Epidote-amphibolite Facies Research Articles

Molybdenum isotope behavior during subduction zone metamorphism

The molybdenum (Mo) isotope composition (defined as δ98Mo measured per mil relative to NIST-3134) of many modern arc systems and the upper continental crust is heavier than the mantle and most subducting slab lithologies. This observation has led to a model whereby fluids leaving the slab transfer isotopically heavy Mo preferentially to the mantle wedge, leaving the residual slab isotopically lighter. We explore this model via an Mo isotope study of the metasedimentary and mélange lithologies of the Catalina Schist in California. These rocks record subduction zone metamorphism over a wide range of high-pressure/low-to-medium temperature conditions.Mo isotope compositions of the metasedimentary rocks decrease with increasing metamorphic grade, from a δ98Momean of −0.04 ± 0.13 ‰ (1σ, n = 3) for the lawsonite albite facies to −0.38 ± 0.07 ‰ (1σ, n = 5) for the epidote–amphibolite facies. The highest grade [amphibolite facies] samples show a slight uptick in δ98Mo values, with a mean of −0.19± 0.14 ‰ (1σ, n = 4). When coupled with major and trace element variations, the changes in δ98Mo appear to reflect metamorphic effects rather than sedimentary source rock heterogeneity. The positive relationship between δ98Mo and [Mo] and negative relationship with Ce/Mo argue for progressive loss of Mo with isotope fractionation during increasing P-T conditions; the reversal of this trend, seen in the increase in δ98Mo between the epidote amphibolite and amphibolite facies may reflect a subsequent partial re-fertilization by fluids carrying isotopically heavy Mo. δ98Mo values in the mélange across all grades are highly variable, ranging from −1.75 to −0.19, consistent with large scale mobilization of Mo with isotope fractionation. A metasomatized amphibolite block and reaction rind show similar light isotope compositions and signs of Mo depletion and transport. Together these three whole-rock data sets demonstrate pervasive open system behavior and mobility of Mo in the Catalina Schist. LA-ICP-MS Mo measurement of minerals in the differing metasedimentary metamorphic grades indicates that Fe oxides and/or hydroxides, titanite, rutile, and epidote are important reservoirs of Mo but when comparing mineral Mo content and modal abundance to whole-rock Mo concentrations, several samples were found to have “missing Mo”, something that has been observed in other Mo inventory studies. We attribute this to sample heterogeneity between the thin section and whole-rock powder scales.

2.13 Ga Lawsonite/Barroisite-Bearing E-Morb Signature Metagabbro Associated with Spinel Metaperidotite from Itaguara (São Francisco Craton, Brazil): Oldest Blueschist-Facies Fragment of Oceanic Moho?

In close association with Paleoproterozoic retroeclogite and accretionary prism-related mica-quartz schist, a 2.13 Ga (metamorphic titanite U-Pb age) lawsonite/barroisite-bearing E-MORB signature metagabbro associated with spinel metaperidotite is found in the Itaguara Sequence from southern São Francisco craton, Brazil. Petrography and pressure-temperature equilibrium phase diagrams suggest that the metagabbro experienced blueschist-facies metamorphism, attaining peak metamorphic conditions at ~16 kbar and ~450 °C during subduction. The retrograde metamorphic path crossed epidote amphibolite-facies, in which the mineral assemblage found in metaperidotite (olivine, clinopyroxene, spinel, serpentine, chlorite, talc, and tremolite) was stable during a ca. 2.1 Ga continental collision-related exhumation that occurred between the Archean Campo Belo/Bonfim and Divinópolis complexes. This geological framework suggests that the metagabbro and adjacent spinel metaperidotite represent a subducted and exhumed blueschist-facies fragment of a Paleoproterozoic oceanic Mohorovičić (Moho) discontinuity, thus establishing the Itaguara metagabbro as the oldest-known occurrence of retrogressed blueschist and providing evidence for the activity of the modern-style plate tectonics more than 2 Gyr ago.

