Amphibolite Facies Metamorphism Research Articles

Metamafic dyke and sill swarms in the Dom Feliciano Belt: Insights for post-collisional strike-slip tectonics and fluid-assisted metamorphism

The oblique collision of the Congo, Kalahari, and Rio de la Plata cratons resulted in the formation of the Dom Feliciano Belt that evolved from rifting, drifting, and amalgamation between ca. 1.0 and 0.5 Ga. The post-collisional stage of the orogenic event (ca. 600 – 530 Ma) is marked by lithospheric scale shear-zones, voluminous magmatism, and volcano-sedimentary sequences. However, the tectonic regimes, metamorphism and the development of space-forming structures are still debated. We present a comprehensive petrographic, mineralogical, and geochemical study of Ediacaran metamafic dyke and sill swarms from the easternmost portion of the São Gabriel Terrane (central Dom Feliciano Belt, southern Brazil). Mafic swarms and lamprophyres from two different areas in the syntectonic Caçapava do Sul aureole show heterogeneous microstructures, metamorphic degree and whole-rock composition due to heterogeneous metamorphism, deformation, and metasomatism. We aimed at deciphering the origin of the sheeted intrusions and the metamorphic processes that led to such heterogeneities. Trace-element composition indicates that the melts were formed at high depths and went through significant crustal assimilation during the ascent through the crust either in a continental arc or an island arc setting. Greenschist facies metamorphism and upper-crust brittle deformation partially preserved subvolcanic textures in metamafic dykes at the outer portion of the aureole. In opposition, amphibolite facies metamorphism with a significant fluid flow component resulted in highly modified and heterogeneous metamafic sills at the inner portion of the aureole and at high strain domains of the Caçapava do Sul Shear Zone. Heterogeneities in different areas are caused by the distance to batholith and to high-strain domains of the shear zone while variations within the same area result from heterogeneous and localized fluid flow during metamorphism. We suggest that the mafic swarms represent markers of the initial stage of the development of the shear zone and were metamorphosed due the subsequent syntectonic emplacement of Caçapava do Sul Granitic Complex. Remarkably, we note that heterogeneities in coeval units that often hinder the understanding of the Precambrian stratigraphy can be explained by heterogeneous fluid-assisted metamorphism, especially near shear zones and syntectonic intrusions.

Metamorphic patterns and zircon U–Pb dating of the Xilingol complex in Inner Mongolia, China: Implications for rifting metamorphism and tectonic evolution in eastern Central Asian Orogenic Belt

The Xilingol complex is one of the maximal outcropped metamorphic terranes in the eastern segment of the Central Asian Orogenic Belt. Detailed petrological and geochronological study of the Xilingol complex will provide valuable insights into the orogenic process and closure of the Paleo-Asian Ocean. We present a comprehensive study of petrology, pseudosection modelling, and zircon U-Pb geochronology of representative rocks collected in Daqing Pasture, including two-mica quartz schist, epidote amphibolite, biotite plagioclase gneiss and quartzite. The Xilingol complex in Daqing Pasture can be preliminarily divided into the north and south segments based on field and petrological observations. The north segment comprises two-mica quartz schist, biotite quartz schist, amphibolite and minor epidote amphibolite, whereas the south segment consists mainly of biotite gneiss, amphibolite and quartzite, with varying degrees of migmatization developed. Combined pseudosection modelling and composition isopleths of garnet and biotite constrain a peak P–T condition of ~660 °C, ~5.5 kbar for the garnet-bearing two-mica quartz schist in the north segment, corresponding to low amphibolite facies metamorphism. The garnet-bearing biotite plagioclase gneisses in the south segment are interpreted to experience high amphibolite facies metamorphism at a peak condition of ~750–790 °C, ~5.5–8.0 kbar, followed by nearly isobaric cooling. The variations of peak P–T estimates within Daqing Pasture, coupled with the spatial heterogeneities of lithological associations between different outcropped terranes, reveal an overall northward decreasing metamorphic grade for the Xilingol complex. Metamorphic zircons from three biotite plagioclase gneisses in the south segment yield apparent 206 Pb/ 238 U age spreads of 397–348 Ma, 363–294 Ma and 341–301 Ma, respectively. These partially overlapping zircon ages are an indication of discrete metamorphic events involving localized thermal metamorphism at depth in the Middle Devonian and long-lasting Buchan-type metamorphism in the Carboniferous, with intermediate data probably related to a mixture of multiple growth domains. Five metamorphic zircons with low Th/U ratios from the epidote amphibolite in the north segment yield a concordia age of 284.8 ± 1.9 Ma, reflecting the diachronism of the Late Paleozoic Buchan-type metamorphism. Summarizing all U-Pb data of inherited magmatic zircons and metamorphic zircons, a distinct U-Pb age continuum is retrieved to elucidate a successive process from sedimentary diagenesis of forearc sedimentary formations to deep burial in the Early Devonian, and then to subsequent multi-stage metamorphic overprinting from the Middle Devonian to the Permian. The isograd patterns, P–T–t paths (clockwise paths and long-term duration) and spatial distribution of metamorphic terranes of the Xilingol complex are more compatible with an active continent rift setting rather than paired metamorphic belts in the arc-basin system, revealing post-collisional extension of the Early-Middle Paleozoic orogenic lithosphere of eastern CAOB in the Late Paleozoic. • The Xilingol complex underwent long-lasting Buchan-type metamorphism in the Carboniferous to Permian. • Metamorphic patterns of the Xilingol complex are compatible with continent rift setting. • Zircon U-Pb age continuum reveals a successive process from diagenesis to deep burial to multiple metamorphic episodes.

