PTtD Path and Fluid Evolution of HighPressure Eclogites from the Dabie Mountains,Central East China

１．Ｉｎｔｒｏｄｕｃｔｉｏｎ　ＴｈｅＱｉｎｌｉｎｇＤａｂｉｅｏｒｏｇｅｎｉｃｂｅｌｔｗａｓｆｏｒｍｅｄｂｙｃｏｌｌｉｓｉｏｎｂｅｔｗｅｅｎｔｈｅＮｏｒｔｈＣｈｉｎａａｎｄＹａｎｇｔｚｅｂｌｏｃｋｓ．Ｔｈｅｃｏｒｅｐａｒｔｏｆｔｈｅｏｒｏｇｅｎｉｃｂｅｌｔｃｏｎｓｉｓｔｓｏｆｓｅｖｅｒａｌｍｅｔａｍｏｒｐｈｉｃｒｏｃｋｇｒｏｕｐｓ,ｉｎｃｌｕｄｉｎｇｔｈｅＤａｂｉｅ（Ｔｏｎｇｂａｉ）ｃｏｍｐｌｅｘ,Ｈｏｎｇ’ａｎ（Ｓｕｓｏｎｇ）ｇｒｏｕｐ,ＳｕｊｉａｈｅｇｒｏｕｐａｎｄＳｕｉｘｉａｎ（Ｙａｏ…     

Petrogenesis of Eclogites in the Light of Punctuated Metamorphic Evolution in Dabie Terrane，China

Ｐｅｔｒｏｇｅｎｅｓｉｓ　ｏｆ　Ｅｃｌｏｇｉｔｅｓ　ｉｎ　ｔｈｅ　Ｌｉｇｈｔ　ｏｆ　ＰｕｎｃｔｕａｔｅｄＭｅｔａｍｏｒｐｈｉｃ　Ｅｖｏｌｕｔｉｏｎ　ｉｎ　Ｄａｂｉｅ　Ｔｅｒｒａｎｅ，Ｃｈｉｎａ￥ＹｏｕＺｈｅｎｄｏｎｇ；ＨａｎＹｕｊｉｎｇ；ＺｈｏｎｇＺ...     

安徽省大别山东段江岭地区榴辉岩及其围岩的变质作用研究

安徽省大别山东段江岭地区的南大别碰撞杂岩主要由榴辉岩、云母片岩－变粒岩及片麻岩系组成,这三套岩石具有不同的变质演化阶段和Ｐ－Ｔ轨迹：榴辉岩有５个变质阶段,早期出现超高压型、高压型、晚期为中压型;云母片岩－变粒岩有４个变质阶段,早期出现高压型,晚期为中压型;片麻岩系仅有一期中压绿帘角闪岩相变质。三套岩石变质历史的差异,反映其所经历的构造过程不同,三者在构造作用下混杂到一起,构成碰撞混杂岩。     

Ultrahigh Pressure Metamorphism and Tectonic Evolution of Southwestern Tianshan Orogenic Belt,China: A Comprehensive Review

Recently, a huge ultrahigh‐pressure (UHP) metamorphic belt of oceanic‐type has been recognized in southwestern (SW) Tianshan, China. Petrological studies show that the UHP metamorphic rocks of SW Tianshan orogenic belt include mafic eclogites and blueschists, felsic garnet phengite schists, marbles and serpentinites. The well‐preserved coesite inclusions were commonly found in eclogites, garnet phengite schists and marbles. Ti‐clinohumite and Ti‐chondrodite have been identified in UHP metamorphic serpentinites. Based on the PT pseudosection calculation and combined U‐Pb zircon dating, the P‐T‐t path has been outlined as four stages: cold subduction to UHP conditions before ～320 Ma whose peak ultrahigh pressure is about 30 kbar at 500oC, heating decompression from the Pmax to the Tmax stage before 305 Ma whose peak temperature is about 600oC at 22kbar, then the early cold exhumation from amphibolite eclogite facies to epidote‐amphibolite facies metamorphism characterized by ITD PT path before 220 Ma and the last tectonic exhumation from epidote amphibolite facies to greenschist facies metamorphism. Combining with the syn‐subduction arc‐like 333‐326 Ma granitic rocks and 280‐260 Ma S‐type granites in the coeval low‐pressure and high‐temperature (LP‐HT) metamorphic belt, the tectonic evolution of Tianshan UHP metamorphic belt during late Cambrian to early Triassic has been proposed.     