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

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

Metamorphic titanite–zircon pseudomorphs after igneous zirconolite

Abstract. The formation of metamorphic zircon after baddeleyite is a well-known reaction that can be used to date the metamorphism of igneous silica-undersaturated rocks. By contrast, metamorphic minerals formed after igneous zirconolite have rarely been reported. In this paper, we document metamorphic titanite + zircon pseudomorphs formed from the metamorphic breakdown of igneous zirconolite in syenodiorite and syenite, in the southeastern Sveconorwegian Province, Sweden. Water-rich fluid influx during tectonometamorphism in epidote–amphibolite-facies metamorphic conditions caused the release of silica during a metamorphic reaction involving igneous feldspar and pyroxene and the simultaneous breakdown of igneous Zr-bearing phases. Typical titanite + zircon intergrowths are elongated or platy titanite crystals speckled with tiny inclusions of zircon. Most intergrowths are smaller than 15 µm; some are subrounded in shape. Locally, bead-like grains of titanite and zircon are intergrown with silicate minerals. The precursor igneous zirconolite was found preserved only in a sample of near-pristine igneous syenodiorite, as remnant grains of mainly < 2 µm in size. Two somewhat larger crystals, 8 and 12 µm, allowed semiquantitative confirmation using microprobe analysis. Analogous with zircon pseudomorphs after baddeleyite, titanite + zircon pseudomorphs after zirconolite potentially offer dating of the metamorphic reaction, although the small size of the crystals makes dating with today's techniques challenging. The scarcity of reports of zirconolite and pseudomorphs reflects that they are either rare or possibly overlooked.

Petrology and Mineralogy of Metamorphic Rocks in The Pringsewu District, Lampung Province, Indonesia

The origin of metamorphic rock is greatly influenced by the temperature and pressure changes in every tectonic setting, especially in active subduction margins. One of the wide outcrops of metamorphic rock occurs in the Pringsewu District, Lampung, and it is a part of the Palaeozoic Gunung Kasih Complex. The presence of metamorphic rocks in Pringsewu has raised several questions and debates for some time due to the lack of research and field evidence found in this area, especially, since the origin of metamorphic rock in Lampung has been rarely studied. This research aims to determine the metamorphic rock facies and the tectonic setting underlying the formation of metamorphic rocks in the study area.
 Petrographic analysis on 19 thin-section samples shows that metamorphic rock in the research area can be divided into two regions i.e., western and eastern regions. The Eastern part is characterized by muscovite-epidote schist and greenschist which consist of quartz, muscovite, actinolite, epidote, and garnet as index minerals. Whereas the western part is characterized by quartzite and biotite-epidote schist that consist of quartz, biotite, and muscovite. Based on the mineral index, metamorphic rock’s protoliths are pelitic rock, mafic rock, and quartz-feldspathic sandstone. The metamorphic rock zonation shows the created temperature is from 280-550oC.
 The foliation structure such as schistose and porphyroblastic texture in the whole rocks sample indicated that metamorphic rocks are created in medium-grade metamorphism and are characterized by the greenschist facies to epidote-amphibolite facies. The abundance of quartz, k-feldspar, and labradorite minerals show that metamorphic rocks came from the microcontinent which was caused by a collision between the intra-oceanic Woyla plate and West Sumatra microcontinent in the Cretaceous. Indications of tectonic activity that create the lithology in Lampung Province need more geological study, especially to determine the absolute age of the metamorphic rock.

Exotic mineral products from the Chupa gneisses of the Belomorides

Exotic mineral products (EМP) in the Chupa paragneisses of the Belomorian rock complex occur as an association of submicroscopic “spherules”, “pipes”, “chips” and their combined varieties. EМPs were derived by multi-stage allochemical stress-metamorphism of host gneisses at high pressure in temperature regimes typical of amphibolite (in the Late Archean) and epidote-amphibolite (in the Early Proterozoic) facies. One distinctive feature of EMPs is the presence of carbon in all of their mineral phases. The metal-like substance of “chips” and the cores of “spherules” and “pipes” is comparable in chemical composition and the Ме/С index (Ме = Fe + impurity elements) with known iron carbides, and is similar in composition to terrestrial minerals, such as chalypite, yarlongite, cohenite, and haxonite. The average bulk carbon content of core-free EMPs and core margin substance (at.%) is 7 for Archean and 7-11 for Proterozoic rocks. The average carbon content of iozite, a mineral phase prevalent in “spherules” and “pipes”, is 11 at.% for Archean and 14 at.% for Proterozoic units. The carbon content of the mineral fragments of host rocks (quartz, plagioclase, garnet, and kyanite), which form inclusions in EMPs, is 7-20 to 46 at. %. The distinctive characteristics of the EMPs, publications on the gas composition and soluble carbonaceous endogenic material of gas-liquid inclusions in the rock-forming minerals of Chupa gneisses, and the presence of graphite therein provide evidence for the significant, yet poorly understood contribution of carbon and its chemical compounds as fluid constituents to the highgrade metamorphism of the Belomorides.