Link between melt-impregnation and metamorphism of Atlantis Massif peridotite (IODP Expedition 357)

IODP Expedition 357 drilled 17 shallow sites scattered over ~ 10 km in the west to east spreading direction across the Atlantis Massif oceanic core complex (OCC, MAR, 30 ºN). Mantle exposed in the footwall of the Atlantic Massif OCC is nearly wholly serpentinized (80–100%) harzburgite and subordinate dunite. A recent whole-rock chemistry study by Whattam et al. (Chemical Geology 594. 10.1016/j.chemgeo.2021.120681, 2022) subdivides Atlantis Massif peridotites into: Type I fluid–rock dominated serpentinite, which exhibits almost nil evidence of melt-impregnation or silica metasomatism; Type II melt–rock dominated, mafic melt-impregnated serpentinite; and Type III melt–rock dominated Si-metasomatized serpentinite. In this study, on the basis of EPMA, three kinds of Cr–spinel are distinguished in Expedition 357 serpentinite: (I) primary, unmetamorphosed mantle array, (II) low-Ti metamorphosed, and (III) high-Ti melt reacted. All Cr–spinel of western site Type I serpentinite is unmetamorphosed (n = 34) and comprises 68% of all unmetamorphosed Cr–spinel. Metamorphosed Cr–spinel (n = 100) are the most abundant and occur in the central and eastern site Type II and Type III serpentinite, whereas melt-reacted Cr–spinel and chromite are restricted to the central sites and occur predominantly in serpentinized dunite. Estimates of the degree of melt extraction of Type I serpentinite using F = 10ln(spinel Cr#) + 24 are ~ 9–17%. Fugacity calculations of primary, unmetamorphosed Cr–spinel yield Δlog(fO2)FMQ of − 1.7 to + 1.0 and calculations using olivine–spinel Mg–Fe exchange thermometry yield a mean closure temperature of 808 ± 39 °C. Mafic melt-impregnation resulted in Cr–spinel with anomalously high TiO2 of 0.27–0.68 wt.% and production of Ti-rich chromite (up to 1.23 wt.% TiO2). Greenschist facies metamorphism (< 500 °C) resulted in Mg–Fe2+ exchange between Cr–spinel and forsterite and anomalously low Cr–spinel Mg#; higher degrees of amphibolite facies metamorphism (~ 500–700 °C) also resulted in anomalously high Cr# due to Al–Cr exchange. As has been previously established, significant Al loss from chromite cores above 550 °C is the result of equilibration with fluids in equilibrium with chlorite, which may be valid for our samples. On the basis of Cr–spinel vs. whole-rock compositions, a clear relationship exists between melt-impregnation and metamorphism of central and eastern serpentinite, which we postulate to be the result of heat associated with magma injection and subsequent localized contact metamorphism. To our knowledge, such a relation between mafic melt-impregnation of peridotite and metamorphism (of peridotite) has not previously been established in general and specifically for the Atlantis Massif peridotite. Closure temperatures of 440–731 °C of metamorphosed Cr–spinel approximate greenschist to amphibolite facies metamorphic conditions.

Genetic linkage between carbonaceous materials and Triassic gold deposits in the Liaodong Peninsula, northeastern North China Craton: Insights from Raman spectroscopy and C[sbnd]O isotope analysis

The Qingchengzi gold ore-concentration district, in the Liaodong Peninsula, northeastern North China Craton, is dominated by altered rock-type gold deposits and hosted within metamorphic rocks of the Paleoproterozoic Liaohe Group. The deposits are mainly controlled by a low-angle, north-dipping thrust structural system, containing numerous carbonaceous materials (CMs) and syn-deformational ore-bearing quartz veins and adjacent clastic rocks. Raman spectroscopy and CO isotope analysis were performed on CM samples from the fault zone to decipher their possible origins and potential genetic links with gold mineralization. Raman spectra analysis distinguishes three types of CM with low (CML), medium (CMM) and high (CMH) crystallinity and maturity. The estimated Raman-derived temperatures of CML range from 308 °C to 401 °C corresponding to greenschist- to blueschist-facies metamorphism. Large temperature variations of 363–678 °C for CMM and 442–664 °C for CMH suggest higher metamorphic grades from low blueschist to amphibolite-facies. The organic carbon isotopic valuess (δ13Corg = -20.9 ‰ – –23.1 ‰) suggest an organic carbon source, mixing with carbonate wall rocks characterized by the δ13Ccarb of −1.92 ‰ to −6.22 ‰. Combined with the Raman spectrum, the CMs are formed by the demethanation of the organic matter during the greenschist- to amphibolite-facies metamorphism. The disseminated CML coexists with Au-bearing quartz veins and is inferred to have facilitated the migration of Au in the fault zone as a solid lubricant and the precipitation of Au from hydrothermal fluids as a reductant. CMM and CMH are recognized in medium- to high-grade metamorphism with higher temperatures and occur with fewer Au-bearing pyrite or quartz veins, and thus they are unrelated to gold mineralization. In summary, the disseminated CML with low crystallinity contributes to the formation of the Triassic gold deposits and serves as a potential indicator for further prospecting.

