博格达与哈尔里克早泥盆世火山岩成因对比及其构造意义

Comparison of Genesis of Devonian Volcanic Rocks in the Bogda and Harlik Areas and its Tectonic Implications

本次研究在东天山博格达东段下涝坝南部新确认一套以安山岩为主的火山岩,为钙碱性系列,LA-ICP-MS锆石U-Pb年龄为397.5±4.7 Ma,属早泥盆世。该火山岩具有一致的轻稀土与重稀土分馏明显的右倾配分模式,K、Ba等大离子亲石元素显著富集,而Nb、Ta等高场强元素相对亏损,其原始岩浆可能来自于石榴石稳定域地幔橄榄岩2%~5%的部分熔融,同化混染程度较小。对应的哈尔里克南麓下泥盆统大南湖组火山岩为一套基性到酸性连续演化的钙碱性系列岩石,稀土丰度较下涝坝火山岩高,轻、重稀土之间分馏较弱,整体富集大离子亲石元素而亏损高场强元素。其岩浆可能来源于尖晶石稳定域地幔橄榄岩【3%的部分熔融,受地壳物质混染明显,岩石后期蚀变较强。Hf/3-Th-Ta图解和Yb标准化的Th-Nb判别图解显示这两套泥盆纪火山岩均形成于与俯冲有关的活动陆缘环境,暗示博格达和哈尔里克地区在早泥盆世时期可能具有相同的大地构造环境。

This work has identified a set of calc- alkali volcanic rocks from the southern Xialaoba area,east section of Bogda,East Tian Shan,which is dominated by andesite with zircon U- Pb ages 397. 5 ± 4. 7 Ma. The samples' REE abundance shows a rightly- oblique pattern type characterized by fractionation. Their trace elements are rich in large- ion- lithophile elements( LILE) such as Ba and K,uniformly and observably depleted in high-field- strength elements( HFSE) such as Nb,Ta,Ti and Y. Their primitive magma might emanate from mantle peridotite 2% ~ 5% partial melting in the garnet stability domain with a slight assimilation and hybridization. The corresponding Early Devonian Dananhu formation volcanic rocks from southern Harlik are calc- alkali series that evolved from basalt to rhyolite. Their REE abundance is higher than the Xiaolaoba volcanic rocks,but have a low fractionation between LREE and HREE and are rich in LIEL and depleted HFSE as a whole. Their primitive magma could originate from mantle peridotite <3% partial melting in the Spinel stability domain,assimilating some crust material,and rocks were strongly altered in later time. Both the two kinds of volcanic rocks formed in an active continental margin environment related to subduction,which implies that the Bogda and Harlik areas shared a same tectonic setting in Early Devonian.