Petrological constraints on the eclogite- and blueschistfacies metamorphism and P-T-t paths in the Western Alps*     

U. POGNANTE 《Journal of Metamorphic Geology》1991,9(1):5-17

Phase relationships in the model mafic system and geothermobarometry allow discrimination of four main groups of high-P rocks in the nappes of the Western Alps: very high-P eclogite-facies (including kyanite eclogites and coesite-pyrope assemblages), eclogite-facies (paragonite-zoisite eclogites), high-T blueschist-facies (glaucophane-garnet ± lawsonite assemblages) and low-T blueschist-facies (glaucophane-lawsonite ± pumpellyite assemblages). The blueschist-facies-eclogite-facies transition is promoted chiefly by increasing T, low bulk X_Mg and relatively low μ_H2O. The variety of assemblages and the heterogeneous approach to equilibrium observed in the Alpine rocks are not only constrained by the intersection of the reaction surfaces in P-T-X space, but also by the effect and timing of the processes which control kinetics (i.e. pervasive deformation and fluid infiltration). The faster rate of dehydration reactions relative to hydration reactions along with the fact that different bulk compositions crossed the reaction curves at different temperatures (and times), all may have induced μ_H2O gradients and contributed to the heterogeneous distribution of deformation through a process of reaction-enhanced ductility. Also mass-transfer may have been an effective process in determining the type of high-P assemblage in particular rock volumes. As regards the P-T-t paths, only the post-climax histories are recorded well in the Alpine nappes. Post-eclogitic exhumation paths at decreasing temperatures characterize structurally higher nappes which were first subducted during the early-Alpine (Cretaceous) event. In contrast, more or less isothermal decompression paths characterize structurally deeper nappes formed by westward propagation of the underthrust surfaces during the early-Alpine event and the subsequent meso-Alpine (Palaeogene) collision between the ‘European’and ‘African’plates. In the Western Alps, prevalent eclogite-facies conditions were attained during the metamorphic climax of the early-Alpine subduction, while blueschist-facies recrystallization characterizes the early-Alpine exhumation of the eclogitized units and the subsequent intracontinental underthrusts linked to the meso-Alpine continental collision.  相似文献   

A deep section of accretionary complex: Susunai Complex in Sakhalin Island, Northwest Pacific Margin     

Gaku  Kimura Masayuki  Sakakibara Hisashi  Ofuka Hideo  Ishizuka Sumio  Miyashita Makoto  Okamura Olex A.  Melinikov Vilera  Lushchenko 《Island Arc》1992,1(1):166-175

Abstract A deep section of accretionary complex, the metamorphosed Susunai Complex, is observed on Sakhalin Is., Russia. High pressure part of pumpellyite-actinolite facies metavolcanics, metacherts and metapelites are well exposed and constitute a tectonic pile preserving primary structures related to underplating of the oceanic crust. Three stages of deformation, D₁ through D₃, suggest successive deformation during subduction, underplating and exhumation of the complex. Oceanic material in the complex is more abundant than other well documented ancient accretionary complexes (e.g. the Shimanto Belt in southwest Japan and the Ghost Rocks Formation in Alaska), which were shallowly underplated. At Susunai, deep down-stepping of a décollément has scraped off the upper part of the oceanic crust, primarily the pillowed basalt horizon. This down-stepping results from crustal weakening as overpressured water is released from the fractured oceanic crust during metamorphism.  相似文献   

甘肃玉门昌马地区的蓝闪片岩     

吴汉泉  冯益民  霍有光  左国朝 《西北地质科学》1991,(2)

蓝闪片岩产于一套含放射虫硅质岩、硅质大理岩、板岩、变质基性火山岩中,与此相伴的超基性岩块,以一系列向南逆冲的断片产出。蓝闪片岩中的蓝闪石往往和黑硬绿泥石、钠长石、绿泥石以及钠钙闪石(蓝透闪石、冻蓝闪石等)、钙闪石(阳起石等)共存。据钠角闪石、绿泥石计算的生成压力是0.65GPa至0.8GPa,略大于美国佛兰西斯科的瓦尔德溪蓝片岩中该矿物对生成的压力。稀土元素分析证明蓝片岩的原岩是典型洋底(中脊)玄武岩和岛弧玄武岩,这是蓝片岩沟-弧俯冲-碰撞的证据之一。  相似文献   

Lawsonite growth in the epidote blueschists from the Ile de Groix (Armorican Massif, France): a potential geobarometer   总被引：2，自引：0，他引：2  