大别地区的变质作用及与碰撞造山过程的关系

大别造山带从南到北可分为５个变质构造单元：扬子北缘蓝片岩带、突松变质杂岩带、南大虽碰撞杂岩带、北大别变质杂岩带和北淮阳变质带。各个变质构造单元中不同岩石的变质作用可划分为３种类型：（１）超高压型。以含柯石英（及金刚石）的榴辉岩为代表,仅见于南大别碰撞杂岩带中,这类岩石的ＰＴ轨迹反映洋壳Ｂ型俯冲的特点。（２）高压型。见于大别山南部的蓝片岩带、宿松变质杂岩带和南大别杂岩中的变质沉积岩及部分片麻岩中,与     

桐柏—大别山高压超高压变质带

桐柏—大别山高压超高压变质带自南向北可划分为３个带：绿帘－蓝片岩相变质带，高压榴辉岩相变质和超高压榴辉岩相变质带，超高压变质带形成于加里东期洋壳俯冲作用过程中，而前两个高压变质带则是印支期陆－陆俯冲－碰撞作用的产物。     

High-pressure/low-temperature metamorphism in northern Hubei Province, central China

Abstract The Qinling–Dabie accretionary fold belt in east-central China represents the E–W trending suture zone between the Sino-Korean and Yangtze cratons. A portion of the accretionary complex exposed in northern Hubei Province contains a high-pressure/low-temperature metamorphic sequence progressively metamorphosed from the blueschist through greenschist to epidote–amphibolite/eclogite facies. The 'Hongan metamorphic belt'can be divided into three metamorphic zones, based on progressive changes in mineral assemblages: Zone I, in the south, is characterized by transitional blueschist–greenschist facies; Zone II is characterized by greenschist facies; Zone III, in the northernmost portion of the belt, is characterized by eclogite and epidote–amphibolite facies sequences. Changes in amphibole compositions from south to north as well as the appearance of increasingly higher pressure mineral assemblages toward the north document differences in metamorphic P–T conditions during formation of this belt. Preliminary P–T estimates for Zone I metamorphism are 5–7 kbar, 350–450°C; estimates for Zone III eclogites are 10–22 kbar, 500 ± 50°C.
The petrographic, chemical and structural characteristics of this metamorphic belt indicate its evolution in a northward-dipping subduction zone and subsequent uplift prior to and during the final collision between the Sino-Korean and Yangtze cratons.     

关于大别山榴辉岩相变质作用

大别山产出的榴辉岩相岩石包括石榴橄榄岩、榴辉岩、榴云片岩、榴辉片麻岩、榴玉英岩和榴辉大理岩等不同系列,它们均分布于花岗质片麻岩中。矿物共生序列研究表明,榴辉岩相岩石经历了从绿帘角闪岩相、柯石英榴辉岩相、角闪榴辉岩相、绿帘角闪岩相到绿片岩相的演化过程。花岗质片麻岩及变质火山—沉积岩系并未经历超高压变质作用,但却与榴辉岩相岩石经历了同一期绿帘角闪岩相变质事件,证明二者在地壳范围内发生了构造合并     

