Low-grade Metamorphism Research Articles

Silica cycling in Neoproterozoic oceanic lithosphere: A case study from Wadi Igla carbonate-serpentinite (southern Eastern Desert of Egypt)

Serpentinites play a pivotal role in carrying fluids and different elements into the Earth’s mantle. However, their role in exchanging silica (Si) between the marine environment and the mantle remains a matter of investigation. The Wadi Igla serpentinite (southern Eastern Desert of Egypt) formed at the expense of abyssal harzburgite by ∼15–22 % melting. It contains abundant Cr-spinel with sub-microscopic serpentine and chlorite-rich pores providing a base to explore Si cycling during serpentinization-carbonatization processes. The low-grade metamorphism of the harzburgite protolith started on the ocean floor, forming lizardite and chlorite (250–300 °C). Increasing the temperature (400–450 °C) caused the formation of brucite and antigorite. With the subduction in the fore-arc and the interaction with subducting sediments-related CO2-rich fluid, the Wadi Igla serpentinite underwent metasomatism, producing chlorite (300 °C), antigorite, tremolite, dolomite, and ferritchromite rims around Cr-spinel (Type 1), with brucite loss. In the upper greenschist-lower amphibolite facies (ca. 500 °C), CO2-rich hydrothermal fluids (with XCO2 of ∼0.55) penetrated a large volume of the protolith leading to full serpentinization together with abundant magnesite replacement. The resultant silica-rich fluids percolated in the Type 1 Cr-spinel from the outward to cores through microfractures and pores, producing Type 2 and Type 3 Cr-spinel with serpentine ± chlorite along cleavages, diminished Al-cores and growing outer ferritchromite zone and/or Cr-magnetite to magnetite zones. The suprachondritic NbN/LaN (up to 39.35) and NbN/BaN (up to 13.37) of whole rock implies for HFSEs metasomatism by subduction sediments input components, while slight enrichment in LREEs (LaN/YbN = 2.5–3) and FMEs (Li, Pb, Sr, and Ba) may have resulted from serpentinization-related hydrothermal alteration. The Wadi Igla serpentinite indicates silica cycled in a closed system, suggesting that the altered Neoproterozoic oceanic lithosphere may not have shared their main components with the surrounding environment whether to the ocean floor or the subduction zone.

SHRIMP U–Pb geochronology of Mesoproterozoic basement and overlying Ocoee Supergroup, NC–TN: dating diagenetic xenotime and monazite overgrowths on detrital minerals to determine the age of sedimentary deposition

Sedimentary rocks of the Ocoee Supergroup crop out in the Appalachian Blue Ridge of western North Carolina and eastern Tennessee. This ∼12 km thick sequence of strata was mapped as nonconformably overlying Mesoproterozoic crystalline basement (about 1.20–1.02 Ga) and beneath the lower Cambrian Chilhowee Group, leading to the traditional interpretation that the Ocoee Supergroup was deposited between about 1.02 and 0.54 Ga. This interpretation was challenged by Unrug and Unrug (1990, Geology, 18: 1041–1045) and Unrug et al. (2000, Geological Society of America Bulletin. Vol. 112, pp. 982–996), who claimed that Paleozoic fossils were recovered from rocks of the Walden Creek Group (upper Ocoee), and thereby created a new tectonic model for the origin of Laurentia. To resolve the age controversy concerning the time of deposition of the Ocoee Supergroup, diagenetic xenotime overgrowths on detrital zircon and diagenetic monazite on detrital monazite were dated by the U–Pb method. Sensitive High Resolution Ion Microprobe ages of diagenetic xenotime from three samples of sandstone (Thunderhead Sandstone, Cades Sandstone, and Shields Formation) indicate that the Ocoee Supergroup was deposited in the interval of about 580–550 Ma. A sample of Sandsuck Formation (uppermost Ocoee) yielded xenotime overgrowths of about 410 Ma, probably related to post-depositional low-grade metamorphism. Various origins of xenotime overgrowths may be distinguished by trace element distributions. Ages and trace element concentrations of monazite overgrowths support the xenotime age results, although concordia systematics are complicated by high concentrations of common Pb and inheritance of ages (∼1.2 and 0.6 Ga). These age data support the observed field relations for a late Neoproterozoic depositional age of the Ocoee Supergroup.

K-Ar dating, petrography, and geochemistry of diabase dikes from Sidakan area, northeastern Iraq: Implications for petrogenesis and Neotethyan tectonics

