Metamorphic evidence for an inverted crustal section, with constraints on the Main Karakorum Thrust, Baltistan, northern Pakistan

Abstract The Shyok Suture Zone separates rocks in the Asian plate from rocks in the Kohistan-Ladakh island arc. In Baltistan, this suture has been reactivated by the late 'break-back'Main Karakorum Thrust (MKT). The P-T histories of metamorphic rocks both north and south of the MKT have been determined in an effort to place constraints on the tectonic history of this zone. The terranes north and south of the MKT have different, unrelated metamorphic histories. Rocks from the Kohistan-Ladakh island arc south of the MKT have undergone a static low- P (2–4 kbar, c. 500° C) thermal metamorphism. The P-T paths and metamorphic textures of these rocks are consistent with metamorphism due to emplacement of plutonic rocks into the island arc. This metamorphism pre-dates folding and deformation of these rocks. Rocks in the Karakorum Metamorphic Complex, north of the MKT, have experienced a complex deformational and metamorphic history. Prograde metamorphic isograds have been deformed by subsequent south-verging folding and by gneiss dome emplacement. However, decompression metamorphic reactions occurred during nappe emplacement. Higher pressure rocks are associated with higher level nappes, creating an inverted pressure metamorphic sequence (8–9-kbar rocks over 5–6-kbar rocks). There is little variation in temperature with structural level (550–625° C). These two different terranes have been juxtaposed after metamorphism by the late south-directed MKT.     

Decompression through clockwise P–T path: implications for early N–S shortening orogenesis in the Mesoproterozoic Mt Isa Inlier (NE Australia)

Mesoproterozoic terranes of the Australian craton exhibit complex tectonometamorphic histories that are generally considered to result from low-pressure/high-temperature (LPHT) metamorphism with an anticlockwise pressure ( P )–temperature ( T ) path. Yet studies regarding the nature of the P–T history and tectonic regime that led to such a LPHT signature have been quite limited. A foliation intersection/inflection axes preserved in the porphyroblast (FIA) analysis combined with textural relationships and P–T pseudosections, using a key locality of the Eastern Fold Belt of the Mt Isa Inlier, has resolved the cause of the LPHT signature in this region. Two periods of porphyroblast growth have been distinguished using a change in FIA trends with time; the first formed during N–S shortening and the second during W–E shortening orogenesis (D1 & D2, respectively). Significantly, D1 porphyroblasts preserving W–E FIAs are minerals of the Barrovian style, whereas D2 formed porphyroblasts preserving N–S FIAs are Buchan in style. This is consistent with the emplacement of the Williams/Naraku Batholiths after D1 around the onset of D2. Higher-pressure garnet cores, that can be modelled in MnNCKFMASH P–T pseudosections, preserve early W–E FIA and formed during D1. This was followed by decompression and then LPHT metamorphism and deformation during which N–S FIAs were preserved within porphyroblasts. This is supported by the presence of at least two generations of staurolite that crystallized before and after andalusite/cordierite. Middle to upper amphibolite facies metamorphic conditions occurred during D1 with crustal thickening followed by near-isothermal decompression leading to LPHT conditions. This was followed by D2 and a second period of middle to upper amphibolite facies metamorphism that obliterated and/or obscured the tectonometamorphic signature of D1 in the matrix of most rocks.     

