俯冲带榴辉岩的变形作用及其对恢复俯冲-折返过程的意义

榴辉岩是大洋和大陆俯冲带的重要岩石类型,在研究俯冲带的形成过程、热结构、壳幔相互作用等方面有重要意义。通过对天然和实验样品中石榴子石、绿辉石等矿物的变形特征、变形机制、变形影响因素等的综合分析,系统总结了高压变质带中榴辉岩矿物显微和超微变形研究的进展,探讨了榴辉岩的变形特征对恢复俯冲与折返过程的意义及一些尚待解决的问题。     

北祁连山高压变质带的变形和变质特征及其对俯冲带折返的意义

北祁连造山带是早古生代大洋板块"冷"俯冲的典型地区,形成了一系列高压低温变质的岩石组合.基性榴辉岩是俯冲洋壳变质的产物,根据其结构构造特征可以划分为(1)粒状榴辉岩,峰期变质矿物组合为Grt+Omp+Phn+Rut±Lws,硬柱石呈包裹体和假象分布于石榴石内,变质温压条件T=465～508℃,P=2.30～2.60GPa.(2)片状榴辉岩,岩石具片状构造,其共生矿物组合Omp+Czo/Ep+Phn+Gln构成透入性面理S1和线理L1,以及宏观和显微同斜褶皱F1.其峰期变质温压条件为T=466～510℃,P=1.9～2.2GPa.(3)蓝片岩相退化变质榴辉岩,蓝片岩相变质矿物Gln+Ep+Phn±Ab强烈交代榴辉岩相矿物,并发生强烈D2期变形作用,岩石形成明显的透入性面理S2和线理L2,以及同斜剪切褶皱F2.蓝闪石和多硅白云母线理的统计揭示D2期变形以斜向走滑剪切为主,与D1期变形的榴辉岩的运动学特征大体相似.D2同变质的温压条件为T=422～487℃,P=1.15～1.37GPa.高压变质带晚期的绿片岩相叠加和D3期变形,形成透入性面理S3和线理L3,其运动特征为近于垂直构造线的逆冲剪切.结合榴辉岩变形特征,变质温压条件和同位素年代学资料,我们认为退变质的强弱与变形程度有密切关系.峰期变质后的榴辉岩经过早期绿帘石榴辉岩相到蓝片岩相斜向走滑剪切,和晚期绿片岩相逆冲,在泥盆纪早期快速折返的出露地表.     

鲁东-苏北榴辉岩的构造特征及其折返机制

鲁东-苏北地区出露了M型和Q型两类榴辉岩,前者的原岩为地幔岩类,后者的原岩为地壳岩类。它们均呈外来岩块产出于片麻岩中。其形成经历了前榴辉岩I、A和S阶段,在I和A阶段它们表现为韧性变形,而在S阶段则表现为碎裂变形。它们的折返过程经历了剪切回流和区域性隆起剥蚀。     

鲁东-苏北榴辉岩的构造特征及其折返机制

鲁东-苏北地区出露了M型和Q型两类榴辉岩,前者的原岩为地幔岩类,后者的原岩为地壳岩类。它们均呈外来岩块产出于片麻岩中。其形成经历了前榴辉岩I、A和S阶段,在I和A阶段它们表现为韧性变形,而在S阶段则表现为碎裂变形。它们的折返过程经历了剪切回流和区域性隆起剥蚀。     

