Retrograde Metamorphism Research Articles

Metamorphism and geochronology of the Foping gneiss dome: Insights into Early Triassic collision of the Qinling Orogen, Central China

The Foping gneiss dome, which outcrops in the narrowest part of the Qinling Orogen, contains high-grade metamorphic rocks that record the collision between the North China Block (NCB) and South China Block (SCB). Herein, petrography, mineral chemistry, phase equilibria modeling and UPb geochronology are used to constrain the collision events responsible for the high-grade metamorphism and exhumation of the Foping gneiss dome. The pelitic and mafic granulites sampled from the dome reveal peak metamorphism at P/T conditions of 6.2–7.4 kbar/ 805–855 °C, and document clockwise P-T-t paths with retrograde metamorphism of amphibolite-facies at 3.4–4.7 kbar/ 605–715 °C. While the pelitic schist from the rim of this dome witnessed peak metamorphism at P/T conditions of 4.9–5.1 kbar/ 590–605 °C and retrograde metamorphism at 3.5–3.7 kbar/ 510–520 °C. UPb dating of zircon and monazite confirm ages of ∼247–240 Ma for peak metamorphism with granulite-facies, ages of ∼208–198 Ma for retrograde metamorphism. The two metamorphic stages may record the transition from crustal thickening to dome exhumation, and the age of peak metamorphism probably marks the timing of the collision between the NCB and SCB along the Qinling Orogen in the Early Triassic. The collision probably terminated in the Late Triassic. The new discovery of the Triassic metamorphic event in the Qinling, combined with previously reported collision event of the Dabie Orogen to the east, supports the idea that the continental collision between the NCB and SCB along the Qinling-Dabie Orogen was synchronous.

Basement reactivation and structural inheritance in the Jurassic to Neogene evolution of the North Patagonian Andes (41° 08’ – 41° 11’ S), Argentina

Structural inheritance is a major control on the Andean structural architecture and magma emplacement, particularly in Patagonia, where the genesis of sedimentary basins and magmatic arcs has been largely influenced by basement fabrics. Based on new geologic, structural, microstructural and geochronologic data, the aim of this contribution is to evaluate the influence of pre-existing mechanical anisotropies of the Paleozoic basement on the Jurassic-Neogene tectonic evolution in the Paso de las Nubes area (North Patagonian Andes, Argentina). U-Pb zircon data of an orthogneiss yielded an age of 166 ± 2 Ma, which is consistent with the Jurassic batholith reported in the North Patagonian Andes, being thus coeval with retrograde metamorphism and deformation in the metasedimentary wallrock. Fault kinematic data indicate a Jurassic transtensional regime, strongly controlled by basement reactivation. On the other hand, a second kinematic population records a mainly strike-slip solution, associated with partitioned Neogene transpression.

Metamorphic history of amphibolite facies metapelites from the Southern margin of the Limpopo Belt: In situ U-Pb dating of Zircon, monazite and rutile

