High-pressure metamorphism of stratiform sulphide deposits from the Diahot region,New Caledonia

Stratiform Cu-Pb-Zn (-Au-Ag) mineralization associated with black carbonaceous schists and acid metatuffs is restricted to distinct horizons within the Cretaceous sequence of the Diahot region. The sulphides occur in sharply bounded lenses which show varying degrees of compositional banding conformable with the foliation of the country schists. The deposits are sedimentary-exhalative of the Rio Tinto-type and have been modified by mid-Tertiary high-pressure metamorphism (lawsonite-albite and glaucophanitic greenschist facies). The ores are not strongly deformed by the metamorphism and sedimentary structures, pyrite framboids and atoll structures are preserved in some deposits. With increasing metamorphic grade sphalerite becomes more iron-rich, pyrrhotite becomes more abundant, and the sulphides show a general increase in grain-size which parallels that of the silicates in the enclosing rocks. In the more highly metamorphosed deposits the sulphide associations are retrograde assemblages. There is no evidence of large-scale metamorphic remobilization of sulphides. No differences were observed in either the sulphide assemblages or in the composition of the sulphides to indicate that the metamorphism was of the high pressure rather than the low pressure type.     

Synmetamorphic carbon mobility and graphite enrichment in metaturbidites as a precursor to orogenic gold mineralisation, Otago Schist, New Zealand

The Macraes orogenic gold deposit is hosted by a graphitic micaceous schist containing auriferous porphyroblastic sulphides. The host rock resembles zones of unmineralised micaceous graphitic pyritic schists, derived from argillaceous protoliths, that occur locally in background pelitic Otago Schist metasediments. This study was aimed at determining the relationship between these similar rock types, and whether the relationship had implications for ore formation. Argillites in the protolith turbidites of the Otago Schist metamorphic belt contain minor amounts of detrital organic matter (<0.1 wt.%) and diagenetic pyrite (<0.3 wt.% S). The detrital organic carbon was mobilised by metamorphic–hydrothermal fluids and redeposited as graphite in low-grade metaturbidites (pumpellyite–actinolite and greenschist facies). This carbon mobility occurred through >50 million years of evolution of the metamorphic belt, from development of sheared argillite in the Jurassic, to postmetamorphic ductile extension in the Cretaceous. Introduced graphite is structurally controlled and occurs with metamorphic muscovite and chlorite as veins and slicken-sided shears, with some veins having >50% noncarbonate carbon. Graphitic foliation seams in low-grade micaceous schist and metamorphic quartz veins contain equant graphite porphyroblasts up to 2 mm across that are composed of crystallographically homogeneous graphite crystals. Graphite reflectance is anisotropic and ranges from ~1% to ~8% (green light). Texturally similar porphyroblastic pyrite has grown in micaceous schist (up to 10 wt.% S), metamorphic quartz veins and associated muscovite-rich shears. These pyritic schists are weakly enriched in arsenic (up to 60 ppm). The low-grade metamorphic mobility and concentration of graphite in micaceous schists is interpreted to be a precursor process that structurally and geochemically prepared parts of the Otago Schist belt for later (more restricted) gold mineralisation. Economic amounts of gold, and associated arsenic, were subsequently introduced to carbonaceous sulphidic schists in the Macraes gold deposit by a separate metamorphic fluid derived from high-grade metaturbidites. Fluid flow at all stages in these processes occurred at metamorphic rates (mm/year), and fluids were broadly in equilibrium with the rocks through which they were passing.     

