Pelitic Schists Research Articles

Strain Partitioning in a Flattening Shear Zone: Re-Evaluation of a Cycladic Style Detachment.

The Almyropotamos tectonic window on southern Evia island in the NW Aegean Sea divides two high pressure-low temperature metamorphic units, representing distinct Hellenic thrust sheets. Ductile thinning along the major low-angle Evia Shear Zone has closely juxtaposed the lower (Basal Unit) marble-flysch sequence structurally below Styra marbles (Cycladic Blueschist Unit). The partially attenuated flysch comprises a matrix dominated by pelitic schist, with dispersed cm- to hm-scale blocks of marble, carbonate schist, quartzite, and metabasite. Structural investigation of the different lithotypes in the flysch reveals tectonic fabrics related to general flattening strain are developed most strongly in the pelitic matrix, whereas the compositionally diverse blocks exhibit differential preservation of older structures. Quartz c-axis distributions from quartz veins in the schists reflect an early, moderate temperature plane strain deformation. Colder deformation is evident in some pelitic schists, capturing Z-centered girdles consistent with the oblate finite strain ellipsoid inferred from macroscopic structures. New in situ 40Ar/39Ar and 87Rb/87Sr geochronology delineate the timing of the two deformation events. Geochronological data reaffirm the first-order observations of strain partitioning behavior at the scale of the shear zone, and confirm that the structure records two resolvable tectonometamorphic events: an early Oligocene HP-LT event, and a late Oligocene-early Miocene greenschist facies overprint coinciding with ductile thinning. The diffuse and discontinuous style of deformation recorded within the shear zone is unusual for major structures facilitating exhumation in the Aegean Sea, and may represent an analogue to mélange-hosted shear zones that accommodate progressive strain during subduction.

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.

CH4 Inclusions in High‐Pressure Metapelite: Revealing the Link Between Fluid Behavior and Redox Mechanisms for Subducting Sedimentary Carbon

Abstract(Ultra)high‐pressure metamorphic rocks provide valuable insights into the properties of slab‐derived fluids. Here, we report CH4‐rich fluid inclusions in garnet of a metapelite from the Zermatt‐Saas ophiolite, western Alps. Two types of metapelite, a CH4‐bearing pelitic schist and a calcareous pelitic schist, were investigated to unravel favorable P‐T‐fO2 conditions for preservation of CH4 in high‐pressure metapelite. In the CH4‐bearing pelitic schist, CH4‐rich fluid inclusions exclusively occur in the core of garnet (GrtI) rather than the rim (GrtII). GrtI records P‐T conditions of ∼2.85 GPa and ∼555°C, whereas GrtII records a prograde P‐T path from ∼1.75 GPa at 510°C to ∼2.0 GPa at 530°C. Compositional profile of garnet in the calcareous pelitic schist reflects a prograde metamorphic path from ∼1.9 GPa at 510°C to ∼2.12 GPa at 545°C. CH4‐rich fluid formation may primarily rise from graphite reduction at high‐pressure reduced conditions (ΔFMQ −3.5 to −4, 2.85 GPa, ∼550°C), while graphite and carbonates stabilize in a relatively oxidized environment (ΔFMQ ∼0, 2.12 GPa, 545°C). The initial redox budget of subducted sediments is primarily controlled by the amount of sedimentary carbonate and organic carbon, which plays the most important role in deciding the carbon speciation at different subduction depths. CH4 formation in COH fluids could primarily be attributed to the reduction of graphite. Subducted metasediments act as conduits for transporting non‐oxidized fluids to arc magmas, which provides crucial evidence to support the heterogeneity for slab‐derived COH fluids and offers new insights into the deep carbon cycle.