Tonian–Ediacaran Tectonometamorphic History of the Sa'al Complex, Sinai (Egypt): Implications for the Tectonostratigraphic Framework of the Northern Arabian–Nubian Shield

AbstractThe Sa'al Metamorphic Complex (SMC; southern Sinai) encompasses the oldest arc rocks in the Arabian–Nubian Shield, comprising two non‐consanguineous metavolcanic successions (the Agramiya Group and the Post‐Ra'ayan Formation) separated by the metasediments of the Ra'ayan Formation. It experienced three distinct deformational events (D1–D3) and two low‐medium grade regional metamorphic events (M1–M2). The Agramiya Group and the Ra'ayan Formation experienced all tectonometamorphic events (D1–D3 and M1–M2), whereas the Post‐Ra'ayan volcanic rocks were only affected by the D3 and M2 events. D1 is an extensional event and is connected to the late Rodinia break‐up (∼Tonian; 900–870 Ma). The M1 metamorphism variably affected the older Agramiya Group, the rhyolitic tuffs experiencing lower to upper greenschist facies conditions and the basic and intermediate volcanic rocks undergoing amphibolite facies metamorphism. The Ra'ayan Formation metasediments experienced upper greenschist to amphibolite facies metamorphism. The upper greenschist facies M2 affected the youngest Post‐Ra'ayan volcanic rocks and other stratigraphic successions. The compressive D2 and D3 events were coeval with the accretion of dismembered terranes in the assembly of Gondwana. D2 can be linked to the Tonian–Cryogenian arc‐arc assembly (∼880–760 Ma; in Elat and Sinai), whereas D3 and the accompanying M2 is constrained to 622–600 Ma (Ediacaran).

Structural evolution and U-Pb geochronology of the metasedimentary Nemiscau subprovince, Canada: implications for Archean tectonics in the Superior Province

The Nemiscau subprovince is a metasedimentary rocks-dominated sequence of the Archean eastern Superior Province. It is bounded by the gneissic and tonalite-trondhjemite-granodiorite (TTG) rocks-dominated La Grande and Opatica subprovinces. The Nemiscau consists of variably migmatized metasedimentary rocks and felsic to intermediate gneisses and plutonic suites. Mafic-to-ultramafic metavolcanic rocks occur along its northern and southern boundaries. Previous structural and metamorphic studies suggested that it was the result of subduction-related, accretionary and collisional tectonics with adjacent plutonic terranes during the Kenorean orogeny. This study integrates various sets of structural, metamorphic, and U-Pb geochronological data suggesting a long-lasting tectonometamorphic evolution between ca. 2843 and 2598 Ma. Four tectonometamorphic events have been recognized. The first event (D1) is only locally preserved and occurred shortly after the deposition of the oldest volcanic sequences of the Nemiscau ( ca. 2756–2736 Ma) under amphibolite facies metamorphic conditions (M1). The kinematic analysis of shear zones bounding the supracrustal sequences of both the Nemiscau and La Grande subprovinces suggests their relative crustal sinking as compared to TTG of the La Grande and Opatica subprovinces, shortly after deposition (at ≤2724 and ≤2706 Ma, respectively). Sedimentation was followed by regional dip-slip-dominated (D2) deformation between ca. 2704 and 2671 Ma, coeval with extensive high-grade granulite facies metamorphism (M2) and anatexis in the Nemiscau subprovince from ca. 2697 to 2685 Ma, followed by exhumation between ca. 2677 and 2671 Ma. This D2 event was followed by regional-scale dextral strike-slip shearing (D3) from ca. 2658 until 2621 Ma at amphibolite facies metamorphism (M3). The youngest deformation event, D4, was accompanied by strain localization along brittle-to-ductile conjugated shear zones and waning crustal cooling from amphibolite to greenschist facies conditions at ca. 2598 Ma and younger. It is suggested that the Nemiscau, La Grande, and Opatica subprovinces represent a single composite terrane, and that their mutual boundaries do not correspond to “collisional sutures” between different crustal blocks or microcontinents. The Nemiscau subprovince is interpreted as a sedimentary sequence unconformably overlying a ca. 2760–2756 Ma (and older) basement made up of volcanic and crosscutting TTG. In terms of tectonic evolution, the overall structural architecture and isotopic ages of the Nemiscau, La Grande, and Opatica subprovinces support a tectonic model in which the vertical transfer of crustal material occurred during the early stages (D2) of regional deformation, which evolved into a predominant lateral crustal flow of the ductile crust during later stages (D3–D4). A non-uniformitarian tectonic model for the Archean more adequately accounts for synchronous vertical and horizontal tectonism as preserved in the Nemiscau basin.

ORIGIN OF THE “COLD” (LOW-TEMPERATURE) GRANULITES IN THE BELOMORIAN MOBILE BELT: THE ROLE OF CARBON DIOXIDE IN METAMORPHIC FLUID

In the Belomorian mobile belt manifestations of the granulitic mineral assemblages occurs among the amphibolite facies rocks. Thermobarometric calculations display that some granulitic scapolite-bearing assemblages in mafic rocks were formed during high-temperature, high-pressure amphibolite facies metamorphism. These granulitic assemblages were resulted from the infiltration of a CO 2 -rich fluid with low a H2O through amphibolites.