Metamorphic and tectonic domains of China

Abstract Ten metamorphic domains can be distinguished in China, comprising four cratonic, three intracratonic and three intercratonic domains. Each domain contains one or more metamorphic belts, each of which, in turn, contains a characteristic metamorphic facies or facies series that was formed during a distinct metamorphic epoch.
The metamorphic domains reflect the tectonic domains and tectonic evolution of China. Ancient continental nucleii in the North China and Tarim–Alxa cratons were probably unified with the Yangtze craton during the Early Proterozoic to form the China Platform. Widespread greenschist facies metamorphism, during the Middle and Late Proterozoic, accompanied by glaucophane–greenschist facies metamorphism, represents a rifting and closure event in the China Platform; a second rifting and closure event in the China Platform occurred during the Caledonian. The China and Siberian platforms were closed during the Hercynian to form the Eurasian Continent. Closure of the ancient Tethys Ocean occurred in the Indosinian epoch, and subduction and collision within Xizang (Tibet) and Taiwan occurred during Mesozoic–Cenozoic time.
The distribution in time of types of metamorphism in China suggests cyclical changes of metamorphism known as the Archaean, Proterozoic and Phanerozoic megacycles. Each megacycle since the Archaean consists of a change from progressive, low- to intermediate-grade metamorphism to lower grade, greenschist metamorphism that was superimposed on a general trend in which high-grade metamorphism became progressively less important with time. The change in metamorphic megacycles shows a general secular decrease in regional heat supply during metamorphism punctuated by episodic high-grade, progressive metamorphism within orogenic belts.     

Sub-ophiolite metamorphic rocks from NW Anatolia, Turkey

The metamorphic rocks from near Kütahya in north-west Anatolia record different stages in the history of closure of the Neo-Tethyan İzmir–Ankara–Erzincan ocean. Sub-ophiolite metamorphic rocks within the Tavşanlı zone are a tectonically composite sequence of quartz–mica schists, amphibole schists, amphibolites and garnet amphibolites. They show increasing metamorphic grade towards the base of the ophiolite. A first metamorphic event, typical of sub-ophiolite metamorphic sole rocks, was characterized by high-grade assemblages, and followed by retrograde metamorphism. A second event was marked by a medium-to high-pressure overprint of the first-stage metamorphic assemblages with assemblages indicating a transition between the blueschist and greenschist facies. The chemistry of the sub-ophiolite metamorphic rocks indicates an ocean island basalt origin, and Ar–Ar dating indicates a high temperature metamorphic event at 93±2 Ma. Counter-clockwise P–T–t paths recorded by the sub-ophiolite metamorphic rocks are interpreted to result from intra-oceanic thrusting during the closure of the İzmir– Ankara–Erzincan ocean, initiating subduction, which formed the high-temperature assemblages. Further subduction then produced the widespread blueschists of the Tavşanlı zone during the Late Cretaceous. Later cold thrusting obducted the ophiolite (with the metamorphic sole welded to its base), ophiolitic melanges and blueschists onto the Anatolide passive margin in the latest Cretaceous. All these events pre-date the final Anatolide–Pontide continent–continent collision.     

P–T conditions of the cratonic rocks and Eastern Ghats granulites along the Eastern Ghats Frontal Thrust,Jeypore (Orissa), India

