Metamorphic Minerals Research Articles

Origin and metamorphism of ophiolitic serpentinites from the Yuli belt, eastern Taiwan: new findings and tectonic implications

ABSTRACT Serpentinites associated with metamafic and metaplagiogranitic rocks occur sporadically within metasedimentary schists in the Yuli belt. Such metaigneous rocks are considered to represent ophiolitic protoliths and some contain high-pressure (HP) metamorphic minerals or assemblages. However, the origin and metamorphism of these serpentinites have long been a mystery. This study systematically investigates the four major serpentinite-bearing exposures at the Fengtien, Wanjung, Tsunkuanshan, and Chinshuichi. The results reveal that the serpentinites are of two different protoliths on the basis of relict chromian spinel composition. Cr-spinel compositions of the Fengtien and Tsunkuanshan samples are characterised by moderate Cr# [Cr/(Cr + Al)] (0.45–0.57), relatively high Mg# [Mg/(Mg+Fe2+)] (0.59–0.79), but low Fe3+# [Fe3+/(Fe3++Cr+Al)] (0.02–0.06), reflecting an abyssal peridotite type protolith. By contrast, Cr-spinel compositions of the Wanjung and Chinshuichi samples show high Cr# (up to 0.74) and Fe3+# (0.03–0.08), but low Mg# (<0.6), indicating a forearc mantle peridotite origin. Peak metamorphic conditions of the serpentinites are represented by the assemblage of antigorite + magnetite + chlorite + olivine + diopside, which is estimated as up to 550°C but the pressure cannot be constrained quantitatively. Based on field relations, it is inferred that the serpentinites, associated HP metaigneous rocks, and garnet-bearing metasedimentary schists were metamorphosed isofacially. Despite two origins, the peridotitic protoliths were all subjected to hydration and subduction processes. We suggest that precursors of the serpentinites, metaigneous rocks, and metasedimentary schists were juxtaposed and metamorphosed at intermediate to great depths (~35–55 km) of a subduction zone. Therefore, the Yuli belt serpentinites and associated rocks likely represent exhumed materials from a palaeo-subduction interface.

A synthesis of the REE-Fe-polymetallic mineral system of the REE-line, Bergslagen, Sweden: New mineralogical and textural-paragenetic constraints

Rare earth elements (REE) have gained increasing significance for numerous technologies, particularly in today’s rapidly expanding “green transition” applications such as wind generators and electric vehicle traction engines. Among the more well-known REE mineralisation types in Sweden, together with alkaline intrusions and apatite-iron oxide ores, is the classic yet enigmatic REE-Fe-polymetallic mineral system of Bastnäs-type. The mineralisation type is regional in context and occurs in a discontinuous SW-NE-striking belt (the REE-line) in the west-central part of the Palaeoproterozoic Bergslagen ore province, Sweden. This contribution is aimed at integrating and synthesising existing geological mineralogical, and textural features with new observations from both well-known and several lesser-known, underexplored or previously unrecognised REE-enriched occurrences within this belt, and to discuss key features within the context of mineral systems modelling. A considerable diversity in both the style and abundance of REE mineralisation as well as in discrete REE mineralogy is evident both regionally across the entire REE-line and locally within different ore districts or mine fields. These variations also extend laterally within or across different stratigraphic levels, and within different host rocks, primarily skarn-altered metacarbonates but also variably altered felsic metavolcanic rocks. Many of the mineralisations share similar textural features, which record a protracted evolution with multiple stages of formation or replacement of REE-minerals. The earliest recognised REE assemblages feature fine-grained cerite-(CeCa) with minor bastnäsite-(Ce) – bastnäsite-(La) or fluorbritholite-(Ce) – fluorbritholite-(Y) or locally britholite-(Ce) – britholite-(Y) minerals. Such assemblages typically display anhedral-granoblastic textures appearing in folded assemblages, all suggesting recrystallisation and ductile deformation during regional metamorphism of REE-minerals that had formed during an early stage of the Svecokarelian orogeny. Overprinting overgrowths and cross-cutting vein-like features of allanite-group minerals likely represent different stage(s) of REE mineralisation and (re)-mobilisation during this orogenic evolution. Several of the REE-enriched occurrences contain variably abundant and diverse polymetallic Cu-Mo-Bi-(Co) sulphide mineralisation that typically occur in late paragenetic positions and show a prevalence to REE-rich assemblages, often dominated by different allanite-group minerals. Sulphide and REE mineralisation are locally strongly associated with metamorphic minerals formed during metamorphism of variably Mg-(Fe)-altered metavolcanic rocks. The diversity in style and intensity of the REE mineralisations, along with variations in textures and specific mineralogy, suggest slight differences in the ore-forming conditions or environment at the time of mineralisation. Additionally, these differences may also reflect variations in the preservation or modification processes that operated later in the evolution of the Svecokarelian orogeny, also featuring remobilisation of REEs and sulphidic minerals. The insights gained from all available evidence synthesised herein, from micro-scale to province-scale, help define key proxies for prospectivity mapping. The combined evidence supports that the primary ore-forming stages in the mineral system coincided with felsic volcanism and associated sub-volcanic to plutonic processes at around 1.9 Ga.