The Southern Marginal Zone (SMZ) comprises, in part, Archean granulite facies metapelitic rocks deposited at ∼ 2733 Ma and metamorphosed to granulite facies at ∼ 2713 Ma of the Limpopo Belt in direct contact with the Kaapvaal craton. The SMZ also contains amphibolite facies metapelites within a fragment that is in direct contact with the thrust-faulted tectonic boundary with the Kaapvaal craton. These metapelites are proposed to represent former granulites that were comprehensively rehydrated under amphibolite facies. However, no previous study has directly investigated the conditions of metamorphism or timing of the proposed higher-grade events in these retrograde rocks. Neither has the timing of retrogression in the retrogressed metapelites been well constrained. Here, detailed petrographic analysis, mineral composition and in situ U-Pb dating of zircon, monazite and rutile are presented for the amphibolite facies metapelites. These rocks are characterized by assemblages consisting of garnet, orthoamphibole, biotite, quartz, plagioclase, rutile, kyanite and graphite. Although there is pervasive retrogression to orthoamphiboles (anthophyllite to gedrite series), rare relics of orthopyroxene are preserved. The compositions of the first garnet (Grt 1) generation suggests granulite facies peak metamorphic conditions of 860 ± 10 °C and 11 ± 0.4 kbar. Metamorphic zircon grains and overgrowth rims and monazite, included in Grt 1, indicate that peak metamorphic conditions occurred between 2714 ± 7 and 2713 ± 4 Ma. Dating of rutile inclusions in Grt 1 from granulite yield a Concordia age of 2677 ± 6 Ma. This indicates post-granulite facies cooling and shows that later retrograde metamorphism did not reach temperatures high enough to reset rutile inclusions in Grt 1 garnet. Orthoamphibole and second garnet with kyanite inclusions (Grt 2) compositions suggest that retrogression occurred under amphibolite facies conditions of 600 ± 10 °C and 6.0 ± 0.5 kbar. Rutile included in Grt 2 and as discrete grains equilibrated with the retrograde textures record a range of Paleoproterozoic ages (2212 ± 19, 2148 ± 12 and 2012 ± 10 Ma). Thus, the metamorphic history of the amphibolite facies metapelites of the SMZ is hotter than thought and similar to their Archean granulite facies counterparts, except for younger Paleoproterozoic ages, likely indicating a pervasive retrogression to amphibolite facies conditions.

Marble-hosted noble spinel deposits from the Luc Yen District (Vietnam): mineral equilibria involving corundum and spinel

Thermodynamic analysis is carried out for the model systems: (1) Na–Ca–Mg–A1–Si–CO2–H2O and (2) Ca–Mg–A1–Si–CO2–H2O. In each systems the mineral equilibria involving dolomite, calcite, spinel, corundum, pargasite, hydroxylclinohumite, and diaspore are considered. A diagram of the multisystem in coordinates μH2O-μCO2is constructed. Analysis showed a significant influence of chemical potentials of water and carbon dioxide in the mineral-forming medium on stability of corundum and spinel in marbles. The conditions of the spinel formation during metamorphism of protolith consisting of carbonate sediments with evaporites lenses and terrigenous material (smectites, bauxites, etc.), are considered in detail. The conditions of the replacement of spinel by corundum during retrograde metamorphism are determined depending on the mole fraction of CO2 in the fluid and temperature at pressure of 5 kbar.

Genesis of high-Al chromitites from Xiaosuihe peridotite (Central Jilin Province, NE China): Implication for initial subduction of the Paleo-Asian ocean

This study presents whole-rock major- and trace-element data of serpentinites and mineral geochemistry of chromite/spinel from both serpentinites and associated podiform chromitites from the Xiaosuihe region in central Jilin Province, NE China, aimed to elucidate the genesis of serpentinites and podiform chromitites. The serpentinites exhibit low Al2O3/SiO2 ratios (0.01–0.02) and high Mg# (0.90–0.91), indicating their derivation from the fore-arc mantle wedge and protolith as harzburgite. Chromitite ores with semi-massive to disseminated textures occur as small pods. The chromite/spinel (Chr) present in both the podiform chromitite and serpentinite can be categorized into four distinct microstructural groups: (1) partially altered chromite, characterized by high-Al primary cores (Cr# = 0.44–0.49), surrounded by high-Cr porous rims; (2) porous chromite, featuring abundant pores enriched in Cr and Fe and depleted in Mg and Al, filled with chlorite; (3) zoned chromite consisting of high-Al cores surrounded by non-porous rims of Cr-magnetite (Cr-mag); and (4) homogeneous chromite, which is non-porous ferritchromite (Fe-Chr). The high-Al primary cores of the partly altered chromite are depleted in Cr but enriched in Mg, exhibiting MORB-like tholeiitic affinities. In contrast, the altered Chr displays low Mg# [Mg/(Mg + Fe2+)] along with elevated Cr# [Cr/(Cr + Al)] and Fe# [Fe3+/(Cr + Al + Fe3+)] values. Based on the above results, the high-Al cores of chromite within the chromitite originally formed through melt/fluid-rock interaction within the forearc mantle wedge. During the exhumation and post-magmatic processes, the reaction between high-Al Chr and olivine resulted in chlorite formation alongside secondary Chr enriched in Fe2+ and Cr content. Subsequently, the infiltration of hydrous fluid led to the development of Fe3+-rich Chr or Cr-mag along grain boundaries or fractures within the original chromite during the retrograde process. The Xiaosuihe serpentinized harzburgites, along with associated chromitites, represent the composition of peridotite in the forearc mantle wedge that could have formed during the initial subduction of an oceanic island arc abutting the northern North China Craton during the Cambrian. Subsequently, these rocks underwent retrograde metamorphism from amphibolite (eclogite) to greenschist facies during their exhumation.