M. Ballevre  P. Pitra  M. Bohn 《Journal of Metamorphic Geology》2003,21(7):723-735

Abstract The garnet blueschists from the Ile de Groix (Armorican Massif, France) contain millimetre‐ to centimetre‐sized pseudomorphs consisting of an aggregate of chlorite, epidote and paragonite. The pseudomorphed phase developed at a late stage of the deformation history, because it overgrows a glaucophane–epidote–titanite foliation. Garnet growth occurred earlier than the beginning of the ductile deformation, and thus garnet is also included in the pseudomorphs. Microprobe analyses show that garnet is strongly zoned, with decreasing spessartine and increasing almandine and pyrope contents from core to rim. Grossular content is higher in garnet cores (about 35 mole%) compared to garnet rims (about 30 mole%). Blue amphibole has glaucophane compositions with a low Fe³⁺ content and become more magnesian when inclusions in garnet (X_Mg = 0.62–0.65) are compared with matrix grains (X_Mg = 0.67–0.70). Matrix epidote has a pistacite content of about 50 mole%. On the basis of their shape and the nature of the breakdown products, the pseudomorphs are attributed to lawsonite. A numerical model (using Thermocalc ) has been developed in order to understand the reactions controlling both the growth and the breakdown of lawsonite. Lawsonite growth could have taken place through the continuous hydration reaction Chl + Ep + Pg + Qtz + Vap = Gln + Lws, followed by the fluid‐absent reaction Chl + Ep + Pg = Grt + Gln + Lws. Peak P–T conditions are estimated at about 18–20 kbar, 450 °C. This indicates that lawsonite growth took place at increasing P and T, hence can be used as a geobarometer in the buffering assemblage garnet–glaucophane–epidote. The final part of the history is recorded by lawsonite breakdown, after cessation of the ductile deformation, and recording the earliest stages of the exhumation.  相似文献   

秦岭——大别高压超高压变质带构造特征及构造演化   总被引：7，自引：0，他引：7  

杨巍然  游振东 《高校地质学报》1995,1(2):53-64

秦岭－大别高压超高压变质带是地中海全球近纬向活动带的组成部分，地中海活动带具有长期活动历史，高压超高压变质岩发育在不同时代的褶皱带中。秦岭－大别高压超高压变带以丹凤－商南－信阳－桐城对拉带为界分为秦岭和鄂北－桐柏－大别两个高压超高压块体，前者分为南北两个高压变质带，后者进一步划分为蓝片岩带、高压低温榴辉岩带、超高压榴辉岩带、高压中温榴辉岩带和盆榴辉岩带。它们均具有前准对称展布特征。不同地区高压超高  相似文献   

滇西澜沧变质带中白云母b_0值特征及其多期变质作用意义     

赵靖  钟大赉  王毅 《矿物岩石》1993,(1)

滇西澜沧变质带中199件白云母b_0值分析结果表明,b_0值均在0.9020nm以上。与澜沧变质带遭受蓝片岩相高压变质作用的事实相吻合。此外,白云母b_0值的统计规律显示,澜沧变质带经历了多期变质作用,一期为高压变质作用,另一期为中压变质作用,这也与已知的地质现象相符。由此可见,白云母b_0值可以很好地反映多期变质作用。  相似文献   

Glaucophane-bearing assemblage overprinted by greenschist-facies metamorphism in the Variscan Kaczawa complex, Sudetes, Poland   总被引：1，自引：0，他引：1  

R. KRYZA  A. MUSZYNSKI  D. VIELZEUF 《Journal of Metamorphic Geology》1990,8(3):345-355

An Early Palaeozoic (Ordovician ?) metamudstone sequence near Wojcieszow, Kaczawa Mts, Western Sudetes, Poland, contains numerous metabasite sills, up to 50 m thick. These subvolcanic rocks are of within-plate alkali basalt type. Primary igneous phases in the metabasites, clinopyroxene (salite) and kaersutite, are veined and partly replaced by complex metamorphic mineral assemblages. Particularly, the kaersutite is corroded and rimmed by zoned sodic, sodic–calcic and calcic amphiboles. The matrix is composed of actinolite, pycnochlorite, albite (An ≤ 0.5%), epidote (Ps 27–33), titanite, calcite, opaques and, occasionally, biotite, phengite and stilpnomelane. The sodic amphiboles are glaucophane to crossite in composition with Na^B from 1.9 to 1.6. They are rimmed successively by sodic–calcic and calcic amphiboles with compositions ranging from magnesioferri-winchite to actinolite. No compositions between Na^B= 0.92 and Na^B= 1.56 have been ascertained. The textures may be interpreted as representing a greenschist facies overprint on an earlier blueschist (or blueschist–greenschist transitional) assemblage. The presence of glaucophane and no traces of a jadeitic pyroxene + quartz association indicate pressures between 6 and 12 kbar during the high-pressure episode. Temperature is difficult to assess in this metamorphic event. The replacement of glaucophane by actinolite + chlorite + albite, with associated epidote, allows restriction of the upper pressure limit of the greenschist recrystallization to <8 kbar, between 350 and 450°C. The mineral assemblage representing the greenschist episode suggests the P–T conditions of the high-pressure part of the chlorite or lower biotite zone. The latest metamorphic recrystallization, under the greenschist facies, may have taken place in the Viséan.  相似文献   

