A secondary precious and base metal mineralization in chromitites linked to the development of a Paleozoic accretionary complex in Central Chile

Platinum-group element (PGE) and gold inclusions are usually present in peridotites and chromitite deposits associated with ophiolites. Here, we present the first detailed study of the mineralogy of precious metals in ultramafic rocks hosted in the Paleozoic Coastal Accretionary Complex of Central Chile. In these ultramafic rocks the mineralization of precious metals is associated with small meter-size pods and veins of massive chromitite hosted in serpentinite-filled shear zones. Crystallographic orientation maps of single chromite grains, obtained using the Electron-Backscattered Secondary Diffraction technique, allow us to identify two types of chromite in the precious-metal bearing chromitites: (1) Type A chromite, characterized by an average misorientation per grain of ≤ 2° and chemically homogeneous cores surrounded by a porous rim with abundant inclusions of chlorite, and (2) Type B chromite, which exhibits higher degrees of misorientation (2–8°) and porosity, and abundant silicate inclusions, but a relatively homogeneous chemical composition. In situ analyses using EMPA and LA-ICP-MS for major, minor and trace elements indicate that composition of the magmatic chromite is only preserved in the cores of Type A chromite grains. Core to rim chemical trends in these Type A chromites are characterized by a progressive increase of the Cr# with a decrease of the Mg#, loss of Al and addition of Fe^2 + in the porous rim. The observed changes in the microstructure and chemistry of chromite are associated with the infiltration of external fluids through shear zones filled with antigorite (± talc) developed in partly serpentinized peridotites (i.e., olivine–lizardite dunites). Thermodynamic calculations using the phase equilibria relations in the system Cr₂O₃–MgO–FeO–Al₂O₃–SiO₂–H₂O (CrMFASH) indicate that Fe^2 +-rich porous chromite + chlorite replaced the original assemblage chromite + olivine in the chromitite while prograde antigorite was formed. According to our results this transformation occurred at ~ 510–560 °C when external fluids penetrated the ultramafic/chromitite bodies through shear zones. These temperatures are slightly higher than estimated for the metamorphic peak in the host metapelitic rocks (i.e., ~ 420 °C at 9.3 kbar), suggesting that a hotter ultramafic body was captured by the metasediments of the accretionary prism during their exhumation through subduction channel. Chlorite geothermometry yielded a wide range of lower temperature from 430 to 188 °C, for chlorite present in the porous chromite rims. These results are in agreement with the retrograde overprint under greenchist-facies metamorphism conditions recorded by metapelitic host rocks and minor volcanogenic massive sulphide deposits in the area (300–400 °C, ~ 3–4 kbar). We suggest that although initially decoupled, the chromitite-bearing ultramafic rocks and their metasedimentary host undergone a common metamorphic PT pathway of exhumation during the formation and evolution of the subduction-related accretionary complex.The chromitites contain appreciable amounts of the platinum-group elements (up to 347 ppb total) and gold (up to 24 ppb), present as inclusions of platinum-group minerals (PGM) and alloys as well as native gold. The PGM identified include native osmium, laurite (RuS₂), irarsite (IrAsS), osarsite (OsAsS), omeiite (OsAs₂), Pt–Fe alloy (possibly isoferroplatinum) and a suite of inadequately identified phases such as PtSb (possibly stumpflite), PdHg (possibly potarite), RhS, Ir–Ni and Ir–Ni–Ru compounds. Only a few grains of osmium and laurite were identified in unaltered cores of chromite and therefore considered as magmatic in origin formed during the high-T event of chomite crystallisation in the upper mantle. The other PGM were located in the porous chromite associated with chlorite or base-metal minerals (BMM) that often fill the pores of this altered chromite or are intergrowth with antigorite in the host serpentinized ultramafic rock. The assemblage of BMM identified in the studied rocks include sulphides [millerite (NiS), polydymite (Ni₃S₄), violarite (FeNi₂S₄), galena (PbS), sphalerite (ZnS), chalcocite (CuS)], arsenides [(orcelite (Ni_5 − xAs₂) and maucherite (Ni₁₁As₈)], the sulpharsenide gersdorfitte (NiAsS), and native bismuth. The irregular shape of several PGM grains observed in porous chromite suggest disequilibrium, whereas others exhibit perfectly developed crystal faces with the associated secondary silicate or base-metal mineral suggesting neoformation of PGMs in situ from metamorphic fluids. We suggest that the origin of these PGM inclusions is magmatic, but some grains were reworked in situ when metalloid (i.e., As, Sb, Pb, Zn and Hg)-rich fluids released from metasediments penetrated the ultramafic rocks through active shear zones, once the ultramafic bodies became tectonically mixed with the host metasedimentary host rocks. During this event, gold sourced from the (meta)sediments was also precipitated within chromitites and serpentinites.     

The role of silica in the hydrous metamorphism of chromite

Retrograde hydrous metamorphism has produced three types of microstructures in chromite grains from chromitites and enclosing rocks of the Tapo Ultramafic Massif (Central Peruvian Andes). In semi-massive chromitites (60–80 vol% chromite), (i) partly altered chromite with homogeneous cores surrounded by lower Al₂O₃ and MgO but higher Cr₂O₃ and FeO porous chromite with chlorite filling the pores. In serpentinites (ii) zoned chromite with homogeneous cores surrounded by extremely higher Fe₂O₃ non-porous chromite and magnetite rims, and (iii) non-porous chromite grains. The different patterns of zoning in chromite grains are the consequences of the infiltration of reducing and SiO₂-rich fluids and the subsequent heterogeneous interaction with more oxidizing and Fe-bearing fluids. During the first stage of alteration under reduced conditions magmatic chromite is dissolved meanwhile new metamorphogenic porous chromite crystallizes in equilibrium with chlorite. This reaction that involves dissolution and precipitation of minerals is here modeled thermodynamically for the first time. µSiO₂-µMgO pseudosection calculated for unaltered semi-massive chromitites at 2 kbar and 300 °C, the lowest P-T conditions inferred from the Tapo Ultramafic Massif and Marañón Complex, predicts that chromite + chlorite (i.e., partly altered chromite) is stable instead of chromite + chlorite + brucite at progressive higher µSiO₂ but lower µMgO. Our observation is twofold as it reveals that the important role of SiO₂ and MgO and the open-nature of this process. P-T-X diagrams computed using the different P-T pathways estimated for the enclosing Tapo Ultramafic Massif reproduce well the partial equilibrium sequence of mineral assemblages preserved in the chromitites. Nevertheless, it is restricted only to the P-T conditions of the metamorphic peak and that of the latest overprint. Our estimations reveal that a high fluid/rock ratio (1:40 ratio) is required to produce the microstructures and compositional changes observed in the chromitites from the Tapo Ultramafic Massif. The circulation of SiO₂-rich fluids and the mobilization of MgO from the chromitite bodies are linked with the formation of garnet amphibolites and carbonate-silica hydrothermalites (i.e., listwaenites and birbirites) in the ultramafic massif. The origin of these fluids is interpreted as a result of the dissolution of orthopyroxene and/or olivine from the metaharzburgites and metagabbros enclosed in the Tapo Ultramafic Massif.     