地壳俯冲与折返过程的变质作用演化:来自高压-超高压榴辉岩相平衡模拟的证据

本文以大别山双河柯石英榴辉岩为基础,在NCKMnFMASHO体系中计算了P-T、T-M(H₂O)和P-M(H₂O)一系列相图。这些相图表明在地壳冷俯冲(地热梯度约为6℃·km^-1)过程中饱和水的变质基性岩通过脱水反应导致矿物组合演化,随着P-T条件增加,约在2.2GPa(80km)处,绿泥石被滑石取代,在2.4GPa处,蓝闪石消失,在2.5GPa处,滑石消失,在2.9GPa(105km)处,硬柱石消失。相应地可出现硬柱石蓝片岩、蓝闪石-硬柱石榴辉岩、硬柱石榴辉岩和多硅白云母榴辉岩等高压-超高压组合。俯冲基性岩中即使含有很少量的水(如0.3%~0.5%),都会在超高压硬柱石榴辉岩相条件下达到饱和。因此硬柱石会广泛出现于经历冷俯冲地壳的变质基性岩中。高压-超高压榴辉岩的折返过程受折返温度及峰期矿物组合的控制。当峰期矿物组合中含有硬柱石(±蓝闪石±滑石,T=540~600℃)时,其早期折返发生在硬柱石稳定域,受脱水反应控制,难以保存峰期矿物组合;晚期折返发生在绿帘石稳定域,岩石处于流体缺失状态,有利于保存硬柱石消失后的高压矿物组合。当峰期矿物组合中含有绿泥石、硬柱石和蓝闪石时(T<540℃),其折返过程中的脱水作用仅发生在硬柱石与绿帘石共生的狭窄区域,在硬柱石稳定域的早期折返与绿帘石稳定域的晚期折返阶段,都不发生脱水作用,岩石处于流体缺失状态,因此,虽然峰期形成的硬柱石难以保存,但峰期形成的其他矿物可能仅受轻微改造。当硬柱石消失 (T>600℃) 后,多硅白云母高压-超高压榴辉岩中含有很少量水,在早期折返过程中的很大压力范围内,岩石保持水含量不变,更容易保留峰期矿物组合。高压-超高压榴辉岩在减压至1.5GPa以下时,由于外来流体注入,发生部分水化,形成含有钠钙质、钙质角闪石榴辉岩, 它们一般不是平衡矿物组合。榴辉岩中名义上的无水矿物在减压过程中释放的水有助于榴辉岩部分水化,但不足以形成水饱和的斜长角闪岩。     

苏鲁榴辉岩在俯冲峰期-折返早期的递进塑性变形过程及对超高压变质岩石折返动力学的启示

榴辉岩的变形特征严重影响俯冲板片的流变学行为,而石榴石的变形机制又直接制约着榴辉岩的变形属性。本文在结合前人研究的基础上,通过对中国大陆科学钻探（CCSD）主孔榴辉岩中石榴子石的典型变形特征的详细对比和分析,进一步证明了榴辉岩在俯冲峰期条件下可以发生强烈的塑性变形。且俯冲峰期-折返早期经历了从以恢复作用主导的位错蠕变,到动态重结晶、扩散蠕变,再到静态重结晶和矿物生长的连续递进变形过程。其中,从动态重结晶到高温静态重结晶的转换,结合短时增温、局部熔融等证据,共同证明了在苏鲁超高压变质岩石折返过程中存在一个短暂的低应力“热”区间,在此前后,折返机制和驱动力发生了转变,由浮力作用下的快速折返转为以挤压透入性剪应力作用下的缓慢抬升阶段。     

柴达木北缘鱼卡河榴辉岩变质时代及其折返过程

为揭示柴北缘超高压变质带的变质时代与折返过程,利用Ar--Ar同位素定年法对鱼卡河和锡铁山地区的榴辉岩进行了研究。结果表明,鱼卡河榴辉岩的峰期变质作用发生在早—中奥陶世(477～466 Ma),而锡铁山榴辉岩中的韭闪石年龄((407±4)Ma)指示了早泥盆世的退变质时间。综合前人研究成果可知柴北缘超高压变质带形成于495～421 Ma,且具有明显的两阶段特征(477～466 Ma与434～421 Ma),其中早阶段为洋壳俯冲的产物,晚阶段为大洋关闭后陆壳深俯冲的结果。祁连陆块与柴达木—东昆仑板块在早泥盆世发生不同方位的俯冲挤压,并导致超高压变质带折返,超高压变质体普遍发生部分熔融和退变质,出现以榴辉岩为中心向外榴辉岩--榴闪岩--斜长角闪岩--花岗片麻岩渐变过渡的退变质带。     

榴辉岩的折返模式及在分析区域构造演化中的意义

榴辉岩作为板块边缘俯冲带中的高压变质岩石，现常以不同的产状零星出露。关于其呈面状暴露的机制，目前流行有三种模式。即造山楔模式、双轨模式和变质核杂岩模式。通过研究榴辉岩区的岩石组合和岩石的Ｐ－Ｔ－ｔ轨迹，可以了解区域构造及其演化背景。本文综述了研究进展，并以胶南一苏北一大别山榴辉岩带为例说明。     

超高压榴辉岩中绿辉石组构测定及其流变学意义

绿辉石高温变形特征在很大决定了榴辉岩流变学表象,绿辉石变形显微构造和晶格优选方位（ＬＰＯ）的研究是认识榴辉岩变形机制和动力学背景参数（应变,应力,应变速率）的重要途径之一,详细介绍了绿辉石ＬＰＯ测定方法和步骤,报道了潜山县双河地区超高压榴辉岩中绿辉石组构初步研究成果及流变学意义,ＬＰＯ测量成果表明：双河榴辉岩绿辉石组构（００１）极密位于页理面附近,并于近平行线理Ｌ（０１０）环带垂直直于Ｌ（００１）     

