中国大陆科学钻探主孔100～2000m岩石弹性波速度：对地震深反射的约束

在常温常压条件对中国大陆科学钻CCSD主孔岩心的700样品进行了弹性波速度测量,并建立了主孔2000m的波速(Vp和Vs)连续剖面,为检验地球物理模型的合理解释提供了岩石物理学方面的宝贵资料。主孔中新鲜榴辉岩纵波速度(Vp)最大(7.86km／s),正副片麻岩波速最小,又分别为5.53km／s和5．71km／s,榴辉岩的波速随着退变质作用的增强而明显减小。主孔2000m总平均Vp速度为6．2km／s,它与地球物理探测方法获得的大别-苏鲁造山带上地壳具有6.2-6．3km／s高速层结论是一致的。大部分岩石具有明显地震波各向异性。水饱和度使岩石纵波(Vp)速度和剪切波速度(Vs)分别增加19％和6％,而使Vp的各向异性降低3％～4％。不同岩性界面的反射系数(Rc)是产生地震反射的主要原因。金红石榴辉岩与片麻岩之间具有很高的反射系数(0.24-0．31)。韧性剪切带中糜棱岩化片麻岩和面理化榴辉岩使岩石各向异性和反射强度明显增加。岩石微裂隙与主孔原位波速变化有密切关系。饱水岩石速度(Vp和Vs)可以代表CCSD主孔原位状态的地震波速度。上述成果为本区地震反射体成因提供了重要的岩石物理性质约束。     

Metamorphism in the New England Orogen,eastern Australia: a review

Abstract

This paper summarises current knowledge on metamorphism within the entire New England Orogen (NEO) of eastern Australia. Rocks recording metamorphic assemblages characteristic of each of the three metamorphic facies series (high, medium and low P/T) have been identified within the orogen. These include high P/T blueschists and eclogites, mid P/T orogenic metamorphism and low P/T contact aureoles and sub-regional high-temperature–low-pressure (HTLP) metamorphism (regional aureoles). Metamorphism is described as it relates to six tectonic phases of development of the NEO that together comprise two major cycles of compression–extension. Medium–high-grade contact metamorphism spans all six tectonic phases while low-grade burial and/or orogenic metamorphism has been identified for four of the six phases. In contrast, exposure of high P/T eclogites and blueschists, and generation of sub-regional low P/T metamorphism is restricted to extensional phases of the orogen. Hallmarks of the orogen are two newly identified zones of HTLP metamorphism, the older of which extends for almost the entire length of the orogen.

1. KEY POINTS

2. The orogen is dominated by low-temperature rocks while high-temperature amphibolite to granulite facies rocks are restricted to small exposures in HTLP complexes and contact aureoles.

3. Blueschist metamorphism falls into two categories; that associated with subduction during the Currabubula-Connors continental arc phase occurring at depths of ～13–30?km; and the other of Cambrian–Ordovician age, exposed within a serpentinite melange and associated with blocks of eclogite. The eclogite, initially from depths of ～75–90?km, appears to have been entrained in the deep crust for an extended period of geological time.

4. A comprehensive review of contact metamorphism in the orogen is lacking and as studies on low-grade metamorphism are more extensive in the southern part of the orogen than the north, this highlights a second research gap.

     

低温榴辉岩中石榴子石的成分分带:快速抬升的证据

对南大别黄镇冷榴辉岩中——颗石榴子石进行了详细的电子探针工作，对其特殊的成分分带，建立了扩散模型并进行数值模拟，结果得到4．5mm／a的快速抬升速率。该结果表明黄镇榴辉岩曾经历了一个快速的抬升过程。     

