Metamorphic Temperatures Research Articles

RAMAN Analysis of Carbonaceous Material and Deduced Peak Metamorphic Temperatures of Metasediments From Western Himalaya, NW Pakistan

ABSTRACTRaman spectroscopy of carbonaceous materials (RSCM) was applied to 10 carbonaceous material (CM) ‐rich low‐ to medium‐grade metasedimentary rock samples of Western Himalaya, Pakistan to assess their optimum thermal evolution. The RSCM thermometry is based on the degree of graphitisation of CM as a function of peak metamorphic temperature. Petrographic observations of the studied samples revealed two dominant CM morphologies. Type‐I CM, represented by fine‐grained discontinuous dust‐like and scattered to thin sporadic layers, was more prevalent in low‐grade metamorphic samples. Type‐II CM, corresponding to continuous and extended thick fibres and elongated grain constellations, was dominant in medium‐ to high‐grade metamorphic samples. The degree of crystallinity of CM and subsequent metamorphic temperatures were quantified by considering the intensity‐based R1 and the area‐based R2 ratio parameters. Low‐grade samples from the Lesser Himalayan Sequence (LHS), having Type‐I CM, resulted in RSCM peak temperatures between 306°C and 403°C. Medium‐ to high‐grade samples from Higher Himalayan Crystalline (HHC), containing Type‐II CM, yielded peak temperatures in the 482°C–560°C range. The extremely deformed sample from a fault zone in the LHS produced relatively higher temperature of 403°C, thereby suggesting heat associated with shearing due to fault activities resulting in CM recrystallisation and elevated temperature values. The study revealed that the peak metamorphic temperatures increase from south towards north following a progressive path of metamorphism in the region.

Intra-grain variability of hydrogen and trace elements in rutile

Rutile is a commonly used petrogenetic indicator mineral to determine metamorphic temperatures, ages, host−/source lithologies, geochemical reservoirs, and subduction conditions. However, intra grain variabilities of trace elements in rutile are rarely considered. We performed trace element and hydrogen mapping of rutile to assess zoning and diffusion in natural rutile from various lithologies. Trace element and hydrogen show distinct zoning patterns, with mostly regular zoning in rutile from pegmatites and low-T hydrothermal clefts, and typically irregular zoning in rutile from high-P veins and metamorphic rocks. Whereas no clear patterns of trace element correlations can be identified, hydrogen, tri-, tetra- and pentavalent cations can show the same zoning patterns within single rutile grains, despite different substitution mechanisms. This indicates that hydrogen and trace element incorporation is externally controlled by availability and diffusivity of hydrogen and trace elements within the rock matrix, as well as rutile growth rates. H2O mapping reveals that hydrogen is retained in rutile at temperatures of up to ~650 °C. Coupled substitution of hydrogen with divalent and trivalent cations requires coupled diffusion processes for charge balance if hydrogen is diffusively re-equilibrated. Slow diffusion rates and thus relatively high temperatures for diffusive closure in rutile lead to retention of primary hydrogen and trace element zoning. At high-T conditions of >650 °C, diffusive re-equilibration of all trace elements can be observed. Complex zoning patterns of Zr in rutile show that Zr incorporation into rutile is not purely temperature dependent. In this study, Zirconium-undersaturation can lead to inaccurate Zr-in-rutile temperatures of up to ~35 °C difference from peak formation temperatures within single rutile grains and might be a useful tool to evaluate rutile growth conditions. Niobium and Ta are highly zoned in rutile, leading to extremely variable Nb/Ta ratios within single rutile grains that cannot be reconstructed from single spot analyses. Overall, mapping offers a novel and promising tool to understanding trace element behavior in rutile.

Mineralization processes in the Bainaimiao Cu-Au deposit in Inner Mongolia, China: Constraints from geology, geochronology, and mineralogy

