Mafic Schists Research Articles

Reduction of iron with no systematic isotope fractionation during continental subduction observed in metamorphic rocks from the Dabie-Sulu orogen, China

• No resolvable Fe isotope fractionation in metabasites during prograde dehydration. • Eclogites have Fe 3+ /∑Fe systematically lower than the other metabasites. • Fe reduction likely occurred during eclogitization with simultaneous S oxidization. In order to better understand the mobility and charge balance of Fe in subducted slabs, here we report precise Fe isotopic data, supplemented by Fe 3+ /∑Fe and C-N-S elemental contents, in a well-characterized suite of metamorphic rocks from the Dabie-Sulu orogen, China, including mafic to felsic greenschists, amphibolites and eclogites. There is no systematic Fe isotopic difference among the mafic schists (δ 56 Fe = 0.08 ± 0.08‰, 2SD, N = 13), amphibolites (δ 56 Fe = 0.09 ± 0.04‰, 2SD, N = 19) and eclogites (δ 56 Fe = 0.07 ± 0.06‰, 2SD, N = 27), implying limited Fe mobility during prograde metamorphic dehydration. The overall δ 56 Fe variation of the metabasites and felsic schists ranging from 0.02 ± 0.03‰ to 0.20 ± 0.03‰ most likely mirrors heterogeneity in their Neoproterozoic magmatic protoliths, and can be interpreted by derivation from diverse sources and isotope fractionation during magma differentiation. In a contrary to limited Fe mobility, a decrease in Fe 3+ /∑Fe ratios has been identified for metabasites during eclogitization (on average 0.43 for greenschists, 0.42 for amphibolites and 0.24 for eclogites respectively). Reduction of Fe could have been compensated by oxidation of S where a concurrent expulsion of sulfate seems to be necessary. This study thus supports the hypothesis that Fe-S redox coupling in subducted slabs promotes transfer of oxidizing agents into the mantle wedge in subduction zones.

Eoarchean subduction-like magmatism recorded in 3750\xa0Ma mafic\u2013ultramafic rocks of the Ukaliq supracrustal belt (Qu\xe9bec)

Our understanding of the nature of crustal formation in the Eoarchean is limited by the scarcity and poor preservation of the oldest rocks and variable and imperfect preservation of protolith magmatic signatures. These limitations hamper our ability to place quantitative constraints on thermomechanical models for early crustal genesis and hence on the operative geodynamic regimes at that time. The recently discovered ca. 3.75 Ga Ukaliq supracrustal enclave (northern Québec) is mainly composed of variably deformed and compositionally diverse serpentinized ultramafic rocks and amphibolitized mafic schists whose metamorphic peak, inferred from phase equilibria modeling, was below 720 °C. Inferred protoliths to the Ukaliq ultramafic rocks include cumulative dunites, pyroxenites, and gabbros, whereas the mafic rocks were probably picrites, basalts, and basaltic andesites. The bulk-rock and mineral chemistry documents the partial preservation of cumulative pyroxenes and probably amphiboles and demonstrates the occurrence of a clinopyroxene-dominated, tholeiitic suite and an orthopyroxene-dominated, boninite-like suite. Together with the presence of negative μ142Nd anomalies in the boninitic basalts, two liquid lines of descent are inferred: (i) a damp tholeiitic sequence resulting from the fractionation of a basaltic liquid produced by mantle decompression; and (ii) a boninitic suite documenting the evolution of an initially primitive basaltic andesite liquid produced by flux melting. Petrographic observations, thermodynamic modeling, bulk-rock and mineral chemistry, and 142Nd isotopic compositions identify the Ukaliq supracrustal belt as the remnant of an Eoarchean arc crust produced by the recycling of Hadean crust in a similar way as modern-style subduction.

Pressure–temperature–time evolution of subducted crust revealed by complex garnet zoning (Theodul Glacier Unit, Switzerland)

AbstractCollisional orogens commonly include mono‐metamorphic and poly‐metamorphic units, and their different evolution can be difficult to recognize and reconcile. The Theodul Glacier Unit (TGU) in the Western Alps consists of an association of metasedimentary and metamafic rocks embedded within the Zermatt‐Saas tectonic unit. In spite of recent petrological studies, it remains unclear whether these rocks underwent one or multiple metamorphic cycles. In this study, different lithologies from the TGU unit (mafic schist, mafic granofels, and chloritoid schist) were investigated for petrography, quantitative compositional mapping of garnet, thermodynamic modelling, and Lu–Hf garnet dating. The data reveal a coherent mono‐metamorphic history with a β‐shaped pressure–temperature (P–T) path characteristic of oceanic subduction. Garnet Lu–Hf ages yield a restricted garnet crystallization time window between 50.3 and 48.8 Ma (±0.5%, 2SD). A prograde metamorphic stage recorded in garnet cores yields conditions of 490 ± 15℃ and 1.75 ± 0.05 GPa. Maximum pressure conditions of 2.65 ± 0.10 GPa and 580 ± 15℃ were reached at 50.3 ± 0.3 Ma. Initial exhumation was rapid and led to isothermal decompression to 1.50 ± 0.10 GPa within 1 Ma. This decompression was associated with lawsonite breakdown in mafic schist and in mafic granofels, causing intense fluid–rock interaction within and between different lithologies. This process is recorded in garnet textures and trace element patterns, and in the major element composition of K‐white mica. Initial exhumation was followed by re‐heating of ~30℃ at a pressure of 1.50 ± 0.10 GPa. Perturbation of the subduction‐zone thermal structure may be related to upwelling of hot asthenospheric mantle material and transient storage of the unit at the crust–mantle boundary.