Trench retreat recorded by a subduction zone metamorphic history

Abstract Upper amphibolite-facies metamorphism in subduction zone rocks may occur under exceptional tectonic settings. Differentiating competing mechanisms for its occurrence requires carefully integrated, high-resolution thermobarometric and geochronologic studies of mélange rocks with well-defined field relationships. We present new pressure, temperature, and age data from the classic Cretaceous Catalina Schist in southern California (USA) that allow us to establish a plausible model for its high-temperature metamorphic history. Our results indicate that garnet-amphibolite blocks in the structurally highest amphibolite-facies mélange preserve evidence of three stages of tectonic evolution: (1) prograde lawsonite eclogite-facies metamorphism that peaked at 2.4–2.7 GPa with temperatures &amp;gt;580 °C during fixed-trench subduction (120–115 Ma); (2) post-peak epidote eclogite-facies metamorphism followed by amphibolite-facies metamorphism at 1.4–1.3 GPa with temperatures of 740–790 °C during trench retreat (115–105 Ma); and (3) isothermal decompression (1.3 GPa to &amp;lt;1.0 GPa at temperatures of ~780 °C) and cooling during trench advance and slab-flattening subduction (ca. 105–100 Ma). Our model implies the presence of a continuous Cordilleran subduction system in the Cretaceous, which had varying tectonic regimes through episodes of trench retreat/advance and slab shallowing/steepening that, in turn, dictated the development of the Cordilleran arc system.

Orogenic belt resulting from ocean-continent collision

Abstract Orogenic belts have been thought to form through plate convergence, involving subduction of oceanic lithosphere at continental margins (accretionary orogens), which may ultimately lead to ocean closure and continent-continent collision (collisional orogens). Intraplate orogens away from plate margins have been known, but the mechanisms controlling their evolution are poorly understood. The South China craton, including the Yangtze and Cathaysia blocks, underwent a Paleozoic orogenesis that formed a &amp;gt;500-km-wide orogenic belt with widespread granitoids that are unconformably overlain by Devonian cover sequences. The pre-Devonian basement rocks were subjected to strong deformation and greenschist-to amphibolite-facies metamorphism at 460–400 Ma. Paleozoic magmatism was characterized by voluminous crustally derived Silurian granitoids associated with incorporation of ancient crustal materials at 450–440 Ma and addition of juvenile mantle-derived melts at 420–410 Ma. Based on the absence of arc-like magmatism and the existence of ophiolites in the West Cathaysia terrane, geochemical evidence that oceanic crust existed beneath the East Cathaysia terrane, and geophysical evidence of contrasting lithospheres on both sides and two discrete slabs beneath their fault boundary, we propose that this Paleozoic orogenic belt was formed by collision between the two terranes that was driven by far-field forces during the assembly of Gondwana, and the East Cathaysia terrane represents oceanic lithosphere that was overthrusted by the continental crustal materials of the West Cathaysia terrane. Numerical modeling shows that this type of collision can explain the dynamics of the Paleozoic orogenesis in the South China craton and may be a mechanism for some orogens in which subduction-related igneous and metamorphic rocks are lacking.

Palaeoproterozoic arc related supracrustal units from the Tasiilaq Region, SE Greenland: Insights into the convergence of the Rae and North Atlantic Cratons

The Tasiilaq region in SE Greenland contains Palaeoproterozoic supracrustal rocks, which record the collision and suturing of the Rae Craton and North Atlantic Craton during the 1.9 Ga Nagssugtoqidian orogeny. The original mineralogy and textures of the supracrustal rocks have been largely overprinted by amphibolite facies metamorphism, obscuring the original protoliths and complicating the interpretation of the early Earth surface environment in which these rocks formed. Here, we present new major and trace element data for the Schweizerland, Kuummiut and Isertoq Terranes, with the former two representing the Rae craton, and the latter the northern margin of the North Atlantic Craton. Supracrustal lithologies include metapelites, marbles, calc-silicate rocks and mafic to felsic volcanic rocks and/or shallow intrusive rocks. Pelites normalised to Post Archaean Australian shales have (sub)horizontal rare earth element patterns, with La/SmCN: 1.61 – 23.91 and Eu/Eu* 0.34 – 1.83 and Th/Sc 1. The volcanic rocks are enriched relative to the primitive mantle (PM) with parallel to sub parallel PM-normalised heavy rare earth elements. They have a suprasubduction zone signature with distinct Nb-Ta troughs. The amphibolites can be further divided into those that interacted with the crust; back-arc basalts with subtle Nb-Ta troughs; and fore-arc basalts with distinct Nb-Ta and Zr-Hf troughs and negative Ti anomalies. The marbles consist of a Mg-rich dolomite group with δ13C 0.48 to 0.91 ‰ and δ18O −6.13 to −7.57 ‰; and Mg-poor calcitic carbonates with δ13C −0.74 to 0.17 ‰ and δ18O −10.81 to −14.87 ‰. This isotopic composition suggests formation at ∼ 2050 Ma during the low δ13C excursion following the Great Oxidation Event. We propose that the supracrustals of the Tasiilaq region were deposited at 2050 Ma in a series of interconnected basins during dual subduction convergence of the North Atlantic and Rae cratons, with development of an island arc with back arc basin in the north, and a continental arc in the south. These basins were inverted during continental collision, thrust, and folded into the Archaean Kuummiut and Isertoq TTG-dominated terrane basement, before being subjected to the region-wide amphibolite facies metamorphism at ∼ 1840 Ma during the Nagssugtoqidian orogeny.

