摘要
扬子陆块西缘康滇南北向构造带内发育大量古元古代基性岩脉(墙),它们对于认识扬子陆块早期的构造演化具有重要意义。对四川会理地区侵入到元古界通安组内的辉长岩进行LA-ICP-MS锆石U-Th-Pb同位素测定,获得的207Pb/206Pb加权平均年龄为1694Ma±16Ma(MSWD=0.9),代表了该辉长岩的形成和侵位时代,这一结果指示会理地区通安组应为古元古代地层。辉长岩全岩地球化学具有类似于N-MORB的特征。辉长岩锆石初始(176Hf/177Hf)i值为0.281881~0.281982,对应的εHf(t)为6.2~9.8,单阶段亏损地幔Hf模式年龄tDM1为1738~1883Ma,平均值为1803Ma。这些数据表明古元古代辉长岩来源于亏损地幔,指示古元古代晚期扬子西缘之下存在亏损地幔。结合扬子西缘大量古老的碎屑锆石,推测扬子陆块西缘存在太古宙—古元古代的结晶基底。与扬子陆块西缘古元古代晚期基性侵入岩形成有关的幔源岩浆事件明显晚于扬子陆块古元古代碰撞造山及造山后的伸展作用,记录了扬子陆块古元古代晚期一次伸展过程,可能是Columbia超大陆裂解作用在扬子陆块的响应。
A large number of mafic dykes occur in the Kang-Dian Axis of the western Yangtze Block, which had a great bearing on the early evolution of the Yangtze Block. In this paper, zircon U-Pb age, Hf isotope, and whole-rock geochemistry were studied for gabbros collected from Jiaopingdu area, which intruded into the Tongan Formation, with the purpose of constraining the age of the crystallization basement and the early evolution of the Yangtze Block. The zircons yielded a weighted mean artpb/a^Pb age of 1694Ma＋ 16Ma （MSWD=0.9）, which is interpreted as the intrusion age. The results constrain the depositional age of the Tongan For- marion to 1694Ma, implying that the Tongan Formation is of Paleoproterozoic. Geochemical data show that these gabbros have N- MORB affinity. Initial Hf isotope ratios（176Hf/177Hf）i of the zircons range from 0.281881 to 0.281982, with corresponding era（t） values of 6.2~9.8. These zircons give the depleted mande model ages（tnva） of 1738~1883Ma（1803Ma on average）, slightly older than the time of the zircon growth. All these data suggest that the Paleoproterozoic gabbro was derived from a depleted mantle. In combination with the presence of abundant Archean to Paleoproterozoic detrital zircom in the Mesoproterozoic strata, the newly identified 1694Ma rnaf- ic intrusion in the western Yangtze block supports the existence of an Archean-Paleoproterozoic crystalline basement beneath the western Yangtze block. The results obtained by the authors also show that the tectonic event, which resulted in the intrusion of large quantifies of late Paleoproterozoic mafic dykes in the western Yangtze block, was obviously younger than the continental collision and post-orogenic extension of Columbia supercontinent recorded in Yangtze block, and thus most probably linked to the breakup of Columbia supercontinent.
出处
《地质通报》
CAS
CSCD
北大核心
2013年第4期617-630,共14页
Geological Bulletin of China
基金
国家重点研究发展计划项目<特提斯演化过程中的VMS型Cu-Pb-Zn成矿作用>(编号:2009CB421003)
中国地质调查局项目<西南三江成矿带地质矿产调查评价成果集成>(编号:12120113049100)
<川滇黔地区基础地质调查成果集成>(编号:12120113050300)
<扬子地块西缘格伦维尔造山过程
南华裂谷作用研究及其成矿效应综合研究>(编号:1212011220901)
关键词
扬子陆块西缘
古元古代
基性侵入岩
U-Pb定年
锆石HF同位素
结晶基底
超大陆裂解
western margin of the Yangtze Block
Paleoproterozoic,mafic intrusion
zircon U-Pb age
zircon Hf isotope
crystalliza-t-ion basement
breakup of the Columbia supercontinent
