crustal extention. Such consistent two-stage extensional pattern across the NE continental Asia not only tends to agree well with the post-orogenic gravitational collapse in the northern Mongol-Okhotsk tectonic regime, but also is conducive to facilitating the vertical differentiation and maturation of the continental crust in the North China-Mongolia plate.