造山带中成对出现的高压麻粒岩与榴辉岩及其地球动力学意义

在一些典型碰撞造山带中,高压麻粒岩与榴辉岩在空间和时间上密切相关,它们之间的关系对揭示碰撞造山带的造山过程和造山机制具有重要意义.本文以中国西部的南阿尔金、柴北缘及中部的北秦岭造山带为例,详细陈述了这3个地区榴辉岩和相关的高压麻粒岩的野外关系、变质演化和形成时代,目的是要建立大陆碰撞造山带中榴辉岩和相关高压麻粒岩形成的地球动力学背景模式.南阿尔金榴辉岩呈近东西向分布在江尕勒萨依,玉石矿沟一带,与含夕线石副片麻岩、花岗质片麻岩和少量大理岩构成榴辉岩一片麻岩单元,榴辉岩中含有柯石英假象,其峰期变质条件为P=2.8～3.0GPa,T=730～850℃,并在抬升过程中经历了角闪岩-麻粒岩相的叠加;大量年代学研究显示其峰期变质时代为485～500Ma.南阿尔金高压麻粒岩分布在巴什瓦克地区,包括高压基性麻粒岩和高压长英质麻粒岩,它们与超基性岩构成了一个大约5km宽的构造岩石单元,与周围角闪岩相的片麻岩为韧性剪切带接触.长英质麻粒岩和基性麻粒岩的峰期组合均具有蓝晶石和三元长石(已变成条纹长石),形成的温压条件为T=930～1020℃,P=1.8～2.5GPa,并在退变质过程中经历了中压麻粒岩相变质作用叠加.锆石SHRIMP测定显示巴什瓦克高压麻粒岩的峰期变质时代为493～497Ma.都兰地区的榴辉岩分布柴北缘HP-UHP变质带的东端,在榴辉岩和围岩副片麻岩中均发现有柯石英保存,形成的峰期温压条件为T=670～730℃和P=2.7～3.25GPa,退变质阶段经过了角闪岩相的叠加;榴辉岩相变质时代为420～450Mao都兰地区的高压麻粒岩分布在阿尔茨托山西部,高压麻粒岩包括基性麻粒岩长英质麻粒岩,基性麻粒岩的峰期矿物组合为Grt+Cpx+Pl±Ky±Zo+Rt±Qtz,长英质麻粒岩的峰期矿物组合为：Grt+Kf+Ky+Pl+Qtz.峰期变质条件为T=800～925℃,P=1.4～1.85GPa,退变质阶段经历了角闪岩-绿片岩的改造,高压麻粒岩的变质时代为420～450Ma.北秦岭榴辉岩分布在官坡-双槐树一带,榴辉岩的峰期变质组合为Grt+Omp±Phe+Qtz+Rt,所计算的峰期温压条件为T=680～770℃和P=2.25～2.65GPa,年代学数据显示榴辉岩的变质时代为500Ma左右.北秦岭高压麻粒岩分布在含榴辉岩单元的南侧松树沟一带,包括高压基性麻粒岩和高压长英质麻粒岩,与超基性岩在空间上密切伴生,高压麻粒岩的峰期温压条件为T=850～925℃,P=1.45～1.80GPa,锆石U-Pb年代学研究显示其峰期变质时代为485～507Ma.以上三个实例显示,出现在同一造山带、在空间上伴生的高压麻粒岩和榴辉岩有各自不同的变质演化历史,但榴辉岩中的榴辉岩相变质时代和相邻的高压麻粒岩中的高压麻粒岩相变质作用时代相同或相近,这种成对出现的榴辉岩和高压麻粒岩代表了它们同时形成在造山带中不同的构造环境中,即榴辉岩的形成于大陆俯冲带中,而高压麻粒岩可能形成在俯冲带之上增厚的大陆地壳根部.     

大别山超高压变质带的构造背景

大别山南部的超高压变质带具有特征的榴辉岩相矿物组合，榴辉岩的岩石化学及稀土元素特征及其伴生的岩石组合，表明这个带是以陆壳成分为主混有少量上地幔及洋壳成分的混杂岩，榴辉岩相围岩和大别群具有不同的变质和变形特征。超高压变质带形成于扬子和中朝板块大陆碰撞的构造环境，是扬子板块陆壳向北俯冲到一定深度的变质产物。     

Metamorphic evolution of eclogite facies rocks in Northern Hubei and Southern Henan, China