This study presents petrography, geochemistry, mineral chemistry and geochronology of diabase dikes from Sidakan area, northeastern Iraq. The area consists of basalts and diabase dikes alternating with sedimentary rocks, hosted by Walash Metavolcanic Group (WMG) as part of the Zagros Fold-Thrust Zone (ZFTZ). These rocks are dark grey to pale brown in color, fine to medium in grain size, and have ophitic and porphyritic textures. They consist of plagioclase and clinopyroxene as essential minerals and minor olivine as well as their alteration and low-grade metamorphism products including amphibole, albite, epidote, chlorite, serpentine, calcite and sericite, with ilmenite as an accessory mineral. The diabase dikes consist (as average wt.%) of 51.41 SiO2, 15.82 Al2O3, 9.95 FeOt, 6.58 MgO, 9.63 CaO, 3.42 Na2O, 0.87 TiO2, 0.17 MnO, 0.35 K2O, 0.09 P2O5, and 1.92 LOI. The Harker diagrams indicate that the diabases were formed from fractional crystallization of tholeiitic parent magma. The classification diagram Nb/Y vs. Zr/TiO2 indicated that the studied diabases have affinities of subalkaline basalts, while the SiO2 vs. K2O diagram showed that they are divided between the low-K basalt and basaltic andesite. The tectonic discriminant diagrams show that the diabases are oceanic island-arc tholeiites with N-MORB geochemical affinities. The K − Ar age of the diabases is Late Eocene (Bartonian-Priabonian) ranging between 34.6 ± 0.84 to 39.2 ± 1.81 Ma representing their intrusion/crystallization age and possibly the age of the formation of Oceanic Island Arc and/or the initial collision stage between the Arabian-Iranian microplates.

Multi-stage metallogeny in the southwestern part of South China, and paleotectonic and climatic implications: A high precision geochronologic study

The South China Block (SCB) is among the large-scale W-Sn mineralized regions of the globe. The Laojunshan W-Sn-dominant ore area (LOA) in the western part of the SCB preserves the records of the tectonic history of the Tethys realm extending through North Vietnam, and Yangtze to Cathaysia blocks, with coeval formation of giant metallic deposits. The prolonged tectonic activities and their control on the genesis and spatio-temporal distribution of giant metallic deposits in the LOA provide a window for a holistic understanding of the tectono-metallogenesis of the SCB. In this study, we present results from a multi-chronologic study to determine the timing of formation of the cassiterite-wolframite-scheelite mineralization. The results suggest three distinct tectono-metallogenic periods in the LOA during the geodynamic evolution of the surrounding tectonic units. The opening of the Proto-Tethys Ocean between the Yangtze-Indochina blocks and the westward Paleo-Pacific subduction beneath the Cathaysia block (420–380 Ma) jointly contributed to the Silurian to early Devonian intracontinental orogeny in the middle of the SCB that involved top-to-the-north thrusting along NE-striking shear zones. This event generated the Dulong-Song Chay granitoids, together with the formation of Xinzhai Sn deposit related to sheared mylonitic granites (ca. 419 Ma) and pegmatites (ca. 389 Ma), which include the early-stage Sn-sulfide skarn (ca. 418 Ma) and the late-stage Sn-bearing schist (ca. 389 Ma). During the Late Permian to Late Triassic (260–220 Ma), with the closure of the Proto-Tethys oceans in the west and ongoing Paleo-Pacific westward subduction in the east, the SCB and Indochina Block (IB) were amalgamated which also marks the time of formation of the Nanwenhe scheelite skarn deposit. The subducted Paleo-Tethys oceanic crust was likely entrained by the nearby rising Emeishan mantle plume (270–259 Ma), which formed the Maguan diabase (ca. 260 Ma) that shows significantly older Re-Os model age of ca. 268 Ma, suggesting that the Nanwenhe mineralization is potentially derived from ca. 260 Ma source. Furthermore, the intraplate shortening induced thin skinned crustal deformation and low grade metamorphism (ca. 230 Ma), with the main stage of scheelite-Sn-Mo mineralization (229.9, 229.8 and 219 Ma) and contemporary formation of the pegmatite (230.7 Ma). The Late Cretaceous involved two episodes of alternate extension and shortening, driven by the subduction polarity change from northwestward subduction of the Okhotomorsk block to northward subduction of the NeoTethys seafloor. The evolution of the LOA consists of the NW–SE transpression ending ca. 100 Ma, the WNW–ESE extension in the earlier episode lasting from 100 Ma to 86 Ma, the WNW–ESE transpression beginning at ca. 85 Ma and the NS extension in the later episode during the latest Cretaceous, which produced the extension-related three periods of Laojunshan granitic magmatism and coeval Sn-W mineralization, with ages in the range of 90–89 Ma, 87–85 Ma and 83–79 Ma. We also evaluate the implications of magmatic-metamorphic-metallogenic degassing on the regional paleoclimatic history.

Rubidium and potassium isotopic variations in chondrites and Mars: Accretion signatures and planetary overprints

