High-P amphibolite-facies metamorphism in the Adrar–Souttouf Metamafic Complex,Oulad Dlim Massif (West African Craton margin,Morocco)     

《Comptes Rendus Geoscience》2018,350(6):245-254

The Oulad Dlim Massif represents the northern segment of the Mauritanide belt that thrusts over the western margin of the Reguibat Shield, north of the West African Craton (WAC). This belt includes various metamorphic units of Archean, Neoproterozoic and Palaeozoic ages that were stacked and thrust eastward during the Variscan orogeny. The core of the Oulad Dlim Massif comprises the Adrar–Souttouf Metamafic Complex that represents a large tectonic unit made of high-grade mafic rocks and vast exposures of amphibolites. A characterisation of the metamorphism in these amphibolites is essential to understand the relationships of the Oulad Dlim Massif with the southern segments of the Mauritanide belt and to provide constraints on the geodynamic evolution of the western margin of the WAC. Here we determine the P–T conditions of metamorphism of two samples of garnet amphibolites collected at the northernmost end of the Adrar–Souttouf Metamafic Complex. The samples show a main mineral assemblage of garnet + low-Ti pargasite + oligoclase + phengite + epidote + quartz + rutile ± paragonite ± K-feldspar. We calculated their P–T conditions using the amphibole–plagioclase NaSi–CaAl exchange thermometer, and the garnet–amphibole–plagioclase–quartz and the amphibole–plagioclase Si–Al partitioning barometers. The thermobarometric results indicate that this mineral assemblage was formed at high-P amphibolite-facies conditions at 650–700 °C and 10–13 kbar. The observed stability of paragonite and phengite reveals fluid-absent conditions or the presence of a fluid phase with reduced H₂O activity during the peak of metamorphism. We found no relicts of eclogite-facies mineral assemblage in the garnet amphibolites. This contrasts with the eclogite-facies metamorphism found due south in the Tarf Magneïna unit. This suggests that the northernmost end of the Adrar–Souttouf Metamafic Complex may have been buried to shallower depths than the units further south, probably during the Variscan orogeny. However, precise absolute radiometric dating of the high-P amphibolite-facies metamorphism is required to confirm these findings.  相似文献   

Evolution of Caledonian deformation fabrics under eclogite and amphibolite facies at Vårdalsneset, Western Gneiss Region, Norway     

Engvik  & Ersen 《Journal of Metamorphic Geology》2000,18(3):241-257.

The Vårdalsneset eclogite situated in the Western Gneiss Region, SW Norway, is a well preserved tectonite giving information about the deformation regimes active in the lower crust during crustal thickening and subsequent exhumation. The eclogite constitutes layers and lenses variably retrograded to amphibolite and is composed of garnet and omphacite with varying amounts of barroisite, actinolite, clinozoisite, kyanite, quartz, paragonite, phengite and rutile. The rocks record a five‐stage evolution connected to Caledonian burial and subsequent exhumation. (1) A prograde evolution through amphibolite facies (T =490±63 °C) is inferred from garnet cores with amphibole inclusions and bell‐shaped Mn profile. (2) Formation of L>S‐tectonite eclogite (T =680±20 °C, P=16±2 kbar) related to the subduction of continental crust during the Caledonian orogeny. Lack of asymmetrical fabrics and orientation of eclogite facies extensional veins indicate that the deformation regime during formation of the L>S fabric was coaxial. (3) Formation of sub‐horizontal eclogite facies foliation in which the finite stretching direction had changed by approximately 90°. Disruption of eclogite lenses and layers between symmetric shear zones characterizes the dominantly coaxial deformation regime of stage 3. Locally occurring mylonitic eclogites (T =690±20 °C, P=15±1.5 kbar) with top‐W kinematics may indicate, however, that non‐coaxial deformation was also active at eclogite facies conditions. (4) Development of a widespread regional amphibolite facies foliation (T =564±44 °C, P<10.3–8.1 kbar), quartz veins and development of conjugate shear zones indicate that coaxial vertical shortening and sub‐horizontal stretching were active during exhumation from eclogite to amphibolite facies conditions. (5) Amphibolite facies mylonites mainly formed under non‐coaxial top‐W movement are related to large‐scale movement on the extensional detachments active during the late‐orogenic extension of the Caledonides. The structural and metamorphic evolution of the Vårdalsneset eclogite and related areas support the exhumation model, including an extensional detachment in the upper crust and overall coaxial deformation in the lower crust.  相似文献   