Metamorphic rocks from the north-eastern part of the Ereendavaa terrane (Eastern Mongolia) :

In this paper, we have conducted geochronological and geochemical studies on the metamorphic rocks of the Khaychingol and Ereendavaa Formations in the Mogoitiin Gol, Khaychin Gol and Emgentiin Bulag areas from the Ereendavaa terrane and these rocks have been considered to be Precambrian in age. However, new LA–ICP–MS zircon U–Pb dating results indicate that the protolith of the studied metamorphic rocks was formed in two stages: 1) during ~ 296 - 285 Ma, the protolith of mafic, felsic and black schists formed; 2) during ~276 - 271 Ma, the protolith of gneiss and psammitic schists began to deposit. The Early Permian bimodal association composed of low-K basalt and comagmatic high-Na, low-K dacite with high-K calc-alkaline rhyolite, represent protolith of the mafic and felsic schists which were formed in back-arc basin environment. The Middle Permian gneiss, and psammitic schists with sedimentary protolith have geochemical signatures of island arc rocks, such as enrichment of LILE relative to HFSE, and markedly negative Nb, Ta and Ti anomalies, suggesting that they were formed in a continental arc environment. Considering a close spatial relationship of the Ereendavaa terrane with the Mongol-Okhotsk Belt in the north-west, we propose that accompanied with the emplacement of arc magmatic rocks, the arc rifting occurred and formed the Early Permian bimodal volcanic rocks. In the Late Permian, after the formation of the back-arc basin, deposition of the immature deposits as wacke, arkose and litharenite dominated sediments in a continental arc environment started.

Geochemistry and Sr–Nd isotopic composition of meta-gabbros from the Omi serpentinite mélange, Niigata, SW Japan: Evidence for subduction erosion in an immature early Paleozoic arc-trench system in proto-Japan

The Renge Belt is a rare subduction-related metamorphic belt that contains blocks of eclogites, meta-gabbros, schists and other rocks in a serpentinite mélange that formed in the early-to-middle Paleozoic of proto-Japan. We report here a detailed geochemical and Sr–-Nd isotope study of two major groups of rocks within the Omi serpentine mélange: 1. medium-grade metamorphosed blocks of meta-gabbros, and 2. high-pressure low-temperature eclogites and mafic schists. Bulk rock geochemistry of the meta-gabbros reveals that they have prominent Nb, Ta and Ti negative anomalies, suggesting an arc origin. In contrast, the eclogites, and mafic schists have trace element spider diagrams and REE patterns that resemble modern oceanic crust. The Sr–Nd isotopic compositions also show concomitant variations with a highly depleted source for the meta-gabbros (εNd values of +1.3 − +9.6 and Sriratios of 0.70301–0.70460) compared with a MORB/OIB-like source of the eclogites and mafic schists (εNd values of −1.94 – +4.83), although the latter have Sr isotopes variably affected by seawater interaction (Sriratios of 0.70518–0.71050). Altogether, the geochemical features suggest that the meta-gabbros were part of a magmatic arc, which developed in the Cambrian and was tectonically eroded and subducted together with oceanic crust and sediments in the Devonian–Carboniferous. The ages of detrital zircons in the pelitic schist (3200–600 Ma) suggests that the South China Craton and northeastern fringes of Gondwana were potential source regions and that the sedimentation occurred most likely in a trench. The abundant Early Paleozoic (peak around ca. 470 Ma) detrital zircons were derived from an active subduction-related arc. The protoliths of meta-gabbros are lower crustal equivalents of an infant arc, which was subducted via subduction erosion in the Devonian–Carboniferous and reached a HP metamorphic depth in the Tournasian. Our results suggest that the formation of proto-Japan was related to the initiation of intra-oceanic arc as early as Cambrian. Following tectonic erosion, the remnant mafic rocks were amalgamated with trench-fill sediments during incorporation in an accretionary complex. Subduction erosion was active where sediment supply was low and where the arc was immature. Subduction of oceanic crust was continuing to at least the middle Carboniferous, when the eclogites and blueschists were formed and later exhumed along with the meta-gabbros in serpentinite diapirs in the mid–late Paleozoic.

