蛇绿混杂岩内基性岩锆石年龄的复杂性:以内蒙古贺根山蛇绿岩为例

Complexity of zircon ages of mafic rocks in ophiolitic mélanges:A case from the Hegenshan ophiolite,Inner Mongolia

蛇绿混杂岩是大洋板块俯冲增生的产物,往往发育不同时代、来源和成因的蛇绿岩岩块或岩片,物质组成和结构构造十分复杂。蛇绿混杂岩内发育的基性岩(辉长岩、辉绿岩和玄武岩)是揭示古洋盆时限、性质和造山带演化过程的关键。然而,造山带往往伴随多期次构造、岩浆和变质等事件的叠加改造,如何准确识别蛇绿岩内基性岩组份,并限定其形成时代,一直是蛇绿混杂岩解析的重点和难点。锆石U⁃Pb年代学是最常用的蛇绿岩定年方法之一,但在许多复合造山带内,与蛇绿岩伴生的基性岩锆石年龄往往十分复杂。多期次岩浆、构造、变质或流体作用的叠加可能会改造蛇绿岩的锆石年龄,也可能会以侵入体形式增加识别蛇绿岩组份的难度。本研究以中亚造山带东南缘的贺根山蛇绿岩带为例,通过分析野外接触关系、锆石形貌、U⁃Pb年代学已有的数据等证据,表明贺根山地区的基性岩可能至少存在早石炭世和早白垩世两期,白垩纪基性岩可能是后期岩浆事件的产物,与贺根山地区超基性岩为主的蛇绿岩之间是否同源和时代关系尚不明确。同时,讨论了其它造山带内晚期基性岩侵入蛇绿岩、蛇绿岩组份受年轻的变质—构造—岩浆—流体事件叠加改造等典型研究实例,强调通过详细野外大比例尺填图、接触关系解析、岩石学、地球化学、锆石形貌、U⁃Pb年代学和Hf⁃O同位素等综合研究,才能更为准确地识别蛇绿岩基性岩组份,限定其形成、改造和变质时代。

Ophiolitic mélange is the product of subduction⁃accretion processes of oceanic plates,and it generally consists of ophiolitic blocks or slices with different ages,sources,and origins,showing a great complexity of components and structural styles.Mafic components(e.g.,gabbro,diabase and basalt)in ophiolitic mélanges are important for deciphering the timing and tectonic affinity of paleo⁃ocean basins as well as the tectonic evolution of orogenic belts.However,many orogenic belts are superimposed and modified by multi⁃stage tectonic,magmatic,and metamorphic events.How to accurately identify ophiolitic components and define their formation ages is a crucial issue but generally remains difficult.Zircon U⁃Pb geochronology is one of the most popular methods in dating ophiolites.However,zircon U⁃Pb ages of mafic rocks associated with ophiolitic mélanges could be very complex,especially in some composite orogenic belts.The overprint of multi⁃stage tectonic,magmatic,metamorphic and hydrothermal events may modify zircons of ophiolitic rocks and increase the difficulty in identifying“true”ophiolitic components.In this review,we take the Hegenshan ophiolitic belt in the southeastern margin of the central Asian orogenic belt as an example to discuss the complexity of zircon ages of mafic rocks in orogenic belts.Based on the detailed field mapping,contact relationship investigation,zircon morphology,and U⁃Pb age data,it is suggested that there are at least two episodes of mafic rocks with ages in the Early Carboniferous and Early Cretaceous in the Hegenshan area.The Early Cretaceous mafic rocks may be the product of post⁃accretionary magmatic events,and their age and genetic relations with ultramafic rock⁃dominated Hegenshan ophiolites remain unclear.We also review typical examples from other orogenic belts to show the complexity of zircons in mafic rocks within“ophiolites”,including late mafic rocks intruding ophiolites,and young events overprinting zircon ages of ophiolitic rocks.This review suggests that an integrated study of detailed field mapping,contact relationship investigation,petrology,geochemistry,zircon morphology,U⁃Pb and Hf⁃O isotopes should be carried out to accurately identify the mafic components of ophiolites,and to constrain their ages of formation,modification,and metamorphism.