Multistage metamorphism of the Huangtuling granulite, Northern Dabie Orogen, eastern China: implications for the tectonometamorphic evolution of subducted lower continental crust

Granulites from Huangtuling in the North Dabie metamorphic core complex in eastern China preserve rare mineralogical and mineral chemical evidence for multistage metamorphism related to Palaeoproterozoic metamorphic processes, Triassic continental subduction‐collision and Cretaceous collapse of the Dabie Orogen. Six stages of metamorphism are resolved, based on detailed mineralogical and petrological studies: (I) amphibolite facies (6.3–7.0 kbar, 520–550 °C); (II) high‐pressure/high‐temperature granulite facies (12–15.5 kbar, 920–980 °C); (III) cooling and decompression (4.8–6.0 kbar, 630–700 °C); (IV) medium‐pressure granulite facies (7.7–9.0 kbar, 690–790 °C); (V) low‐pressure/high‐temperature granulite facies (4.0–4.7 kbar, 860–920 °C); (VI) retrograde greenschist facies overprint (1–2 kbar, 340–370 °C). The P–T history derived in this study and existing geochronological data indicate that the Huangtuling granulite records two cycles of orogenic crustal thickening events. The earlier three stages of metamorphism define a clockwise P–T path, implying crustal thickening and thinning events, possibly related to the assembly and breakup of the Columbia Supercontinent at c. 2000 Ma. Stage IV metamorphism indicates another crustal thickening event, which is attributed to Triassic subduction/collision between the Yangtze and Sino‐Korean Cratons. The dry lower crustal granulite persisted metastably during the Triassic subduction/collision because of the lack of hydrous fluid and deformation. Stage V metamorphism records the Cretaceous collapse of the Dabie Orogen, possibly due to asthenosphere upwelling or removal of the lithospheric mantle resulting in heating of the granulite and partial melting of the North Dabie metamorphic core complex. Comparison of the Huangtuling granulite in North Dabie and the high‐pressure–ultrahigh‐pressure metamorphic rocks in South Dabie indicates that the subducted upper (South Dabie) and lower (North Dabie) continental crusts underwent contrasting tectonometamorphic evolution during continental subduction‐collision and orogenic collapse.     

Petrological and geochronological constraints on high pressure, high temperature metamorphism in the Snowbird tectonic zone, Canada

The upper deck of the East Athabasca mylonite triangle (EAmt), northern Saskatchewan, Canada, contains mafic granulites that have undergone high P–T metamorphism at conditions ranging from 1.3 to 1.9 GPa, 890–960 °C. Coronitic textures in these mafic granulites indicate a near‐isothermal decompression path to 0.9 GPa, 800 °C. The Godfrey granite occurs to the north adjacent to the upper deck high P–T domain. Well‐preserved corona textures in the Godfrey granite constrain igneous crystallization and early metamorphism in the intermediate‐pressure granulite field (Opx + Pl) at 1.0 GPa, 775 °C followed by metamorphism in the high pressure granulite field (Grt + Cpx + Pl) at 1.2 GPa, 860 °C. U–Pb geochronology of zircon in upper deck mafic granulite yields evidence for events at both c. 2.5 Ga and c. 1.9 Ga. The oldest zircon dates are interpreted to constrain a minimum age for crystallization or early metamorphism of the protolith. A population of 1.9 Ga zircon in one mafic granulite is interpreted to constrain the timing of high P–T metamorphism. Titanite from the mafic granulites yields dates ranging from 1900 to 1894 Ma, and is interpreted to have grown along the decompression path, but still above its closure temperature, indicating cooling following the high P–T metamorphism from c. 960–650 °C in 4–10 Myr. Zircon dates from the Godfrey granite indicate a minimum crystallization age of 2.61 Ga, without any evidence for 1.9 Ga overgrowths. The data indicate that an early granulite facies event occurred at c. 2.55–2.52 Ga in the lower crust (c. 1.0 GPa), but at 1.9 Ga the upper deck underwent high P–T metamorphism, then decompressed to 0.9–1.0 GPa. Juxtaposition of the upper deck and Godfrey granite would have occurred after or been related to this decompression. In this model, the high P–T rocks are exhumed quickly following the high pressure metamorphism. This type of metamorphism is typically associated with collisional orogenesis, which has important implications for the Snowbird tectonic zone as a fundamental boundary in the Canadian Shield.     

