北秦岭孤山坪地区辉长岩地球化学特征、锆石U-Pb年龄及Lu-Hf同位素组成

北秦岭孤山坪地区辉长岩的研究程度较低，而辉长岩通常是下地壳或上地幔玄武质岩浆结晶的产物，可以在一定程度上反映特定地质时代壳幔演化格局。为了研究其成因和形成机制，对其锆石U-Pb年代学、岩石地球化学及锆石Hf同位素进行了研究。结果表明:辉长岩的侵位结晶年龄为（457.4±1.4）Ma，即形成于晚奥陶世；总体上富集轻稀土元素和大离子亲石元素K、Rb、Ba、Sr，相对亏损高场强元素Nb、P、Ti、Zr，指示其可能形成于活动陆缘弧环境；锆石ε_Hf（t）=6.4~9.1，一阶段模式年龄（t_DM1=822~708 Ma）和二阶段模式年龄（t_DM2=1 268~1 025 Ma）均大于锆石结晶年龄。综合研究认为，孤山坪地区辉长岩原始岩浆应来源于受俯冲流体交代作用改造的富集型地幔，可能形成于二郎坪群早期岛弧与秦岭地块之间的弧陆碰撞所导致的增生造山作用过程，代表了晚奥陶世二郎坪群由岛弧向弧后盆地转化的初始岩浆记录。本研究可以为北秦岭早古生代构造演化的厘定提供进一步的约束资料。 

Geochemical characteristics,zircon U-Pb ages and Lu-Hf isotope composition of gabbro in Gushanping area,north Qinling

The gabbros, which are usually the product of the basaltic magma crystallization in lower crust or upper mantle, and to a certain extent, can reflect the specific geological era crust-mantle evolution history, are not well documented in Gushanping area, North Qinling. In order to confirm its petrogenesis and mechanism of formation, this study carried out zircon LA-ICP-MS U-Pb chronology, petrogeochemistry and zircon Hf isotope studies. The results indicate that emplacement crystallization age of the gabbros in Gushanping area is (457.4±1.4) Ma, indicating that the gabbros were formed in Late Ordovician. A geochemical study suggests the gabbros are generally enriched in LREE, rich in K, Rb, Ba, Sr, and relatively depleted in HFSE, Nb, P, Ti and Zr, which indicat the gabbros may form in an island arc setting of active continental margin. The ε_Hf(t) values vary from 6.4 to 9.1, the single model age t_DM1 is 822-708 Ma and the two model age t_DM2 is 1 268-1 025 Ma, both of which are older than the crystallizational age of zircon. Comprehensive studies suggest that the magma of the gabbros in Gushanping area originated from an enriched mantle which may be modified by subduction metasomatic fluids, and may have been formed in accretionary orogenesis caused by arc-continent collision between the early island arc of Erlangping Group and Qinling geolgogical block. The authors thus infer that the gabbros in Gushanping area ought to be initial magmatism records of the transition from island arc to back-arc basin in Erlangping Group in Late Ordovician. And this study can provide a new basis for the research on the Early Paleozoic tectonic evolution of North Qinling.