Facies Metamorphism Research Articles

A review of the chondrite–achondrite transition, and a metamorphic facies series for equilibrated primitive stony meteorites

AbstractHere, the petrological features of numerous primitive achondrites and highly equilibrated chondrites are evaluated to review and expand upon our knowledge of the chondrite–achondrite transition, and primitive achondrites in general. A thermodynamic model for the initial silicate melting temperature and progressive melting for nearly the entire known range of oxidation states is provided, which can be expressed as Tm = 0.035Fa2−3.51Fa + 1109 (in °C, where Fa is the proportion of fayalite in olivine). This model is then used to frame a discussion of textural and mineralogical evolution of stony meteorites with increasing temperature. We suggest that the metamorphic petrology of these meteorites should be based on diffusive equilibration among the silicate minerals, and as such, the chondrite–achondrite transition should be defined by the initial point of silicate melting, not by metal–troilite melting. Evidence of silicate melting is preserved by a distinctive texture of interconnected interstitial plagioclase ± pyroxene networks among rounded olivine and/or pyroxene (depending on ƒO2), which pseudomorph the former silicate melt network. Indirectly, the presence of exsolution lamellae in augite in slowly cooled achondrites also implies that silicate melting occurred because of the high temperatures required, and because silicate melt enhances diffusion. A metamorphic facies series is defined: the Plagioclase Facies is equivalent to petrologic types 5 and 6, the Sub‐calcic Augite Facies is bounded at lower temperatures by the initiation of silicate melting and at higher temperatures by the appearance of pigeonite, which marks the transition to the Pigeonite Facies.

Mars Rover Techniques and Lower/Middle Cambrian Microbialites from South Australia: Construction, Biofacies, and Biogeochemistry.

The Perseverance rover (Mars 2020) is equipped with an instrumental and analytical payload capable of identifying a broad range of organic molecules in geological samples. To determine the efficacy of these analytical techniques in recognizing important ecological and environmental signals in the rock record, this study utilized analogous equipment, including gas chromatography/mass spectrometry, Raman spectroscopy, X-ray fluorescence (XRF), Fourier transform infrared spectroscopy, along with macroscopic and petrographic observations, to examine early-middle Cambrian microbialites from the Arrowie Basin, South Australia. Morphological and petrographic observations of these carbonate successions reveal evidence of hypersaline-restricted environments. Microbialites have undergone moderate diagenesis, as supported by XRF data that show mineral assemblages, including celestine and the illitization of smectite. Raman spectral data, carbon preference indices of ∼1, and the methylphenanthrene index place the samples in the prehnite/pumpellyite metamorphic facies. Pristane and phytane are the only biomarkers that were detected in the least thermally mature samples. This research demonstrates a multitechnique approach that can yield significant geological, depositional, paleobiological, and diagenetic information that has important implications for planning future astrobiological exploration.

Mineralogical variation in platinum group element within altered chromitite of the Kondapalli layered igneous complex (Southern India): Implication on magmatic evolution and its petrogenetic significance

The Kondapalli Layered Igneous Complex (KLIC) is deformed and dismembered stratiform type of mafic-ultramafic complex emplaced into high-grade metamorphic rocks of Eastern Ghat Belt (EGB). Which contains chromite with relatively oxidized compositions that occur as bands, pockets, pods, stringer, and dissemination. This chromite hosts traces of PGM eg. laurite, irarsite [(Ir, Ru, Rh, Pt)AsS], Os-Ir-Ru alloys and Fe–Ni–As–S minerals. The present study helps to understand the changes and/or reworking of chromite and PGM due to the effect of alteration. Petrographic observations and EPMA study indicate that the chromite core is still consistent with magmatic fractionation processes, whereas sequential alteration of the chromite rim suggests upper greenschist to lower amphibolite facies of metamorphism (500° to 550 °C). The platinum-group minerals (PGM), however, display evidence of partial hydrothermal/metamorphic reworking. Especially, laurite and irarsite which occur near the chromite grain boundary. In contrast, the primary ones which probably formed at high-T and low fs2 and occur in the chromite cores are relatively less affected by alteration.The calculated parental magma composition was similar to that of modern primitive boninitic/tholeiitic basalt formed by a high-degree of mantle melting. Present study ascribes the formation of the subduction-generated arc mafic-ultramafic magmatism in the Kondapalli to the closure of the ocean that led to the formation of the suture between Eastern Dharwar Craton (EDC) and EGB. The parent magma of the KLIC is interpreted as mantle-derived arc-related boninites with S-undersaturated nature like other layered intrusions. This is indicating that an initial percolation of sulphur-poor melts for long-period without significant contamination for the genesis of KLIC. The mineral chemistry supports the very limited crustal contamination and non-availability of the sufficient meta-sediments during assimilation precludes the sulphur saturation in the KLIC, which is mainly constrained for the development of potential Ni-Cu-PGE mineralization. As a result, the KLIC fever the deposition of only primary orthomagmatic chromite-PGE mineralization. These findings determined that the investigated KLIC are low-priority Ni-Cu-PGE targets. The compositions of the chromites can be used to determine the petrologic history of the intrusions and may prove to be a useful exploration tool in such mineralised belt.

