二连浩特北部石炭纪地壳演化过程——来自哈拉图庙群火山碎屑岩锆石U-Pb-Hf同位素的制约

Carboniferous Crustal Evolution in Northern Erenhot:Constraint from Zircon U-Pb-Hf Isotopes of Volcanoclastic Rocks in Halatumiao Group

中亚造山带东部尤其是兴安地块石炭纪地壳和构造演化是争议较大的热点问题。传统上认为除了贺根山蛇绿岩外,区域上早石炭世地质记录出露较少,普遍发育晚石炭世岩浆和沉积作用,制约了我们对内蒙古中部石炭纪完整地壳演化过程的认识。岩浆锆石的微量元素可以有效反映其从岩浆中结晶的温度和起源深度。因此,本文选取二连浩特北部原定早石炭世的哈拉图庙群火山碎屑岩为研究对象,开展系统的锆石U-Pb定年、微量元素和Hf同位素研究,探讨哈拉图庙群形成时代、锆石记录的结晶温度和地壳厚度及其随时代的变化,结合区域地层格架和岩浆作用进一步制约兴安地块西部石炭纪地壳演化过程。定年结果显示,哈拉图庙群一段和二段的熔结凝灰岩形成时代分别为(322±3)和(309±4)Ma,比三段和四段凝灰岩(302和298 Ma)古老,表明哈拉图庙群火山碎屑岩至少是晚石炭世早期和晚期两个火山喷发阶段的产物,其中含有大量早石炭世岩浆成因锆石,与邻区出现的早石炭世岩浆活动吻合。本文熔结凝灰岩中石炭纪锆石及同时代酸性岩浆岩记录的结晶温度和地壳厚度随时代变新,展示出有规律的变化:1)360~350 Ma锆石结晶温度高,Eu/Eu^(*)值和地壳厚度小,Hf同位素强烈亏损,暗示了地壳减薄过程的存在,与二连—贺根山带开始拉张产生蛇绿岩一致;2)350~340 Ma锆石结晶温度整体下降,而Eu/Eu^(*)值和地壳厚度增大,基于349 Ma酸性火山岩全岩Sr/Y值计算的结果也显示出较大的地壳厚度,这一地壳加厚过程与区域性隆升导致广泛缺失早石炭世沉积地层的浅部过程是耦合的;3)336~304 Ma锆石结晶温度整体高于700℃,而Eu/Eu^(*)值和地壳厚度减小,与同时代酸性岩锆石饱和温度和地壳厚度变化一致,同区域上双峰式火成岩记录的区域性伸展相吻合。上述研究显示了锆石微量元素及其与地壳厚度的关系及时空变化在揭示中亚造山带东部地壳演化过程方面的潜力。

The Carboniferous crustal and tectonic evolution in the eastern part of the Central Asian orogenic belt,especially the Xing’an block,is a hot controversial issue.It is traditionally believed that except for the Early Carboniferous Hegenshan ophiolite,the Early Carboniferous geological records are limited in central Inner Mongolia whereas the Late Carboniferous magmatism and sedimentation are widely developed.This restricts our understanding of the Carboniferous crustal evolution in this region.We carried out a systematic analysis of zircon U-Pb dating,trace elements and Hf isotopes for the volcanoclastic rocks previously believed to be Early Carboniferous Halatumiao Group in northern Erenhot,and discussed their formation age,crystallization temperatures and crustal thickness recorded by zircons and their changes over time.Combined with the regional stratigraphic framework and magmatism,the Carboniferous crustal evolution in the western Xing’an block was further revealed.The dating results show that the formation ages of ignimbrites in the first and second members of the Halatumiao Group are(322±3)and(309±4)Ma,respectively,older than those of tuffs(302 and 298 Ma)in the third and fourth members,indicating that the pyroclastic rocks of the Halatumiao Group are at least the products of the two volcanic eruption stages,i.e.,the early and late Late Carboniferous.There are a lot of Early Carboniferous magmatic zircons,which is consistent with the occurrence of the Early Carboniferous magmatism in the neighboring areas.The crystallization temperatures and crustal thickness recorded by the Carboniferous ignimbrite zircons and coeval felsic igneous rocks show a systematic change over time:1)The 360 to 350 Ma zircons have relatively high crystallization temperatures,low Eu/Eu^(*)values and crustal thickness,and highly depleted Hf isotope,suggesting a crustal thinning process that matches well with the extension of the Early Carboniferous Erenhot-Hegenshan ophiolite belt.2)From 350 to 340 Ma,the decrease of crystallization temperatures and increases of Eu/Eu^(*)and crustal thickness are consistent with the large crustal thickness calculated by the whole-rock Sr/Y values of 349 Ma felsic volcanic rocks,which resulted in the general absence of the Early Carboniferous sedimentary strata.3)From 336 to 304 Ma,the zircon crystallization temperatures are generally higher than 700℃,while Eu/Eu^(*)and crustal thickness are low,which is consistent with both the variation trends of the zircon saturation temperatures and crustal thickness of coeval felsic rocks with time and the regional extension recorded by the bimodal igneous rock association.These studies indicate that zircon trace elements,their relationship with crustal thickness,and their temporal and spatial variation are potential tools to reveal crustal evolution of the eastern part of the Central Asian orogenic belt.