Corundum in Sulfide Ore at the Thompson Mine, Manitoba, Canada: An Unusual Occurrence of Cr- and Ni-Bearing Corundum

Abstract A novel type of corundum host environment is described from the Thompson Mine, northern Manitoba, where corundum occurs within a high-grade metamorphic reaction zone between boudinaged metaperidotite bodies, sulfide ore, and Al-rich pelitic schist of the Pipe Formation (Ospwagan Group). Zoned corundum porphyroblasts, up to 20 mm across, contain coevally formed sulfide inclusions, zircon, rutile, abundant exsolved rutile needles, and secondary sulfide inclusions formed within fractures. Retrograde alteration products of corundum include greenish muscovite (in contact with the schist) and diaspore (in contact with sulfides). The median concentrations of trace elements in Thompson corundum are as follows (in µg/g): 119 Mg, 1720 Ti, 69.9 V, 390 Cr, 8.2 Mn, 1924 Fe, 9.1 Ni, and 106 Ga. The geochemical signature of Thompson corundum (enrichment in Fe, Cr, and Ni) resembles that of ruby in other metamorphic and metasomatic mafic-ultramafic-hosted deposits, and the Cr concentration is comparable to the low range of rubies. Thompson corundum contains high Ni concentrations relative to most Cr-enriched corundum occurrences, a result of formation in an exceptionally Ni-rich environment. The high abundance of exsolved rutile inclusions imparts a dominantly brownish-gray color to the corundum, which precludes it from being of gem quality. Chromium-bearing corundum has been suggested as a possible indicator mineral for metamorphosed massive sulfide deposits, however, it is an indirect indicator that risks yielding false positives. Further investigation into Ni concentrations in corundum, a potential indicator of formation in a Ni-rich environment, is recommended.

Pressure-temperature-time evolution of the Milin pelitic schists: Implications for the tectonic evolution of the eastern Indus-Yarlung suture zone, eastern Himalaya

Abstract We report an integrated comprehensive dataset composed of petrography, pressure-temperature (P-T) calculations, monazite U-Th-Pb ages, and trace-element data from pelitic schists in the eastern Indus-Yarlung suture zone in the Milin area of the eastern Himalaya. These rocks represent the exposure of subduction-related rocks within the eastern Indus-Yarlung suture zone accretionary complex. The dominant mineral assemblages of the pelitic schists are garnet + kyanite + staurolite + biotite + quartz and garnet + kyanite + staurolite + biotite + paragonite + sillimanite with quartz, plagioclase, and ilmenite assemblages. Phase equilibrium modeling of sillimanite-bearing pelitic schists yielded peak P-T conditions of ~670–680 °C at ~8.6 kbar, similar to that of kyanite-bearing schists (~670 °C, ~8.8 kbar). Monazite grains with complex internal structures retained variable ages ranging from 28 Ma to 15 Ma, which correlate systematically with changes in the concentrations of Y, Th, U, and heavy rare earth elements and ratios of Th/U. Combined with petrologic analysis, we conclude that the pelitic schists experienced a long-lived prograde metamorphism from ca. 28 Ma to ca. 22 Ma. Peak metamorphism occurred in the period 22–21 Ma, followed by quasi-isothermal decompression until 15 Ma. The discrepancies among metamorphic P-T-t paths in the eastern Indus-Yarlung suture zone indicate the presence of not only collision-related regional metamorphism at medium P-T conditions, but also subduction-related high-pressure–low-temperature terranes in the Milin region. These two domains experienced different P-T evolution and tectonic histories and were juxtaposed in the early Neogene during the India-Asia continental collision.

The processing behaviour of the sphalerites from the Big Syncline deposit, South Africa

Abstract Big Syncline is one of the few unexploited deposits of the Aggeneys-Gamsberg Ore District of the Northern Cape Province of South Africa and comprises Zn > Pb > Cu mineralisation. Future exploitation of this deposit requires a good understanding of the mineralisation textures and mineralogy for beneficiation purposes. Two of the most commonly occurring mineralisation types at Big Syncline, the mineralised pelitic schist (PEL) and garnet-magnetite-associated mineralisation (GM), are investigated with regards to sphalerite petrography and mineralogy and their potential impact on Zn flotation recovery and grade. Detailed optical petrographic analyses revealed three colour variations in the sphalerite: the opaque (i.e., black) and ruby-red varieties prevalent in the PEL; and the honey-yellow variety more common in the GM. Scanning electron microscopy with energy dispersive X-ray spectroscopy (EDS) analyses indicated increasing Fe content from the honey-yellow to ruby-red to opaque sphalerite varieties. The latter two varieties were also observed to have elevated Mn contents. Laboratory-scale differential flotation tests were applied on the two mineralisation types to determine their processing behaviour. Despite poor selectivity, the GM produced a bulk Zn concentrate of superior quality to the PEL. However, the latter showed better selectivity with the bulk of the Zn reporting to the Zn concentrate. The reduced overall quality of the PEL concentrate is attributed to the increased dilution. This is due to the high abundance of gangue sulphides in the flotation feeds as well as higher Fe and Mn contents in sphalerite. The higher Mn content in the PEL sphalerites also leads to higher Mn recovery and grade in the Zn concentrates. Although the GM overall has higher Mn contents, it is hosted in silicates leading to substantial Mn rejection to the tailings.

