湘北华容地区小墨山花岗岩体SHRIMPU—Pb年龄及地球化学特征

小墨山岩体侵位于中元古代冷家溪群中，由两期侵人体组成，早期为粗中粒-中粒斑状黑云母二长花岗岩；末期为细粒黑（二）云母二长花岗岩。通过锆石SHRIMPU—Pb法测得岩体侵位年龄为122．5±2．1Ma（20），MSWD=1．9，成岩时代为早白垩世。主元素中，SiO2变化于67．20％～75．16％，K20含量高，且K2O〉Na2O，K2O／Na2O为1．16～1．72；ASI值变化于0．96～1．10之间，平均1．02，属准铝质-微过铝质、高钾钙碱性系列。岩石明显富集大离子亲石元素，亏损高场强元素，Rb／Sr=0．27～15．13；Nb／Ta=15．9～17．1，为锶和铌亏损型。EREE总体较高，重稀土含量相对较高，轻重稀土分馏稍弱，∑Ce／∑Y为0．49～6．18，（La／Yb）。为0．66～15．54。有较高的εNd（t），为-6．8～-8．7；T2DM相对较小（1．47～1．62Ga）。综合研究表明，小墨山花岗岩石为壳源型富黑云母过铝花岗岩类（CPG），其成因应为下地壳物质和上地壳物质混合而成，与花岗岩底侵作用或注入地壳中的幔源岩浆有关，形成的构造背景为陆内挤压造山向非造山转换的后造山拉张环境，是在紧随侏罗纪挤压造山运动之后的构造松驰和拉张减薄条件下所形成。

SHRIMP U-Pb zircon dating and geochemical characteristics of Xiaomoshan granites in Huarong county, northern Hunan

Xiaomoshan granitic pluton in northern Hunan intrude in Middle Proterozoic Lengjiaxi Formation, and is composed of early-stage coarse-medium-grained and medium-grained porphyritic biotitic monzonitic granite and late-stage fine-grained biotite-two-mica mon- zonitic granite. SHRIMP U-Pb zircon dating of the granites is 122.5±2.1 Ma which belong to Early Cretaceous. The granites belong to metaluminous to weakly peraluminous high-K calc-alkaline series with SiO2= 67.20-75.16%, rich in K2O and K2O/Na2O=1. 16 -1.12, ASI=0.96-1.10 with average value of 1.02. The large-ion lithophile elements are rich and high field-strength ements are depleted in the rocks, while Sr and Nb are depleted with Rb/Sr=0.27-15.13, Nb/Ta= 15.9-17.1; ∑REE are rich as a whole with ∑Ce/∑Y= 0. 49 - 6.18 and （La/Yb）N = 0. 66- 15. 54 ; εNd =-6.8- -8. 7, T2DM = 1.47- 1.62Ga. Studies show that the Xiaomoshan granites belong to crust-derived rich peraluminous biotitic granitoid （CPG） and were derived from upper and lower crust, and were related with the underplating of mantle magma. The rocks were formed in a post-orogenic tectonic setting with the weakening of the stress and lithospheric extending and thinning after Jurassic compression orogeny.