Multi-stage infiltration of Na- and K-rich fluids from pegmatites at mid-crustal depths as revealed by feldspar replacement textures

In volcanic zones, geophysical observations have identified the presence of deep-seated fluids around intrusions in the mid-crust. However, the fluid compositions and mechanisms of fluid migration at high temperatures in the crust are still poorly understood. In this study, we investigated plagioclase alteration in mafic schists (clinopyroxene [Cpx] schist, hornblende [Hbl] schist, and actinolite [Act] schist) at the contact with a quartz diorite intrusion on Kinkasan Island, NE Japan. The quartz diorite consists of hornblende and plagioclase, and crystallized at 670–760 °C and 0.30–0.45 GPa. The quartz diorite contains abundant pegmatitic veins, which consist of plagioclase at their margins and K-feldspar + quartz ± garnet in their centers. Some pegmatitic veins cut the schistosity of the metamorphic rocks. In the metamorphic rocks, two stages of fluid-mediated alteration during cooling were recognized. The first stage of fluid infiltration (Stage I) was characterized by the formation of a reaction zone around pegmatitic dikes in the Cpx schists. The Cpx was altered to hornblende (Hbl) and Ca-plagioclase (An83–95) was altered to Na-plagioclase (An36–67). The replacement proceeded along the grain boundaries of plagioclase and clinopyroxene. The second stage (Stage II) resulted in the formation of K-feldspar veins and replacement of plagioclase (An35–57) by K-feldspar (An0Ab1Or99) and albite (An2Ab95Or3–An12Ab87Or1) in the Act and Hbl schists, and the latter were probably derived from the Cpx schists. Assuming the pressure was the same as the crystallization of the quartz diorite intrusion, the alteration temperatures were estimated as 690–730 °C for Stage I and 400–570 °C for Stage II. Mass balance considerations with assumptions of the volume conservation during the replacements of plagioclase and clinopyroxene/amphibole grains indicate the Cpx schists gained Na and H2O and lost Ca in Stage I, and the Act schists gained K and Si and lost Al and Ca at almost constant Na in Stage II. This indicates that the fluid composition changed from Na- to K-rich during cooling. During the Stage II alteration, when ~45% of the plagioclase grains were altered, significant nano- to micro-scale pores were generated in plagioclase, which produced a porosity of ~3.0%–3.5% in the plagioclase and increased the whole-rock porosity by 1.3% ± 0.2%. These pores are now isolated, irregularly shaped, and occur preferentially along the replacement front, suggesting that the pores migrated during or after replacement. The altered zone in plagioclase developed from trans-granular microcracks rather than grain boundaries. Our results suggest that infiltration of K-rich fluids could be self-promoting, with such fluids forming their own pathways via dissolution and precipitation processes during plagioclase replacement. Given that plagioclase is the dominant mineral in various rock types in the mid-crust, reaction-induced porosity networks in plagioclase could be the dominant fluid pathway at mid-crustal depths. The nature of this porosity and fluid migration are potentially controlled by temperature and/or the composition of the fluids.

Revisiting the type area VMS deposit of Besshi, SW Japan: In-situ trace element chemistry, isotopes and Re-Os age of sulfides

The Besshi Cu-rich volcanogenic massive sulfide (VMS) deposit occurring in the type area of Besshi, central Shikoku, Southwest Japan comprises tabular orebodies closely associated with mafic schist in the Cretaceous Sambagawa high P/T metamorphic belt. Pyrite, chalcopyrite, and sphalerite are the dominant sulfide minerals, together with minor amounts of pyrrhotite and bornite. The associated greenstone samples show geochemical features between N-MORB and E-MORB. The δ34S values of sulfides from the greenstones and Cu ores are in the range of 2.10 to 7.11‰, indicating deposition from hydrothermal fluids with a heterogeneous metal source. Re-Os analysis of sulfides show initial (187Os/188Os)i ratio of 1.14 ± 0.86 and yield an isochron age of 147.9 ± 4.2 Ma, considered as the timing of sulfide deposition on the paleo-seafloor. The lithological association, geochemical features of the host greenstones and isotopic data from the sulfides indicate that the Besshi-type ores formed in a pelagic setting, and are possibly associated with hydrothermal activity related to mid ocean ridge volcanism and can be correlated to a major oceanic anoxic event.

Petrology and U–Pb geochronology of high-grade metavolcano-sedimentary rocks from central Xolapa Complex, southern Mexico

The Xolapa Complex (XC) is a high-grade metamorphic belt exposed along the southern margin of the North American plate in Mexico. Its evolution includes an episode of widespread anatexis related to crustal thickening and subsequent uplift to upper crustal levels, which culminated in the Paleocene. On the basis of field and petrographic work, this study integrates a petrological modelling approach with LA-ICP-MS U–Pb zircon and monazite geochronology to elucidate the tectonothermal evolution of this metamorphic belt. The study area in central XC includes a suite of alternating migmatitic paragneiss and garnet-bearing mafic schist. Petrological evidence and phase equilibria calculations demonstrate that a staurolite–kyanite grade metamorphism occurred at ~640–670 °C and 8–9 kbar before extensive migmatization took place. Differential anatexis of fertile and refractory layers occurred progressively via biotite and amphibole dehydration-melting reactions, and continued during peak metamorphism at granulite-facies conditions of ~800–820 °C and ~5–7 kbar. According to Ti-in-zircon thermometry, melt generation/mobilization progressed during cooling until crystallize at ~700 °C. The age of prograde metamorphism was likely recorded by monazite cores that give mid- to Late Cretaceous ages. Cooling and melt crystallization could be constrained by leucosome zircon dated at 61.8±0.6 Ma. Both zircon and low-Y monazite from whole rock and melanosome portions, respectively, are consistent with an episode of growth around 60.9±0.5 Ma. Monazite, however, would have continued its crystallization process for a period of ca. 10 Ma. Thin zircon and high-Y monazite rims show ages between ~34–25 Ma and are interpreted to reflect a stage of reheating by magmatic advection related to large Oligocene plutonism in the region. Age and nature of central XC basement show good correlation with other Early-Cretaceous volcano-sedimentary basins of central and southern Mexico. A period of crustal thickening would have buried the sequence at depths of ~30 km, causing Barrovian-type metamorphism during mid- to Late Cretaceous time and eventually, anatexis that evolved at granulite-facies conditions during orogenic collapse in the Paleocene (≥62 Ma). The time span from leucosome crystallization to late-stage reheating in central XC would imply a protracted high-temperature evolution indicating a middle- to upper-crustal residence time of at least ca. 30 Ma.