Petrological evidence for deep subduction of organic carbon to subarc depths

The significance of subducted organic carbon for the deep carbon cycle has been demonstrated by the presence of ultra-deep diamonds and arc emissions. However, there is no convincing evidence that organic carbon can be subducted to subarc depths. This study provides the evidence for deep subduction of sedimentary organic carbon to mantle depths through petrological observation, Raman and isotopic analyses of ultrahigh-pressure (UHP) coesite- and graphite-bearing pelitic schists. The analyzed graphite shows light δ13C values (−24.7 to −22.5‰), indicating a sedimentary organic carbon source. Petrological characteristics of graphite co-existing with coesite in garnet and P–T calculations suggest that organic graphite was subducted to the subarc depths exceeding 90 km. This research represents the petrological evidence for the deep subduction of organic carbon to mantle depths and may provide insight into the origin of diamonds with light δ13C values and contribute as key evidence for the Lomagundi event.

Paired metamorphism in peel back tectonics at the Archaean-Proterozoic boundary: New insights into Early Earth Plate tectonics from the Western Dharwar Craton, South India

In this study, we deduce a spatially and temporally linked paired metamorphism from the Western Dharwar Craton (WDC), South India, for which we use representative rocks from the medium-grade (MG) (e.g. pelitic schist) and high-grade (HG) (e.g. mafic and metasedimentary granulites) domains in the craton. Using metamorphic reconstructions (P-T-t paths of evolution and thermobaric, T/P ratios at the metamorphic peak), garnet-biotite diffusion chronometry, monazite chemical dating techniques of the investigated rocks and published metamorphic and geochronological dataset, we demonstrate that the two domains, despite having a record of uniform burial to lower crustal depths, differ in their thermal (T-t) history. The medium-grade domain records relatively cooler, intermediate T/P type of metamorphism in the stability field of kyanite with TMax at ∼650-630°C, multiple metamorphic cycles, all along clockwise P-T paths of evolution, and representing recurring burial-exhumation and heating-cooling cycles and finally, relatively short-lived nature of residence at the medium- to lower crustal levels (Δt∼10-20 Myrs) and faster metamorphic cooling (dT/dt ∼10-20°C/Myr) and exhumation rates (dz/dt∼0.6-1.1 to 1-2 mm/yr). The high-grade domain, in contrast records relatively warmer, high T/P metamorphism (T/P ratios at ∼800-870°C/GPa) in the stability field of sillimanite with TMax at ∼800–870°C, recurring burial-exhumation stages as part of one prograde heating event along a monocyclic CW P-T path of evolution and finally, extremely long-lived lower-mid-crustal residence (Δt∼238 Myrs) and ultra-slow cooling (∼2°C/Myr) and exhumation (∼0.1 mm/yr) history. Both domains show broadly coeval metamorphism as part of the same orogenesis between c.2.53 and 2.41 Ga. We use these findings from the WDC to suggest the development of a paired metamorphism as part of a peel-back convergence style of plate tectonics at the Archaean-Proterozoic boundary in the planet Earth.

Appraisal of Water Resources Development Action Plan for Groundwater Recharge in Raghunathapalli Watershed, Jangaon District, Telangana State, India