As moderately volatile elements, isotopes of Rb and K can trace volatilization processes in planetary bodies. Rubidium isotopic data are however very scarce, especially for non-carbonaceous meteorites. Here, we report combined Rb and K isotopic data (δ87/85Rb and δ41/39Κ) for 7 ordinary, 6 enstatite, and 4 Martian meteorite falls to understand the causes for the variations in volatile abundances and isotopic compositions. Bulk Rb and K isotopic compositions of planetary bodies are estimated to be (Table 1): Mars +0.10 ± 0.03 ‰ for Rb and −0.26 ± 0.05 ‰ for K, bulk OCs -0.12-0.24+0.15 ‰ for Rb and -0.72-0.41+0.28 ‰ for K, bulk ECs +0.02-0.26+0.29 ‰ for Rb and -0.33-0.23+0.37 ‰ for K. The bulk K isotopic compositions of subgroup OCs are estimated to be -0.72-0.55+0.26 ‰ for H chondrites, -0.71-0.39+0.23 ‰ for L chondrites, and -0.77-0.30+0.63 ‰ for LL chondrites. A broad correlation between the Rb and K isotopic compositions of planetary bodies is observed. The correlation follows a slope that is consistent with kinetic evaporation and condensation processes, suggesting volatility-controlled mass-dependent isotope fractionation (as opposed to nucleosynthetic anomalies).Individual ordinary and enstatite chondrites show large Rb and K isotopic variations (−1.02 to +0.29 ‰ for Rb and −0.91 to −0.15 ‰ for K). Samples of lower metamorphic grades display correlated elemental and isotopic fractionations between Rb and K, while samples of higher metamorphic grades show great scatter, suggesting that chondrite parent-body processes have decoupled the two elements and their isotopes at the sample scale. Several processes could have contributed to the observed isotopic variations of Rb and K, including (i) chondrule “nugget effect”, (ii) volatilization during parent-body thermal metamorphism (heat-induced vaporization and gas transport within parent bodies), (iii) thermal diffusion during parent-body metamorphism, and (iv) impact/shock heating. Quantitative modeling of the first two processes suggests that neither of them could produce isotopic variations large enough to explain the observed isotopic variations. Volatilization during parent-body thermal metamorphism [the scenario (ii)], which has been commonly invoked to explain the isotopic variations of volatile elements, is gas transport-limited and its effect on isotopic fractionations of moderately volatile elements should be negligible. Modeling of diffusion processes suggests that (iii) could produce K isotopic variation comparable to the observed variation. The large isotopic variations in non-carbonaceous meteorites are thus most likely due to diffusive redistribution of K and Rb during metamorphism and/or shock-induced heating and vaporization.

In Search of Portable Water Supplies within a Brine and Mine Water-Invaded Region for Serving Some Communities Around Ishiagu, Afikpo and Environs in Abakaliki Basin, Southeast Nigeria

The study investigates the hydrogeochemical characteristics of some towns in the Abakaliki Basin, comprising, Ishiagu, Aka Eze, Amaseri, Afikpo and Okposi communities, with the aim of sourcing for portable water in the area. The basin is underlain by Albian sediments, essentially shales, in the lowlands, which were affected by low-grade metamorphism that had produced slates. The highlands comprise basic intrusives from episodes of magmatism and metallic ore mineralisation. Injection of brines into the aquifer system and low, seasonal aquifer recharge from rainfall results in poor water quality in the area. The study analyzes the geochemical distribution in water sources in the area and identifies sources of pollutants to guide the better choice of portable water. Results of hydrogeochemical analysis of both surface and groundwater from the communities were compared with World Health Organization to identify portable water locations in the area. While the salt lake at Okposi is the main source of brine intrusion in the study area, the Pb/Zn mine at Ishiagu is the main source of mine-water pollution in the study area. Most chemical parameters, (especially Cl-, Na+, Ca2+, Mg2+, SO42-, HCO3-) maintain high concentrations within the salt lake area, with the values declining away from the salt lake. The main anthropogenic source of pollution in the area, especially at Ishiagu, is the indiscriminate surface mining of lead-zinc without proposer waste management practices. Possible sourcing for portable water in the study area includes a deep borehole at Ishiagu, away from lead-zinc intrusives. At the Okposi axis, searching for portable water in boreholes should target shallower aquifers that do not communicate with the deeper-seated brine zones, likewise targeting zones farther away from these brine-invaded areas. A controlled pumping rate could potentially ensure that the cone of depression was not low enough to reach the brine zone at depth. In addition, desalination could also potentially render the salt water drinkable if properly handled to eliminate the high concentration of salts in the water to the level of acceptable limit by the WHO. Based on the study, the best area to target for portable water in the study area is Afikpo, with most geochemical elements naturally occurring within WHO’s standard concentration while portable water could be harnessed in areas further away from mining sites, especially at deep groundwater.

Microscopic and geochemical analyses of the Tonian Longfengshan biota from the Luotuoling Formation (Hebei Province, North China) with taphonomic implications

The Tonian Longfengshan biota is among the earliest known fossil assemblages with a predominance of the benthic macroalgal fossil LongfengshaniaDu, 1982, which is well known for its blade, stipe, and holdfast differentiation. However, the microstructure and geochemical composition of these fossils are poorly understood. We documented the microscopic characteristics, mineralogy, and geochemistry of 29 fossil specimens from the Luotuoling Formation at the Longfengshan locality of Hebei Province, North China using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), micro X-ray fluorescence spectroscopy (μ-XRF), Raman spectroscopy, and X-ray Diffraction (XRD). SEM-EDS revealed that all fossil specimens are preserved as two-dimensional compressions that are composed of carbonaceous patches and also have high calcium content. Iron-rich clay minerals were additionally detected in association with the fossils. μ-XRF mapping confirmed the fossils’ high calcium content. Raman spectroscopy showed that carbonaceous patches are comprised of disordered organic carbon that has been altered by low grade metamorphism (<300 °C). XRD showed a homogenous mineralogy (dominated by quartz, feldspar, and muscovite with other minor e.g., kaolinite, smectite). Petrographic analysis revealed that microbial mats existed in the fossil-bearing shale, implying that benthic macroalga Longfengshania attached its holdfast in a sticky microbial mat that encrusted the sea-bed. Integration of these data suggested that the fossil-bearing silty shale was deposited in the subtidal zone below storm wave base. Iron-replete but sulfate-depleted porewater within sediments may have inhibited the degradation process of the Longfengshan macroalgae.