The Central Asian Orogenic Belt underwent complex tectonic processes and is one of the most intensely accretionary areas globally. Porphyry copper deposits within the belt were likely subjected to deformation during tectonic processes. The Bainaimiao Cu-Au deposit is a typical example of a deformed porphyry deposit whose formation processes include a porphyry emplacement (Event I), a greenschist facies metamorphism (Event II), and a brittle deformation (Event III). Geochronology and trace element geochemistry of zircon, volatiles of magmatic apatite, along with the assemblages, textures, abundances, and compositions of phyllosilicates from these three events were investigated to unveil the physicochemical conditions under which the key geological events relevant to the deposit formation took place. LA-ICP-MS zircon U-Pb dating shows that the granodiorite porphyry in the northern and southern zones formed at 447.4 ± 1.6 to 445.8 ± 3.6 Ma and 436.1 ± 3.8 to 434.1 ± 3.3 Ma, respectively. The granodiorite porphyry in the northern zone has higher oxygen fugacity and Clmelt content but similar Smelt and Fmelt contents compared to the granodiorite porphyry in the southern zone. Microscopic and mineralogic observations point to Event I to be of high plagioclase (22–67 vol%) and quartz (6–40 vol%) content with a range of hydrothermal minerals related to potassic, phyllic, and propylitic alterations. Event II features high amphibole (38–83 vol%) or epidote-chlorite (up to 77 vol%) content with minerals precipitating along the schistosity planes. Event III is characterized by wide veins (3–80 cm) and the highest quartz (61–65 vol%) and calcite (12–19 vol%) content. Geothermometry results show the temperature of potassic and phyllic alterations of Event I to be ∼622 °C and ∼288 °C, respectively. Based on geothermometry and P-T pseudosections, the temperatures of metamorphism and metallic precipitation of Event II were ∼271–634 °C and 297–328 °C, respectively. Both mechanical and chemical mobilization of metallic elements results in Cu mineralization during Event II. The metallic precipitation temperatures of Event III spanned from 297 to 328 °C according to chlorite geothermometry. The ratios of Fe3+/Fetotal and Mg/(Mg + Fetotal) of biotite, chlorite Fe/(Fe + Mg), and white K-mica composition show the mineralizing fluid of Event III to be the most oxidized while that of Event II is the most reduced, F-rich and features the lowest water/rock ratio. This study suggests that deformation processes can increase the Cu mineralization grade of the deformed porphyry deposits through mobilization and re-precipitation of metallic elements.

XRD Studies on Metamorphic Changes of the Dissimilarly Graphitized Carbonaceous Materials

One of the unique yet functional properties of the carbonaceous materials is an irregular change in their internal heterogeneity and graphene layouts upon graphitizing themselves. The precise investigations of such type phase transitions inclusive of reshuffling/restructuring of the sporadically arranged graphene C–atoms, and thereby acquiring crystallite growths & contractions, crystallinity, graphitization degree, extent of structural orders, dissimilar range pores & porosity networks, etc., are indispensable for understanding the actual transformation aptitudes of the low temperature amorphous carbons into the crystalline semi-graphite and graphite like functional materials. In this report, the XRD spectroscopy is applied to the variably carbonized and graphitized carbon materials (activated carbons (AC1─AC3) & carbon blacks (CB1─CB4)), and the progressive internal improvisations in their graphene layouts caused by the dissimilar metamorphic temperature regimes are sought out. For this, the XRD diffractograms linked descriptors (diffraction angles (\(2\theta\)), intergraphene layer gap (d), & crystallite sizes (Lc)), and the correlations between many descriptive subordinates (% crystallinity (%C), graphitization degree (%DOG), metamorphic temperature (Tmet), BET surface area (SBET), and specific capacitance (CSP)) are taken into account. The former set speculates the existence of lower amorphicity and higher graphitic homogeneity in CBs than in the ACs, and elucidates their contrasting inbound graphene configurations. And, the latter set quantifies the extent to which the founding graphitic units of them undergo significant improvements owing to acquire better functional domains. The applicative propensities for them are forecasted as CB4 (C = 1.3%, DOG = 98%, Tmet = 595⁰C, SBET = 5348 m2/g, CSP = 600 F/g) > CB3 (C = 5.1%, DOG = 96%, Tmet = 588⁰C, SBET = 3735 m2/g, CSP = 419 F/g) > CB2 (C = 5.8%, DOG = 93%, Tmet = 576⁰C, SBET =1508 m2/g, CSP = 169 F/g) > CB1 (C = 19.7%, DOG = 64%, Tmet = 486⁰C, SBET = 1186 m2/g, CSP = 133 F/g). The author believes these datasets as quite useful while specifying their compressibility & reinforcement, binding & lubricity, catalysts & catalysis, electrochemical & electro-adsorption, etc., and can be referred time to time while identifying distinctive functionalities of one over the others.

Mountain building process of the Taiwan orogeny.