Experimental study of metamorphic reactions and dehydration processes at the blueschist–eclogite transition during warm subduction

The transition between blueschist and eclogite plays an important role in subduction zones via dehydration and densification processes in descending oceanic slabs. There are a number of previous petrological studies describing potential mineral reactions taking place at the transition. An experimental determination of such reactions could help constrain the pressure–temperature conditions of the transition as well as the processes of dehydration. However, previous experimental contributions have focused on the stability of spontaneously formed hydrous minerals in basaltic compositions rather than on reactions among already formed blueschist facies minerals. Therefore, this study conducted three groups of experiments to explore the metamorphic reactions among blueschist facies minerals at conditions corresponding to warm subduction, where faster reaction rates are possible on the time scale of laboratory experiments. The first group of experiments was to establish experimental reversals of the reaction glaucophane+paragonite to jadeite+pyrope+quartz+H₂O over the range of 2.2–3.5 GPa and 650–820°C. This reaction has long been treated as key to the blueschist–eclogite transition. However, only the growth of glaucophane+paragonite was observed at the intersectional stability field of both paragonite and jadeite+quartz, confirming thermodynamic calculations that the reaction is not stable in the system Na₂O–MgO–Al₂O₃–SiO₂–H₂O. The second set of experiments involved unreversed experiments using glaucophane+zoisite ±quartz in low‐Fe and Ca‐rich systems and were run at 1.8–2.4 GPa and 600–780°C. These produced omphacite+paragonite/kyanite+H₂O accompanied by compositional shifts in the sodium amphibole, glaucophane, towards sodium–calcium amphiboles such as winchite (?(CaNa)(Mg₄Al)Si₈O₂₂(OH)₂) and barroisite (?(CaNa)(Mg₃Al₂)(AlSi₇)O₂₂(OH)₂). This suggests that a two‐step dehydration occurs, first involving the breakdown of glaucophane+zoisite towards a paragonite‐bearing assemblage, then the breakdown of paragonite to release H₂O. It also indicates that sodium–calcium amphibole can coexist with eclogite phases, thereby extending the thermal stability of amphibole to greater subduction zone depths. The third set of experiments was an experimental investigation at 2.0–2.4 GPa and 630–850°C involving a high‐Fe (Fe#=Fe_total/(Fe_total+Mg)≈0.36) natural glaucophane, synthetic paragonite and their eclogite‐forming reaction products. The results indicated that garnet and omphacite grew over most of these pressure–temperature conditions, which demonstrates the importance of Fe‐rich glaucophane in forming the key eclogite assemblage of garnet+omphacite, even under warm subduction zone conditions. Based on the experiments of this study, reaction between glaucophane+zoisite is instrumental in controlling dehydration processes at the blueschist–eclogite transition during warm subduction.     

拉萨地块吉朗榴辉岩的岩石学研究及其对古特提斯洋壳俯冲折返过程的限定

拉萨地块东部松多(超)高压榴辉岩记录了古特提斯洋俯冲及折返过程。松多榴辉岩带已发现松多、新达多、白朗和吉朗4个榴辉岩出露区,它们的峰期温压条件及变质p-T轨迹的研究对揭示拉萨地块古特提斯时期的俯冲及折返过程有重要意义。松多榴辉岩带东段吉朗榴辉岩的主要矿物为石榴子石、绿辉石、多硅白云母、角闪石、金红石、绿帘石、石英以及退变形成的后成合晶结构(透辉石+角闪石+斜长石)和少量的黑云母。石榴子石具有含丰富矿物包裹体的"脏"核和极少包裹体的"净"边,具有典型的进变质成分环带特征,从核部到边部镁铝榴石组分升高,锰铝榴石和钙铝榴石组分降低。石榴子石边部发育窄的角闪石+斜长石(An=28)组成的冠状体,表明石榴子石边部发生了后期角闪岩相退变质作用。通过变质相平衡模拟计算得到石榴子石以及多硅白云母记录的峰期温压条件为563℃、2. 4 GPa。结合岩相学特征,确定吉朗榴辉岩经历了4期变质演化阶段:(1)进变质阶段以石榴子石核部及其包裹体为代表性矿物组合;(2)峰期变质阶段矿物组合为石榴子石边部、绿辉石、多硅白云母、蓝闪石、硬柱石、金红石和石英;(3)早期退变质阶段以硬柱石分解产生绿帘石为特征;(4)晚期退变质阶段以绿辉石发育后成合晶和石榴子石生长冠状体为特征。认为吉朗榴辉岩为典型的低温高压榴辉岩,经历了顺时针p-T演化轨迹,折返过程为近等温降压过程。与松多带内其他(超)高压岩石相比,吉朗榴辉岩峰期温压条件较低,其围岩为变石英岩,区别于区内其他(超)高压榴辉岩的石榴子石白云母片岩及蛇纹岩围岩。推测吉朗榴辉岩来自于俯冲带浅部,由俯冲隧道中低密度沉积物裹挟折返。     