蓝片岩及其有关高压变质带研究的新进展   总被引：5，自引：0，他引：5  

魏春景 《地学前缘》1994,(Z1)

通常认为蓝片岩主要分布在中新生代的造山带中，其成因与大洋板块的俯冲作用有关；而大陆上的蓝片岩一般与蛇绿岩和混杂岩伴生代表了古板块的缝合线。但近来研究发现，蓝片岩的矿物组合、时空分布和原岩建造多种多样。归纳起来可分为两种类型：科迪勒拉型和大陆内部型。不同的蓝片岩是在不同的大地构造环境下由不同的成因机制形成的，还是仅在洋壳俯冲环境中，由于不同的物质卷入俯冲带，引起俯冲过程中动力学机制变化形成的，是当前蓝片岩研究中至为重要的问题。  相似文献   

Petrogenesis of lawsonite and epidote eclogite and blueschist, Sivrihisar Massif, Turkey   总被引：6，自引：0，他引：6  

P. B. DAVIS  D. L. WHITNEY 《Journal of Metamorphic Geology》2006,24(9):823-849

The Sivrihisar Massif, Turkey, is comprised of blueschist and eclogite facies metasedimentary and metabasaltic rocks. Abundant metre‐ to centimetre‐scale eclogite pods occur in blueschist facies metabasalt, marble and quartz‐rich rocks. Sivrihisar eclogite contains omphacite + garnet + phengite + rutile ± glaucophane ± quartz + lawsonite and/or epidote. Blueschists contain sodic amphibole + garnet + phengite + lawsonite and/or epidote ± omphacite ± quartz. Sivrihisar eclogite and blueschist have similar bulk composition, equivalent to NMORB, but record different P–T conditions: ～26 kbar, 500 °C (lawsonite eclogite); 18 kbar, 600 °C (epidote eclogite); 12 kbar, 380 °C (lawsonite blueschist); and 15–16 kbar, 480–500 °C (lawsonite‐epidote blueschist). Pressures for the Sivrihisar lawsonite eclogite are among the highest reported for this rock type, which is rarely exposed at the Earth's surface. The distribution and textures of lawsonite ± epidote define P–T conditions and paths. For example, in some lawsonite‐bearing rocks, epidote inclusions in garnet and partial replacement of matrix epidote by lawsonite suggest an anticlockwise P–T path. Other rocks contain no epidote as inclusions or as a matrix phase, and were metamorphosed entirely within the lawsonite stability field. Results of the P–T study and mapping of the distribution of blueschists and eclogites in the massif suggest that rocks recording different maximum P–T conditions were tectonically juxtaposed as kilometre‐scale slices and associated high‐P pods, although all shared the same exhumation path from ～9–11 kbar, 300–400 °C. Within the tectonic slices, alternating millimetre–centimetre‐scale layers of eclogite and blueschist formed together at the same P–T conditions but represent different extents of prograde reaction controlled by strain partitioning or local variations in fO₂ or other chemical factors.  相似文献   

北阿尔金HP/LT蓝片岩和榴辉岩的Ar-Ar年代学及其区域构造意义   总被引：16，自引：1，他引：16       下载免费PDF全文

张建新孟繁聪  于胜尧陈 文 陈松永 《中国地质》2007,34(4):558-564

在北阿尔金的红柳泉一带,新识别出蓝片岩和榴辉岩的出露。它们与泥质片岩、钙质片岩和石英片岩等一起构成HP/LT变质带,与相邻的蛇绿混杂岩呈断层接触。根据估算的温度和压力显示榴辉岩形成的峰期温压条件为T=430～540℃,P=2.0～2.3GPa。分别对榴辉岩和蓝片岩中的多硅白云母和钠云母进行39Ar-40Ar年代学测定,获得榴辉岩中多硅白云母的坪年龄为(512±3)Ma,等时线年龄为(513±5)Ma;蓝片岩中钠云母的坪年龄为(491±3)Ma,等时线年龄为(497±10)Ma。这些年龄资料显示北阿尔金HP/LT变质带形成时代可能早于北祁连HP/LT变质带,反映了北阿尔金—北祁连早古生代洋壳俯冲存在穿时性。  相似文献