Garnet Biotite Gneiss Research Articles

Geochemical and geochronological evidence of a Neoproterozoic island-arc on the easternmost Sierras Pampeanas. New U-Pb LA-ICP-MS data from amphibolites and metacarbonate depositional sequences

The first basement outcrops west of the Río de La Plata craton are metabasaltic and metasedimentary units of the Sierra Chica belt, part of the Eastern Sierras Pampeanas. These are orthoamphibolites and subordinate garnet-biotite gneisses and metacarbonate sequences that are intruded by Cambrian calc-alkaline and peraluminous rocks of the Pampean Orogeny. In accordance with one of the leading tectonic models for the region, new provenance data from this research suggest that the metabasaltic-metasedimentary association represents the proximal deposits of a Late Neoproterozoic island-arc. This contribution brings new geochemical and geochronological evidence from orthoamphibolites located on different sites along the Sierra Chica belt, which are interbedded with metacarbonate and gneisses outcrops, showing tholeiitic compositions, island-arc related tectonic settings, and Cryogenian to Ediacaran crystallization ages. Moreover, silicate-rich metacarbonates interbedded within the same sequences yielded a maximum depositional age of 569 Ma, reinforcing the assumed age for the sequence. All things considered, these rocks are interpreted as old basic lava flows and gabbro dikes linked to an island-arc activity, interspersed within the carbonatic and siliciclastic shelf sediments, together constituting the Neoproterozoic Córdoba Arc from the Sierra Chica belt. A depositional analogy with the present-day Caribbean island-arcs deposits suggests that these sequences could have been part of Oceanic Arc Depositional Systems (OADS) during the diachronous closure of the Goiás ocean and the final assembly of West Gondwana. Overlapping thermo-tectonic processes associated with the Pampean Orogeny are ubiquitous throughout all the studied outcrops, as evidenced by the petrography, the zircon morphology, and the U-Pb results, indicating a complex scenario towards a Late Cambrian slab break-off event.

Late orogenic high-temperature overprint in (ultra)high-pressure ([U]HP) metamorphic rocks of the North Qinling Orogen, Central China: Insights into the geodynamics of the exhumation of (U)HP metamorphic rocks

Exhumed (ultra)high-pressure ([U]HP) metamorphic rocks are markers of paleo-subduction zones and can provide crucial implications for the geodynamic evolution of subduction-exhumation. A variety of (U)HP metamorphic rocks from many collisional orogens have commonly documented a low-pressure high-temperature overprint, which appears to indicate a pervasive geodynamic process behind (U)HP metamorphic rock exhumation. A good case study of garnet amphibolite and associated garnet-biotite gneiss with an unambiguous low-pressure high-temperature overprint in the Xixia-Xiaguan area of the North Qinling Orogen, central China, provides a significant opportunity to uncover this geodynamic process. A detailed study of petrography, mineral chemistry, and phase equilibria modeling suggests that the investigated garnet amphibolite and garnet-biotite gneiss have jointly experienced multistage exhumation processes with near-isothermal decompression from the (U)HP eclogite facies (26.8−29.0 kbar/740−775 °C) to the medium-pressure (MP) amphibolite facies (8.7−10.0 kbar/740−775 °C), subsequent remarkable near-isobaric heating to the low-pressure (LP) granulite facies (6.8−8.2 kbar/832−835 °C), followed by final decompression and cooling to the LP amphibolite facies (5.8−6.8 kbar/740−757 °C). U-Pb age dating of zircons from the garnet amphibolite and garnet-biotite gneiss yielded metamorphic ages of ca. 500 Ma, 450 Ma, and 400 Ma, which we consider to represent (U)HP eclogite-facies, LP granulite-facies, and late LP amphibolite-facies metamorphism, respectively. According to the regional geology and the metamorphic history we have determined, this multistage exhumation process of two-step exhumation stages delimited by a phase of LP near-isobaric heating was associated with slab rollback. Accordingly, we propose a tectonic model of exhumation driven by slab rollback for the geodynamic evolution of (U)HP metamorphic rock exhumation in the North Qinling Orogen. Additionally, a combination of previous studies and our new results suggests that exhumation driven by slab rollback appears to be a fundamental mechanism for the (U)HP metamorphic rock exhumation within collisional orogens.