Well-preserved garnet growth zoning in granulite from the Dabie Mountains, central China

Ultra-high pressure eclogites and granulites both occur in the Dabie Mountains, central China. A garnet porphyroblast from felsic granulite in the Dabie Mountains has been analysed for compositional zoning by electron microprobe. Two segments of the porphyroblast have opposite compositional variations. Segment I (from centre outward 9  mm to analytical point 18) has decreasing X_Sps and increasing X_Pyr, while Segment II (from analytical point 18, 1  mm outward to the rim) has increasing X_Sps and X_Alm and decreasing X_Pyr and X_Grs. The compositional zoning in segment I is considered as growth zoning and that in Segment II as diffusive retrograde zoning. Garnet growth zoning records a P–T  path prior to the peak granulite metamorphism. The minimum P – T  conditions are estimated to be 1.35  GPa and 850  °C for peak metamorphism, based on the highest Mg/(Fe+Mg) composition in the garnet (analytical point 18) and matrix hypersthene, biotite and plagioclase. A symplectitic corona surrounds the porphyroblast and appears to have formed at 0.6  GPa and 700  °C. The well-preserved growth zoning in garnet suggests a short residence time for the granulite at peak metamorphism and thus rapid tectonic uplift history. The P–T  path is consistent with that of ultra-high-pressure eclogite in the area. Tectonic movements during a collisional event could have brought both the granulite and the eclogite to their present positions.     

P–T–t evolution of spinel–cordierite–garnet gneisses from the Sauwald Zone (Southern Bohemian Massif, Upper Austria): is there evidence for two independent late-Variscan low-P/high-T events in the Moldanubian Unit?

The Sauwald Zone, located at the southern rim of the Bohemian Massif in Upper Austria, belongs to the Moldanubian Unit. It exposes uniform biotite + plagioclase ± cordierite paragneisses that formed during the post-collisional high-T/low-P stage of the Variscan orogeny. Rare metapelitic inlayers contain the mineral assemblage garnet + cordierite + green spinel + sillimanite + K-feldspar + plagioclase + biotite + quartz. Mineral chemical and textural data indicate four stages of mineral growth: (1) peak assemblage as inclusions in garnet (stage 1): garnet core + cordierite + green spinel + sillimanite + plagioclase (An_35–65); (2) post-peak assemblages in the matrix (stages 2, 3): cordierite + spinel (brown-green and brown) ± sillimanite ± garnet rim + plagioclase (An_10–45); and (3) late-stage growth of fibrolite, muscovite and albite (An_0–15) during stage 4. Calculation of the P–T conditions of the peak assemblage (stage 1) yields 750–840°C, 0.29–0.53 GPa and for the stage 2 matrix assemblage garnet + cordierite + green spinel + sillimanite + plagioclase 620–730°C, 0.27–0.36 GPa. The observed phase relations indicate a clockwise P–T path, which terminates below 0.38 GPa. The P–T evolution of the Sauwald Zone and the Monotonous Unit are very similar, however, monazite ages of the former are younger (321 ± 9 Ma vs. 334 ± 1 Ma). This indicates that high-T/low-P metamorphism in the Sauwald Zone was either of longer duration or there were two independent phases of late-Variscan low-P/high-T metamorphism in the Moldanubian Unit.     

Reaction textures and P–T conditions of Opx-Crd gneisses from Karimnagar, Andhra Pradesh, India