江尕勒萨依榴辉岩的主要特征及其地质意义

于新疆且末县江尕勒萨依发现两种不同类型的榴辉岩，它们的地质产状、矿物组成、岩石化学成分、地球化学特征及峰期变质温压条件均有明显的差别。北带榴辉岩属暗色富铁型，南带榴辉岩为浅色富镁铝型，两者都具有高压变质带的特征，是塔里木地块东南缘持续俯冲先后与两个或多个前寒武纪微地块碰撞的产物。阿尔金岩群实际上是碰撞拼贴的杂岩带，是加里东碰撞造山带山根之一部分。     

Eclogite and carpholite-bearing metasedimentary rocks in the North Qilian suture zone, NW China: implications for Early Palaeozoic cold oceanic subduction and water transport into mantle

Low‐temperature eclogite and eclogite facies metapelite together with serpentinite and marble occur as blocks within foliated blueschist that was originated from greywacke matrix; they formed a high‐pressure low‐temperature (HPLT) subduction complex (mélange) in the North Qilian oceanic‐type suture zone, NW China. Phengite–eclogite (type I) and epidote–eclogite (type II) were recognized on the basis of mineral assemblage. Relic lawsonite and lawsonite pseudomorphs occur as inclusions in garnet from both types of eclogite. Garnet–omphacite–phengite geothermobarometry yields metamorphic conditions of 460–510 °C and 2.20–2.60 GPa for weakly deformed eclogite, and 475–500 °C and 1.75–1.95 GPa for strongly foliated eclogite. Eclogite facies metasediments include garnet–omphacite–phengite–glaucophane schist and various chloritoid‐bearing schists. Mg‐carpholite was identified in some high‐Mg chloritoid schists. P–T estimates yield 2.60–2.15 GPa and 495–540 °C for Grt–Omp–Phn–Gln schist, and 2.45–2.50 GPa and 525–530 °C for the Mg‐carpholite schist. Mineral assemblages and P–T estimates, together with isotopic ages, suggest that the oceanic lithosphere as well as pelagic to semi‐pelagic sediments have been subducted to the mantle depths (≥75 km) before 460 Ma. Blueschist facies retrogression occurred at c. 454–446 Ma and led to eclogite deformation and dehydration of lawsonite during exhumation. The peak P–Tconditions for eclogite and metapelite in the North Qilian suture zone demonstrate the existence of cold subduction‐zone gradients (6–7 °C km^?1), and this cold subduction brought a large amount of H₂O to the deep mantle in the Early Palaeozoic times.     

俯冲带中大理岩与榴辉岩的Mg-Fe-C-O同位素迁移

碳酸盐岩和碳酸盐化岩石是俯冲带中碳的主要地质载体.俯冲-折返的超高压变质岩记录了俯冲隧道中流体-岩石的相互作用与元素迁移,是研究地球内部元素循环的重要媒介.本文采集了大别造山带罗家岭互层状产出的大理岩和榴辉岩,建立了横跨岩性界面的矿物组成、显微结构和Mg-Fe-C-O同位素剖面.大理岩中主要组成矿物为白云石,其次为方解...     