Structural record of polyorogenic pre-Alpine and Alpine deformations within a major thrust nappe close to a suture zone (Internal-External Zones Boundary of the central Betic Cordillera, S Spain)

ABSTRACT The Malaguide Complex occupies the highest position in a thick-skinned Alpine thrust stack. It includes polydeformed, sedimentary and low-grade metamorphic rocks of Palaeozoic basements and their Meso-Cenozoic covers. NE of Granada the complex results severely sheared and stretched along the Betic suture zone in the metamorphic hinterland (orogenic backstop). The suture developed after the Burdigalian, due to oblique collision of the Alboran Domain (including the Malaguide Complex) with the South-Eastern Iberian Paleomargin. The orogenic evolution in the backstop region started with late Palaeogene to Early Miocene N-directed pre-collisional shortening and early thrust stacking, associated with low-grade metamorphism in the lowest Malaguide units increasing downwards into the underlying Alpujarride Complex. An accretionary prism developed at the expense of deep-marine (Flysch) sediments and the sedimentary covers of the Iberian and Alboran conjugate margins (External Zones and Frontal units, respectively). Later orogenic stages were characterized by SSE-directed back-folding and -thrusting in the Middle Miocene affecting both sides of the Betic suture related to dextral wrenching causing plurikilometric displacements. In this way, the suture evolved towards a several kilometres wide megashear zone (Cogollos-Diezma Corridor), prior to a final phase of SW-directed post-orogenic extension in the Late Miocene. Despite this protracted Alpine evolution, the Malaguide basements also preserve a record of orogenic activity during Late Carboniferous stages of the Variscan Orogeny in the form of a set of NE-SW trending folds with SE vergence and associated slaty cleavage, which are not found in the post-Carboniferous cover. The temperature conditions of this event (>300°C) in the less metamorphic Malaguide units are determined by colour alteration index (CAI) data from conodonts that have also allowed biostratigraphic dating of the Devonian-Carboniferous successions.

Petrogenesis of meta-sedimentary rocks in the deep crust of the eastern Gangdese arc

The production of new crustal material at magmatic arcs is dominated by juvenile continental crust growth through the emplacement of voluminous mantle-derived magmatic rocks. There is evidence, however, that such settings can also accommodate the burial of supracrustal material to mid/lower crustal depths resulting in the production of the new crust through metamorphism and anatexis. Investigation of migmatitic garnet- and sillimanite-bearing schists and paragneisses demonstrate that rocks exposed within the exhumed, former middle to lower crust of eastern Gangdese arc, southern Tibet, underwent sillimanite-grade metamorphism and partial melting. Monazite in the studies rocks yield 208 Pb/ 232 Th ages of ca. 72–57 Ma, which overlaps with 206 Pb/ 238 U dates from zircon rim overgrowths of ca. 73–61 Ma. Inherited zircon cores in paragneisses and adjacent low-metamorphic grade Carboniferous sedimentary rocks of the Lhasa terrane yield similar U Pb dates, with main populations at 1220–1070 Ma, 570–470 Ma, and 370–320 Ma. When combined with previously published studies, these new data demonstrate that meta-sedimentary rocks are common in the middle/lower crust of the eastern Gangdese arc and were mainly derived from Carboniferous strata of the Lhasa terrane. The Carboniferous protolith rocks were buried to deep crustal levels via thrusting during crustal shortening during Late Cretaceous (ca. 90–70 Ma) flat subduction of the Neo-Tethyan slab. Subsequent (ca. 70–60 Ma) mantle-derived magmatism induced by slab rollback, likely facilitated metamorphism and anatexis of the sedimentary rocks in the arc. This study provides important new information on the origin of the meta-sedimentary rocks and on how they affected the evolution of the eastern Gangdese arc during the Late Cretaceous to Early Cenozoic. • 73–57 Ma sillimanite-grade meta-sedimentary rocks occur in the eastern Gangdese arc. • The meta-sedimentary rocks derived from the Carboniferous strata of the Lhasa terrane. • Crustal shortening and thrusting delivered the sedimentary rocks to the deep arc crust. • Melting of sedimentary rocks contribute to contemporaneous magmatism.