Structural and hydrothermal evolution of the shear-zone-related iron-oxide enrichment in metavolcano-sedimentary rocks of the intracontinental araçuaí orogen: The case of the espírito santo iron deposit

The Rio São João metavolcano-sedimentary sequence is located in the eastern border of the northern Espinhaço fold thrust belt, in the intracontinental Araçuaí-West Congo Orogen. Mafic schist, quartz-, amphibole- and carbonate itabirites, as well as a carbonate–silicate rock, occur in the study area. These contain mineral assemblages compatible with the amphibolite-facies metamorphic conditions. Magnetite- and hematite-rich itabirites form a series of iron deposits in the region near Igaporã-Caetité, Bahia state, Brazil. The Espírito Santo iron deposit has compact hematite layers truncating quartz itabirite with magnetite varying from 10 to 55 vol% of the rock volume; siliciclastic and mafic rocks also host iron-rich domains, variably enriched in magnetite and hematite. The deposit is hosted in compressional duplexes with structural top towards the SW. These structures have an Sn-1//Sn foliation represented by compositional banding and parallel schistosity that is observed at all scales. Other common features are S/C/C, boudins, pinch-and-swell structures, as well as sheath, syn-tectonic folds, and a mineral stretching lineation (Lxn). Hydrothermal alteration was contemporaneous to the development of duplex and the Sn-1//Sn foliation. A distal alteration zone is dominated by a potassic (muscovite and-or biotite) ± chlorite assemblage and the intermediate zone by carbonate development. The proximal alteration zone constitutes the iron oxide (magnetite and hematite) domains; sulfide minerals as well as quartz veins (silicification) in association with other phases also formed. Hydrothermal magnetite is associated with C’ structures, which grows either including silicates and skeletal carbonates, or forming corrosion borders in these minerals. Lamellar hematite forms polycrystalline aggregates, characterizing the S/C structure in the Sn-1//Sn foliation, as well as the axial planar foliation (Sn) in intrafolial folds and the mineral stretching line Lxn. Data obtained by the LA-ICPMS technique show that, in general, in quartz-rich and amphibole itabirites hydrothermal magnetite is richer in REE than an early-stage magnetite. Hydrothermal magnetite REE pattern is close to that of the country rock of the mineralization. The formation of hematite- and magnetite-rich domains is related to the percolation of hydrothermal fluids that leached all components of the original silicates, quartz and the carbonates of all host rocks, and precipitated new hydrothermal phases taking advantage of the Ediacaran shear structures.

Petrogenesis of the post-collisional porphyritic granitoids from Jhalida, Chhotanagpur Gneissic Complex, eastern India

AbstractThe Jhalida porphyritic granitoid pluton is exposed in a regional shear zone belonging to the Chhotanagpur Gneissic Complex of the Satpura Orogen (c. 1.0 Ga), regarded as the collisional suture between the South and North Indian blocks. The pluton intruded the migmatitic gneisses, metapelites, calc-silicate rocks and amphibolites belonging to the amphibolite facies. The mineral assemblage indicates the calc-alkaline nature of the granitoids. Mafic (Pl–Qz–Bt±Hbl) schists occur as xenoliths within the pluton. The granitoids are classified as alkali-calcic to alkalic, dominantly magnesian grading to ferroan, metaluminous to slightly peraluminous, and shoshonitic to ultrapotassic. Geochemically, the granitoids are enriched in large-ion lithophile elements (LILE), particularly K, and light rare earth elements (LREE), but are comparatively depleted in Nb, Ta, and heavy rare earth elements (HREE). The strong negative correlation between SiO2and P2O5, metaluminous to weakly peraluminous character, high liquidus temperature (798–891°C) and high fO2(ΔQFM +0.8 to +1.6) of the melt suggest their I-type nature. Field relations and tectonic discrimination diagrams imply their post-collisional emplacement. Low Nb/U (average 8.5), Ce/Pb (average 9.0), and Al2O3/(Al2O3+ FeO(t) + MgO + TiO2) ratios and relatively low Mg number (average 0.15) of these granitoids indicate a crustal mafic source. Batch melting (at 825–950°C) of 10–20% of an old, incompatible elements-rich high-K high-alumina hornblende granulite can generate the porphyritic granite melt. The heat source for melting was an upwelling of the asthenospheric mantle in the post-collisional set-up. Textural and chemical characteristics of the mafic xenoliths show that invading porphyritic granitoid magma metasomatized the amphibolite protoliths.