Late Jurassic High-Pressure Metamorphism of Variscan I-Type Granitoids in the Northern Part of the Pelagonian Unit (Republic of North Macedonia)

Abstract The high P/T metamorphic Pelagonian Unit in the Republic of North Macedonia comprises (1) a Variscan basement consisting of gneisses, schists and minor meta-mafic rocks, which are all intruded by I-type granitoids and rare related dikes; (2) a metamorphosed sedimentary sequence of Permian to Lower Triassic age, and (3) a sequence of calcite and dolomite marbles resulting from Late Triassic to Middle Jurassic carbonate sediments. All these rocks underwent a common high-P/T metamorphism of Late Jurassic age. This paper deals with the metamorphism of the Variscan I-type granitoids which contained the igneous mineral assemblage plagioclase I + alkali feldspar I + quartz I + biotite I + titanite I + allanite I + zircon I + apatite I ± magnetite I. During Late Jurassic high-P/T metamorphism, these undeformed granitoids were thoroughly metamorphosed under isotropic pressure conditions as documented by undeformed granitic textures that are overgrown by metamorphic minerals such as garnet II, epidote II, and phengite II. Various, eventually metasomatic mineral reactions took place in different textural positions: (1) Former igneous plagioclase grains became completely transformed to Na-rich plagioclase IIa (An09–14) containing numerous small grains of epidote IIa and phengite IIa. Either this transformation was an allochemical one and was accompanied by the syn-metamorphic introduction of an aqueous fluid phase containing Fe, Mg and K or, alternatively, the more Ca-rich parts of plagioclase I became considerably sericitized before high-P/T metamorphism, and the resulting mixture of more Na-rich relic plagioclase with its sericite-rich domains became later metamorphosed under high-P/T conditions. In the first case, an aqueous phase is needed during metamorphism, while in the second case high-P/T metamorphism might have proceeded under H2O-undersaturated conditions; (2) igneous alkali feldspar I was changed to albite-poor orthoclase II or microcline II; (3) igneous Ti-rich biotite I reacted with plagioclase to metamorphic garnet II + Ti-poorer biotite II + titanite II + phengite II + quartz II ± epidote II ± rutile II, which is rimmed by Ttn II. At textural positions, where igneous plagioclase I was not available, igneous biotite I was transformed to Ti-poorer biotite II + titanite II ± ilmenite-hematite II; (4) during uplift, high-P/T metamorphic rutile II became marginally overgrown by titanite II ± ilmenite II; (5) igneous allanite I grains stayed unaltered, but when located near to former plagiocase I, they became partially rimmed by metamorphic epidote II. Equilibrium phase diagram calculations showed that the observed metamorphic paragenesis (plagioclase II + K-rich feldspar II + biotite II + garnet II + epidote II + phengite II + garnet II + quartz II + rutile II + titanite II) is only stable under H2O-unsaturated conditions. The I-type granitoids and their metamorphic country rocks were metamorphosed under high-P/T conditions of 1.3 to 1.5 GPa and 560 to 590 °C.

Age and emplacement depth of intrusions in the southeastern Canadian Cordillera: Implications for Jurassic to Paleocene exhumation

Synorogenic exhumation is important for understanding the evolution of mountain belts but is often difficult to discern because traditional thermochronology methods mostly date the final stages of exhumation. Intrusions emplaced during orogenesis provide an alternate method to elucidate these exhumation patterns since they preserve the timing and depth of their emplacement. We apply this concept the southern Omineca belt of the Canadian Cordillera by combining new and existing zircon UPb and mica Ar/Ar ages from 43 intrusions as well as new and existing estimates of emplacement depth from 23 contact aureoles. Seventeen new zircon UPb laser ablation inductively coupled plasma mass spectrometry results are presented in the paper, spanning the existing range of known ages for four episodes of phanerozoic magmatism in this region: Middle Jurassic (174–154 Ma), mid-Cretaceous (118–95 Ma), a newly identified Late Cretaceous (78–76 Ma), and Paleocene (66–60 Ma). This new work has resolved two new distinct periods of mid-Cretaceous magmatism at ~100 and ~ 76 Ma from what was previously considered to be a single mid-Cretaceous event (Bayonne magmatic suite). Furthermore, the Fry Creek batholith, previously considered to be emblematic of the Bayonne suite, the newly dated ~60 Ma Nelson stock and Shoreline stock, and the previously dated 55–62 Ma Ladybird magmatic suite form part of a newly identified phase of 66–55 Ma Paleocene magmatism in the region. Compiled intrusion ages were then combined with estimates of emplacement depth determined from metamorphic mineral assemblages in contact aureoles developed in pelitic host rocks. Jurassic intrusions typically have higher pressure staurolite + andalusite-bearing contact aureoles (3.3–4.0 kbar) corresponding to depths of 12.5–15 km, whereas Cretaceous and Paleocene intrusions mainly have lower pressure andalusite + cordierite-bearing contact aureoles (2.3–2.9 kbar) corresponding to depths of 8.5–11 km. These data indicate regional exhumation of 2–6 km occurred between the Middle Jurassic (180–160) and mid-Cretaceous to Paleocene (120–60 Ma) periods of Cordilleran orogenesis. In contrast, pluton emplacement depths changed by <2 km, suggesting minimal exhumation occurred in the southern Canadian Cordillera between the mid-Cretaceous and Paleocene. This extended lull in exhumation may be related to the partitioning of compressional deformation east into the foreland along a basal decollement beneath the study area during this time.