Deformation conditions and 40Ar/39Ar dating of thrusting recorded by clay minerals: An example of the Lakora thrust (west-central Pyrenees)

Newly-formed clay minerals in fault zones can provide crucial information about conditions, mechanisms, and timing of deformation at low-grade metamorphism. The present study focuses on the major Lakora thrust in the west-central Pyrenees, that exposes an assemblage of Paleozoic-Upper Cretaceous rocks of the North Pyrenean Zone in the hanging wall and Campanian-Maastrichtian turbidites of the Axial Zone cover in the footwall. Detailed structural, petrological, mineralogical, and chemical investigations were performed on fault rock samples from the Lakora thrust footwall to unravel the deformation conditions related to thrusting. In addition, an attempt to date the thrust activity was made through direct 40Ar/39Ar step-heating on different fractions of muscovite/illite. The structural and petrographic observations suggest a brittle-ductile deformation in two stages along the fault zone: a first stage of bedding-parallel extension and bedding-perpendicular shortening that is marked by boudinage associated with a bedding-parallel cleavage (S0–S1) and a second stage of oblique shearing as marked by reverse faults, folds and oblique cleavage (S2). Pressure-solution and dissolution-recrystallization in the presence of fluids are the main deformation mechanisms that enhanced the neoformation of chlorite and illite associated with cleavage development during the two stages of deformation. The crystallinity, polytypism, and chemistry of illite show that the synkinematic clay minerals with the S2-stage formed under upper anchizone conditions (∼250–300 °C). Such thermal conditions are higher than those expected from the inferred maximal burial (∼6 km), thus arguing for a possible circulation of hot fluids along the thrust footwall. 40Ar/39Ar step-heating results confirm that the newly-formed illite is more abundant in the finer fractions of the most deformed sample, with a total gas age of c. 61.7 ± 0.5 Ma in the finest <0.2 μm fraction. This age is older than the early-mid Eocene age expected from structural constraints for the emplacement of the Lakora thrust sheet due to the mixing of newly-formed illites with small amounts of inherited micas and/or contamination by excess argon carried by fluids.

Effects of Diagenetic Alterations on Hydrocarbon Reservoirs and Water Aquifers

Diagenesis includes all the biological, physical, chemical, biochemical, and physicochemical alterations that occur immediately after deposition and prior to low-grade metamorphism [...]

Characterization and distribution of clay minerals in the soils of Fildes Peninsula and Ardley Island (King George Island, Maritime Antarctica)

AbstractThe environmental conditions in Maritime Antarctica are more favorable to soil development than in continental areas, which is reflected in the content and type of clay minerals present. In this context, soil clay minerals of Fildes Peninsula, South Shetland Islands were studied with the aim of relating them to periglacial and paraglacial processes as possible indicators of initial pedogenic processes. In this work, textural, mineralogical and crystallochemical characterization of clay minerals as well as chemical and physical soil analyses were carried out. The soil samples represented various surface cover types present on Fildes Peninsula. All samples were composed mainly of clay minerals, plagioclase, quartz and minor zeolites and pyroxene. The clay mineral content was very variable and reached up to 63% w/w. The clay minerals present are mainly smectite, vermiculite, chlorite and minor kaolinite, mica, corrensite and interstratified illite–smectite, with smectite and vermiculite dominating in almost all of the samples. The primary minerals display chemical alteration, and smectite formed by alteration of plagioclase. The clay mineral types were related to the parent material, which was affected by low-grade metamorphism and hydrothermal alteration that transformed biotite and chlorite into vermiculite via interstratified chlorite–vermiculite. Furthermore, this process and/or ongoing surface weathering transformed vermiculite into smectite. The genetic relationship observed between vermiculite and smectite suggests progressive alteration and transformation into a phase with intermediate composition between vermiculite and smectite. Therefore, vermiculite could be at least in part the smectite precursor. Samples closer to the current Collins Glacier front are composed mainly of vermiculite, with the greatest chemical variation occurring where the soils were developed from a mixture of initially glacially transported volcanic rocks through periglacial and fluvial processes. The clay minerals from the centre and south of Fildes Peninsula are mixtures of montmorillonite and vermiculite, as well as of chlorite and corrensite in various proportions. The clay minerals in soils developed on the west coast are a mixture of Fe-rich montmorillonite and vermiculite.

Origin and metamorphic evolution of Chachahe eclogites, North Qaidam UHP metamorphic Belt, NW China: Implications for fate of overriding plate material in subduction channel

• A clockwise P–T path was established for Chachahe eclogites in North Qaidam UHP belt, NW China. • The Chachahe eclogites experienced a 1.92 Ga metamorphic event prior to eclogitization at ∼ 450 Ma. • Overriding plate material can enter subduction channel via tectonic erosion and be carried to mantle depths of ≥ 85 km. Although numerical modelling and marine seismic data show that overriding plate material can enter subduction channel and even recycle to mantle, and finally contribute to mantle heterogeneity and arc magmas, protoliths of the majority of high pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks on the earth’s surface originate from the subducting plate. Whether overriding plate material can enter subduction channel and their fate in subduction channel remain enigmas. The North Qaidam UHP Metamorphic Belt (NQUB), located in northwestern China, was formed by north-dipping subduction of the Qaidam Block beneath the Quanji block in the Early Paleozoic. In this paper, we report that some eclogites in Chachahe area, NQUB experienced a 1.92 Ga low-grade metamorphism prior to eclogitization at ∼ 450 Ma. This Paleoproterozoic metamorphic event is not reported in the subducted Qaidam Block but widespread in the overriding Quanji Block, indicating that protoliths of Chachahe eclogites originated from the overriding plate. A clockwise P–T path from 12 kbar/520 °C to near UHP conditions of 25.5 kbar/690 °C, followed by isothermal decompression, was estimated for the Early Paleozoic metamorphism using a combination of garnet major and trace element zoning, phase equilibrium modelling and thermobarometry. This P–T path pattern is similar to other eclogites in this area, protoliths of which originated from the subducted plate. These results clearly show that overriding plate material can enter subduction channel and be carried to mantle depths of at least 85 km and exhume fast along subduction channel with other slices from subducted slab.