华北克拉通基底主要构造单元变质作用演化及其若干问题讨论

华北克拉通基底可分为三个太古宙微陆块(东部陆块、阴山陆块和鄂尔多斯陆块)和三个早元古宙活动带(孔兹岩带、华北中部带和胶-辽-吉带).这些构造单元具有不同的变质作用时间和P-T演化特征.东部陆块和阴山陆块晚太古宙基底岩系的变质作用发生在～2.5Ga,变质演化以等压冷却(IBC)逆时针P-T轨迹为特征,反映变质作用的成因与大规模地幔岩浆底侵有关.孔兹岩带主期变质作用发生在～1.95Ga,变质演化以近等温减压(ITD)顺时针P-T轨迹为特征,反映阴山陆块与鄂尔多斯陆块碰撞形成西部陆块的热构造过程.华北中部带变质作用发生在～1.85Ga,变质演化同样以近等温减压(ITD)顺时针P-T轨迹为特征,反映了西部陆块和东部陆块最终碰撞形成统一的华北克拉通基底的构造过程.早元古宙胶-辽-吉带变质作用表现‘双变质带'特征:西北带的北辽河群、老岭群和粉子山群的变质作用以中压顺时针P-T轨迹为特征,而东南带的南辽河群、吉安群和荆山群的变质作用以低压逆时针P-T演化为特征.华北克拉通基底变质作用演化地质图能更好地反映上述不同构造单元的变质作用演化特征.尽管岩浆弧、大陆裂谷和地幔柱模式都能解释东部陆块晚太古宙基底变质作用所具有的近等压冷却(IBC)逆时针P-T演化特征,地幔柱模式能够更合理解释东部陆块所存在的宽达800千米而时代近于相同的晚太古代火成岩带、大量科马提质超镁铁质岩石和双峰式火山岩、广泛发育的穹窿构造等.华北克拉通变质基底中具有石榴石-单斜辉石-斜长石-石英组合的高压基性麻粒岩和具有蓝晶石-钾长石组合的高压泥质麻粒岩的出露只局限在早元古宙华北中部带的北段和胶-辽-吉带的南端;这些高压麻粒岩形成在俯冲和陆-陆碰撞的构造环境中.西部陆块孔兹岩带含假蓝宝石麻粒岩是碰撞后(～1.92Ga)拉伸引发地幔岩浆底侵导致局部地带发生超高温(UHT)变质作用的产物.     

中条山早元古代变质岩石的PTt轨迹和构造演化

中条山早元古代变质岩石呈北东向分布，经历了中低级变质作用。本文在变质作用研究的基础上，结合地质构造和显微构造，利用近来发展的石榴石环带定量测量Ｐ－Ｔ轨迹的技术，建立了绛县群和中条群泥质岩石经历的ＰＴｔ轨迹。ＰＴｔ轨迹表明绛县群经历拉张作用，地壳发生抬升，产生绿片岩相变质；在中条第Ⅰ期运动中，地壳略有增厚或不变升温和抬升，产生低角闪岩相变质；而中条群岩石经历低绿片岩相埋深变质后，于中条第Ⅰ期运动中，     

Metamorphic interpretation of high-pressure-temperature metapelites with preserved growth zoning in garnet, eastern Grenville Province, Canadian Shield

Abstract High-pressure-temperature metapelites that occur in close proximity to eclogitized mafic rocks in the southern part of the Gagnon terrane (Parautochthonous Belt, eastern Grenville Province) were investigated in order to constrain depths of burial and P-T paths. Mineral assemblages and partial melting relationships in these metapelites are consistent with peak temperatures in the range between 700 and 800° C. However, growth zoning is apparently well preserved in garnets and only narrow rims (width = 100–500 μm) are obviously affected by diffusional retrograde resetting. Despite uncertainties regarding mineral assemblages and compositions of matrix minerals at early stages of garnet growth, it can be shown that the observed growth zoning profiles of garnets imply increase of both pressure and temperature up to a common maximum at pressures between 1300 and 1600 MPa, and that thermal relaxation did not occur during the initial stages of unloading. On the other hand, calculated retrograde P-T conditions are consistent with steep decompression paths. The inferred 'hair-pin'-shaped P-T path is consistent with independent evidence of rapid, tectonically driven exhumation, resulting in the preservation of growth zoning in garnets from such a high-temperature regime.     

P-T grids for silica-undersaturated granulites in the systems MAS (n+ 4) and FMAS (n+ 3)-tools for the derivation of P-T paths of metamorphism