Late Archaean granulite facies metamorphism in the Vestfold Hills,East Antarctica

Granulites of the Vestfold Hills record a pulsed end-Archaean to early Palaeoproterozoic M1–M2 evolution that is distinct from other Archaean areas in East Antarctica and cratonic domains placed adjacent to East Antarctica in Gondwana reconstructions. Pressure and temperature conditions of the end-Archaean to earliest Palaeoproterozoic (2501–2496 Ma) M1 granulite facies metamorphism in the Vestfold Hills have been constrained from mineral assemblages and thermobarometry of Fe-rich paragneisses. Reintegrated compositions of exsolved subcalcic clinopyroxenes and pigeonites in a metaironstone yield temperatures of 895 ± 35 °C, whilst reintegrated compositions of perthitic feldspars in semipelitic paragneisses give minimum estimates of 860 ± 30 °C. These results rule out the extreme ultrahigh temperature (UHT) conditions previously proposed for M1 in the Vestfold Hills. Pressures of metamorphism during M1 are estimated as 8.1 ± 0.9 kb at 850 ± 40 °C from hercynite + sillimanite + almandine + corundum and retrieved Fe–Mg–Al relations in orthopyroxene coexisting with garnet. A second metamorphic event, M2, occurred at 600–660 °C and 6–8 kb based on thermometry of recrystallised pyroxene neoblasts and thermobarometry applied to M2 garnet–quartz symplectites formed on orthopyroxene and garnet. The intervening emplacement of the magmatic Crooked Lake Gneiss Group precursors occurred at similar or shallower pressures prior to D2–M2, an event that caused tectonic interleaving and reactivation of the Vestfold Hills basement at mid-crustal depths in the earliest Palaeoproterozoic, prior to its unroofing to shallower levels (3–5 kb) by 2470 Ma. The lack of correlative Archaean histories in areas that were formerly adjacent in Gondwanan reconstructions is consistent with the Vestfold Hills region either being exotic to the East Antarctic Shield until the final (Neoproterozoic to Cambrian) amalgamation of Gondwana, or being accreted to part of East Antarctica in a Proterozoic event distinct from the Rayner–Eastern Ghats tectonism that united much of India with Antarctica at 1000–900 Ma.     

Chromitites from the Luanga Complex,Carajás,Brazil: Stratigraphic distribution and clues to processes leading to post-magmatic alteration

The Neoarchean (ca. 2.75 Ga) Luanga Complex, located in the Carajás Mineral Province in Brazil, is a medium-size layered intrusion consisting, from base to top, of ultramafic cumulates (Ultramafic Zone), interlayered ultramafic and mafic cumulates (Transition Zone) and mafic cumulates (Mafic Zone). Chromitite layers in the Luanga Complex occur in the upper portion of interlayered harzburgite and orthopyroxenite of the Transition Zone and associated with the lowermost norites of the Mafic Zone. The stratigraphic interval that hosts chromitites (∼150 meters thick) consists of several cyclic units interpreted as the result of successive influxes of primitive parental magma. The compositions of chromite in chromitites from the Transition Zone (Lower Group Chromitites) have distinctively higher Cr# (100Cr/(Cr + Al + Fe³⁺)) compared with chromite in chromitites from the Mafic Zone (Upper Group Chromitites). Chromitites hosted by noritic rocks are preceded by a thin layer of harzburgite located 15–20 cm below each chromitite layer. Lower Cr# in chromitites hosted by noritic rocks are interpreted as the result of increased Al₂O₃ activity caused by new magma influxes. Electron microprobe analyses on line transverses through 35 chromite crystals indicate that they are rimmed and/or extensively zoned. The composition of chromite in chromitites changes abruptly in the outer rim, becoming enriched in Fe³⁺ and Fe²⁺ at the expense of Mg, Cr, Al, thus moving toward the magnetite apex on the spinel prism. This outer rim, characterized by higher reflectance, is probably related to the metamorphic replacement of the primary mineralogy of the Luanga Complex. Zoned chromite crystals indicate an extensive exchange between divalent (Mg, Fe²⁺) cations and minor to none exchange between trivalent cations (Cr³⁺, Al³⁺ and Fe³⁺). This Mg-Fe zoning is interpreted as the result of subsolidus exchange of Fe²⁺ and Mg between chromite and coexisting silicates during slow cooling of the intrusion. A remarkable feature of chromitites from Luanga Complex is the occurrence of abundant silicate inclusions within chromite crystals. These inclusions show an adjacent inner rim with higher Cr# and lower Mg# (100 Mg/(Mg + Fe²⁺)) and Al# (100Al/(Cr + Al + Fe³⁺)). This compositional shift is possibly due to crystallization from a progressively more fractionated liquid trapped in the chromite crystal. Significant modification of primary cumulus composition of chromite, as indicated in our study for the Luanga Complex, is likely to be common in non-massive chromitites and the rule for disseminated chromites in mafic intrusions.     

