小兴安岭-张广才岭铁力地区侏罗纪辉绿岩年代学、地球化学、锆石Hf同位素特征及其构造意义

小兴安岭-张广才岭地区中生代岩浆活动的成因及动力学背景对于揭示古太平洋的构造演化具有重要意义。本文选取小兴安岭-张广才岭铁力地区出露的辉绿岩墙为研究对象,进行锆石LA-ICP-MS U-Pb年代学、全岩地球化学和锆石Hf同位素等分析,对该基性岩的形成时代、岩石成因、岩浆源区以及形成的大地构造背景进行讨论。研究表明:辉绿岩锆石具有高Th/U比值( 0.3),CL图像显示微弱的振荡环带结构,具有岩浆锆石特征,~(206)Pb/~(238)U加权平均年龄为187±2Ma,即形成于早侏罗世;该岩体主要经历了橄榄石和单斜辉石的分离结晶作用,未遭受明显的地壳混染作用,并且具有Rb、Ba、U、Pb、K和Sr等流体活动元素相对富集,Th、Nb和Ta等非流体活动性元素相对亏损的地球化学特征,暗示其形成于被俯冲流体富集交代的亏损地幔部分熔融,源区可能为尖晶石-石榴石二辉橄榄岩,部分熔融程度约为6%～20%。结合该地区同时代的岩浆岩、变形构造、矿床特征和黑龙江蓝片岩的相关报道,本文认为小兴安岭-张广才岭地区在中生代期间处于活动大陆边缘环境,其岩浆岩的形成主要是由于存在于佳木斯地块和松嫩-小兴安岭地块间的牡丹江洋西向俯冲造成的。

Geochronology, geochemistry and zircon Hf isotope of the Jurassic diabase from the Tieli area, Lesser Xing'an-Zhangguangcai Range, and its geological implications

The petrogenesis and geodynamic setting of Mesozoic magmatism in Lesser Xing''an-Zhangguangcai Range, China, hold a key to understanding the tectonic evolution of the Paleo-Pacific. In this contribution, we report a new LA-ICP-MS zircon U-Pb age, whole-rock geochemistry and Lu-Hf isotope results for the diabase from the Tieli area, in Lesser Xing''an-Zhangguangcai Range with the aims to constrain its age, petrogenesis, source and tectonic affinity. Zircons from the diabase samples had high Th/U ratios (>0.3) and weak oscillatory zoning structure in the CL images, indicating their magmatic origin, and yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 187±2Ma which was interpreted as the crystallization age. The geochemical and isotopic studies imply that fractional crystallization of clinopyroxene and olivine played an important role during magma evolution with insignificant crustal contamination. Moreover, they are characterized by enrichment of fluid-mobile elements (Rb, Ba, U, Pb, K and Sr), but depletion of fluid-immobile elements (Th, Nb and Ta), suggesting that the mafic magma was originated from a depleted lithospheric mantle enriched by slab-derived fluids. Modeling further reveals that the parent magma was derived from 6%~20% partial melting of spinel-garnet lherzolite. Integrated with data of regional coeval magmatism, structure, metallogeny and metamorphism, our findings indicate that the Mesozoic magmatic rocks in the Lesser Xing''an-Zhangguangcai Range were probably formed in an active continental margin, resulting from the westward subduction of the Mudanjiang Ocean between the Jiamusi and Songnen blocks.