Newly recognized blueschist-facies metamorphism (glaucophane-omphacite-garnet), Belvidere Mountain Complex, northern Appalachians

Abstract An occurrence of blueschistfacies metamorphism in the Appalachian orogen is newly recognized in northwestern New England, United States. Inclusions of glaucophane and omphacite occur in a relict garnet core from a retrogressed garnetbarroisite amphibolite of the Belvidere Mountain Complex in Vermont. Pressuretemperature pseudosection and mineral composition isopleth calculations demonstrate that the Belvidere Mountain Complex blueschistfacies mineral assemblage of glaucophane–magnesio-hornblende–omphacite–chlorite–rutile–quartz–clinozoisite–garnet was stable at ~1.65–2.0 GPa and ~450–480 °C. Garnetabsent amphibolite with barroisite and chlorite inclusions in clinozoisite records highpressure epidote-amphibolite–facies metamorphism at ~1.0–1.4 GPa and ~515–550 °C. These new findings quantify deep subduction of the Belvidere Mountain Complex during the Cambrian to Ordovician Taconic orogenic cycle and suggest that more blueschistfacies mineral assemblages could be revealed in the Appalachians with detailed analysis of retrogressed rocks.

Oxidation and decomposition of stratiform SedEx sulfidic mineralization in the epidote-amphibolite facies producing cassiterite, V-rich micas, In-Sn-Ag-Sb-Pb-Bi-Zn-Fe-As-Cu-Ni-Co sulfides and Fe-Ca-Pb carbonates in situ (Bystrý potok locality, Gemeric unit, W. Carpathians)

The studied area of Bystrý potok locality (Gemeric unit, W. Carpathians) is built of lenses of lydites and limestones, being a part of the graphite bearing phyllites of Silurian Holec Beds. The Holec Beds represent the bedrock of the albite-quartz bearing keratophyre (trachyte sensu IUGS classif.) sequence with stratiform SedEx (sedimentary-exhalation) sulfidic mineralization in the Gemeric Lower Paleozoic stratigraphy. This SedEx mineralization was contemporaneous with Late Silurian–Devonian keratophyre/basalt magmatism and originated at 280–340 °C from the seafloor exhalates in Lower Paleozoic rift magmatism. The Silurian-Devonian rock sequences were metamorphosed in Permian. The limestone lenses were altered to Permian skarn in the epidote-amphibolite facies (526–546 °C, 3–6 kbar) and in related chlorite-apatite zone (420–540 °C) they consisted of garnet Grs41.4-60.2Sps19.8-32.8Alm16.7-22.2Adr0-8.5, hedenbergite Wo44.5-50.1Fs31.6-38.2En12.7-20.7, epidote, actinolite, fluorapatite, titanite, chlorite, ankerite and siderite. The disseminated original millerite was partially replaced by hauchecornite, hauchecornite-(Sb) and cobaltite was formed in the silicate matrix of the skarn. The Permian metamorphism of the epidote-amphibolite facies and the chlorite-apatite zone has also released a fluid phase rich in O2, H2O, CO2, H3PO4, H2S, HF and V from the organic matter-bearing rock (Holec Beds) into the overlying bed with keratophyres and primary stratiform SedEx sulfidic mineralization where V-rich micas, V-rich chlorite and schreyerite formed. The primary stratiform SedEx sulfidic mineralization composed predominantly of pyrite 1 less pyrrhotite, chalcopyrite, sphalerite 1, galena 1, arsenopyrite, ferrokesterite, stephanite, gudmundite, bismuthinite 1, PbBiSb-rich sulphide (A2B2S5-type) and kobellite has been oxidized and decomposed by this fluid phase to form a new metamorphic minerals in situ. The pyrrhotite decomposed to form szomolnokite, pyrite 2, goethite and siderite. The galena 1 was partly oxidized and decomposed to anglesite and cerussite. The In-rich ferrokesterite, Sn-rich sakuraiite and In-rich sphalerite 1 oxidized, decomposed, and reacted with the fluid phase to form cassiterite, chalcopyrite, In-rich sakuraiite and sphalerite 2. The PbSbBi-rich sulfide (A2B3X6-type), wittite, bismuthinite 2, native Bi and Se-rich galena 2 are new minerals, formed by the decomposition of original PbBiSb-rich sufhide (A2B2X5-type), kobellite and bismuthinite 1. Tetrahedrites 1–3 ranging from tetrahedrite-(Fe), kenoargentotetrahedrite-(Fe) to rozhdestvenskayaite-(Fe) show a gradual ordering in three separate zones controlled by immiscibility gaps in the Cu-Ag substitution of tetrahedrite group. These tetrahedrites were formed by the decomposition of the original SedEx ferrokesterite, stephanite and gudmundite. The gudmundite was also commonly oxidized to form valentinite and the decomposition of ferrokesterite, galena 1 and gudmundite also produced bournonite and plumosite-like. Altogether 22 reactions illustrate the relationship between the source SedEx mineralization and the new metamorphic minerals, which have formed at the expense of the original stratiform SedEx sulfidic mineralization in situ. The metamorphogenic fluid phase, released from the Holec Beds, has been enriched with additional elements from the stratigraphic horizon of the stratiform SedEx sulfidic mineralization and is genetically part of the Permian metamorphic-magmatic-hydrothermal (MMH) cycle (281–256 Ma).