Summary The Karimnagar granulite terrain is an integral part of the Eastern Dharwar Craton (EDC). It has received much interest because of the only reported granulite facies rocks in the EDC. These granulites contain quartz-free sapphirine-spinel-bearing granulites, kornerupine – bearing granulites, mafic granulites, orthopyroxene-cordierite gneisses, charnockites, amphibolites, dolerite dykes, granite gneisses, quartzites and banded magnetite quartzite. The orthopyroxene-cordierite gneisses occur as enclaves within granite-gneiss in association with banded magnetite quartzites, charnockites and amphibolites. The observed reaction textures, spectacular as they are, have an extraordinary information content within a tiny domain. Coronas, symplectites and resorption textures are of particular interest as they reflect discontinuous or continuous reactions under changing physical conditions. The main mineral assemblages encountered in these gneisses are orthopyroxene – cordierite – biotite – plagioclase – perthite – quartz and garnet – orthopyroxene – cordierite – biotite – quartz – plagioclase – perthite ± sillimanite. Multiphase reaction textures in conjunction with mineral chemical data in the KFMASH system indicate the following reactions: Based on chemographic relationships and petrogenetic grids in the K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (KFMASH) system, a sequence of prograde (early stage), isothermal decompression (middle stage) and retrograde (late stage) reactions (‘back reactions’ and hydration reactions) are inferred. Relatively lower P–T estimates (0.35 GPa/550–750 °C) obtained from the different geothermobarometers are attributed to late Fe–Mg re-equilibration during cooling. Therefore, the convergence method has been applied to retrieve simultaneously the P–T conditions of the thermal peak of metamorphism. The near thermal peak condition of metamorphism estimated by the convergence method are 850 °C/0.62 GPa. The P–T estimates define a retrograde trajectory with substantial decompression.     

西藏定结高喜马拉雅石榴辉石岩-镁铁质麻粒岩的岩石特征及其地质意义

在高喜马拉雅带的定日县曲当—扎乡一带出露的高喜马拉雅结晶岩系中, 发现了高压变质的石榴辉石岩及其降压变质的镁铁质麻粒岩组合, 早期高压条件下形成的石榴辉石岩矿物组合为Grt+Cpx (富铝) +Ru+Q, 斜长石已完全消失, 形成温度为845~896℃, 压力大于1.2GPa, 已达到榴辉岩相的压力条件.中期的麻粒岩相组合为Opx+Pl±Cpx±Ga, 其中Opx、Cpx和Pl为石榴石的后成合晶, 形成温度为993~776℃, 压力为0.90~1.21GPa, 为中压麻粒岩相产物, 晚期矿物仅见普通角闪石、斜长石和石英, 是角闪岩相退变质的产物, 表明HHC经历了降压升温-降压降温的快速抬升过程, 证明其抬升作用与地幔热源的参与有关.      

Metamorphism and granite genesis in the Hidaka Metamorphic Belt, Hokkaido, Japan

The Main Zone of the Hidaka Metamorphic Belt is an uplifted crustal section of island-arc type. The crust was formed during early Tertiary time, as a result of collision between two arc–trench systems of Cretaceous age. The crustal metamorphic sequence is divided into four metamorphic zones (I–IV), in which zone IV is in the granulite facies. A detailed study of the evolution of the Hidaka Belt, based on a revised P–T–t analysis of the metamorphic rocks, notably a newly found staurolite-bearing granulite, confirms a prograde isobaric heating path, after a supposed event of tectonic thickening of accretionary sedimentary and oceanic crustal rocks. During the peak metamorphic event (c. 53 Ma), the regional geothermal gradient attained 33–40° C km^?1, and the highest P–T condition obtained from the lowest part of the granulite unit is 830° C, 7 kbar. In this part, X_H2O of Gt–Opx–Cd gneiss is about 0.15 and that of Gt–Cd–Bt gneiss is 0.4. The P–T–X_H2O condition of the granulite unit is well within a field where fluid-present partial melting of pelitic and greywacke metamorphic rocks takes place. This is in harmony with the restitic nature of the Gt–Opx–Cd gneiss in the lowest part of the granulite unit. The possibility that partial melting took place in the Main Zone is significant for the genesis of the peraluminous (S-type) granitic rocks within it. The S-type granitic rocks in this zone are Opx–Gt–Bt tonalite in the granulite zone, Gt–Cd–Bt tonalite in the amphibolite zone, and Cd–Bt–Mus tonalite in the Bt–Mus gneiss zone. The mineralogical and chemical nature of these strongly peraluminous tonalitic rocks permit them to be regarded as having been derived from S-type granitic magma generated by crustal anatexis of pelitic metamorphic rocks in deeper crust.     