Water is elixir of life which needs regular assessment for maintaining ecological balance. Micro watershed wise (a few thousand hectares of extent) water resource action plan is need of the hour especially for a hard rock terrain like Raghunathapalli watershed of Jangaon District. Geologically, the area is underlain by Quartzite and Pelitic Schist of Older Metamorphics and Leuco Granite / Grey Alkali FelpsarGranite Terrain (Peninsular Gneissic Complex) belonging to Archeans. The Archeans are intruded at several places by several Pegmatite Veins and Basic Dolerite Dykes. The average annual rainfall of the study area for the past 19 years is 946 mm and that of monsoonal rainfall is 746 mm. In order to do artificial ground water recharge, the study and analysis of parameters that control the ground water recharge is very important for effective ground water recharge. The spatio-temporal variations in rainfall, regional/local distribution in geological formations and geomorphic composition of various units have led to uneven occurrence and distribution of water resources. Different thematic layers like Geology, Geomorphology, Stream Network, Micro Watersheds, Digital Elevation Model, Contours (2m), Base Layers, Soil and Ground Water Potential maps are prepared using Geospatial technologies like Remote Sensing & GIS. Existing Recharge Structures are marked using 1:25,000 scale Survey of India Toposheets and new locations are updated with high resolution satellite imagery. Micro watershed wise Water Balance studies are carried out and Water Resources Development Action Plan is prepared using Ridge to Valley concept with new artificial recharge structures like Check Dams (22), Percolation Tanks (10), Contour Trenches (68Kms), Subsurface Dyke (2), Farm Pond (268), Recharge Pit (179), Recharge Shaft (16), Desilting Tank (24 nos) etc at suitable terrain conditions. Newly proposed locations for constructions of Rainwater harvesting structures in the gap areas will improve the ground water regime and improves the sustainability of wells for agriculture in the drought prone area of Raghunathapalli watershed. Thus Geospatial technologies are cost effective tools for management and conservation of water resources.

Metamorphic P–T evolution of the pelitic granulites from the Helanshan in the North China Craton revealed by phase equilibrium modeling and garnet trace elements: Implications for Paleoproterozoic collisional orogenesis

Granulite-facies metamorphic rocks are widely exposed in Paleoproterozoic orogens, and their prograde metamorphic evolution is critical for a better understanding of the Paleoproterozoic collisional orogenic and geodynamic processes. In this paper, we present the results of petrology, trace element geochemistry of garnet, P–T conditions calculated by phase equilibrium modeling and garnet trace element geothermobarometry, for the pelitic granulites of the Helanshan Complex in the western Khondalite Belt of the North China Craton (NCC). These results are used for determination of their metamorphic evolution and tectonic implications. Petrographic observations show that the metamorphic evolution of the pelitic granulites can be divided into four stages (M1–M4), and these metamorphic stages are recorded by the mineral inclusions and reaction textures of garnets. Compared with the nearly flat major-element zoning, the zoning of yttrium (Y) and heavy rare earth elements (HREEs) in the garnet cores and mantles show bell-shaped distributions as well as inner-rim troughs and increases on the outer-rims. Different garnets have the same REE patterns in the same textural domain. The bell-shaped distributions of trace elements (Y and HREEs) in the garnets were controlled mainly by Rayleigh fractionation processes under subsolidus conditions, whereas the increases in these elements in the garnet rims were related to the breakdown of major and accessory phases under suprasolidus conditions. By combination of phase equilibrium modeling and garnet trace element geothermobarometry, a complete clockwise P–T path is constructed for the pelitic granulites. This P–T path is characterized by the heating as pressure rising during evolution from early metamorphic stage (M1: 4.2 ± 0.3 kbar/524 ± 17 °C) to peak metamorphic stage (M2-1; 9.8–10.6 kbar /794–815 °C), which is followed by a isothermal decompression (ITD) to the condition of 5.8–6.5 kbar/808–833 °C and later near isobaric cooling (IBC) to subsolidus. As a result, the pelitic granulites revealed that the formation of the Khondalite Belt experienced initially crustal thickening and post collisional extensional uplift. Integrated with previously reported results, the prograde geothermal gradient of the Helanshan pelitic granulites (14 ± 1 °C/km) is similar to that the pelitic schists in the Himalayan Orogen (13 ± 2 °C/km), which suggests that the Paleoproterozoic and present-day orogens have similar geothermal gradient during crustal thickening.