Petrology, phase equilibria modelling, and fluid inclusion study of mafic granulites from Bhavani Suture Zone, Southern India

We report new petrology, mineral chemistry, P–T conditions, and fluid inclusion data on mafic granulites from the Mettupalayam region along the Bhavani Suture Zone, Southern Granulite Terrane, India. Phase equilibria modelling of mafic granulites yielded peak P–T conditions of 780–860 °C and 7.6–10.1 kbar followed by a near isothermal decompression along a clockwise P–T path. The trapped fluid inclusions in the peak metamorphic minerals display a melting temperature range from −57.4 °C to −56.6 °C, close to the triple-point temperature of pure CO2. The primary inclusions homogenized at −18.9 °C to +0.2 °C, corresponding to density values of 0.93–1.03 g/cm3. Homogenization of the secondary inclusions occurred within the range from −6.3 to +18.1 °C, corresponding to low CO2 densities of 0.79–0.96 g/cm3. From the textural characteristics of the high-density primary carbonic fluid inclusions, we interpret these inclusions as the CO2-rich syn-metamorphic fluid present during the high-grade metamorphism. The secondary fluids characterised by lower densities have undergone re-equilibration during the exhumation stage (decompression) from the peak granulite-facies metamorphism along a clockwise P–T trajectory. This interpretation is consistent with the occurrence of hornblende + plagioclase symplectite around the porphyroblastic garnet, suggesting decompression. We infer that the high-density CO2 was the dominant syn-metamorphic fluid components present during the granulite-facies metamorphism in the Mettupalayam region. Such carbonic fluids, possibly derived by degassing from carbonates or mantle sources, probably played a significant role in stabilizing high-grade mineral assemblages along this collisional suture zone.

Metamorphic mineral reactions and mineral paragenesis in the rocks of the Meyeri tectonic zone (the south-eastern part of the Fennoscandian shield, Russia)

Metamorphic mineral reactions and mineral paragenesis in the rocks of the Meyeri tectonic zone (the south-eastern part of the Fennoscandian shield, Russia)

High-pressure mafic granulite and supracrustal rocks in the southern Hengshan area, North China Craton: Metamorphic P-T-t evolution and geotectonic significance

Amphibolite- to granulite-facies rocks are widely distributed in the Hengshan Complex, middle Trans-North China Orogen, and the high-pressure (HP) mafic granulite has been recently identified in the southern Hengshan area. The HP mafic granulite and amphibolite occur as rootless tectonic boudins/lenses within the TTG (tonalite–trondhjemite–granodiorite) gneiss/metapelite, indicative of typical “block-in-matrix” texture of metamorphic-tectonic mélange. Three to four generations of metamorphic mineral assemblages that correspond to the prograde (M1), peak (M2), and retrograde (M3-M4) stages, are recognized in these rocks. Conventional geothermobarometry and phase equilibrium modeling yield peak P–T conditions of 10.8–13.8 kbar/754–799 °C for the HP mafic granulite and 7.3–9.0 kbar/690–725 °C for the supracrustal rocks, respectively. A clockwise P–T path with near-isothermal decompression (ITD) and subsequent near-isobaric cooling (IBC) segments is reconstructed for the HP mafic granulite, indicating a dynamic subduction–collision–exhumation process that unfolded during an orogenic event. Secondary ion mass spectrometry (SIMS) U-Pb dating of zircon yields metamorphic ages of ca. 1.91–1.83 Ga, representing the long-lived tectono-metamorphic event caused by collision between the Eastern and Western Blocks along the Trans-North China Orogen. It is hypothesized that the tectono-metamorphic mélange in the area originated from the tectonically juxtaposition of metamorphic rocks that had diverse protoliths, different peak P–T conditions and discrepant metamorphic ages. This complexity may be a hallmark of Paleoproterozoic orogens, drawing intriguing parallels with the intricate characteristics observed in Phanerozoic orogenic belts.