Lithostratigraphy and metamorphism of the Khudi-Tal area, west central Nepal

The study is mainly focused on lithostratigraphy, petrography and geological structures to decipher the metamorphic facies and metamorphic history of the Khudi-Tal area along the Marsyangdi Valley, west central Nepal. Geological mapping was carried out covering 142 sq.km, on the scale of 1:50000. The Main Central Thrust (MCT) separates lithological succession of the Lesser Himalaya and Higher Himalaya around Bahundanda, Probhi and Naiche of the Khudi-Tal area. Lithological succession is dominated by metapelitic to psammatic schist, siliceous to dolomitic marble, quartzites, graphitic to garnet schist, kyanite garnet gneiss and banded gneiss from south to north. The kyanite isograd follow the MCT separating Kyanite zone with mineral assemblages Ky+Grt+Qtz+Ms+Bt+Pl of the Higher Himalaya and Garnet zone as Grt+Bt+Ms+Chl+K-Fel+Pl+Qtz in pelitic rocks, Bt+Ms+Chl+Plag+Qtz in Psammatic rocks and Cal/ Dol+Qtz+Pl+Bt+Ms in calcareous rock of the Lesser Himalaya. Based on the mineral assemblages, the Higher Himalaya belongs to Amphibolite Facies whereas the Lesser Himalaya belongs to Epidote-Amphibolite Facies. The lithological succession of the Higher and the Lesser Himalaya has undergone poly-phase metamorphism. In the Lesser Himalayan zone rocks, the regional burial metamorphism M1 (pre-MCT/ eo-Himalayan) is followed by barrovian type garnet grade inverted metamorphism M2 and later post-MCT retrogressive metamorphism M3. In the Higher Himalaya rocks, the early phase kyanite bearing medium temperature/ high pressure regional prograde metamorphism M1 (pre-MCT/ eo-Himalayan) is followed by barrovian type inverted metamorphism M3 (syn-MCT/ neo-Himalayan) and over printed by later phase retrogressive metamorphism M3 (post-MCT).

GENETIC TYPE AND CONDITIONS OF FORMATION OF GRANITOIDS OF THE
 KISEGACH, CHASHKOV, AND ELANCHIK MASSIFS (ILMENOGORSK
 ANTICLINORIUM)

The article provides a description of the geological structure of the Kisegach, Chashkov and Elanchik migmatite-gneiss granite massifs located in the southern and southeastern parts of the IlmenogorskSysert block. The petrographic, petrogeochemical characteristics of all varieties of their constituent rocks are given. It is shown that the Kisegach massif is composed mainly of gneissic biotite granites of phase I, much less granosyenites and porphyritic granites of phase II, which are intruded by dikes of phase III leucogranites. The Chashkov and Elanchik massifs are represented by granitoids, the formation of which occurred in two stages: migmatites and gneissic granites formed in the first stage, and biotite, muscovite, and two-mica granites formed in the second stage. Phase I granitoids belong to the potassium-sodium and potassium series and belong to the migmatite-granite formation. According to biotite-garnet and biotite-amphibole thermobarometers, it was found that the formation of gneissic granites and gneissic biotite granites of the Kisegach massif occurred at a temperature of 510–540°C as a result of metasomatic processes, and porphyritic granites crystallized from the melt at a temperature of 680–700°C in the abyssal zone on depth of about 23–24 km (Р=7.1–7.4 kbar). Subsequently, all its rocks, including monzogabbro and monzodiorite, experienced regional metamorphism under the conditions of the epidote-amphibolite facies (T=550–575°C, P=5.4–6.0 kbar). Based on the study of the garnet-biotite paragenesis, which is widely represented in the granitoids of the Elanchik and Chashkov massifs, the formation temperatures of gneissic granites (530–550°C and 510–530°C, respectively) and gneissic biotite granites (540–560°C and 520–540°C) were obtained, as well as an approximate estimate of the depth of rock formation was achieved, corresponding to the near-surface or hypabyssal zones (a few kilometers). The obtained data on the physicochemical conditions of formation of the rocks of the massifs made it possible to draw a conclusion about their metasomatic nature and exclude magmatic genesis. The obtained physicochemical characteristics repeat the parameters of metamorphism for the Precambrian stratigraphic units of the Ilmenogorsk-Sysert block, as well as the Kochkar anticlinorium (East Ural uplift). This indicates a single stage in the formation of migmatite-gneiss-granite domes within the entire Southern Urals, which apparently occurred of at the Upper Precambrian — Lower Paleozoic boundary.

