Index Minerals Research Articles

Frictional Strength and Frictional Instability of Glaucophane Gouges at Blueschist Temperatures Support Diverse Modes of Fault Slip From Slow Slip Events to Moderate‐Sized Earthquakes

AbstractFluid overpressure from the water released by subducted sediments and oceanic crust is an important mechanism for generating earthquakes via brittle failure and frictional instability. If unstable, such fault materials may also host diverse fault reactivation mechanisms from slow slip events to moderate‐sized earthquakes in cold subduction zones. We examine this hypothesis for glaucophane gouge ‐ a key index minerals for blueschist facies ‐ at lower confining stresses where behavior is poorly understood. We measure friction and stability at temperatures of 100°C–500°C and effective normal stresses of 50–200 MPa, to explore the controls of temperature, stresses and excess pore fluid pressures on fault friction. The frictional coefficient of glaucophane at representative temperatures and stresses is ∼0.70, insensitive to temperature but with a slight increase at lower effective stresses. Elevating temperature promotes a transition from velocity‐strengthening to weak velocity‐weakening behavior, indicating the destabilizing effect of high temperature downdip in subduction zones. Reducing effective normal stress or concomitantly elevating pore fluid pressure further strengthens the velocity‐weakening response and would be manifest as moderate‐sized earthquakes. Our results support the potential for enhanced unstable sliding of glaucophane gouges at lower effective stresses and blueschist facies temperatures ‐ with weak to moderate velocity‐weakening responses upon fluid pressurization vital for understanding the abundance of slow slip to moderate‐sized earthquakes apparent in cold subduction zones.

METAMORPHISM AND DEFORMATION OF GOLD-BEARING NEOPROTEROZOIC WONAKA SCHIST BELT, NORTHWEST-NIGERIA.

The role of metamorphism and deformation is indispensable in the occurrences of gold mineralization worldwide. In this work, deformation and metamorphic conditions for gold-bearing Neoproterozoic Wonaka Schist Belt; located around Kutcheri town of Tsafe Local Government of Zamfara State, was investigated. This is achieved using metamorphic litho-minerals obtained from ternary plots via X-Ray fluorescence (XRF) geochemical data, and directly using minerals phases from X-Ray Diffraction (XRD) technique. Index minerals identified from petrographic analysis previously suggest low to medium-grade metamorphism (M1). XRD analysis indicates quartz, albite, oligoclase, microcline, chlorite, and biotite, suggesting greenschist to lower amphibolite facies (M2). Sillimanite, andalusite, kyanite, staurolite, chlorite, biotite, and garnet were identified from the ternary plots using XRF major oxides, indicating upper amphibolite to granulite facies metamorphism (M3). This is typical of prograde metamorphism, granulite facie metamorphic grade is indicated. Na2O/Al2O3 versus K2O/Al2O3 for petrogenetic character suggests shale provenance, while the trace elements spider diagram indicates Wonaka litho-units as co-genetic compositionally, as high concentrations of V and Cr linked the petrogenetic affinity to mafic sources. Three circles of deformations are indicated; ductile deformation (D1) of the paleosome Schist producing foliations and lineation, brittle type (D2) in mid Pan-African and was accompanied by several fractures and felsic intrusions. Late Pan-African (D3) involves the folding of banded orthogneisses, the development of boudinage as well as intense shearing (ductile fault). Geospatial analysis of the fractures suggests that they represent regional Pan-African sutures cross-cutting Nigeria into the Atlantic and up to South American plate. The research therefore concludes that Au-fluid emanating through this regional event, utilizes D2 as channel ways and loci. D3 with M3 engulfed the entire structures repositioning the geometry to its present disposition.