The Taiwan orogeny, an example of arc-continental collision, exhibits complex geological structures and rapid exhumation. Many models have tried and failed to fully capture the dynamics of these processes. We developed a comprehensive thermomechanical model that considers the transition from brittle to ductile behavior with depth, lithology-dependent erosion and observed decollement and backstop geometries. This model successfully reproduces the intricate structures observed within the Taiwan orogeny, aligns with structural complexities, metamorphic temperature profiles, thermochronological records, strain distributions, and the rates of exhumation and cooling and elucidates the roles of ductile deformation and ramp structures in forming the Hsuehshan Range and the Western fold and thrust belt. The insights from this model offer potential applicability to other orogenic wedges worldwide.

On the Nature of the Kharamatolou Structure and the Ratio of the Amount of Ultramafic Rocks of the Voykar-Synya Massif to That of the Ray-Iz Massif, Polar Urals

There have been petrographic, geochemical, geochronological (Rb-Sr, Ar-Ar) and isotopic (Sm-Nd) studies done on the metamorphic formations of the Kharamatolou structure of the Polar Urals, which were usually assigned to the Precambrian. The temperatures of metamorphism have been shown to range from 450 to 626 °C, and the pressures – from 3.7–9.1 kbar once PT-conditions reached the amphibolite facies. The amphibolites of the Kharamatolou formation are geochemically divided into two groups. According to the distribution spectra of lanthanides and Nd isotope composition, one group has the characteristics similar to depleted N-MORB basalts εNd(0)=+7.0, and another group – those similar to moderately enriched E-MORB basalts εNd(0)=+(4.5–2.4). It has been found that the Kharamatolou structure is probably composed of the Early to Middle Paleozoic continental rise deposit of the Russian Platform, which passed the main stage of folding and metamorphism in the Late Devonian (Rb-Sr, 366±11 Ma). The gabbro-ultrabasic Ray-Iz and Voykar-Synya massifs have been shown to be one during most of their Pre-Triassic history. Now these ultrabasite massifs are separated by the young (Triassic) Kharamatolou uplift which is an erosive-tectonic semi-window where ultrabasic rocks of the Polar-Ural belt are exposed at the base of the thrust fault. The Kharamatolou structure is one of the transverse uplifts of the Urals which probably resulted from the Middle to Late Triassic compression therealong. As a result of the Middle to Late Triassic uplift of the Haramatolou metamorphites, the overlying ultrabasites were washed away by erosion. The mélange serpentinites observed in the center of the Kharamatolou structure are uneroded remnants of the link that once existed between the Ray-Iz and Voikar-Synya massifs. This significantly increases the metallogenic potential of the Voikar-Synya massif, as the Ray-Iz hosts the largest known chromite deposits in Russia.

A hot, hydrothermally influenced microbial-tidal flat setting in the Palaeoarchaean Moodies Group, Barberton Greenstone Belt, South Africa

Abstract Sandy, microbial-mat-laminated sediments are common in estuarine and tidal environments of the Palaeoarchean Moodies Group (ca. 3.22 Ga); they are interspersed with numerous expressions of mafic to intermediate (sub-) volcanism, including sills, stockwork dykes, lavas, and air-fall tuffs. We describe abundant fluid-escape structures up to 6 m in height associated with this facies in the Saddleback Syncline of the central Barberton Greenstone Belt. The fluid-escape conduits fed small sand volcanoes during prolonged and/or recurring discharge of gases, liquids, and solids. They are filled by sand, sericitic clay, and fine-grained organic matter of former microbial mats. In comparison to the mean composition of adjacent beds of identical composition, the conduits are enriched in Fe, Cr, Ti, and Mg. This suggests that fluid-escape was not only a consequence of overpressure buildup from decaying microbial mats in the shallow subsurface or of water-level fluctuations but also due to periodic or continuous release of hydrothermal fluids circulating in the thermal aureole above the cooling Lomati River Sill of Moodies age. Such an inference is also supported by textures characteristic of in-place argillaceous and sericitic alteration and by Raman spectroscopy of carbonaceous matter (RSCM) indicating temperatures ca. 50 to 100°C above the regional maximum metamorphic temperature of 320 to 380°C. Pre-compaction carbonate and/or silica cementation also preserved the abundant carbonaceous laminae interpreted as benthic microbial mats. Analogue recent hot spring deposits suggest that surficial hydrothermal activity in the medium-energy siliciclastic tidal zone would have significantly boosted microbial growth.