Abstract Considering the minerals cordierite (Cd), sapphirine (Sa), hypersthene (Hy), garnet (Ga), spinel (Sp), sillimanite (Si) and corundum (Co) in the system FeO-MgO-Al₂O₃-SiO₂ (FMAS), the stable invariant points are [Co], [Ga], [Cd] and [Sa]. Constraints imposed by experimental data for the system MAS indicate that under low P _H2o conditions the invariant points occur at high temperature (> 900° C) and intermediate pressure (7-10 kbar). This temperature is higher than that commonly advocated for granulite facies metamorphism. In granulites Fe-Mg exchange geothermometers may yield temperatures of 100–150° C below peak metamorphic conditions and evidence for peak temperatures is best preserved by relict high-temperature assemblages and by Al-rich cores in orthopyroxene. Application of the FMAS grid to some well-documented granulite occurrences introduces important constraints on their P-T histories. Rocks of different bulk compositions, occurring in close proximity in the field, may record distinct segments of their P-T paths. This applies particularly to rocks with evidence for reaction in the form of coronas, symplectites and zoned minerals. Consideration of curved reaction boundaries and _XMs isopleths may explain apparently contradictory results for the stability of cordierite obtained from low-temperature experiments and thermochemical calculations on the one hand and hightemperature experimental data on the other.     

Pressure, Temperature, and Structural Evolution of West-Central New Hampshire: Hot Thrusts over Cold Basement

Pressure-temperature (P-T) paths have been calculated from pelitesand amphibolites of several major Acadian structures in west-centralNew Hampshire by using both inclusion thermobarometry and differentialthermodynamics (the Gibbs method). P-T paths calculated forrocks exposed in the Orfordville and Bronson Hill anticlinoriaare ‘clockwise’ and show 1–2.5 kb of exhumationwith 30–100 C of heating. Because this type of path ischaracteristic of the lower plate of overthrust terranes, theserocks are interpreted to be (para)autochthonous. P-T paths forrocks exposed in an intervening synclinorium (the Hardscrabblesynclinorium) show isothermal loading of 1–3 kb followedby possible isobaric cooling. This behavior is characteristicof rocks occupying a middle-plate structural position withina multiple thrust package, and so these rocks are interpretedto be allochthonous. The interpretation that the Hardscrabblerocks are allochthonous differs from previous models, but betterexplains the petrologic data and is consistent with the stratigraphicand structural data on which other models have been based. Correlation of the P-T paths with deformational events throughkinematic and textural analysis indicates that during nappestage deformation, the synclinorial rocks were transported westward,and that the anticlinorial and synclinorial rocks were buriedto depths of 25–30 and 20–25 km respectively. Theexhumation with heating recorded by the anticlinorial samplesoccurred during the dome stage of deformation, and differentiallyuplifted the anticlinorial rocks relative to the synclinorialrocks; this differential uplift may have been accommodated throughreactivation of early thrust faults with normal movement sense.P-T paths of the Hardscrabble synclinorium rocks are suggestiveof a relatively elevated initial geothermal gradient for theirpre-nappe source terrane, which is interpreted to have beenbetween the Kearsarge-Central Maine basin and the Bronson Hillparautochthon.     

On the formation of granulites

The tectonic settings for the formation and evolution of regional granulite terranes and the lowermost continental crust can be deduced from pressure–temperature–time (P–T–time) paths and constrained by petrological and geophysical considerations. P–T conditions deduced for regional granulites require transient, average geothermal gradients of greater than 35°C km^?1, implying minimum heat flow in excess of 100 mWm^?2. Such high heat flow is probably caused by magmatic heating. Tectonic settings wherein such conditions are found include convergent plate margins, continental rifts, hot spots and at the margins of large, deep-seated batholiths. However, particular P–T–time paths do not allow specific tectonic settings to be distinguished at this time. Under different conditions, both clockwise, CW (P_max attained before T_max), and anticlockwise, ACW (P_max attained slightly after T_max), paths are possible in the same tectonic setting. Both CW and ACW end-member paths can yield nearly isobaric cooling, IBC, paths. Such cooling paths are clearly not an artefact of thermobarometry, but can be constrained by solid–solid and devolatilization equilibria and geophysical modelling. In terms of understanding the evolution of the deep crust, a potentially significant group of regional granulite terranes are those that show evidence for ACW-IBC paths. Such paths are the likely result of: (i) episodic igneous activity resulting in intrusions within all levels of the crust, (ii) thickening of the crust by magmatic underplating, (iii) slow uplift as a result of the formation of a deep, garnet-rich crustal root and (iv) excavation resulting from a later tectonic event unrelated to that resulting in the formation of the granulites. The later event might be triggered by the delamination of the garnet-rich, lowermost crust.     