The Ductile‐to‐Brittle Transition Recorded in the Balmuccia Peridotite Body, Italy: Ambient Temperature for the Onset of Seismic Rupture in Mantle Rocks

AbstractTo constrain the conditions of the brittle‐ductile transition in peridotite, we studied the deformation sequence of a small‐displacement fault system developed in the Balmuccia peridotite body, northern Italy. The ambient pressure‐temperature conditions of the deformation were estimated using equilibrium mineral assemblages and mineral compositions. Fault‐related deformed rocks, including pseudotachylytes, cataclasites, and mylonites, were classified into three groups according to the metamorphic facies at the time of their formation: Group 1 consists of mylonites, cataclasites, and pseudotachylytes of the spinel‐lherzolite facies; Group 2 consists of cataclasite and pseudotachylyte, but no mylonite, of the plagioclase‐lherzolite facies; and Group 3 consists of cataclasite and pseudotachylyte of a lower‐grade hydrous facies. Cataclasite of the spinel‐lherzolite facies coeval with fault‐vein pseudotachylyte was identified for Group 1 and ambient temperature of the earthquake was determined by applying geothermometers to the mylonite and cataclasite. Among the deformed rocks of the spinel‐lherzolite facies (Group 1), mylonite is truncated by veins of pseudotachylyte. However, the mineral compositions of the mylonite and the cataclasite associated with the pseudotachylyte are essentially identical, yielding a temperature for the ductile‐to‐brittle transition in the peridotite of ~720 °C at a pressure of ~0.6–1.6 GPa. In the pressure range, low‐pressure side, which overlaps with conditions of surrounding crustal rocks, is more consistent with obtained fault structures and regional tectonics. This condition lies on a geotherm of an oceanic lithosphere of the age of ~10 Ma and is consistent with a predicted temperature and depth using published flow laws of olivine.

Klimovskii Metasomatic Complex of the Belomorian Mobile Belt: Composition, Age, Geological Position

Metasomatites of the White Sea mobile belt of the Fennoscandian (Baltic) shield are considered. Metasomatites are traced from southeast to northwest for a distance of ≥550 km. In mineral composition, metasomatites correspond to the paragenesis of amphibolite facies of regional high-pressure metamorphism. To determine the tectonic position of metasomatites and their place in the sequence of geological events, we carried out isotope-geochemical studies of zircons, isolated both from metasomatites and their enclosing gneisses. It was established that the host metamorphic formations formed in the Neoarchean and metasomatites formed in the Late Paleoproterozoic (≈1.9 Ga). The rare-earth spectra of the studied zircons indicate that they formed from a fluid in a retrograde branch of regional metamorphism. Fluids were discharged along the deep thrust planes, thereby marking the zones of their exits to the modern surface. The results obtained indicate tectonothermal reworking of Archean complexes in the Late Paleoproterozoic.

Vein Mineralogy and Hydrothermal Gold Mineralization at the Kyaw Soe Thu Prospect in the Wabo Deposit, Mandalay Region, Central Myanmar