Using U-Pb detrital zircon systematics to constrain the architecture and provenance of the Ronda peridotites’ crustal hanging wall, S Spain

Detrital zircon geochronology is frequently used for sediment provenance analysis in orogenic belts, but it can also be applied to locate the boundary between basement and sedimentary cover in successions and tectonometamorphic units where the paleontological record is missing, and primary stratigraphic relationships have been erased by intense deformation and/or metamorphism. This is critical to differentiate metamorphic mineral associations, P-T conditions and deformational events associated with superposed orogenic cycles. In this work, detrital zircon geochronology has been applied to the Ronda peridotites’ crustal hanging wall (hinterland of the Alpine Betic Cordillera, southern Spain), apparently made up of two major tectonic units – the Jubrique unit and the Maláguide complex – separated by a major Alpine tectonic contact camouflaged in between low-grade metamorphic rocks.Our results show that pre-Paleozoic peaks (c. 650, 1000, 2000, and 2600 Ma) are common along the entire studied crustal sequence, and Paleozoic ages are only recorded in the upper part of the Jubrique unit (c. 290, 350, and 475 Ma) and in the Maláguide complex (c. 350 and 475 Ma). Thus, the Paleozoic rocks of the Maláguide complex, constituting the highest crustal unit overlying the Ronda peridotites, represent syn-to-late orogenic Variscan sediments deformed and slightly metamorphosed prior to the Alpine cycle. In contrast, the underlying Jubrique unit is made of a post-Variscan sedimentary cover deposited on a Variscan basement, with the contact between both (hidden unconformity?) located within low- to medium-grade schists. These data support the presence of a major tectonic contact between the Maláguide complex (Late Carboniferous maximum depositional ages) and the Jubrique unit (Early Permian maximum depositional ages), i.e. the Maláguide complex would have overthrust the Jubrique unit during the Alpine orogeny. These results indicate a Permian or younger exhumation/emplacement up to crustal levels of the Ronda peridotites subcontinental mantle slab.Altogether, these detrital zircon age spectra support a relatively, but not entirely local derivation of the studied crustal sequences (fragments of the Iberian Massif) and suggest some Mesozoic drifting with respect to its present position. Nonetheless, the different age distribution of the Paleozoic populations between the Jubrique (Alpujárride complex) and Maláguide samples suggests that the former deposited relatively close to other samples from the eastern-central Alpujárride complex and Nevado-Filábride complex, which, in turn, show strong similarities with the Cantabrian zone and NE Iberia (Iberian Massif). In contrast, the Maláguide samples were probably deposited close to the Ossa-Morena Zone (Iberian Massif) . Thus, Alpujárride/Nevado-Filábride and Maláguide complexes would have been juxtaposed during the Triassic-Cretaceous Neotethys rifting/drifting and/or the subsequent Alpine collisional events.

Fault Tectonites of the Yenisei Shear Zone (Yenisei Ridge): Evidence and Thermomechanical Numerical Model of Generation of Tectonic Overpressure

Based on the proposed numerical model of the stress-strain state of polymineral rocks, which describes the formation of blastomylonites in the Yenisei Regional Shear Zone (PRSZ) in the Yenisei Ridge, the possibility of local tectonic overpressure exceeding the lithostatic pressure in rocks subjected to shear deformations is shown. For tectonites of the southern (Angara-Kan block) and northern (Isakovka terrane and Garevka complex) segments of the PRSZ, estimates of the maximum overpressure were obtained from 2–3 to 4–5 kbar, which range from 25 to 50% of the lithostatic pressure. It is shown that excess pressures can be preserved in a local volume on a geological time scale sufficient for their fixation in metamorphic minerals. Model values of overlithostatic pressure in garnet-amphibole tectonites and geobarometric estimates of peak values during stress metamorphism allow us to offer new evidence of pressure inhomogeneity in natural mineral associations. Using the results of numerical modeling for the evolution of fault metabasite blastomylonites, it was established that the overpressure at the stage of syn-deformation metamorphism in the shear zone are possible at temperatures up to 600–650°C and not reaching 800°C; the presence of fluid or partial melt prevents the occurrence of overpressure. The amount of excess pressure due to shear stresses depends on the mineral composition and structure of the rock.

Chromian minerals and talc veins in the metasandstones of the Ochi nappe, southern Evia, Greece: The role of fluids in retrograde metamorphism during exhumation

Unusual minerals were formed during the retrograde metamorphism of high-pressure metamorphic rocks in the Ochi nappe, southern Evia, Greece, specifically widespread sub-mm-scale talc veins and chromian varieties of chlorite, phengite, biotite and allanite. Albite-quartz-calcite and calcite-quartz schists are cut by common 10–200 μm wide veins of talc, rimmed by Cr-chlorite with <0.6 apfu Cr. Hydraulic fracturing by fluids derived from dehydration of subducted chromite-rich serpentinite in the Ochi nappe created fractures in the schists where talc precipitated. Mineral chemistry and textures are related to regional metamorphic and tectonic phases, which included Eocene blueschist metamorphism followed by Oligocene syn-orogenic wedge extrusion. The overprint resulted in retrograde growth of albite porphyroblasts, emplacement of talc veins, and formation of phengite with <0.08 apfu Cr and later biotite with <0.3 apfu Cr. Late Oligocene shearing flattened many pre-existing minerals. Neogene exhumation within the North Cycladic detachment system produced decompression fractures that nucleated precipitation of allanite (some with <0.4 apfu Cr and > 0.8 apfu REE), apatite, titanite and zircon, reflecting the availability of halogen-rich fluids. The evolution of the retrograde metamorphic minerals assemblages can thus be tracked, with pervasive Cr-Mg infiltration in the Oligocene, and more focussed Miocene metasomatism by halogen-rich fluids.