Composite nature of the North China Granulite-Facies Belt: Tectonothermal and geochronological constraints

Granulite-facies rocks are intermittently exposed in a roughly E–W trending belt that extends for approximately 2000 km across the North China Craton, from the Helanshan, Qianlishan, Wulashan–Daqingshan, Guyang and Jining Complexes in the Western Block, through the Huai'an, Hengshan, Xuanhua and Chengde Complexes in the Trans-North China Orogen, to the Jianping (Western Liaoning), Eastern Hebei, Northern Liaoning and Southern Jilin Complexes in the Eastern Block. The belt is generally referred to as the North China Granulite-Facies Belt, previously interpreted as the lowest part of an obliquely exposed crust of the North China Craton. Recent data indicate that the North China Granulite-Facies Belt is not a single terrane. Instead, it represents components of three separate terranes: the Eastern and Western Blocks and Trans-North China Orogen. Each of these units records different metamorphic histories and reflect the complex tectonic evolution of the NCC during the late Archean and Paleoproterozoic. Mafic granulites in the Eastern Block and the Yinshan Terrane (Western Block) underwent medium-pressure granulite-facies metamorphism at about 2.5 Ga, with anticlockwise P–T paths involving near isobaric cooling following peak metamorphism, reflecting an origin related to intrusion and underplating of mantle-derived magmas. Pelitic granulites in the Khondalite Belt (Western Block) underwent medium-pressure granulite-facies metamorphism at about 2.0–1.9 Ga, with clockwise P–T paths, which record the Paleoproterozoic amalgamation of the Yinshan and Ordos Terranes to form the Western Block. Mafic and pelitic granulites in the Trans-North China Orogen experienced high- to medium-pressure granulite-facies metamorphism at 1.85 Ga, with clockwise P–T paths involving nearly isothermal decompression following peak metamorphism, which are in accord with the final collision between the Eastern and Western Blocks to form the North China Craton at 1.8 Ga. The NCGB cannot therefore represent a separate unique terrane; instead it reflects the amalgamation of three separate granulite terranes that evolved independently and at different times.     

华北中部造山带五台-恒山地区古元古代变质作用与构造演化

华北克拉通古元古代造山带的时空分布与构造属性尚有很大争论,一种观点认为华北克拉通从新太古代至古元古代受大洋俯冲闭合控制,在1.85~1.95 Ga之间先后发生3次陆-陆碰撞形成3条造山带,即孔兹岩带、胶-辽-吉带和中部造山带;另一种观点强调华北克拉通这3条元古代造山带在1.80~1.98 Ga之间经历了相同的漫长演化过程,指示当时特有的热俯冲和碰撞环境.通过总结中部造山带中的五台-恒山地区变质作用研究进展,阐述该区古元古代造山时代与构造属性.五台-恒山地区的主要变质岩石-构造单元包括恒山杂岩、五台杂岩和滹沱群.恒山杂岩和五台杂岩主体为新太古代TTG片麻岩和表壳岩,它们在古元古代晚期经历了两期变质作用改造.第一期变质作用为中压型,是由于陆-陆碰撞导致弧后伸展盆地闭合、地壳加厚造山所致,从南向北形成一个递增序列:包括五台群下部和南恒山杂岩南部的低角闪岩相、南恒山杂岩北部高角闪岩相和北恒山杂岩的高压麻粒岩相,其压力峰期所对应的地热梯度为~15 ℃/km.变质锆石所记录的年龄峰值随着变质程度增高而降低,依次为~1.95 Ga、~1.92 Ga和~1.85 Ga,这是因为在变质过程中锆石生长受流体和熔体行为控制:在亚固相线下,变质锆石可记录峰期变质年龄,而在超固相线条件下记录伴随熔体结晶的退变质年龄.由此确定该区中压相系变质作用压力峰期时间为~1.95 Ga,对应地壳加厚造山的峰期.加厚地壳由于重力均衡导致变质岩从深部地壳折返至中部地壳,在P-T轨迹上表现为压力峰期之后发生等温减压（ITD）至0.5~0.7 GPa,岩相学上表现为峰期石榴石分解形成斜长石"白眼圈"等,指示缺流体条件.南恒山北部高角闪岩相岩石中的变质锆石记录的折返时间为~1.92 Ga,指示第一次造山结束.第二期变质作用为中-低压型,系为板内变形所致,表现为折返至中地壳的岩石伴随挤压型剪切变形和流体注入形成平衡矿物组合.朱家坊韧性剪切带就是这次板内变形的强构造域,其中也记录了顺时针型P-T轨迹,但所反映的地壳加厚程度有限,第二期变质-变形峰期时间为~1.85 Ga.由于朱家坊韧性剪切带左行走滑,导致北恒山麻粒岩地体抬升.五台-恒山地区在1.80~1.96 Ga之间经历两期变质-变形事件,这一认识或对讨论华北克拉通其他地区的古元古代造山带演化有一定启示意义.      