Ti‐in‐zircon and Ti‐in‐quartz thermometry using electron probe microanalysis: A promising approach in retrieving thermal conditions of granulite facies metamorphism and felsic magmatism in the Sandmata Complex, Aravalli Craton (NW India)

The titanium (Ti) concentrations in zircon and quartz and corresponding thermometers have been regarded as powerful monitors that constrain petrogenetically meaningful temperature conditions of crystallization and metamorphism. The precise measurement of trace elements by electron probe microanalyzer (EPMA) is quite challenging as their x‐ray intensities are only slightly higher than the Bremsstrahlung background, leading to significant errors while quantifying the elemental abundances (e.g., Ti‐in‐zircon). We present EPMA‐based protocols for analysing Ti concentrations in zircon and quartz by considering recent analytical and instrumental improvements. High counting times at peak and background positions, simultaneous acquisition of Ti in multiple (three) spectrometers and data processing using the non‐central chi2 test for risk determination in sub‐counting method were employed in Ti‐in‐zircon and Ti‐in‐quartz protocols. Applying these protocols to the Mongolian garnet, San Carlos olivine and NIST 610 glass standards yielded Ti concentrations of 0.60 ± 0.02 wt%, 18 ± 3 ppm and 437 ± 26 ppm (in ±2σ), respectively. The Ti concentrations obtained from these standards are consistent with those published based on high‐precision analytical methods. We use the existing Ti‐in‐zircon and Ti‐in‐quartz thermometers to understand the thermal evolution of various lithologies in the Sandmata Complex, Aravalli Craton (north‐western India). The investigated samples include garnet–biotite gneiss, migmatite gneiss, mafic granulite and Anjana granite. The Ti‐in‐zircon temperatures calculated from the patchy‐zoned zircon cores yield HT–UHT conditions (892–959°C) for garnetiferous granite gneiss, migmatite gneiss and mafic granulite, whereas the zircon overgrowths yield temperatures of 725–871°C. The homogeneous zircon cores from Anjana granite yield temperatures (914–984°C) comparable to zirconium saturation conditions (920–960°C), indicating zircon crystallization from “hot” granitic melt. The oscillatory overgrowths on the zircon core obtain variable temperature conditions for inner overgrowth (811–877°C) and outer overgrowth (714–782°C), suggesting the episodic growth of zircon grains from different magma pulses. Additionally, the application of pressure‐dependent Ti‐in‐quartz thermometry yields quartz recrystallization conditions for garnet–biotite gneiss (430–556°C), migmatite gneiss (423–593°C), mafic granulite (430–679°C) and Anjana Granite (444–533°C). The combination of Ti‐in‐zircon thermometry, Ti‐in‐quartz and conventional thermobarometry demonstrates cooling histories for various high‐grade metamorphic and magmatic rocks in the Sandmata Complex. Based on this study, we emphasize that trace element thermometers can decipher high‐temperature metamorphism, which is otherwise erased in conventional thermometers due to diffusional readjustments during long‐lasting near‐peak and post‐peak metamorphism.

Kaalamo and Velimyaki paleoroterozoic gabbroid massifs of the Northern Ladoga area: thermodynamic modeling of crystallization and evolution trends of igneous rocks

Kaalamo and Velimyaki gabbroid massifs are located in the south-eastern part of the Raahe-Ladoga suture zone of the Karelian craton and the Sveсofennian mobile belt. These massifs were formed almost simultaneously 1.89 Ga ago, occupy the same tectonic position and are located in a single unit of the Early Proterozoic metamorphosed rocks: the intrusions are contained by the Early Proterozoic supracrustal stratas: garnet-biotite gneisses and micaceous shales of the Ladoga series, as well as amphibolites of the Sortavala series. Despite this, the gabbroids of the compared massifs differ sharply in their material composition, especially the content of alkalis in a differentiated series of rocks. The results of thermodynamic modeling of magmatic crystallization (Magma Chamber Simulator, MELTS packages) showed that a differentiated sub-alkaline series of rocks of the Velimyaki massif cannot be obtained from magma of the Kaalamo massif by slightly changing such parameters as pressure, water content in the melt, and oxygen fugacity. The assumption of a significant difference in these parameters for magmas of the Kaalamo and Velimyaki massifs is not confirmed at the mineralogical-petrographic level and other characteristics of the rocks of the compared ones. The effect of contamination at the level of the magma chamber also does not significantly shift the trend of rock compositions from the calc-alkaline region to the sub-alkaline region. Analysis of the results of thermodynamic modeling of magmatic mineral formation in the two massifs allows us to make it possible to conclude that the Kaalamo and Velimyaki massifs were formed either from various parent melts, or their initial melt was one, but the magma of the Velimyaki massif underwent contamination with enrichment with alkalis along the migration route of magma until the level of the upper crust was reached.