Физико-химические условия формирования Ларинского гранито-гнейсового купола (Южный Урал)

The Larino granite-gneiss dome is located on the border of the Aramil-Sukhtelya and Magnitogorsk megazones and is a two-headed anticlinal structure, the nuclear parts of which are composed of granitoids of the Pervomai and Larino massifs, and their framing – apovolcanic amphibolites, garnet-mica-tins, garnet-amphislans siliceous-carbonaceous schists of the Bulatovo Series (S1–D1bl). A series of concentric high-gradient zones of metamorphism are observed from granitoids to shales. Mineral parageneses of the amphibolite facies at a distance of several kilometers are replaced by associations of epidote-amphibolite and greenschist facies. It has been proven that granitoids belong to the granite-migmatite formation, are close to trondhjemites of the inner parts of the continents (granites, leucogranites) and continental margins (gneisized, migmatized granitoids) and were formed in intraplate and collisional geodynamic settings. Crystallization of granites took place at temperatures of 780–840°С and a pressure of 2.1–3.7 kbar, plagiogranites – 880–940°С and 2.9–3.3 kbar, which corresponds to the abyssal depth zone (7–11 km). For granitoids formed in the process of metasomatic granitization (gneissized and migmatized rocks), formation temperatures are noticeably lower – 650–680°С, and the pressure is higher – 6.6–7.0 kbar (almandine amphibolite facies).

УЛАМКИ ЕКЗОТИЧНИХ МЕТАМОРФІЧНИХ СЛАНЦІВ В ОЛІГОЦЕНОВИХ ВІДКЛАДАХ СІЛЕЗЬКОГО ПОКРИВУ (УКРАЇНСЬКІ КАРПАТИ)

The study aims at analyzing the petrographic composition of exotic metamorphic schist clasts in the Oligocene deposits filling the Silesian Nappe in the south-western part of the Ukrainian Carpathians and reconstructing their possible source area. The methodology includes petrographic, sedimentological and comparative geological methods. To achieve this goal, field research was carried out along the first right tributary of the Hysnyi Stream (the village of Uzhok, Velykobereznyanskyi district, Transcarpathian region) and petrographic analysis of the selected samples of the exotic metamorphic schist clasts included in the Oligocene debris-flow deposits was performed. The results of microscopic studies showed that these exotic rocks are represented by mica-quartz schist and garnet-muscovitebiotite-plagioclase-quartz schist with a low content of chlorite, carbonate and epidote. Mineral composition and structure/texture features suggest the rocks were metamorphosed under green schist and epidote-amphibolite facies. Similar exotic rock fragments were identified by Polish scientists in the south-eastern part of the Polish Carpathians in the Silesian Nappe, where possible source area of the exotic rocks is related to the Bukowiec Paleo-Ridge, which had been located between the Dukla and Silesian basins of the Carpathian sedimentary realm. Scientific novelty. For the first time, the petrographic composition and textural and structural features of the exotic schists fragments included in the Oligocene deposits of the Silesian Nappe in the Ukrainian Carpathians (area of the village of Uzhok, Ukrainian Carpathians) are described. These schists are similar to the exotic rocks included in the Oligocene deposits of the Silesian Nappe in the Polish Carpathians, which may indicate the same source area.

Structural and Thermal Evolution of an Infant Subduction Shear Zone: Insights From Sub-Ophiolite Metamorphic Rocks Recovered From Oman Drilling Project Site BT-1B.