The latest Jurassic protoliths of the Sangsang mafic schists in southern Tibet: Implications for the spatial extent of Greater India

Field observations, petrology, zircon geochronology, whole-rock geochemistry, and Sr-Nd isotopes were used to reveal the lithology, age, and tectonic setting of the protoliths of the Sangsang mafic schists in the Yarlung Zangbo Suture Zone (YZSZ), central southern Tibet. The mafic schists occur as exotic blocks within the accretionary complex of the YZSZ. Relic amygdaloidal features indicate the schist protoliths were volcanic rocks. The mineral assemblage mainly comprises riebeckite + magnesioriebeckite + chlorite + sericite + albite + relic clinopyroxene. The youngest group of zircon ages constrains the formation time of the protoliths to 149.2 ± 2.2 Ma (i.e., latest Jurassic). Abundant Paleozoic and older zircons suggest the protolith volcanic rocks were erupted onto a continental terrane. Whole-rock geochemical and Sr-Nd isotopic data indicate the protoliths were of ocean island basalt affinity. The mafic schists mostly have high-Ti, alkaline, basaltic compositions with 43.57–46.93 wt% SiO2, 3.27–7.24 wt% Na2O + K2O, and 4.04–4.69 wt% TiO2. The schists are enriched in light rare earth elements relative to heavy rare earth elements, and have (La/Yb)N = 5.85–8.53 and small positive Ta and negative Zr-Hf anomalies. (87Sr/86Sr)i values vary from 0.7044 to 0.7055, while (143Nd/144Nd)i ranges from 0.512670 to 0.512727, with εNd(t) values of +4.4 to +5.5. The protoliths of the mafic schists were probably formed in a within-plate extension setting associated with mantle plume upwelling and melting of continental lithosphere. This setting was related to the Late Jurassic continental breakup of Argoland off the northern margin of east Gondwana, and thus marked the paleoposition of the northern edge of Greater India before the breakup of India from east Gondwana during Early Cretaceous. The N-S extent of Greater India at that time was ~2400 km. This further indicates how the India–Eurasia continental convergence has been accommodated.

Deformation of mafic schists from subducted oceanic crust at high pressure and temperature conditions

We conducted triaxial deformation experiments using a Griggs-type solid pressure-medium apparatus on two mafic schists that experienced different peak metamorphic conditions from the Sambagawa metamorphic belt; a greenschist and an epidote-amphibole schist. At a confining pressure of 1GPa, 400°C and strain rate of 10–5 1/s, differential stresses for all mafic schists were higher than 1GPa. The greenschist samples were weaker than epidote-amphibolite samples under all experimental conditions. Microstructures of recovered samples of greenschist indicate that deformation of mafic schist is accommodated by semi-brittle deformation. Samples of the epidote-amphibole schist deformed at 500°C show shear localization along a single fault oriented parallel to the foliation, while mechanical data shows no rapid stress drop. On the surface of the fault, we observe the formation of slickensides with little gouge. All sample types exhibit a high stress exponent (n > 15) and strain rate strengthening (i.e., positive a–b values); frictional behavior that inhibits earthquake nucleation. Differential stress increased with increasing confining pressure, while the friction coefficient decreased with increasing confining pressure and temperature. Greenschist and epidote-amphibolite facies metamorphism typically occurs at hot subduction zones such as Cascadia and southwest Japan. The lack of seismicity within the subducting oceanic crust at hot subduction zones could be explained by semi-brittle deformation, or frictional sliding with a positive (a – b) during deformation of these types of metamorphic rocks. Weak and aseismic fault zones in subducting slabs might promote slab-mantle decoupling at the corner of the mantle wedge, and detachment and underplating of oceanic crust from the subducting slab to forearc crust.

A new occurrence of retrogressed eclogite from the Sanbagawa belt of southwest Japan and its significance

AbstractThe present paper reports, for the first time, the occurrence of an omphacite‐bearing mafic schist from the Asemi‐gawa region of the Sanbagawa belt (southwest Japan). The mafic schist occurs as thin layers within pelitic schist of the albite–biotite zone. Omphacite in the mafic schist only occurs as inclusions in garnet, and albite is the major Na phase in the matrix, suggesting that the mafic schist represents highly retrogressed eclogite. Garnet grains in the sample show prograde‐type compositional zoning with no textural or compositional break, and contain mineral inclusions of omphacite, quartz, glaucophane, barroisite/hornblende, epidote and titanite. In addition to the petrographic observations, Raman spectroscopy and focused ion beam system–transmission electron microscope analyses were used for identification of omphacite in the sample. The omphacite in the sample shows a strong Raman peak at 678 cm−1, and concomitant Raman peaks are all consistent with those of the reference omphacite Raman spectrum. The selected area electron diffraction pattern of the omphacite is compatible with the common P2/n omphacite structure. Quartz inclusions in the mafic schist preserve high residual pressure values of Δω1 > 8.5 cm−1, corresponding to the eclogite facies conditions. The combination of Raman geothermobarometries and garnet–clinopyroxene geothermometry gives peak pressure–temperature (P–T) conditions of 1.7–2.0 GPa and 440–540 °C for the mafic schist. The peak P–T values are comparable to those of the schistose eclogitic rocks in other Sanbagawa eclogite units of Shikoku. These findings along with previous age constraints suggest that most of the Sanbagawa schistose eclogites and associated metasedimentary rocks share similar simple P–T histories along the Late Cretaceous subduction zone.