Long-lived (>100 m.y.) postcollisional exhumation and cooling in the Paleoproterozoic Trans−North China orogen: Evidence from phase equilibria modeling and monazite petrochronology of granulite-facies metapelites in the Fuping Complex

Long-lived collisional orogens that formed over tens to hundreds of millions of years are common in the geologic record. The Trans−North China orogen marks the collision between the Eastern and Western blocks of the North China craton, and it preserves metamorphic rocks with ages between 1.98 Ga and 1.80 Ga. These units allow detailed assessment of the time scale and duration of crustal thickening, exhumation, and cooling associated with a major Proterozoic orogeny. In this study, we present integrated petrography, mineral chemistry, phase equilibria modeling, and texturally controlled in situ mass spectrometry of monazite U-Th-Pb and trace-element analyses performed on a suite of orthopyroxene-bearing pelitic granulites and garnet-biotite gneisses from the Fuping Complex within the Trans−North China orogen. These rocks record clockwise pressure-temperature (P-T) paths involving granulite-facies peak conditions of 9.9−11.0 kbar and 850−880 °C for pelitic granulites, and 10.9−11.6 kbar and 860−880 °C for garnet-biotite gneisses, followed by postpeak decompression to ∼8−9 kbar and later cooling, with final solidification of melt at <840 °C. Four monazite populations were identified in these samples. Group I grains are irregular and elongated and occur in contact with or embay garnet. They have high rare earth element (REE) and Y contents and metamorphic ages of 1.90−1.86 Ga, which correspond to the breakdown of garnet during postpeak decompression. Most monazite grains crystallized from melt are represented by groups II + III + IV and are associated with orthopyroxene, biotite, plagioclase, and quartz in the matrix. They have crystallization ages between 1.86 Ga and 1.76 Ga and relatively low REE and Y concentrations. These data imply a long-lived (>100 m.y.) postcollisional exhumation and cooling involving decompression from 10−12 kbar to ∼9 kbar during 1.90−1.86 Ga, followed by retrograde cooling from 1.86 to 1.76 Ga under prolonged residence in the middle to lower crust. Initial collision and peak metamorphism occurred before 1.90 Ga, ultimately leading to the final cratonization of the North China craton and its incorporation into the Columbia supercontinent.

METATERRIGENIOUS ROCKS OF THE OLKHON TERRANE OF THE CENTRAL ASIAN OROGENIC BELT: U-Pb ZIRCON AGE, GEOCHEMICAL CHARACTERISTICS, AND FORMATION MODELS OF SEDIMENTARY PROTOLITHS

The paper presents a petrographic, geochemical, and Sm-Nd isotopic data on the gneisses from different tectonic zones of the Olkhon terrane of the Central Asian Orogenic Belt, as well as the composition of garnets and the age of zircons in these metaterrigenous rocks. The garnet-biotite gneisses of the Anga-Sakhurta zone, as well as the garnet-bearing and garnet-free gneisses (granulites) of the Chernorud zone may result from metamorphism of immature terrigenous rocks of polymictic or greywacke compositions similar in geochemical characteristics to rocks of continental arcs. At the same time, the gneisses of these zones show both similarities and some differences in geochemical and isotopic characteristics, including variations in ɛNd(T) values from –0.2 to –9.0, which may indicate different proportions of one or another source in their protoliths. The age of most detrital zircons in the gneisses of the Chernorud and Anga-Sakhurta zones corresponds to 530–1000 Ma, and the youngest detrital zircons have an age of 522–537 Ma. The geochemical and geochronological data on the gneisses of the Chernorud and Anga-Sakhyurta zones suggests that the protoliths of these gneisses could be formed from the same sources of predominantly Neoproterozoic age. We assumed that gneiss protoliths could initially be sediments of the continental slope of Neoproterozoic composite terrane assembled to the Siberian Platform at 600‒610 Ma. The transport of clastic material being gneiss protoliths from this composite terrane, took place towards the Paleo-Asian Ocean. The youngest zircons with an age of about 530 Ma could be generated from igneous complexes intruding the Neoproterozoic composite superterrane. The garnet-biotite gneisses of the Krestovskaya zone are similar in chemical composition to immature graywacke sandstones, The ɛNd(T) value in these gneisses is –3.7. Detrital zircons in the gneisses of the Krestovskaya zone form age peaks at 780–820 and 498–515 Ma. Based on geochemical and geochronological data we assume that these gneisses could be formed in an intracontinental basin formed at the orogenic stage during accretionary-collisional events at the amalgamation the Orso block and the Birkhin island arc into the Krestovskaya zone.All terrigenous rocks being gneiss protoliths were metamorphosed at 460–510 Ma under granulite or amphibolite facies associated with accretionary and collisional events, which led to the formation of the Early Paleozoic Olkhon composite terrane.