Polycyclic metamorphic evolution of the Sierra Albarrana Schists (SW Iberian Massif): From low-pressure Ordovician rifting to medium-pressure Variscan overprint

This study presents new information on the polycyclic metamorphic evolution of the Sierra Albarrana Domain (SW Iberian Massif) obtained through a comprehensive assessment of mineral assemblages and petrological modelling (P–T–X pseudosection approach). The pelitic schists in this domain depict the uncommon paragenesis St-Grt-And together with And-Grt-Sil-bearing schists. The first assemblage has been accurately reproduced by pseudosection modelling using the whole rock composition directly analysed by XRF. In contrast, early attempts to model And-Grt-Sil-bearing schists were not as successful. Fe3+ can be a critical element influencing the phase relations in pelites. For this reason, we investigated the influence of the effective amounts of Fe3+ to estimate the oxidation estate during the metamorphism by computing a series of P/T–X pseudosections to explore whether somewhere in P–T–X space the matrix association can be reproduced.The Sierra Albarrana Domain recorded a regional low-pressure metamorphic event accompanied by a complex structural evolution, probably related with the Middle Cambrian-Lower Ordovician rifting developed in the NW Gondwanan margin. This episode has been constrained at ∼4 kbar and 580 °C in the St-Grt-And pelitic schists. Related granitic magma pulses contributed to a local thermal increase of the extending crust (preserved in the And-Grt-Sil-bearing schists), where temperature reaches up to 595 °C. This area shows the syntaxial replacement of coarse-grained andalusite porphyroblasts by sillimanite. This was followed by exhumation, characterised by decompression at ∼2 kbar and 510 °C. After the main pre-Variscan Buchan-type regional metamorphism, a localised medium-pressure metamorphic event, limited to granitic pegmatites and concordant ky-bearing quartz segregation veins was recorded, possibly due to the tectonic thickening corresponding to the Variscan cycle.

Petrology of UHP eclogite-facies felsic schist in the Western Tianshan subduction zone, China

Although quartzo-feldspathic metasedimentary rocks are widespread in high pressure-ultrahigh pressure (HP-UHP) metamorphic belts worldwide, their petrogenesis and metamorphic evolution is poorly understood. We discovered an UHP eclogite-facies felsic schist in the Western Tianshan metamorphic belt, China. Petrological observations and phase equilibria modeling both indicate the felsic schist experienced UHP metamorphism in the coesite stability field. In particular, it experienced prograde metamorphism at 21–24 kbar, 445°C–470°C, a pressure peak at 25–28 kbar and 490°C–525°C, and eventually heating with decompression to 20 kbar and 560°C. The obtained clockwise P-T path was consistent with those of other lithologies (eclogite and pelitic schist) from the same belt, which provides new evidence for the coherent exhumation of the UHP unit of the Western Tianshan metamorphic belt. The final uplift of the Western Tianshan oceanic crust to the surface is attributed to fluid activity and late tectonic deformation.

Grain size reduction of albite porphyroblasts in pelitic schists of the Sanbagawa metamorphic belt, Kanto Mountains, Japan

Grain size reduction of albite porphyroblasts in pelitic schists of the Sanbagawa metamorphic belt, Kanto Mountains, Japan

P-T path from garnet zoning in pelitic schist from NE Sardinia, Italy: Further constraints on the metamorphic and tectonic evolution of the north Sardinia Variscan belt

Mylonitic micaschists in the south-eastern sector of the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan chain were investigated for the reconstruction of their metamorphic evolution and P-T history. Micaschists underwent polyphase ductile deformation consisting of an old D1 deformation (~345–340 Ma) associated to shearing and folding and to a penetrative S1 axial plane foliation. The S1 foliation is progressively transposed by the D2 phase, which is associated with upright up to NE verging folds and dextral shear zones. Micaschists are characterized by abundant centimetric garnet crystals with strong compositional zoning. The garnet porphyroblasts (~15 vol%) are associated with plagioclase, quartz, biotite, staurolite, white mica, chloritoid and retrograde chlorite. Garnet presents an internal foliation identified by the isoorientation of quartz inclusions sometimes arranged into a sigmoidal pattern suggesting rotation of the garnet during growth, discordant respect to the external S2 foliation. The S2 foliation is identified by the preferred orientation of micas and chlorite and by the alternance between quartzo-feldspathic and micaceous layers. The garnet core contains several small inclusions of quartz, rutile, apatite, and minor monazite and zircon. Additional inclusions, observed in the garnet domain around the core are ilmenite, chloritoid, staurolite and white mica. EMPA analyses reveal an even more complex chemical zoning consisting of garnet core, garnet mantle, and rim. The compositional isopleths that match the composition of the garnet core intersect with isopleths for the highest Si contents in K-white mica at about T 430–490 °C, P 1.3–1.4 GPa. Garnet rim isopleths and isopleths for the lowest Si contents in K-white mica indicate P-T conditions close to 560–630 °C/0.6–1.1 GPa. The resulting P-T path is clockwise, subdivided into two separate stages. The first stage is a prograde segment suggested by the garnet core to mantle compositional variation whereas the second reflects garnet rim growth during exhumation. • Garnet crystals up to 1 cm are characterized by core, mantle, rim, and outer rim • Clockwise P-T path shows a prograde segment up to pressure peak (1.7GPa) • High – pressure conditions pre-date Barrovian metamorphism in Sardinia