Eclogite fades rocks in this area are diverse in rock type. The field occurrence and rock-chemistry reflect theirin-situ origin. Based on their regional geology and field occurrence, two groups of eclogites are divided in terms of their peak temperature of metamorphism. Medium-temperature eclogites (MT), as Group B, occur in the Dabie Group. They were formed from epidote-amphibolite facies. The metamorphism of eclogite facies has two stages: the coesite eclogite facies stage (the peak condition:T = 600 -700°C,P = 2.7-3.0 GPa) and the glaucophane eclogite facies stage (the pressure decreases, may be lower than 2.5 GPa while the temperature has little change). Low temperature eclogites (LT), as Group C, occur in the Qijiaoshan Formation. They were formed from blue schist facies (the peak condition:T = 490–560°C,P< 1.5 GPa). The appearance of hydrous minerals in the eclogites indicates the important role of water in metamorphism. Medium-temperature eclogites are different from low-temperature ones in metamorphism. At last, the evolution of the high-pressure metamorphic belt is discussed as well. This research project was financially supported by the National Natural Science Foundation of China (No. 49372100).     

Prograde and Retrograde Eclogites in the Sambagawa Metamorphic Belt, Besshi District, Japan

In the Besshi district of the Sambagawa metamorphic belt, thereare two types of eclogites: one occurring in the Sebadani metagabbromass and retrograded from high-temperature anhydrous ecologite,and the other in the basic schists and produced by prograde,dehydration of epidote amphibolite. The Sebadani metagabbro mass was originally layered gabbro,which was once equilibrated in the ecologite facies before emplacementinto the Sambagawa terrain as a hot eclogite mass. Basic schistssurrounding the Sebadani mass, which had suffered the Sambagawametamorphism of albite epidote amphibolite facies, were contact-metamorphosedat high pressure by the emplacement of the Sebadani mass. Asa result, the basic schists were dehydrated to form eclogiticbasic schists, i.e. garnet and omphacite porphyroblast-bearingbasic schists. Thus, two types of ecologite, retrograde andprograde, converged into the same metamorphic condition, 610–650?C, 7–17 kb, in a part of the ecologite facies duringthe Sambagawa metamorphism. Correspondingly, the values of distributioncoefficients of Fe and Mg between garnet and omphacite increasefrom core pairs to rim pairs in the retrograde eclogites anddecrease from core pairs to rim pairs in the prograde ecologites.After this stage, both the prograde and retrograde eclogitesshared a common metamorphic history; they were retrograded viathe epidote amphibolite facies to the greenschist facies. The Sebadani metagabbro mass, as a large tectonic block, hadbeen emplaced into a m?lange zone in the Sambagawa metamorphicbelt after the peak of the Sambagawa metamorphism, probablyfollowing initiation of uplift of the metamorphic rocks fromtheir deep-seated environment.     

桐柏-大别山区高压变质相的构造配置

作为华北和扬子陆块间的碰撞造山带桐柏大别山区以发育高压、超高压变质带为特征,从南到北变质相从低级到高级,代表俯冲带深度不同的变质产物,整体形成高压变质相系列。不过现今各变质相岩石的分布极受后期地壳规模的伸展构造控制,大别杂岩的穹隆作用更使高压变质相带的空间分布复杂化。超高压变质岩今日多呈大小不等的块体嵌布于相对低压的大别杂岩之内,造山带根部物质的热软化,使许多深层地幔物质得以像挤牙膏一样挤出于大别杂岩内。它们之中广泛发育着减压退变质的显微结构,与大别杂岩内一些麻粒岩相表壳岩所保存的减压退变质证迹一样,同是挤出作用和碰撞后隆升的构造证迹。高压相系的发育使南桐柏山和大别山迥然不同于桐商（ 商丹） 断裂以北的北秦岭北淮阳变质带。新近发表的同位素年代学（４０Ａｒ ３９ Ａｒ） 资料：３１６ ～４３４ Ｍａ ,已证明北秦岭是古生代变质带,它与桐柏－ 大别印支期碰撞造山带差异甚大。这两个变质地温梯度差异甚大的变质地体的拼合,说明华北和扬子陆块碰撞的主缝合带是商丹－ 桐商断裂带     

Metamorphic history of eclogites and country rock gneisses in the Aktyuz area,Northern Tien‐Shan,Kyrgyzstan: a record from initiation of subduction through to oceanic closure by continent–continent collision