The Eastern Ghats Frontal Thrust (EGFT) demarcates the boundary between the Archaean/Paleoproterozoic cratonic rocks to the west, and the Meso/Neoproterozoic granulites of the Eastern Ghats Mobile Belt (EGMB) to the east. At Jeypore (Orissa, India), mafic schists and granites of the cratonic domain document a spatial increase in the metamorphic grade from greenschist facies (garnet, clinozoisite – absent varieties) in the foreland to amphibolite facies (clinozoisite- and garnet-bearing variants) progressively closer to the EGFT. Across the EGFT, the enderbite–charnockite gneisses and mafic granulites of EGMB preserves a high-grade granulite facies history; amphibolite facies overprinting in the enderbite–charnockite gneisses at the cratonic fringe is restricted to multi-layered growth of progressively Al, Ti – poor hornblende at the expense of pyroxene and plagioclase. In associated mafic granulites, the granulite facies gneissic layering is truncated by sub-centimeter wide shear bands defined by synkinematic hornblende + quartz intergrowth, with post-kinematic garnet stabilized at the expense of hornblende and plagioclase. Proximal to the contact, these granulites of the Eastern Ghats rocks are intruded by dolerite dykes. In the metadolerites, the igneous assemblage of pyroxene–plagioclase is replaced by intergrown hornblende + quartz ± calcite that define the thrust-related fabric and are in turn mantled by coronal garnet overgrowth, while scapolite is stabilized at the expense of recrystallized plagioclase and calcite. Petrogenetic grid considerations and thermobarometry of the metamorphic assemblages in metadolerites intrusive into granulites and mafic schists within the craton confirm that the rocks across the EGFT experienced prograde heating (T_max value ∼650–700 °C at P ∼ 6–8 kbar) along the prograde arm of a seemingly clockwise P–T path. Since the dolerites were emplaced post-dating the granulite facies metamorphism, the prograde heating is correlated with renewed metamorphism of the granulites proximal to the EGFT. A review of available age data from rocks neighboring the EGFT suggests that the prograde heating of the cratonic granites and the re-heating of the Eastern Ghats granulites are Pan – African in age. The re-heating may relate to an Early Paleozoic Pan-Gondwanic crustal amalgamation of older terrains or reactivation along an old suture.     

Ultramafic–Mafic Assemblage of Plutonic Rocks and Hornblende Schists of Shirshov Rise,Bering Sea,and Stalemate Ridge,Northwest Pacific: Geodynamic Interpretations of Geochemical Data

The paper presents data on plutonic and metamorphic rocks dredged during Cruise 249 of the German R/V Sonne to the Stalemate Ridge, Northwest Pacific Ocean and the Shirshov Rise, western Bering Sea. Dredges in the northwestern sector of the Stalemate Ridge and central portion of the Shirshov Rise show that the plutonic and metamorphic rocks obtained here are amazingly similar. Our petrologic and geochemical data led us to view the rocks as members of a mafic–ultramafic assemblage typical of cumulate portions of ophiolite complexes and backarc spreading centers. The plutonic complexes of the Shirshov Rise and Stalemate Ridge show similarities not only in the petrography and mineralogy of their protoliths but also in the character of their metamorphic transformations. Plutonic rocks from both areas display mineralogical evidence of metamorphism within a broad temperature range: from the high-temperature amphibolite facies to the greenschist facies. Relations between the index mineral assemblages indicate that the metamorphic history of plutonic complexes in the Stalemate Ridge and Shirshov Rise proceeded along a retrograde path. Hornblende schists accompanying the plutonic rocks of the Stalemate Ridge and Shirshov Rise are petrographically close to foliated amphibolites in subophiolitic metamorphic aureoles. Within the framework of geodynamic interpretations of our results, it is realistic to suggest that the examined plutonic complexes were exhumed from subduction zones of various age.     

The nature and dimensions of regional and local gold-related hydrothermal alteration in tholeiitic metabasalts in the Norseman goldfields: the missing link in a crustal continuum of gold deposits?