Exploring the Composition of Egyptian Faience

Egyptian Faience, a revolutionary innovation in ancient ceramics, was used for crafting various objects, including amulets, vessels, ornaments, and funerary figurines, like shabtis. Despite extensive research, many aspects of ancient shabti production technology, chemistry and mineralogy remain relatively understudied from the 21st to the 22nd Dynasty, belonging to a recovered 19th-century private collection. The fragments’ origin is tentatively identified in the middle Nile valley in the Luxor area. Our study focused on a modest yet compositionally interesting small collection of shabti fragments to provide information on the glaze’s components and shabti’s core. We found that the core is a quartz and K-feldspars silt blended with an organic component made of plastic resins and vegetable fibres soaked with natron. The studied shabti figurines, after being modelled, dried, and covered with coloured glaze, were subjected to a firing process. Sodium metasilicate and sulphate compounds formed upon contact of the glaze with the silica matrix, forming a shell that holds together the fragile inner matrix. The pigments dissolved in the sodic glaze glass, produced by quartz, K-feldspars, and natron frit, are mainly manganese (Mn) and copper (Cu) compounds. The ratio Cu2O/CaO > 5 produces a blue colour; if <5, the glaze is green. In some cases, Mg and As may have been added to produce a darker brown and an intense blue, respectively. Reaction minerals provided information on the high-temperature firing process that rapidly vitrified the glaze. These data index minerals for the firing temperature of a sodic glaze, reaching up to a maximum of 1050 °C.

Mineral Indicators of Geologically Recent Past Habitability on Mars.

We provide new support for habitable microenvironments in the near-subsurface of Mars, hosted in Fe- and Mg-rich rock units, and present a list of minerals that can serve as indicators of specific water-rock reactions in recent geologic paleohabitats for follow-on study. We modeled, using a thermodynamic basis without selective phase suppression, the reactions of published Martian meteorites and Jezero Crater igneous rock compositions and reasonable planetary waters (saline, alkaline waters) using Geochemist's Workbench Ver. 12.0. Solid-phase inputs were meteorite compositions for ALH 77005, Nakhla, and Chassigny, and two rock units from the Mars 2020 Perseverance rover sites, Máaz and Séítah. Six plausible Martian groundwater types [NaClO4, Mg(ClO4)2, Ca(ClO4)2, Mg-Na2(ClO4)2, Ca-Na2(ClO4)2, Mg-Ca(ClO4)2] and a unique Mars soil-water analog solution (dilute saline solution) named "Rosy Red", related to the Phoenix Lander mission, were the aqueous-phase inputs. Geophysical conditions were tuned to near-subsurface Mars (100 °C or 373.15 K, associated with residual heat from a magmatic system, impact event, or a concentration of radionuclides, and 101.3 kPa, similar to <10 m depth). Mineral products were dominated by phyllosilicates such as serpentine-group minerals in most reaction paths, but differed in some important indicator minerals. Modeled products varied in physicochemical properties (pH, Eh, conductivity), major ion activities, and related gas fugacities, with different ecological implications. The microbial habitability of pore spaces in subsurface groundwater percolation systems was interrogated at equilibrium in a thermodynamic framework, based on Gibbs Free Energy Minimization. Models run with the Chassigny meteorite produced the overall highest H2 fugacity. Models reliant on the Rosy Red soil-water analog produced the highest sustained CH4 fugacity (maximum values observed for reactant ALH 77005). In general, Chassigny meteorite protoliths produced the best yield regarding Gibbs Free Energy, from an astrobiological perspective. Occurrences of serpentine and saponite across models are key: these minerals have been observed using CRISM spectral data, and their formation via serpentinization would be consistent with geologically recent-past H2 and CH4 production and sustained energy sources for microbial life. We list index minerals to be used as diagnostic for paleo water-rock models that could have supported geologically recent-past microbial activity, and suggest their application as criteria for future astrobiology study-site selections.

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

The origin of metamorphic rock is greatly influenced by the temperature and pressure changes in every tectonic setting, especially in active subduction margins. One of the wide outcrops of metamorphic rock occurs in the Pringsewu District, Lampung, and it is a part of the Palaeozoic Gunung Kasih Complex. The presence of metamorphic rocks in Pringsewu has raised several questions and debates for some time due to the lack of research and field evidence found in this area, especially, since the origin of metamorphic rock in Lampung has been rarely studied. This research aims to determine the metamorphic rock facies and the tectonic setting underlying the formation of metamorphic rocks in the study area.&#x0D; Petrographic analysis on 19 thin-section samples shows that metamorphic rock in the research area can be divided into two regions i.e., western and eastern regions. The Eastern part is characterized by muscovite-epidote schist and greenschist which consist of quartz, muscovite, actinolite, epidote, and garnet as index minerals. Whereas the western part is characterized by quartzite and biotite-epidote schist that consist of quartz, biotite, and muscovite. Based on the mineral index, metamorphic rock’s protoliths are pelitic rock, mafic rock, and quartz-feldspathic sandstone. The metamorphic rock zonation shows the created temperature is from 280-550oC.&#x0D; The foliation structure such as schistose and porphyroblastic texture in the whole rocks sample indicated that metamorphic rocks are created in medium-grade metamorphism and are characterized by the greenschist facies to epidote-amphibolite facies. The abundance of quartz, k-feldspar, and labradorite minerals show that metamorphic rocks came from the microcontinent which was caused by a collision between the intra-oceanic Woyla plate and West Sumatra microcontinent in the Cretaceous. Indications of tectonic activity that create the lithology in Lampung Province need more geological study, especially to determine the absolute age of the metamorphic rock.