Petrogenesis and Geodynamic Evolution of the Archean Shawmere Anorthosite Complex and Associated Gneisses, Kapuskasing Uplift, Superior Province, Canada

Abstract In this study, we integrated extensive field, petrographic, whole-rock major and trace element, and Nd–Pb–Sr–O isotope, and zircon U–Pb ages, trace element and Lu–Hf isotope data from the Neoarchean Shawmere Anorthosite Complex and surrounding gneisses to unravel their petrogenetic origin and tectonic history. The ~2765 Ma Shawmere Anorthosite Complex is interpreted to have been emplaced into a sequence of interlayered greywacke and basalt deposited in an intra-continental arc rift system above a north-dipping subduction zone. The complex consists mainly of anorthosite, leucogabbro, gabbro, and hornblendite that were emplaced as several batches of magmas and crystal mushes originating from sub-arc mantle sources. In contrast to the previous studies, our field and petrographic data suggest an igneous origin for the most hornblende in the complex, implying hydrous parental magmas. A hydrous magma origin is also consistent with the high-anorthite content (mostly 70–90%) of the plagioclase in the complex. Percolation of hydrous basaltic melts through gabbroic cumulates in crustal magma chambers led to extensive (>50%) replacement of igneous clinopyroxene by igneous hornblende. Continued subduction resulted in the closure of the intra-arc rift system and the intrusion of the complex by tonalite, granodiorite and diorite between 2765 and 2680 Ma in an Andean-type margin. The complex and surrounding gneisses underwent hornblende granulite-facies metamorphism mainly between 2680 and 2620 Ma, overlapping with mid-crustal east-west extension between 2660 and 2640 Ma. The granulite-facies metamorphism is recorded by the replacement of hornblende, plagioclase and clinopyroxene by garnet and the development of a garnet-orthopyroxene-plagioclase metamorphic assemblage with a granoblastic texture. Tectonic rebounding of mid-crustal rocks to upper crustal levels after 2620 Ma led to the formation of an extensive network of extensional fractures and retrograde metamorphism. Migration of CO2-rich hydrous fluids along the extensional fractures and grain boundaries resulted in the precipitation of many metasomatic minerals mainly at the expense of hornblende and plagioclase, including epidote, clinozoisite, tremolite, actinolite, paragonite, margarite, titanite, quartz, calcite, sillimanite, dolomite, and chlorite. Prevalent replacement of hornblende by garnet during prograde metamorphism and metasomatic replacement of hornblende and plagioclase by retrograde mineral assemblages disturbed the Sm–Nd, U–Th–Pb, and Rb–Sr isotope systems.

Orogen‐scale uniformity of recorded granulite facies conditions due to thermal buffering and melt retention