Constraints on a long-lived Paleoproterozoic metamorphic process in a middle- to lower-grade metamorphic complex, Songshan area, southern North China craton: Evidence from minor- and trace-element thermometry and geochronology

The Dengfeng Complex and Songshan Group comprise a middle- to lower-grade metamorphic complex with abundant lithological associations in the Songshan area (central Henan, China), representing an excellent opportunity to investigate the thermal history of the North China craton. The polymetamorphic Dengfeng Complex is a typical granite-greenstone belt that records a late Paleoproterozoic metamorphic event (ca. 1.95−1.80 Ga) overprinting on a late Neoarchean metamorphic event (ca. 2.51−2.41 Ga). In contrast, the Songshan Group preserves well-developed stratigraphic sequences and sedimentary structures with greenschist-facies metamorphism. Numerous studies have focused on the late Neoarchean metamorphic event from the Dengfeng Complex, whereas the thermal evolution of ca. 1.95−1.80 Ga units from the Dengfeng Complex and Songshan Group has been largely overlooked. Quartzite of the Songshan Group unconformably overlies schists of the Dengfeng Group, and they show coherent NNE-SSW−striking schistosity structures with dip angles of ∼50°−65°. Here, the metamorphic temperatures from the Dengfeng Group were constrained to be ∼510−550 °C by the Ti-in-biotite geothermometer. The Ti-in-quartz and Zr-in-rutile geothermometers record metamorphic temperatures of ∼480−550 °C or 520−555 °C for the Songshan Group, respectively. Laser ablation−inductively coupled plasma−mass spectrometry U-Pb dating of monazite and rutile constrains the timing of metamorphism of the Songshan Group to be ca. 1.94−1.83 Ga, while zircon constrains the timing of metamorphism of the Dengfeng Complex to be ca. 2.53 Ga and ca. 1.95−1.87 Ga. By integrating geochronologic data from the Dengfeng Complex and Songshan Group, the ca. 2.53 Ga metamorphic age of amphibolite may be connected with Neoarchean subduction-accretion processes, whereas the consistent temperature obtained in this study ca. 1.95−1.83 Ga indicates that the Songshan area may have experienced a long-lived metamorphic event, which could have resulted from the final collision between the Western and Eastern blocks of the North China craton in the late Paleoproterozoic. The persistence of a Paleoproterozoic hot orogen for millions of years was probably the norm in the North China craton, which provides new insights into the tectonic-thermal evolution in the southern North China craton.

Paleoarchean sedimentation and Mesoarchean high-temperature metamorphism in northeastern Brazil: Tracing sources and depositional ages in polymetamorphic rocks

Supracrustal rocks offer a window into tectonic processes of the early Earth, since they are common in the Archean lithosphere. However, these rocks are usually affected by several episodes of metamorphism that can compromise their UPb systematics, leading to equivocal interpretations of depositional ages and sources. In northeast Brazil, supracrustal rocks are frequent within the Archean basement of the São José do Campestre Massif. These metapelitic migmatites show a high-temperature mineral assemblage, with garnet + sillimanite ± spinel and retrograde cordierite, with abundant anatectic melt migration at conditions of upper amphibolite to granulite facies. Zircon UPb dating coupled to trace elements analysis through LA-ICP-MS, as well as zircon internal zoning patterns suggest a maximum depositional age of 3305 ± 16 Ma followed by high-temperature metamorphism in the Mesoarchean, Paleoproterozoic and Neoproterozoic. Mesoarchean high temperature metamorphism occurred between 3084 ± 4 and 3006 ± 6 Ma and generated a wide range of textures that could be grouped in, at least, three stages of zircon growth. The first, during prograde heating, led to dissolution of detrital cores through the process of Ostwald Ripening and reprecipitation in oscillatory zoned rims. The second, probably at peak conditions, occurred above the stability of monazite, as evidenced by high Th/U ratios within zircon grains, rounded shape and sector-zoned cores. The third, during retrograde cooling, is mostly driven by garnet breakdown, and resulted in the crystallization of convoluted rims. Ti-in-zircon temperatures indicate minimum temperatures of 712 ± 21 °C for prograde/retrograde stages and 881 ± 50 °C for peak conditions. The Paleoarchean sedimentation and Mesoarchean metamorphism are coeval with similar events in Kaapvaal and Dharwar Cratons (South Africa and South India, respectively), but show no correlation to any Archean domains in South America to date. Solid-state recrystallization during the Paleoproterozoic (ca. 2.0 Ga) and the Neoproterozoic (ca. 0.6 Ga) correlates with orogenic events in both the Borborema Province and São Francisco Craton, suggesting a common evolution since the Rhyacian.

