Metamorphic Phase Research Articles

Mineralogy, geochemistry and genesis of low-grade manganese ores of Anujurhi area, Eastern Ghats, India

The Anujurhi manganese ores occur in the high-grade gneisses of the Precambrian Eastern Ghats Supergroup in Odisha, India. They are characterized by conformable lenses containing minerals such as cryptomelane, romanechite, pyrolusite, todorokite, and pyrophanite, along with other opaque minerals like graphite, goethite and ilmenite. The gangue minerals associated with these ores include quartz, feldspar, garnet, kaolinite, apatite, sillimanite, zircon, biotite, alunite, and gorceixite. The primary elements present in the ore, Si, Mn, Fe, and Al, average at 16.20 %, 15.06 %, 11.94 %, and 6.6 % respectively. Additionally, trace amounts of P, K, Ti, Mg, Ca, and Na were detected. The average Fe/Mn ratio of 0.81 and the Si versus Al plot of the Anujurhi manganese ores suggest a hydrogenous-hydrothermal mixed source for the ferromanganese sediments. The characteristics of the manganese ore bands, absence of carbonate facies of ore, and geochemical association of Mn-Ba together with Na/Mg ratios and CaO-Na2O-MgO ternary plot of the manganese ores strongly indicate that the mineralization is a metamorphosed shallow marine-lacustrine deposit. Following deposition and diagenesis, the manganese minerals underwent at least two phases of Ultra High Temperature (UHT) and granulite facies metamorphism along with the host rocks. Tectonic uplift, erosion, extended exposure to atmospheric oxygen and percolation of meteoric water led to the supergene alteration and remobilization of the primary manganese minerals in a colloidal state, followed by epigenetic replacement along the structural weak planes of the granulite facies rocks, resulting in the formation of the current deposits. This is evidenced by the observed secondary replacement and colloidal textures in the Mn oxides.

Lithium pegmatite formation in Kelumute-Jideke pegmatite field, Chinese Altai: Insight from geochronology, petrology and lithium isotope geochemistry

The Kelumute-Jideke pegmatite field is an important lithium (Li)-rich pegmatite mineralization area within the Chinese Altai orogenic belt. There have been four large (Kalu’an No.803, No. 805, No.806, No.807 veins), one medium (Kelumute No.112 vein), nine small deposits (such as Jiamukai, Qunkuer, Azubai) with totally 130,000 tons Li2O found in this area. However, it remains enigmatic whether Li-rich pegmatite formed through fractional crystallization of coeval granite or direct anatexis of metasedimentary rocks. Here we selected pegmatites and their surrounding rocks of metasedimentary rocks and granitic batholith in this pegmatite field to conduct geochronology, whole major and trace element, and Li isotope analyses. The granite intrusions in this field exhibits multiple-stage emplacement ages, including 447.7 ± 4.2 Ma, 405.8 ± 0.0 – 397.4 ± 4.1 Ma, 358.4 ± 3.1–308.4 ± 4.6 Ma, 239.5 ± 1.6 – 213.7 ± 1.5 Ma. The youngest magmatism is coeval with the formation age of rare metal pegmatites. Geochemistry analyses of granites show a peraluminous S-type granite signature with high SiO2 (71.21–75.01 wt%) contents and A/CNK (> 1) ratios. The metasedimentary rocks of Kulumuti Group in this field have experienced weak weathering and exhibit extremely high Li content (maximum of 1193 ppm), which is much higher than that of the Traissic granite (96.1 ppm). The average composition of metasedimentary rocks is used to calculate the partition coefficient of different mineral proportions during the modelled equilibrium melting of the metamorphic phase. This approach determines the mineral proportions and partition coefficients under various conditions. Subsequently, Rayleigh dehydration melting simulations were performed on the Li-rich and Li-poor metasedimentary rocks. The Rayleigh dehydration melting simulations show that melts produced from partial melting of Kulumuti Group display Li content of 80–3435 ppm and δ7Li values of 1.0–8.3 ‰, consistent with natural pegmatites. The low-degree partial melting of claystone-rich metasedimentary rocks (Li-rich) in the Kulumuti Group can produce a preliminary Li-rich melt. However, through Rayleigh fractional crystallization simulation of coeval granite, it is found that the calculated δ7Li value (+6‰) of pegmatites is much higher than that of natural Li-rich pegmatite (average + 3.0 ‰). Therefore, the formation of Li-rich pegmatites in the Kelumute-Jideke pegmatite field is predominantly attributed to the partial melting of Li-rich protoliths with low δ7Li values, rather than high differentiation of Halong granite. The study highlights the importance of anatexis of metasedimentary rocks in formation of Li-rich pegmatite, which is key to mineral exploration for Li pegmatite in Altai orogenic belt.