Muscovite 40Ar-39Ar and cassiterite U-Pb dating of the Indosinian Aotou quartz-vein type tin deposit, southern China and its geological significance

The determination of deposit age is fundamental in economic geology, while the result is often disappointing because of the difficulties in selection of suitable minerals. In this study, on the base of detailed field investigation, we successfully combined the muscovite 40Ar-39Ar dating and cassiterite LA-ICP-MS U-Pb dating in the Aotou tin deposit, Jiangxi province, South China. The result of the muscovite 40Ar–39Ar plateau ages (228.7 ± 1.5 Ma) and the cassiterite U-Pb lower intercept ages (227.4 ± 6.9 Ma) are consistent, accurately defined that the Aotou tin deposit was formed during late Indosinian. With the gradual discovery of another Indosinian W-Sn deposit, we have taken this opportunity to discuss the prospecting index of the Indosinian granite by compiling data of relevant plutons. The result reflects that the Indosinian metallogenic plutons are mainly acidic rocks and produced in the form of small rock mass, with index minerals of muscovite and volatile-rich minerals such as tourmaline and fluorite. Geochemically, they show that the higher SiO2 content (71.51–78.14%), A/CNK (1.13–1.66), Rb/Sr (21.94 in average), but lower Nb/Ta (4.03 in average) and ΣREE (85 ppm in average), which could be useful indexes for identifying the metallogenic pluton. Furthermore, the negative correlation between SiO2 and ΣREE, K2O and Na2O can be used as important ore-prospecting indexes, as a result of they indicate the differentiation of ore-bearing hydrothermal fluids.

Water-Rock Interactions in the Metamorphic Complex of Geothermal Field: Case Study in Wahlua Complex, Buru Island

In Indonesia, surface thermal manifestation is commonly found in a volcanic geological setting. However, hot springs and alteration minerals in study area occur on metamorphic rock. In order to study its correlation with the emergence of the thermal fluid, the objective of this study is to characterize each metamorphic zone based on rock chemistry and the appearance of index minerals. Consequently, the correlation with thermal fluid chemistry can be observed. Data for this study are surface rock samples and thermal manifestation data. Metamorphic zones were determined by integrating geomorphology, petrography, and X-Ray Fluorescence (XRF) analyses. Three metamorphic zones were identified, namely, Garnet Zone, Biotite Zone, and Chlorite Zone. Garnet Zone is characterized by high concentrations of CaO and Al2O3 due to the presence of garnet. The abundance of biotite and muscovite in Biotite Zone may contribute to the highest concentration of K2O than the other two zones. Meanwhile, Chlorite Zone is characterized by the concentrations of SiO2, Fe2O3, and MgO. However, the composition of MgO might be influenced by mineral-fluid reaction. There are two clusters of thermal manifestation, which are western and eastern cluster manifestations. Western cluster manifestations are in the Biotite Zone. Meanwhile, the eastern cluster manifestation is in the Chlorite Zone. However, both clusters’ fluid chemistry shows significant differences in some elements such as SiO2, Na, K, Fe, and Mg concentration. It might suggest that the enrichment and depletion of these constituents in thermal fluid might correlate with each metamorphic zone’s characteristics. An inconsistent pattern between thermal fluid composition and rock chemistry was found in Mg concentration. It is most likely that the Mg concentration was influenced by groundwater dilution rather than the mineral-fluid interaction. The study concludes that the appearance of index minerals shows consistencies with composition variation of some elements in the manifestation fluid, such as SiO2, Na, K, and Fe.