The Kyaw Soe Thu Prospect is a part of the Wabo deposit which is situated along Mogok Metamorphic Belt (MMB) in Central Myanmar. Gold bearing veins are hosted in the banded garnet-biotite gneiss, calc-silicate, marble and granitoid of the Wabo area. The granitic rocks intruded into banded garnet-biotite gneiss. The MMB was intensely deformed and it is a highly fractured metamorphic terrain in which metamorphic facies ranges from amphibolite to granulite facies. Gold mineralization at the Kyaw Soe Thu Prospect in the Wabo deposit occurred in the forms of gold-quartz veins. The quartz veins are a few centimeters in width. The color of the quartz vein at the Kyaw Soe Thu Prospect is commonly smoky and milky. The quartz vein exhibits chalcedonic, saccharoidal and granular textures. Three mineralizations were indicated. Stage I is the thin layered crustiform chalcedonic quartz. Stage II is chalcedonic quartz vein and stage III is Ankerite-quartz-sericite vein. Stage II and stage III are gold rich hydrothermal stages, and Ankerite-quartz-sericite veins were formed with a small amount of sericites in the Kyaw Soe Thu Prospect. The Ankerite-quartz-sericite veinlets range from a centimeter up to three centimeter in width within the stage II chalcedonic quartz vein. Textures of mineralized quartz vein samples were petrographically examined. The textures include a feathery texture that is most closely associated with electrum in both stage II chalcedonic quartz veins and stage III Ankerite-quartz-sericite veins. Ankerite varies in color from pink, white to buff. Among them, pink color is more common in the Kyaw Soe Thu Prospect. Ankerite in the Kyaw Soe Thu Prospect exhibits rhombohedral idiomorphic, which exhibits growth patterns. Ore minerals of the veins are electrum, pyrite, galena, and trace amount of very fine-grained chalcopyrite. On the basis of textures of the chalcedonic quartz veins, the Kyaw Soe Thu Prospect is ascribed to be a low-sulfidation epithermal deposit.

Petrological Characteristic and Whole Rock Geochemistry of Metamorphic Rocks in Melangè Complex of Ciletuh Area, West Java, Indonesia

Melangí¨ Complex of Ciletuh Area, West Java, Indonesia is located at 106 ° 23 '38.9 "- 106 ° 25' 27.6" E and 7 ° 14 '27.6 "- 7 ° 12' 46.2" S. This area has unique geological features where the Early Cenozoic uplifted subduction rocks such as ophiolites and metamorphic rocks are exposed. This study aimed to determine the condition of the stratigraphic position and geological structure of metamorphic rock unit in the field, to find out the metamorphic facies and their evolution, to know the parental rocks (protoliths) of metamorphic rocks, and to interpret tectonic environments for the formation of metamorphic rocks based on stratigraphy, geological structure, petrography, and geochemistry. Study of literatures, geological mapping, petrology and geochemical analyses were used as methods of this research. The entire analysis is combined as a guidance in interpretation of petro-tectonic environment. Distribution of metamorphic rock outcrops found in several places, such as, Citisuk River, Cikopo River and Cikarikil River. Based on petrography analysis, metamorphic rocks types consist of schistose amphibolite, various type of greenschist, phyllite and quartzite. The protolith of metamorphic rocks in Ciletuh is quite diverse, namely metasediments such as pelitic, psammitic, calc-silicate sediment, and meta-igneous such as gabbro and basalt. The presence of epidote, chlorite, and calcite in schistose amphibolite show retrograde metamorphism process in lower temperature-pressures condition. The occurance of quartz and calcite vein in several samples was shown as an indication of hydrothermal alteration. Based on geochemical characteristics, the result showed that sedimentary source of metasedimentary rocks were derived from the volcanic arc environment. While, metabasalt rocks were originated from the Island Arc tectonic environment. According to the association, these metamorphic rocks were formed by regional metamorphism, as the result of subduction process and orogenic event. Thus, retrograde metamorphism indicates the lifting or accretion process that caused by subsecuent tectonic activity.