榴辉岩的两种变质演化轨迹和俯冲大陆地壳的差异折返——以柴北缘都兰超高压地体为例

都兰榴辉岩地体位于柴北缘—南阿尔金超高压变质带的东端,是唯一确定含柯石英的超高压变质地体,约700 km,其特点是含有两个特征不同的变质亚带,并经历了不同的折返过程。柯石英假像和温压计算表明两带榴辉岩峰期变质的压力都在柯石英的稳定域（2.8~3.3 GPa）,但它们退化变质的p–T 轨迹具有明显不同的特征。北带榴辉岩经历了两个阶段的折返：早期从地幔深度快速折返到中部地壳层次,伴随岩石的等温降压,并发生角闪岩相退化变质;晚期抬升到地壳浅部。都兰南带榴辉岩折返过程中经历了高压麻粒岩相变质的改造,高压麻粒岩阶段的p–T条件为p＝1.9~2.0 GPa,T＝873~948℃, 并进一步经历了角闪岩相退化变质,说明都兰南带榴辉岩折返速率较慢,发生了壳幔过渡带（或加厚的深部地壳）层次的强烈热松弛。这种热松弛发生在许多大陆俯冲带的超高压岩石的折返过程中,并且是榴辉岩发生深熔作用的主要机制。都兰两个变质带不同的变质演化轨迹反映了俯冲的大陆地壳具有差异折返的特征。     

High-temperature dehydration melting and decompressive P–T path in a granulite complex from the Eastern Ghats, India

. The granulite complex of Paderu, in the south central sector of the Eastern Ghats belt, India, consists of closely related pelitic granulites and peraluminous granitoids which could be linked via dehydration melting in pelitic and greywacke-like precursors. The pelitic granulites, including high-Mg-Al sapphirine granulites with early deformation microstructures, also record a high-temperature decompression from ~10 to ~8 kbar at ~1,000 °C, preceding isobaric cooling from above 900 to ~600 °C at 8 kbar. Highly magnesian biotite in the pelitic granulites, the presence of spinel in some of the granitoids, and granitoids of two distinct compositions, namely granite and quartz-monzonite, all suggest dehydration melting in highly magnesian pelitic and greywacke-like precursors. Moreover, high-temperature melting in highly magnesian pelitic precursors is indicated by the migmatitic spinel-bearing layers which, besides having significant abundance of quartz and feldspar, also contain aluminous orthopyroxene and cordierite. These melting reactions, occurring above 9 kbar, may constrain the prograde arm of the P-T trajectory. This and the high-temperature decompression constitute a clockwise P-T path. This clockwise P-T path is consistent with the tectonic model in which crustal thickening and granulite metamorphism in the Eastern Ghats belt is interpreted as the result of homogeneous shortening in a compressional setting.     

Integrated P-T Paths of the High-Pressure Rocks and Their Tectonic Implications for the Mountain-Building of the North Qilian, China

After the integration of petrographic study, geothermobarometry and Gibbs method, the synthetic P-T paths for the rocks from different geological profiles in the North Qilian, China, have been derived. The composite P-T paths from different methods indicate that all the high-pressure rocks in the Qilian area recorded P-T paths with clockwise loops starting at the blueschist facies, later reaching peak metamorphism at the blueschist facies, eclogite fades or epidote-amphibolite facies and ending up with the greenschist facies. The incremental Ar-Ar dating shows that the plateau ages for the high-pressure rocks range from 410 to 443 Ma. The plateau ages could be used as a minimum age constraint for the subduction that resulted in the formation of these high-pressure rocks in the Qilian area. It is proposed that the late-stage decompressional and cooling P-T paths with ends at the greenschist facies for these high-pressure rocks probably reflect the uplift process which could occur after shifting the arc-t     