Hydrothermal Uranium Mineralization in Umra, Udaipur District, Rajasthan, India: Inferences from Petrography, Mineral Chemistry and Sulfur Isotopic Studies

Abstract Uranium mineralization at Umra, Udaipur district, Rajasthan is hosted by carbon and siliceous phyllite (with minor calcareous components) of the Paleoproterozoic Aravalli Supergroup. The mineralized zone trends NE-SW to N-S, dipping 45-55° towards SE to E. Uraninite, coffinite and brannerite are the main uranium phases observed that occur in association with sulphides (pyrrhotite, pentlandite, gersdorffite, chalcopyrite and pyrite), magnetite and ilmenite. Uraninite is found to occur as veins and disseminated ultrafine grains. Colloform and botryoidal texture with shrinkage/ synaeresis cracks indicate the presence of pitchblende also, formed at relatively low-temperature. Textural features suggest remobilization of disseminated syngenetic and / or diagenetic uranium minerals during low-grade metamorphism and deformation. Textural features suggest multiple phases of sulphide mineralization also. Mineral chemical data indicate that uraninite and pitchblende are compositionally similar containing 80.09-87.17 wt.% UO2 (av. 84.45 wt.%) and 11.37-11.82 wt.% PbO (av. 11.58 wt.%) with negligible SiO2, CaO, TiO2, FeO, ThO2 and RE2O3. High U/Th ratio (&amp;gt;1000) in uraninite/ pitchblende indicates limited cationic substitution and therefore low temperature (&amp;lt;350°C) hydrothermal origin of mineralization. Chemical age, calculated on the basis of UO2, ThO2 and PbO concentration in uraninite, indicates the minimum age of mineralization is 956±12 Ma. S isotopic data of pyrite suggest syn-sedimentary or diagenetic origin by bacterial reduction of seawater or pore water sulphate under anoxic conditions (-24.7‰ to -16.2‰), as well as hydrothermal (-3.7‰ to +8.8‰) origin. Similarity of S isotopic composition of hydrothermal pyrite with that of pyrite (+5.2‰) from meta-basic rock indicates that a component of S has been derived from the associated basic/meta-basic rocks in the area. Signatures of multiple generations of uranium mineralization, and multiple episodes of deformation, metamorphism and hydrothermal alteration reveal a complex metallogenic history in Umra. Compositional details of uraninite and their occurrence in the form of veinlets in association with hydrothermal pyrite suggest dominantly epigenetic hydrothermal mineralization events coeval with deformations of the Aravalli sediments.

Fluid-rock interactions at shallow depths in subduction zone: Insights from trace elements and B isotopic composition of metabasites from the Mariana forearc

To reveal the spatial variations of slab-derived fluids and to trace the in-situ dehydration in the shallow subduction zone, we investigated the petrography, mineral chemistry, and whole-rock B isotopes of metabasites that were recovered from the Fantangisña and Asùt Tesoru serpentinite mud volcanoes and originated from the shallow subduction channel at the forearc of the Mariana subduction zone. The alteration mineral assemblages in the investigated metabasites suggest zeolite- to prehnite-pumpellyite-facies metamorphism and lawsonite-blueschist facies metamorphism beneath Fantangisña and Asùt Tesoru seamounts, respectively. The fluid mobile elements (e.g., B, As, Sb, Pb) are preserved in the low grade metamorphic phyllosilicate minerals (e.g., glauconite, pumpellyite, celadonite), thus fixing the B concentrations of the subducted oceanic crust during shallow subduction (<18 km). Both B concentrations (16.7 to 43.9 μg/g) and δ 11 B values (−5.0 to +3.2‰) of the investigated metabasites are significantly higher than fresh normal mid-ocean ridge basalts (N-MORB) and ocean island basalts (OIB), and are generally comparable to the uppermost altered oceanic slab. Notably, the recovered metabasites from the Mariana forearc exhibit a decreasing trend in δ 11 B values with increasing distance to the trench from Fantangisña (62 km) through Asùt Tesoru (72 km) to South Chamorro (78 km) Seamounts. This trend together with Rayleigh dehydration modeling indicate that the 11 B-enriched aqueous fluids were released from the subducting slab during prograde metamorphic dehydration. The estimated B isotopic compositions of the slab-derived fluids released at arc magma genesis depths are generally comparable to that of the Mariana arc lavas. However, the fluids released by dehydration of subducted sediments at shallow depths should be characterized by lower δ 11 B values than fluids released from the slab at depths of magma genesis beneath the island arc. Then, the recycling of the hydrated forearc mantle is necessary to explain the high δ 11 B values of the Mariana arc lavas. While, the variable B isotope compositions of Mariana arc lavas should be controlled by the different ratios of sediment/AOC ratios. • The investigated metabasites experienced low-grade metamorphism. • Both B concentrations and δ 11 B values are significantly higher than their protolith. • δ 11 B values exhibit a decreasing trend with increasing distance to the trench axis. • Subducted serpentine is necessary to explain the high δ 11 B values of the arc lavas.