Incomplete reaction of plagioclase in experimental dehydration melting of amphibolite     

W. Johannes  J. Koepke 《Australian Journal of Earth Sciences》2013,60(4):581-590

Dehydration melting of a hornblende‐plagioclase mixture of amphibolitic composition was investigated at 1000°C and at 800 MPa and 1200 MPa. At 1200 MPa the reaction products are garnet, clinopyroxene, melt and relatively Ab‐rich plagioclase (An₄₇). At 800 MPa the products are orthopyroxene, clinopyroxene, magnetite, amphibole (pargasite) and An‐rich plagioclase (An₇₅). The melts are rich in plagioclase components (especially in Ab) and, when compared to tonalites, relatively poor in silica. The grainsize of the starting materials was ≤?5 μm in the 800 MPa and ≤?10 μm in the 1200 MPa runs. All run products show unchanged plagioclase cores, which are the remnants of a very sluggish reaction assumed to be controlled by dissolution/precipitation processes at the plagioclase grain boundaries. The results indicate that only local equilibrium could have been obtained in recent investigations on dehydration melting experiments in plagioclase‐bearing systems. The results also suggest that plagioclase compositions once formed may persist for a very long time, even in hot magma chambers, if the prevailing water activity is low.  相似文献   

Geothermobarometry, P–T paths and metamorphic field gradients of high-pressure rocks from the Adula Nappe, Central Alps     

J. Dale  T. J. B. Holland 《Journal of Metamorphic Geology》2003,21(8):813-829

Metabasic rocks from the Adula Nappe in the Central Alps record a regional high‐pressure metamorphic event during the Eocene, and display a regional variation in high‐pressure mineral assemblages from barroisite, or glaucophane, bearing garnet amphibolites in the north to kyanite eclogites in the central part of the nappe. High‐pressure rocks from all parts of the nappe show the same metamorphic evolution of assemblages consistent with prograde blueschist, high‐pressure amphibolite or eclogite facies conditions followed by peak‐pressure eclogite facies conditions and decompression to the greenschist or amphibolite facies. Average PT calculations (using thermocalc ) quantitatively establish nested, clockwise P–T paths for different parts of the Adula Nappe that are displaced to higher pressure and temperature from north to south. Metamorphic conditions at peak pressure increase from about 17 kbar, 640 °C in the north to 22 kbar, 750 °C in the centre and 25 kbar, 750 °C in the south. The northern and central Adula Nappe behaved as a coherent tectonic unit at peak pressures and during decompression, and thermobarometric results are interpreted in terms of a metamorphic field gradient of 9.6 ± 2.0 °C km^?1 and 0.20 ± 0.05 kbar km^?1. These results constrain the peak‐pressure position and orientation of the nappe to a depth of 55–75 km, dipping at an angle of approximately 45° towards the south. Results from the southern Adula Nappe are not consistent with the metamorphic field gradient determined for the northern and central parts, which suggests that the southern Adula Nappe may have been separated from central and northern parts at peak pressure.  相似文献   