A Non‐Magnetized Chondrite Parent Body Revealed by Paleomagnetic Investigation of LL6 Chondrite NWA 14180

AbstractMagnetic records from meteorites provide valuable information about the formation and evolution of the solar system and planets. The parent planetesimals of chondrites are typically considered to be undifferentiated based on their primary chemical composition and texture. However, recent paleomagnetic investigations of various chondrites indicate that they carry a primary remanence generated by a dynamo, suggesting partial differentiation of their parent planetesimals. The presence of a dynamo within the parent planetesimal of LL chondrites remains uncertain due to the ambiguous origin of the remanent magnetism. Here, we report petrographic, paleomagnetic, and rock magnetic properties for the novel LL6 chondrite NWA 14180. The high metamorphic temperature experienced by NWA 14180 could have removed the pre‐accretionary remanence. The fusion crust baked‐contact test suggests that NWA 14180 preserves primary magnetic information about its parent body. Alternating field demagnetization results from interior subsamples reveal distinct low‐ and medium‐coercivity components that may represent a viscous remanent magnetization acquired in the geomagnetic field. No natural remanent magnetization was unblocked in the high coercivity range, implying that NWA 14180 cooled in zero‐field conditions. Therefore, we suggest that the parent body of NWA 14180 did not have a dynamo. Furthermore, this result suggests that the LL chondrite parent planetesimal accreted later and was smaller in size than other chondrite classes.

Structural styles and chronological sequence of the Mianyuzui ductile strike-slip fault along the Shangdan suture Zone: Implications for the Paleozoic tectonic evolution of the Qinling Orogenic Belt, China

The Shangdan suture zone (SDSZ) subdivides the Qinling Orogenic Belt into North Qinling and South Qinling belts, marking the main tectonic boundary between the North China and South China Blocks. The Mianyuzi ductile strike-slip fault (MYZF) within the SDSZ separates the North China Block from the South China Block and contains valuable collisional information. This paper integrates the structural and geochronological data of the MYZF to investigate the Paleozoic tectonic evolution of the Qinling Orogenic Belt and the collision between the North China and South China Blocks. Two phases of deformation (D1-D2a-b) are identified along the southern boundary of the SDSZ. The metamorphic temperature of mylonites is ∼ 600–700 ℃ for the early stage and ∼ 300–500 ℃ for the late stage of shearing deformation. U-Pb zircon analysis of the mylonites and diabase dike in the ductile shear reveals that zircons have major younger protolith populations at 480–460 Ma, 450–430 Ma, and 405–425 Ma, which suggests that rocks were probably derived from the North Qinling belt. The metamorphism and deformation cooling age from the MYZF have been constrained to be 339–328 Ma and 259 ± 1.6 Ma using U-Pb and 40Ar/39Ar dating. Combined with previously published data, we concluded that the subduction of the Shangdan Ocean was active until the Early Silurian and subsequent collisions occurred during the Late Silurian. Meanwhile, the shearing might have been related to the intraplate extension-exhumation during the Carboniferous-Late Permian in the Qinling Orogenic Belt.

Multi-stage ultrahigh temperature metamorphism in the lower crust of the Kaapvaal craton recorded by U–Pb ages of garnet

U–Pb ages were determined by split-stream LA-SF/MC-ICPMS in garnets from UHT granulite xenoliths (Star mine, South Africa; 124 Ma). They give a considerable age range of 400 million years with well-defined maximas at 3.09, 3.01 and 2.75 Ga. The oldest peak overlaps with the changeover from tonalites to K-granites at 3.14–3.04 Ga and with zircon ages of the mid-crustal granulites of the Vredefort dome (3.1 Ga) in the wake of the 3.2 Ga collision of three terrains that compose the Witwatersrand block. Subduction (or sagduction) of the uppermost crust in an ultrahot orogen setting brought shales and greenstones to the lower crust. Ultrahigh temperature (UHT) conditions are the result of high mantle potential temperatures and self- heating by the radioactive inventory of the subducted lithologies. Metamorphism, anatexis to very high degrees and melt extraction left UHT granulites as residue. Rejuvenation of UHT conditions was brought about by Dominion Group magmatism between 3.0 and 2.95 Ga. Magmatic uprise caused intense shearing in the lower crust followed by recrystallisation of the shear zones to generate the younger garnet age group. Ventersdorp flood basalt volcanism caused similar processes at around 2.72 Ga and generated the third garnet age group. Zircon gives U–Pb ages mainly around 2.72 Ga (both literature and our own data) i.e. zircon adjusted or newly crystallized at the youngest UHT event. Only few zircon grains retained older ages up to 2.94 Ga. Still unconstrained, but very high closure temperatures (≥ 1100 °C) for the U–Pb system in garnet keep the memory of the oldest ages in UHT granulites. Such ages can only be reset by recrystallization. This way, garnet records a prolonged high-temperature history of the lower crust of the Kaapvaal craton.