Some remarks on high-temperature—low-pressure metamorphism in convergent orogens

Many high-temperature–low-pressure (high- T –low- P ) metamorphic terranes show evidence for peak mineral growth during crustal thickening strain increments at pressures near the maximum attained during the heating–cooling cycle. Such terranes are not readily explained as the conductive response to crustal thickening since the resulting Moho temperatures would greatly exceed the crustal liquidus and because heating due to conductive equilibration on length scales appropriate to lithospheric-scale strains must greatly outlast the deformation. Consequently, high- T –low- P metamorphism may be generated during crustal thickening only when significant heat is advected within the crust, as for example may occur during the segregation of granitic melts. We show that without the addition of asthenospheric melts and at strain rates appropriate to continental deformation the conditions required for significant lower crustal melting during deformation are only likely to be attained if heat flow into the lower crust during crustal thickening is increased substantially, for example, by removing the mantle part of the lithosphere. A simple parameterization of lithospheric deformation involving the vertical strain on the scale of the crust, _c, and the lithosphere, ₁ respectively, allows the potential energy of the evolving orogen to be readily evaluated. Using this parameterization we show that an important isostatic consequence of the deformation geometries capable of generating such high- T –low- P metamorphism during crustal thickening (with _c1) is an imposed upper limit to crustal thicknesses which is much lower than for homogeneous deformations (f_c= f₁) for the same initial lithospheric configuration.     

Garnet-staurolite-mica schist from Rangli Rangliot,Eastern Himalaya: Constraints from phase equilibria and Thermobarometry

Rangli Rangliot is an integral part of lesser Himalaya. The area around Rangli Rangliot consists of garnetstaurolite-mica schist and it is characterized by mineral assemblage garnet-biotite-muscovite-staurolite-quartz± plagioclase. Different reaction textures are of particular interest as they reflect discontinuous or continuous reactions under changing physical conditions. The relative X_Mg in the minerals varies in the order: muscovite> biotite> staurolite> garnet, and the X_Mn decreases in the order: garnet>staurolite>biotite>muscovite. The P-T evolution of the garnet-staurolite-mica schist has been constrained through the use of internally consistent TWEEQU programme and Perple_X software in the KFMASH model system. The combination of these two approaches demonstrates that the garnet-staurolite-mica schist experienced peak pressure and temperature at 5.8 kbar and 590 °C. The proposed clockwise P-T path implies that rocks from the study area could have resulted from thickened continental crust undergoing decompression.     

四川丹巴地区中低压变质作用及P-T轨迹

四川丹巴地区地处华北板块、扬子板块和羌塘板块的汇合处,这里集中了中压型的巴罗变质带和低压型的巴肯变质带,该区中低压变质作用的研究对于探讨松潘-甘孜造山带的形成过程与地壳演化过程具有重要的理论意义和大陆动力学意义。本文通过地质温压力计估算了研究区的峰期变质温度和压力,从微区的角度分析了各变质带的P-T轨迹和地热梯度, 结果表明,丹巴地区巴罗型变质带和巴肯型变质带都具有顺时针的P-T轨迹,但是巴肯型变质带的地热梯度高于巴罗带。结合研究区的变质带的分布,变质矿物共生组合及变质反应,本文认为丹巴地区的确并存巴罗带和巴肯带,两种地热梯度反映了构造条件上的差异。     

Pressure-Temperature Contours and PT Paths of the Early Precambrian Rocks through the Yinshan Area and Their Implication to the Crustal Evolution

Four distinct lithe-tectonic belts (zones) in the Yinshan area, North China, were identified by pressure-temperature contours and litho-tectonic features, such as the Sanggan granulite belt, Jining metasedimentary belt. Wulashan-Daqingshan front tectonic zone and Se' eratengshan belt. This area witnessed two important thermo-tectonic events. The older one is c. 2.5 Ga while the younger one c. 1.9 Ga. The Se' ertengshan Neoarchaean terrane features a clockwise PT path with the decompression ranging from > 1500 MPa to 800-1000 MPa in the Se' ertengshan belt, which implies an island arc setting. The Sanggan belt is a Mesoarchaean microcontinent reworked by Neoarchaean magma underplating, which shows an counterclockwise PT path. During the Palaeoproterozoic period, two Archaean continent (arc) collided. The Archaean basement of the Sanggan and Wulashan-Daqingshan belts overthrust northwards, the PTt paths of basement show a decompression from 1000-1200 MPa to 500-700 MPa. The PT paths of the Jining and Erda     