Lithostratigraphy of the Copperton Formation, Areachap Group

Abstract The type area of the Copperton Formation is on the farms Vogelstruisbult 104, Somuspan 105 and Dooniespan 108 in Prieska District. Outcrop is poor and the type material is preserved in exploration borehole cores from the Prieska Copper Mines and the Annex Cu-Zn deposits. It is highly deformed and variably metamorphosed. Thus it is a lithodemic unit, but interpreted as a supracrustal sequence and described as a formation including lithologically distinct members. The Copperton Formation comprises a wide range of rock types including metabasic and intermediate gneisses with minor amounts of metapelitic and calc-silicate rocks. Metamorphic parageneses generally reflect amphibolite facies metamorphism, but granulite and retrograde greenschist facies zones also occur. The protoliths are interpreted as an arc-related volcano-sedimentary package and the Smouspan Member metadacite is dated at 1284 ± 9 Ma. Members are distinguished as follows: The Magazine Member is dominated by calc-silicate rocks which are dominant in outcrop but rarely found in borehole cores. The Smouspan Gneiss is a fairly homogeneous hornblende-biotite intermediate gneiss which is up to 400 meters thick in an isoclinal fold structure. It comprises the footwall to the Prieska Copper Mines Member, in which the massive sulphide orebody occurs, enclosed in an alteration assemblage comprising dark gedrite fels and strongly foliated, leucocratic quartz-perthite-sillimanite gneiss. The ore is interpreted as a volcanogenic massive sulphide deposit formed in a Mesoproterozoic island arc system. The Vogelstruisbult Member is the hanging wall unit, comprising mainly laminated amphibolites and metapelites, but also containing a variety of rock types including hornblende gneiss, biotite gneiss, chlorite schist, and calc-silicate gneiss. Away from the Prieska Mines orebody, a similar variety of rock types is found, and not subdivided but classified as Copperton Formation, a mappable unit. The same assemblage of rock types, including massive sulphide mineralization, was intersected in drill holes on the farms Kielder (portion of Doonies Pan 108), Eierdop Pan and Kantienpan to the north. The Copperton Formation is the southernmost unit of the Areachap Group which is exposed between Prieska Copper Mines and Areachap Mine north of Upington, where the Jannelsepan and Bethesda formations occur. The Copperton Formation is partly obscured in many places by Dwyka Group tillite cover which thickens southwards. A sequence of structural and metamorphic events affected the Copperton Formation and Areachap Group during the 1.2 to 1.0 Ga Namaqua-Natal orogeny. These involved collision of the Areachap Terrane with the Kaapvaal-Rehoboth Craton at about 1220 Ma, a thermal and deformational event coeval with the continental-scale Umkondo mantle event at about 1100 Ma, followed by uplift, erosion and the development of right-lateral shear zones of the Doornberg Lineament, with cooling below 300°C by 920 Ma. A Cambrian peneplain developed in the region which was first covered by Nama Group sandstones, then glaciated and covered by Permian Dwyka Group tillites which are presently being eroded to expose the Copperton Formation.

Sulfides in the marbles of spurrite-merwinite facies: the Kochumdek River, East Siberia

In the marbles of the Kochumdek contact metamorphic aureole pyrrhotite, rasvumite, sphalerite, alabandite, wurtzite, galena, acanthite have been identified. Mineralogical diversity of sulphides is the consequence of the efficient crystal-chemical fractionation of trace elements under the PT‑conditions of spurrite-merwinite metamorphic facies. Trace element and isotope fingerprints of sulfides indicate that sedimentary parent rocks were the main source of sulfur and metals for sulfides mineralization.

Sulfides in Metamorphic Rocks of the Fore Range Zone (Greater Caucasus). A New Type of Mineral Container for Peak Metamorphism Mineral Assemblages

The rocks of the Armovka Formation (the Fore Range zone, Greater Caucasus) have undergone low-grade metamorphism that partially erased information about initial rock formation conditions. We discovered high-pressure mineral inclusions such as omphacite, phengite, garnet, and paragonite enclosed by pyrite and chalcopyrite. Mineral inclusions in sulfides may provide important information about metamorphic pressure−temperature conditions because they are shielded by the host minerals and isolated from significant low-grade overprinting. Calculations performed on phengite inclusions using the phengite Si-content barometry indicate a pressure ranging from 1.7 ± 0.2 to 1.9 ± 0.2 GPa for temperature of 600 ± 40 °C. These data are consistent with estimations obtained for eclogite bodies embedded in rocks of the Armovka Formation. Geothermobarometry of the latest yielded conditions of 680 ± 40 °C and a minimum pressure of 1.6 ± 0.2 GPa to upper pressure boundary at 2.1 GPa. This fact allows us to assume that the metamorphic rocks of the Armovka Formation were immersed in the subduction zone to the conditions of the eclogite facies of metamorphism, forming a coherent subduction complex together with eclogites.