Two phases of granulite-facies metamorphism superimposied on retrograde eclogite: Constraints on the early Paleozoic tectonic evolution of the Qinling Orogenic Belt, central China

Existing studies provide adequate petrological evidences on ca. 500 Ma ultra-high pressure (UHP) metamorphism in the North Qinling Orogenic Belt (NQOB) in central China, but the genesis of 470–420 Ma multi-phase granulite-facies metamorphism in the NQOB and their relationship with the ca. 500 Ma UHP metamorphism remain controversial, resulting in the early Paleozoic evolution of the Qinling Orogenic Belt (QOB) highly debatable. In this study, we present mafic granulites and host felsic gneisses with a “red-eye socket” texture from the Shuanglong area, eastern NQOB, which recorded two phases of granulite-facies metamorphism superimposing on former eclogite-facies metamorphism. The former eclogite-facies metamorphism is indicated by eclogite-facies zircon trace element patterns and 496–495 Ma zircon ages, which are the same with those of the HP–UHP eclogite-facies metamorphic rocks in NQOB. The first granulite-facies metamorphism occurred at 460–448 Ma is characterized by coarse-grained minerals in matrix. Compositions and zonings of these minerals define an anticlockwise P–T path involving a prograde stage (751–763 °C), a high-temperature peak stage (9.2 kbar and 864 °C), and a near-isobaric cooling retrograde stage (8.3 kbar and 818 °C). The second granulite-facies metamorphism occurred at 422–421 Ma is represented by coronal garnet and coexisting fine-grained mineral aggregates. Coronal garnet compositional zonings suggest a clockwise P–T path consisting of a high-pressure peak stage (9.5–11.2 kbar and 748–783 °C) and a decompressing and heating retrograde stage (9.2–9.5 kbar and 789–800 °C). Combining dating results of leucosomes in these rocks and existing data, we proposed a new model for early Paleozoic tectonic evolution of the NQOB. The North Qinling Terrane (NQT), probably separated from the South China Block (SCB) during the breakup of Rodinia, drifted northwards and underwent UHP metamorphism at 500 Ma and then rapidly exhumed to crust level. Later, the Shangdan Ocean subducted northwards beneath the exhumed NQT at 470–440 Ma, resulting in the first granulite-facies metamorphism and contemporaneous migmatization and magmatism. Finally, the closure of the Shangdan Ocean led to collision between the NQT and South Qinling Terrane/SCB and the second granulite-facies metamorphism and anatexis at 422–418 Ma.

Polymetamorphism and metastability in Paleozoic schists of the central Appalachian Baltimore Terrane, USA

Metapelitic assemblages are often considered highly reactive during prograde metamorphism, such that they effectively record a history of tectono-metamorphic processes. Across eastern North America, metamorphosed rift-to-drift stratigraphy has been central to disentangling the terminal history of Appalachian convergence. We report results from a single metapelitic outcrop in the Loch Raven Schist of Maryland's Baltimore Terrane, where regional metamorphism is historically interpreted to have derived only from the Ordovician Taconic orogeny. An aluminum-rich litho-horizon alludes to two phases of metamorphism: an early, low/medium-pressure (P) sillimanite-stabilizing phase largely overprinted by a subsequent medium/high-P kyanite-stabilizing event, terminating at c. 391–383 Ma. This provides evidence of substantial crustal disturbances in the central Appalachians during Avalonian collision of comparable timing to equivalent events in the north. In a subordinate and aluminum-poor litho-horizon, large (2–3.5 cm) garnet provides geochronological evidence only for a protracted phase of metamorphism at 440–424 Ma. We interpret this age as recording the early, low/medium-P phase of metamorphism that either: (a) extends the duration of the Taconic event; or (b) provides new evidence for metamorphism associated with Silurian tectonism and/or extension in the Central Appalachians. However, this assemblage appears to have been metamorphically unresponsive during Devonian overprinting, despite peak metamorphic conditions of ∼670 °C and ∼8 kbar. We suggest that a potential combination of the paucity of fluid, limited strain accumulation, and coarse refractive assemblages stabilized a kinetically sluggish and metastable composition.

Origin and Affinities of the Malmberget Iron Oxide-Apatite Deposit, Northern Sweden: Insights From Magnetite Chemistry and Fe-O Isotopes

European iron ore production is primarily sourced from magnetite dominated iron oxide-apatite ore deposits in the northern Norrbotten ore province of northernmost Sweden. The Malmberget iron oxide-apatite deposit is at present the largest iron ore resource in Europe and is an amphibolite facies grade analogue of the world-famous Kiirunavaara iron oxide-apatite deposit. The Malmberget rock association is characterised by multiple phases of deformation, metamorphism, and alteration that resulted in a geometrically and petrologically complex deposit that is genetically ambiguous. Primary ore textures and emplacement structures of the Malmberget iron oxide-apatite deposit have largely been recrystallised during metamorphic overprinting and now comprise dominantly medium- to coarse-grained granoblastic magnetite. However, isotopic characteristics are preserved and when combined with trace element chemistry, these can be used to understand magmatic vs. hydrothermal origin of the deposit. To unravel the primary origin of the Malmberget magnetite ore, we combined magnetite trace element chemistry and Fe-O stable isotopes to investigate the massive magnetite in the Fabian-Kapten and ViRi ore bodies of the Malmberget iron oxide-apatite deposit. Trace element correlations indicate a high-temperature magmatic to a transitional high-temperature magmatic-hydrothermal origin of the Malmberget iron oxide-apatite ore deposit, with data plotting into fields of clear magmatic affinity in trace element discrimination diagrams. Fe-O data fall into established magmatic fields regardless of subsequent metamorphic modifications, underlining a dominantly (ortho-)magmatic origin of the investigated deposits. Despite an overall magmatic to magmatic-hydrothermal origin for the two ore bodies studied, Fe-O isotope equilibrium calculations of the magnetite suggest a possible temperature discrepancy between the Fabian-Kapten ore body and the ViRi ore body, the latter showing a more pronounced magmatic character. These variations in trace element contents and Fe-O isotopes can be explained by the proximity of the more magmatic signatures to the centre of the ore forming magmatic system.