Middle Carboniferous-Early Triassic eclogite–blueschist blocks within a serpentinite mélange at Port Macquarie,eastern Australia: Implications for the evolution of Gondwana's eastern margin

The New England Orogen of easternmost Australia is dominated by suites of Palaeozoic to earliest Mesozoic rocks that formed in supra-subduction zone settings at Gondwana's eastern margin. On the northern New South Wales coast at Rocky Beach, Port Macquarie, a serpentinite mélange carries rare tectonic blocks of low-grade, high-pressure, metamorphic rocks derived from sedimentary and igneous protoliths. Dominant assemblages are glaucophane + phengite ± garnet ± lawsonite ± calcite ± albite blueschists and lawsonite-bearing retrogressed garnet + omphacite eclogites. In some blocks with sedimentary protoliths, eclogite forms folded layers within the blueschists, which is interpreted as Mn/(Mn + Fe) compositional control on the development of blueschist versus eclogite assemblages. Review of previous studies indicates pressure–temperature conditions of 0.7–0.5 GPa and ≤ 450 °C. Three samples of high-pressure metasedimentary rocks contain Archaean to 251 ± 6 Ma (Permo-Triassic) zircons, with the majority of the grains being Middle Devonian to Middle Carboniferous in age (380–340 Ma). Regardless of age, all grains show pitting and variable rounding of their exteriors. This morphology is attributed to abrasion in sedimentary systems, suggesting that they are all detrital grains. New in situ metamorphic zircon growth did not develop because of the low temperature (≤ 450 °C) of metamorphism. The Permo-Triassic, Devonian and Carboniferous zircons show strong heavy rare earth element enrichment and negative europium anomalies, indicating that they grew in low pressure igneous systems, not in a garnet-rich plagioclase-absent high pressure metamorphic environment. Therefore the youngest of these detrital zircons provides the maximum age of the metamorphism. A titanite + rutile porphyroblast within an eclogite has a U–Pb age of 332 ± 140 Ma (poor precision due to very low U abundances of mostly < 1 p.p.m.) and provides an imprecise direct age for metamorphism. In the south of the Port Macquarie area, the Lorne Basin ≥ 220 Ma Triassic sedimentary and volcanic rocks unconformably overlie serpentinite mélange, and provide the minimum age of the high-pressure metamorphism. Our preferred interpretation is that the 251 Ma zircons are detrital and thus the Port Macquarie high-pressure metamorphism is constrained to the end of the Permian–Early Triassic. Emplacement of the serpentinite mélange carrying the Rocky Beach high-pressure rocks might have been due to docking of a Permian oceanic island arc (represented by the Gympie terrane in southern Queensland?) and an Andean-style arc at the eastern Australian margin (expressed in the New England Orogen by 260–230 Ma north-south orientated magmatic belts). Alternatively, if the 251 Ma grains are regarded as having grown in thin pegmatites, then the dominant Devonian–Carboniferous detrital population still indicates a maximum age for the high pressure metamorphism of ca. 340 Ma. A ≤ 340 Ma age of metamorphism would still be much younger than the previously suggested ca. 470 Ma (Ordovician) age, which was based on Ar–Ar dating of phengites.     

Percolation of diagenetic fluids in the Archaean basement of the Franceville basin

The Palaeoproterozoic Franceville basin, Gabon, is mainly known for its high-grade uranium deposits, which are the only ones known to act as natural nuclear fission reactors. Previous work in the Kiéné region investigated the nature of the fluids responsible for these natural nuclear reactors. The present work focuses on the top of the Archaean granitic basement, specifically, to identify and date the successive alteration events that affected this basement just below the unconformity separating it from the Palaeoproterozoic basin. Core from four drill holes crosscutting the basin–basement unconformity have been studied. Dating is based on U–Pb isotopic analyses performed on monazite. The origin of fluids is discussed from the study of fluid inclusion planes (FIP) in quartz from basement granitoids. From the deepest part of the drill holes to the unconformable boundary with the basin, propylitic alteration assemblages are progressively replaced by illite and locally by a phengite + Fe chlorite ± Fe oxide assemblage. Illitic alteration is particularly strong along the sediment–granitoid contact and is associated with quartz dissolution. It was followed by calcite and anhydrite precipitation as fracture fillings. U–Pb isotopic dating outlines three successive events: a 3.0–2.9-Ga primary magmatic event, a 2.6-Ga propylitic alteration and a late 1.9-Ga diagenetic event. Fluid inclusion microthermometry suggests the circulation of three types of fluids: (1) a Na–Ca-rich diagenetic brine, (2) a moderately saline (diagenetic + meteoric) fluid, and (3) a low-salinity fluid of probable meteoric origin. These fluids are similar to those previously identified within the overlying sedimentary rocks of the Franceville basin. Overall, the data collected in this study show that the Proterozoic–Archaean unconformity has operated as a major flow corridor for fluids circulation, around 1.9 Ga.     

The nature of geomagnetic anomalies in metamorphosed chromite-bearing dunites: a case study of the southern Klyuchevskoy complex,Central Urals

Chromite mineralization in metamorphosed dunites from the southern Klyuchevskoy dunite-harzburgite ultramafic complex (Central Urals) has been investigated using geomagnetic surveys along with laboratory studies of ore-forming and accessory spinels of the same genetic type. Magnetization in the study area is carried mainly by accessory Fe-Cr-spinel of a variable Fe^2 +(Cr_2 -xFe_x^3 +)O₄ composition. Metamorphism caused changes in element contents and in both crystal and magnetic structure of the primary nonmagnetic accessory spinel, unlike the almost fresh ore-forming spinel. Thus, ore bodies stand against their host rocks, which is a prerequisite for the use of geomagnetic surveys for exploration of podiform chromite deposits in dunite-harzburgite complexes. Ground magnetic surveys at a test site composed of faulted rocks bearing disseminated chromite mineralization in metamorphosed dunites resolved a chromite ore zone and a fault block boundary showing up as geomagnetic anomalies. Laboratory studies using high technologies (thermomagnetic analysis at 4 to 1000 K, as well as magnetic resonance and magnetic force spectroscopy) revealed, for the first time, magnetic clusters (superparamagnetic phases) in primary nonmagnetic accessory spinel, which are responsible for the magnetic properties of the host rocks. Microscale variations in Cr-spinel correlate with the geomagnetic anomalies recorded by field surveys at the test site._{© 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.}     

Monazite stability,composition and geochronology as tracers of Paleoproterozoic events at the eastern margin of the East European Craton (Taratash complex,Middle Urals)

