桂西南那坡盆地河口组沉凝灰岩的时代与来源及其大地构造意义

Age and origin of tuffites from the Hekou Formation in the Napo basin,Southwest Guangxi and its tectonic implications

桂西南那坡盆地位于八布-Song Hien构造带,发育早-中三叠世岩浆岩和巨厚海相碎屑岩系,是研究华南地块南缘古特提斯造山过程的关键地区。本文对那坡盆地中三叠统河口组中-上部的两套沉凝灰岩开展了详细的岩石学、地球化学和锆石U-Pb-Hf同位素分析研究。两件沉凝灰岩样品LA-ICP-MS锆石U-Pb定年结果分别为240.6±1.0Ma和236.3±0.8Ma,指示河口组碎屑岩沉积时代为中三叠世拉丁期,部分为晚三叠世卡尼期早期。全岩地球化学结果显示,该沉凝灰岩属于亚碱性英安质,富集轻稀土(LREEs)和大离子亲石元素(LILEs)、Rb、Th和U,亏损高场强元素(HFSEs),具有显著的Nb、Ta和Ti负异常。沉凝灰岩锆石具有较低的ε_(Hf)(t)值(-18.6~-6.3)和较老的地壳模式年龄(t_(DM)^(C)=1664~2434Ma),结合锆石微量元素特征认为其形成于俯冲环境。整体上,那坡盆地河口组沉凝灰岩表现出俯冲作用形成的岛弧火山岩地球化学特征,推断其来源于桂西南凭祥地区的中-晚三叠世酸性火山岩。结合已有区域地质调查成果,认为华南地块南缘八布-Cao Bang古特提斯分支洋盆于晚二叠世至晚三叠世早期向北俯冲,随后自西向东呈“剪刀”式逐渐闭合。华南地块与北越地块完全碰撞拼合的时间发生在晚三叠世中晚期。

The Napo basin,Southwest Guangxi,locates in the Babu-Song Hien tectonic belt and shows widespread Early-Middle Triassic magmatic and massive marine clastic rocks,which plays a key role in studying the evolution of the Paleo-Tethys orogeny along the southern margin of South China block.In this paper,we have carefully analyzed the petrology,geochemistry and zircon U-Pb-Hf isotope for two sets of tuffites in the middleupper part of the Middle Triassic Hekou Formation,Napo basin.LA-ICP-MS zircon U-Pb dating results of the two tuffite samples yield the ages at 240.6±1.0Ma and 236.3±0.8Ma,respectively,indicating that the clastic rocks of Hekou Formation should be deposited during the Middle Triassic Ladinian stage,some of which may extend to the early Carnian of Late Triassic.Whole-rock geochemical results indicate that the tuffites are subalkaline dacitic,enriched in light rare earth elements(LREEs),large ion lithophile elements(LILEs),Rb,Th and U,depleted in high field strength elements(HFSEs),and have significant negative anomalies of Nb,Ta and Ti.The tuffite zircons have lowerε_(Hf)(t)values(-18.6~-6.3)and older crustal model ages(t _(DM) ^(C)=1664~2434Ma).Combined with trace elements,we suggest these zircons may have been formed in subduction-related setting.In summary,the tuffite samples from the Hekou Formation in Napo basin display the geochemical characteristics of island-arc volcanic rock,which are inferred to be derived from the Middle-Late Triassic acidic volcanic rocks in Pingxiang area,Southwest Guangxi.Given the existing geological survey results,it is concluded that the Babu-Cao Bang Paleo-Tethys branch ocean along the southern margin of the South China block should subduct northward during the Late Permian-early Late Triassic,and subsequently closed with scissor-style from west to east.The collision between the South China block and North Vietnam terrane should occur in the middle to late stage of Late Triassic.