Suturing of the Archean Bastar craton with the Eastern Ghats Province to form the Greater Indian Landmass: Insights from geochemistry, U-Pb geochronology and phase equilibria modelling

The contact between the Archean Bastar craton (BC) and Proterozoic Eastern Ghats Province (EGP), central India, is marked by a suture zone termed Terrane Boundary Shear Zone (TBSZ). BC in this area is largely composed of hornblende-biotite granite with some mafic dykes. Rocks in the TBSZ include quartzofeldspathic (leptynite) gneiss, garnet-orthopyroxene-bearing granitoid, mafic granulites (Group A of cratonic affinity, and Group B of EGP affinity), Mg-Al granulite and an isolated exposure of orthopyroxene-bearing gneiss. Detailed geochemical analysis shows remarkable similarity between Hbl-Bt granite and Grt-Opx-bearing granitoid, with A-type affinity, and between mafic dykes and Group A mafic granulites. However, the Opx-bearing gneiss is geochemically distinct having I-type affinity, similar to TTG gneisses described from BC. Metamorphic phase equilibria analysis and trace element modelling shows that (i) melting of Opx-bearing gneiss would produce a ferroan granitic melt resembling the Hbl-Bt granite, (ii) metamorphism at appropriate P-T conditions would convert the granite to Grt-Opx-bearing granitoid and the mafic dyke to Group A mafic granulite. U-Pb geochronology of zircon constrains emplacement ages of the magmatic precursors of Opx-bearing gneiss and Grt-Opx-bearing granitoid as ca. 2.73 and ca. 2.5 Ga, respectively. These rocks were subjected to an early granulite facies metamorphism, followed by an amphibolite facies metamorphism, shearing and hydrous fluid flux. Geochronological data shows that the latter event took place at ca. 0.52 Ga, while the earlier granulite facies event can only be tentatively suggested to be of late Stenian/Tonian age. Collating all the evidence (including published geophysical and geochronological data), we suggest that the initial collision between BC and EGP took place during late Stenian/Tonian time as a consequence of formation of the Greater Indian Landmass, a part of Rodinia supercontinent. The TBSZ, probably initiated due to the late Stenian/Tonian collision, was reactivated and reworked tectonothermally at ca. 0.52 Ga, caused by far field stress effect of the Kuunga orogeny, which was strong enough to obliterate most of the imprints of the late Stenian/Tonian orogeny.

Rapid heating (

Ultrahigh-temperature (UHT) metamorphism is the most thermally extreme form of regional crustal metamorphism, with temperatures exceeding 900°C. The duration and heat source of UHT metamorphism are critical constraints on the tectonic evolution of orogenic systems. Here, we report the discovery of a sapphirine-bearing granulite from the east-central Himalaya which preserves UHT evidence. The reconstructed pressure-temperature-time path indicates that the temperature increased by almost 200°C within ~2 Ma which is consistent with rapid asthenospheric heat input. Numerical simulations illustrate potential mechanisms for such heating: juxtaposition of the deep crustal accretionary system with the upwelling asthenospheric mantle through newly developed apertures generated by slab break-off and/or associated vertical tearing of down-going Indian lithosphere. Spatial-temporal consistencies among the UHT metamorphic phases, postcollisional magmatism, geophysical constraints, and crustal deformation indicate that slab break-off or tearing controls broad swaths of Himalayan tectonics. The consequent upwelling asthenosphere may have been a significant heat source for the Miocene Himalaya and for similar ancient collisional orogenic systems.