Updating the alluvial mapping of the middle Loire catchment: Predictive geomorphology approach and mineralogical characterization of the deposits

This study aims to provide a full synthesis of recent work on the fluvial terraces of the middle section of the Loire River, between its exit from the “Massif Central” montane area and its entry into the Touraine region, with the aim of better understanding of the evolution of the river during the Pleistocene period.New fieldwork and topographic analyses (including a predictive geomorphology approach) permit updating the spatial extent and geometry of the different Quaternary alluvial sedimentary formations in this area. In addition, new analytical data were acquired both by X-ray diffraction (XRD) on heavy minerals and by gamma spectrometry measurements on alluvial sandy sediments. These analyses allow us to characterize and distinguish the alluvial formations.This work has identified three distinct periods in the history of the Loire: the oldest, corresponding to the deposition of sediments characterised by an abundance of metamorphic minerals, originating from the Massif Central; an intermediate period, corresponding to the deposition of the high terraces of the Loire system, not characterised by mineralogical analyses but by the approach of the predicted geomorphology showing different terrace geometries; lastly, a recent period comprising the low and medium altitude terraces, characterised by an abundance of volcanic minerals also originating from the Massif Central.

Tracing a remnant of subducted Indian felsic crust: insights from zircon studies

Zircon is a common mineral in nature that survives varied pressure and temperature conditions in the subduction process. It has excellent ability to reveal progressive metamorphic history and, hence, is useful in reconstructing the subduction tectonics in collisional orogenic belts. In the Tso Morari Gneiss of the Indus Suture Zone, Himalaya, eclogite boudins have registered the imprint of subduction-related ultrahigh-pressure (UHP) metamorphism; this imprint is, however, missing in the host gneisses. To search for the missing link, zircons of the gneisses were studied. The zircon overgrowth and the numerous mineral inclusions indicate the metamorphic responses of the gneisses. The Raman spectra of minerals show the cores of the zircon consist of apatite and quartz and the surrounding overgrowth preserves quartz–coesite, c-polymorphs and other metamorphic minerals. The distribution pattern of these minerals in the zircons is consistent with the Th/U ratios ranging from 0.30 to 0.01, recognizing the inner magmatic and outer metamorphic domains. The U–Pb ages from the inner magmatic (at c. 500 Ma) and from the outer metamorphic growth (at c. 45–42 Ma) suggest the former is the protolith age and the latter the metamorphic age of the gneisses. The tectonic interpretation reveals the subduction of Indian felsic crust to UHP depth (&gt;100 km) at c. 45 Ma. Supplementary material : Raman spectra of C-inclusions in the zircon of Tso Morari Gneiss are available at https://doi.org/10.6084/m9.figshare.c.7168228 Thematic collection: This article is part of the Mesozoic and Cenozoic tectonics, landscape and climate change collection available at: https://www.lyellcollection.org/topic/collections/mesozoic-and-cenozoic-tectonics-landscape-and-climate-change

Hydrothermal alteration and geochemical proximity indicators to ore at the Metsämonttu Zn–Pb–Cu–Au–Ag deposit, Uusimaa belt, southern Finland