The Precambrian Taratash complex (Middle Urals) is one of the rare windows into the Palaeoproterozoic and earlier history of the eastern margin of the East European Craton. Monazite from intensively deformed rocks within a major amphibolite-facies shear zone in the Taratash complex has been investigated by means of electron-probe microanalysis and laser-ablation SF-ICP-MS.Metamorphic and magmatic cores of monazite from metasedimentary and metagranitoid rocks yield U–Pb ages of 2244 ± 19 and 2230 ± 22 Ma (± 2 σ) and record a previously unknown pre-deformational HT-metamorphic event in the Taratash complex. Subsequent dissolution–reprecipitation of monazite, during shear zone formation under amphibolite-facies conditions, caused patchy zonation and chemical alteration of the recrystallised monazite domains, leading to higher cheralite and huttonite components. This process, which was mediated by a probable (alkali + OH)-bearing metamorphic fluid also caused a total resetting of the U–Pb-system. The patchy domains yield concordant U–Pb-ages between 2052 ± 16 and 2066 ± 22 Ma, interpreted as the age of the shear zone. In line with previously published ages of high grade metamorphism and migmatisation, the data may point to a Palaeoproterozoic orogenic event at the eastern margin of the East European Craton.Post-deformational fluid-induced greenschist-facies retrogression caused partial to complete breakdown of monazite to fluorapatite, REE + Y-rich epidote, allanite and Th-orthosilicate.The retrograde assemblages either form coronas around monazite, or occur as dispersed reaction zones, indicating that the REE, Y, and Th were mobile at least on the thin section scale. The greenschist-facies metamorphic fluid was aqueous and rich in Ca. Monazite affected by advanced breakdown responded to the retrogression by incorporating the cheralite or huttonite components during a fluid-induced dissolution–reprecipitation process. This event did not reset the U–Pb-system but caused partial Pb loss reflected by discordant U–Pb-dates.     

Mineralogy,fluid inclusions,and isotopes of the Cihai iron deposit,eastern Tianshan,NW China: Implication for hydrothermal evolution and genesis of subvolcanic rocks-hosted skarn-type deposits

Most skarn deposits are closely related to granitoids that intruded into carbonate rocks. The Cihai (>100 Mt at 45% Fe) is a deposit with mineral assemblages and hydrothermal features similar to many other typical skarn deposits of the world. However, the iron orebodies of Cihai are mainly hosted within the diabase and not in contact with carbonate rocks. In addition, some magnetite grains exhibit unusual relatively high TiO₂ content. These features are not consistent with the typical skarn iron deposit. Different hydrothermal and/or magmatic processes are being actively investigated for its origin. Because of a lack of systematic studies of geology, mineral compositions, fluid inclusions, and isotopes, the genetic type, ore genesis, and hydrothermal evolution of this deposit are still poorly understood and remain controversial.The skarn mineral assemblages are the alteration products of diabase. Three main paragenetic stages of skarn formation and ore deposition have been recognized based on petrographic observations, which show a prograde skarn stage (garnet-clinopyroxene-disseminated magnetite), a retrograde skarn stage (main iron ore stage, massive magnetite-amphibole-epidote ± ilvaite), and a quartz-sulfide stage (quartz-calcite-pyrite-pyrrhotite-cobaltite).Overall, the compositions of garnet, clinpyroxene, and amphibole are consistent with those of typical skarn Fe deposits worldwide. In the disseminated ores, some magnetite grains exhibit relatively high TiO₂ content (>1 wt.%), which may be inherited from the diabase protoliths. Some distinct chemical zoning in magnetite grains were observed in this study, wherein cores are enriched in Ti, and magnetite rims show a pronounced depletion in Ti. The textural and compositional data of magnetite confirm that the Cihai Fe deposit is of hydrothermal origin, rather than associated with iron rich melts as previously suggested.Fluid inclusions study reveal that, the prograde skarn (garnet and pyroxene) formed from high temperature (520–600 °C), moderate- to high-salinity (8.1–23.1 wt.% NaCl equiv, and >46 wt.% NaCl equiv) fluids. Massive iron ore and retrograde skarn assemblages (amphibole-epidote ± ilvaite) formed under hydrostatic condition after the fracturing of early skarn. Fluids in this stage had lower temperature (220°–456 °C) and salinity (8.4–16.3 wt.% NaCl equiv). Fluid inclusions in quartz-sulfide stage quartz and calcite also record similar conditions, with temperature range from 128° to 367 °C and salinity range from 0.2 to 22.9 wt.% NaCl equiv. Oxygen and hydrogen isotopic data of garnet and quartz suggest that mixing and dilution of early magmatic fluids with external fluids (e.g., meteoric waters) caused a decrease in fluid temperature and salinity in the later stages of the skarn formation and massive iron precipitation. The δ¹⁸O values of magnetite from iron ores vary between 4.1 and 8.5‰, which are similar to values reported in other skarn Fe deposits. Such values are distinct from those of other iron ore deposits such as Kiruna-type and magmatic Fe-Ti-V deposits worldwide. Taken together, these geologic, geochemical, and isotopic data confirm that Cihai is a diabase-hosted skarn deposit related to the granitoids at depth.     

In-situ LA-ICP-MS trace elemental analyses of magnetite: Fe–Ti–(V) oxide-bearing mafic–ultramafic layered intrusions of the Emeishan Large Igneous Province,SW China

The Taihe, Baima, Hongge, Panzhihua and Anyi intrusions of the Emeishan Large Igneous Province (ELIP), SW China, contain large magmatic Fe–Ti–(V) oxide ore deposits. Magnetites from these intrusions have extensive trellis or sandwich exsolution lamellae of ilmenite and spinel. Regular electron microprobe analyses are insufficient to obtain the primary compositions of such magnetites. Instead, laser ablation ICP-MS uses large spot sizes (~ 40 μm) and can produce reliable data for magnetites with exsolution lamellae. Although magnetites from these deposits have variable trace element contents, they have similar multi-element variation patterns. Primary controls of trace element variations of magnetite in these deposits include crystallography in terms of the affinity of the ionic radius and the overall charge balance, oxygen fugacity, magma composition and coexisting minerals. Early deposition of chromite or Cr-magnetite can greatly deplete magmas in Cr and thus Cr-poor magnetite crystallized from such magmas. Co-crystallizing minerals, olivine, pyroxenes, plagioclase and apatite, have little influence on trace element contents of magnetite because elements compatible in magnetite are incompatible in these silicate and phosphate minerals. Low contents and bi-modal distribution of the highly compatible trace elements such as V and Cr in magnetite from Fe–Ti oxide ores of the ELIP suggest that magnetite may not form from fractional crystallization, but from relatively homogeneous Fe-rich melts. QUILF equilibrium modeling further indicates that the parental magmas of the Panzhihua and Baima intrusions had high oxygen fugacities and thus crystallized massive and/or net-textured Fe–Ti oxide ores at the bottom of the intrusive bodies. Magnetite of the Taihe, Hongge and Anyi intrusions, on the other hand, crystallized under relatively low oxygen fugacities and, therefore, formed net-textured and/or disseminated Fe–Ti oxides after a lengthy period of silicate fractionation. Plots of Ge vs. Ga + Co can be used as a discrimination diagram to differentiate magnetite of Fe–Ti–(V) oxide-bearing layered intrusions in the ELIP from that of massif anorthosites and magmatic Cu–Ni sulfide deposits. Variable amounts of trace elements of magmatic magnetites from Fe–Ti–(P) oxide ores of the Damiao anorthosite massif (North China) and from Cu–Ni sulfide deposits of Sudbury (Canada) and Huangshandong (northwest China) demonstrate the primary control of magma compositions on major and trace element contents of magnetite.     