Structural and metamorphic architecture of the Zanskar Himalaya, Suru Valley region, NW India: Implications for the evolution of the Himalayan metamorphic core

New 1:50,000-scale geological mapping in the Zanskar Himalaya of NW India, covering 2,400 km2, is integrated with structural and petrographic analysis to document the evolution and key tectonometamorphic relationships within the Himalayan metamorphic core. The integrated dataset constrains the regional three-dimensional geology and relationships between lithostratigraphy, folds, faults, deformation fabrics, metamorphic isograds, and growth of porphyroblasts within the context of five main deformation phases. Following the initial collision of India and Asia, NW−SE-oriented deformation is recorded by D1 (greenschist-facies) fabrics and D2 (greenschist- to amphibolite-facies) fabrics. D2 represents the main tectonometamorphic deformation phase associated with crustal thickening and produced the dominant regional penetrative fabric through crenulation and transposition of D1 fabrics. Thrust-sense D2 fabrics were reactivated during D3 as the Greater Himalayan Sequence was exhumed along the normal-sense Zanskar Shear Zone, which is part of the South Tibetan Detachment System. D3 fabrics, associated with movement on the Zanskar Shear Zone, were temporally continuous with crenulation and mesoscale folding, recording progressive kilometer-scale backfolding and backthrusting toward the NE between the Greater Himalayan Sequence−Tethyan Himalayan Sequence and the adjacent Indus Suture Zone. Finally, D4 and D5 are recorded as kilometer-scale open folding of older planar and linear structures. The orientation of mineral isograd surfaces ranges from subparallel to oblique with respect to D2 planar structural elements. The growth of pelitic and metabasic peak metamorphic phases from greenschist to upper-amphibolite facies is synchronous with or postdates D2 fabrics. D3 fabrics wrap thermal peak porphyroblasts and realign linear mineral phases. Tectonic thinning adjacent to D3 normal faults is documented by reduced structural spacing of isograds and alignment of isograd surfaces parallel to the faults. D4 and D5 structures modify the trace of all regional metamorphic isograds. Collectively, these observations imply that the thermal peak of metamorphism was reached after the main phase of deformation (D2), and predated movement on the Zanskar Shear Zone (D3). The results document numerous classical elements of collisional orogenesis, including implied clockwise P-T paths, polyphase deformation, and a complete Barrovian metamorphic isograd sequence supplemented by complementary metabasic isograds. The Zanskar Himalaya, unlike other areas of the Himalayan metamorphic core, records metamorphic conditions primarily attained following substantial crustal thickening rather than during subsequent decompression and exhumation. The reduced expression and/or discontinuous nature of exhuming fault systems, which produces variable levels of crustal exposure, may account for this lateral heterogeneity across the mountain belt. Deciphering the complex kinematics of continental tectonics requires the integration of observations and data over large length scales and a range of structural levels.

Tectono-metamorphic evolution of the Central Bundelkhand Craton, India from geochemistry and bulk composition modelling of amphibolite enclaves

The Bundelkhand Craton (BuC) represents a significant Archean terrane within the northern Indian Craton and yet its tectono-metamorphic evolutionary history remains relatively understudied. Our investigation involves detailed petrography, geochemistry, and bulk composition modeling of both garnet-bearing and garnet-absent amphibolites with a two-fold objective: (i) to constrain the protolithic nature and tectonic settings involved in the genesis of these rocks, and (ii) to propose a tectono-metamorphic evolutionary history for the BuC. The BuC amphibolites originate from basalt and andesitic-basalt protoliths. Their trace element compositions reveal negative anomalies in Nb, Ta, and Ti, while their rare earth element (REE) normalized patterns indicate enrichment in light REEs over heavy REEs. The basaltic protolith is interpreted to have formed during orogeny in a compressional tectonic regime at the active margins of island arcs in a subduction-related setting. This interpretation is supported by various discrimination plots for amphibolites, such as Nb/Th vs Zr/Nb, Zr vs Zr/Y, and Th/Nb vs Ce/Nb, as well as high Th/Yb and low Nb/Yb contents— all of which suggest an island arc setting influenced by subduction. These amphibolites have undergone three distinct phases of metamorphism, as evidenced by petrography, mineral chemistry, and bulk composition modeling. This interpretation is further supported by geochemical discrimination diagrams which indicate that a subduction tectonic setting was active during peak metamorphism. During the pre-peak phase, the garnet-bearing amphibolites experienced pressure and temperature conditions ranging from 6.25 to 6.5 kbar and 580 to 590ºC, while the garnet-absent amphibolites underwent conditions from 5.0 to 5.8 kbar and 400 to 450ºC. Peak metamorphism was observed at pressures ranging from 6.8 to 7.4 kbar and temperatures from 760 to 805ºC for the garnet-bearing amphibolites, and at pressures from 7.0 to 7.4 kbar and temperatures from 785 to 810ºC for the garnet-absent amphibolites. The metamorphic retrograde conditions for the garnet-bearing amphibolites are defined by P-T conditions ranging from 4.45 to 4.75 kbar and 585 to 615ºC, while for the garnet-absent amphibolites, it ranges from 3.1 to 4.0 kbar and 620 to 710ºC. The mineral assemblages and P-T conditions delineate a clockwise P-T path for both, garnet-bearing and garnet-absent amphibolites from the Babina and Mauranipur regions. This suggests that the rocks underwent a burial process amid subduction tectonic settings in an arc-related environment, followed by a decompression stage that brought them to the surface.