Possible Eoarchean Records of the Geomagnetic Field Preserved in the Isua Supracrustal Belt, Southern West Greenland

AbstractRecovering ancient records of Earth's magnetic field is essential for determining the role of the magnetosphere in protecting early Earth from cosmic radiation and atmospheric escape. We present paleomagnetic field tests hinting that a record of Earth's 3.7‐billion‐year (Ga) old magnetic field may be preserved in the northeastern Isua Supracrustal Belt as a chemical remanent magnetization acquired during amphibolite‐grade metamorphism in the banded iron formation. Multiple petrological and geochronological lines of evidence indicate that the northernmost part of Isua has not experienced metamorphic temperatures exceeding 380°C since the Eoarchean, suggesting the rocks have not been significantly heated since magnetization was acquired. We use “pseudo” baked contact tests (intrusions emplaced 3.26–3.5 Ga ago) and a fold test (folding 3.6 Ga ago) to demonstrate that some samples preserve a ca. 3.7 Ga record of the magnetic field. We recover a field strength of >15 µT. This suggests that Earth's magnetic field may have been weak enough to enhance atmospheric escape during the Archean.

Insights on the origin of oldhamite in enstatite meteorites from Ca stable isotopes

In order to understand the origin of oldhamite (CaS) in enstatite meteorites, we report Ca isotopic compositions (δ44/40Ca) of oldhamite (obtained from water leachate of bulk chondrites and aubrites and mineral separates from the Norton County aubrite) and silicate minerals from different types of enstatite chondrites and aubrite. The δ44/40Ca of the bulk enstatite chondrites range from 1.05 ‰ to 1.24 ‰, with an average of 1.13 ± 0.12 ‰, higher than that of the estimate of the bulk silicate earth (∼0.94 ‰). Major and trace element analyses show that the water leachates of enstatite chondrites are mainly composed of oldhamite, and they take over 20.6–68.5 % Ca of the bulk meteorite Ca budget. The Ca isotope fractionation between oldhamite and residual silicate (Δ44/40Caoldhamite-silicate) for the studied enstatite chondrites is minimum (−0.44 ‰) for Abee (impact-melt breccia) and maximum (+0.16) for St.Marks (EH5). The Ca isotope fractionation between oldhamite (individual mineral grains and leachate) and silicates in Norton County varies from −0.47 ‰ to −0.31 ‰ with an average of −0.41 ‰. These Δ44/40Caoldhamite-silicate correlate well with previous theoretical calculation and suggests that the oldhamites in Norton County are in isotopic equilibrium with co-existing silicates, and therefore were formed during magmatic processes. However, in enstatite chondrites, the large variation on Δ44/40Caoldhamite-silicate and its negative correlation with metamorphic temperature reflects the redistribution and equilibration of Ca isotopes during metamorphism. The variable Δ44/40Caoldhamite-silicate found in unequilibrated chondrites reflect kinetic Ca isotope fractionation between oldhamite and nebular gas and therefore is evidence for the formation of oldhamite by condensation in the solar nebula.