Petrological evolution of silica-undersaturated sapphirine-bearing granulite in the Paleoproterozoic Salvador–Curaçá Belt,Bahia, Brazil

The Salvador–Curaçá Belt, located in São Francisco Craton, Brazil, was subjected to granulite facies metamorphism during the Paleoproterozoic orogeny (c. 2.0 Ga). Well preserved in enclaves of silica-undersaturated sapphirine-bearing granulite occur in a charnockite outcrop located along a kilometric-scale shear zone. The sapphirine-bearing granulite preserves domains with distinct mineral assemblages that record interactions between melt and peritectic phases (orthopyroxene₁ + spinel₁ + biotite₁). Sapphirine was crystallized in the Si-poor cores of the enclaves, sillimanite and spinel–cordierite symplectites in the intermediate Si-rich domains between cores and margins, and garnet and quartz-bearing cordierite/biotite symplectites in Si-rich margins of the enclaves. Melt-rock interactions and metamorphism occurred at ultrahigh temperatures of 900–950 °C at 7.0–8.0 kbar pressures. The mineralogical evolution of the domains reflects not only the influence of changes in bulk composition in the equilibrium volume of the reactions but also P–T changes during orogeny evolution. Electron microprobe dating of monazite both in the sapphirine-bearing granulite and charnockite indicates UHT metamorphism timing at c. 2.08–2.05 Ga that is related to global Paleoproterozoic UHT metamorphic events that occurred during the Columbia supercontinent assembly.     

Zircon as the best mineral for P–T–time history of UHP metamorphism: A review on mineral inclusions and U–Pb SHRIMP ages of zircons from the Dabie–Sulu UHP rocks

Zircon is the best mineral to record the complex evolution history of ultrahigh-pressure (UHP) metamorphic rocks as mineralogical and geochemical tracers of UHP metamorphism are almost obliterated in matrix assemblages resulted from subsequent retrogression during exhumation. Zircons from Dabie–Sulu UHP rocks, including outcrop and core samples from drill holes ranging from 432 to 5158 m in depth contain abundant mineral inclusions of protolith, prograde, peak (UHP) and retrograde minerals in different domains; these minute inclusions were identified by laser Raman spectroscopy and/or electronic microprobe analysis. Systematic studies on inclusions in zircons from previous and present studies indicate that the Dabie–Sulu UHP terrane extends for >2000 km, is about 50 km wide, and has at least 10 km thick, probably the largest UHP terrane recognized in the world thus far. The internal structure of zircon revealed by cathodoluminescence (CL) imaging displays a distinct zonation, which comprises an inherited (magmatic or detrital) core, prograde, peak (UHP), and outmost retrograde domains, each with distinctive mineral inclusion assemblages. Low-pressure, igneous mineral inclusions are common in the inherited (magmatic or detrital) zircon cores. In contrast, quartz eclogite-facies inclusion assemblages occur in prograde domains, coesite eclogite-facies inclusion assemblages are preserved in UHP domains, and amphibolite-facies inclusion assemblages are enclosed in outmost retrograde rims. Parageneses and compositions of inclusion minerals preserved in distinct zircon domains were used to constrain the metamorphic P–T path of many Dabie–Sulu UHP rocks. The results indicate that Neoproterozoic supracrustal rocks together with minor mafic-ultramafic rocks were subjected to a prograde subduction-zone metamorphism at 570–690 °C and 1.7–2.1 GPa, and UHP metamorphism at 750–850 °C and 3.4–4.0 GPa, following by rapid decompression to amphibolite-facies retrograde metamorphism at 550–650 °C and 0.7–1.05 GPa. Sensitive high-resolution ion microprobe (SHRIMP) U–Pb spot analyses of the zoned zircons show four discrete and meaningful ages of the Dabie–Sulu metamorphic evolution: (1) Neoproterozoic protolith ages (800–750 Ma); (2) 246–244 Ma for early-stage quartz eclogite-facies prograde metamorphism; (3) 235–225 Ma for UHP metamorphism; and (4) 215–208 Ma for late-stage amphibolite-facies retrogression. This indicates that Neoproterozoic voluminous igneous protoliths of orthogneiss in response to the breakup of Rodinia supercontinent, together with various sedimentary rocks, and minor mafic-ultramafic intrusive and extrusive rocks, were subjected to coeval Triassic subduction to mantle depths and exhumation during the collision between the South China Block and North China Block. The estimated subduction and exhumation rates for the Dabie–Sulu UHP terrane would be up to 4.7–9.3 km Myr^?1 and 5.0–11.3 km Myr^?1, respectively. The zonal distribution of mineral inclusions and the preservation of index UHP minerals such as coesite imply that zircon is the best mineral container for each metamorphic stage, particular for supracrustal rocks as their metamorphic evolution and UHP evidence have been almost or completely obliterated. Similar conclusions have been documented elsewhere for other UHP terranes.     

Neoarchean suprasubduction zone arc magmatism in southern India: Geochemistry,zircon U-Pb geochronology and Hf isotopes of the Sittampundi Anorthosite Complex

The Sittampundi Anorthosite Complex (SAC) in southern India is one of the well exposed Archean layered anorthosite-gabbro-ultramafic rock associations. Here we present high precision geochemical data for the various units of SAC, coupled with zircon U-Pb geochronology and Hf isotopic data for the anorthosite. The zircon ages define two populations, the older yield a concordia age of 2541 ± 13 Ma, which is interpreted as the best estimate of the magmatic crystallization age for the Sittampundi anorthosite. A high-grade metamorphic event at 2461 ± 15 Ma is suggested by the upper intercept age of the younger zircon population. A Neoproterozoic event at 715 ± 180 Ma resulted in Pb loss from some of the metamorphic zircons. The magmatic age of the anorthosite correlates well with the timing of crystallization of the arc-related ~ 2530 Ma magmatic charnockites in the adjacent Salem Block, while the metamorphic age is synchronous with the regional metamorphic event. The geochemical data suggest that the rocks were derived from a depleted mantle source. Sub-arc mantle metasomatism of slab derived fluids and subsequent partial melting produced hydrous, aluminous basalt magma. The magma fractionated at depth to produce a variety of high-alumina basalt compositions, from which the anorthositic complex with its chromite-rich and amphibole-rich layers formed as cumulates within the magma chamber of a supra-subduction zone arc. The coherent initial¹⁷⁶Hf/¹⁷⁷Hf ratios and positive εHf values (1.7 – 4.5) of the magmatic zircons in the anorthosite are consistent with derivation of a rather homogeneous juvenile parent magma from a depleted mantle source. Our study further confirms that the southern part of the Dharwar Craton was an active convergent margin during the Neoarchean with the generation and emplacement of suprasubduction zone arc magmas which played a significant role in continental growth.     

