摘要
结晶程度较低的基性岩如玄武岩、辉绿岩的岩浆锆石含量一般很低,不利于用锆石原位测年方法确定其成岩时代。但这类岩石可能含有俘获锆石,通过对俘获锆石的详细研究,可以推测基性岩浆通道周围地质体的演化历史,其作用如同穿越岩石圈上部的＂探针＂。玉树混杂岩带为一晚古生代混杂带,由变质-变形陆源碎屑岩、玄武岩、辉绿岩及辉长岩侵入体组成,脉状辉绿岩侵入到玄武岩中。从一件辉绿岩样品中分离了约130颗锆石,对其中72颗进行了SHRIMP及LA-ICP-MS原位U/Pb同位素分析,结果显示其表面年龄范围在400~2 900 Ma间,明显老于其成岩年龄,表明玉树混杂带发育于古老陆壳基底之上。分析显示年龄谱具两个主要峰值：500 Ma和1 000 Ma,与扬子地块年龄谱类似。该结果表明,玉树混杂带发育于东羌塘地块北缘,并不能代表成熟洋壳残留体;东羌塘地块与扬子板块具亲缘性。
Mafic rocks,especially those with low crystallization degree such as basalt and diabase,generally contain very rare zircon.It is thus not very easy to determine their rock-forming epochs by in-situ zircon U/Pb analysis.However,this kind of magmatic rock is likely to entrap numerous zircon xenocrysts in the rising process of magma,whose geochronology may provide valuable constrains on the tectonic evolution of the penetrated geological units.Such zircon-bearing mafic rocks play a role just like the role of lithosphere probe.This paper reports a case study of such zircon xenocrysts.The Yushu tectonic mélange in north-central Tibet is a Late Paleozoic one consisting of metamorphosed and deformed continent-derived clastics,metabasalt,diabase dykes,and numerous gabbroic plutons.Its tectonic affinity and tectonic implications remain in controversy mainly due to the absence of precise geochronological data.In order to get the precise time of the mélange,the authors sampled a diabase dyke for zircon separation,which cuts across the basalts with pillow structures.Approximately 130 zircon grains were collected from the diabase sample,12 and 60 of which were analyzed for U,Th,and Pb isotopic content by employing SHRIMP and LA-ICP-MS techniques,respectively.The results reveal that the zircon U/Pb ages are in a large range of 400 to 2 900 Ma,obviously older than the time of magmatism,suggesting that all the analyzed zircons are xenocrystals entrapped from the country rocks of the diabase.Based on the field relationship and the petrographic nature of the diabase,the authors hold that these xenocrystic zircons came from a crystalline basement at the earlier stage of the magma rising process instead of being captured from the meta-clastics of the Yushu mélange.Thus,the mafic rocks of the Yushu mélange were developed on a continental crystalline basement,probably in a rifting basin.Furthermore,the geochronology of the xenocrystic zircons provides valuable constraint on the thermo-tectonic evolution of the penetrated crystalline basement.The 72 analyses form an age distribution spectrum that exhibits three major age-peaks： 450 to 750 Ma,850 to 1 400 Ma,and 1 500 to 1 800 Ma,respectively,which is quite comparable with the age distribution spectrum of the Yangtze block.For the zircon grains of each age-peak,internal zoning structure in the zircons with older ages was commonly clearer than those with younger ages.This fact implies that the thermo-tectonic events of the penetrated crystalline basement were all composed of magmatism and associated metamorphism.Although further studies are required,the new data presented here demonstrate that the Yushu mélange was a continent marginal one along the northern margin of the eastern Qiangtang block of Cathaysian affinity instead of a remnant oceanic slice.
出处
《岩石矿物学杂志》
CAS
CSCD
北大核心
2011年第3期438-448,共11页
Acta Petrologica et Mineralogica
基金
国家科技支撑计划项目(2006BAB01A08)
国家自然科学重点资助项目(40730419)
973项目(2009CB421001)
中国地质调查项目(1212010818096)