Early Orosirian belts of the central Guiana Shield, northern Amazonian Craton: U-Pb geochronology and tectonic implications

The Cauarane-Coeroeni Belt (CCB), the Orocaima Igneous Belt (OIB) and the Rio Urubu Belt (RUB) are the main tectonic features of the central part of the Guiana Shield. The CCB is a sinuous NW-SE/NE-SW/NW-SE trending high-grade supracrustal belt, bordered to the north by the OIB, formed by volcanics and high-crustal level granitoids, and to the south by the RUB, made up of deeply emplaced granitoids and gneisses. We present 27 new U-Pb SHRIMP and LA-ICP-MS zircon ages for these belts.The geochronological data obtained for 10 granitoid samples of the OIB indicate that magma pulses of different typologies were coeval along the belt and confirm its continental scale extension. Crystallization ages obtained for the high-K calc-alkaline granitoids vary from 1990 ± 5 Ma to 1957 ± 17 Ma and those of reduced A-type granites vary from 1980 ± 14 Ma to 1974 ± 8 Ma.Only four samples of granitoids and gneisses of the RUB were analysed. Crystallization ages of 1964 ± 7 Ma and 1956 ± 8 Ma were calculated respectively for a granodiorite and for an orthogneiss and are in agreement with the 1.96–1.92 Ga age period regionally considered for the evolution of the RUB. However, an age of 1973 ± 7 Ma was calculated for a foliated S-type granite, indicating that slightly older granitoid rocks are also present within the RUB. In addition, an age of 1935 ± 10 Ma was calculated for the high-grade metamorphism along the belt.A microgranitic dyke that crosscuts the foliation of a gneiss of the RUB was dated at 1884 ± 7 Ma, and probably represents one of the feeder dykes for the large 1.89–1.87 Ga Uatumã magmatic event.Twelve samples of the CCB were analysed. The data for the detrital nuclei of the zircon crystals of the paragneisses and schists suggest an overall similar inheritance pattern along the length of the CCB, recording the contribution of Archean to early Orosirian sources, with a major participation of Rhyacian domains.The oldest detrital zircon crystals yielded an age of 3.65 Ga, but the age of most of the Archean zircon grains fall in the 3.50–2.50 Ga range and they were probably derived from the Imataca and Amapá blocks. Results in the 2.50–2.30 Ga age interval were obtained for detrital zircon grains of many samples, indicating the contribution of Siderian source-areas, possibly the Bacajá Domain, located outside the shield. Those terranes formed within the shield during the Rhyacian phase of the Transamazonian Cycle were probably the source of the 2.30–2.05 Ga detrital zircon grains. In addition, zircon grains with ages between 2.05 Ga and 2.03 Ga were possibly derived from the Trairão-Anauá terranes.The U-Pb SHRIMP zircon dating allowed to better constrain the metamorphic events along the CCB. The syn-kinematic M1 high-grade metamorphic phase could not be directly dated, and we suggest it occurred between 2025 Ma and 2000 Ma. The imprint of the thermal M2 metamorphic event is recorded by the presence of rims and cores of zircon crystals with very low Th/U ratios (<0.1), showing secondary texture. Upper intercept ages of 1983 ± 9 Ma, 1978 ± 9 Ma and 1967 ± 27 Ma were obtained for M2. Zircon grains with secondary textures and very low Th/U ratios (<0.1) from a sample collected in the vicinities of the RUB yielded an upper intercept age of 1955 ± 14 Ma, tentatively interpreted as reflecting M3.The ages obtained for the M2 and M3 metamorphic phases coincide, respectively with those measured for the rocks of the OIB and for the high-grade metamorphism in the RUB. We envisage that the M2 and M3 metamorphic overprints in the paraderived rocks of the CCB reflect important fluid input and thermal perturbation respectively during the emplacement of the OIB granitoids to the north, and the assemblage of granitoid bodies of slightly different ages, deformation and high-grade metamorphism within the RUB, south of the CCB.The early Orosirian terranes of the central part of the Guiana Shield were formed during Akawai Orogeny. The 2.04–2.03 Ga Trairão-Anauá terranes and the CCB supracrustal rocks represent the pre-collisional and collisional stages of the orogeny, the OIB records post-collisional, late-orogenic magmatism. The RUB is probably reflecting a post-collisional setting associated with transpressive deformation. The Akawai Orogeny is interpreted as the late stage of the Transamazonian Cycle, after the main Rhyacian orogeny of the cycle, and was responsible for the final amalgamation of this part of the Columbia paleocontinent.

Machine Learning in Petrology: State-of-the-Art and Future Perspectives

Abstract This article reports on the state-of-the-art and future perspectives of machine learning (ML) in petrology. To achieve this goal, it first introduces the basics of ML, including definitions, core concepts, and applications. Then, it starts reviewing the state-of-the-art of ML in petrology. Established applications mainly concern the so-called data-driven discovery and involve specific tasks like clustering, dimensionality reduction, classification, and regression. Among them, clustering and dimensionality reduction have been demonstrated to be valuable for decoding the chemical record stored in igneous and metamorphic phases and to enhance data visualization, respectively. Classification and regression tasks find applications, for example, in petrotectonic discrimination and geo-thermobarometry, respectively. The main core of the manuscript consists of depicting emerging trends and the future directions of ML in petrological investigations. I propose a future scenario where ML methods will progressively integrate and support established petrological methods in automating time-consuming and repetitive tasks, improving current models, and boosting discovery. In this framework, promising applications include (1) the acquisition of new multimodal petrologic data; (2) the development of data fusion techniques, physics-informed ML models, and ML-supported numerical simulations; and (3) the continuous exploration of the ML potential in petrology. To boost the contribution of ML in petrology, our main challenges are: (1) to improve the ability of ML models to capture the complexity of petrologic processes, (2) progressively link ML algorithms with the physical and thermodynamic nature of the investigated problems, and (3) to start a collaborative effort among researchers coming from different disciplines, both in research and teaching.