Geochemistry and Phase Equilibrium Modelling of Garnet-Biotite Gneiss from Mauranipur, Bundelkhand Craton, Northern India: Implication for Tectonic Setting and Metamorphism

The Mauranipur region is situated along the central part of the Bundelkhand Craton (BuC) in the northern Indian shield, which consists of garnet- biotite gneisses with various deformational structures in the form of folding, faulting, augen and tail structures. These deformation structures are tectonic imprints that reveal the tectonic nature of the garnet-biotite gneisses. The groundmass of Grt-Bt gneisses is characterized by presence of garnet, biotite, plagioclase, K-feldspar, quartz, and ilmenite. The phase equilibrium modelling and geochemical attributes depict the tectonic activity and metamorphic evolution of the studied rocks. The P-T pseudosection has been calculated in the NCKFMASHT system, which revealed that the peak mineral assemblage stabilized in the P-T range of 6.35–6.75 kbar and 755–780ºC, and it further goes to retrograde metamorphism under P-T condition ranging from 4.80–5.28 kbar and 718–735ºC. These gneisses represent a calc-alkaline to high-K calc-alkaline series of protolithic origin. The negative anomaly of Nb and Ti for all samples indicates that a subduction tectonic setting has occurred in the BuC. The (La/Lu)N ratio and differences in the trace elements indicate heterogeneous sources and large variation in the degree of partial melting. The Y vs Nb and (Y+Nb) vs Rb tectonic discrimination diagrams indicate that the Grt-Bt gneisses have an affinity towards the volcanic arc granite and developed during subduction setting. The geochemical interpretation provides significant evidence that protoliths of Grt-Bt gneisses were further metamorphosed by the continent-continent collision. Keywords: Garnet-biotite gneiss, Pseudosection, P-T condition, Geochemistry, Bundelkhand

Geology, Petrographic Characteristics and Tectonic Structure of Paleoproterozoic basement of Kabul Block (North Eastern Afghanistan), Initial Findings

The Paleoproterozoic basement of Kabul Block is a cratonic basement in north eastern Afghanistan. Precambrian Kabul Block including metamorphic rocks only takes place at the base of the study area. The Neo-Proterozoic Welayati formation overlies the Paleo-Proterozoic Sherdarwaza formation with a tectonic contact, and they both overlay Khair Khana formation. In the region, Alghoi meta granitoid intruded into both Khair Khana and Sherdarwaza formations. The Khair Khana formation contains of granulites, granitic gneisses ve amphibolites. The mineral assemblages of granulites are plagioclase + quartz + K-feldspar + orthopyroxene + clinopyroxene (diopside/augite). Granitic gneisses are observed as hornblende gneiss, garnet-biotite gneiss and pyroxene-gneiss. Hornblende-gneisses include hornblende + quartz + K-feldspar mineral assemblage, garnet-biotite gneisses are evident with plagioclase + quartz + K-feldspar + biotite (red) + garnet mineral assemblage, and pyroxene gneisses include quartz + plagioclase + biotite + clinopyroxene mineral assemblage. Amphibolites consist of hornblende + plagioclase + quartz ± zoisite ± sphene ± ilmenite ± apatite mineral assemblage. The Sherdarwaza formation contains marbles, amphibolites, biotite gneisses, micaschists and migmatites. Marbles consist of calcites ± quartz ± apatite. In amphibolites, harnblende + plagioclase + quartz ± epidote ± zoisite ± garnet mineral assemblage are observed. Biotite gneisses include plagioclase + quartz + microcline + biotite ± sphene ± apatite ± zircon mineral assemblage. Micachists consist of quartz + plagioclase + microcline + biotite ± sphene ± apatite mineral assemblage. Migmatites include plagioclase + quartz + microcline +biotite ± sphene ± apatite mineral assemblage. Alghoi meta granitoid is generally observed as stocks and small masses, and sometimes it shows foliation. It consists mainly of meta granites. The meta granites that show granular texture include plagioclase + quartz + K-feldspar + biotite + hornblende. The Welayeti formation contains quartzite, kyanite-garnet-staurolite schist, garnet-mica schist, kyanite-garnet-mica schist and garnet-muscovite schist. Quartzite include quartz ± muscovite ± biotite. They are observed hornblende + quartz + plagioclase ± garnet ± epidote ± zoisite ± rutile ± magnetite in amphibolites. Kyanite-garnet-staurolite schists are evident with staurolite + kyanite + garnet + mica + biotite + quartz + plagioclase ± apatite ± epidote ± ilmenite ± monazite mineral assemblage. Garnet-mica schists and kyanite-garnet-mica schists include quartz + plagioclase + biotite + muscovite + kyanite + garnet ± chlorite ± apatite ± tourmaline ± sphene ± rutile. In garnet-muscovite schists, muscovite + quartz + plagioclase + garnet ± tourmaline ± apatite ± zircon are observed. The granulites in Kabul Block are together with granite gneisses, and they can be separated under one formation. Granulites show different mineral compositions, Usually, they are without garnet, in some cases they include garnet. The amount of plagioclase is higher in them according to other rocks. In some cases, the composition of granulites is very similar to that of quartz mangarites that indicates high temperature and pressure of the metamorphism process and shows that this block is related to a segment of Columbia and Rodinia supercontinents during Paleoproterozoic collisional events.