Spacial analysis of Lithology and forest in Doddahalla watershed, Chikmagalur district, Vedavati river sub-basin, Karnataka, India

The Lithology are a significant geographical component of this watershed and are from the stratigraphy of topographical opportunity scale goes under Archean to Lower Proterozoic. The various kinds of geography in the investigation region which incorporates Basal Polymict Conglomerate, Granite(Shimoga), Meta basalt and Tuff Migmatites And Granodiorite - Tonalitic Gneiss, Quartz Chlorite Schist With Orthoquartzite and Ultra Mafic Schist. In the investigation zone, Migmatites And Granodiorite - Tonalitic Gneiss are predominant (453.4 km), Quartz Chlorite Schist With Orthoquartzite happened (89.9 sq km), Granite (Shimoga) (46.9 sq km), Ultra Mafic Schist is (2.1 sq km), Basal Polymict Conglomerate and Metabasalt and Tuff (0.9 sq km). By land use land cover information increment in agriculture ranch 12 to 27 %, forest 5 to 6 % and scrub 1 to 11 % all information from 1991 to 2018. Keywords: Granite (Shimoga), Lithology, Land Use Land Cover and Forest

Моделирование минеральных парагенезисов и термобарометрия метавулканических пород серии Рукер, Южные горы Принс-Чарльз, Восточная Антарктида

There are diverse geological complexes that expose at Prince Charles Mountains and form the crystalline basement of the East Antarctic Platform. One of them is the Ruker Metasedimentary Group that is a part of the Palaeproterozoic-Neoproterozoic suprastructure of the Ruker Terrane and is divided into two sequences of metasedimentary and metavolcanic rocks, highly deformed and metamorphosed in greenschist facies. In order to estimate metamorphic conditions in this group, mineral and major element compositions of metavolcanic rocks were studied and isochemical phase diagram were calculated with the Theriak/Domino program. Mafic schists contain the mineral assemblage Chl—Ep—Ab—Qz—Ttn ± Act ± Bt ± Ms ± Cal that is typical of metabasites, and which varies with the bulk rock composition and ratio of components in a water-carbon dioxide fluid involved in phase reactions. The calculated mole fraction of CO 2 in the fluid equilibrated with carbonate-bearing parageneses lies in a range between 0.13 and 0.27. The chloritoid schist is essentially composed of Cld, Chl, Ms, and Rt. It is debated whether this rock is genetically related to metamorphism of laterites derived from the basalts. Results of forward thermodynamic modeling, supplemented by chlorite-phengite thermobarometry, indicate that metamorphism of the Ruker Group occurred under conditions of high-P part of the greenschist facies (temperature 300—450 °C, pressure up to 7—8 kbar). These conditions significantly higher than the stable continental geotherm and is close to temperatures and pressures corresponding to those along the slow subduction geotherm. According to available geological data, the same geodynamic setting may have taken place in a course of the evolution of the Neoproterozoic intraplate sedimentary basin in connection with sinking of parts of the basement at large depth as a consequence of tectonic aggregation during the closure of the basin.

Petrology, geochemistry and P–T–t path of lawsonite‐bearing retrograded eclogites in the Changning–Menglian orogenic belt, southeast Tibetan Plateau

AbstractThe Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau, is considered as the main suture zone marking the closure of the Palaeo‐Tethys Ocean between the Indochina and Sibumasu blocks. Here, we investigate the recently discovered retrograded eclogites from this suture zone in terms of their petrological, geochemical and geochronological features, with the aim of constraining the metamorphic evolution and protolith signature. Two types of metabasites are identified: retrograded eclogites and mafic schists. The igneous precursors of the retrograded eclogites exhibit rare earth element distribution patterns and trace element abundance similar to those of ocean island basalts, and are inferred to have been derived from a basaltic seamount in an intra‐oceanic tectonic setting. In contrast, the mafic schists show geochemical affinity to arc‐related volcanics with the enrichment of Rb, Th and U, and depletion of Nb, Ta, Zr, Hf and Ti, and their protoliths possibly formed at an active continental margin tectonic setting. Retrograded eclogites are characterized by peak metamorphic mineral assemblages of garnet, omphacite, white mica, lawsonite and rutile, and underwent five‐stage metamorphic evolution, including pre‐peak prograde stage (M1) at 18–19 kbar and 400–420°C, peak lawsonite‐eclogite facies (M2) at 24–26 kbar and 520–530°C, post‐peak epidote–eclogite facies decompression stage (M3) at 13–18 kbar and 530–560°C, subsequent amphibolite facies retrogressive stage (M4) at 8–10 kbar and 530–600°C, and late greenschist facies cooling stage (M5) at 5–8 kbar and 480–490°C. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb spot analyses of zircon show two distinct age groups. The magmatic zircon from both the retrograded eclogite and mafic schist yielded protolith ages of 451 ± 3 Ma, which is consistent with the ages of Early Palaeozoic ophiolitic complexes and ocean island sequences in the CMOB reported in previous studies. In contrast, metamorphic zircon from the retrograded eclogite samples yielded consistent Triassic metamorphic ages of 246 ± 2 and 245 ± 2 Ma, which can be interpreted as the timing of closure of the Palaeo‐Tethys Ocean. The compatible peak metamorphic mineral assemblages, P–T–t paths and metamorphic ages, as well as the similar protolith signatures for the eclogites in the CMOB and Longmu Co–Shuanghu suture (LCSS) suggest that the two belts formed part of a cold oceanic subduction system in the Triassic. The main suture zone of the Palaeo‐Tethyan domain extends at least 1,500 km in length from the CMOB to the LCSS in the Tibetan Plateau. The identification of lawsonite‐bearing retrograded eclogites in the CMOB provides important insights into the tectonic framework and complex geological evolution of the Palaeo‐Tethys.