大别山下地壳岩石及其深俯冲

本文对大别山下地壳岩石的主要组成及其变质岩石学和年代学特征进行了系统总结,并首次报道了作者等最近在罗田穹隆中发现的镁铁质下地壳岩石俯冲变质成因的榴辉岩及其有关片麻岩的 SHRIMP 锆石 U-Pb 同位素年代学的最新研究成果。大别山这种不寻常的镁铁质下地壳俯冲成因榴辉岩为研究大陆深俯冲和陆壳物质再循环以及对地幔不均一性的影响提供了重要对象和可能性。根据阴极发光图像及矿物包体组合,罗田榴辉岩中继承锆石可分为两种即具有岩浆结晶环带的岩浆锆石和含有石榴子石 紫苏辉石 斜长石等麻粒岩相变质矿物包体的变质锆石,而且这两种继承锆石常具有舍石榴子石 绿辉石等榴辉岩相矿物包体的增生边。SHRIMP U-Pb 定年结果表明,继承岩浆锆石的~(206)Pb/~(238)U 年龄为791±9Ma、含麻粒岩相矿物包体的继承变质锆石的~(206)Pb/~(238)U 年龄为794±10Ma、含榴辉岩相矿物包体的锆石增生边的~(206)Pb/~(238)U 年龄值为212±10Ma。罗田榴辉岩的围岩——含石榴子石英云闪长质片麻岩中锆石的 CL 图像具有特征的核-幔-边结构即表现为继承岩浆锆石核-高压变质幔-退变质增生边,高压变质幔给出罗田片麻岩中印支期高压变质的精确年龄为218±3Ma。因此,罗田榴辉岩的原岩时代与早期麻粒岩相变质时代一致,均为晚元古代,证明华南陆块北缘晚元古代岩浆板底垫托的存在。而且,大别山下地壳岩石参与了印支期大陆深俯冲,但与南大别带具有不同的折返过程即前者经历了麻粒岩相退变质过程。此外,北大别带榴辉岩的印支期峰期变质时代(212±4Ma)比南大别带年轻(226±3Ma),这可用大陆叠瓦式深俯冲模式来解释。     

俄罗斯阿尔丹地块南部辉长-斜长岩杂岩体的构造-物质组分特点及其演化

Ｋａｌａｒｓｋｙ岩体是沿阿尔丹南部边缘分布的规模巨大的ＤｊｕｇｄｊｕｒＫａｌａｒｓｋｙ辉长－斜长岩岩带的一部分。本文研究了代表典型地球早期（前寒武纪）发展史、在行星水准上发育的这个特殊杂岩体的构造－物质组分、形成的阶段性及其继后构造演化。辉长－斜长岩杂岩体产于前寒武纪克拉通化地块交接处及其褶皱的流动性较高的框架中。该杂岩体由两个岩浆岩共生组合组成：硅铝质（斜长岩及辉长－斜长岩）及镁铁质（基性－超基性岩）。体积上前者明显小于后者。这两个岩石共生组合的锶同位素比值都特别低,说明其原始岩浆的地幔属性,是在一个相对宁静的背景中在早期分异出来的产物。辉长－斜长岩杂岩体的演化明显分３个阶段：早期是圈闭结晶岩并将其极度加热的阶段,是庞大的具对称构造的地块的形成阶段;中期是地块的最终固结阶段,在此期间发生了褶皱变形,是高压变质作用的初期阶段;晚期是发生刚性变形,形成辉长－斜长杂岩岩体的岩浆岩岩墙群、链状构造及雁行构造,并发生最大高压变质作用的阶段。文中对与辉长－斜长岩杂岩体共生的似榴辉岩也进行了探讨。     

山东诸诚地区胶南群榴辉岩围岩高压变质作用特征

大别－苏北－胶南－荣成含榴辉岩带长达１３００ｋｍ，胶南合榴辉岩地体位于这一变质带中段。本文通过对胶南地体西北缘的诸城地区榴辉岩围岩的岩石学、矿物学及热力学等方面的研究，首次在该区发现了一些高压的特征矿物：（石榴）镁钠门石暗硬玉片麻岩中暗硬玉具有较高的硬玉分子含量，最高达２２．５ｍｏｌ％，并具明显的成份环带；石榴多硅白云母二长片麻岩及石榴多硅白云母变粒岩中，多硅白云母略具粉红色多色性，其中Ｓｉ＝３．４４～３．５５，ｂ０＝９．０６。同时探讨了胶南群变质作用特征及Ｐ—Ｔ—ｔ演化趋势。     

苏鲁造山带中段宝山-桃行榴辉岩地球化学特征

宝山-桃行榴辉岩呈透镜状包体赋存于荣成花岗质片麻岩套（Chgg）内,通过榴辉岩主量元素地球化学、微量元素及稀土元素地球化学、石榴子石矿物化学分析,确定榴辉岩原岩为壳成变质成因,具有岛弧拉斑玄武岩（IAT）特征.宝山-桃行榴辉岩是其原岩与围岩一起经过高温高压变质作用而形成.     