Deformation of granitic rocks within Derbugan Fault belt, Erguna Massif, Northeast China: Implication of the subduction of Mongol‐Okhotsk oceanic plate

The NE–SW striking Derbugan Fault has been comprehensively accepted as a Mesozoic strike‐slip fault in Erguna Massif rather than a suture belt of Erguna and Xing'an massifs. The rocks exposed within the fault belt, which consist of Jiageda Formation and intrusions which have been multiply deformed and metamorphosed. These deformed rocks provide an ideal opportunity to understand the geodynamics of the uplift history and tectonic evolution of Derbugan Fault in Mongol‐Okhotsk regime and influence of the circum‐Pacific tectonic regime, particularly during the later stages of the Mongol‐Okhotsk Ocean collision. In this study, we report kinematics, meso‐microstructures, quartz EBSD analysis and geochronology data of the Jiageda Formation exposed within the Derbugan Fault belt in Cuogang area, Erguna Massif. The Jiageda Formation and granitic rocks have experienced regional metamorphism and deformation, and is mainly characterised by simple‐shear dominated general shearing with LS tectonites. Our new zircon U‐Pb data of mylonitic rocks from Cuogang area yielded ages between 381 to 401 Ma which was produced as a result of the subduction of the Paleo‐Asian Ocean. Deformed granitic intrusive formed in the Early Triassic (244.8 ± 5.6 Ma) was related to the southward subduction of the Mongol‐Okhotsk oceanic plate beneath the Erguna Massif. The Derbugan Fault represents a ductile to brittle deformation at decreasing crustal level due to the dominant oblique (subhorizontal) slip. The early sinistral strike‐slip deformation with a medium temperature between 450 and 550°C developed at a deep ductile regime resulted from NW–SE extension. The subsequent deformation was essentially brittle that corresponded to a low‐temperature deformation corresponded to a greenschist metamorphic facies. Crystallographic textures indicate a restricted recrystallisation of quartz mainly via dislocation creep through activation prismatic <a> slip and rhomb <a> slip system, with a later activation of basal <a> slip system. The Early Cretaceous extensional environment may consist with the main medium temperature deformation stage of Derbugan Fault, which was related to either delamination or upwelling of the thickened lithosphere of the Mongol‐Okhotsk tectonic regime, and rollback of the Paleo‐Pacific Plate.

Gabbroids of Pre-Jurassic Basement of Arctic and Their Sulfide Mineralization (Syunay-Salin Area, Yamal Peninsula)

The chemical and mineral composition of gabbroids of the pre-Jurassic basement of the Syunay-Salin area located in the Arctic part of the West Siberian megabasin on Yamal Peninsula is studied. Borehole 45 has drilled a layered gabbrodiorite intrusion, which was previously considered monotonic diorite one. The gabbroids are metamorphosed under conditions of the lower degree of the greenschist facies of metamorphism and also underwent propylitization, which is resulted in the formation of sulfdes including pyrite, chalcopyrite, also pyrrhotite, pentlandite, cubanite, samaniite and galena. According to mineral geothermometers, sulfde dissemination in Syunay-Salin gabbroids formed at temperature of ~200 °С.

“Rootless” Ophiolites above the Exhuming Pelagonian Core Complex, Northern Greece

The Mesohellenic ophiolites (MHO) in the Western Hellenides are part of an oceanic slab emplaced onto Pelagonian (Pangaean) continental rocks in the mid-Jurassic with a documented NE ophiolite emplacement. Ophiolitic outliers to the east of the MHO are oceanic lithospheric fragments, not complete ophiolite bodies, preserved above exhumed Pelagonia continental rocks. As these fragments lack connection to original root zone provenance, we refer to these as the “rootless” ophiolites.Pelagonian exhumation, possibly triggered by transcurent shear along its continental margin with the Pindos basin, began by the Late Jurassic and continued into the mid-Cretaceous. Exhumation affected the emplaced oceanic slab in the following ways: i) The metamorphic facies of the basal mélange separating the ophiolite from the Pelagonian basement grades from phyllitic to schist and amphibolite-schist over the exhumed Pelagonia. ii) Ophiolitic remnants are metasomatized where in contact with the exhumed Pelagonian rocks. iii) Remnant ophiolitic fragments are rotated and largely disassociated from their original relative pseudostratigraphic positions in their parent slab. iv) No amphibolite emplacement soles are preserved beneath ophiolitic remnants found directly above Pelagonia.East of Vourinos, remnants of the slab were tectonically entrapped between the exhuming Pelagonian core and its sedimentary overburden, and demonstrate extensional, largely gravitational displacements as well as rotation from original emplacement vectors. Primary constrictive slab emplacement features are obscured, but a general westerly sense of kinematics via listric and extensional faults have been imprinted. In the exhumation model, this "SW topping" direction cannot be interpreted as indicative of an eastern origin of the Pindos Basin ophiolites from the Vardar Zone, but rather as a local response to the uplift of Pelagonia and active deformation of the sedimentary overburden.

