北秦岭东段二郎坪单元辉长岩成因及其构造意义:来自锆石U-Pb年代学、地球化学和Sr-Nd-Hf同位素的制约

Petrogenesis and tectonic significance of gabbros from Erlangping unit in eastern segment of North Qinling:constrained by zircons U-Pb ages,geochemistry and Sr-Nd-Hf isotopes

北秦岭造山带二郎坪单元的形成时代与构造属性一直是争论的焦点,解决此问题的关键在于精确约束与俯冲相关的岩浆作用的时限和性质。本文对北秦岭东段二郎坪单元辉长岩开展了岩相学、锆石U-Pb年代学、全岩主微量元素地球化学和Sr-Nd-Hf同位素分析研究。地球化学特征表明,二郎坪单元辉长岩具有较低的w(SiO_(2))(49.61%~50.58%),表现出富Na贫K的特征(Na_(2)O/K 2O=8.44~14.09);稀土配分型式呈右倾型,相对富集大离子亲石元素(LILEs)、亏损高场强元素(HFSEs),具有Nb、Ta、Zr、Hf的负异常,Sr-Nd同位素组成略微富集,显示出岛弧岩浆岩的特征,具有CAB的性质,其原始岩浆来源于含角闪石的石榴子石相地幔源区的部分熔融,形成于岛弧环境,岩浆源区经历了俯冲流体的交代。利用锆石SIMS U-Pb定年手段获得二郎坪单元辉长岩加权平均年龄为(464±2.9)Ma,限定其成岩时代属中奥陶世。结合区域地质资料和前人研究成果,认为二郎坪单元辉长岩是宽坪洋向北秦岭微陆块下南向俯冲的产物,但不排除商丹洋向北秦岭微陆块下北向俯冲带来的影响。

The formation age and tectonic properties of Erlangping unit in North Qinling Orogenic Belt have long been the focus of controversy.The key to solve this problem is to precisely constrain the time and nature of the subduction-related magmatism.In this paper,the petrography,zircon U--Pb geochronology,whole-rock major and trace elements geochemistry,and Sr-Nd-Hf isotopes analysis of the gabbros from Erlangping unit in eastern segment of the North Qinling Orogenic Belt have been studied.Geochemical characteristics show that the gabbros from the Erlangping unit has low w(SiO_(2))(49.61%-50.58%),and enriched Na and depleted K(Na_(2)O/K 2O=8.44-14.09),with right-leaning chondrite normalized REE patterns,relatively enriched large ion lithophile elements(LILEs)and depleted high field strength elements(HFSEs),with negative anomalies of Nb,Ta,Zr and Hf,and slightly enriched Sr-Nd isotope compositions,exhibiting the characteristics of island arc igneous rocks with CAB properties.Their primary magmas could be mainly derived from the partial melting of amphibole-bearing garnet phase mantle metasomatized by subduction fluids,formed in the island arc setting.By zircon SIMS U-Pb dating technology,the weighted average age of the gabbros from Erlangping unit is(464±2.9)Ma,which defines the diagenetic age belongs to Middle Ordovician.Based on the regional geological data and the previous research results,the authors propose that the gabbros from Erlangping unit is the product of the southward subduction of Kuanping Ocean underneath North Qinling Microcontinent,but the influence of the northward subduction of Shangdan Ocean underneath North Qinling Microcontinent could not be ruled out.