Uranium-lead isotopic age of monazites of plagiogneyses of the Ingulo-Inguletse series (Ingul megablock of the Ukrainian Shield)

Among the supercrustal formations of the Ingul megablock, Paleoproterozoic rocks, represented by the Ingul-Ingul series, are significantly predominant. In general, the composition of the Ingul-Ingulets series includes (from bottom to top): Zelenorichen, Artemiv, Rodioniv, Kamyano-Kostovatsky (age analogue of Spasivsk) and Roschakhiv (age analogue of Chechelivsk) worlds. The Chechelivsk suite is represented by biotite and garnet-biotite gneisses, places with sillimanite and cordierite, as well as underlying layers of pyroxene gneisses and schists. Suite rocks are widespread within the Ingul Synclinorium, in the framework of the Korsun-Novomyrhorod pluton and in the northern part of the Bratsky Synclinorium. Currently, the age of monazites from plagiogneisses distributed in the Novgorodkiv quarry and south of the village Lavrovka has been determined using the uranium-lead isotope method. The obtained monazite isotopic age values of 2022.9 ± 6.7 Ma and 2031.5 ± 1.5 Ma, respectively, can only serve as the upper age limit for the formation of the Ingul-Ingul series. Analysis of the internal structure of zircon crystals from plagiogneiss distributed south of Lavrivka, showed that the majority of zircons are complex. In almost all crystals there are mainly thin shells of light pink zircon. Light pink zircon cores are found in the middle of the light pink crystals. Crystals with a brown color have a more complex structure. Along with isometric cores of light pink zircon, cores with fine «magmatic» zoning occur, as well as highly fractured uniform brown cores and complex cores.On this basis, it is assumed that this plagiogneiss was formed by clastogenic material from several sources.

Evidence of Eoarchean crust beneath the Yakutian kimberlite province in the Siberian craton

The Siberian craton is one of the largest Precambrian cratons in the world. However, unlike other cratons, the Siberian craton can only provide a relatively limited amount of geochronological data since a large part of it is covered by younger sedimentary deposits. Xenoliths from numerous kimberlite and lamprophyre pipes and alkali basalts allow us to investigate the composition and evolution of the Siberian Craton crust. The available isotopic and geochemical data show that the crust of the Anabar tectonic province of the Siberian craton is not age stratified, and consists of variably reworked Paleoarchean rocks and juvenile Proterozoic rocks at all crustal levels. Hence, a detailed geochronological study of crustal xenoliths (mainly zircon U-Pb age and Hf isotopic composition) in the kimberlite pipes of the Yakutian kimberlite province is required. In this study, we present U-Pb ages and Hf isotope data of zircons derived from crustal xenoliths in the Zarnitsa (Daldyn kimberlite field) and the Nyurbinskaya (Nakyn kimberlite field) kimberlite pipes. The obtained data provided grounds for identifying four main tectono-thermal events during the evolution of the Archean crust in the Anabar tectonic province of the Siberian craton. The first event at 3.2 Ga denotes the beginning of zircon crystallisation. These zircons are eventually recrystallised during the second event at 2.9 Ga, and radiogenic Hf (formed from 176Lu decay in the host rock) was introduced into the system. The third event at 2.8–2.7 Ga featured further recrystallisation of zircons and the incorporation of additional radiogenic 176Hf. The minor addition of radiogenic Hf during the final event at 1.9 Ga indicates a relatively large range for the initial 176Hf/177Hf ratio. Our data indicate that Eoarchean or even Hadean crust, which were reworked in the Paleoproterozoic, may be present beneath the Daldyn and Nakyn kimberlite fields. Zircon grains with low 176Hf/177Hf (0.28047–0.280661) were found in the xenoliths of the mafic garnet granulite (Zar-1–02) and garnet biotite gneisses (Nrb-13 and Nrb-65).

Influence of carbonate rocks on soil properties in the humid tropical climate of atlantic forest, Rio de Janeiro –Brazil

Soil carbonate accumulation plays an important role in C sequestration, accounting for approximately 40% of the C stored in soils worldwide. Under humid tropical climate these carbonates are rapidly dissolved, intensifying CO2 emissions. Despite this, little is known about the weathering of carbonates in areas with complex geology. This study aimed to evaluate the influence of the parent material on soil attributes and weathering processes under humid tropical climate conditions along toposequences formed by dolomitic marbles and calc-silicate rocks interspersed with garnet-biotite gneiss. Samples and soils profiles were collected on sites of both parental materials and at different points of a toposequence, namely: upper third, middle third and lower third of a slope. The profiles were analyzed for their morphological, physical, chemical and mineralogical attributes. The attributes of soils formed from metamorphic sedimentary rocks were more influenced by geological processes than pedological ones. It was found that the nature of the parent material was the main responsible for the attributes of the profiles, especially the strongly alkaline pH, being greater than 8.0 in most soil horizons, and the high contents of exchangeable Ca2+ and CaCO3 equivalent, with maximum values of 40.6 cmolc kg−1 and 763 g kg−1, respectively. As for mineralogical composition, it was verified the occurrence of minerals such as talc, vermiculite and Fe and Ti oxides, with metamorphic processes and the addition of hydrothermal fluids being the main responsible for the presence of these minerals. The supply of high contents of basic cations, with maximum values of 61.4 cmolc kg−1, high Si/Al ratio (reaching values close to 5.0) and the high contents of CaCO3 conditioning the strongly alkaline pH hinder the dissolution of silicates, even those with low resistance to weathering, despite the hot and humid climate conditions. The action of weathering processes in humid tropical climate is more marked in acidic materials, being responsible for the in situ formation of kaolinite from the alteration of mica and feldspar.