Palynofacies Analyses of HA-001 well, Offshore Depobelt, Niger Delta Basin, Nigeria

Palynofacies studies of the strata penetrated by HA-001 well were carried out with the aim to establish palynostratigraphic zones and paleodepositional environments. The standard acid palynological method of sample administration was employed to recover the palynofacies, soaking and washing of samples with water for sedimentological analyses and integration of Gamma Ray Log, palynofacies and sedimentological data for interpretation of paleodepositional environments. The study established two new assemblage biozones namely Cyperaceaepollis sp. – Nympheaepollis clarus, and Stereisporites sp. within the sedimentary intervals penetrated in the wells. The Cyperaceaepollis sp. – Nympheaepollis clarus biozones were further subdivided into Nympheaepollis clarus – Echitriletes pliocenicus and Cyperaceaepollis sp. – Elaeis guineensis subzones. The Late Miocene age is associated with the biozones. The lithologic, textural and wireline log data indicated that the intervals studied in the HA-001 well belong to the Benin Formation and Agbada Formation. Index minerals and accessories are dominated by ferruginous materials, glauconite pellets, carbonaceous detritus, shell fragments and pyrites plus minor mica flakes. Lower delta plain, delta front and prodelta environment of deposition were inferred for the studied well intervals. The research has conformed to international stratigraphic guide for establishment of biozones which serves to increase the knowledge of high resolution biostratigraphy.

A new UHP unit in the Western Alps: First occurrence of coesite from the Monviso Massif (Italy)

AbstractThe Western Alps are one of the most studied exhumed subduction-accretionary complexes worldwide. Ultrahigh-pressure (UHP) metamorphism has been documented there since the 1980s. We now report the first discovery of coesite in the meta-ophiolitic suite of the Monviso Massif, corresponding to the fourth UHP unit defined on the Western Alps. Previous petrographic studies and results from thermodynamic modeling already suggested that these Alpine units may have experienced UHP metamorphism, but no occurrences of index minerals, such as coesite, have been reported to date. The newly discovered coesite inclusions from the Monviso Massif occur as intact single crystals (10–60 µm) hosted by garnet. The observations suggest that they have escaped re-equilibration and maintained all the original features from the trapping time. The reduced size of the crystals and the lack of re-equilibration significantly differ from the typical textural features described in past findings (i.e., radial cracks, palisade texture of quartz surrounding coesite relicts). Detailed garnet inclusions analysis and thermodynamic modeling constrained the metamorphic peak conditions at P = 2.8–2.9 GPa and T = 500–520 °C within the coesite stability field.The Lago Superiore Unit represents the fourth UHP unit discovered on the Western Alps. The UHP metamorphism on the Western Alps was considered rare due to the escape process of unusual units from mantle depths. In our view, the implication of our discovery provides new insight into UHP processes that seem to be more common than expected. Further tectonic reconstructions should take into account the common features observed in the UHP units to better constrain the subduction- and exhumation-related mechanisms that drove the actual stacking of mountain belts.

Determining metamorphic grade from greenschist to amphibolite using Hyperion data: the case of an inverted metamorphic gradient of Almora nappe (lesser Himalaya)

The intricate nature of metamorphic terrain remains a complex problem for many remote sensing scientists. The metamorphic rock assemblages are composed by intrinsic mixtures of index minerals which are defined at different pressure and temperature zones. Metamorphic assemblage’s nature and geothermal gradient can be identified using reflectance spectroscopy. The study aims to determine the nature and gradient of metamorphism using spectral identification of the index mineral phases. A model for metamorphic assemblage was developed to predict the nature and grade of metamorphism in order to address the problem with the inadequate quantification of mineral spectral mixing. The study addresses the problem by incorporating semi-empirical method of classification of metamorphic rock assemblage based on reflectance characteristics. We use the simulated spectral library of Hyperion satellite data which is favourable for classifying geological outcrops in metamorphic terrain of the Almora nappe of lower Himalayas. The Hyperion data analysis reveals the nature and trend of change of the metamorphic grades within different zones based on mass proportion of index mineral assemblages. An inverse thermal grade of the metamorphic Almora group is observed along North Almora Thrust and South Almora Thrust, where reverse thrusting might have resulted in lowering of pressure and temperature condition. The developed model can be utilized for geological survey’s involving metamorphic terrain in inaccessible areas.