晋冀蒙高级地体石榴石基性麻粒岩地质特征及其构造意义

在晋、冀、蒙交界处的高级变质岩区,沿孔兹岩系和怀安麻粒岩杂岩的接触带上广泛发育石榴石基性麻粒岩,其主要特征: 1)以透镜状、岩脉状分布于孔兹岩系和麻粒岩杂岩的接触带上; 2)和孔兹岩系及麻粒岩杂岩的接触关系为侵入或韧性剪切接触; 3)具斜方辉石-单斜辉石-角闪石-斜长石-石榴石的矿物组合及围绕石榴石发育斜长石和角闪石或斜方辉石的交生结构,部分还可见到斜方辉石后成合晶定向排列; 4)在孔兹岩和麻粒岩的接触带上还有透镜状的角闪辉石岩、脉状的斜长岩; 5)具等温降压的PT轨迹,并有200-300MPa的压力降,具有陆-陆碰撞的特征; 6)同位素年龄资料显示,岩石的定位年龄约为1800Ma.因此,石榴石基性麻粒岩是下地壳深源岩浆在吕梁运动时沿不同地体的对接带上侵的深源侵入体,并和孔兹岩岩片、 (超)基性岩体一同构成复杂的具混杂岩性质的构造岩浆杂岩带,其出现部位能指明不同地体的对接部位.     

Partial Melting of Spinel Lherzolite in the System CaO-MgO-Al2O3-SiO2 {+/-} K2O at 1{middle dot}1 GPa

The compositions of multiply saturated partial melts are valuablefor the thermodynamic information that they contain, but aredifficult to determine experimentally because they exist onlyover a narrow temperature range at a given pressure. Here wetry a new approach for determining the composition of the partialmelt in equilibrium with olivine, orthopyroxene, clinopyroxeneand spinel (Ol + Opx + Cpx + Sp + Melt) in the system CaO–MgO–Al₂O₃–SiO₂(CMAS) at 1·1 GPa: various amounts of K₂O are added tothe system, and the resulting melt compositions and temperatureare extrapolated to zero K₂O. The ‘sandwich’ experimentalmethod was used to minimize problems caused by quench modification,and Opx and Cpx were previously synthesized at conditions nearthose of the melting experiments to ensure they had appropriatecompositions. Results were then checked by reversal crystallizationexperiments. The results are in good agreement with previouswork, and establish the anhydrous solidus in CMAS to be at 1320± 10°C at 1·1 GPa. The effect of K₂O is todepress the solidus by 5·8°C/wt %, while the meltcomposition becomes increasingly enriched in SiO₂, being quartz-normativeabove 4 wt % K₂O. Compared with Na₂O, K₂O has a stronger effectin depressing the solidus and modifying melt compositions. Theisobaric invariant point in the system CMAS–K₂O at whichOl + Opx + Cpx + Sp + Melt is joined by sanidine (San) is at1240 ± 10°C. During the course of the study severalother isobaric invariant points were identified and their crystaland melt compositions determined in unreversed experiments:Opx + Cpx + Sp + An + Melt in the system CMAS at 1315 ±10°C; in CMAS–K₂O, Opx + Cpx + Sp + An + San + Meltat 1230 ± 10°C and Opx + Sp + An + San + Sapph +Melt at 1230 ± 10°C, where An is anorthite and Sapphis sapphirine. Coexisting San plus An in three experiments helpdefine the An–San solvus at 1230–1250°C. KEY WORDS: feldspar solvus; igneous sapphirine; mantle solidus; partial melting; systems CMAS and CMAS–K₂O     

Experimental Study on the Solubility of Cr^2 in Olivine,Orthopyroxene and Spinel Solid Solutions

Experiments have been performed on the system MgO-SiO2-Cr-O at 0-2.88 GPa and 1100-1450℃,focusing on the stability of Cr^2 in olivine(O1),orthopyroxene(Opx) and spinel(Sp) and its partitioning between these phases.Analytical reagent grade chemicals,MgO,SiO2,Cr2O3.and Cr were used to make starting mixtures.Excess Cr(50%) was then added in these mixtures to ensure that the resultant phases were in equilibrium with the metal Cr.Flux of BaO B2O3(%) was added for facilitating experimental equilibrium and crystal growth.Cr was used as capsule material.All phases in the product were identified by X-ray and analyzed by electron microprobe,The contents of CrO in the different phases(O1,Opx and Sp)were calculated according to stoichiometry.The obtained results of calculation indicate that Cr^3 in Ol and Opx is negligible.The experimental results show;(a) with increasing temperature and decreasing pressure,Cr^2 solubility in Ol,Opx and Sp increases;(b) with in creasing temperature,the partitioning coefficient of Mg and Cr^2 between Ol and Opx decreases,that between Opx and Sp increases,and that between Ol and Sp remains almost unchanged;(c) the effect of pressure on all partitioning coefficients is negligible.     