Eclogites and related high‐P metamorphic rocks occur in the Zaili Range of the Northern Kyrgyz Tien‐Shan (Tianshan) Mountains, which are located in the south‐western segment of the Central Asian Orogenic Belt. Eclogites are preserved in the cores of garnet amphibolites and amphibolites that occur in the Aktyuz area as boudins and layers (up to 2000 m in length) within country rock gneisses. The textures and mineral chemistry of the Aktyuz eclogites, garnet amphibolites and country rock gneisses record three distinct metamorphic events (M1–M3). In the eclogites, the first MP–HT metamorphic event (M1) of amphibolite/epidote‐amphibolite facies conditions (560–650 °C, 4–10 kbar) is established from relict mineral assemblages of polyphase inclusions in the cores and mantles of garnet, i.e. Mg‐taramite + Fe‐staurolite + paragonite ± oligoclase (An_<16) ± hematite. The eclogites also record the second HP‐LT metamorphism (M2) with a prograde stage passing through epidote‐blueschist facies conditions (330–570 °C, 8–16 kbar) to peak metamorphism in the eclogite facies (550–660 °C, 21–23 kbar) and subsequent retrograde metamorphism to epidote‐amphibolite facies conditions (545–565 °C and 10–11 kbar) that defines a clockwise P–T path. thermocalc (average P–T mode) calculations and other geothermobarometers have been applied for the estimation of P–T conditions. M3 is inferred from the garnet amphibolites and country rock gneisses. Garnet amphibolites that underwent this pervasive HP–HT metamorphism after the eclogite facies equilibrium have a peak metamorphic assemblage of garnet and pargasite. The prograde and peak metamorphic conditions of the garnet amphibolites are estimated to be 600–640 °C; 11–12 kbar and 675–735 °C and 14–15 kbar, respectively. Inclusion phases in porphyroblastic plagioclase in the country rock gneisses suggest a prograde stage of the epidote‐amphibolite facies (477 °C and 10 kbar). The peak mineral assemblage of the country rock gneisses of garnet, plagioclase (An_11–16), phengite, biotite, quartz and rutile indicate 635–745 °C and 13–15 kbar. The P–T conditions estimated for the prograde, peak and retrograde stages in garnet amphibolite and country rock are similar, implying that the third metamorphic event in the garnet amphibolites was correlated with the metamorphism in the country rock gneisses. The eclogites also show evidence of the third metamorphic event with development of the prograde mineral assemblage pargasite, oligoclase and biotite after the retrograde epidote‐amphibolite facies metamorphism. The three metamorphic events occurred in distinct tectonic settings: (i) metamorphism along the hot hangingwall at the inception of subduction, (ii) subsequent subduction zone metamorphism of the oceanic plate and exhumation, and (iii) continent–continent collision and exhumation of the entire metamorphic sequences. These tectonic processes document the initial stage of closure of a palaeo‐ocean subduction to its completion by continent–continent collision.     

西藏松多榴辉岩变质作用研究

西藏拉萨地块松多附近新发现一条榴辉岩带,长约100 km,宽约2~3 km。松多榴辉岩主要经历了进变质的绿帘石榴辉岩相-峰期的榴辉岩相-退变质的角闪岩相3个阶段。岩石学研究表明,峰期的特征矿物组合是石榴子石绿辉石多硅白云母金红石,峰期温压条件是760~800 ℃,33~39 GPa。这表明松多地区可能曾经历超高压变质作用,之后快速返回,p T轨迹呈“发卡”状,后期退变质经历了角闪石榴辉岩相阶段。研究松多榴辉岩表明,拉萨地块内部有一条新的缝合带,这对于了解拉萨地块和古特提斯洋的演化有重要意义。     

A cold Early Palaeozoic subduction zone in the North Qilian Mountains, NW China: petrological and U-Pb geochronological constraints