Accretionary unit formats in subduction complexes: examples from the Franciscan and Miura-Boso complexes

ABSTRACT Subduction accretionary complexes are composed of major and minor structural, stratigraphic, and tectonostratigraphic units. The major architectural units, Accretionary Units (AUs), are bounded by major faults, but differ from terranes in that they may contain units stratigraphically correlative with units in other AUs and they are smaller than the largest terranes. AUs occur in three basic formats – (1) singular sheets or blocks of stratigraphic layers, dismembered formation, or mélange, (2) folded units composed of one or more stratigraphic or block-in-matrix units, and (3) faulted stratigraphic masses (i.e., broken formations). Composite AUs with multiple units and multiple attributes are common. Multiple suites of structures may arise in AUs from progressive early deformation or later superimposed deformational events. AUs may be subdivided through detailed mapping into sub-units such as fault blocks, mélanges, dismembered formations, broken formations, intact formations, and members. Each AU should be defined on the basis of unique characters that derive from a thorough description of the AU, including its distinct rock types and character; and where possible, lithofacies, metamorphic facies, structures, and unit history. Descriptions of partially described AUs from the Franciscan Complex of California and the Miura-Boso area of Japan provide examples of the character of AUs. Ideally, the architecture and history of a subduction complex can be reconstructed by assembling detailed map and text descriptions of the constituent AUs, their 3D-positions, their relationships, and their histories.

To the problem of origin of the Bug series rocks

The age of the Bug series rocks is an object for discussion. Isotopic methods do not give a distinct answer, because in the same outcrop, the same sample and even in the same crystal of zircon several dates of formation are to be found with evident significance. Involvement of additional criteria is required to determine the time of the Bug series formation. While examining the phase composition and the initial source of the material we need taking into account temperature and pressure distribution in the depth of formation of rocks, which are on the surface nowadays. Mineral and chemical composition of rocks and tectonic structure of including patterns have been considered. It has been shown that the composition of the Bug series is an indicator of time of formation. Carbonatites and calcyphyres, carbonate-magnetite rocks are not produced earlier than ~2.0 Ga ago. A considerable part of the stratum is presented by high-clayish and graphite shale, quartzite, and ultrabasite that occur jointly only in this period of time. Arrangement of the structures is subdued to tectonic control by zones of deep faults. Smaller fault zones are determinative in the pattern of structures themselves and in manifestations of metasomatic processes. By the moment of their formation the level outcropped on the modern surface occurred at a depth more than 20 km that makes sedimentary nature of composing rocks doubtful. It has been shown that the most part of rocks attached to the Bug series are crystallization products of magmatic melts as well as carbonate, silicate (water-silicate with chlorine), hydrocarbon fluids in faults (permeable zones) of crystalline basement in diapason of PT -conditions, corresponding to high amphibolites-granulites metamorphic facies. The pattern of these structures in projection is similar to the section of volcano-tectonic apparatuses at a depth not less than 20 km. In the structures small by their area, the rocks formed at different depths (from 300 to 200 km) from melts and fluids in different chemical media are presented. High temperature multiple metasomatism has been superposed. A conclusion has been made that the pattern of the structures of Khashchevato-Zavalye block and the Golovaniv suture zone does not reflect the succession of primary sedimentary rocks occurrence but a result of multiple crystallization and metasomatic re-crystallization of melts and fluids.