南黄海千里岩岛榴辉岩的地球化学特征及成因分析

南黄海千里岩榴辉岩对研究华北板块和扬子板块碰撞缝合线有重要意义。千里岩榴辉岩变形作用较弱,经历了多期退变质作用。千里岩榴辉岩原岩为拉斑系列的玄武岩,与大别苏鲁造山带榴辉岩相似。千里岩榴辉岩微量与稀土元素配分模式均与大别苏鲁造山带榴辉岩有较大差异。千里岩榴辉岩大离子亲石元素相对于N-MORB富集,高场强元素特征与N-MORB基本一致,无Nb、Ta、Zr、Ti负异常;稀土元素配分形式为LREE亏损的左倾型。千里岩榴辉岩主微量元素判别图与微量元素特征值均指示其形成的构造环境为弧后盆地。榴辉岩的207Pb/204Pb比值不同于华北板块而接近扬子板块,继承性锆石(747±19)Ma的SHRIMP U-Pb年龄说明其原岩经历了仅在扬子板块有记录的晋宁期热事件。该区所在的千里岩隆起区代表了扬子板块的基底。     

中国的榴辉岩

自20世纪90年代,对中国大别山造山带等地榴辉岩的研究,已成为国际地学界关注的热点。榴辉岩形成于高一超高压地质环境,其分布、地质产状、矿物组合、地球化学特征、成岩温压、成岩年龄及相关矿产等具有自身的特点,对其成因尚有地幔捕获说、岩浆说、变质说等不同认识。     

Exhumation of eclogite and blueschist (Cyclades,Greece): Pressure–temperature evolution determined by thermobarometry and garnet equilibrium modelling

High‐P rocks such as eclogite and blueschist are metamorphic markers of palaeo‐subduction zones, and their formation at high‐P and low‐T (HP–LT) conditions is relatively well understood since it has been the focus of numerous petrological investigations in the past 40 years. The tectonic mechanisms controlling their exhumation back to the surface are, however, diverse, complex and still actively debated. Although the Cycladic Blueschist Unit (CBU, Greece) is among the best worldwide examples for the preservation of eclogite and blueschist, the proposed P–T evolution followed by this unit within the Hellenic subduction zone is quite different from one study to another, hindering the comprehension of exhumation processes. In this study, we present an extensive petrological data set that permits refinement of the shape of the P–T trajectory for different subunits of the CBU on Syros. High‐resolution quantitative compositional mapping has been applied to support the thermobarometric investigations, which involve semi‐empirical thermobarometry, garnet equilibrium modelling and P–T isochemical phase diagrams. The thermodynamic models highlight the powerful use of reactive bulk compositions approximated from local bulk compositions. The results are also combined with Raman spectrometry of carbonaceous material (RSCM) to retrieve the metamorphic peak temperature distribution at the scale of the island. A major result of this study is the good agreement between all the independent thermobarometric methods, permitting reconstruction of the prograde and retrograde P–T trajectories. Garnet compositional zoning was used to retrieve prograde, peak and retrograde growth stages in line with the results of the P–T isochemical phase diagrams, RSCM temperature and peak‐pressure crystallization of the garnet–omphacite–phengite assemblage. Our results are consistent with previous thermobarometric estimates from other occurrences of CBU rocks (Tinos, Andros), suggesting a multistage exhumation process with (1) early syn‐orogenic exhumation within the subduction channel, (2) isobaric heating at mid‐crustal depths (~10–12 kbar) following thermal re‐equilibration of the lithosphere from a cold syn‐orogenic regime in the subduction zone to a warmer post‐orogenic regime in the back‐arc domain and (3) exhumation and cooling related to a post‐orogenic phase of extension following slab retreat. Expanding to the general aspects of subduction zones, we suggest that such metamorphic evolution of HP–LT units should be regarded as a characteristic feature of exhumation driven by slab rollback.