Minerals explained 62

Eudialyte is a mineral unfamiliar to most people. This is likely to change in the near future, as this mineral is of increasing economic importance. Eudialyte is now known to be one of a group that has grown in number to a remarkable extent in recent years, and eudialytes are index minerals for a specific category of igneous rocks and are repositories for elements, such as the rare earths, which have become increasingly critical to modern technology. The eudialyte group now comprises about 50 members of which about 29 members are fully recognized, and exploitation of eudialyte group minerals will soon be a reality. The mineral's name comes from Greek: Eu διάλυτος, meaning ‘well decomposable’—a reference to the fact that, unlike the highly refractory zircon (the common source of zirconium), eudialyte is soluble in common acids, making it relatively cheap to work.

150 Myr of Episodic Metamorphism Recorded in the Yukon-Tanana Terrane, Northern Canadian Cordillera: Evidence from Monazite and Xenotime Petrochronology

Abstract We used in situ laser-ablation split-stream petrochronology to target monazite and xenotime associated with specific metamorphic index minerals in the Florence Range metamorphic suite in the northern Canadian Cordillera. Based on this petrochronological approach, we reconstruct the polyphase metamorphic evolution of the Yukon-Tanana terrane in northwestern British Columbia. Our new data reveal a complex P–T–t path, requiring three metamorphic episodes and passing through the kyanite, sillimanite, and andalusite stability fields over 150 Myr from the Permian to the Early Cretaceous. The earliest preserved metamorphic event is cryptic, reaching the kyanite stability field (&amp;gt;600°C, &amp;gt;0.6 GPa) at ca. 270–240 Ma followed by retrograde metamorphism at ca. 240–215 Ma. Renewed garnet growth occurred during a second prograde metamorphic event at ca. 195–185 Ma. Garnet breakdown, possibly linked to decompression and/or cooling in the suprasolidus stability field of sillimanite and K-feldspar (&amp;gt;675°C, &amp;lt;0.8 GPa), is well-documented at ca. 185–170 Ma. Finally, a third metamorphic episode is characterized by the local growth of andalusite, then cordierite, and reached &amp;gt;600°C below 0.3–0.4 GPa after ca. 120 Ma. This study illustrates the importance of analyzing petrochronometers in their textural context, especially inclusions in porphyroblasts, and the complementary P–T–t information that can be extracted from monazite and xenotime. Our new data are compatible with two separate contractional events involving the Yukon-Tanana terrane during the Permian–Triassic and Late Triassic to Early Jurassic, followed by Cretaceous contact metamorphism.

Faults caused by the fault: Microstructural and mineral characterization of deformation in Chungliao Tunnel, Taiwan, caused by Chishan Fault

This study employs microstructural and mineralogical characterization to analyze deformation within the Chungliao Tunnel (Formosan Freeway, Taiwan) caused by the movement along the Chishan Fault, as well as its effect on accelerated creep along the fault. The results reveal pyrite-filled fissures and a large amount of wavy extinction quartz and bent muscovite. The mineral particles are fragmented and finely granulated. Additionally, slippage between grains and dislocation creep are observed within Chishan Fault. Therefore, the primary deformation mechanisms of this fault are cracking/frictional sliding, dislocation creep, and grain-boundary sliding. The presence of calcite, clay minerals, physically absorbed water, and crystalline water in the rock enhances fault movement and significantly weakens the strength of rock layers. As a result, the Chungliao Tunnel over the past 18 years has risen at an average annual rate of 8 cm, and is now 130 cm higher than at the time of construction. This study is expected to contribute to future infrastructure planning along the fault. For example, mineral grains in the rock layer tend to align themselves along the fracture orientation, implying the possible stress direction. Moreover, the content of index minerals (e.g. calcite, pyrite, and clay minerals) in the rock layer can be utilized to predict the scale of rock-layer sliding and the severity of potential fault-related disasters.