云南高黎贡山典型斜长角闪岩地震波速、各向异性及其晶格优选定向的研究   总被引：1，自引：0，他引：1  

嵇少丞  道林克祯  邵同宾  赵卫华  近藤洋裕  王红才 《地质论评》2013,59(4):769-780

斜长角闪岩和角闪岩相变质岩石是大陆中—下地、岛弧深部地壳以及俯冲大洋地壳中最重要的组成岩石之一,查明目前地壳中斜长角闪岩的体积含量、空间分布及其应变状态对于深入研究大陆地壳的形成与演化过程极其重要.笔者等实验测量了云南高黎贡韧性剪切带典型变形斜长角闪岩7个关键性方向上的地震(P和S波)波速随静水围压(0～600 MPa)的变化规律,并利用电子背散射衍射(EBSD)技术测定了主要造岩矿物角闪石、斜长石和石英的品格优选方位,查清斜长角闪岩中地震波速各向异性与剪切波分裂的成因,确定斜长角闪岩中有限应变椭球与波速椭球的对应关系,为今后利用原地地震波速的各向异性调查地壳深部区域构造应变场提供必要的实验和理论基础.  相似文献   

西藏那曲县北聂荣微地块聂荣岩群中斜长角闪岩——Rodinia超大陆裂解的地质纪录   总被引：7，自引：0，他引：7  

辜平阳  李荣社  何世平  查显锋  于浦生  时超  潘术娟  王轶 《岩石矿物学杂志》2012,31(2):145-154

西藏那曲县北聂荣微地块聂荣岩群中斜长角闪岩原岩恢复为正斜长角闪岩,地球化学特征显示岩石属于拉斑系列基性火山岩,具有轻稀土元素富集、重稀土元素亏损的右倾型稀土元素配分模式。岩石富集大离子亲石元素Rb、Ba及高场强元素Nb、Ta,Sr、Th和U呈现明显的负异常,在构造环境判别图解中样品均投到板内玄武岩区,具有与裂谷火山岩相似的地球化学特征。通过高精度的LA-ICP-MS(激光剥蚀等离子体质谱仪)锆石微区原位U-Pb同位素测年,西藏那曲县北聂荣微地块聂荣岩群片麻状斜长角闪岩中获得863±10 Ma的谐和年龄,为原岩(基性火山岩)的形成时代,说明聂荣岩群形成于新元古代早期。该年龄相当于Rodinia超大陆裂解的地质记录。此外,还获得了2 539~1 006 Ma之间的捕获锆石年龄信息。  相似文献   

P-T-X conditions of calc-silicate formation: evidence from fluid inclusions and phase equilibria; Llano Uplift, central Texas, USA     

C. M. R. LETARGO  W. M. LAMB 《Journal of Metamorphic Geology》1993,11(1):89-100

Abstract The Llano Uplift in central Texas is a Grenville aged (c. 1.1 Ga) metamorphic terrane consisting predominantly of amphibolite facies mineral assemblages. The formation of these assemblages has been attributed to the emplacement of relatively late granite plutons throughout the area. Two types of granitic intrusion have previously been recognized: (1) Town Mountain Granites, which occur as relatively large, circular-shaped bodies of coarse-grained granite, and (2) Younger Granites which are present as smaller and more irregular bodies of finer-grained granite. In the central part of the uplift, wollastonite-bearing calc-silicate rocks occur within the Valley Spring Gneiss. The development of these calc-silicate rocks has been linked to infiltrating fluids presumably derived from spatially associated Younger Granites. The stability of coexisting quartz, calcite, wollastonite, grossular and anorthite and coexisting quartz, calcite, wollastonite, andradite and hedenbergite shows that the calc-silicate rocks equilibrated under H₂O-rich conditions with χCO₂ <0.10. Fluid inclusions present within the calc-silicate minerals are H₂O-rich with salinities of <17 wt% equivalent NaCl. The absence of any detectable CO₂ in the fluid inclusions may indicate entrapment of the inclusions at lower pressures and more H₂O-rich conditions compared to the stability of the peak metamorphic mineral assemblage. Homogenization temperatures, measured for texturally primary inclusions, range from 360 to 368° C corresponding to a density range from 0.53 to 0.82 g/cm³. Isochores for these fluid inclusions, when combined with the stability of the solid-solid equilibria Grs + Qtz = Wo + An, yield formation conditions of 500–550° C at 1–2 kbar. This indicates that the granitic intrusions involved in the formation of the Blount Mountain calc-silicates were emplaced at a pressure of at least 1–2 kbar.  相似文献   