Subduction interface thermal structure changes drastically within the first few million years of underthrusting (i.e., subduction infancy). Metamorphic soles beneath ophiolites record snapshots of dynamic conditions and mechanical coupling during subduction infancy. Beneath the Samail Ophiolite (Oman), the sole comprises structurally higher high‐temperature (HT) and lower low‐temperature (LT) units. This inverted metamorphic gradient has been attributed to evolving metamorphic Pressure‐Temperature (P‐T) conditions during infancy; however, peak P‐T and timing of LT sole subduction are poorly constrained. Oman Drilling Project core BT‐1B sampled the base of the ophiolite in a location lacking the HT sole. Metasedimentary and meta‐mafic samples collected from 104 m of core reveal that the LT sole subducted to similar peak P as HT rocks preserved elsewhere in Oman, but experienced ∼300°C lower peak T. Prograde fabrics record Si‐in‐phengite and amphibole chemistries consistent with peak P‐T of ∼7–10 kbar and ∼450–550°C in the epidote‐amphibolite facies. Retrograde fabrics record a transition from near‐pervasive ductile to localized brittle strain under greenschist facies conditions. Titanite U‐Pb ages (n = 2) constrain timing of peak LT sole subduction to ∼91 Ma (post‐dating initial HT sole subduction by ∼12–13 Myr) and dynamic retrogression through ∼90 Ma. Combined with existing geo/thermo‐chronology, our results support a model of protracted subduction and accretion while the infant subduction zone experienced multi‐phase, slow‐fast‐slow cooling. Temporal overlap of HT sole cooling (rehydration?) and ophiolite formation suggests that cooling may lead to interface weakening, facilitating upper‐plate extension and spreading. The LT sole formed in a rapidly‐refrigerating forearc after ophiolite formation and may reflect the transition to self‐sustaining subduction.

Paleoproterozoic OIB- and MORB-Type Rift Volcanics of the Kursk Block, Eastern Sarmatia: Petrology and Geodynamics

The metabasites of the Tim Formation occur in the Paleoproterozoic Orel, Tim–Yastrebovka, and Avilsky synforms in the Archean Kursk block of Eastern Sarmatia. The volcanics are picrites, picrobasalts, basalts, and basaltic andesites metamorphosed to the greenschist and epidote-amphibolite facies. Their zircon (SIMS) age is 2099 ± 8 Ma. Based on their (Gd/Yb)n and Ti/Y ratios, the volcanics of the Tim Formation are subdivided into two types. The first one is OIB-type metabasites with (Gd/Yb)n > 2 and Ti/Y > 500. The parental melts of these rocks corresponded to ferropicrite and were derived by melting a garnet-bearing mantle source. The rocks of the second type are MORB-type volcanics with (Gd/Yb)n < 2 and Ti/Y < 500, which implies that the magma was derived from a garnet-free source at a relatively shallow depth. Their parental melts could be variously enriched MORB-type picrites. These magmas, which are contrastingly different in petrogenesis but had approximately equal age, were likely generated at various interaction stages between the asthenospheric mantle and eclogitized oceanic crust, subcontinental lithospheric mantle, and continental crust in the destruction environment of a subducted oceanic slab at the accretionary–collisional interaction between the Kursk domain and Volga–Don orogen.

U-Pb Age, by Zircon, of the Pre-Klesiv Breed Association of the Osnytsky Block

The Osnytsky block is an integral part of the Osnytsky-Mikashevytsky volcanic-plutonic belt, represented by early proterozoic igneous rocks of different compositions. This is a new type of igneous belts for the early precambrian – a paleotypical analogue of intracontinental volcanic-plutonic belts by the phanerozoic, which was formed in the conditions of mature continental crust in the initial stage of radical structural rearrangement of the cratonized gneiss base of the Eastern European platform. Volcanogenic formations have undergone regional metamorphism at a level not higher than the epidote-amphibolite facies, classified in the klesiv series, which combines effusives of basic, acidic and medium composition and occupies a higher stratigraphic position compared to the metamorphites of the teteriv series. Intrusive rocks (gabbro-diorite-granodiorite granite association) – to the osnitsky complex. Rocks of the osnitsky complex are widespread, they account for 80-85% of the area of the Osnitsky block. The pre-klesiv association, represented mainly by gneisses of the teteriv series and plagiogranitoids of the sheremetivsky complex, has a limited distribution. The LA-ICP-MS U-Pb method was applied to date zircon from plagiogranite and plagiomigmatite of the sheremetivsky complex, and from gneiss and amphibolite. The plagiogranite yielded an age of 2110 ± 8, and plagiomigmatite of 2165 ± 22 Ma. Hence, granitoids of the sheremetivsky complex in the Osnitsk block were formed between 2.16 and 2.11 Ga. The age of the metamorphism of gneisses was defined as 2198 ± 29 Ma, whereas zircon from amphibolite crystallized at 1981 ± 36 Ma. Correspondingly, gneiss can be attributed to the teteriv series, whereas amphibolite can belong to either osnitsky complex, or nartsyzivsky complex.