The Archaean lode-gold deposits at Norseman, Western Australia, consist of auriferous quartz veins in dextral-reverse ductile-brittle shear zones within tholeiitic metabasalts of upper-greenschist to amphibolite facies metamorphic grade. Three types of deposits (Northern, Central, Southern) are delineated on the basis of their spatial distribution, veining style, alteration mineraloty and metamorphic grade of host rocks. Northern deposits, hosted in upper-greenschist to lower-amphibolite facies rocks, comprise massive to laminated quartz veins with selvedges of quartz-chlorite-calcite-biotite-plagioclase assemblages. Central deposits, hosted in lower-amphibolite facies rocks, consist of laminated to massive quartz veins with selvedges of quartz-actinolite-biotite-plagioclasecalcite assemblages. Southern deposits, hosted in middleamphibolite facies metabasalts, consist of banded quartz-diopside-calcite-microcline-zoisite veins. All deposits exhibit variable ductile deformation of veins and contiguous alteration haloes, consistent with a syn-deformational genesis at high temperatures. From Northern to Southern deposits, the alteration assemblages are indicative of higher temperatures of formation, and there are progressively greater degrees of dynamically recovered textures in alteration and gangue minerals. These observations imply that a thermal variation of gold-related hydrothermal alteration exists within the Norseman Terrane over a distance of 40 km, with T_Northern<T_Central<T_Southern This thermal zonation is corroborated by T−X_{CO 2} phase relations between vein selvedge assemblages, which signify formation temperatures of approximately 420°–475°C, 470°–495°C and >500°C for Northern, Central and Southern deposits, respectively. The sum of structural, petrographic and mineral chemistry data indicates that the alteration assemblages formed in high-temperature, open hydrothermal systems and have not been subsequently metamorphosed. The thermal differences between the deposit groups may reflect (1) a temperature gradient, at relatively constant P, corresponding to the proximity of the deposits to regional granitoid complexes, or (2) formation of the deposits at progressively deeper crustal levels from north to south. In either case the deposits represent a continuum of gold deposition from upper-greenschist to amphibolite facies, now exposed in an oblique section through the Archaean crust at Norseman.     

Constraints on crustal hydration below the Colorado plateau from Vp measurements on crustal xenoliths

Seismic velocities have been measured as a function of confining pressure to 8 kbar for crustal xenoliths from the Moses Rock Dike and Mule Ear Diatreme, two kimberlite pipes on the Colorado Plateau. Rock types measured include rhyolite, granite, diorite, metasedimentary schists and gneisses, mafic amphibolites and granulites. Many of our samples have been hydrothermally altered to greenschist facies mineral assemblages during transport to the earth's surface. The velocity of compressional waves measured on altered amphibolites and granulites are too low by 0.1–0.3 km/s for such rock types to be characteristic of deep crustal levels. A direct correlation exists between progressive alteration and the presence of microcracks extending into the xenoliths from the kimberlitic host rock. Velocities of pristine samples are compatible with existing velocity profiles for the Colorado Plateau and we conclude that the crust at depths greater than 15 km has probably not undergone a greenschist facies metamorphic event. The xenolith suite reflects a crustal profile similar to that exposed in the Ivrea-Verbano and Strona-Ceneri zones in northern Italy.     

Metamorphic zones and physical conditions of metamorphism in Leros Island,Greece

On the basis of the systematic variation and the appearance and disappearance of some metamorphic minerals in metapelitic assemblages, the metamorphic terrain of Leros can be divided into chlorite, biotite, garnet and staurolite-kyanite zones of progressive regional metamorphism. The matapelites are interbedded with blueschists containing magnesioriebeckite in Fe³⁺-rich mafic assemblages in the chlorite zone and more normal greenschist and amphibolite facies in higher grade zones. Combining the observed mineral assemblages in pelitic and mafic schists with the available experimental or calculated relevant phase equilibria, one can deduce temperature conditions of metamorphism ranging from about 350° C up to about 700° C and pressures ranging between a minimum value defined by the pressure of the triple point of the Al₂SiO₅ polymorphs and a possible minimum around 7 kb.The observed metamorphic sequence may be interpreted as the result of progressive transportation of the original sediments and the interbedded mafic rocks from a regime typified by low temperatures and relatively high pressures, to regimes characterized by higher temperature and medium pressures.     