AbstractGranulite facies metapelitic gneisses collected over a km exposed area of the Kakamas Domain of the Namaqua–Natal Metamorphic Province in southern Namibia all contain similar garnet–sillimanite–cordierite–biotite–quartz–K‐feldspar–ilmenite plagioclase magnetite mineral assemblages. These assemblages are interpreted to have equilibrated at suprasolidus retrograde conditions, and most samples contain distinct biotite‐ or sillimanite‐free peak assemblages. Pseudosection modelling constrains extremely uniform residuum solidus conditions of kbar and °C for the entire Kakamas Domain. Estimated peak metamorphic conditions overlap with these but are more smeared out at between 4 and 7 kbar at 760°C to potentially more than 900°C. The uniformity of residuum solidus conditions is not coincidental, but is a consequence of retrograde re‐equilibration due to minor melt retention after peak metamorphism. Re‐equilibration could only stop once all retained melt had crystallized, which required the concomitant growth of a hydrous mineral to account for its H2O component. Biotite is the most stable hydrous mineral in these rocks, such that the residuum – conditions in the Kakamas Domain reflect the upper‐ stability of biotite, and also corresponds to the intersection of the well‐known biotite–sillimanite melting reaction that consumed all biotite during prograde metamorphism. The calculated melt fertility of the sample suite indicates that the variable amounts of heat consumed to overcome the latent heat of fusion could have caused a 25°C spread in the peak temperature achieved by the most and least fertile samples. Peak temperature in the Kakamas Domain may have been as much as 100°C higher than residuum solidus conditions for specific samples but cannot be confidently constrained as it is obscured by the effects of both thermal buffering during prograde metamorphism and melt retention during retrograde metamorphism. Both processes are an inescapable part of the evolution of all granulite facies rocks, but their effects are most pronounced in fertile rocks like metapelites that are traditionally the preferred lithology for quantifying the – history of exhumed terranes.

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

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

Early Paleozoic metamorphic P–T–t evolution of the northern Cathaysia Block (SE China): Implications for thermal history and geodynamic setting

The tectono-thermal evolution of the Early Paleozoic orogeny in South China has been debated. In this contribution, detailed studies of petrography, mineral chemistry, phase equilibria modeling, conventional geothermobarometry, and zircon–monazite geochronology were conducted on metamorphic rock samples from the Zhoutan, Longyou, and Chencai groups in the northern Cathaysia Block, South China. All rocks record clockwise P–T paths, including the Pmax (pre-Tmax), Tmax, and post-peak retrograde stages. Sillimanite-garnet-mica gneiss from the Zhoutan Group is characterized by decompression and heating from the Pmax (0.97 GPa/710 °C) to the Tmax stage (0.70–0.85 GPa/795–815 °C). Garnet-amphibole gneiss from the Longyou Group and sillimanite-garnet-biotite gneiss from the Chencai Group experienced Tmax metamorphism at 0.90–1.00 GPa/865–880 °C and 0.90–1.00 GPa/805–820 °C, respectively. LA-ICP-MS zircon–monazite UPb dating yields 453–446 Ma for the retrograde metamorphism of the Zhoutan Group, ∼457 Ma and ∼432 Ma for the peak and retrograde metamorphism, respectively, of the Longyou Group, and 441–436 Ma for the peak metamorphism of the Chencai Group. This indicates that the Early Paleozoic crustal thickening event may have occurred 10–20 Ma earlier in the western region than in the eastern region of the northern Cathaysia Block. Integrated Early Paleozoic P–T–t data from the Cathaysia Block provide medium-pressure facies series conditions without the products of high-pressure facies series metamorphism, which are common in subduction-to-collision orogens. Based on the current metamorphic characteristics and other geological data for Early Paleozoic orogeny in South China, we infer that the current tectonic domains developing metamorphism–magmatism–deformation may not be the “first scene” of subduction-to-collision. The stress that caused the Early Paleozoic orogeny likely originated from the ancient plate boundaries of South China.

Al-Rich Titanites from Mont Blanc Alpine Fissures: Evidence of Ti-Nb-Y-REE Mobility during Water–Rock Interactions

Titanites can be excellent markers of element transfer in medium-temperature retrograde metamorphism. Euhedral titanites from several alpine fissures from Mont Blanc, particularly those of Périades and Courtes, crystallised at the end of the main quartz stage and are synchronous with the formation of green biotites and albite before chlorite formation. Micro-XRF, SEM, electron probe, and LA-ICP-MS analyses show that these titanites have a wide range of Al2O3 content from 1 to 8%, are dominated by -OH versus F, and have a wide range of Nb (up to 4500 ppm), Y (up to 3000 ppm), Zr (up to 1800 ppm), and Sn (up to 1400 ppm) concentrations. The allanite from the granite, partly destabilised into epidote, is the most likely source of Nb, Y, Zr, Sn, and REE. Titanites are enriched in HREE and show variations in LREE depending on the studied sites. Like quartz, they formed at around 400 ± 20 °C, which is compatible with the formation of green biotites after the destabilisation of granite Fe-Mg silicates. This early stage of fluid circulation, synchronous with the Mont Blanc massif uplift, is therefore marked by the titanite formation at the transition between the biotite and chlorite stability fields.