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

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

Sanidinite facies metamorphism at Nagarahal Village of Bilgi taluk, Bagalkot district, Karnataka, India


 
 
 Nagarahal village in the Bilgi taluk of Bagalkot district in Karnataka state shows the contact between the younger Bilgi granites and the Deccan basalts. The thermal impact that has happened on the pre-existing granites by the basalts has been studied by identifying the field occurrence of the contact zone of granites and basalts. The unaffected portion of the granites and the basalts that are far away from the contact are studied as well. The petrographic studies of the contact rocks between the granites and basalts show the polygonal recrystallized quartz with the triple point contact with other grains indicating the decussate texture indicating the high-temperature conditions, crushed quartz grains having contact with each grain appear as sugary crystals from the granoblastic texture, the irregular oriented mafic and felsic crystals show the hornfelsic texture and micrographic, and intergrowth textures showing the effect of thermal metamorphism near to the contact between the granite and basalt. The post-hydrothermal solution activity is at the peripheral region of the contact between granite and basalt which is seen by the formation of tourmaline. The mineral assemblages that are identified by the petrographic studies are muscovite, quartz, orthoclase, sanidine, sillimanite, chlorite, and Fe-Ti oxide minerals. The mineral chemistry data shows the high potash K-feldspar that consists of Or from 84% to 97.97%, Ab2%- 4.26%, and An0.09%-1.34% was identified as sanidine variety of K-feldspar, the plagioclase feldspar shows the dominant composition of Ab ranging from 84.92% to 97.10%, An content in plagioclase was found to be ranging from 0.87% to 14.02%, and the Or content is ranging from 1.06%-7.41%. indicating the presence of the albite variety of plagioclase, the Fe2+ dominating chlorite, and the presence of tourmaline indicates the hydrothermal effects at the periphery of the contact. The two feldspar thermometry indicates the formation temperature was 750°C at the contact and the chlorite formation temperature calculated is found to be 313°C which shows a decrease in the temperature away from the contact of the basalt and the granitic rocks. Whereas the granite and the basalts that are far away from the contact do not show any changes. The mineral assemblages, mineral chemistry data, and thermometric results indicate the high temperature and low-pressure sanidine facies of contact metamorphism in the studied area.
 
 

H2O-rich rutile as an indicator for modern-style cold subduction

The trace-element and isotope geochemistry of rutile are robust tools to determine metamorphic temperatures, age, and host-/source lithologies. The use of rutile as single grain indicator for pressure, temperature, time and composition (P–T–t–X) of the host rock, which is vital in the use of detrital rutile to trace plate-tectonic regimes throughout Earth’s history, requires the identification of a pressure dependent trace element in rutile. We investigate the pressure dependence of hydrogen in rutile using polarized in-situ Fourier Transform Infrared (FTIR) spectroscopy. H2O contents in rutile vary between < 10–2500 μg/g H2O with higher contents in samples with higher peak metamorphic pressures, making H2O-in-rutile a viable pressure indicator. The highest H2O contents at ~ 450–2000 μg/g are observed in mafic low temperature eclogite-facies rutile related to modern-style cold subduction conditions. Hydrogen zoning in FTIR maps indicates that H+ is retained at temperatures below 600–700 °C. Ratios of H2O/Zr, using H2O as pressure indicator and Zr as temperature proxy, are a proxy for thermal gradients of metamorphic rutile (i.e. P/T). Low temperature eclogite samples are also characterized by high Fe contents and therefore Fe/Zr-ratios might be used as a first order approximation for H2O/Zr-ratios to identify mafic low temperature eclogite facies rutile. Based on common discrimination diagrams, Nb, W, and Sn can be used to distinguish different host/source rock lithologies of rutile. Combining both H2O/Zr-ratios and Nb, W, and Sn contents can thus identify modern-style cold subduction signatures in rutile. The developed systematics can consequently be used to trace cold-subduction features in the (pre-Proterozoic) detrital record.

Polyphase stratabound scheelite-ferberite mineralization at Mallnock, Eastern Alps, Austria