The Coolgardie Goldfied,Western Australia: district-scale controls on an Archaean gold camp in an amphibolite facies terrane

Archaean lode gold deposits in the Coolgardie Goldfield, Western Australia, occur in mafic and ultramafic rocks which have been metamorphosed to the amphibolite facies. Mineralisation was broadly synchronous with peak metamorphism, the main phase of granitoid emplacement, and regional deformation. Several different structural styles are represented by the deposits of the Coolgardie Goldfield. Mineralisation occurs along sheared felsic porphyry-ultramafic rock contacts, in gabbro-hosted quartz-vein sets, in fault-bounded quartzvein sets, and in laminated quartz reefs sited in brittleductile shear zones. The structures hosting mineralisation formed in response to a progressive deformation event, related to granitoid emplacement in an east-west compressional far-field stress régime, but with local heterogeneous stress orientations. This occurred after an earlier period of thrust-stacking, with probable north-south directed tectonic transport. Two contrasting styles of goldrelated wallrock alteration are associated with the auriferous lodes of the Coolgardie Goldfield. A high-temperature assemblage (formation temperature >500°C), characterised by the proximal alteration assemblage garnet+ hornblende + plagioclase + pyrrhotite, contrasts with a medium-temperature assemblage (formation temperature <500°C), consisting of calcic amphibole + biotite + plagioclase + calcite + arsenopyrite + pyrrhotite. The distribution of the two styles of gold-related alteration is controlled by distance from voluminous syntectonic granitoids located to the west of the Coolgardie Goldfield, with the high-temperature style of alteration more proximal to the granitoid-greenstone contact than the medium-temperature style. The occurrence of gold deposits that formed under amphibolite facies conditions throughout the Coolgardie Goldfield supports a crustal continuum model for Archaean lode-gold deposits, which proposes that gold is deposited in metamorphic environments that range from the sub-greenschist to granulite facies. In addition, the data from Coolgardie suggest that syntectonic, synmineralisation granitic plutons may play a significant role in controlling the style of gold associated wallrock-alteration at deep crustal levels.     

Petrology,geochemistry and P–T–t path of lawsonite‐bearing retrograded eclogites in the Changning–Menglian orogenic belt,southeast Tibetan Plateau

The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau, is considered as the main suture zone marking the closure of the Palaeo‐Tethys Ocean between the Indochina and Sibumasu blocks. Here, we investigate the recently discovered retrograded eclogites from this suture zone in terms of their petrological, geochemical and geochronological features, with the aim of constraining the metamorphic evolution and protolith signature. Two types of metabasites are identified: retrograded eclogites and mafic schists. The igneous precursors of the retrograded eclogites exhibit rare earth element distribution patterns and trace element abundance similar to those of ocean island basalts, and are inferred to have been derived from a basaltic seamount in an intra‐oceanic tectonic setting. In contrast, the mafic schists show geochemical affinity to arc‐related volcanics with the enrichment of Rb, Th and U, and depletion of Nb, Ta, Zr, Hf and Ti, and their protoliths possibly formed at an active continental margin tectonic setting. Retrograded eclogites are characterized by peak metamorphic mineral assemblages of garnet, omphacite, white mica, lawsonite and rutile, and underwent five‐stage metamorphic evolution, including pre‐peak prograde stage (M₁) at 18–19 kbar and 400–420°C, peak lawsonite‐eclogite facies (M₂) at 24–26 kbar and 520–530°C, post‐peak epidote–eclogite facies decompression stage (M₃) at 13–18 kbar and 530–560°C, subsequent amphibolite facies retrogressive stage (M₄) at 8–10 kbar and 530–600°C, and late greenschist facies cooling stage (M₅) at 5–8 kbar and 480–490°C. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb spot analyses of zircon show two distinct age groups. The magmatic zircon from both the retrograded eclogite and mafic schist yielded protolith ages of 451 ± 3 Ma, which is consistent with the ages of Early Palaeozoic ophiolitic complexes and ocean island sequences in the CMOB reported in previous studies. In contrast, metamorphic zircon from the retrograded eclogite samples yielded consistent Triassic metamorphic ages of 246 ± 2 and 245 ± 2 Ma, which can be interpreted as the timing of closure of the Palaeo‐Tethys Ocean. The compatible peak metamorphic mineral assemblages, P–T–t paths and metamorphic ages, as well as the similar protolith signatures for the eclogites in the CMOB and Longmu Co–Shuanghu suture (LCSS) suggest that the two belts formed part of a cold oceanic subduction system in the Triassic. The main suture zone of the Palaeo‐Tethyan domain extends at least 1,500 km in length from the CMOB to the LCSS in the Tibetan Plateau. The identification of lawsonite‐bearing retrograded eclogites in the CMOB provides important insights into the tectonic framework and complex geological evolution of the Palaeo‐Tethys.     