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

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

Discovery of Eclogite Facies Kyanite Quartzite in Central Jilin Province:Evidence of Ultra-high-pressure Metamorphism

The work area is located in the convergence domain of the Siberian plate and North China plate. The Jilin Province of the Soren-Xilamnlun river-Changchun-Heilog jiang plate collision zone. Before 2000, there was no empirical evidence of high-ultrahigh pressure transformation in the middle Jilin section. For this reason, the Changchun-Panshi-Jiaohe area was selected as the primary geological survey area. After comprehensively analysing the previous regional geological and mineral exploration data, we drew on the experience and achievements of studying the collision between the Suiu-dabie plate. Jiaohe blueschist origin was concerned. After fine field investigation, collecting fresh and representative samples, conducting microscopic identification, electron probe analysis and other work, the key identification and measurement results were obtained. Comprehensive results proved that the geological body producing kyanite undergoes high-pressure kyanite-quartzite was a metamorphic rock of gernet pgroxenite facies. This discovery is of fundamental importance for the establishment of the Jihei high pressure human ultra-high-pressure collision subduction zone. The discovery was briefly reported in 2000 and is now being published in detail. The garnet-phase blueschist quartzite is massive, laminated, ienticular, and rootless hook like bodies occur in within the luoguangou Dashihu narrow ductile shear zone. The rock being sheared is late paleozoic Early perntian Daheshan formation and late variscan Granite Porphyry, which were deformed and metamorphosed to form. blueschist dolomite quartz schist and dolomitic blueschist quartzite. The Peak mineral combination is Ky2+Qz2+Ms2 (Phn2) +Zt+Rt+Miss, without Sili and Pl, which basically meets the constraining charactenstics of eciogite facies. The peak metamorphic temperature reaches 700-750℃, indicating the Si (pfu) =3. 440-3. 535 of phengite, and indicated metamorphic pressure reaches p=2. 75-3. 30GPa, which shows that the rock has undergone ultra-high pressure metamorphic process, and its temperature and pressure conditions have been comparable to those formed by coesite. The temperature of secondary peak period of is about 550℃-600℃ and the pressure is about 1. 6-1. 7GPa. The multi-phase nature of the blueschist, muscovite and quartz indicates that the rock is subjected to prosess of subduction exhumation, and exhumation suffering from strong progressional and retrograde metamorphism. The temperature and pressure numbers of the metamorphism indicate that the paths are successively hornblende and green schist phases, with temperatures of about 500℃-450℃ and pressures of about 0. 5-0. 7GPa, and temperatures of about 300℃-350℃ and pressures of about 0. 3-0. 4GPa, respectively.

Finite strain, kinematic vorticity, rheological behavior and thermochronology of the Diancang Shan complex: Insights into channel flow of the southeastern Tibetan plateau

This study investigates the crustal flow within the southeastern orogenic belt of the Tibetan Plateau, with a focus on the Diancang Shan (DCS) complex. As an A-type gneiss dome, the DCS complex offers valuable insights into the deformation and evolution of intra-continental orogenic belts and the flow of deep crustal material. We have identified three main stages of deformation within the DCS complex since the Cenozoic. The initial stage (D1), spanning 41–30 Ma, was characterized by high-temperature pure shear deformation, which developed penetrative foliations, symmetric folds, and structural lenses. The subsequent major stage (D2), which shaped the overall architecture of the DCS complex from approximately 27–21 Ma, was primarily influenced by strike-slip stress regimes, resulting in shear fabrics with a top-to-the SE orientation in the releasing bend. The final stage (D3), initiated around 13 Ma, was marked by retrograde metamorphism at lower temperatures. Kinematic vorticity calculations indicate that the D1 stage was dominated by pure shear, while the D2 stage was primarily influenced by simple shear. The deformation temperature for the major deformation stage (D2) is estimated to range between 450 and 550 °C. During this stage, strain rates lie between 10−13–10−16 s−1 at 450 °C and 10−12–10−14 s−1 at 550 °C, indicating relatively low strain rates within the DCS complex. Our findings suggest that the formation of the DCS dome can be attributed to the ductile extrusion of channel flow, driven by the gravitational collapse of the Tibetan Plateau. Partial melting of the thickened middle and lower crust may have provided the channels and material sources. Furthermore, shear strain analysis (0.15–0.49) indicates that pure shear played a more significant role, contributing ∼ 65 % to the formation of the DCS dome.