The north Qilian high‐pressure (HP)/low‐temperature (LT) metamorphic belt is composed mainly of blueschists, eclogites and greenschist facies rocks. It formed within an Early Palaeozoic accretionary wedge associated with the subduction of the oceanic crust and is considered to be one of the best preserved HP/LT metamorphic belts in China. Here we report new lawsonite‐bearing eclogites and eclogitic rocks enclosed within epidote blueschists in the North Qilian Mountains. Five samples contain unaltered lawsonite coexisting with omphacite and phengite as inclusions in garnet, indicating eclogite facies garnet growth and lawsonite pseudomorphs were observed in garnet from an additional 11 eclogites and eclogitic rocks. Peak pressure conditions estimated from lawsonite omphacite‐phengite‐garnet assemblages were 2.1–2.4 GPa at temperatures of 420–510 °C, in or near the stability field of lawsonite eclogite, and implying formation under an apparent geothermal gradient of 6–8 °C km^?1, consistent with metamorphism in a cold subduction zone. SHRIMP U‐Pb dating of zircon from two lawsonite‐bearing eclogitic metabasites yields ages of 489 ± 7 Ma and 477 ± 16 Ma, respectively. CL images and mineral inclusions in zircon grains indicate that these ages reflect an eclogite facies metamorphism. An age of 502 ± 16 Ma is recorded in igneous cores of zircon grains from one lawsonite pseudomorph‐bearing eclogite, which is in agreement with the formation age of Early Ordovician for some ophiolite sequences in the North Qilian Mountains, and may be associated with a period of oceanic crust formation. The petrological and chronological data demonstrate the existence of a cold Early Palaeozoic subduction zone in the North Qilian Mountains.     

柴北缘都兰高压麻粒岩的锆石U-Pb定年及其地质意义

在柴北缘高压-超高压变质带的东端都兰地区,高压麻粒岩以透镜体的形式存在于石榴白云母片岩、花岗质片麻岩以及斜长角闪岩中。高压麻粒岩的主体为基性麻粒岩,并含少量中酸性麻粒岩。基性麻粒岩主要由石榴子石、单斜辉石、斜长石和石英等组成,而中酸性麻粒岩峰期矿物组合为:石榴子石+斜长石+钾长石+蓝晶石+石英±单斜辉石。根据显微构造和反应结构特征,主要识别出3期变质作用:①峰期高压麻粒岩相阶段(M1);②退变质高角闪岩相阶段(M2);③绿片岩相/低角闪岩相阶段(M3)。选取典型的中酸性麻粒岩样品进行了锆石LA-ICP-MSU-Pb原位定年分析,获得加权平均年龄为446.9±6.5Ma,且CL图像显示锆石内部发育石榴子石、单斜辉石、斜长石等矿物包体,反映锆石可能形成在峰期高压麻粒岩相变质条件下。岩石学和年代学结果显示都兰高压麻粒岩和邻近的榴辉岩同时形成于同一俯冲带的不同热构造环境,高压麻粒岩并非榴辉岩热松弛作用形成的,两者具有各自独立的变质演化历史。     

大别—苏鲁超高压变质带内的块状榴辉岩及其构造意义

大别—苏鲁超高压(> 27× 108Pa) 变质带内的榴辉岩, 在大陆深俯冲、碰撞和折返剥露过程中, 大都遭受了强烈的变形和变质作用的重置与再造.但是, 大型榴辉岩体核部以及包裹于大理岩和石榴橄榄岩体内部的块状榴辉岩, 往往保留其初始简单的矿物组合、中-细粒状变晶结构和块状构造.详细地分析了块状榴辉岩的几何学、岩相学及变质作用特征, 指出它们是超高压榴辉岩递进及多期变质变形分解作用的残留体, 位于尺度不同的弱应变域内, 是大陆深俯冲及碰撞作用的真正记录.      