Trace element composition of scheelite in orogenic gold deposits

Scheelite from 25 representative orogenic gold deposits from various geological settings was investigated by EPMA (electron probe micro-analyzer) and LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometer) to establish discriminant geochemical features to constrain indicator mineral surveys for gold exploration. Scheelite from orogenic gold deposits displays five REE patterns including a bell-shaped pattern with a (i) positive or (ii) negative Eu anomaly; (iii) a flat pattern with a positive Eu anomaly and, less commonly, (iv) a LREE-enriched pattern, and (v) a HREE-enriched pattern. The REE patterns are interpreted to reflect the source of the auriferous hydrothermal fluids and, perhaps, co-precipitating mineral phases. Scheelite from deposits formed in rocks metamorphosed at upper greenschist to lower amphibolite facies have low contents in REE, Y, and Sr, and high contents in Mn, Nb, Ta, and V, compared to scheelite formed in rocks metamorphosed below the middle greenschist facies. Scheelite from deposits hosted in sedimentary rocks has high Sr, Pb, U, and Th, and low Na, REE, and Y, compared to that hosted in felsic to intermediate rocks. Statistical analysis including elemental plots and multivariate statistics with PLS-DA (partial least square-discriminant analysis) reveal that the metamorphic facies of the host rocks as well as the regional host rock composition exert a strong control on scheelite composition. This is a result of fluid-rock exchange during fluid flow to gold deposition site. PLS-DA and elemental ratio plots show that scheelite from orogenic gold deposits have distinct Sr, Mo, Eu, As, and Sr/Mo, but indistinguishable REE signatures, compared to scheelite from other deposit types.

Seismic imaging evidence that forearc underplating built the accretionary rock record of coastal North and South America

AbstractThe submerged forearcs of Pacific subduction zones of North and South America are underlain by a coastally exposed basement of late Palaeozoic to early Tertiary age. Basement is either an igneous massif of an accreted intra-oceanic arc or oceanic plateau (e.g. Cascadia(?), Colombia), an in situ formed arc massif (e.g. Aleutian Arc) or an exhumed accretionary complex of low and high P/T metamorphic facies of late Palaeozoic (e.g. southern Chile, Patagonia) and Mesozoic age (e.g. Alaska). Seismic studies at Pacific forearcs image frontal prisms of trench sediment accreted to the seaward edge of forearc basement. Frontal prisms tend to be narrow (10–40 km), weakly consolidated and volumetrically small (∼35–40 km3/km of trench). In contrast, deep seismic imaging of submerged forearcs commonly reveals large volumes (∼2000 km3/km of trench) of underplated material accreted at subsurface depths of ∼10–30 km to the base of forearc basement. Underplates have been imaged below the southern Chile, Ecuador–Colombia, north Cascade, Alaska, and possibly the eastern Aleutian forearcs. Deep underplates have also been observed below the Japan and New Zealand forearcs. Seismic imaging of northern and eastern Pacific forearcs supports the conclusion drawn from field and laboratory studies that exposed low and high P/T accretionary complexes accumulated in the subsurface at depths of 10–30 km. It seems significant that imaged underplated bodies are characteristic of modern well-sedimented subduction zones. It also seems likely that large Pacific-rim underplates store a significant fraction of sediment subducted in Cenozoic time.

Metamorphism, geochemistry, and carbon source on sedimentary‐metamorphic graphite deposits in eastern Shandong, China

Eastern Shandong is one of the most economically important regions for ore‐producing sedimentary‐metamorphic graphite deposits in China. The Liugezhuang and Daliangzikou graphite deposits in the Pingdu–Laixi Graphite Belt are typical examples. The ores in the Liugezhuang deposit and Daliangzikou deposit are typically gneiss/granulite type and biotite granulite type, respectively. Graphite occurs as stratabound layers of slender flakes associated with biotite, clinopyroxene, and plagioclase and as slender flakes and disseminated grains associated with biotite and filling tiny fractures in mineral grains. The mineral associations indicate that the ore rocks can be classified into four metamorphic facies: (a) amphibolite granulite facies, Cpx (Hy) + Opx (Di) + Pl ± Hbl ± Qz; (b) amphibolite facies, Hbl + Pl + Cpx (Opm) + Ep + Bt + Ms + Qz ± Chl ± Mic; (c) high‐grade greenshist facies, Hbl + Pl + Ep + Bt + Chl + Qz ± Cpx; and (d) low‐grade greenshist facies, Ep + Chl + Pl + Bt + Ms + Chl + Qz + Rt. Study of the variation in graphite crystals and fixed carbon with increasing metamorphic grade suggests that more than one metamorphism process (graphitization) occurred during ore formation. Geochemical analysis indicates that the primary condition under which the Liugezhuang and Daliangzikou graphite deposits formed was a marine facies sedimentary environment of which the material source was terrestrial weathering detritus and compounds and also a participation of terrestrial material in the graphite deposits. Trace element data suggest that the sedimentary condition was from a reducing condition in the Liugezhuang graphite deposit to a weaker reducing condition in the Daliangzikou graphite deposit, whereas samples from a Jingshan Group stratum show multiple oxidation–reduction conditions but dominantly oxidation. Carbon isotope analysis indicates that the carbon source of the Liugezhuang graphite deposit was organic, whereas that of the Daliangzikou graphite deposit was mainly organic carbon but with some inorganic participation.