Petro-Structural Study of the Paleoproterozoic Formations of the Faboula Gold Deposit (Bougouni-K&amp;#233;koro Basin, Leo-Man Shield)

Recent petro-structural investigations on the Faboula gold deposit located in the Bougouni-Kékoro basin, in southern Mali, north-west of the Leo-Man Shield, have provided new data on the nature and spatial organization of the lithostratigraphic units as well as their deformation style. The deposit is covered by a thick lateritic layer and is hosted by a metavolcano-sedimentary sequence of Paleoproterozoic age intersected by intrusive bodies and filled fractures of various shapes and types. The lithostratigraphic units consist of metagreywackes, metasiltstones, meta-argillites, slates and schists. Metagreywackes and metasiltstones are generally feldspathic, both may contain biotite and locally amphibole, just as slates may contain andalusite which is locally stretched. Plutonic units most often occur as stocks or as dikes on the drill core, up to 1 m. The metavolcano-sedimentary rocks are schistose and deformed under greenschist facies conditions, and locally they reach the epidote-amphibolite facies. The structural study revealed that the deposit is affected by several stages of deformation evolving from a ductile type to a brittle type via a ductile-brittle type. The dominant ductile and brittle-ductile deformations show a combination of isoclinal folding and strike-slip faults. Both the isoclinal folding and the strike-slip faults whose sigmoidal en-echelon tension gashes indicate a dextral movement in the NNE-SSW direction are the result of the same ENE-WSW regional shortening. Consequently, they highlight a transpressive deformation. This deformation noted here D2Fb, could be equivalent to the regional D2 or D3 deformations identified at the scale of the Leo-Man Shield if we refer to the style of deformation. There is an abundance of quartz veins networks. Their relation within the structural features indicates that the mineralization is structurally controlled during a hydrothermal event linked certainly to the circulations of fluids during the transpressive event D2Fb.

Reappraisal of the oldest high-pressure type schist in Japan: New zircon U-Pb age of the Kitomyo Schist of the Kurosegawa Belt

The Kitomyo Schist from Kurosegawa Belt, Shikoku, has been long considered as the oldest records of subduction metamorphism in Japan, based on an early 1970s K–Ar dating of white mica. The schist consists of mafic and pelitic layers and occurs as a tectonic block within serpentinite. Reappraisal of the schist confirmed the schist is characterized by an epidote-amphibolite peak metamorphic facies. The mafic portion is characterized by zoned amphibole + epidote + chlorite + titanite ± phengite ± rutile. The presences of relict rutile surrounded by titanite and the barroisitic cores of zoned amphibole suggest a high-pressure intermediate type metamorphism at the metamorphic peak (P = ~0.8–1.5 GPa and T = ~500–570 °C). The presence of Mn-rich garnet and the lack of biotite, oligoclase and paragonite also support high-pressure intermediate type metamorphism that eliminate the possibility of a typical blueschist-facies metamorphism. New SHRIMP and LA-ICPMS zircon U–Pb geochronology on a pelitic sample show detrital grains of Mesoproterozoic and Early Paleozoic ages, suggesting a maximum deposition age for the trench-fill sediment of ~440 Ma. Also the U–Pb data confirmed ~360 Ma overgrown rims that might have formed during the subduction zone epidote-amphibolite facies metamorphism. Reappraisal revealed that the Kitomyo Schist is not the oldest high-pressure type schist in Japan and rather comparable to the Late Paleozoic Renge Metamorphic Rocks and their equivalents in the Kurosegawa Belt. The Devono–Carboniferous high-pressure metamorphic rocks in Japan might have been paired with their coeval batholiths along the ‘Greater South China’ margin that was extensively eroded during later tectonic processes.

GEOLOGICAL STRUCTURE AND FORMATION CONDITIONS OF THE CHELYABINSK GRANITOID MASSIF (SOUTHERN URALS)

