扎河坝蛇绿岩锆石U-Pb年龄、Hf-O同位素组成及其地质意义

Zircon U-Pb ages and Hf-O isotope compositions of the Zhaheba ophiolite in the northern margin of the Junggar terrane and their tectonic implications

摘要: 扎河坝蛇绿岩位于准噶尔北缘,是扎河坝—阿尔曼太蛇绿混杂岩带的一部分。通过对蛇绿混杂岩中堆晶辉长岩和凝灰岩夹层的锆石U-Pb年龄及Hf-O同位素进行研究,获得堆晶辉长岩的结晶年龄为485±4Ma,凝灰岩的喷发年龄为401±2.5Ma。辉长岩极度亏损Hf同位素,ε_Hf(t)为13～20,δ¹⁸O峰值为+5.37‰,显示其具有地幔特征。凝灰岩锆石的Hf同位素亏损程度低于堆晶辉长岩,ε_Hf(t)为1.4～9.2。扎河坝蛇绿岩具有典型的SSZ型蛇绿岩特征,是通过俯冲增生到洋内弧弧前的蛇绿岩。

Abstract: The Zhaheba ophiolite, located in the northern margin of Junggar terrane, is part of the Zhaheba-Aerman ophiolitic melange belt. Through U-Pb zircon age and Hf-O composition analysis for two samples from the ophiolitic melange belt, this study obtained a crystallization age of 485±4Ma for cumulate gabbros and an eruptive age of 401±2.5Ma for tuffs. The gabbros are characterized by extreme depletion of Hf isotopic composition with ε_Hf(t)of 13～20 and δ¹⁸O peak value of +5.37‰, indicating a mantle-sourced feature. Hf isotope depletion degree of zircons from the tuffs is lower than that from cumulate gabbros, with ε_Hf(t)of 1.4～9.2. All this features suggest that the Zhaheba ophiolite is of typical characteristic of the SSZ type ophiolite, which results from subduction accretion to the front of intra-oceanic arc.