The Paleoproterozoic Metsämonttu Zn–Pb–Cu–Ag–Au deposit (1.5 Mt at 3.5 wt% Zn, 0.8 wt% Pb, 0.3 wt% Cu, 13.2 wt% S, 25 g/t Ag, and 1.4 g/t Au, production 1952–1974) is the largest past-producing mine in the Aijala–Orijärvi area (Orijärvi formation, Aijala member) within the Uusimaa belt, southern Finland. The Aijala member is characterized by 1.9–1.88 Ga felsic-dominated volcanic-sedimentary supracrustal rocks with intercalated sedimentary carbonates and iron formations. The area is underexplored, and little deposit-scale research has been carried out, despite the lateral continuum to the world-class ore district of Bergslagen in south-central Sweden. To better understand the Metsämonttu VMS-related alteration system, we reassessed the previously described metamorphic mineral assemblages and their protolith rock compositions by using mobile and immobile element geochemistry. This resulted in the definition of four metamorphosed alteration mineral assemblages and eight chemostratigraphic rock units. The chemostratigraphic results suggest that the lithological and/or structural setting of the Metsämonttu succession is more complex than previously considered. The stratigraphic footwall is mainly characterized by an extensive cordierite + anthophyllite ± biotite ± phlogopite + pyrite ± pyrrhotite (Mg–Fe–S) assemblage dominated by mafic rocks, designated Mafic B1 and Mafic B2, and Andesite A1. The main sulfide mineralization is hosted by tremolite + diopside ± biotite ± phlogopite ± chlorite skarn (Ca–Mg–K). A sericite-bearing muscovite + quartz ± biotite ± phlogopite + pyrite (K–Si–S) assemblage is composed of felsic to mafic protoliths (Mafic B1–B2, Andesite A1, Dacite A1, Dacite B1, Dacite C1) and extends several tens of meters into the stratigraphic hanging-wall. A quartz + pyrite ± muscovite (Si–S) assemblage represents the immediate ore-proximal alteration and is derived from rocks with a rhyolitic composition (Rhyolite A1).Limited drill core samples near Metsämonttu mineralization, along with restricted surface alteration, pose challenges in studying geochemical variations from distal to ore proximal areas and limits the ability to model the shape and size of the alteration zone. Large mass changes suggest that the alteration was hydrothermal, and due to several protolith compositions, the alteration is interpreted to be predominantly discordant to stratigraphy. A 60-m-wide alteration halo surrounds the Metsämonttu deposit. Major and trace elements, namely MgO, Na2O, K2O, SiO2, Fe2O3, Cu, Zn, Pb, S, Ag, Tl, Hg, Se, Te, Sn, Sb, Rb, and Sr, and the indices modified alteration index (MAI), Ishikawa alteration index (AI), chlorite‑carbonate-pyrite index (CCPI), S/Na2O can be used for chemical vectoring, which can assist regional or near-mine exploration. Elements MnO, Ba, Cd, Bi, As, Ni, Co, W, Ga, Mo, and indices Hashigushi index and advance argillic alteration index (AAAI) were less useful as geochemical exploration indicators.

A New Insight into the Genesis of Graphite Deposits in Madagascar Using U–Pb Zircon Dating and Electron Microprobe Analysis

Madagascar is globally recognized as an important producer of high-quality flaky graphite. However, current research on graphite deposits in Madagascar remains insufficient. Previous studies have linked the genesis of Madagascan graphite deposits to the metamorphism of sedimentary organic matter. Here, we provide a case study of graphite deposits in Madagascar, combining new data from the Ambahita graphite deposit (AMG) in southern Madagascar with data from the Antanisoa graphite deposit (ANG) in central Madagascar and the Vohitasara graphite deposit (VOG) on the east coast of Madagascar. We note that the mineral assemblages of graphite-bearing rocks in the AMG, ANG, and VOG are not typical of metamorphic mineral assemblages but rather the results of filling and metasomatism by mantle-derived fluids that occurred after peak metamorphism. Electron microprobe analysis indicates that the graphite of the AMG, VOG, and ANG is usually associated with phlogopite or Mg-biotite; the phlogopite shares a common genesis with other widespread phlogopite deposits across Madagascar. We reveal that the distribution of graphite deposits in Madagascar is primarily controlled by ductile shear zones between blocks. Ductile shear zones that extend deep into the mantle can provide an ideal migration channel and architecture for the emplacement of mantle-derived fluids. The graphite mineralization formed no earlier than the peak metamorphism (490 Ma) and no later than the intrusion of pegmatite veins (389 ± 5 Ma). The distribution of graphite deposits, graphite orebody morphologies, mineral associations, and geochemical data suggest that the genesis of graphite deposits in Madagascar is linked to mantle-derived fluid filling rather than the metamorphism of sediments, as previously suggested. These findings have important implications for similar deposits in Madagascar and potentially globally.

In Situ Geochemical Evaluation of Retrograde Hydration Effects in the Peri-Siberian Forearc Mantle (Khara-Nur and Alag-Khadny Peridotite Complexes)

In order to assess the geochemical effects of retrograde metamorphic rehydration, fluid metasomatism, and the fluid-mobile elements (FMEs) budget in the case of oceanic and continental subduction, we report the petrography, bulk, and in situ LA-ICP-MS trace-element data for the two poorly studied ophiolites in the northern (Khara-Nur, Eastern Sayan, Russia) and central (Alag-Khadny accretionary wedge, SW Mongolia) parts of the peri-Siberian orogenic framing. Both complexes are relics of the ancient oceanic mantle, which was subjected to processes of partial melting, metasomatism, and retrograde metamorphism. Typical mineral assemblages include olivine + orthopyroxene + chlorite + tremolite ± secondary olivine (640–800 °C), olivine + antigorite ± secondary clinopyroxene (&lt;640 °C), and olivine + chrysotile ± secondary clinopyroxene (&lt;250 °C) and are stable at pressures up to 2 GPa. Hydration and partial serpentinization of mantle peridotites lead to tremolite formation after orthopyroxene, followed by olivine replacement by antigorite. Serpentine-group minerals (antigorite and chrysotile) were distinguished by Raman spectroscopy, and the contents of incompatible elements (mobile and immobile in fluids) in metamorphic minerals (tremolite, antigorite, and chrysotile) were examined in situ by LA-ICP-MS. The behavior of conservative HFSE (Zr, Nb, Ta, and Ti) and—in part—HREE does not distinguish between the two types (oceanic and continental) of subduction environments. Different patterns of FMEs (Cs, Rb, Ba, U, Sb, Pb, Sr, and LREE) enrichment in metaperidotites reflect variations in the slab fluid composition, which was primarily governed by the contrasting nature of subducted lithologies. The affinity of Alag-Khadny to the subduction of a continental margin is recorded by increased FME contents and selective enrichment by some moderately mobile elements, such as U, Th, and LREE, with respect to the oceanic-type subduction environment of Khara-Nur. Distinct patterns of FME enrichment in tremolite and antigorite from two complexes indicate different sequences of fluid-induced replacement, which was controlled by Opx composition. We demonstrate that evaluation of the initial composition of precursor minerals affected by multi-stage melting and melt metasomatism should be considered with care to estimate the differential fluid overprint and associated elemental uptake from subduction fluids.