西藏新达多地区榴辉岩的变质过程研究及其对古特提斯俯冲带演化的限定意义

位于拉萨地块中部松多榴辉岩带西端的新达多地区出露两种类型榴辉岩:含蓝闪石榴辉岩和双矿物榴辉岩。含蓝闪石榴辉岩主要矿物组合为石榴石、绿辉石、蓝闪石、多硅白云母及少量的绿帘石、角闪石、石英、金红石。石榴石不具有成分环带结构,蓝闪石存在于基质中,边部大多退变为冻蓝闪石并普遍发育有角闪石和石英的后成合晶。双矿物榴辉岩的主要矿物组合为石榴石、绿辉石、石英及少量的绿帘石、角闪石、金红石、钛铁矿、榍石。石榴石具有典型的进变质环带特征,从核部到边部镁铝榴石和钙铝榴石组分先升高后降低,铁铝榴石组分变化与之耦合,石榴石边部发育角闪石和钠长石的冠状体,推断石榴石记录了进变质的生长过程后又受到了退变质改造。结合传统温压计和变质相平衡模拟两种温压计算方法对榴辉岩的峰期变质条件进行限定,得到含蓝闪石榴辉岩的峰期温压条件为:615±5℃,33±0.5kbar;双矿物榴辉岩的峰期变质温度为630±10℃,压力不超过27kbar。变质相平衡模拟计算结果显示:(1)含蓝闪石榴辉岩经历了退变初期近等温减压过程,这一过程以硬柱石和少量滑石的脱水反应生成蓝闪石和绿帘石为主要特征;中晚期退变质阶段以大范围硬柱石消失后局部富余流体的消耗在蓝闪石边部形成冻蓝闪石以及蓝闪石边部发育后成合晶为特征,部分石榴石边部的韭闪石冠状体大致也发生在这一过程;(2)双矿物榴辉岩则记录了从进变质生长阶段到峰期变质阶段,最后再到退变质演化阶段的完整变质过程。结合前人对松多榴辉岩的工作,对新达多地区新近发现的两类榴辉岩的岩石学研究表明:拉萨地块内部的榴辉岩为典型的大洋俯冲带产物,代表了古特提斯洋盆的存在。俯冲折返过程中复杂的构造机制使得不同类型榴辉岩在同一地区出露共生。     

Dynamics of Saxothuringian subduction channel/wedge constrained by phase‐equilibria modelling and micro‐fabric analysis

Subduction and exhumation dynamics can be investigated through analysis of metamorphic and deformational evolution of associated high‐grade rocks. The Erzgebirge anticline, which forms at the boundary between the Saxothuringian and Teplá‐Barrandian domains of the Bohemian Massif, provides a useful study area for these processes owing to the occurrence of numerous meta‐basites preserving eclogite facies assemblages, and coesite and diamond bearing quartzofeldspathic lithologies indicating subduction to deep mantle depths. The prograde and retrograde evolution of meta‐basite from the Czech portion of the Erzgebirge anticline has been constrained through a combination of thermodynamic modelling and conventional thermobarometry. Garnet growth zoning indicates that the rocks underwent burial and heating to peak conditions of 2.6 GPa and at least 615 °C. Initial exhumation occurred with concurrent cooling and decompression resulting in the growth of amphibole and zoisite poikiloblasts overgrowing and including the eclogite facies assemblage. The development of clinopyroxene–plagioclase–amphibole symplectites after omphacite and Al‐rich rims on matrix amphibole indicate later heating at the base of the lower crust. Omphacite microstructures, in particular grain size analysis and lattice‐preferred orientations, indicate that the prograde evolution was characterized by a constrictional strain geometry transitioning into plane strain and oblate fabrics during exhumation. The initial constrictional strain pattern is interpreted as being controlled by competing slab pull and crustal buoyancy forces leading to necking of the subducting slab. The transition to plane strain and flattening geometries represents transfer of material from the subducting lithosphere into a subduction channel, break‐off of the dense slab and rebound of the buoyant crustal material.     

Deformation and metamorphism in the Bard area of the Sesia Lanzo Zone, Western Alps, during subduction and uplift

The structure, microstructure and petrology of a small area close to the village of Bard in Val d'Aosta (Italy) has been studied in detail. The area lies across the contact between the Gneiss Minuti (GM) and the Eclogitic Micaschist (EMS) Complexes of the Lower element of the Sesia portion of the Sesia-Lanzo Zone (Western Alps). Both complexes have undergone high-pressure metamorphism, but the metamorphic assemblages indicate a sudden increase in pressure in going across the contact from the GM to the EMS. Therefore, we interpret the contact as a thrust dividing the lower element of the Sesia into two sub-elements. This interpretation is supported by structural evidence.
The early Alpine (90-70 Ma) metamorphic history is best preserved in the EMS and is one of increasing pressure associated with thrusting. The maximum P/T recorded in the EMS is >1500 MPa (>15kbar) and 550°C and in the GM is < 1500-1300 MPa (< 15-13 kbar) and 500-550°C. We suggest that the rocks were probably in an active Benioff zone during this time.
From then on the histories of the GM and EMS are the same. Deformation continued and the thrust and thrust slices were folded during decreasing pressure. We interpret the first postthrusting deformation in terms of uplift associated with continued shortening of the crust and underplating after the Benioff zone had become inactive and a new Benioff zone had developed further to the north-west.
A still later deformation and the Lepontine metamorphism (38 Ma) are related to continued uplift. Much of this deformation is characterized by structures indicative of vertical shortening and lateral spreading as the mountains rose above the general level of the surface.