西秦岭天水关子镇变玄武岩的地球化学特征及大地构造环境

Geochemical characteristics and tectonic environment of metabasalts in Guanzizhen, Tianshui, Western Qinling

天水关子镇蛇绿岩分布于西秦岭造山带北缘,一般认为是商南-丹凤缝合带西延部分,代表了华北板块与扬子板块的结合部位。本文通过关子镇蛇绿岩中变玄武岩的主量、微量和稀土元素及Sr、Nd同位素综合分析,对关子镇蛇绿岩的源区特征及构造背景进行分析和探讨。研究表明,蛇绿岩中变玄武岩隶属于低碱、富钠低钾的拉斑玄武岩系列;稀土元素分异不明显,微量元素亏损Ba、Th、P,高场强元素无亏损现象。εNd(t)值均为正值,与MORB相近。

Objectives: The ophiolite in Guanzizhen is located in the northern margin of the western Qinling Orogen,which is taken as the western extension part of the Shang-Dan(Shangnan-Danfeng) suture zone representing the joint position between North China and Yangtze Blocks. In this paper,we focus on the geochemical features of the ophiolite in Guanzizhen to analysis its source area characteristics and tectonic background.Methods: Samples for major and trace element analysis were first crushed and ground to a particle size of less than 200 mesh. The oxides of major elements were measured by X-ray fluorescence spectrometry. The trace and rare earth elements were determined with an XⅡ Series ICP-MS. For rare earth elements content analyses,50 mg samples were precisely weighed and reacted with 1 mL HF and 0. 5 mL HNO3 in screw top PTFE-lined stainless bombs. The analytical precision was better than 5%.Prior to n(87 Sr)/n(86 Sr) and n(144 Nd)/n(143 Nd) analysis,50 mg of sample powder was weighed. The powder was then leached using 1. 0 mol/L acetic acid to dissolve the carbonate fraction. After this step,Sr and Nd were separated and purified by conventional cation-exchange resin method respectively. Isotopic ratios of obtained Sr and Nd solutions were measured on a thermal ionization mass spectrometer(Triton,MAT261). The measured isotopic ratios were corrected for mass fractionation using n(86 Sr)/n(88 Sr) = 0. 1194 for Sr and n(146 Nd/n(144 Nd)= 0. 7219 for Nd. The Sr standard NBS SRM 987 yielded n(87 Sr)/n(86 Sr) = 0. 710244 ± 4(n=10,2σ),while the Nd standard JNDi-1 gave n(143 Nd)/n(144 Nd) = 0. 512124 ± 2( n = 20,2σ) . All analyses were performed at the Mineral Resources Supervision Testing Wuhan Centre of China Land & Resources Ministry.Results and conclutions: Major elements of the rocks shows that the meta-basalts belong to tholeiitic basalt characterized by rich in sodium and poor in potassium;rare-earth element differentiation is not obvious,trace elements are deficient in Ba,Th,and P,and high field strength elements are not deficient. the values of εNd( t)are all positive,similar to the mid-oceanic ridge basalt( MORB).