Multi-Stage Metamorphism of the South Altyn Ultrahigh-Pressure Metamorphic Belt, West China: Insights into Tectonic Evolution from Continental Subduction to Arc–Backarc Extension

Abstract The South Altyn ultrahigh-pressure (UHP) metamorphic belt is claimed to host the deepest subducted continental crust based on the discovery of former stishovite, and thus can provide unique insights into the tectonic evolution from deep continental subduction and exhumation to arc–backarc extension. In this article, we present detailed studies of petrography, mineral chemistry, phase equilibria modelling and zircon U–Pb dating for three representative samples involving garnet amphibolite (A1531 & A1533) and associated garnet-biotite gneiss (A1534) from the UHP belt. Three phases of metamorphism are inferred for the rocks. The first phase high-pressure (HP)–UHP-type eclogite facies is represented by the mineral assemblages of garnet and phengite inclusions in zircon and garnet cores with the high grossular (XGrs = 0.33–0.34). The Si contents of 3.40–3.53 and 3.24–3.25 p.f.u. in phengite inclusions yield pressure conditions of >1.7–2.3 GPa for A1533 and 2.5–2.55 GPa for A1534 at a fixed temperature of 770°C. The second phase medium-pressure (MP)-type overprinting of garnet amphibolite facies shows P–T conditions of 0.8–1.2 GPa/750–785°C based on the stability fields of corresponding mineral assemblages, the measured isopleths of Ti contents in biotite and amphibole cores, and XGrs in garnet. The third phase low-pressure (LP) type overprinting includes early-stage heating to peak granulite facies followed by cooling towards a late-stage amphibolite facies. The peak granulite facies is represented by the high Ti amphibole mantle, high Zr titanite and the intergrowths of clinopyroxene + ilmenite in A1533 & A1531, with P–T conditions of 800–875°C/0.80–0.95 GPa. The late-stage is defined by the solidus assemblages, giving P–T conditions of 0.5–0.7 GPa/720–805°C. U–Pb geochronology on metamorphic zircons from A1533 and A1534 gives three ages of c. 500 Ma, c. 482 Ma and c. 460 Ma. They are interpreted to represent the HP-UHP, MP and LP types of metamorphism respectively, based on cathodoluminescence (CL) images, mineral inclusions and trace element patterns. Combining the regional geology and metamorphic evolution from the Altyn Orogen, a tectonic model is inferred, including the following tectonic scenarios. The small Altyn Microcontinent (AM) was subducted to great mantle depths with dragging of the surrounding vast oceanic lithosphere to undergo the HP-UHP eclogite facies metamorphism during the early subduction stage (c. 500 Ma) of the Proto-Tethys Ocean. Then, the subducted slabs were exhumed to a thickened crust region to be overprinted by the MP-type assemblages at c. 482 Ma. Finally, an arc–backarc extension was operated within the thickened crust region due to the retreat of subduction zones. It caused evident heating and the LP-type metamorphic overprinting at c. 460 Ma, with a fairly long interval of 30–40 Myr after the HP-UHP metamorphism, distinct from the short interval of <5–10 Myr in the Bohemian Massif.

Newly found Tonian metamorphism in Akebono Rock, eastern Dronning Maud Land, East Antarctica

In this paper, we report a new metamorphic age for kyanite-bearing garnet-biotite gneiss collected from Akebono Rock, which is located on Prince Olav Coast, in the Lützow-Holm Complex (LHC), East Antarctica. The metamorphic P–T path estimated from pseudosection modeling, together with garnet compositional zoning and experimentally calibrated geothermobarometry, suggests that the gneiss underwent a clockwise amphibolite-facies P–T history. Two populations of U–Pb zircon ages of 1121–1014 Ma and 972–904 Ma (n = 65) were identified, with the latter population having a weighted mean age of 937 ± 6 Ma. The younger population was dated from overgrowth rims and single zircon grains interpreted as metamorphic in origin. Ti-in-zircon thermometry supports crystallization of these zircon domains and grains at a temperature close to the metamorphic peak of 642 °C at 937 Ma. Monazite EMP dating from the other four samples show an age between 977 and 917 Ma. These lines of evidence indicate that prograde metamorphism of the Akebono Rock gneiss occurred during the Tonian, that is, substantially earlier than the LHC regional metamorphic age of late Neoproterozoic to Cambrian. Our results indicate that a fundamental revision of the sequence of metamorphic events in the Prince Olav Coast area of the LHC is required. We also identified three possible relationships between Akebono Rock and other geological units that record Tonian metamorphism, although we cannot determine which relationship is most likely. In particular, linear magnetic anomalies were identified near the boundary between the LHC and the Western Rayner Complex/inland nunataks of western Enderby Land, suggesting the presence of geological discontinuities.

