大陆碰撞和超高压变质的化学地球动力学：来自中国大陆科学钻探的结果  

Geodynamics of continental collision and ultrahigh-pressure metamorphism:Constraints from Chinese Continental Scientific Drilling project.

作　　者：郑永飞[1] 赵子福[1] 陈仁旭[1] 

机构地区：[1]中国科学院壳幔物质与环境重点实验室,中国科学技术大学地球和空间科学学院,合肥230026

出　　处：《岩石学报》2007年第12期3078-3094,共17页Acta Petrologica Sinica

基　　金：国家973计划项目(2003CB716501);国家自然科学基金项目(40573011)资助成果

摘　　要：利用中国大陆科学钻探工程连续取样的优势,对主孔200～4000m超高压变质岩进行了系统的地球化学研究,结果对大陆俯冲带变质的化学地球动力学提供了新的制约。目前取得的主要成果如下:(1)建立了200～4000m深度的超高压矿物氧同位素变化剖面,发现^(18)O亏损深度达3300m。结合大别-苏鲁造山带地表露头岩石达30000km^2的^(18)O亏损,表明扬子陆块北缘曾发生三维空间巨量物质(～100000km^3)的^(18)O亏损;(2)钻孔超高压变质岩的原岩形成于新元古代中期(～780Ma),属于双重双峰式岩浆活动的产物;原岩既有中元古代晚期新生地壳,也有古元古代中期古老地壳。这种双重双峰式岩浆活动出现在弧-陆碰撞带基础上发育的裂谷构造带,是对Rodinia超大陆开始裂解的响应;(3)高压榴辉岩相重结晶作用发生在216±3Ma,高压榴辉岩相重结晶作用持续的时间为1.9～9.3Myr或更短;(4)不同岩性接触带的矿物O同位素、H同位素、全水含量以及羟基含量的研究表明,退变质流体在同位素组成上是内部缓冲的,且退变质流体主要来自于名义上无水矿物中结构羟基和分子水的出溶。定量计算表明,这些出溶的结构羟基和分子水能为角闪岩相退变质作用提供足够的流体来源;(5)在不同岩性接触带,稳定同位素、水含量及岩相学上出现同步变化,表明不同岩性接触带可能是流体活动最剧烈的地方。对于远离榴辉岩-片麻岩接触带的榴辉岩,退变质流体主要来自榴辉岩内部名义上无水矿物中结构羟基和分子水的出溶。对于靠近接触带的榴辉岩,则可能主要受来自比榴辉岩更富水的片麻岩中矿物的脱水作用。根据相邻样品之间的距离、岩相学和δ^(18)O值,可以确定出不同岩性和相同岩性之间氧同位素不均一性的尺度为20到50cm,对应于大陆碰撞过程中流体活动的最大尺度;(6)对两个连续岩芯段的主量和微量元素剖面By taking advantage of having depth profiles from core samples of the Chinese Continental Scientific Drilling (CCSD) project, a series of combined studies were carried out for ultrahigh-pressure (UHP) metamorphic rocks from CCSD main hole (MH) at continuous depths of 200 to 4000 m. The results provide new constraints on the geodynamics of continental subduction-zone metamorphism, especially on the issues that are not able to be resolved from the surface outcrops. This paper presents a summary of relevant results obtained so far from our geochemical studies of CCSD-MH core samples. They can be outlined as follows. (1) An delta(18)O profile of 200 to 4000 m is established for the UHP metamorphic minerals, with finding of (18)O depletion as deep as 3300 n. Along with areal (18)O depletion of over 30000 km(2) along the Dabie-Sulu orogenic belt, the three-dimensional (18)O depletion of over 100000 km(3) occurs along the northern margin of the Yangtze Block. (2) Zircon U-Pb age and Hf isotope studies of UHP rocks from two CCSD-MH core segments suggest that mid-Neoproterozoic protoliths of bimodal UHP metaigneous rocks were formed during supercontinental rifting in the northern margin of the Yangtze Block, with contrasting origins of magmatic source. The first type of bimodal magmatism was formed by prompt reworking of juvenile late Mesoproterozoic crust, whereas the second type of bimodal magmatism was principally generated by rifting anatexis of ancient middle Paleoproterozoic crust. The dual bimodal magmatism occurred during the supercontinental rifting along preexisting arc-continent collision orogen, in response to the attempted breakup of the supercontinent Rodinia at about 780 Ma. (3) A combined study of petrography and geochronology reveals occurrence of HP eclogite-facies recrystallization phase at 216 +/- 3Ma, with timescale of 1.9 to 9.3Myr or less for this process. (4) Despite the widespread retrogression in petrography subsequent to peak UHP conditions, retrograde fluid was internally buffered in stable iso

关 键 词：大陆地壳 超高压变质 稳定同位素 流体活动 元素活动性 地球动力学 科学钻探 

分 类 号：P588.348[天文地球—岩石学] P597.2[天文地球—地质学]