A peculiar type of stratabound tungsten mineralization in metacarbonate rocks was discovered and explored at Mallnock (Austria) during the late 1980s. It is the only tungsten occurrence in the Eastern Alps in which scheelite is associated with wolframite (96 mol% ferberite). The tungsten prospect is located in the Austroalpine Drauzug-Gurktal Nappe System recording polyphase low-grade regional metamorphism. Raman spectroscopy of carbonaceous material yield maximum metamorphic temperatures of 296 ± 27 °C and 258 ± 27 °C, which are assigned to Variscan and Eoalpine metamorphism, respectively. Scheelite and ferberite occur as polyphase stockwork-like mineralization in Fe-rich magnesite in the northern ore zone (Mallnock North), whereas in the western ore zone (Mallnock West), scheelite-quartz veinlets are exclusively hosted in dolomitic marbles. LA-ICP-MS analyses of scheelite and ferberite yield low contents of Mo, Nb, Ta, and rare earth elements, but high contents of Na and Sr. Uranium is particularly high in scheelite (up to 200 µg/g) and makes this mineral a suitable target for U–Pb dating. In situ U–Pb dating of scheelite yielded an early Permian age (294 ± 8 Ma) for Mallnock West and a Middle Triassic age (239 ± 3 Ma) for Mallnock North. A monzodioritic dike close to Mallnock yielded a U–Pb apatite date of 282 ± 9 Ma and supports the polyphase formation of this mineralization. The U–Pb scheelite ages indicate that a model for tungsten metallogeny in the Eastern Alps must also consider remobilization of tungsten by metamorphic fluids. In the Alps, the Permian to Triassic period (ca. 290–225 Ma) is characterized by an overall extensional geodynamic setting related to the breakup of Pangea. Lithospheric thinning caused higher heat flow, low-P metamorphism, and anatexis in the lower crust, which led to enhanced crustal fluid flow in the upper crust. These processes were not only responsible for the formation of metasomatic hydrothermal magnesite and siderite deposits in the Eastern Alps but also for this unique magnesite-ferberite-scheelite mineralization at Mallnock.

Tracking cycles of Phanerozoic opening and closing of ocean basins using detrital rutile and zircon geochronology and geochemistry

Abstract Sedimentary basins provide a deep time archive of tectonic and Earth-surface processes that can be leveraged by detrital mineral U-Pb dating and geochemistry to track paleogeography, magmatism, and crustal evolution. Zircon preserves the long-term (billions of years) record of supercontinent cycles; however, it is biased toward preserving felsic crustal records. Detrital rutile complements the detrital zircon record by providing constraints on the time and temperature of rifting and mafic magmatism, metamorphism, exhumation of the middle and lower crust, subduction, and amagmatic orogenesis. We use detrital zircon U-Pb and detrital rutile U-Pb geochronology and trace element analysis of Permian to Eocene siliciclastic rocks in the southern Pyrenees to capture supercontinent cycles of ocean basins opening and closing. Detrital rutile age spectra show peaks at ca. 100 Ma associated with rifting and hyperextension in the Pyrenean realm, 200 Ma associated with the Central Atlantic Magmatic Province, and 330 Ma, 375 Ma, and 400 Ma associated with subduction and Rheic Ocean crust formation. Zr-in-rutile thermometry and rutile Cr-Nb systematics provide further insight into metamorphic facies (peak metamorphic temperatures) and source rock lithology (mafic versus felsic affinity). Detrital zircon age spectra have peaks at ca. 300 Ma, 450 Ma, and 600 Ma associated with major orogenic events and felsic magmatism, and Th/U ratios provide information on relative zircon formation temperatures. Comparison of these independent records shows that detrital rutile reflects rifting, magma-poor orogenesis, and oceanic lithospheric processes, while detrital zircon detects continental lithospheric processes. Integrated detrital zircon and rutile data sets archive past geological events across multiple Wilson cycles.

Paleoproterozoic Reworking of Archean Crust and Extreme Back‐Arc Metamorphism in the Enigmatic Southern Trans‐Hudson Orogen

AbstractThe crustal evolution of the southernmost ∼2000–1800 Ma Trans‐Hudson orogen (THO) is enigmatic due to burial by Phanerozoic sediments. We provide new insights through petrochronologic analysis of a paragneiss drill core sample. Detrital zircon age peaks at 2625, 2340, and 1880 Ma and Hf isotopes suggest Paleoproterozoic arc development proximal to Archean source(s). Phase equilibria modeling and ternary feldspar thermometry suggest peak conditions of ≥1 GPa, ≥900°C, the first recognition of extreme, ultra‐high temperature metamorphism in the THO. The largely isobaric P‐T path, rapid heating rate, and ∼20 Myr duration (1872–1850 Ma) of peak conditions suggest that this metamorphism occurred in a back‐arc tectonic setting. The sample records post‐peak (1850–1815 Ma) mid‐crustal residence, slow cooling, and exhumation. Further retrogression occurred during Proterozoic regional exhumation (1630–1470 Ma) and Phanerozoic (360–220 Ma) reheating and/or fluid influx. Evidence for Paleoproterozoic arc(s) supports geophysical data for Archean cratonic and Paleoproterozoic arc crust in this region.