Sulfur-poor intense acid hydrothermal alteration: A distinctive hydrothermal environment

A fundamentally distinct, sulfide-poor variant of intense acid (advanced argillic) alteration occurs at the highest structural levels in iron oxide-rich hydrothermal systems. Understanding the mineralogy, and geochemical conditions of formation in these sulfide-poor mineral assemblages have both genetic and environmental implications. New field observations and compilation of global occurrences of low-sulfur advanced argillic alteration demonstrates that in common with the sulfide-rich variants of advanced argillic alteration, sulfide-poor examples exhibit nearly complete removal of alkalis, leaving a residuum of aluminum-silicate + quartz. In contrast, the sulfur-poor variants lack the abundant pyrite ± other sulfides, hypogene alunite, Al-leached rocks (residual “vuggy” quartz) as well as the Au-Cu-Ag ± As-rich mineralization of some sulfur-rich occurrences. Associated mineralization is dominated by magnetite and/or hematite with accessory elements such as Cu, Au, REE, and P. These observations presented here indicate there must be distinct geologic processes that result in the formation of low-sulfur advanced argillic styles of alteration.Hydrolysis of magmatic SO₂ to sulfuric acid is the most commonly recognized mechanism for generating hypogene advanced argillic alteration, but is not requisite for its formation. Low sulfur iron-oxide copper-gold systems are known to contain abundant acid-styles of alteration (e.g. sericitic, chloritic), which locally reaches advanced argillic assemblages. A compilation of mapping in four districts in northern Chile and reconnaissance observations elsewhere show systematic zoning from near surface low-sulfide advanced argillic alteration through chlorite-sericite-albite and locally potassic alteration. The latter is commonly associated with specular hematite-chalcopyrite mineralization. Present at deeper structural levels are higher-temperature styles of sodic-calcic (oligoclase/scapolite – actinolite) alteration associated with magnetite ± chalcopyrite mineralization. These patterns are in contrast to the more sulfur-rich examples which generally zone to higher pyrite and locally alunite-bearing alteration.Fluid inclusion evidence from the systems in northern Chile shows that many fluids contain 25 to >50 wt% NaCl_eq with appreciable Ca, Fe, and K contents with trapping temperatures >300 °C. These geological and geochemical observations are consistent with the origin of the low-sulfur advanced argillic assemblages from HCl generated by precipitation of iron oxides from iron chloride complexes from a high-salinity fluid by reactions such as 3FeCl₂ + 4H₂O = Fe₃O₄ + 6HCl + H₂. Such HCl-rich (and relatively HSO₄⁼-poor) fluids can then account for the intense acid, Al-silicate-rich styles of alteration observed at high levels in some iron-oxide-coppe-gold (IOCG) systems. The geochemical differences between the presence of sulfide-rich and sulfur-poor examples of advanced argillic alteration are important to distinguishing between system types and the acid-producing capacity of the system, including in the modern weathering environment. They have fundamental implications for effective mineral exploration in low-sulfur systems and provide yet another vector of exposed alteration in the enigmatic IOCG clan of mineral deposits. Furthermore, understanding the geochemistry and mineralogy of this distinct geologic environment has applications to understanding the acid generating capacity and deleterious heavy metals associated with advanced argillic alteration.     

Metamorphic P–T path of mafic granulites from Eastern Hebei: Implications for the Neoarchean tectonics of the Eastern Block,North China Craton

This study documents the metamorphic evolution of mafic granulites from the Eastern Hebei Complex in the Eastern Block of the North China Craton. Mafic granulites from Eastern Hebei occur as boudins or enclaves within Neoarchean high-grade TTG gneisses. Petrographic observations reveal three characteristic metamorphic mineral assemblages in the mafic granulites: the pre-peak hornblende + plagioclase + ilmenite + quartz + sphene assemblage (M₁) existing as mineral inclusions within coarse-grained peak assemblage (M₂) represented by garnet + clinopyroxene + orthopyroxene + plagioclase + hornblende + ilmenite + quartz, and post-peak assemblage (M₃) marked by garnet + quartz ± ilmenite symplectites surrounding the peak pyroxene and plagioclase. Based on pseudosection modeling calculated in the NCFMASHTO model system using the program THERMOCALC, P–T conditions of the pre-peak (M₁), peak (M₂) and post-peak (M₃) assemblages are constrained at 600–715 °C/6.0 kbar or below, 860–900 °C/9.6–10.3 kbar, and 790–810 °C/9.6–10.4 kbar, respectively. These P–T estimates, combined with their mineral compositions and reaction relations, define an anticlockwise P–T path incorporating isobaric cooling subsequent to the peak medium-pressure granulite-facies metamorphism for the mafic granulites from Eastern Hebei. Such an anticlockwise P–T path suggests that the end-Neoarchean metamorphism of the Eastern Hebei Complex correlated closely with underplating and intrusion of voluminous mantle-derived magmas. In conjunction with other geological considerations, a mantle-plume model is favored to interpret the Neoarchean tectonothermal evolution of the Eastern Hebei Complex and other metamorphic complexes in the Eastern Block. The prograde amphibolite-facies metamorphism (M₁) was initiated due to the upwelling of the relatively cooler mantle plume head, followed by the peak medium-pressure granulite-facies metamorphism (M₂) as triggered by the uprising hotter plume “tail”, and finally when plume activity ceased, the heated metamorphic crust experienced nearly isobaric cooling (M₃).     