大别山铙钹寨被麻粒岩相变质作用改造的变质超镁铁岩体：石榴辉石岩证据

对大别山铙钹寨超镁铁岩体中石榴辉石岩的研究表明，铙钹寨岩体经历了从尖晶石－富铝辉石相（750度，1．1GPa)，尖晶石－石榴石相（850度，1．5GPa)到麻粒岩相（800度，0．85GPa)的变质演化，该岩体与南大别超高压变质杂岩的演化过程存在很大差异，它基本上处于一较高温地热体制之下，属于南大别俯冲陆壳的上盘杂岩，铙钹寨岩体及其它超镁铁岩体的普遍麻粒岩化与区域麻粒岩相的相关性可能说明大别变质基底已被彻底改造，北大别（安徽省内）目前所展示的高级变质作用仅是印支期后麻粒岩相变质事件的反映。     

Metabasites with eclogite facies relics from Variscides in Sardinia,Italy: a review

Metabasites with eclogite facies relics occur in northern Sardinia as massive to strongly foliated lenses or boudins embedded within low- to medium-grade rocks (Anglona) and migmatites (NE Sardinia). U–Pb zircon dating yielded 453 ± 14, 457 ± 2 and 460 ± 5 Ma as the protolith ages; 400 ± 10 and 403 ± 4 Ma have been interpreted as the ages of the HP event and 352 ± 3 and 327 ± 7 Ma as the ages of the main Variscan retrograde events. A pre-eclogite stage is documented by the occurrence of tschermakite, zoisite relics within garnet porphyroblasts (Punta de li Tulchi) and an edenite–andesine inclusion within a relict kyanite porphyroblast (Golfo Aranci). Four main metamorphic stages have been distinguished in the eclogite evolution: (1) eclogite stage, revealed by the occurrence of armoured omphacite relics within garnet porphyroblasts. The Golfo Aranci eclogites also include kyanite, Mg-rich garnet and pargasite; (2) granulite stage, producing orthopyroxene and clinopyroxene–plagioclase symplectites replacing omphacite. At Golfo Aranci, the symplectitic rims around relict kyanite consist of sapphirine, anorthite, corundum and spinel; (3) amphibolite stage, leading to the formation of amphibole–plagioclase kelyphites between garnet porphyroblasts and pyroxene–plagioclase symplectites and to the growth of cummingtonite on orthopyroxene. Tschermakite to Mg-hornblende, plagioclase, cummingtonite, ilmenite, titanite and biotite are coexisting phases; (4) greenschist to sub-greenschist stage, defined by the appearance of actinolite, chlorite, epidote ss, titanite, sericite and prehnite. The following P–T ranges have been estimated for the different stages. Eclogite stage 550–700°C; 1.3–1.7 GPa; granulite stage 650–900°C; 0.8–1.2 GPa, clustering in the range 1.0–1.2 GPa; amphibolite stage 550–740°C; 0.3–0.7 GPa; greenschist stage 300–400°C; 0.2–0.3 GPa. Comparable ranges characterise the other Variscan massifs in Europe; eclogite stage: T = 530–800°C; P from 0.7–1.1 to 1.7 ± 0.3 GPa; granulite stage T = 760–870°C and P from 1.1–1.4 to 7.2–9.9 GPa, clustering around 1.0–1.2 GPa. Whole-rock chemistry: Sardinian eclogites are N- to T-MORB; European ones N- to E-MORB or calc-alkaline.     