Element mobility as a proxy for delineating shear zone boundaries in orogenic belts: A case study of Iwaraja shear zone, southwestern Nigeria

Shear zones are localised regions of intense deformation and are important archives of element mobility history during crustal deformation. The boundary is determined by examining shear deformation fabrics in affected rocks. However, the "mixture" of relicts of undeformed rocks coupled with newly formed shear fabrics affect the determination of the internal boundaries of the shear zone and its boundary with the unsheared protolith, which in turn limits the evaluation of the shear zone's width. A new approach involving element mobility across the shear zone is proposed as a proxy to address this problem and corroborate existing methods. The petrographic and geochemical analyses of granitic gneiss across the Iwaraja shear zone and its adjacent rock were carried out. The element mobility patterns and volume of chemical changes were evaluated using isocon and Cartesian element-distance plots to interpret the chemical changes in the granitic gneiss across the shear zone. The result indicates a very high enrichment of TiO2, Fe2O3, MnO, MgO, P2O5, and REEs with relatively similar mobility patterns and a high depletion of Na2O, Rb, and Th in the shear zone. Al2O3 is not significantly mobilized but the mobility pattern of K2O is erratic. Based on the chemical changes in the granitic gneiss, the ductile shear zone is a CaO–MgO–Fe2O3-REE-enriched and Na2O–Rb–Th-depleted system. Under greenschist facies condition, the chemical mobility of TiO2, MnO, P2O5, Th, Rb, Zn, Ba, Zr, Th, Hf, and REEs is sensitive to the changes in the deformation intensity between the strongly- and weakly-deformed zones whereas REE [Lu, Yb, Gd, Eu, Ho, Tb] is useful to determine the boundary between the weakly-deformed zone and the unsheared rock. These index minerals are useful in other similar ductile shear zones in any orogenic belt. Based on the element mobility patterns, the width of the affected granite gneiss in the shear zone is ~680 m.

Stability conditions and compositional variations of deerite in high-pressure meta-ironstone during subduction–exhumation processes (SW Tianshan, China)

Index minerals in metamorphic rocks, such as coesite, glaucophane, or lawsonite, are of great importance to understand high-pressure (HP) to ultrahigh-pressure (UHP) metamorphism, the thermal gradient, and the geodynamics of subduction zones. Deerite is thought to be diagnostic for blueschist-facies metamorphism in cold subduction environments and a sensitive indicator of the oxygen fugacity (fO2) in Fe-rich rocks. However, little is known concerning its stability and compositional variation during subduction and exhumation cycles. In this study, we provide the first detailed investigation of deerite in eclogite-facies meta‑ironstones (deerite–garnet riebeckitite) from the Southwestern Tianshan (U)HP metamorphic belt. Petrological observation and textural evidence suggest that the meta‑ironstones –enclosing eclogite blocks– form part of an initially coherent volcano-sedimentary sequence that underwent a common tectono-metamorphic evolution, including HP metamorphism and subsequent exhumation. Two types of deerite, which display a different texture (inclusion vs. matrix) and composition (TiO2 content), occur in the deerite–garnet riebeckitite. Dee_1 and aegirine–augite appear exclusively as fine-grained inclusions in garnet, and together with garnet, riebeckite, quartz and magnetite form the peak metamorphic mineral assemblages at P–T conditions of ~510 °C and 2.6 GPa. Matrix deerite (Dee_2), on the other hand, mainly occurs as coarse-grained rods and acicular needle-like grains in quartz, and rarely as inclusions in riebeckite and garnet intimately in the riebeckitite, representing the blueschist-facies retrogression of the meta‑ironstones during exhumation. The garnet–clinopyroxene oxybarometer and thermodynamic calculations show that the fO2 of the deerite–garnet riebeckitite increases from ΔFMQ–0.64 at the peak eclogite-facies stage to ΔFMQ+1.5 at the blueschist-facies retrogression. This fO2 variation corresponds to the chemical variation of deerite, whereby the TiO2 content increases gradually from <0.1 wt% in the Dee_1 in garnet to about 5 wt% in the matrix Dee_2. Our study reveals the detailed metamorphic P–T–fO2 evolution of the prograde eclogite- and retrograde blueschist-facies deerite-bearing meta‑ironstones during subduction–exhumation processes. This study further emphasizes the potential of deerite to reveal the P–T–fO2 evolution of subduction-related high-pressure rocks.