Eocene to Miocene metamorphic evolution and tectonic implication of the Ilam Nappe in Nepal Himalaya: Constraints from P–T conditions and monazite petrochronology

The Ilam Nappe, consisting of the High Himalayan Crystalline Sequence (HHCS) in far-eastern Nepal, was largely emplaced southward along the Main Central Thrust (MCT). Here we present the P–T–t path of the Ilam Nappe using conventional geothermobarometers, pseudosection modelling, and monazite petrochronology. Amphibole-bearing migmatite just above the MCT experienced prolonged fluid-present melting from prograde metamorphism at 39–31 Ma to peak upper amphibolite-facies metamorphism (694 °C and 10.3 kbar) during 24–19 Ma, followed by cooling. Kyanite-sillimanite migmatites record prograde metamorphism at 35–28 Ma and peak lower granulite-facies metamorphism (738–765 °C and 8.3–9.7 kbar) under kyanite stability via the muscovite dehydration reaction at 25–20 Ma, followed by isothermal decompression under sillimanite stability. The clockwise P–T path and Early-Middle Miocene peak metamorphism of the Ilam Nappe can be correlated to those in the lower-middle HHCS hinterland. The Eocene-Oligocene prograde metamorphism with a low geothermal gradient of 15–25 °C/km in the Ilam Nappe and the entire HHCS hinterland could represent the earlier crustal thickening accompanied by large-scale overthrusting before the normal faulting of the South Thrust Detachment System (STDS), followed by diachronous extrusions of the upper HHCS along the High Himal Thrust, and of the lower-middle HHCS along the MCT with nappe emplacement. The lower granulite-facies Ilam Nappe was exhumed during the Early-Middle Miocene from the deeper or lower structural level of the HHCS, compared to the earlier Oligocene exhumation of the upper amphibolite-facies Karnali and Kathmandu klippes from the shallower or uppermost structural level of the HHCS.

Fractionation of trace and platinum-group elements during metamorphism of komatiitic chromites from the early Archean Gorumahishani greenstone belt, Singhbhum Craton (eastern India)

It is well established that the major and minor element contents of chromites are subject to change during greenschist to amphibolite facies metamorphism. During upper amphibolite facies metamorphism, chromite can be completely converted to chrome magnetite. However, not all elements are affected to the same degree, the concentrations of +2 ions (e.g. Zn, Co, Mn) being particularly vulnerable to modification. The degree to which trace elements, particularly the platinum-group elements (PGE), are affected has not been closely examined. The compositions and textures of chromites from komatiites of the Gorumahishani greenstone belt of the Singhbhum Craton (India) have experienced a range of metamorphic conditions from greenschist to amphibolite facies, providing the opportunity to study the changes of trace and platinum-group element composition with metamorphic grade. Five types of altered chromites are identified from the komatiitic suite of rocks in the ~120-km-long greenstone belt. The type-I chromites are non-porous and characterized by the least modified cores. These chromites are mostly present in the northern Maharajgunj-Tua Dungri section where rocks show metamorphism from greenschist to greenschist-amphibolite transition facies. The type-II and type-III chromites are porous and mostly found in the southern Kapili section of the greenstone belt where rocks show metamorphism up to the mid-amphibolite facies. Type-IV and type-V chromites are completely modified to ferritchromit and chrome magnetite, respectively, and are present in the komatiitic rocks from the entire greenstone belt. The central cores of the type-I and type-II grains have relatively higher concentrations of mobile trace elements (e.g. Zn, Co, and Mn) with higher Mg# [Mg/(Mg + Fe2+)], lower Cr# [Cr/(Cr + Al)], and lower Fe3+/R3+ (R3+ = Fe3+ + Cr3+ + Al3+) ratios than their respective rims. Significantly higher concentrations of the immobile trace elements (e.g. Ti and V) in the cores of the type-II grains relative to their chrome magnetite rims from the Kapili section and to the type-I varieties from other sections might be due to the metamorphism of the komatiitic rocks under higher-grade conditions (amphibolite facies). In situ LA-ICPMS analysis for PGE reveals a relatively higher concentration of Ru and Rh in the rims of the type-I chromites than in the cores which is due to the diffusion of these elements from the normal spinel structure of the cores towards the bivalent octahedral sites of the inverse spinel structure of the chrome magnetite rims during metamorphic processes. The lower concentrations of Os, Ir, Ru, and Rh in the cores of the type-II chromites from the Kapili section might be related to the metamorphism of the rocks under higher-grade conditions that facilitated the diffusion of these elements to associated sulphide or platinum-group mineral or alloy phases. The calculated partition coefficients of Sc, Ti, V, Mn, Ni, Ga, Os, Ir, Ru, and Rh from the least altered chromite cores assuming equilibrium with the parental komatiitic melt also suggest the variable effects of metamorphism when compared with global experimental and empirical values of the natural samples.