华南加里东期造山型金矿中金的来源—来自浙江诸暨陈蔡杂岩的证据

造山型金矿中金的来源是目前广泛争论的问题,尽管变质成因模式受到了较多的关注,但研究实例还不多。加里东期是华南地区重要的金矿成矿期,浙江诸暨陈蔡杂岩发育了从绿片岩相到角闪岩相的岩石组合,变质时代为加里东期,同时,其变质温度和压力等都已经得到精确厘定,为研究华南加里东期金矿形成与变质作用的关系提供理想的研究剖面对象。通过系统采集陈蔡杂岩岩石样品,在详细的岩相学研究的基础上对其中的黄铁矿中的金及亲硫元素进行了激光剥蚀-电感耦合-等离子体质谱（LA-ICP-MS）原位微区微量元素分析。研究结果表明：陈蔡杂岩角闪岩相岩石岩相学特征中有绿泥石,角闪石退变质为黑云母,反映退变质作用中有变质流体的回流。华南加里东期变质过程中,从绿片岩相到角闪岩相的相变过程中金及其他亲硫元素从主要赋存矿物黄铁矿中释放出来,进入变质流体,为江南造山带造山型金矿提供了金属来源。利用硫化物单矿物进行金的含量原位分析是解决变质过程中金活化迁移的重要手段。并提出华南加里东期变质过程中金的来源模式图。     

Crustal-scale shear zones and their significance to Archaean gold mineralization in Western Australia

Many large Archaean epigenetic gold deposits show a broad spatial relationship to regional lineaments in greenstone belts, although in detail they are sited in subsidiary brittle-ductile fault structures. Fluids, originating from a deep source, follow a complex path and re-equilibrate with different lithologies and with metamorphic fluid during migration to higher crustal levels. Temperature and pressure conditions at or below the amphibolite/greenschist metamorphic boundary, where most gold deposits are located, favour the establishment of brittle-ductile and brittle subsidiary structures, the preferred structural setting of gold deposits. Physical gradients between the regional ductile structures and more brittle subsidiary structures ensure transient, strongly localized, fluid flow into the latter, where lower temperatures and suitable host rocks with high Fe/(Fe + Mg) ratios favour gold-deposition. The multi-source origin and continuous re-equilibration of the fluid with crustal rock, which includes granitoid and greenstone-belt lithologies of different ages, is reflected in the diverse isotopic and geochemical signature of the gold deposits.     

Kyanite-eclogite to amphibolite fades evolution of hydrous mafic and pelitic rocks,Adula nappe,Central Alps

In the southern Adula nappe (Central Alps), two stages of regional metamorphism have affected mafic and pelitic rocks. Earlier eclogite facies with a regional zonation from glaucophane eclogites to kyanite-hornblende eclogites was followed by a Tertiary overprint which varied from greenschist to high-grade amphibolite facies. Despite a common metamorphic history, contrasting equilibration conditions are often recorded by high-pressure mafic eclogite and adjacent predominantly lower-pressure pelite assemblages. This pressure contrast may be explained by different overprinting rates of the two bulk compositions during unloading. The rates are controlled by a mechanism in which dehydrating metapelites provide the H₂O required for simultaneous overprinting of enclosed mafic eclogites by hydration.Quantitative mass balance modelling based on corona textures is used to show that overprinting of metapelites during unloading involved dehydration reactions. The relatively rapid rate of dehydration reactions led to nearly complete reequilibration of metapelites to amphibolite facies assemblages.After the formation during high-pressure metamorphism of mafic eclogites, later lower-pressure reequilibration by hydration to amphibolites was slow, and therefore incomplete, because it depended on large scale transport of H₂O from adjacent, dehydrating metapelites.The facies contrast observed between rocks of different bulk composition is thus a consequence of the general tendency of metamorphic rocks to retain the most dehydrated assemblage as the final recorded state.     