Tourmaline growth in pelitic schist and quartzite: A textural, chemical and B-isotopic study from the Gangpur Schist Belt, eastern India

Tourmaline is the principal repository of boron in crustal rocks and therefore useful for tracing B-cycling during prograde dehydration and retrogression of supracrustal rocks. Here, we use the major-trace element, and B isotope composition of tourmaline from schists, quartzites, and tourmaline-quartz veins of the Gangpur Schist Belt in eastern India to constrain the source of boron and the physicochemical evolution of B-rich fluids during prograde dehydration metamorphism. Tourmaline growth and re-equilibration in rocks of the Gangpur Schist Belt was a multi-stage process involving several fluid sources. The δ11B varies between −6‰ and −18‰, indicating a dominantly continental source for boron. Tourmaline in schists, quartzites, and tourmaline-quartz veins grew over a wide range of P-T conditions and record multiple episodes of metamorphic dehydration between ca. 1.6 Ga and ca. 0.95Ga. The tourmaline in tourmaline-quartz veins and quartzites has lighter B-isotope composition, typical of continental detritus, while those in the schists and quartzites record pelite-dehydration signature with values decreasing gradually from ca. −12‰ in the cores to ca. −17‰ in the rims. Heavier isotopic compositions (δ11B of ca. −6‰) measured in some grains in the pelites and quartzites indicate boron contribution from meta‑carbonate sources. The mixing of a heavier B-rich metacarbonate-derived fluid with pelite-derived metamorphic fluids could explain the lower B-isotope values in such tourmaline. The study also attempts to constrain the controls on the intake of trace elements in tourmaline. The results suggest that the partitioning of Mn, Y, V, Co and Ti in tourmaline is affected by the growth of porphyroblast phases such as garnet, staurolite, and biotite, while Li, Sr, Zn and Sn reflect the signature of the metamorphic fluid.

Latest Mesoproterozoic (ca. 1.2–1.1 Ga) amphibolite-facies metamorphism from the Dete-Kamativi Inlier, NW Zimbabwe: Implications for a Rodinia-related intracratonic orogen in Southern Africa

The Magondi Belt has been regarded as a Paleoproterozoic orogen formed by the collision of the Zimbabwe Craton and an unknown continental block. The Dete-Kamativi Inlier (DKI) located approximately 200 km west-southwest of the main Magondi Belt has been regarded as an extension of the belt. Here, we report new geochronological data for pelitic schists and a felsic orthogneiss from the DKI using monazites (CHIME method) and zircons (LA-ICP-MS analysis), and discuss the tectonic evolution of the region. Zircons from a felsic orthogneiss in the Kamativi area yielded magmatic and metamorphic ages of 2279 ± 25 Ma and 2020 ± 28 Ma, respectively. Similar Paleoproterozoic ages of ca. 2.1–1.8 Ga were also obtained from subhedral and rounded monazite grains in the pelitic schists. In contrast, irregular-shaped monazite intergrown with biotite in a different pelitic schist gave three latest Mesoproterozoic isochron ages of 1196 ± 37 Ma, 1143 ± 32 Ma, and 1070 ± 25 Ma, suggesting a long-lived (>120 million years) thermal event with several monazite-growing stages. Consistent isochron ages of 1062 ± 41 Ma and 1061 ± 26 Ma were obtained from monazites in the felsic orthogneiss and metapelite samples from an adjacent region. The application of phase equilibrium modeling for the peak mineral assemblage in the garnet-andalusite-biotite-cordierite-bearing pelitic schist with 1196–1070 Ma metamorphic ages indicated a peak P–T condition of 520–600 °C and 1.5–2.5 kbar, suggesting low-pressure amphibolite-facies metamorphism. The condition is lower than that obtained from the hornblende-plagioclase geothermometry of amphibolites (>700 °C) from the southwest part of the DKI, which probably corresponds to an earlier (ca. 2.0 Ga) high-grade metamorphic condition. The youngest thermal event, 994–982 Ma, from monazite rim in a mylonitic orthogneiss might correspond to the timing of later deformation. The latest Mesoproterozoic (1.2–1.1 Ga) amphibolite-facies metamorphism was likely associated with an intracratonic orogeny related to the activity of broadly coeval orogenic events, such as the Namaqua-Natal orogenic belt related to the amalgamation of the Rodinia supercontinent. Regional magmatic activity of the Umkondo large igneous province at 1112–1108 Ma could have also been an additional heat source.