Collision with Gondwana or with Baltica? Ordovician magmatic arc volcanism in the Marmarosh Massif (Eastern Carpathians, Ukraine)

The pre-Alpine Marmarosh Massif is a tectonically complex unit of the crystalline basement within the Eastern Outer Carpathians. In the eastern (Ukrainian) segment of this massif, two units have been identified—the Bilyi Potok Nappe and the Dilove Nappe. Petrological investigations coupled with zircon U–Pb dating were performed on metavolcanic rocks (porphyroids) and their phyllite host rocks, sampled from three locations within the Dilove Nappe. The geochemical characteristics of the meta-rhyodacite porphyroids revealed a volcanic arc affinity of the protolith, with U–Pb zircon ages of 452.8 ± 1.5 Ma and 461.5 ± 1.6 Ma and zircon saturation temperatures in the range of 823–892 °C. The phyllite host rocks (meta-tuff) yield a U–Pb zircon maximum estimate for the eruption age at 584.7 ± 2.9 Ma. Peak amphibolite-facies metamorphism (M1) was estimated at the pressure of 600–900 MPa with a temperature range of 560–600 °C. Retrogression (M2), possibly related to Alpine nappe stacking and shearing, is assumed to have taken place at 200–250 MPa and 384–222 °C. The volcanic arc is interpreted as an early Caledonian arc that was subsequently accreted to the margin of Baltica during the closure of the easternmost Tornquist Ocean rather than Cenerian (early Paleozoic) orogenic events on the margin of East Gondwana.

Garnet crystallization mechanisms and localized polymetamorphism in the southwestern Meguma Terrane, Nova Scotia, Canada

AbstractWe present data on the pressure and temperature (P–T) conditions experienced by metamorphic rocks of the Meguma Terrane, Nova Scotia, Canada, also utilizing three‐dimensional microstructural data on one sample to better constrain the mechanisms that controlled garnet crystallization. Inverse and forward thermodynamic modelling place peak P–T conditions in the southwestern Meguma Terrane at ~650°C and 4.5 kbar. Interpretation of these results with petrographic observations and previous P–T constraints across the terrane suggests that amphibolite facies metamorphism occurred during the Devonian Neoacadian orogeny (406–388 Ma). Integration of quantitative 3D textural data with an estimated metamorphic heating rate of &lt;5°C/Myr is consistent with amphibolite facies metamorphism resulting from tectonic loading during the Neoacadian orogeny, though the exact nature of the orogeny is still not well understood. Further, the intrusion of granitic plutons into the Meguma metasediments at 373 Ma likely locally drove metamorphic recrystallization (polymetamorphism). The 3D size, shape, and location of garnet crystals in one sample reveal that the rate‐limiting step for garnet crystallization was likely the diffusion of aluminium through the intergranular matrix at length scales less than the mean nearest neighbour distance between garnet crystals. Nucleation was aided by epitaxial overgrowth onto a muscovite substrate, though it appears there may have been a decoupling between minerals providing a substrate and those providing nutrients during garnet growth.

Metasomatized mantle lithosphere and altered ocean crust as a fluid source for orogenic gold deposits

Whether auriferous hydrothermal fluid is generated during crustal metamorphism or by subcrustal processes to form orogenic gold deposits is hotly debated. Noble gas isotope and halogen abundance ratios can provide important constraints to help resolve this controversy. The Danba gold deposit is a rare example of a Jurassic age hypozonal orogenic gold deposit on the western margin of the Yangtze Craton, China. It is characterized by thick barren quartz veins with fractures infilled by thin auriferous quartz veinlets. The syn-gold quartz veinlets are associated with plagioclase and pyrrhotite and are distinguished from the barren quartz vein hosts on the basis of cathodoluminescence, trace-element content and Raman spectroscopy and microthermometry of fluid inclusions. The dominant fluid inclusions in the gold-bearing quartz and plagioclase are H2O–CH4 with vapor fill of ∼ 15–45 vol% and high pressure-corrected trapping temperatures of ∼ 500 °C, low salinities (<4 wt% NaCl equiv.) and high Mg/Fe ratios. Crushing syn-gold quartz and plagioclase released fluid inclusions with 40Ar/36Ar ratios of between 490 and 710, molar Br/Cl ratios of (0.3–0.7) × 10−3 and molar I/Cl ratios of (18–60) × 10−6. Crushing syn-gold pyrrhotite released fluid inclusions with 3He/4He ratios of 0.21–0.38 Ra (Ra = air value, 1.39 × 10−6) that are higher than typical crustal values of < 0.1 Ra, and 40Ar/36Ar of 330–640 that are similar to the gold-related quartz-hosted inclusions. The combined noble gas and halogen data do not favor an auriferous fluid derived from K- and U-rich Proterozoic granitic basement, which is expected to have radiogenic 40Ar/36Ar and 3He/4He ratios. Instead, the data are more consistent with a fluid source from underlying mantle lithosphere containing a high proportion of altered ocean crust (AOC). The auriferous fluid that formed the Danba gold deposit is thus considered to have been derived from a source comprising both metasomatized mantle lithosphere that had been fertilized in a Neoproterozoic subduction event and a portion of relict AOC beneath the western margin of the Yangtze Craton. In contrast with syn-gold fluid inclusions, those in pre-gold barren quartz are H2O–CO2 dominated. Crushing pre-gold quartz released fluid inclusions with 40Ar/36Ar ratios of 2410–5450, Br/Cl ratios of (1.2–1.5) × 10−3 and I/Cl ratios of (60–80) × 10−6 that are distinctly different to the gold-ore stage. The higher 40Ar/36Ar ratios of the pre-gold fluids are consistent with a source for the barren host quartz veins from prograde greenschist- to amphibolite-facies metamorphism of the Proterozoic granitic basement.

