内蒙古克什克腾旗大石寨组凝灰岩锆石LA-ICP-MS U-Pb年龄、地球化学特征及其构造意义

LA-ICP-MS Zircon U-Pb Ages,Geochemical Characteristics of the Dashizhai Formation Tuffs in Hexigten Banner,Inner Mongolia and Thier Tectonic Significance

本文对内蒙古克什克腾旗长岭子地区大石寨组安山质岩屑晶屑凝灰岩进行了锆石LA-ICP-MS U-Pb年代学、锆石原位Hf同位素分析及地球化学研究。测年结果表明样品中锆石年龄主要分七组:3140±19～2826±36 Ma、2761±36～2273±23 Ma、2025±27～1673±89 Ma、1036±144～673±17 Ma、574±19～422±11 Ma、392±11～312±10 Ma和224±18～133±2 Ma。结合锆石的原位微区Hf同位素结果ε_(Hf)(t)值介于-18.0～16.8,n(^(176)Lu)/n(^(177)Hf)和n(^(176)Hf)/n(^(177)Hf)分别为0.000235～0.005759和0.280524～0.283022,T_(DM2)=4291～293 Ma,我们认为大石寨组岩屑晶屑凝灰岩中第一组锆石(3140～2826 Ma)指示东北地区或其邻区可能发育过中太古代—古太古代的古老基底。第二组锆石(2761～2273 Ma)记录了Kenorland超大陆汇聚的地质事件,揭示了大兴安岭地区太古代—古元古代古老结晶基底的信息。第三组锆石(2025～1673 Ma)则是Columbia超大陆形成地质事件的响应。第四、五组锆石(1036～673 Ma、574～422 Ma)来源于东北地区,分别记录了区域新元古代Rondinia和泛非期Gondwana两期重大地质事件。第六组锆石(392～312 Ma)峰值年龄为347 Ma,加权平均年龄为350.0±6.5Ma(MSWD=4.0,n=43),占总数48%,代表凝灰岩的形成时代,属于早石炭世。第七组锆石(224～133 Ma)可能与后期流体改造有关。岩石地球化学分析表明,大石寨组岩屑晶屑凝灰岩大部分样品属于偏铝质(A/CNK=0.65～1.95)、钙碱性(δ=0.88～2.50)火山岩,具有轻稀土富集,重稀土相对亏损,无明显Eu异常,富集Ba、Rb、K等大离子亲石元素,亏损Nb、Ta、Ti、P等高场强元素的特点,与安第斯型活动大陆边缘火山岩相似。结合前人已发表地质资料,我们认为克什克腾旗大石寨组安山质岩屑晶屑凝灰岩为古亚洲洋俯冲过程中洋壳脱水产生的流体与上覆地幔楔相互作用后形成的。形成于活动大陆边缘弧环境,指示古亚洲洋在早石炭世尚未闭合。

In this essay,we studied the Dashizhai Formation andesitic debris crystal tuffs developed in Changlingzi area,Hexigten Banner,Inner Mongolia,in such aspects as LA-ICP-MS zircon U-Pb isotopic geochronology,Hf isotopic composition analysis and geochemistry.Detrital zircon grains from the Dashizhai Formation tuffs yield seven U-Pb age clusters of 3140~2826 Ma,2761~2273 Ma,2025~1673 Ma,1036~673 Ma,574~422 Ma,392~312 Ma and 224~133 Ma.Meanwhile,in situ zircon Hf isotopic analysis yieldεHf(t)values between-18.0 and 16.8,n(176Lu)/n(177Hf)and n(176Hf)/n(177Hf)ratios are 0.000235~0.005759 and 0.280524~0.283022 respectively,and two-stage model ages are 4291~293 Ma.The first group of zircons(3140~2826 Ma)revealed the information of the ancient basements(Mesoarchean to Paleoarchean)in Northeast China;the second group(2761~2273 Ma)recorded the geological event of Kenorland supercontinent convergence,and revealed the information of Archean to Paleoproterozoic ancient basement in Great Hinggan Mountains(Da Xing'an Range)area;the third group(2025~1673 Ma)corresponed to the geological event of Columbia supercontinent formation;the forth and the fifth groups of zircons(1036~673 Ma,574~422 Ma)may originate from the Northeast China,and these two age groups corresponded to two key tectonic events of the Gondwana and Rodinia episodes,respectively;the peak value of the sixth age cluster(392~312 Ma)is 350 Ma and its weighted mean age is 350.0±6.5 Ma(MSWD=4.0,n=43),which can be identified as the protolith age of the Dashizhai Formation;the fifth cluster(224~133 Ma)may be related to fluid reformation.Whole-rock major,trace and rare earth elements analyses show that most samples exhibit metaluminous(A/CNK=0.65~1.95)and sub-alkaline(δ=0.88~2.50)characteristics.In the Chondrite-normalized REE patterns,the tuffs show slightly enriched in LREE,depleted in HREE,with no obvious Eu anomalies.In the spider diagram,they show enrichment in large ion lithophile elements(Ba,Rb and K),and depletion in high field-strengh elements(Nb,Ta,Ti and P).As shown above,the Dashizhai Formation tuffs exhibited an affinity with Andean active continental margin volcanic rocks.Combined with previous published regional geological data,we concluded that the Dashizhai Formaiton andesitic debris crystal tuffs in Hexigten Banner were formed by the fluids,which derived from dehydration of oceanic crust during the subduction of the Paleo-Asian Ocean,interacting with the overlying mantle wedge.And the tectonic environment for the formation of the tuffs was active continental marginal arc,indicating that the Paleo-Asian Ocean wasn't closed in Early Carboniferous.