Diagenetic nutrient supplies to the Proterozoic biosphere archived in divergent nitrogen isotopic ratios between kerogen and silicate minerals.

Nitrogen isotopes and abundances in sedimentary rocks have become an important tool for reconstructing biogeochemical cycles in ancient ecosystems. There are two archives of nitrogen in the rock record, namely kerogen‐bound amines and silicate‐bound ammonium, and it is well documented that the isotopic ratios of these two archives can be offset from one another. This offset has been observed to increase with metamorphic grade, suggesting that it may be related to the bonding environment in differing nitrogen host phases and associated equilibrium isotope fractionation. However, theoretical bounds for this effect have not been established, and it remains possible that some isotopic offsets predate metamorphism. In support of this hypothesis, we report an unexpectedly large isotopic offset of 4–5‰ in siltstones of very low metamorphic grade from the late Mesoproterozoic Diabaig Formation in NW Scotland (1.0 Ga). Carbon to nitrogen ratios of bulk rocks are 2–3 times lower than in other Mesoproterozoic sections. The rocks also contain early‐formed phosphate concretions and display wrinkled surfaces on bedding planes, indicative of fossilised microbial mats. Collectively, these data are most parsimoniously interpreted as evidence of diagenetic ammonium release from microbial mats into porewaters, followed by partial oxidation to nitrite or nitrate at the sediment–water interface. This process would render residual ammonium in clays isotopically heavy, while the resulting nitrite or nitrate would be relatively lighter and captured in new biomass, leading to the observed isotopic divergence. The same diagenetic degradation pathway likely also liberated phosphate that was trapped within concretions. Diagenetic release of nutrients is known to occur in modern settings, and our data suggest that nitrogen isotopes may be a way to track this local sedimentary nutrient source in past environments. Lastly, we speculate that diagenetic nutrient recycling within Proterozoic microbial mats may have created a favourable niche for eukaryotic organisms in shallow waters.

Timing and significance of pre-Olarian metamorphism in the Willyama Supergroup, Broken Hill, Australia

In the Willyama Supergroup of the Curnamona Province, the intensity of granulite facies Olarian metamorphism at c. 1600 Ma has hampered efforts to elucidate the timing and nature of preceding lower-grade metamorphic events. Previous dating of pre-peak monazite inclusions in aluminous metapelites has been complicated by potentially incomplete isotopic ‘shielding’ by their host porphyroblasts, owing to pervasive deformation and recrystallisation of these rocks during the c. 1600 Ma event. Near the Broken Hill Pb-Zn orebody, quartz-feldspar-biotite-garnet (‘Potosi-type’) gneiss proved more resistant to recrystallization during granulite facies metamorphism: new Sensitive High Resolution Ion MicroProbe (SHRIMP) U-Pb analyses of monazite from this rock delineated a population of relatively Y-rich (median 0.96 wt%) cores at 1636.8 ± 4.4 Ma (95% confidence), overprinted by Y-poor (median 0.10 wt%) rims and domains at 1597.3 ± 3.3 Ma. Garnet is a sink for Y in metamorphic rocks, which suggests that Y-rich c. 1637 Ma monazite growth predated garnet formation (and possibly occurred at relatively low metamorphic grade), whereas the Y-poor nature of the c. 1597 Ma rims and domains implies that dissolution and reprecipitation during Olarian metamorphism postdated porphyroblastic garnet growth. Coexisting low-Th/U zircon rims are dominated by a 1600.7 ± 3.6 Ma population, emphasising that the event responsible for c. 1637 Ma monazite growth was largely invisible to zircon. Interlayered mafic granulite 70 m away yielded partially recrystallised zircons with dark-CL internal domains which establish a minimum age of 1639 ± 8 Ma for the igneous precursor. Recrystallisation was accompanied by actinide depletion, with bright-CL, U-poor and low-Th/U domains dated at 1596 ± 8 Ma. Our new 1640–1635 Ma dates constitute robust isotopic evidence for pre-Olarian mafic magmatism and metamorphism within the lower Willyama Supergroup. We infer an extensional environment in the upper mid-crust for this tectonothermal activity, coeval with ongoing sedimentation and deposition of the upper Paragon Group at the surface. Extension of the eastern Curnamona Province at c. 1640 Ma is coeval with sediment-hosted base-metal mineralisation in the Mount Isa Inlier and the McArthur Basin, and is consistent with tectonic regimes prevailing throughout Proterozoic southern, central and eastern Australia at that time.