Igneous intrusion contact metamorphic system and its reservoir characteristics: A case study of Paleogene Shahejie Formation in Nanpu sag of Bohai Bay Basin, China

Taking the Paleogene Shahejie Formation in Nanpu sag of Bohai Bay Basin as an example, this study comprehensively utilizes seismic, mud logging, well logging, physical property analysis and core thin section data to investigate the metamorphic reservoir formed by contact metamorphism after igneous rock intrusion. (1) A geological model of the igneous intrusion contact metamorphic system is proposed, which can be divided into five structural layers vertically: the intrusion, upper metamorphic aureole, lower metamorphic aureole, normal sedimentary layers on the roof and floor. (2) The intrusion is characterized by xenoliths indicating intrusive facies at the top, regular changes in rock texture and mineral crystallization from the center to the edge on a microscopic scale, and low-angle oblique penetrations of the intrusion through sedimentary strata on a macroscopic scale. The metamorphic aureole has characteristics such as sedimentary rocks as the host rock, typical palimpsest textures developed, various low-temperature thermal metamorphic minerals developed, and medium-low grade thermal metamorphic rocks as the lithology. (3) The reservoir in contact metamorphic aureole has two types of reservoir spaces: matrix pores and fractures. The matrix pores are secondary “intergranular pores” distributed around metamorphic minerals after thermal metamorphic transformation in metasandstones. The fractures are mainly structural fractures and intrusive compressive fractures in metamudstones. The reservoirs generally have three spatial distribution characteristics: layered, porphyritic and hydrocarbon impregnation along fracture. (4) The distribution of reservoirs in the metamorphic aureole is mainly controlled by the intensity of thermal baking. Furthermore, the distribution of favorable reservoirs is controlled by the coupling of favorable lithofacies and thermal contact metamorphism, intrusive compression and hydrothermal dissolution. The proposal and application of the geological model of the intrusion contact metamorphic system are expected to promote the discovery of exploration targets of contact metamorphic rock in Nanpu sag, and provide a reference for the study and exploration of deep contact metamorphic rock reservoirs in the Bohai Bay Basin.

Metamorphic Mineral Reactions and Mineral Assemblages in Rocks of the Meyeri Tectonic Zone, Southeastern Fennoscandian Shield, Russia

Metamorphic Mineral Reactions and Mineral Assemblages in Rocks of the Meyeri Tectonic Zone, Southeastern Fennoscandian Shield, Russia

An evolutionary system of mineralogy, Part VIII: The evolution of metamorphic minerals

Abstract Part VIII of the evolutionary system of mineralogy focuses on 1220 metamorphic mineral species, which correspond to 755 root mineral kinds associated with varied metamorphic rock types, most of which likely formed prior to the Phanerozoic Eon. A catalog of the mineral modes of 2785 metamorphic rocks from around the world reveals that 94 mineral kinds often occur as major phases. Of these common metamorphic minerals, 66 are silicates, 14 are oxides or hydroxides, 8 are carbonates or phosphates, 4 are sulfides, and 2 are polymorphs of carbon. Collectively, these 94 minerals incorporate 23 different essential chemical elements. Patterns of coexistence among these 94 minerals, as revealed by network analysis and Louvain community detection, point to six major communities of metamorphic phases, three of which correspond to different pressure-temperature (P-T) regimes of metamorphosed siliceous igneous and sedimentary rocks, while three represent thermally altered carbonate and calc-silicate lithologies. Metamorphic rocks display characteristics of an evolving chemical system, with significant increases in mineral diversity and chemical complexity through billions of years of Earth history. Earth's first metamorphic minerals 27 formed in thermally altered xenoliths and contact zones (hornfels and sanidinite facies) associated with early Hadean igneous activity (&amp;gt; 4.5 Ga). The appearance of new Hadean lithologies, including clay-rich sediments, arkosic sandstones, and carbonates, provided additional protoliths for thermal metamorphism prior to 4.0 Ga. Orogenesis and erosion exposed extensive regional metamorphic terrains, with lithologies corresponding to the Barrovian sequence of index mineral metamorphic zones appearing by the Mesoarchean Era (&amp;gt; 2.8 Ga). More recently, rapid subduction and rebound of crustal wedges, coupled with a shallowing geothermal gradient, has produced distinctive suites of blueschist, eclogite, and ultrahigh pressure metamorphic suites (&amp;lt; 1.0 Ga). The evolution of metamorphic minerals thus exemplifies changes in physical and chemical processes in Earth's crust and upper mantle.