Tectono-Metamorphic Evolution of the Cretaceous Kluane Schist, Southwest Yukon

Abstract A wealth of information regarding the Mesozoic evolution of the Northern Canadian and Alaskan Cordillera is held within a series of variably metamorphosed and deformed rocks that formed in Jura-Cretaceous basins. Located at the interface between the pericratonic Intermontane and exotic Insular terranes, these basinal rocks are key to understanding the timing and tectonic style of Insular terrane accretion, a topic of longstanding debate. This study unravels the structural and metamorphic evolution of one of these basins, the Kluane Basin, within southwest Yukon Territory. The Kluane Schist is the primary assemblage of the Kluane Basin. It consists of metamorphosed and deformed low-Al pelites that were intruded by granodioritic plutons of the Paleocene Ruby Range batholith. Previous workers have suggested the variable metamorphic character of the Kluane Schist represents an extensive and static thermal aureole related to Ruby Range batholith emplacement. Our work, however, indicates that the Kluane Schist experienced Buchan-style metamorphism coeval with protracted deformation and can be divided into seven distinct petrologic zones, which, based on their unique combination of mineral assemblage and structure, are incompatible with static thermal metamorphism. Instead, we propose the Kluane Schist experienced two distinct metamorphic phases: (1) an early greenschist-facies phase that resulted in the development of a bedding-parallel chlorite-muscovite-titanite fabric, preserved by its lowest-grade units, and (2) a later amphibolite-facies phase that manifests as the progressive transposition of the earlier chlorite-muscovite-titanite fabric into a penetrative biotite-rich schistosity that transitions upgrade into a segregated gneissic fabric comprised of biotite-cordierite and plagioclase-quartz (± sillimanite-K-feldspar-melt). By integrating the results of detailed petrography and petrological modeling, we demonstrate that the second main metamorphic phase experienced by the Kluane Schist preserves a record of pressures and temperatures that align with other Buchan-style terranes worldwide. Our data defines a field gradient across the Kluane Schist ranging from 3.0–3.5 kbar at 375–400 °C to 4–4.5 kbar at 700–750 °C. This record of a coupled Buchan-style metamorphic-deformational evolution and tops-to-the SW non-coaxial shear structures is consistent with the override of the thermally mature Yukon-Tanana terrane as the principal driver of Kluane Schist metamorphism, with some limited heat likely contributed by the late-syn- to post-tectonic intrusion of the Ruby Range batholith.

Structural and Geochemistry of Air Piau Gold Mineralisation in Kelantan, North-East Peninsular Malaysia

The formation of the Air Piau Gold Deposit in North-East Peninsular Malaysia due to Permo–Triassic subduction and collision between Sibumasu and East Malaya blocks and resulted in several phases of deformation, contraction, and metamorphism. Kinematic and petrography study of quartz veins in Air Piau area observed whether following the foliation of metamorphic rock or crosscut the foliation. Early generation of veining system in Air Piau follow the host rock foliation (V1). Meanwhile the later formation of the hydrothermal vein has a cross-cutting relationship with foliation (V2). Some veins observed were sheared and brecciated (V3). Positive correlation between Au and As concentration that can be characterized these deposit formed at mesozonal zone. V3 vein type showed the brecciated texture of vein indicates the brittle structure deformation. The conceptual model of orogenic gold, this type of V3 vein can be classified as epizonal zone. Regionally, the Air Piau gold deposit shares geologically similarities (in term of tectonic setting, geological structures controlled, host rock, ore gechemistry) with many other sediment/metasediment- hosted, orogenic gold deposits in Peninsular Malaysia such as Tersang, Selinsing and Penjom. Furthermore, Air Piau gold deposit characteristics as discussed before supported the theory related with orogenic/mesothermal gold from various researchers However, a number of inferences on classified Air Piau gold deposit model as orogenic gold deposit remains speculative in the absence importance data like ore geochemistry data (Hg, Sb and Te element concentrations) and fluid inclusion data.

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.