North-west propagation of the Grenville orogen: Grenvillian structure and metamorphism near Key Harbour,Georgian Bay,Ontario, Canada*     

R. A. JAMIESON  N. G. CULSHAW  D. CORRIGAN 《Journal of Metamorphic Geology》1995,13(2):185-207

Abstract We report structural and metamorphic data from a c. 25-km transect across the eastern Grenville Front Tectonic Zone (GFTZ) to the Britt domain at the northern end of Georgian Bay near Key Harbour, Ontario. Constrasting Grenvillian structural and fabric elements characterize the eastern GFTZ, northern Britt domain and a narrow Transition Zone between them. Moderately to steeply dipping foliations with strong down-dip lineations in all three divisions appear to be associated with NW-directed thrusting. In the Transition Zone and northern Britt domain, early S = L fabrics with steep lineations are overprinted by younger structures (S > L) with shallow, SE-SSE-plunging lineations in which sparse, dominantly (but not exclusively) normal-sense kinematic indicators are recognized. Pressure and temperature estimates from Grenvillian metamorphic assemblages in metadi-abase indicate that conditions of P ± 12 kbar and T c 800° C were achieved before or during the thrust-related deformation, with P-T-t paths that indicate near-isothermal decompression to P c. 4 kbar and T c. 700° C. Correlation of fabric elements with points on the P-T-t paths suggests that exhumation occurred during two stages, the first associated with thrusting (≥1035 Ma) and the second with extension and thrusting (pre-1003 Ma). The GFTZ contains steeply to moderately dipping, thrust-related fabrics and lacks shallow, extensional structures; the latest episode of thrusting in the GFTZ is inferred to have taken place at 990-980 Ma. The data are interpreted in terms of a tectonic model involving two stages of propagation of the Grenville orogen towards its foreland (≥1035 Ma and ≥980 Ma), with an intervening period of extension, although the tectonic regime probably remained compressional on the scale of the orogen.  相似文献   

辽宁红透山块状硫化物矿床高级变质下盘蚀变带研究   总被引：1，自引：0，他引：1  

郑远川  顾连兴  汤晓茜  李春海  刘四海  吴昌志 《岩石学报》2008,24(8)

辽宁红透山块状硫化物矿床(MSD)位于华北地台东北部的浑北花岗岩-绿岩地体内,矿区岩石受到了高级角闪岩相变质.我们的研究表明,广泛分布于红透山层状矿体下盘数百米处的和直接产于矿体下盘的堇青-直闪片麻岩(COG),分别代表了经历过变质的MSD成矿系统的半整合状和筒状海底热液蚀变带.元素地球特征显示,层状COG并非同一种岩石,而是由五种不同岩性的岩石组成,筒状COG的原岩主要由流纹质岩石组成,而在走向上与COG过渡的角闪片麻岩和黑云片麻岩则代表了各种岩性蚀变岩变质后的未蚀变原岩.相对于未蚀变原岩而言,层状COG具富Fe、Mg,贫Na、K、Ca、Rb、Sr和Ba的特征;筒状COG则富Fe、Mg、Si,贫K、Rb、Sr和Ba.蚀变带内Fe、Mg的增加,K的强烈丢失表明,蚀变带在变质前应以绿泥石化为特征.  相似文献   

东疆哈尔里克造山带中角闪石的特征及其地质意义     

赵明  王赐银 《矿物岩石地球化学通报》1998,(1)

东疆哈尔里克造山带中角闪石的特征及其地质意义＠赵明＠王赐银￥南京大学地球科学系哈尔里,克造山,带角闪,石角闪岩,相温压条件东疆哈尔里克造山带中角闪石的特征及其地质意义赵明王赐银（南京大学地球科学系,南京２１００９３）关键词哈尔里克造山带角闪石角闪岩相温压...  相似文献