Sapphirine-bearing orthopyroxene-kyanite/sillimanite granulites from South Harris, NW Scotland: evidence for Proterozoic UHT metamorphism in the Lewisian

Sapphirine-bearing orthopyroxene-kyanite (Opx-Ky) and -sillimanite (Opx-Sil) granulites have been found in the Lewisian complex of South Harris in northwest Scotland. In the Opx-Ky granulites, orthopyroxene and kyanite are intergrown in a stable mineral assemblage, which indicates metamorphic condition at 800–900 °C >12 kbar. Sillimanite inclusions within orthopyroxene suggest that sillimanite formed earlier; conditions are estimated at 950 ± 30 °C at 10 kbar from orthopyroxene isopleths for aluminous orthopyroxene (<9.7 wt%). In the Opx-Sil granulite, the orthopyroxene + sillimanite + garnet + sapphirine assemblage is stable at the peak metamorphic stages, indicating P-T condition of 930–950 °C, >8 kbar according to the FMAS petrogenetic grid, and similar conditions were obtained by using orthopyroxene-garnet geothermobarometers. The two types of orthopyroxene-aluminosilicate granulites indicate that the peak metamorphic conditions were over 900 °C, compatible with ultra-high temperature metamorphism. As accessory sapphirine occurs in several assemblages and with different compositions; it is interpreted to be formed at different stages of the metamorphism. These granulites were formed during Early Proterozoic high-grade metamorphism due to the emplacement of the South Harris Igneous Complex at c. 2170–1870 Ma, and are not related to the major metamorphic episode of the Badcallian/Inverian metamorphism at c. 2700–2500 Ma in the mainland Lewisian. Received: 17 July 1998 / Accepted 8 March 1999     

The Spačice eclogite: constraints on the P–T–t history of the Gföhl granulite terrane, Moldanubian Zone, Bohemian Massif

Summary In the Kutná Hora Complex, the Běstvina Formation, which is similar to Gf?hl granulite, contains eclogite that has escaped widespread retrograde recrystallization. The eclogite assemblage, garnet + omphacite + quartz + rutile ± plagioclase, yields an estimate for peak metamorphic conditions of 18–20 kbar and 835–935 °C, which is comparable to that determined from felsic granulite, 14–20 kbar and 900–1000 °C. Garnet in eclogite exhibits both prograde and retrograde compositional zoning, from which constraints on thermal history of the Gf?hl terrane can be derived by diffusion modelling. At 900 °C, a garnet grain of 800–1000 μm radius would homogenize in 7.5–11.7 million years, but the existence of compositional gradients on a length scale of 100–200 μm suggests that the duration of peak metamorphism may have been limited to ∼500,000 years. Diffusion modelling of retrograde zoning in garnet yields a cooling rate of 150–100 °C/m.y. for a radius of 800–1000 μm and initial temperature of 900 °C. The relatively brief duration of high-pressure/high-temperature metamorphism and rapid cooling and exhumation of the Gf?hl terrane may be a consequence of lithospheric delamination during Early Carboniferous collision of Bohemia (Teplá-Barrandia) and Moldanubia (Franke, 2000).     

假蓝宝石(Sapphirine)的矿物学特征及其在超高温变质作用研究中的应用

假蓝宝石是Mg-Al质麻粒岩中一种特殊的高温矿物,对超高温变质作用的研究有重要的意义。本文通过对全球66个超高温麻粒岩中47个含假蓝宝石麻粒岩地区的文献调研,总结了几种最常见的含假蓝宝石矿物组合产出的结构位置和变质反应关系,以及假蓝宝石的矿物化学特征。假蓝宝石的化学成分一般位于7∶9∶3端元左右,X_(Mg)大于0. 7,XFe_(3+)变化范围很宽,为0～0. 7。含假蓝宝石矿物组合的形成和演化指示了岩石经历的P-T轨迹。岩石中保留的假蓝宝石取代尖晶石、Grt/Opx+Sil取代Spr+Qz组合,以及随后的Spr+Crd±Opx后成合晶取代Grt/Opx+Sil组合的结构,一般可能指示了逆时针P-T轨迹中冷却和随后减压的部分;岩石中Grt/Opx+Sil/Ky或富Mg十字石反应形成Spr+Qz组合的结构可能指示了顺时针P-T轨迹中减压升温的部分。超高温变质岩不同的P-T轨迹暗示着它们的成因机制并不单一,前者可能是幔源基性岩浆底侵或增生作用的结果,后者可能与长期的热造山作用相关。     

Metamorphic history of high-pressure granulites in Payer Land, Greenland Caledonides