Petrology of an Arc-Oceanic Crust Contact Zone in the Laohushan Back-arc Basin, the Eastern Section of the North Qilian Mountains, NW China

A contact zone sandwiched between an arc and an oceanic crust was discovered in the Laohushan area in the present study. It consists of a series of north-dipping imbricated thrust sheets and is exposed on the surface as a narrow arcuate belt, which extends for about 30 km in an E-W direction and measures about 1-3 km wide. Lithologically, it can be divided into four subzones. Subzone 1 consists of meta-andesite and metasandstone; subzone 2, psammitic schists; subzone 3, psammitic and pelitic schists, quartz diorite and hornfelses; and subzone 4, metagabbro, epidote amphibolite and pelitic schists. The metamorphism has the following grading sequence: low greenschist facies in subzone 1 → high greenschist facies in subzone 2 →low amphibolite facies in subzone 3→ epidote amphibolite facies in subzone 4. Petrographic and geochemical evidence shows that rocks in subzones 1, 2 and 3 are arc rocks, whereas those of subzone 4 are oceanic crustal rocks. The metamorphic mineral assemblages and especially miner     

Pan-African decompressional P-T path recorded by granulites from central Dronning Maud Land, Antarctica

A high-grade metamorphic complex is exposed in Filchnerfjella (6–8°E), central Dronning Maud Land. The metamorphic evolution of the complex has been recovered through a study of textural relationships, conventional geothermobarometry and pseudosection modelling. Relicts of an early, high-P assemblage are preserved within low-strain mafic pods. Subsequent granulite facies metamorphism resulted in formation of orthopyroxene in rocks of mafic, intermediate to felsic compositions, whereas spinel + quartz were part of the peak assemblage in pelitic gneisses. Peak conditions were attained at temperatures between 850–885 °C and 0.55–0.70 GPa. Reaction textures, including the replacement of amphibole and garnet by symplectites of orthopyroxene + plagioclase and partial replacement of garnet + sillimanite + spinel bearing assemblages by cordierite, indicate that the granulite facies metamorphism was accompanied and followed by decompression. The observed assemblages define a clock-wise P-T path including near-isothermal decompression. During decompression, localized melting led to formation of post-kinematic cordierite-melt assemblages, whereas mafic rocks contain melt patches with euhedral orthopyroxene. The granulite facies metamorphism, decompression and partial crustal melting occurred during the Cambrian Pan-African tectonothermal event.     

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.     

Metamorphism and deformation of golpayegan metapelitic rocks, Sanandaj-Sirjan Zone, Iran

The metapelitic schists of the Golpayegan region can be divided into four groups based on their mineral assemblages: (1) garnet-chloritoid schists, (2) garnet schists, (3) garnet-staurolite schists, and (4) staurolite-kyanite schists. Paleozoic pelagic shales experienced progressive metamorphism and polymetamorphism from greenschist to amphibolite facies along the kyanite geotherm. Mylonitic granites are concentrated in the central part of the region more than in other areas, and formed during the dynamic metamorphic phase by activity on the NW-SE striking Varzaneh and Sfajerd faults. The presence of chloritoid in the metapelites demonstrates low-grade metamorphism in the greenschist facies. The textural and chemical zoning of garnets shows three stages of growth and syntectonic formation. With ongoing metamorphism, staurolite appeared, and the rocks reached amphibolite facies, but the degree of metamorphism did not increase past the kyanite zone. Thus, metamorphism of the pelitic sediments occurred at greenschist to lower amphibolite facies. Thermodynamic studies of these rocks indicate that the metapelites in the north Golpayegan region formed at 511?C618°C and 0.24?C4.1 kbar.     