Metamorphic P-T evolution and tectonic implications of UHT metamorphism from the Shillong Meghalaya Gneissic Complex, India: evidence from phase equilibria modelling, monazite U-Pb-Th geochronology, K-Ar dating and geochemistry

Abstract The study area Sonapahar is an integral part of Shillong Meghalaya Gneissic Complex (SMGC), which is located in the Northeastern part of India. This complex mainly comprises metamorphic formations spanning from Upper Amphibolite to Ultra-high temperature granulite, interspersed with various igneous intrusions. In this study, particular attention is directed towards unravelling the metamorphic history of Mg-Al granulite. For the very first time, we establish the pressure–temperature (P-T) trajectory of the Mg-Al granulite from Sonapahar, SMGC. Employing conventional thermobarometry along with winTWQ analysis, the inferred metamorphic conditions for this granulite reveal temperatures exceeding 900°C and pressures of approximately &gt;8 kbar. These conditions firmly indicate the presence of ultra-high-temperature metamorphism. By utilizing the Perple_X software in the MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 compositional system, we construct a P-T pseudosection. This gives a clockwise P-T path, signifying an episode of cooling (+ minor decompression). Such a pattern also suggests rapid cooling of the tectonically-thickened crust. Concurrently, a geochemical exploration of trace and rare earth elements in the rocks offers further insights. These investigations give an idea about the protolith, having a clay-to-sandstone in nature. Additionally, chemical data from monazite within the studied rock provide a weighted mean age of 682 Ma for the peak metamorphic stage. This age aligns with the global Pan-African orogenic events. The biotite K-Ar isotopic geochronology from the symplectite position provides decompression history or cooling age of 442 Ma. This age corresponds to a period after the last peak metamorphic phase that occurred during the Pan-African thermal event.

ОСОБЕННОСТИ ФОРМИРОВАНИЯ ЗОЛОТОРУДНОЙ МИНЕРАЛИЗАЦИИ В УСЛОВИЯХ АМФИБОЛИТОВОЙ ФАЦИИ МЕТАМОРФИЗМА: МЕСТОРОЖДЕНИЕ ЫКАН (БАЙКАЛО-ПАТОМСКИЙ ПОЯС)

Black shale deposits whose metamorphic grade exceeds greenschist-facies of regional metamorphosed rocks are considered by most researchers exclusively from the point of view of the potential to mine alluvial gold. The article presents the results of the study on the Ykan deposit located within the epidote-amphibolite facies zone and its comparison with previously studied deposits localized in the greenschist facies alteration zone. It was found that the Ykan gold deposit is lithologically and stratigraphically confined to the contact between carbonaceous phyllite shales and sandstones of the Aunakit Formation in the anticlinal limb, which is one of the main factors controlling ore localization. The ore mineralization of the deposit is represented by the change of the early diathagenetic pyrite (py-I, py-II) → metamorphogenic pyrrhotite (po, cpy, asp) → metamorphogenic-metasomatic pyrite-polymetallic (cpy, asp, py-III, gln, sph, pn, mrc) → post-ore pyrite (py-IV) associations. The ore stage is characterized by the highest possible TP parameters in the early ore metamorphic phase and their decrease in the ore-productive phase. Data on variations in the isotopic composition of sulfur and lead indicate that they were transported into the ore fluid from the host meta-sedimentary strata of the Aunakit Formation. Comparison of the obtained parameters of the Ykan deposit with the previously studied deposits of Sukhoi Log, Golets the Highest, Ugakhan and Krasnyi made it possible to conclude that we are correct in assigning the Ykan deposit to the group of deposits of the Sukholozhsky genetic type.

Structural Architectures of Precambrian Metamorphites in and Around Rongmil –Rongjeng Area, East Garo Hills Districts Meghalaya

The present work elucidates the Precambrian metamorphites in and around Rongmil-Rongjeng area in the light of their polydeformational episodes that witness four pfases of deformation showing multiple interference patterns in the ductlile to brittle fields. Field as well as microstructural study reveals that all the pre existing depositional fabrics have been affected by BD1, the first phase of deformation under progressive metamorphic phase leading to the formation of most dominant, highly penetrative schistose and gneissose fabrics of rocks of amphibolites facies. The second phase of deformation, BD2 forming NW-SE trending up and down facing folds of probable plane non cylindrical nature, was responsible for layer parallel shortening (buckling) of the mechanically active BS0 and BS1 fabrics in the NW-SE longitudinal compressive tectonics. The third phase folds, BF3 refolded the earlier set of folds into non –plane non cylindrical nature and occur as non coaxial, open warp type gently plunging with near vertical axial plane striking NW-SE direction. BD4, the fourth phase deformation is considered as brittle phase and represents the joints, fractures and fault structures.