Fluid‐Driven Mass Transfer During Retrograde Metamorphism and Exhumation of the UHP Western Gneiss Region Terrane, Norway

AbstractDehydration reactions within subducted oceanic crust are important for fluid‐mediated element transfer within the subducting plate and potentially to the mantle wedge. The effects of metamorphic reactions and fluid flow on element recycling that occur during retrogression and exhumation of subducted continental crust from mantle depths are poorly understood. We study two metabasite pods with fresh eclogite cores and retrogressed amphibolite‐facies rims and surrounding host gneiss within the Western Gneiss Region (WGR), Norway, to better understand element mobility and mass transfer during exhumation of subducted continental crust. Bulk‐rock data were collected from samples taken across the pod and into the host gneiss. Phengite breakdown in eclogite and epidote recrystallization in veins and/or gneiss within pod cores contributed large ion lithophile elements and REE to retrogressed eclogite closest to the pod cores. In gneiss hosting the pods, phengite and epidote breakdown provided fluid that mediated elemental transfer and redistribution to the pod rim or tail. Compared to the studied pod in the southern WGR, the pod in the northern WGR underwent higher P‐T conditions, partial melting and higher strain rates. This resulted in the infiltration of external fluid farther into the pod interior from the rim and facilitated larger mass gain in trace elements in the amphibolite tail of the pod relative to fresh eclogite in the core. The results show clear evidence for retrogression dehydration reactions driving significant fluid‐mediated element redistribution as observed on the outcrop scale during exhumation following ultrahigh‐pressure metamorphism, which directly impacts element signatures within the exhuming crust.

A Late Triassic gold metallogenic event within the extended tectonic evolution of the West Qinling Orogen, central China: Constraints from U-Pb and Ar-Ar geochronology in the Yangshan gold district

The West Qinling Orogen in central China resulted from Late Devonian to Triassic collisional orogenesis involving accreted terranes along the southwestern margin of the North China Block with subsequent ocean closure and collision of the South China Block with those terranes. The Yangshan gold district, with a pre-mining resource of more than 308 t gold, is situated along the Mianlue Suture Zone on the southern margin of the orogen. The gold orebodies are mainly controlled by the ENE-trending Anchanghe-Guanyinba Fault and its subparallel second-order faults. The relatively low pressure–temperature mineralization is dominated by disseminated pyrite, arsenopyrite, and stibnite hosted in phyllite and granite dikes, accompanied by intense muscovite-silica-carbonate alteration.Four major phases of deformation related to collisional tectonics are recorded in the Yangshan gold district. Early N-S-oriented compressional deformation (D1) is characterized by an alignment of muscovite, overgrowths on framboidal pyrite, and quartz veinlets. D2 is characterized by a series of continuous ENE-trending reverse faults and folds at multiple scales related to SSE-NNW-oriented compression. D3 is related to gold mineralization and is characterized by a series of ENE-trending, largely normal faults, formed during transtensional tectonics. D4 post-dates gold mineralization and is associated with the development of nearly N-S-trending, low-displacement normal faults.New high-precision SHRIMP zircon U-Pb dating of three types of granite dikes in the Yangshan gold district yields broadly coincident ages of 216 ± 3.0 Ma to 210 ± 9.7 Ma, which is a range identical to that of existing dates of 217–211 Ma for more regional granite magmatism in this district. New Ar-Ar dating of gold-related muscovite gives an age of 210.9 ± 1.6 Ma for the Yangshan gold deposit, indicating that hydrothermal activity overlapped or post-dated the final stages of granite intrusion. Pre-226 Ma peak metamorphism of the ore-hosting rocks occurred prior to extensional ductile shear deformation that is constrained by a published 40Ar/39Ar muscovite cooling age of 223 ± 2 Ma. In good agreement, U-Pb constrained ages of retrograde metamorphism and associated late magmatism of around 215 Ma are reported for rocks along the Mianlue Suture Zone.These geological and geochronological data suggest that relatively shallow-level gold mineralization was broadly coeval with Late Triassic transtensional, orogen-parallel deformation along the regional-scale fault system, late in the tectonic evolution of the West Qinling Orogen. This is consistent with its classification as epizonal orogenic gold mineralization.