Base metal mineral segregation and Fe-Mg exchange inducing extreme compositions of olivine and chromite from the Xiadong Alaskan-type complex in the southern part of the Central Asian Orogenic Belt

The Xiadong Alaskan-type complex shares much in common with typical Alaskan-type complexes worldwide, while showing some unique features in terms of mineral compositions. Olivine from the Xiadong dunites is characterized by extremely high Fo component of 91.7–96.7 and anomalously negative correlation of Fo with NiO, while chromite is featured by high 100 × Fe³⁺/(Fe³⁺ + Cr + Al) (>70), high 100 × Fe²⁺/(Fe²⁺ + Mg) (>70), high 100 × Cr/(Cr + Al) (>90), low MnO (<0.6 wt%) and TiO₂ contents (<0.5 wt%). To investigate these particular features, we conducted petrographic observation and mineral composition analyses for the Xiadong dunite. A number of Fe and/or Ni sulfides and alloys occurring as inclusions in olivine and chromite indicate that base metal mineral segregation took place prior to crystallization of olivine and chromite and probably induced Fe and Ni depletions in olivine. The FeO and MgO variations in profile analyses from chromite to adjacent olivine are compatible with Fe-Mg exchange. The diffusion mechanism of Fe from olivine to chromite and Mg from chromite to olivine may have elevated both Fo of olivine and 100 × Fe²⁺/(Mg + Fe²⁺) ratio of chromite and further enhanced the decoupling of Fo and NiO in olivine. We thus suggest that base metal mineral segregation and Fe-Mg exchange play important roles in the extreme compositions of the Xiadong dunite. The Ni depletion of olivine and degree of Fe-Mg exchange between olivine and chromite may be used as indicators of mineralization in mafic-ultramafic intrusions.     

Chromite typology and composition characterized through multispectral specular reflectance

Determination of multispectral specular reflectance is an important tool for ore identification in reflected light microscopy, and may be used for automated characterization of ores. However, reflectance values can be affected by compositional variations in a way that is seldom understood. The aim of the present work is to investigate this problem in chromite, an ore whose composition may show typically large natural variations as member of the spinel family, and whose relatively high reflectance variations are apparently unpredictable. For this research, eighteen samples of chromian spinel covering a large range of compositions in the base of the Hagerthy prism were selected for microprobe analysis and reflectance measure on polished sections. The samples belong to a variety of deposits and types (ultramafic massif, ophiolitic, and metamorphosed types: Ojén and Ronda Massifs, Spain; Mayarí, Moa-Baracoa, Camagüey and Sagua de Tánamo, Cuba; Golyamo Kemenyane, Avren and Yakovitsa, Bulgaria). The specular reflectances are characterized as multispectral values, measured at thirteen intervals (50 nm each) in the VNIR region (Visible and Near-Infrared: 400–1000 nm), using the automated CAMEVA System.The relationship between compositional and reflectance values is studied by multivariate analysis and subsequently tested on independent samples. For this purpose, the samples have been grouped in two sets: a larger population of fifteen samples constitutes the initial data set for mathematical processing, while a selection of three samples of widely differing compositions is used to test the resulting functions, so as to gain a critical appraisal of their validity.The results obtained show that the specular reflectance of chromite depends on composition and can be used to estimate compositional parameters, as #Cr = Cr/(Cr + Al) or #Mg = Mg/(Mg + Fe²⁺), but this relationship is complex and does not allow simple direct determinations, due to the multiplicity of possible changes and coupled substitutions (e.g. Al–Cr–Fe³⁺, or Mg-Fe²⁺, Ni, Zn, Ti …) in the chromian spinel family. On the other hand, the relationship of chromite composition with deposit type is also detectable through the reflectance values, but with a higher uncertainty. Reflectance increases with increasing Cr and Fe³⁺ contents in all cases, but the relationship of reflectance with Mg seems to be specific of the deposit type: while reflectance increases with increasing #Mg in the Ojén chromites, it shows the opposite behaviour in podiform chromites. Although these results should be regarded as preliminary until further studies on larger sample populations can be achieved, they are suggestive of possible practical applications for exploration, e. g. to approach Cr-spinel compositions or chromite deposit typology, early in an exploration campaign, by inexpensive reflected light determination of specular reflectance in a few samples.     

Boron,sulphur and copper isotope systematics in the orogenic gold deposits of the Archaean Hattu schist belt,eastern Finland

The Hattu schist belt is located in the western part of the Archaean Karelian domain of the Fennoscandian Shield. The orogenic gold deposits with Au–Bi–Te geochemical signatures are hosted by NE–SW, N–S and NW–SE oriented shear zones that deform 2.76–2.73 Ga volcanic and sedimentary sequences, as well as 2.75–2.72 Ga tonalite–granodiorite intrusions and diverse felsic porphyry dykes. Mo–W mineralization is also present in some tonalite intrusions, both separate from, and associated with Au mineralization. Somewhat younger, unmineralized leucogranite intrusions (2.70 Ga) also intrude the belt. Lower amphibolite facies peak metamorphism at 3–5 kbar pressures and at 500–600 °C temperatures affected the belt at around 2.70 Ga and post-date hydrothermal alteration and ore formation. In this study, we investigated the potential influence of magmatic-hydrothermal processes on the formation of orogenic gold deposits on the basis of multiple stable isotope (B, S, Cu) studies of tourmaline and sulphide minerals by application of in situ SIMS and LA ICP MS analytical techniques.Crystal chemistry of tourmaline from a Mo–W mineralization hosted by a tonalite intrusion in the Hattu schist belt is characterized by Fe^3 +–Al^3 +-substitution indicating relatively oxidizing conditions of hydrothermal processes. The range of δ¹¹B data for this kind of tourmaline is from − 17.2‰ to − 12.2‰. The hydrothermal tourmaline from felsic porphyry dyke swith gold mineralization has similar crystal chemistry (e.g. dravite–povondraite compositional trend with Fe^3 +–Al^3 + substitution) and δ¹¹B values between − 19.0‰ and − 9.6‰. The uvite–foitite compositional trend and δ¹¹B ‰ values between − 24.1% and − 13.6% characterize metasomatic–hydrothermal tourmaline from the metasediment-hosted gold deposits. Composition of hydrothermal vein-filling and disseminated tourmaline from the gold-bearing shear zones in metavolcanic rocks is transitional between the felsic intrusion and metasedimentary rock hosted hydrothermal tourmaline but the range of average boron isotope data is essentially identical with that of the metasediment-hosted tourmaline. Rock-forming (magmatic) tourmaline from leucogranite has δ¹¹B values between − 14.5‰ and − 10.8‰ and the major element composition is similar to that of the metasediment-hosted tourmaline.The range of δ³⁴S_VCDT values measured in pyrite, chalcopyrite and pyrrhotite is from − 9.1 to + 8.5‰, which falls within the typical range of sulphur isotope data for Archaean orogenic gold deposits. In the Hattu schist belt, positive δ³⁴S_VCDT values characterize metasediment-hosted gold ores with sulphide parageneses dominated by pyrrhotite and arsenopyrite. The δ³⁴S_VCDT values are both positive and negative in ore mineral parageneses within felsic intrusive rocks in which variable amounts of pyrrhotite are associated with pyrite. Purely negative values were only recorded from the pyrite-dominated gold mineralization within metavolcanic units. Therefore the shift of δ³⁴S_VCDT values to the negative values reflects precipitation of sulphide minerals from relatively oxidizing fluids. The range of measured δ⁶⁵Cu_NBS978 values from chalcopyrite is from − 1.11 to 1.19‰. Positive values are common for mineralization in felsic intrusive rocks and negative values are more typical for deposits confined to metasedimentary rocks. Positive and negative δ⁶⁵Cu_NBS978 values occur in the ores hosted by metavolcanic rocks. There is no correlation between sulphur and copper isotope data obtained in the same chalcopyrite grains.Evaluation of sulphur and boron isotope data together and comparisons with other Archaean orogenic gold provinces supports the hypothesis that the metasedimentary rocks were the major sources of sulphur and boron in the orogenic gold deposits in the Hattu schist belt. Variations in major element and boron isotope compositions in tourmaline, as well as in the δ³⁴S_VCDT values in sulphide minerals are attributed to localized involvement of magmatic fluids in the hydrothermal processes. The results of copper isotope studies indicate that local sources of copper in orogenic gold deposits may potentially be recognized if the original, distinct signatures of the sources have not been homogenized by widespread interaction of fluids with a large variety of rocks and provided that local chemical variations have been too small to trigger changes in the oxidation state of copper during hydrothermal processes.     