A high‐P granulite facies gneiss complex occurs in north‐west Payer Land (74°28′?74°47′N) in the central part of the East Greenland Caledonian (Ordovician–Devonian) orogen. High‐P metamorphism of the Payer Land gneiss complex resulted in formation of the assemblages Grt + Cpx + Amp + Qtz + Ru ± Pl in mafic rocks, and Grt + Ol + Cpx + Opx + Spl in rare ultramafic pods. Associated metapelites experienced anatexis in the kyanite stability field. Peak metamorphic assemblages formed around 800–850 °C at pressures of c. 1.4–1.7 GPa, corresponding to crustal depths of c. 50 km. Mafic granulites contain abundant reaction textures, including the replacement of garnet by symplectites of Opx + Spl + Pl, indicating that the high‐P event was followed by decompression while the granulites remained at elevated temperatures. Charnockitic gneisses from Payer Land show evidence of late Archean (c. 2.8–2.4 Ga) crustal growth and subsequent Palaeoproterozoic (c. 1.85 Ga) metamorphism. The gneiss complex experienced intense reworking during the Caledonian continental collision. On the basis of Caledonian monazite ages recorded from the high‐P anatectic metapelites, the clockwise P–T evolution and formation of the high‐P granulite facies assemblages is related to Caledonian crustal thickening, which resulted in formation of eclogites approximately 300 km north of Payer Land. The Payer Land granulites comprise a metamorphic core complex, which is separated from the overlying low‐grade supracrustal rocks (the Neoproterozoic Eleonore Bay Supergroup) by a late Caledonian extensional fault zone, the Payer Land Detachment. The steep, nearly isothermal, unloading P–T path recorded by the granulites can be explained by erosional and tectonic unroofing along the Payer Land Detachment.     

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).     

Metamorphic evolution of the Río Santa Rosa Granulites,northern Sierra de Comechingones,Argentina

Highly anhydrous granulites from Río Santa Rosa in the eastern Sierras Pampeanas of Argentina occur as a thick lens surrounded by melt-depleted migmatites. Grt–Crd granulite composed of Qtz+Pl+Grt+Crd+Ilm±Spl±Ath±Phl is the dominant rock, whereas Opx–Grt granulite appears as discontinuous lenses in the center of the granulite body. Grt–Crd granulite includes blocks of metabasite that are relics of refractory lithologic beds interlayered in the supracrustal sequence. A distinct assemblage composed of Qtz, Pl, Grt, Crd, Opx, Spl, Crn, Sil, Bt, Phl, Ath, and Fe–Ti oxides in different combinations was generated in a reaction zone between Grt–Crd granulites and metabasites at peak metamorphism (850–900 °C and 7.6±0.5 kbar). The P–T trajectory of Grt–Crd granulites suggests an early prograde garnet-forming stage followed by nearly isothermal decompression that caused garnet breakdown. Melting and melt draining accompanying garnet growth was active during heating (to 900 °C) at intermediate pressures (∼7.6 kbar). Peak P–T estimates for Opx–Grt granulites are similar to those obtained with Grt–Crd granulites, which indicates that both granulites passed through the highest thermal stage. These results constrain the late evolution of Opx–Grt granulite to a garnet-consuming stage. Furthermore, they imply that garnet formation in Opx–Grt granulite happened at an early prograde P–T trajectory. Garnet growth in Opx–Grt granulite cannot result from heating at high pressure, which would lead to an apparent contradiction in the prograde P–T paths of the two granulites. This discrepancy may be solved by demonstrating that Opx–Grt granulite is the product of synmetamorphic mafic magmatism that was contaminated while cooling. The Río Santa Rosa granulites are inferred to have formed in a thickened crust in which mafic magmatic activity providing a local heat input.     