Low-pressure (&gt; 4 MPa) and high-temperature (&gt; 1250 °C) incongruent melting of marly limestone: formation of carbonate melt and melilite–nepheline paralava in the Khamaryn–Khural–Khiid combustion metamorphic complex, East Mongolia

Evidence for low-pressure incongruent melting of limestone containing up to 40 wt% of silicate minerals (marly limestone) is reported from the Khamaryn–Khural–Khiid Combustion Metamorphic (CM) complex in East Mongolia. Marly limestone exposed to high-temperature metamorphism during wild coal fires has acquired a mineral assemblage consisting of gehlenitic melilite and Al-diopside-to-kushiroite-dominant clinopyroxene as main phases and rare phases of celsian, spinel, perovskite, geikielite, etc. Melting of silicate minerals and calcite produced silicate melts of different compositions and stoichiometric calcitic (CaCO3) or non-stoichiometric carbonatite-like (CaCO3 + CaO) melts. Coalescence of silicate melt drops was followed by the formation of silica-undersaturated Ca-rich and Na-bearing paralava melts. Melilite-nepheline paralavas discovered in the Khamaryn–Khural–Khiid and Nyalga CM complexes in Mongolia have exceptional features of mineralogy and chemistry due to a rare combination of P–T conditions associated with wild coal fires and combustion metamorphism: a high temperature (> 1250 °C) and fluid pressure above 4 MPa that prevented decomposition of calcite. Such paralavas from the two CM complexes are composed of similar mineral assemblages made up of melilite, clinopyroxene, plagioclase, nepheline, Fe–Ca olivines (Ca-fayalite and kirschsteinite), K-Ba feldspars (celsian and hyalophane), spinel-group minerals, and rhonite-kuratite, with broad composition variations. Paralavas of this kind have never been reported before from anywhere. Their local mineralogical and geochemical specificity may be due to variations in the composition of carbonate protolith and in physicochemical conditions above the coal fire foci (T, P, gas composition, oxygen fugacity, and melt cooling rate). Low-pressure melting of calcite and the formation of carbonate (calcitic or carbonatite-like) melts have implications for phase relations in carbonate rocks altered by high-temperature metamorphism and metasomatism, as well to the origin of carbonatites. Calcite in carbonate sediments exposed to low pressures and high temperatures (above the invariant point Q1 in the CaCO3 phase diagram) does not decompose and can melt. Correspondingly, metamorphic decarbonation reactions do not produce CaO required for the crystallization of many Ca-rich index minerals of spurrite–merwinite facies. The melting point of calcite decreases markedly with increasing H2O content in the H2O–CO2 fluid, which may lead to melting of calcite in carbonate sediments and to formation of calcite-rich carbonatites at P–T crustal conditions.

Barrovian metamorphism of the metapelites in NE Sikkim (Eastern Himalaya): Constraints from chemographic projection and geothermobarometry

• Growth of index minerals reveals a time relation between crystallization and deformation. • P-T & T-M H2O pseudosections suggest the peak metamorphic conditions as ~7.5 kbar & ~750 °C. • Channel flow model has been proposed for the exhumation of Sikkim Himalaya. The Sikkim Himalaya is mostly constituted of Proterozoic metapelites and metavolcanics of the Daling Group and high-grade gneisses and metasediments of the Darjeeling Group. Metapelitic schists and gneisses of the Sikkim Himalaya show an inverted sequence of metamorphic rocks ranging from lower greenschist facies (chlorite zone) to upper amphibolite facies (sillimanite + K-feldspar zone). The relative X Mg in the minerals varies as: garnet > staurolite > biotite > muscovite. The P-T evolution of the study area has been constrained through the use of an internally consistent winTWQ programme and Perple_X software in the MnNCKFMASHTO model system. The combination of these two approaches demonstrates that the schists and gneisses have experienced a range of pressure and temperature with increasing grade. Constraints on the interpretation of mineral reactions and metamorphic history provided by pseudosection modelling for mineral proportions suggest the peak metamorphic temperature as ~750 °C and pressure ~7.5 kbar. The proposed prograde P-T path implies that the rocks from the study area may have resulted from a single metamorphic event with continuous changes in pressure and temperature. The conclusions made on the basis of the study of metamorphic event in the area and the regeneration of the reaction history adheres to classical Barrovian type metamorphism. The tectonic implication of such a metamorphic evolution is also discussed in the present work. Finally, we conclude that the possible explanation for the exhumation in this section of Himalaya may most likely be explained with the help of the channel flow model.