The Chelyabinsk granitoid massif is located in the northern part of the East-Ural megazone and it is about 1100 km2 in area. It is an integral part of the Main granite belt of the Urals. The massif is one of polyformational, polychronous and is a close analogue of the granite-gneiss domes: Varlamovsky, Borisovsky, Yeremkinsky, Sanarsky (Kochkarsky anticlinorium) and Beloretsky (Bashkirsky meganticlinorium). Four main granitoid complexes are distinguished in the Chelyabinsk pluton by age and petrographic parameters: smolensky (Є?S) diorite-tonalite (migmatite), warshavsky (C2v) granite-granitegneiss, poletayevsky (C2pl) granodiorite-granite and kremenkulsky (P2kr) granite-leucogranite. It is established that smolensky granite-gneisses, warshavsky granites, poletaevsky granodiorites and granites belong to the tonalite-granodiorite formation. The first of them were formed in the intracontinental geodynamic setting, and the second and third - on the marginal continental setting. Warshavsky granite-gneisses are close to the granitemigmatite formation in petrogeochemical features, and the kremenkulsky leucogranites are close to granite. The geodynamic setting of their formation corresponded to the interior of the continents. The igneous group includes warshavsky granites and kremenkulsky leucogranites, in the quartz of which melt inclusions are established, as well as granodiorites and granites of the poletayevsky complex, having formation temperatures of 800 °C and 700 °C and a pressure of 6.1 and 4.3 kbar (abyssal zone), respectively based on the study of the chemical composition of biotite and garnet. The smolensky and warshavsky gneisses and migmatites apparently have a metasomatic genesis and subsequently underwent a transformation under the conditions of the green shale and epidote-amphibolite facies (400 - 460 °С and 490 - 510 °С

Metamorphic imprint of ridge subduction on the Neo-Tethyan ophiolites from the Saka Unit (Central Pontides, northern Turkey)

The Intra-Pontide suture zone is the northernmost ophiolite suture zone exposed in the Anatolian peninsula. It consists of several variably deformed and metamorphosed tectonic units derived from the Neo-Tethyan Intra-Pontide oceanic (IPO) basin, and its continental margins that are currently represented by the Istanbul-Zonguldak and the Sakarya terranes. Recent data suggests that the IPO was a wide supra-subduction oceanic basin whose closure began in the uppermost Early Jurassic by a north dipping intra-oceanic subduction that divided the IPO into two different oceanic areas, namely IPO1 and IPO2. The Saka Unit is a small tectonic unit exposed in the eastern portion of the Intra-Pontide suture zone in the Central Pontides, which is interpreted as a tectonic mélange produced during the intra-oceanic subduction that led to the closure of IPO1. The unit was affected by polyphase deformation and epidote–amphibolite facies metamorphism peak conditions experienced during the uppermost Middle Jurassic. Metamorphic studies and thermodynamic investigations constrain the peak pressure conditions to T ≈ 650–700 °C and P ≈ 0.9–1.3 GPa. These conditions reveal a ‘hot’ and anomalous geothermal gradient (15–22 °C/km) with respect to those of the other ophiolite-bearing units of the Intra-Pontide suture zone involved in the subduction. This anomalous gradient was herein attributed to the thermal pulse produced by the subduction of the IPO1 mid-oceanic ridge. The final re-equilibration occurred under greenschist facies conditions during the Early Cretaceous exhumation, when the oceanic lithosphere of the IPO was completely consumed and the continental collision between the Istanbul-Zonguldak and Sakarya terranes occurred.

Multi‐stage metamorphism by progressive accretion of continental blocks, example from the Western Hindu Kush

AbstractBasement rocks from the Western Hindu Kush preserve evidence of multiple metamorphic and magmatic events that occurred along the boundary between the Archean–Proterozoic Afghan Central and Afghan–Tajik Blocks. To verify the different metamorphic stages or events, mineral textures and phase equilibria in metamorphic basement rocks and their age relations to magmatic episodes have been investigated. Quartzofeldspathic gneiss and migmatite with lenses of amphibolite (with assumed Proterozoic age for their metamorphism) are intruded by the Triassic Hindu Kush granitoid batholith and small Cretaceous and Oligocene granite intrusions. The age of thermal overprint (210–170 Ma) by the Triassic batholith is confirmed by new monazite data. Both Triassic and Cretaceous granitoids and surrounding basement rocks underwent subsequent metamorphism up to epidote–amphibolite facies. The degree of this metamorphism increases southward at the contact to the Kabul Block, which under‐plates the Western Hindu Kush from the south. An early Miocene age was obtained by Pb–Th analyses in thorite and huttonite, which are close or slightly younger than the Oligocene granite in this area. The Cretaceous meta‐granodiorite near the border with the Kabul Block contains xenoliths of granulite facies rocks that could come from the Neoarchean granulite facies basement of the Kabul Block. The multi‐stage metamorphic and magmatic evolution classifies the Hindu Kush mountain belt as a long‐lived suture zone that was active since the early Palaeozoic. The results of this study support the interpretation about possible relations of the Afghan Central Blocks to the southern margin of Eurasia during the evolution of Para‐ and Neotethys.