Enstatite nodules in the harzburgites of the Southern Urals

Research subject. At two points on the northern shore of Lake Uvil’dy in the Southern Urals, two outcrops of unique rocks – nodular harzburgites – were discovered in the tectonic lenses of serpentinite melange among garnet-biotite gneisses of the Ilmenogorsko-Vishnevogorsky Сomplex. These outcrops form a body of about 9 m in thickness among amphibolized orthopyroxenites.Results. The fine-grained serpentinized enstatite-olivine matrix of the harzburgites under study contains numerous spherical formations (nodules) 2–7 cm in diameter, composed of large enstatite crystals and rare forsterite grains. The composition and structure of these spheroids are highly similar to the enstatite chondrules from stony meteorites, although differing in size (ten times larger).Conclusions. Such structures are very rare in hyperbasites and occur in slightly altered varieties. Judging by the microstructural features and relationships of the main minerals, it is concluded that these minerals were crystallized from a magmatic melt.

Metamorphic evolution of Daqingshan supracrustal rocks and garnet granite from the North China Craton: Constraints from phase equilibria modelling, geochemistry, and SHRIMP U–Pb geochronology

Supracrustal rocks and garnet granite within the high-grade metamorphic complex in Daqingshan, at the northern margin of the Palaeoproterozoic Khondalite Belt, North China Craton (NCC), preserve various mineral assemblages that record three distinct stages of their metamorphic evolution. The mineral assemblages representative of prograde metamorphism (M1) comprise plagioclase (Pl) + quartz (Q) ± biotite (Bio) inclusions preserved in garnet porphyroblasts in gneiss and granite members. The metamorphic peak mineral assemblages (M2) are represented by garnet (Gt) + Pl + Bio + Q + sillimanite (Sil) + K-feldspar (Kfs) in each rock’s matrix. Retrograde mineral assemblages (M3) that formed during post-peak, near-isothermal decompression (ITD) occur between relict garnet porphyroblasts and matrix minerals. These M3 assemblages consist of Bio + cordierite (Crd) + Pl + Gt + Q + Sil + Kfs. Phase equilibria modelling shows that the supracrustal rocks record clockwise pressure–temperature (P–T) trajectories from 650 to 750 °C/5.3–8.9 kbar (M1) up to 800–830 °C/9.8–11.2 kbar (M2), to 760–830 °C/4.8–6.2 kbar (M3). Geochemical similarities between the Daqingshan supracrustal rocks (i.e. garnet-biotite gneisses) and garnet granites, such as relative enrichment in K, Rb, and Ba, and depletion in Th, U, Ta, Nb, P, and Ti, imply a petrological affinity. Sensitive high-resolution ion microprobe analysis (SHRIMP) U–Pb dating of zircons reveals that the protolith of the Daqingshan supracrustal rocks formed at ~2.5 Ga and subsequently metamorphosed at ~2.45–2.37 Ga. Anatexis likely occurred at ~2.43–2.40 Ga during the retrograde near-ITD stage. Combined with previous studies, the timing of metamorphism and anatexis in the Daqingshan area may be connected with the regional mafic magmatism.

Geological position and mineral composition of kyanite amphibolites of the Chupinskiy paragneiss belt (Belomorian province of Eastern Fennoscandia)