GEOCHEMISTRY AND PETROGRAPHY OF METAMORPHIC SOLE AMPHIBOLITES FROM THE SLATINA QUARRY, MT. ZRINSKA GORA, CROATIA

The investigated Mt. Zrinska Gora amphibolites are a part of the metamorphic sole of the Banovina ophiolite complex. Mineral composition and other petrographic characteristics of amphibolites and adjacent peridotite were investigated by polarised microscopy and chemical analyses of rocks were obtained by a combination of inductively coupled plasma mass and emission spectrometry. The main minerals in these garnet-bearing amphibolites are amphibole and plagioclase with accessory garnet, sphene, spinel, opaque mineral ± clinopyroxene ± quartz ± actinolite ± zoisite-clinozoisite ± prehnite ± pumpelyite and ± clay minerals. Kelyphitic corona is developed around garnet. The amphibolites have nematoblastic, nematogranoblastic, porphyroblastic and porphyroclastic textures and homogenous to foliated structures. The presence of clinopyroxene in some of the investigated amphibolites points to their possible formation under P‒T conditions of upper amphibolite facies. The greenschist facies retrograde metamorphism and subsequent surface weathering of amphibolites is also evident. The chemical composition of rocks indicates that protoliths of most amphibolites were tholeiitic basalts, initially formed in island arcs and the adjoining back-arc basins. During the intraoceanic subduction in Jurassic Neotethys Ocean basalts/basaltic tuffs of island arc tholeiite affinity and back-arc basin basalts, positioned on the top of the down-going oceanic slab, were welded to the base of the hot over-riding mantle wedge, metamorphosed to amphibolite rocks and obducted together with ophiolites on the Adria continental margine. The new described amphibolite type having peridotite protolith characteristics originated as the consequence of the hydration of the bottom part of the overlying mantle wedge.

In-sequence tectonic evolution of Ediacaran nappes in the southeastern branch of the Brasília Orogen (SE Brazil): Constraints from metamorphic iterative thermodynamic modeling and monazite petrochronology

The metamorphic and kinematic evolution of medium-high grade rocks of the Andrelândia Nappe System (ANS), the orogenic wedge of the Southern Brasilia Orogen (SBO), was investigated in this work. Field and microstructural observations were combined with metamorphic petrology (i.e., iterative thermodynamic modeling) and monazite petrochronology to reconstruct the tectono-metamorphic history of the ANS rocks. The Liberdade Nappe experienced prograde metamorphism at ca. 610 Ma, achieving peak metamorphic conditions of ca. 650 °C and 9.5–10 kbar. This stage was followed by isothermal decompression linked to tectonic transport toward SE, at ca. 570 Ma. On the contrary, the Andrelândia Nappe experienced prograde metamorphism later, at ca. 580 Ma, reaching peak metamorphic conditions of ca. 680 °C and 11–12 kbar. The obtained results indicate that each nappe of the Andrelândia System records a single metamorphic cycle of burial and decompression, although it took place at different ages over a period of ca. 60 myr, from 630 to 570 Ma. The nappes experienced prograde and retrograde metamorphism whose ages progressively decreased toward the bottom of the nappe stack. We attribute this pattern to propagation of older buried material from the orogenic wedge (i.e., Liberdade Nappe), via thrust-and-fold, upon recently accreted rocks (i.e., Andrelândia Nappe), conducting a younger metamorphism event on the footwall of the ductile thrusted nappes. This mechanism is consistent with the ANS in-sequence fold-and-thrust architecture.