Metamorphic record of the Asemi‐gawa eclogite unit in the Sanbagawa belt, southwest Japan: Constraints from inclusions study in garnet porphyroblasts

AbstractThe Sanbagawa belt is one of the famous subduction‐related high‐pressure (HP) metamorphic belts in the world. However, spatial distributions of eclogite units in the belt have not yet satisfactorily established, except within the Besshi region, central Shikoku, southwest Japan because most eclogitic rocks were affected by lower pressure overprinting during exhumation. In order to better determine the areal distribution of the eclogite units and their metamorphic features, inclusion petrography of garnet porphyroblasts using a combination of electron probe microanalyser and Raman spectroscopy was applied to pelitic and mafic schists from the Asemi‐gawa region, central Shikoku. All pelitic schist samples are highly retrogressed, and include no index HP minerals such as jadeite, omphacite, paragonite, or glaucophane in the matrix. Garnet porphyroblasts in pelitic schists occur as subhedral or anhedral crystals, and show compositional zoning with irregular‐shaped inner segments and overgrown outer segments, the boundary of which is marked by discontinuous changes in spessartine. This feature suggests that a resorption process of the inner segment occurred prior to the formation of the outer segment, indicating discontinuous crystallization between the two segments. The inner segment of some composite‐zoned garnet grains displays Mn oscillations, implying infiltration of metamorphic fluid during the initial exhumation stage. Evidence for an early eclogite facies event was determined from mineral inclusions (e.g., jadeite, paragonite, glaucophane) in the garnet inner segments. Mafic schists include no index HP minerals in the matrix as with pelitic schists. Garnet grains in mafic schists show simple normal zoning, recording no discontinuous growth during crystal formation. There are no index HP mineral inclusions in the garnet, and thus no evidence suggesting eclogite facies conditions. Quartz inclusions in garnet of the pelitic and mafic schists show residual pressure values (∆ω1) of >8.5 cm−1 and <8.5 cm−1 respectively. The combination of Raman geobarometry and conventional thermodynamic calculations gives peak P–T conditions of 1.6–2.1 GPa at 460–520°C for the pelitic schists. The ∆ω1 values of quartz inclusions in mafic schists are converted to a metamorphic pressure of 1.2–1.4 GPa at 466–549°C based on Raman geothermometry results. These results indicate that a pressure gap definitely exists between the mafic schists and the almost adjacent pelitic schists, which have experienced a different metamorphic history. Furthermore, the peak P–T values of the Asemi‐gawa eclogite unit are compatible with those of Sanbagawa eclogite unit in the Besshi region of central Shikoku, suggesting that these eclogite units share a similar P–T trajectory. The Asemi‐gawa eclogite unit exists in a limited area and is composed of mostly pelitic schists. We infer that these abundant pelitic schists played a key role in buoyancy‐driven exhumation by reducing bulk rock density and strength.

Progressive changes in lithological association of the Sanbagawa metamorphic complex, Southwest Japan: Relict clinopyroxene and detrital zircon perspectives

AbstractThe main tectono‐stratigraphic unit (Shirataki unit) of the Sanbagawa metamorphic complex in central Shikoku is characterized by abundant mafic schist layers that show the mid‐ocean ridge basalt (MORB) affinity. These MORB‐derived schist layers are absent in a southern (structurally lower) domain within the unit. Instead, sporadic occurrences of small metabasite lenses that contain relict igneous minerals (Ti‐rich augite and kaersutite) indicative of alkali basalt magmatism are newly recognized in the southern domain. Compositions of relict clinopyroxene in metabasalt are useful to identify the tectonic setting and origin of the protolith basalt, and those in each unit of the Sanbagawa metamorphic complex are presented. The metamorphic grade of the Shirataki unit generally increases structurally upwards in the southern side of the highest‐grade zone, and metamorphic zonation is subparallel to lithostratigraphic succession. The protolith assemblage of the Shirataki unit shows a distinct change from the southern low‐grade domain (lower Shirataki subunit) composed of terrigenous sedimentary rocks (mudstone and sandstone) with minor alkali basalt to the northern higher‐grade domain (upper Shirataki subunit) consisting of terrigenous and pelagic sedimentary rocks with abundant MORB. The youngest detrital zircon U–Pb ages (ca 95–90 Ma) suggest that both domains have Late Cretaceous depositional ages at the trench. Progressive peeling of oceanic plate stratigraphy during subduction can account for the observed change of lithological association in the Shirataki unit.