New CA-ID-TIMS U–Pb zircon ages for the Altenberg–Teplice Volcanic Complex (ATVC) document discrete and coeval pulses of Variscan magmatic activity in the Eastern Erzgebirge (Eastern Variscan Belt)

The Altenberg–Teplice Volcanic Complex (ATVC) is a large ~ NNW–SSE trending volcano-plutonic system in the southern part of the Eastern Erzgebirge (northern Bohemian Massif, south-eastern Germany and northern Czech Republic). This study presents high precision U–Pb CA-ID-TIMS zircon ages for the pre-caldera volcano-sedimentary Schönfeld–Altenberg Complex and various rocks of the caldera stage: the Teplice rhyolite, the microgranite ring dyke, and the Sayda-Berggießhübel dyke swarm. These data revealed a prolonged time gap of ca. 7–8 Myr between the pre-caldera stage (Schönfeld–Altenberg Complex) and the climactic caldera stage. The volcanic rocks of the Schönfeld–Altenberg Complex represent the earliest volcanic activity in the Erzgebirge and central Europe at ca. 322 Ma. The subsequent Teplice rhyolite was formed during a relatively short time interval of only 1–2 Myr (314–313 Ma). During the same time interval (314–313 Ma), the microgranite ring dyke intruded at the rim of the caldera structure. In addition, one dyke of the Sayda-Berggiesshübel dyke swarm was dated at ca. 314 Ma, while another yielded a younger age (ca. 311 Ma). These data confirm the close genetic and temporal relationship of the Teplice rhyolite, the microgranite ring dyke, and (at least part of) the Sayda-Berggießhübel dyke swarm. Remarkably, the caldera formation in the south of the Eastern Erzgebirge (caldera stage of ATVC: 314–313 Ma) and that in the north (Tharandt Forest caldera: 314–312 Ma) occurred during the same time. These data document a large ~ 60 km NNW–SSE trending magmatic system in the whole Eastern Erzgebirge. For the first time, Hf-O-isotope zircon data was acquired on the ring dyke from the ATVC rocks to better characterize its possible sources. The homogeneous Hf-O-isotope zircon data from the microgranite ring dyke require preceding homogenization of basement rocks. Some small-scale melts that were produced during Variscan amphibolite-facies metamorphism show similar Hf-O-isotope characteristics and can therefore be considered as the most probable source for the microgranite ring dyke melt. In addition, a second source with low oxygen isotope ratios (e.g. basic rocks) probably contributed to the melt and possibly triggered the climactic eruption of the Teplice rhyolite as well as the crystal-rich intrusion of the ring dyke.

Metamorphic evolution of the Sittampundi Layered Complex, India, during the Archaean–Proterozoic boundary: insight from pseudosection modelling and zircon U–Pb SHRIMP geochronology

AbstractIn the Palghat–Cauvery Shear/Suture Zone of the Southern Granulite Terrane, the Sittampundi Layered Complex occurs as a mappable unit. The Sittampundi Layered Complex consists of mafic, ultramafic and anorthositic rocks with chromitite layers and has been interpreted as an arc/ophiolite complex that formed in an Archaean suprasubduction zone arc setting. In the Sittampundi Layered Complex, reddish-black metabasites occur as layers or boudins with a rim of amphibolite within anorthosite. The peak metamorphic assemblage of the metabasites is garnet + clinopyroxene + quartz + rutile ± plagioclase ± orthopyroxene, and symplectite consisting of amphibole and plagioclase formed around the garnet during retrograde metamorphism. The protolith of metabasite may have intruded in a suprasubduction zone arc setting during the late Neoarchaean (c. 2540–2520 Ma), and then underwent high-pressure granulite-facies peak metamorphism (900–800 °C and 11–14 kbar) in the early Palaeoproterozoic (c. 2460–2440 Ma), followed by amphibolite-facies metamorphism (550–480 °C and 5.5–4.5 kbar) in the middle Palaeoproterozoic (c. 1900–1850 Ma). The results obtained in this study, together with previous studies, indicate that: (1) the high-pressure granulite-facies metamorphism in the study area indicates that subduction occurred during the Archaean–Proterozoic boundary with a higher apparent average geothermal gradient (∼20–16 °C km−1) than the modern-day Earth, and (2) the apparent average geothermal gradient of the subduction zone was ∼29–14 °C km−1 during the Archaean–Proterozoic boundary, which was still too high to enter the realm of eclogite-facies metamorphism.

Identification of UHT Granulites in the Pan-African Dahomeyide Suture Zone in SE Ghana: Implications for Evolution of Collisional Orogens