Zircon from diamondiferous kyanite gneisses of the Kokchetav massif: Revealing growth stages using an integrated cathodoluminescence, Raman spectroscopy and electron microprobe approach

AbstractZircon crystals from diamondiferous kyanite gneisses of the Barchi-Kol area (Kokchetav massif, Northern Kazakhstan) have been investigated by a combined application of cathodoluminescence (CL), Raman spectroscopy and electron probe microanalysis (EPMA). The zircon crystals exhibit up to four distinct domains characterised by significantly different CL signatures and parameters of the ν3(SiO4) (1008 cm–1) Raman band (i.e. full width at half maximum, position and intensity). Extremely metamict zircon cores (Domain I) host inclusions of low-pressure minerals (quartz and graphite) and the outer mantles (Domain III) are populated by ultrahigh-pressure relicts (diamond and coesite), whereas inner mantles (Domain II) and overgrowth rim zones (Domain IV) are inclusion free. Both the zircon cores and rims have very low Ti concentrations, implying formation temperatures below 760°C. The Ti content in the inner mantles (up to 40 ppm) is indicative of temperatures in the 760–880°C range. The temperature estimates for the outer mantles are 900–940°C, indicating a pronounced overlap with the peak metamorphic values yielded by the Zr-in-rutile geothermometer for the same rocks (910–950°C). The internal textures of the zircons and the occurrence of index minerals within the distinct domains allow us to unravel the stages of the complex metamorphic history recorded in the zircon. Our data show that the zircon cores are inherited seeds of pre-metamorphic (magmatic?) origin, the inner mantles were formed on the prograde metamorphic stage, the outer mantles record ultrahigh-pressure metamorphism and the outermost rims mark the retrograde metamorphic stage. The observed zircon internal textures are thus clearly correlated with distinct growth events, and in some examples reflect a major part of the metamorphic history. It is concluded that the combined application of the CL, Raman spectroscopy and EPMA techniques to zircon offers significant potential for deciphering the metamorphic evolution of deeply-subducted rocks.

Index Minerals of the Hydrothermal Arsenide Process of the Formation of Cobalt Deposits of the Bou Offro–El Graara Ore Belt in the Anti-Atlas Province, Morocco

The distribution of index minerals of the hydrothermal arsenide process of the formation of Ni‒Co–As (±U–Ag), Co–S–As (±Au–W), and Cu–Co–As (±Sb–Ag) quartz–carbonate veins of deposits of the Bou Offro–El Graara ore belt (Morocco) are discussed. The Co arsenide objects of the Bou Azzer ore cluster with typical low-temperature Ni–Co–As (U–Ag) deposits of the five-element hydrothermal type are of special interest: El Jir, Mechoui, Tarouni, Central and East Bou Azzer, Aghbar, Tamdrost, etc. The ores of most of the studied deposits are strongly enriched in Co relative to Ni (Co/Ni ratio = 10/1), have high grades of Au (up to 100 g/t) and Mo (up to 0.1%) with relatively low Ag, Ni, and U contents. Based on the analysis of the interrelations of ore and gangue minerals, the sequence of the formation of mineral assemblages is identified and their mineralogical–geochemical zonation is studied. The range of the most favorable physicochemical parameters of ore deposition is determined from the fluid inclusions in the gangue minerals. The indicative role of mineralogical–geochemical features of ore assemblages is shown for the forecast and survey of blind ore bodies.

Exhumation of deeply subducted crust: Review and outlook

The discovery of ultrahigh-pressure (UHP) index minerals such as coesite and micro-diamond in supracrustal rocks indicate that a large amount of crustal rocks were subducted to mantle depths and subsequently returned to the Earth’s surface. However, exhumation mechanism of the deeply subducted crustal rocks has been a hot topic in geodynamic community and remains controversial. Previous studies have been performed by petrologic observations, analogue experiments and numerical simulations for exhumation processes and mechanisms of deeply subducted crustal rocks, and a series of important results have been achieved. In this paper, we summerize in detail recent progresses and existing problems on types of subduction zones, exhumation processes and mechanisms of deeply subducted crustal rocks, and related factors. Based on these, three forthcoming prospects are presented: (1) The geological processes within the exhuming crust; (2) the interaction between the exhuming crust and the surface process; (3) the interaction between the exhuming crust and the mantle.