Subduction re-initiation at dying ridge of Neo-Tethys: Insights from mafic and metamafic rocks in Lhaze ophiolitic mélange, Yarlung-Tsangbo Suture Zone

Ophiolitic mélanges provide critical information on the history of subduction and metamorphism and, thus, on the evolution of ancient oceans. Blocks of mafic rocks, garnet-clinopyroxenites and amphibolites have recently been discovered in the Lhaze ophiolitic mélange of the Yarlung-Tsangbo Suture Zone, Southern Tibet. These blocks represent different origins from the Neo-Tethys. The unmetamorphosed MORB-type mafic rocks are considered as the oceanic crusts formed during the spreading of the dying mid-ocean-ridges. The garnet-clinopyroxenites and amphibolites are interpreted to be parts of dismembered metamorphic soles beneath the lowest part of the ophiolites. In spite of different mineral assemblages and metamorphic facies, both clinopyroxenites and amphibolites show strong affinities to the oceanic crust rocks, as evidenced by their geochemical and isotopic data. In addition, the inherited igneous zircon U–Pb results indicate that the protolith ages of these metamorphic rocks are 124–125 Ma, which are coeval with the formation ages of oceanic crusts and the metamorphic ages of amphibolites within the YTSZ. Geothermobarometry calculations indicate that these rocks went through metamorphism to peak conditions of granulite-facies in excess of 900°C at minimum pressure of 9 kbar for garnet-clinopyroxenites and of high-amphibolite facies of 650°C at 7.5 kbar for amphibolites, corresponding to a quite high geothermal gradient (25–30°C/km). Such a high geotherm implies that the lithospheric mantle and the subducted oceanic crusts were nascent and still hot when the mantle obducted onto the ocean crust of the Neo-Tethys. A scenario of subduction re-initiation at the dying mid-ocean-ridge close to the Eurasian continental margin is proposed to reveal the evolution of the Neo-Tethys and to provide new insights into the mechanism of formation of ophiolites and metamorphic soles worldwide.

Insights into orogenic processes from drab schists and minor intrusions: Southern São Francisco Craton, Brazil

Minor altered intrusions and drab retrogressed schists can easily be overlooked in geological studies but this contribution explores these rocks within the Archaean and Palaeoproterozoic southern São Francisco Craton (SSFC), Brazil using geological relationships and accessory mineral in situ analyses in the context of cratonic assembly. Three magmatic pulses are documented: i) Archaean and ii) Palaeoproterozoic felsic intrusions, both hosted by Archaean protoliths, and iii) Palaeoproterozoic felsic intrusions in Palaeoproterozoic supracrustal sequences. Archaean felsic intrusions confirm the Palaeoarchaean age of the mafic/ultramafic sequence of the Rio das Velhas Greenstone Belt and Rhyacian intrusions mark the collisional stage of the Mineiro Belt with the SFC at c. 2130 Ma. Greenstone belt schists show a wide distribution of rounded ‘soccer ball’ Archaean detrital and metamorphic zircon grains ranging in age from 3200 to 2750 Ma with an interpreted overprinting high-grade metamorphic event at c. 2700 to 2680 Ma. Most high-grade metamorphic rims have Th/U > 0.1, negative εHf(t) values and REE pattern consistent with eclogite/granulite metamorphic facies, reinforcing the hypothesis of a dehydrated-refractory crust formed during the stabilization of the SSFC, even though no such protoliths are preserved. This event links crustal thickening and partial melting of Archaean lower crust. Archaean rutile crystals from the greenstone belt schist were reset during the Palaeoproterozoic event but still preserve the early Archaean high-grade metamorphic signature. The presence of unstable ilmenite replaced by rutile in the schist, associated to felsic intrusions with the same age at c. 2130 Ma suggest high pressure, low temperature prograde metamorphism during the collisional stage of the Palaeoproterozoic orogen. Elongate and prismatic zircon grains from the Rhyacian intrusions have low εHf(t) signature and crystallised from partial melting of sedimentary protoliths. Accretionary events produced thicker and more differentiated crust by the end of Rhyacian time. Easily overlooked rocks in this study, when studied, have revealed a rich multi-event history of cratonic evolution.