Contrasting subduction–exhumation paths in the blueschists of the Anarak Metamorphic Complex (Central Iran)

AbstractThe Anarak Metamorphic Complex, localized in Central Iran, is a fossil accretionary wedge composed of several tectonometamorphic units. Some of these, the Chah Gorbeh, the Morghab and the Ophiolitic complexes, contain mafic rocks that have been metamorphosed at high-pressure–low-temperature conditions. Such units have been stacked together and later refolded during the final stages of exhumation. Structural analysis at the mesoscale recognized at least three deformation events. Microstructural analyses, mineral chemistry and thermodynamic modelling reveal that the mafic schists followed contrasting P–T paths during their tectonometamorphic evolutions. In the schists of the Chah Gorbeh and Ophiolitic complexes an early greenschist-facies stage was later overprinted by blueschist-facies phase assemblages with suggested peak conditions of 390–440°C at 0.6–0.9 GPa for the meta-basalt within the Ophiolitic Complex and 320–380°C at 0.6–0.9 GPa for the blueschists of the Chah Gorbeh Complex. P–T conditions at metamorphic peak were 410–450°C at 0.78–0.9 GPa for the Morghab blueschists, but they are reached before a greenschist-facies re-equilibration. Compositional zoning of amphiboles and epidotes of this greenschist-facies stage suggests a renewed pressure increase at the end of this metamorphic stage. Based on these data we reconstructed a clockwise P–T path for the Morghab mafic schists and a counter-clockwise path for the Chah Gorbeh blueschists and ophiolitic meta-basalts. Such contrasting metamorphic evolutions of tectonic units that were later accreted to the same wedge are indicative of the complex tectonic dynamics that occur within accretionary–subduction complexes.

Signatures of Multiple Mineralization Processes in the Archean Orogenic Gold Deposit of the Pampalo Mine, Hattu Schist Belt, Eastern Finland

The Hattu schist belt is an emerging gold-producing ore district in the western part of the Archean Karelian Province of the Fennoscandian Shield. The belt consists of 2.76 to 2.70 Ga tonalite, granodiorite, and leucogranite intruded into a mafic-felsic epiclastic-volcanic sequence of an only slightly older age. Complex and successive folding, shearing, and hydrothermal processes affected these rocks prior to the lower amphibolites facies peak metamorphism (550° ± 50°C; 3–5 kbar) at ca. 2.70 Ga. Orogenic gold deposits are hosted by the highly strained zones that developed during the Archean deformation of the belt. However, previous K-Ar and Rb-Sr geochronological studies indicated that a second tectonothermal overprint affected the Hattu schist belt between 1.7 and 1.8 Ga during the Svecofennian orogeny. Results of our field, mineralogical, textural, and Pb isotope studies suggest that ore deposition at Pampalo was initiated by hydrothermal processes at the time of the emplacement of feldspar porphyry and tonalite intrusions and dikes, at around 2.72 Ga. Albitization and quartz-tourmaline-biotite-muscovite veining characterize this hydrothermal activity. The major stage of gold ore deposition can be confined to the subsequent development of high-strain zones in an intermediate-felsic tuffaceous unit (mafic schist) characterized by biotite-carbonate-pyrite alteration. Ar-Ar studies revealed the complete resetting of the Ar-Ar system in muscovite and biotite due to the Svecofennian orogeny, with closure of the isotopic system at 1.81 Ga. Results of Pb isotope studies of hydrothermal K-feldspar, galena, and altaite by laser ablation-inductively coupled plasma-mass spectrometry indicate that K-feldspar alteration and remobilization of metals also took place during the Svecofennian reactivation of high-strain zones. The petrographic manifestation of this process is the replacement of albite by hydrothermal K-feldspar. Fluid inclusion data from hydrothermal K-feldspar suggest that carbonic-aqueous, low-salinity fluids interacted with the Archean ore at 350° to 400°C and 1.8 to 2.4 kbar during the Svecofennian overprint. Results of mass transfer calculations indicate that potassium gain correlates with increase of gold concentrations in the mafic schist and feldspar-porphyry units. Therefore, the Svecofennian overprint locally also enhanced the grade of gold mineralization, mostly along competency differences of the mafic schist and feldspar porphyry blocks/dikes. Later percolation of relatively low temperature (<300°C) saline basin fluids in some fractures of the crystalline basement also left their Pb isotope and fluid inclusion signatures on the mineralization while further modifying the composition of the ore. Occurrences of hydrothermal proto-ores, as well as fluid flow events along structures reactivated by overprinting orogenic processes, are not exceptional features in Archean orogenic gold provinces. This study shows that the combination of in situ Pb isotope studies of U-poor alteration minerals and Pb-rich ore minerals, together with evaluation of relationships between gold enrichment and hydrothermal alteration and fluid inclusion studies, can be very useful in determining the significance and conditions of overprinting processes, as well as their potential implications for genetic and exploration models.