AbstractThis study presents the petrology, geochemistry, U–Pb ages, Lu–Hf and oxygen isotope compositions of Adaklu mafic granulites (ADMGs), from the Pan-African Dahomeyide suture zone in southeastern Ghana. The ADMGs show mafic precursor with low-K tholeiitic affinity. They display convex rare earth and trace elements characteristics without any obvious anomalies of Eu, Ti, Nb and Ta. The geochemical characteristics of ADMGs mimic those of N-MORB. Zircon U–Pb dating on the ADMGs reveals granulite facies metamorphic ages of ca. 595–602 Ma. However, a few zircons yield relatively older apparent 206Pb/238U ages of ca. 620 Ma, representing prograde metamorphic age. ADMGs preserve mean zircon εHf (t) values of +7.0 to +9.7 and δ18O values of 6.1–8.0‰. Based on petrographic observations, geothermobarometric calculations using conventional thermobarometry, mineral equilibria modeling, Ti-in-zircon and Zr-in-rutile thermometers reveal peak granulite facies P–T conditions of 0.95–1.2 GPa/940–1000°C, and retrograde amphibolite facies conditions of 0.83–0.93 GPa/575–710°C. The prograde metamorphic stage is inferred to be amphibolite or eclogite facies metamorphism. Hence, a clockwise a P–T–t path is proposed for the ADMGs. The overall results indicate that ADMGs are ultra-high temperature (UHT) granulites, and the protolith is altered oceanic crust consumed during the Pan-African collisional events. Asthenospheric upwelling induced by lithospheric delamination in the earliest extension of the thickened orogen, or by slab break-off in the background of collision may have provided the additional heat for UHT granulite facies metamorphism.

Provenance of metasiliciclastic rocks at the northwestern margin of the East Gabonian Block: Implications for deposition of BIFs and crustal evolution in southwestern Cameroon

The Congo craton hosts banded iron formations (BIFs) in southern Cameroon. The Gouap iron deposit is in the central part of the Nyong Complex, next to the Ntem Complex in the northwestern part of the East Gabonian Block of the Congo craton. The Gouap BIFs are interbedded with metasiliciclastic rocks consisting of gneisses, schists, and quartzites. We present the first integrated study combining whole-rock and Sm-Nd isotope geochemistry with SIMS and LA-ICP-MS U-Pb-Hf-O isotope analyses of detrital zircons from metasiliciclastic rocks from the Gouap iron deposit to assess the provenance and tectonic setting during deposition of BIFs as well as the crustal evolution at the northwestern margin of the East Gabonian Block. Geochemical data for the metasiliciclastic rocks indicate that their protolith comprised a sequence of greywacke. Along with regional geology, our U-Pb-Hf-O detrital zircon and Sm-Nd whole-rock isotope data suggest that these rocks were predominantly derived from Neoarchean charnockite and TTG suites of the Ntem Complex with a variable and episodic contribution from ca. 2.2–2.0 Ga charnockites of the Nyong and Ogooué complexes. Metasiliciclastic rocks and BIFs were deposited between ca. 2.1 and 2.0 Ga in a foreland basin developed along the northwestern continental margin of the East Gabonian Block. So far unrecognized ca. 2.50–2.45 and 2.45–2.22 Ga magmatic rocks were exposed in their provenance. The Gouap succession experienced amphibolite-facies metamorphism when the Congo and São Francisco cratons collided during the Eburnean-Transamazonian orogeny to form the core of the Nuna/Columbia supercontinent. With the subduction dipping to the west, under the West Gabonian Block, slices of Paleoproterozoic oceanic crust and sediments scraped off the margin of the East Gabonian Block formed an accretionary prism. BIFs of the Nyong Complex likely represent a deep-water temporal equivalent of shallow-marine Mn-deposits of the Francevillian basin, Gabon, and together they belong to the same redox-fractionated deep-water upwelling system that was charged with iron and manganese via hydrothermal alteration of oceanic crust.

Reply to Comment by A.P. Nutman et al. on “Tectonics of the Isua Supracrustal Belt 1: P‐T‐X‐d Constraints of a Poly‐Metamorphic Terrane” by A. Ramírez‐Salazar et al. and “Tectonics of the Isua Supracrustal Belt 2: Microstructures Reveal Distributed Strain in the Absence of Major Fault Structures” by J. Zuo et al.

AbstractStructural and metamorphic analyses from the works under discussion (Ramírez‐Salazar et al., 2021, https://doi.org/10.1029/2020tc006516; Zuo et al., 2021, https://doi.org/10.1029/2020tc006514) show that the Isua supracrustal rocks can be interpreted to record one single deformation and metamorphic event featuring quasi‐homogeneous deformation and amphibolite facies metamorphism, followed by late static retrogression or thermal event(s). Observed deformation and metamorphic records are consistent with three hypotheses: (a) they represent Neoarchean plate tectonic overprints following Eoarchean plate tectonic evolution (e.g., Nutman et al., 2022, https://doi.org/10.1029/2021TC007036); (b) they represent Eoarchean heat‐pipe and/or plate tectonic deformation that survived later tectonic event(s) (e.g., Ramírez‐Salazar et al., 2021, https://doi.org/10.1029/2020tc006516; Zuo et al., 2021, https://doi.org/10.1029/2020tc006514), and; (c) they represent one major Neoarchean tectonic event, such that the Isua supracrustal belt (ISB) records Eoarchean protolith‐related processes but does not record Eoarchean metamorphism nor deformation. While a heat‐pipe model for crustal formation is central to hypothesis 2, it is also a viable crustal formation mechanism for hypothesis 3 where the ISB would still form in a heat‐pipe setting in Eoarchean time, but the major deformation of the heat‐pipe lithosphere happened during Neoarchean time, probably by (proto‐)plate tectonic processes. If the data presented in Zuo et al. (2021), https://doi.org/10.1029/2020tc006514 and Ramírez‐Salazar et al. (2021), https://doi.org/10.1029/2020tc006516 only reflect Neoarchean histories, then these cannot be used to refute or support any Eoarchean geodynamic background for the formation of the ISB.