Blueschists of the Amassia-Stepanavan Suture Zone (Armenia): linking Tethys subduction history from E-Turkey to W-Iran

The Amassia–Stepanavan blueschist-ophiolite complex of the Lesser Caucasus in NW Armenia is part of an Upper Cretaceous-Cenozoic belt, which presents similar metamorphic features as other suture zones from Turkey to Iran. The blueschists include calcschists, metaconglomerates, quartzites, gneisses and metabasites, suggesting a tectonic mélange within an accretionary prism. This blueschist mélange is tectonically overlain by a low-metamorphic grade ophiolite sequence composed of serpentinites, gabbro-norite pods, plagiogranites, basalts and radiolarites. The metabasites include high-P assemblages (glaucophane–aegirine–clinozoisite–phengite), which indicate maximal burial pressure of ∼1.2 GPa at ∼550°C. Most blueschists show evidence of greenschist retrogression (chlorite—epidote, actinolite), but locally epidote-amphibolite conditions were attained (garnet—epidote, Ca/Na amphibole) at a pressure of ∼0.6 GPa and a temperature of ∼500°C. This LP–MT retrogression is coeval with exhumation and nappe-stacking of lower grade units over higher grade ones. ⁴⁰Ar/³⁹Ar phengite ages obtained on the high-P assemblages range between 95 and 90 Ma, while ages obtained for epidote-amphibolite retrogression assemblages range within 73.5–71 Ma. These two metamorphic phases are significant of (1) HP metamorphism during a phase of subduction in the Cenomanian–Turonian times followed by (2) exhumation in the greenschist to epidote-amphibolite facies conditions during the Upper Campanian/Maastrichtian due to the onset of continental subduction of the South Armenian block below Eurasia.     

Petrogenesis of kyanite‐ and corundum‐bearing mafic granulite in a meta‐ophiolite,SE Turkey

Although ophiolitic rocks are abundant in Anatolia (Turkey), only in rare cases have they experienced high‐grade metamorphism. Even more uncommon, in Anatolia and elsewhere are high‐grade meta‐ophiolites that retain an oceanic lithosphere stratigraphy from upper crustal mafic volcanic rocks through lower crustal gabbro to mantle peridotite. The Berit meta‐ophiolite of SE Turkey exhibits both features: from structurally higher to lower levels, it consists of garnet amphibolite (metabasalt), granulite facies metagabbro (as lenses in amphibolite inferred to be retrogressed granulite) and metaperidotite (locally with metapyroxenite layers). Whole‐rock major and trace‐element data indicate a tholeiitic protolith that formed in a suprasubduction setting. This paper presents new results for the metamorphic P–T conditions and path of oceanic lower crustal rocks in the Berit meta‐ophiolite, and an evaluation of the tectonic processes that may drive granulite facies metamorphism of ophiolite gabbro. In the Do?an?ehir (Malatya, Turkey) region, granulite facies gabbroic rocks contain garnet (Grt)+clinopyroxene (Cpx)+plagioclase (Pl)+corundum (Crn)±orthopyroxene (Opx)±kyanite (Ky)±sapphirine (Spr)±rutile. Some exhibit symplectites consisting of Crn+Cpx, Ky+Cpx and/or coronas of garnet (outer shell) around a polygonal aggregate of clinopyroxene that in some cases surrounds a polygonal aggregate of orthopyroxene. Coronitic and non‐coronitic textures occur in proximity in mm‐ to cm‐scale layers; corona structures typically occur in plagioclase‐rich layers. Their formation is therefore related primarily to protolith type (troctolite v. gabbro) rather than P–T path. Phase diagrams calculated for a kyanite‐rich granulite, a plagioclase‐rich non‐coronitic granulite, and a plagioclase‐rich coronitic granulite (taking into account changes in effective bulk composition during texture development) predict peak conditions of ~800°C, 1.1–1.5 GPa; these conditions do not require invoking an unusually high geothermal gradient. In the coronitic metagabbro, reaction textures formed along the prograde path: Crn–Cpx symplectites grew at the expense of garnet, sapphirine and plagioclase. Peak conditions were followed by isobaric cooling of ~150°C. Hornblende–plagioclase thermometry results for host amphibolite (Hbl+Pl±Crn±Grt±relict Cpx) indicate retrograde conditions of 620–675°C and 0.5–0.8 GPa accompanied by infiltration of H₂O‐rich fluid. This anticlockwise P–T path differs from an isothermal decompression path previously proposed for these rocks based on the presence of symplectite. Metamorphism of the ophiolitic rocks was driven by closing of the southern Neotethys Ocean, as oceanic lithosphere was obducted (most SE Anatolian ophiolites) or underthrust (Berit meta‐ophiolite). This was followed by subduction of a continental margin, driving cooling of the Berit granulite after the thermal peak at depths of ~40 km.