Sublithospheric Diamonds: Plate Tectonics from Earth's Deepest Mantle Samples

Sublithospheric diamonds and the inclusions they may carry crystallize in the asthenosphere, transition zone, or uppermost lower mantle (from 300 to ∼800 km), and are the deepest minerals so far recognized to form by plate tectonics. These diamonds are distinctive in their deformation features, low nitrogen content, and inclusions of these major mantle minerals: majoritic garnet, clinopyroxene, ringwoodite, CaSi perovskite, ferropericlase, and bridgmanite or their retrograde equivalents. The stable isotopic compositions of elements within these diamonds (δ11B, δ13C, δ15N) and their inclusions (δ18O, δ56Fe) are typically well outside normal mantle ranges, showing that these elements were either organic (C) or modified by seawater alteration (B, O, Fe) at relatively low temperatures. Metamorphic minerals in cold slabs are effective hosts that transport C as CO3 and H as H2O, OH, or CH4 below the island arc and mantle wedge. Warming of the slab generates carbonatitic melts, supercritical aqueous fluids, or metallic liquids, forming three types of sublithospheric diamonds. Diamond crystallization occurs by movement and reduction of mobile fluids as they pass through host mantle via fractures—a process that creates chemical heterogeneity and may promote deep focus earthquakes. Geobarometry of majoritic garnet inclusions and diamond ages suggest upward transport, perhaps to the base of mantle lithosphere. From there, diamonds are carried to Earth's surface by eruptions of kimberlite magma. Mineral assemblages in sublithospheric diamonds directly trace Earth's deep volatile cycle, demonstrating how the hydrosphere of a rocky planet can connect to its solid interior. ▪Sublithospheric diamonds from the deep upper mantle, transition zone, and lower mantle host Earth's deepest obtainable mineral samples.▪Low-temperature seawater alteration of the ocean floor captures organic and inorganic carbon at the surface eventually to become some of the most precious gem diamonds.▪Subduction transports fluids in metamorphic minerals to great depth. Fluids released by slab heating migrate, react with host mantle to induce diamond crystallization, and may trigger earthquakes.▪Sublithospheric diamonds are powerful tracers of subduction—a plate tectonic process that deeply recycles part of Earth's planetary volatile budget.

In situ white mica Rb/Sr geochronology of the Leszczyniec metaigneous complex, West Sudetes: evidence of upper plate deformation at the onset of Variscan collision

The Karkonosze-Izera Massif in the West Sudetes preserves evidence of subduction of the Saxothuringian Ocean beneath the Teplá-Barrandian Domain. Within the massif, the Leszczyniec metaigneous complex (LMC) is identified as the upper allochthon. It exhibits a unique structural history in comparison to the underlying allochthons, suggesting that the LMC records a distinctive tectonic history. To investigate the timing of this history, two orthogneisses were studied from a single outcrop of the LMC for in situ white mica Rb/Sr geochronology. The outcrop bears a southeast-dipping foliation (S1) and a north/northeast plunging stretching lineation (L1), defined by white mica and quartz. Quartz recrystallization textures, white mica chemistry (celadonite content of 0.23–0.47), preservation of igneous plagioclase phenocrysts, and the metamorphic mineral assemblage, all indicate maximum epidote–amphibolite facies conditions. Single-spot Rb/Sr dates were calculated from white mica using initial 87Sr/86Sr values obtained by titanite and epidote. Results are similar for both rocks, providing weighted averages of 352.4 ± 4.1 Ma (MSWD: 0.6; n: 24) and 349.3 ± 2.5 Ma (MSWD: 0.4; n: 31). The rocks are interpreted to have the same structural and metamorphic history; thus, a pooled weighted average of 350.1 ± 5.3 Ma (2σ) is reported as the timing of white mica (re)crystallization during S1 and L1 development in epidote–amphibolite facies conditions. This event is bracketed by the timing of blueschist-facies metamorphism for the subjacent middle (c. 364 Ma) and lower (c. 345–341 Ma) allochthons, associated with an east/southeast-plunging L1. Considering the different structural and metamorphic histories of the LMC compared to the subjacent allochthons, it is likely that the complex was extracted from the Teplá-Barrandian upper plate due to subduction erosion prior to collective exhumation and stacking of the Karkonosze-Izera Massif.Graphical abstract