SHRIMP U-Pb zircon dating from Sulu-Dabie dolomitic marble, eastern China: constraints on prograde, ultrahigh-pressure and retrograde metamorphic ages

Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircon from Sulu‐Dabie dolomitic marbles is characterized by distinctive domains of inherited (detrital), prograde, ultrahigh‐pressure (UHP) and retrograde metamorphic growths. The inherited zircon domains are dark‐luminescent in CL images and contain mineral inclusions of Qtz + Cal + Ap. The prograde metamorphic domains are white‐luminescent in CL images and preserve a quartz eclogite facies assemblage of Qtz + Dol + Grt + Omp + Phe + Ap, formed at 542–693 °C and 1.8–2.1 GPa. In contrast, the UHP metamorphic domains are grey‐luminescent in CL images, retain the UHP assemblage of Coe + Grt + Omp + Arg + Mgs + Ap, and record UHP conditions of 739–866 °C and >5.5 GPa. The outermost retrograde rims have dark‐luminescent CL images, and contain low‐P minerals such as calcite, related to the regional amphibolite facies retrogression. Laser ablation ICP‐MS trace‐element data show striking difference between the inherited cores of mostly magmatic origin and zircon domains grown in response to prograde, UHP and retrograde metamorphism. SHRIMP U‐Pb dating on these zoned zircon identified four discrete ²⁰⁶Pb/²³⁸U age groups: 1823–503 Ma is recorded in the inherited (detrital) zircon derived from various Proterozoic protoliths, the prograde domains record the quartz eclogite facies metamorphism at 254–239 Ma, the UHP growth domains occurred at 238–230 Ma, and the late amphibolite facies retrogressive overprint in the outermost rims was restricted to 218–206 Ma. Thus, Proterozoic continental materials of the Yangtze craton were subducted to 55–60 km depth during the Early Triassic and recrystallized at quartz eclogite facies conditions. Then these metamorphic rocks were further subducted to depths of 165–175 km in the Middle Triassic and experienced UHP metamorphism, and finally these UHP metamorphic rocks were exhumed to mid‐crustal levels (about 30 km) in the Late Triassic and overprinted by regional amphibolite facies metamorphism. The subduction and exhumation rates deduced from the SHRIMP data and metamorphic P–T conditions are 9–10 km Myr^?1 and 6.4 km Myr^?1, respectively, and these rapid subduction–exhumation rates may explain the obtained P–T–t path. Such a fast exhumation suggests that Sulu‐Dabie UHP rocks that returned towards crustal depths were driven by buoyant forces, caused as a consequence of slab breakoff at mantle depth.     

P–T path of high-pressure/low-temperature rocks and tectonic implications in the western Tianshan Mountains, NW China

The Chinese western Tianshan high-pressure/low-temperature (HP–LT) metamorphic belt, which extends for about 200 km along the South Central Tianshan suture zone, is composed of mainly metabasic blueschists, eclogites and greenschist facies rocks. The metabasic blueschists occur as small discrete blocks, lenses, bands, laminae or thick beds in meta-sedimentary greenschist facies country rocks. Eclogites are intercalated within blueschist layers as lenses, laminae, thick beds or large massive blocks (up to 2 km² in plan view). Metabasic blueschists consist of mainly garnet, sodic amphibole, phengite, paragonite, clinozoisite, epidote, chlorite, albite, accessory titanite and ilmenite. Eclogites are predominantly composed of garnet, omphacite, sodic–calcic amphibole, clinozoisite, phengite, paragonite, quartz with accessory minerals such as rutile, titanite, ilmenite, calcite and apatite. Garnet in eclogite has a composition of 53–79 mol% almandine, 8.5–30 mol% grossular, 5–24 mol% pyrope and 0.6–13 mol% spessartine. Garnet in blueschists shows similar composition. Sodic amphiboles include glaucophane, ferro-glaucophane and crossite, whereas the sodic–calcic amphiboles mainly comprise barroisite and winchite. The jadeite content of omphacite varies from 35–54 mol%. Peak eclogite facies temperatures are estimated as 480–580 °C for a pressure range of 14–21 kbar. The conditions of pre-peak, epidote–blueschist facies metamorphism are estimated to be 350–450 °C and 8–12 kbar. All rock types have experienced a clockwise P–T path through pre-peak lawsonite/epidote-blueschist to eclogite facies conditions. The retrograde part of the P–T path is represented by the transition of epidote-blueschist to greenschist facies conditions. The P–T path indicates that the high-pressure rocks formed in a B-type subduction zone along the northern margin of the Palaeozoic South Tianshan ocean between the Tarim and Yili-central Tianshan plates.