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研究表明,在大陆俯冲带变质过程中,大离子亲石元素(LILE)和轻稀土(LREE)具有较高的活动性,而高场强元素(HFSE)和重稀土(HREE)是不活动的。这些元素的活动可能是由折返过程中大陆地壳的部分熔融导致的,因此指示了超临界流体的出现。折返过程中大陆地壳的这种部分熔融作用导致了不同板片组分之间的质量迁移,但是这种有效的迁移只发生在很小的尺度范围内,在岩相学接触带和裂隙带的局部开放系统中。
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)Anδ~(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 m.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 bv 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±3 Ma,with timescale of 1.9 to 9.3 Myr or less for this process.(4)Despite the widespread retrogression in petrography subsequent to peak UHP conditions,retrograde fluid was internally buffered in stable isotope compositions,and the retrograde fluid was of deuteric origin and thus was derived from the decompression exsolution of structural hydroxyl and molecular H_2O in nominally anhydrous minerals.A quantitative estimate suggests that the fluid derived from the decompression exsolution can provide a sufficient amount of water for amphibolization of UHP eclogites.(5)Changes in mineral O isotope,H isotope and water content occur in eclogite-gneiss transitions,concordant with petrographic changes.This suggests that the contact between different lithologies is the most favorable place for fluid activity;and that fluid for retrogression of the eclogites away from the eclogite-gneiss boundary was derived from the decompression exsolution.For the eclogites adjacent to gneiss,in contrast,the retrograde metamorphism was principally caused by aqueous fluid from the gneiss which is relatively rich in water.Inspection of the relationship between the distance,petrography andδ~(18)O values of adjacent samples shows O isotope heterogeneities between the different and same lithologies on scales of 20 to 50 cm,corresponding to the maximum scales of fluid mobility during the continental collision.(6)Studies of major and trace elements in the two continuous core segments indicate high mobility of LILE and LREE but immobility of HFSE and HREE.Some eclogites have andesitic compositions with high SiO_2,alkalis,LREE,and LILE but low CaO,MgO and FeO contents.These features likely result from chemical exchange with gneisses,possibly due to the metasomatism of felsic melt produced by partial melting of the associated gneisses during the exhumation.On the other hand,some eclogites appear to have geochemical affinity to refractory rocks formed by melt extraction as indicated by strong LREE and LILE depletion and the absence of hydrous minerals.These results provide evidence for melt-induced element mobility in the UHP metamorphic rocks,and thus the possible presence of supercritical fluid during exhumation.In particular,large variations in the abundance of such elements as SiO2,LREE and LILE occur at the contact between eclogite and granitic gneiss,indicating their mobility between different slab components,although it only occurs on small scales and is thus limited in local open-systems.