摘要
采用LA-MC-ICP-MS手段对敦煌地块中敦煌群的白云母石英片岩、石榴斜长角闪岩、石榴黑云斜长片麻岩和长英质伟晶岩脉中的锆石进行了U-Pb和Lu-Hf同位素分析,获得白云母石英片岩碎屑岩浆锆石的207Pb/206Pb表面年龄为1545～756Ma,主要集中在1200～1000Ma,表明地层的最大沉积时代为756Ma,蚀源区存在中、新元古代的岩浆事件。白云母石英片岩锆石的εHf(t)分为两组,一组为正值,εHf(t)=1.2～10.1,单阶段模式年龄为tDM=1.09～1.66Ga;一组为负值,εHf(t)=-1～-16,两阶段模式年龄为tDM2=1.91～2.42Ga。表明蚀源区存在古元古代、中元古代的再造地壳。石榴斜长角闪岩的两粒捕获锆石的年龄为2272Ma和1208Ma,εHf(t)为-3和14,tDM2和tDM为2.82Ga和1.1Ga,暗示捕获区存在太古代再造地壳和中元古代晚期新生地壳。石榴斜长角闪岩碎屑锆石的Th/U比值为0.02～0.42,206Pb/238U年龄为441±5Ma,代表了岩石遭受变质作用的时代。石榴黑云斜长片麻岩中的碎屑锆石与长英质伟晶岩脉中的继承锆石特征相同,锆石年龄集中在3个峰值区间:2.2～2.1Ga,1.8～1.6Ga,1.2～0.8Ga,相应的的εHf(t)分别为-9～4,-5.4～15,-27～20,相应锆石的模式年龄分别为3.1～2.4Ga,2.6～1.4Ga,3.1～1.7Ga,均大于其形成年龄,表明蚀源区锆石来自于再循环的新太古代、古元古代和中元古代地壳,样品代表的地层的最大沉积时代为新元古代早期。岩石中检测出早古生代的变质锆石,206Pb/238U年龄为464～422Ma,可能代表了沉积岩的变质时代。敦煌群锆石U-Pb和Lu-Hf同位素表明蚀源区岩石类型和时代的多样性,也表明部分敦煌群不是前寒武纪的变质基底,而是塔里木盆地变质基底之上新元古代的沉积盖层,后卷入了我国西北部早古生代的造山事件。
This paper reports in situe U-Pb ages and Hf isotopic data on detrital zircons from muscovite quartz schist,garnet amphibolite,garnet-bearing paragneiss and pegmatite-vein of the Dunhuang Group,which belong to metamorphic basement of NE marginal Tarim basin by the LA-MC-ICP-MS.Magmatic detrital zircon 207Pb/206Pb age span are between ca.1545~756Ma,mostly with ca.1200~1000Ma from muscovite quartz schist indicating Mesoproterozoic and Neoproterozoic magmatic events in the erosion source areas and start sediment of strata after ca.756Ma.Divided two group εHf（t） values of zircons from muscovite quartz schist,positive εHf（t） values with 1.2~10.1 and tDM=1.09~1.66Ga;other group with negative εHf（t）=-1^-16 and tDM2=1.91~2.42Ga,that suggesting exist Paleoproterozoic recycled crust and Mesoproterozoic juvenile crust in the source.Ages of two xenosrystal zircon grains from garnet amphibolite are 2272Ma and 1208Ma,εHf（t） values are -3 and 14,tDM2 and tDM are 2.82Ga and 1.1Ga,respectively.This indicating exist Mesoarchean recycled crust and later Mesoproterozoic juvenile crust in the passage area.Th/U ratio with 0.02~0.42 and 206Pb/238U ages with 441±5Ma of zircons from garnet amphibolite implying occur metamorphism time of volcanic-clastic rocks.Character of detrital-zircons from garnet-bearing paragneiss and inherited zircons from pegmatite-vein is similarity,their ages are three peak spane： 2.2~2.1Ga,1.8~1.6Ga,1.2~0.8Ga,relevant εHf（t） values are -9~4,-5~15,-27~20;tDM=3.1~2.4Ga,2.6~1.4Ga,3.1~1.7Ga,respectively.That indicated zircons from recycled Neoarchean,Paleoproterozoic and Mesoproterozoic crust and maximal deposit time of strata is Neoproterozoic.Early Plaeozoic metamorphic zircons exist in these rocks with 206Pb/238U ages are 464~422Ma is likely to presentation of metamorphic time.The U-Pb ages and Hf isotopic composition on detrital zircons reflected variety of rocks in source and segment of Dunhuang Group is not Precambrian metamorphic basement,only one of later cover above.Then Early Paleozoic orogenic event impacted the Dunhuang Group,which occurred in NW China.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2011年第1期59-76,共18页
Acta Petrologica Sinica
基金
中国石化股份有限公司科技开发部项目(YPH08110)资助