EVIDENCE OF ULTRAHIGH-PRESSURE EVOLUTION OF GARNET PERIDOTITES IN THE POLAR URALS

In garnet peridotites of the Marunkeu eclogite-gneiss complex (Polar Urals), segregations of carbonates in association with chlorapatite were first established. They contain relics of dolomite, which is “replaced” by calcite with symplectite (graphic) intergrowths of brucite and hydrocarbonate. The formation of peridotites with unusual carbonate structures is the result of the evolution of this complex at different depths. During deep subduction and progressive with the formation of garnet peridotites, segregated dolomite experienced a solid-phase breakdown into aragonite and magnesite at the pressure peak (≥5 GPa). During exhumation and retrograde metamorphism, garnet peridotites, together with segregations, underwent low-temperature transformations; aragonite was replaced by calcite, and magnesite by brucite and hydrocarbonate. The presence of carbonates with deep-seated structures confirms that the garnet peridotites and eclogites of the Polar Urals belong to ultrahigh-pressure complexes.

Sm-Nd age determination of eclogites from Sredna Gora Mountains – HP metamorphic event or final retrogression

The eclogites from Sredna Gora Mts. occur as small isolated bodies enclosed in amphibolites, two-mica schists and migmatized gneisses. They have tholeiitic composition and characteristics of MOR basalts. Three equilibrium mineral associations are distinguished in eclogites: (1) high-pressure (HP) – Grt+CPx(Omp)+Zo+Rt; (2) middle-pressure-high temperature (MP/HT) – Opx+CPx+Pl and middle pressure – middle temperature (MP/MT) presented by Amp+Pl+Qzt+Ep+Ilm+Ttn. The P-T conditions of HP stage reached values of 1.6–2.02 GPa and 650–690 °С. After the HP stage and CPx-Pl symplectite formation, the eclogites underwent a short-lived granulite facies overprint under temperatures similar to those of the HP event. The P-T conditions of the retrograde amphibolite facies overprint are in the range 620–650 °C and 0.6–0.8 GPa. The obtained age of 319±28 Ma by Sm-Nd garnet+whole rock method is interpreted as age of the final retrograde metamorphism under P-T conditions of high temperature amphibolite facies.

A modified paleoplacer model for the metaconglomerate-hosted gold deposits at Jacobina, Brazil

Evidence is presented that the metaconglomerate-hosted Jacobina gold deposits in Brazil represent paleoplacers that became partly remobilized during later metamorphic overprint analogous to Witwatersrand-type deposits elsewhere, notably in South Africa. This includes strong lithological and sedimentological control on the gold, presence of detrital minerals with gold inclusions, and detrital gold particles. Detrital, synsedimentary, and post-depositional pyrite types can be differentiated. Whereas the first two types can be linked to gold accumulation, the latter was associated with gold dispersion. Synsedimentary pyrite has the highest Au content, from which elevated Au concentrations in Archean rivers can be inferred. The nature and extent of post-depositional alteration, mainly in the course of the Paleoproterozoic Orogeny, distinguishes the Jacobina deposits from other Witwatersrand-type gold deposits. Phase equilibria and Zr-in-rutile thermometry indicate peak metamorphic temperatures of ca. 600 °C. Both Mg-chlorite and Fe-chlorite formed in disequilibrium at approximately 280–340 °C during retrograde metamorphism. An igneous signature in the chemistry of some of the tourmaline and the remobilization of gold associated with Fe-oxides, near intrusive rocks, point at a local magmatic influence on the post-depositional mineralization stage. Whether magmatic hydrothermal fluids added Au to the system at that stage remains to be determined. Remobilization during regional metamorphism was insufficient to form substantial ore bodies but led to purification of the initially detrital gold particles that now contain relatively little Ag and Cu and lack Hg.