Relict sapphirine + kyanite and spinel + kyanite associations in pyropic garnet from the eastern Sør Rondane Mountains,East Antarctica

Sapphirine, spinel, orthoamphibole ± quartz and kyanite are included in porphyroblastic garnet in biotitic gneiss enclosed in a lens of metamorphosed ultramafic rocks in the Cambrian granulite-facies metamorphic complex of the eastern Sør Rondane Mountains, Queen Maud Land, East Antarctica. A bulk analysis of the biotitic gneiss reveals features characteristic both of ultramafic rocks, e.g., high contents of Cr and Ni, and of metasomatism associated with fluids having a crustal source, e.g., relatively elevated contents of Li, Rb, Mo, Cs, Ba, Tl, and Pb. This trace element signature is consistent with the biotitic gneiss being a slice of blackwall skarn that developed between harzburgite and the enclosing biotite–hornblende ± garnet ± orthopyroxene ± clinopyroxene gneiss and was subsequently infolded or inserted by faulting. The matrix assemblage of the biotitic gneiss is garnet + corundum + hercynite + biotite +plagioclase +allanite + zircon. The included associations (all with biotite and rutile) are (1) sapphirine + kyanite, (2) spinel + kyanite, (3) sapphirine +spinel, (4) kyanite, and (5) orthoamphibole + plagioclase ± quartz. The garnet porphyroblasts are compositionally zoned with broad pyropic cores (X_Mg(=Mg / (total Fe + Mg)) = 0.45–0.55) surrounded by Fe-richer rims (X_Mg ≈ 0.3 at the outermost part). The garnet cores preserve compositions homogenized under peak conditions of the granulite-facies metamorphism (760–800 °C and 7–8 kb), whereas the Fe-enriched rims are attributed to an amphibolite-facies overprint at 500–600 °C. Theoretical calculations of garnet + corundum + spinel ± sapphirine + kyanite equilibria in the FMAS system constrain possible P–T conditions for a sapphirine + spinel + kyanite + garnet (X_Mg ≈ 0.55) assemblage to form near 450 °C and 4 kb on the prograde path. In contrast, a modified calibration of the Das et al. (Das, K., Fujino, K., Tomioka, N., Miura, H., 2006. Experimental data on Fe and Mg partitioning between coexisting sapphirine and spinel: an empirical geothermometer and its application. Eur. J. Mineral., 18, 49–58). sapphirine–spinel thermometer gives 860–895 °C for the included associations; pressures would have to be at least 12 kb to stabilize kyanite at these temperatures. Neither estimate is satisfactory and the stability range of kyanite + spinel–hercynite ± sapphirine assemblages remains an unresolved question. The Sør Rondane Mountains constitute the third region for kyanite, sapphirine and spinel–hercynite inclusions in garnet in granulite-facies rocks of the Neoproterozoic–Cambrian orogen extending from the Sør Rondane Mountains to Lützow–Holm Bay and onward to Sri Lanka, southern India and southern Madagascar, and thus determining the stability range of kyanite + spinel–hercynite ± sapphirine is critical for deducing the tectonic evolution of this orogen.     

High-pressure and ultrahigh-temperature metamorphism at Komateri,northern Madurai Block,southern India

We report for the first time the evidence for prograde high-pressure (HP) metamorphism preceding a peak ultrahigh-temperature (UHT) event in the northernmost part of the Madurai Block in southern India. Mg–Al-rich Grt–Ged rocks from Komateri in Karur district contain poikiloblastic garnet with numerous multi-phase inclusions. Although most of the inclusion assemblages are composed of gedrite, quartz, and secondary biotite, rare staurolite + sapphirine and spinel + quartz are also present. The X_Mg (=Mg/[Fe+Mg]) of staurolite (0.45–0.49) is almost consistent with that reported previously from Namakkal district in the Palghat–Cauvery Shear Zone system (X_Mg = 0.51–0.52), north of the Madurai Block. The HP event was followed by peak UHT metamorphism at T = 880–1040 °C and P = 9.8–12.5 kbar as indicated by thermobarometric computations in the Grt–Ged rock and associated mafic granulite. Symplectic intergrowth of spinel (X_Mg = 0.50–0.59, ZnO < 1.7 wt.%) and quartz, a diagnostic indicator of UHT metamorphism, probably formed by decompression at UHT conditions. The rocks subsequently underwent retrograde metamorphism at T = 720–760 °C and P = 4.2–5.1 kbar. The P–T conditions and clockwise exhumation trajectory of the Komateri rocks, comparable to similar features recorded from the Palghat–Cauvery Shear Zone system, suggest that the Madurai Block and the Palghat–Cauvery Shear Zone system underwent similar HP and UHT metamorphic history probably related to the continent–continent collision during the final stage of amalgamation of Gondwana supercontinent.