Creep behavior of unsaturated cohesive soils subjected to various stress levels

This paper presents the results of an experimental investigation carried out on disturbed clay samples to analyze their creep behavior. The experimental program included compression triaxial tests with varied degree of saturation of the samples and stress level. Triaxial tests were performed at imposed strain rate varied from 0.005 to 0.10%/min. Various stress paths have also been tested including isotropic consolidation and deviatoric stress. The obtained results showed that shear deformation increases with the initial degree of saturation and stress level of clay samples. At fixed stress level and for both isotropic and deviatoric stress paths, the initial degree of saturation and the second invariant of the strain tensor varied with a similar trend. The variation of axial creep strain is correlated with time in a semi-logarithmic function.     

Geothermobarometry of mafic granulites and metapelite from the Palghat Gap, South India: petrological evidence for isothermal uplift and rapid cooling

The Palghat Gap region is located near the centre of the large southern Indian granulite terrane. at the northern edge of the Kodaikanal charnockite massif. The dominant rock types in the region are hornblende-biotite ± orthopyroxene gneisses and charnockites along with minor amounts of intercalated mafic granulite, metapelite and calc-silicate. The P-T estimates from garnetiferous mafic granulites and metapelite samples are generally in the range 9-10 kbar and 800-900 C using both conventional thermobarometric methods and the TWEEQU thermobarometry program. These P-T estimates, which should be taken as minimum values, are among the highest yet reported for South Indian and Sri Lankan granulites. The occurrence of orthopyroxene + plagioclase symplectites around embayed garnet grains in the mafic granulites and cordierite rims around garnet grains in metapelite suggest an isothermal decompression-type path. Similarly, a core-rim P-T trajectory indicates c. 3 and 7 kbar decompression at high temperature in the mafic granulites and metapelite, respectively. In both rock types, the key to the determination of the retrograde P-T path was the recognition of small amounts of second generation plagioclase with a more anorthitic composition than the matrix plagioclase. The preservation of high garnet-pyroxene temperatures in the mafic granulites (despite small garnet grain size) suggests rapid cooling of the terrane. Calculated minimum cooling rates range from 8 to 80 C Ma^-1. Such cooling rates are more rapid than those associated with normal isostatic processes and suggest that the terrane was tectonically exhumed at high temperature.     

Petrology of Fe-Mg-carpholite-bearing metasediments from NE Oman

Abstract Fe-Mg carpholite occurs in metasediments of tectonically disrupted basement, shelf and foreland basin units that structurally underlie the Semail ophiolite in NE Oman. In the lower grade, structurally higher units, Fe-rich carpholite coexists with paragonite, quartz, illite, kaolinite and chlorite, whereas in deeper units, Fe-Mg carpholite occurs with pyrophyllite, sudoite, phengite and/or chloritoid. Mineral compositions in these units indicate that chlorite is more magnesian than coexisting Fe-Mg carpholite at low temperatures and pressures but, at higher metamorphic grades, X_Mg decreases in the order sudoite > carpholite > chlorite > chloritoid. This suggests a reversal in Fe-Mg partitioning between Fe-Mg carpholite and chlorite at temperatures below or close to those of the breakdown of kaolinite + quartz to pyrophyllite and at X_Mg= 0.35.
Phase relations and mineral equilibria indicate that the P-T conditions of formation of the Fe-Mg-carpholite-bearing rocks of NE Oman range from 280–315° C, 3–6 kbar for the structurally highest units to 325–440° C, 6–9.5 kbar for the deepest units, indicating a systematic down-section increase in metamorphic grade. Textural relations in these rocks, interpreted in the context of pertinent equilibria, are consistent with the clockwise P-T paths previously constrained for these units from petrological studies of interlayered isofacial mafic rocks.