Multistage gold mineralization at the Lapa mine, Abitibi Subprovince: insights into auriferous hydrothermal and metasomatic processes in the Cadillac–Larder Lake Fault Zone

The Lapa gold deposit contains reserves of 2.4 Mt at 6.5 g/t Au and is one of the few deposits located directly within the Cadillac–Larder Lake Fault Zone (CLLFZ), a first-order crustal-scale fault that separates the Archean Abitibi Subprovince from the Pontiac Subprovince to the south. Gold mineralization is predominantly hosted in highly strained and altered, upper greenschist–lower amphibolite facies mafic to ultramafic rocks of the Piché Group. Auriferous ore zones consist of finely disseminated auriferous arsenopyrite–pyrrhotite?±?pyrite and native gold disseminated in biotite- and carbonate-altered wall rocks. Native gold, which is also present in quartz ± dolomite–calcite veinlets, is locally associated with Sb-bearing minerals, especially at depth ≤1 km from surface where the deposit is characterized by a Au–Sb–As association. At vertical depth greater than 1 km, gold is associated with arsenopyrite and pyrrhotite (Au–As association). The mineralogy and paragenesis of the Lapa deposit metamorphosed ore and alteration assemblages record the superposition of three metamorphic episodes (M1, M2, and M3) and three gold mineralizing events. Spatial association between biotitized wall rocks and auriferous arsenopyrite indicates that arsenopyrite precipitation is concomitant with potassic alteration. The predominant Au–As association recognized across the deposit is related to gold in solid solution in arsenopyrite as part of a pre-M2 low-grade auriferous hydrothermal event. However, the occurrence of hornblende?+?oligoclase porphyroblasts overprinting the biotite alteration, and the presence of porous clusters and porphyroblasts of arsenopyrite with native gold and pyrrhotite indicate an auriferous metasomatic event associated with peak M2 prograde metamorphism. Late retrograde metamorphism (M3) overprints the hornblende–oligoclase M2 assemblage within the host rocks proximal to ore by an actinolite–albite assemblage by precipitation of free gold and Sb–sulfosalts at lower P–T. The complex relationships between ore, structural features, and metamorphic assemblages at Lapa are related to the tectonometamorphic evolution of the Cadillac–Larder Lake Fault Zone at different times and crustal levels, and varying heat and fluid flow regimes. The Lapa deposit demonstrates that early, low-grade gold mineralization within the Cadillac–Larder Lake Fault Zone has benefited from late gold enrichment(s) during prograde and retrograde metamorphism, suggesting that multi-stage processes may be important to form gold-rich orogenic deposits in first order crustal-scale structures.     

Petrochemical characterization of mafic rocks from the Ligurian ophiolites,southern Apennines

New mineralogical and chemical data for ophiolitic rocks from the southwesternmost Liguride Units are presented in order to constrain their ocean-floor origin and subsequent emplacement in an accretionary wedge. Their complete petrochemical evolution is particularly well preserved in the southern Apennine metabasites. Metadolerites show amphibolite and greenschist facies mineral assemblages of ocean-floor metamorphism. Metabasalts display greenschist facies ocean-floor metamorphism and spilitic alteration. Veins cutting the mafic rocks show mineral assemblage of the prehnite–pumpellyite metamorphic facies. HP/LT orogenic metamorphism, reflecting underplating of the ophiolitic suite at the base of the Liguride accretionary wedge during subduction of the western Tethys oceanic lithosphere produced a mineral assemblage typical of the lawsonite–glaucophane facies. Bulk-rock chemistry suggests that the mafic protoliths had a MORB-type affinity, and were affected by ocean-floor rodingitic and/or spilitic alteration. Hydrothermal alteration-induced LREE mobility and LREE enrichment may be correlated with the ocean-floor metamorphism.