Rheology of Metasedimentary Rocks at the Base of the Subduction Seismogenic Zone

AbstractThe Arosa Zone, Central Alps, represents a Cretaceous to Paleogene fossil subduction interface from paleodepths of 10 to >35 km. Metasedimentary schists directly below a preserved seismogenic zone exhibit viscous (foliations, folds, and lineations) and brittle (crack‐seal quartz ± calcite veins) deformation features. Si‐in‐phengite barometry and published temperature estimates yield deformation and metamorphism conditions of 0.8–0.9 GPa and 300–350°C for an albite‐rich pelitic schist and 0.9 GPa and 420°C for a quartz and calcite‐rich calcareous schist. Petrography, microprobe element maps, and electron backscattered diffraction data constrain the modal mineralogies, microtextures, and active deformation mechanisms of these schists. There is evidence of minor crystal plasticity of quartz in both rocks and calcite in the calcareous schist. However, the most prominent deformation microstructures formed during pressure solution creep of quartz and albite. Pressure solution creep was activated due to pinning by ubiquitous fine grained phengite. Rheologic modeling indicates that pressure solution of quartz and albite can accommodate tectonic strain rates at stresses of <30 of MPa and is weaker than dislocation creep at these same conditions; results that are consistent with our microstructural observations. However, to accommodate slow slip strain rates by either pressure solution or dislocation creep, stresses would need to be >>100 MPa. Triple oxygen isotope data from veins in these rocks combined with published data suggest a metamorphic mafic source for these vein filling and pressure solution accommodating fluids, indicating that dehydration from subducting oceanic lithosphere likely provided the fluids that facilitated viscous deformation in these rocks.

Pressure-temperature evolution of andalusite-kyanite-sillimanite-bearing pelitic schists from Nishidohira, southern Abukuma Mountains, Northeast Japan: Implications for Cretaceous rapid burial and exhumation in the Northeast Asian continental margin

We provide new petrological data of andalusite-kyanite-sillimanite-bearing pelitic schists from Nishidohira in the southern Abukuma Mountains, Northeast Japan. Investigation of the pressure-temperature (P–T) evolution of amphibolite-facies metamorphism in the region described the timing and tectonics of early Cretaceous metamorphic processes. The pelitic schists are composed of garnet, biotite, muscovite, quartz, plagioclase, K-feldspar, ilmenite, and graphite, in addition to Al2SiO5 polymorphs. The polymorphs occur predominantly as rounded or rhombic minerals, and are often composite aggregates. This suggests that the aggregates are pseudomorphs of andalusite (chiastolite). The andalusite, which rarely contains xenoblastic kyanite, is subidioblastic to xenoblastic, and partly replaced by kyanite, sillimanite, and muscovite. The matrix kyanite is coarse grained and porphyroblastic, surrounded by muscovite, which is in turn replaced by coarse-grained sillimanite. Fibrolite occurs as the latest phase around kyanite and sillimanite, or replacing muscovite. These textures suggest a crystallization sequence of kyanite > andalusite > sillimanite > kyanite > sillimanite > fibrolite for the Al2SiO5 polymorphs. Phase equilibria modeling of a prograde mineral assemblage (garnet + plagioclase + K-feldspar + biotite + kyanite + muscovite + ilmenite + quartz) yielded a P–T condition of 4.8–6.7 kbar and 530–570 °C. The maximum-pressure condition prior to the peak stage has been inferred as ~9 kbar (at 600 °C) based on Raman elastic geobarometry. Garnet-biotite-plagioclase-sillimanite/fibrolite-quartz assemblages yielded the peak condition of 4.5–6.9 kbar and 630–660 °C, which is within the stability field of the inferred peak assemblage (garnet + plagioclase + K-feldspar + biotite + sillimanite + muscovite + ilmenite + quartz). Our results indicate a clockwise P–T evolution with rapid burial (from the stability field of andalusite to ~9 kbar) and subsequent rapid erosion and exhumation at ~6 kbar. Electron microprobe Th-U-PbTotal dating of monazites intergrowing with the matrix biotite yielded a metamorphic age of 139 ± 24 Ma. The high-pressure amphibolite-facies metamorphism recorded in the rocks is likely associated with the thrusting of an allochthonous crustal block (including the protolith of the Hitachi metamorphic rocks) upon the Nishidohira metamorphic rocks, with serpentinite as lubricant. This process could be related to large-scale sinistral strike-slip movement and complex metamorphic processes in Northeast Asian continental margin during late Jurassic to early Cretaceous.