Orogen-parallel discontinuity of the Apennines subduction zone in Southern Italy as seen from mantle wedge seismic structure

We investigate the seismic structure of the mantle wedge of the Apennines subduction zone (Central Mediterranean) using teleseismic receiver function (RF). We inverted RF for both isotropic and anisotropic properties of the mantle wedge, from below the overriding Moho to the “plate boundary”, i.e. the interface that separate the slab from the mantle wedge. Given the distribution of the seismic network, we are able to map out the change in the elastic properties at the transition between southern apennines and the Calabrian arc, given by the change in the subduction style (i.e from the subduction of continental materials to oceanic plate). We found that the anisotropy in the mantle wedge is similar between all seismic stations, generally highly anisotropic (> 10%), with a direction of the symmetry axis that rotates clockwise from North to South, following the Calabrian arc geometry and likely indicating the mantle flow driven by the slab retreat. The elastic properties of the subducted crust are more heterogeneous. To the North, the subducted crust shows a highly anisotropic (> 10%) behavior, and it occurs at larger depth (around 70 km depth), where to the South anisotropy is less intense (around 7%) and the subducted crust is shallower (around 60 km depth). These results point out a change in the subduction style that can be given by either a change in the metamorphic phase (more evolved blueschist facies stage to the North, initial greenschist facies stage to the South) or a different origin for the subducted materials (continental to the North and oceanic to the South). The differences in the anisotropic behavior of the subducted crust are reflected in the topography of the plate boundary, which becomes shallower from North to South, suggesting the existence of either a step in the slab topography or a more gentle ramp.

The Neoarchean mafic-ultramafic São José do Jacuípe Suite in the Itabuna-Salvador-Curaçá orogen, Brazil: New U-Pb ages and geochemical data

The São José do Jacuípe Suite (SSJJ) comprises an association of mafic and ultramafic rocks that crops out in the northern portion of the Itabuna Salvador-Curaçá Orogen (ISCO), São Francisco Craton. They are represented by small NW-SE elongated bodies comprising metamorphosed anorthosites, leucogabbros, gabbronorites, gabbros, ferrogabbros, and subordinate pyroxenites and serpentinites. The rocks are part of an orogenic belt that comprises orthogranulites and orthogneisses of the Caraíba complex, and metasedimentary rocks of the Tanque Novo-Ipirá complex, which appear to be amalgamated into a folded mélange. Petrographical analysis of the SSJJ rocks revealed relict cumulus textures in the pyroxenites. Moreover, inclusion relationships allowed the identification of two metamorphic phases; the first phase likely occurred at amphibolite facies conditions and was overprinted by granulite facies metamorphism that represents the peak metamorphic conditions recorded by the SSJJ rocks. Analytical results for major and trace elements indicate cogeneticity between the mafic rocks, and arc-related mafic cumulate field compositions. In the geotectonic environment diagrams the rocks plot in the MORB and IAT fields, indicating an origin involving melting of different sources or magma mixing. The characteristic signature of the studied rocks is represented by negative HFSE anomalies (Nb, Ti, Zr and Hf), depletion in HREE and enrichment in LILE (Ba, Rb, and Cs), indicating their derivation from metasomatic mantle in a suprasubduction setting. These data corroborate previous isotope data that point to mildly enriched mantle sources. Incompatible trace elements signatures are similar to ophiolites generated in supra-subduction zone (SSZ) environments worldwide. New U/Pb LA-ICP-MS ages indicate igneous crystallization at 2549 ± 23 Ma and 2563 ± 13 Ma for SSJJ gabbronorites, thus indicating that they postdate the other members of the magmatic arc that formed the ISCO by 20–30 Ma.

THE Fe-Cu DISCONNECT: UNRAVELING A COMPOSITE IRON OXIDE COPPER-GOLD DEPOSIT IN THE OLYMPIC Fe-Cu-Au PROVINCE, GAWLER CRATON

Abstract The northern Olympic Cu-Au province, Gawler craton, Australia, includes a series of magnetite-dominated deposits/prospects associated with minor Cu-Au mineralization such as the 8.37 million tonne Cairn Hill deposit. Cairn Hill has long been considered a deep, magnetite end member of the iron oxide copper-gold (IOCG) family that is largely represented in the southern Olympic province by the 1590 Ma hematite-dominated Olympic Dam, Carrapeteena, and Prominent Hill deposits. In contrast to the southern district, the deposits in the northern Olympic Cu-Au province are hosted in rocks that experienced multiple phases of high-temperature metamorphism and deformation. New U-Pb zircon geochronology shows the magnetite-hornblende lodes at Cairn Hill were formed at ca. 1580 Ma at amphibolite facies conditions. The magnetite lodes are crosscut by ca. 1515 Ma granitic dikes. A second high-temperature event is recorded by U-Pb monazite geochronology at ca. 1490 Ma and involved deformation and metamorphism along the Cairn Hill shear zone at conditions of 4.6 to 5.3 kbar and 740° to 770°C. The 1490 Ma event reworked the iron lodes and 1515 Ma granitic dikes. However, Cu mineralization at Cairn Hill occurs in brittle fractures and quartz-biotite veins, overprinting the 1490 Ma deformation and metamorphism. Despite a spatial association between magnetite and Cu, the long thermal history that affected magnetite mineralization and the clear petrographic links between magnetite and high-temperature granulite facies minerals contrast with the late, low-temperature hydrothermal Cu mineralization and indicate the two are not paragenetically related. Therefore, the spatial but not temporal association between magnetite and Cu has effectively overlain two distinct episodes of mineralization to create the Fe-Cu deposit observed today. Although this fits within the broad IOCG deposit family, exploration strategies for Cairn Hill-style composite deposits should be distinct from IOCG deposits with cogenetic Fe and Cu.