Late Carboniferous intracontinental magmatism in the northernmost Sierras Pampeanas, Argentina: The case study of the Tres Cerritos pluton

The Tres Cerritos pluton, a composed, shallow intrusive body, represents the northernmost Late Carboniferous intracontinental magmatism emplaced in the Eastern Sierras Pampeanas of Argentina. Our geochronological study has yielded a U–Pb zircon age of 318 ± 1.7 Ma. It is located between two juxtaposed structural metamorphic domains of the Tolombón Complex in the Sierra de Quilmes that are in contact through old shear zones with N–S to NE-SW orientation. The emplacement of the Tres Cerritos pluton was highly conditioned by the high rheological constrast between the intrusive magma and metasediments in low metamorphic grade. Main magmatic structures and the external shape of the pluton domains are roughly parallel to the regional shear zones. On the contrary, the contacts of the intrusive body crosscut the principal fabrics of the metamorphic host, which show negligible deflection close to the granitic body. Therefore, the magma emplacement would have been through stoping mechanism, assisted by hydraulic forces and favored by Famatinian shear zones reactivated.The Tres Cerritos pluton is mostly composed of alkali-calcic, felsic and peraluminous granites, plotted between the magnesian and ferroan fields. Geochemical characteristics point to a clear A-type affinity, with low La/Yb (MBU: La/Yb = 28.72–21.27; MU: La/Yb = 13.58–2.41) and Sr/Y (MBU: Sr/Y = 5.40–3.86; MU: Sr/Y = 1.80–1.32) ratios. εHf isotopic values between −12 and −15, the most negative results obtained from the Carboniferous magmatism in the Eastern Sierras Pampeanas, relate the pluton to an old metasedimentary source. The comparative analysis with other intrusive bodies of the Devonian-Carboniferous silicic and alkali-calcic magmatism scattered along the Eastern Sierras Pampeanas, suggests a homogeneous and long-lasting crustal source for these granitic rocks, with a similar crustal affinity. In this regard, the contribution of juvenile basic magmas at the source decreased during the thermal, likely extensional, event that promoted the Devonian-Carboniferous intracontinental magmatic suite.

In situ Lu–Hf geochronology of calcite

Abstract. The ability to constrain the age of calcite formation is of great utility to the Earth science community, due to the ubiquity of calcite across a wide spectrum of geological systems. Here, we present the first in situ laser ablation inductively coupled tandem quadrupole mass spectrometry (LA-ICP-MS/MS) Lu–Hf ages for calcite, demonstrating geologically meaningful ages for iron oxide copper gold (IOCG) and skarn mineralisation, carbonatite intrusion, and low-grade metamorphism. The analysed samples range in age between ca. 0.9 and ca. 2 Ga with uncertainties between 1.7 % and 0.6 % obtained from calcite with Lu concentrations as low as ca. 0.5 ppm. The Lu–Hf system in calcite appears to be able to preserve primary precipitation ages over a significant amount of geological time, although further research is required to constrain the closure temperature. The in situ approach allows calcite to be rapidly dated while maintaining its petrogenetic context with mineralisation and other associated mineral processes. Therefore, LA-ICP-MS/MS Lu–Hf dating of calcite can be used to resolve the timing of complex mineral paragenetic sequences that are a feature of many ancient rock systems.

Signature of Himalayan orogenic features in Brahmaputra River sediments, Bangladesh: Evidence from single-grain heavy mineral chemistry

The present study describes results obtained from the chemistry of detrital heavy minerals i.e. pyroxene, amphibole, biotite, garnet, epidote and Fe-Ti oxides in fluvial sediments of the northern Brahmaputra River (Bangladesh) with an aim to determine conditions of their petrogenesis and provenance. The primary and secondary genera of ferromagnesian minerals occurred in calc-alkaline and peraluminous subduction zone. In which, the garnets are Fe-rich, indicating mostly almandine component (Alm65–Pyp16–Grs8–Sps6 averagely), occurred in medium to high grade metasedimentary rocks in the Lesser Himalaya (LH), along the Main Central Thrust (MCT) and the eastern Himalayan syntaxis. Besides, the fingerprint of omphacite and actinolite owe to ascertain the co-existence of garnet developed in ultrahigh-pressure (UHP) eclogites that may also be drained from the Tso Morari massif. Augite to aegirine-augite pyroxenes emphasizes Fe enrichment in basaltic systems and high to ultrahigh grade metamorphic rocks, which are exposed in the LH, Shillong Plateau, Mikir Hills, South Tibetan Detachment System (STDS), eastern Himalayan syntaxis and Tso Morari massif. Geochemistry and thermobarometry of the primary magmatic amphiboles and biotites manifest the source of granitoid and granodiorite like bodies, and their windows are exposed in the Bomi–Chayu, Gangdese arcs and the western Arunachal Himalaya. Again, metamorphosed Fe-Ti oxide minerals are well-exposed along the NE Lesser Himalaya, where magmatic derivative of Fe-Ti oxide minerals were modified through the diffusional processes in low-grade metamorphism (534–562 °C with 10–22.1–10−21.5fo2). Integrating the aforementioned discussion with the thermochronology, it is evident that the eastern Himalayan syntaxis is the major source of sediment flux, which is carried mostly by the upper Himalayan tributaries i.e. Yigong, Parlung, Dibang and Lohit. Also, the lower Himalayan tributaries i.e. Subansiri and Manas drain the sequestered derivatives dominantly from the Arunachal Himalayan. Tso Morari eclogites (NW Himalaya) have also contribution somewhat of dense minerals to the Tsangpo-Brahmaputra River system. Thus, scrutinizing the fingerprint of single-grain detrital minerals provides key information regarding the source terrains and tectonics of the Himalayan sequences.