PETROGENETIC MODELING OF THE GRANITIC ROCKS IN AND AROUND LODHMA, DIST RANCHI, JHARKHAND, INDIA

The Lodhma area, in the Ranchi district, constitutes the central segment of Chotanagpur Granite Gneissic Complex ( CGGC) and falls in the northeastern segment of Survey of India toposheet number 73E/4. The CGGC is mainly composed of variants of granitic rocks, amphibolites and metasedimentaries. The granitic rocks find good exposure in the area. They comprise predominantly of granite gneisses, crystalline massive granites and migmatites showing noticeable variations in mineralogy, texture, structure and associations. The regional strike of the lithounits of the area show almost E-W trend. However ambient structural controls have brought about local changes in the strike direction. The area has witnessed at least three phases of deformations which is reflected by three generations of folds, which are generally isoclinals. Petrogenetic characterization of granitic rocks and its variants in the area has been attempted using various geochemical tools involving major/ trace element studies and Niggli values. S –type parentage of these Granites have been established on the basis of distribution behavior of various chemical components present in it, thereby indicating role of pelitic schists in the generation of anatectic melt, which ultimately crystallized into granites. S-type lineage of these granites has also been further cemented with the help of several well established binary and ternary variation diagrams

P-T-D evolution of the southeast Passo Feio Complex and the meaning of the Caçapava Lineament, Dom Feliciano Belt, southernmost Brazil

Metamorphic–deformation paths and interpretations of regional structures are essential for the understanding of how orogens evolve. While in active orogens, structures can be easily mapped for hundreds of kilometres, regional structures in ancient mountain belts may appear only as subsurface geophysical data. In this paper, we constrain the metamorphic–deformation history of the southeastern part of the Passo Feio Complex located in the Neoproterozoic Dom Feliciano Belt in southernmost Brazil. The complex is crosscut by a block-limiting structure defined by magnetometric data, the Caçapava Lineament. Our petrographic interpretations and thermodynamic modelling of pelitic schists of the Passo Feio Complex indicate the beginning of garnet growth at ca. 530–550 °C and 3–4.3 kbar during D1, and metamorphic peak at ca. 560–570 °C and 5–5.5 kbar (M1), during progression to D2. After M1 (~17–19 km depth), the complex was exhumed to at least 14 km (~4 kbar) depth, as the contact metamorphism by the Caçapava Granite (ca. 562 Ma) produced andalusite. The correlation of D3 structures of the Passo Feio Complex (S3 and L3), magmatic structures in the Caçapava Granite, orientation of geophysical anomalies in the region and kinematics of S3 corroborate the interpretation of the Caçapava Lineament as a dextral shear zone. Due to the location of anomalies and previous three-dimensional modelling of the granitic body, we interpret that a curved ENE to NS branch of the NE-striking Caçapava Shear Zone is responsible for the accommodation of the Caçapava Granite. Lastly, we discuss the regional implications of such interpretation, which brings up the question of block-limiting character of the Caçapava Shear Zone and the geotectonic position of the Passo Feio Complex.