The Lyagkomina occurrence of kyanite amphibolites (the Belomorian Province of the Fennoscandian Shield) is confined to an amphibolite boudin among kyanite-garnet-biotite and garnet-biotite gneisses of the Chupa Sequence. The main rock types at the Lyagkomina occurrence are meso- and melanocratic garnet-kyanite amphibolites with large kyanite (more than 1 cm in length) and kyanite-garnet amphibolites with small kyanite. There are also leucocratic garnet-biotite-amphibole-plagioclase rocks with kyanite, melanocratic epidote-staurolite-amphibole rocks, zoisite plagioclasites with staurolite and kyanite. The country gneisses around the boudin were metasomatically altered: quartz disappears in kyanite gneisses, and amphibole appears in garnet-biotite gneisses. Calcic amphiboles in the kyanite amphibolites are represented by high-alumina tschermakite; biotite — by intermediate-alumina phlogopite; garnet belongs to the pyrope-almandine series with a high grossular content. Plagioclases have an intermediate and basic composition. Intermediate plagioclases are common in kyanite amphibolites without staurolite and in kyanite amphibolites with staurolite rims, while the basic plagioclases are found in staurolite amphibolites. Many rocks are characterized by reaction structures, representing the change of the mineral assemblages. Metasomatic zoning has been reconstructed in the occurrence with a regular sequence of the mineral assemblages: Hbl + Grt + Pl + Qtz ± Bt (garnet amphibolite) → Hbl + Grt + Pl + Ky + Qtz ± Bt → Hbl + Grt + Pl + Ky ± Bt → Hbl + Grt + Pl + Ky + St ± Bt → Hbl + Grt + Ky + Pl + St + Czo ± Bt → Hbl + Grt + Pl + St + Czo ± Bt → Hbl + St + Czo + Pl ± Bt. In the described series, the alumina content increases and the Mg content of the main rock-forming minerals decreases, and desilication of the rock occurs. The described mineral zoning was formed during metamorphism as a result of the reaction between the initial amphibolite and fluid flow in the shear zone (i. e., at synmetamorphic metasomatism). Petrological and isotope data indicate that the Lyagkomina kyanite amphibolites are genetically related to the occurrences of corundum-bearing rocks in the Belomorian Province of the Fennoscandian Shield.

Incipient charnockite formation in the Trivandrum Block, southern India: Evidence from melt-related reaction textures and phase equilibria modelling

The formation of incipient charnockite is significant for understanding the deep crustal process, which, however, remains under broad controversy. Patches of incipient charnockite occur ubiquitously in garnet-biotite gneiss in southern India. Here, we present new textural observations, geochemical analyses and phase equilibria modelling on the incipient charnockite and its garnet-biotite gneiss host from the Irinrayam quarry in the Trivandrum Block, with the aim to provide some new insights into the petrogenesis of incipient charnockite. Both rock types have similar bulk compositions which are characterized by high SiO2 (c. 70 wt%) and K2O (5.5–6.2 wt%), and low K/Rb (c. 150). Both rock types share similar peak mineral assemblage of garnet, K-feldspar, plagioclase, quartz, ilmenite and biotite except for the presence of orthopyroxene in the incipient charnockite. Phase equilibria modelling and thermobarometers reveal that these rocks record similar peak P–T conditions (870–900 °C, 6.2–7.2 kbar) followed by extensive cooling. Nonetheless, temperature versus mole H2O (T–M(H2O)) pseudosections show that orthopyroxene-bearing mineral assemblage is stable at lower M(H2O) than that without orthopyroxene. The post-peak cooling stage is characterized by fine-grained secondary garnet + quartz ± biotite ± feldspar symplectite in the incipient charnockite, but by simple biotite + quartz symplectite in the garnet-biotite gneiss. The garnet-bearing symplectite overgrows around peak ferromagnesian minerals in the incipient charnockite, corresponding to the increase of garnet proportion with the consumption of anatectic melt in the phase equilibria modelling. Garnet in the symplectite is extremely depleted in heavy rare earth elements (HREEs), which might be inherited from the anatectic melt and other reactants. These phase equilibria modelling and garnet REE results suggest that the garnet-bearing symplectite is a melt-related retrograde texture. The appearance of anhydrous garnet during cooling indicates that the system of the incipient charnockite, especially the anatectic melt itself, is more anhydrous than that of the garnet-biotite gneiss. Simple process of fluid-absent melting and melt loss was likely insufficient to dehydrate the melt itself, and we propose that the infiltration of low-water-activity fluid is likely a significant process to reduce the water activity of the whole petrogenetic system, leading to the incipient charnockite formation in this area.

LA–ICP–MS U–Pb DATES FOR DETRITAL ZIRCON FROM QUARTZITES OF THE TEMRYUK FORMATION (NOVOUKRAINKA IRON DEPOSIT OF THE KORSAK BLOCK, WEST AZOV AREA)

The West Azov iron district corresponds to the Korsak synclinorium or block in which iron deposits of the Kamyana Mohyla belt (Pavlivka, Novoukrainka, and Serhiivka areas), Kuksungur, and Korsak Mohyla are located. The iron– bearing units are confined to the metamorphosed volcanogenic–sedimentary rocks of the Satchky Formation of the Central Azov Group, while rocks of the Temryuk Formation form the base of the section. The group is intruded by numerous dykes of pink microcline granites and pegmatites up to 15 to 20 m in thickness. The basement for the group consists of Archean plagioclase granitoids hosting xenoliths of gneisses, crystalline schists, and amphibolites. The iron ores of the Korsak block are characterized by the same rock association and mineral assemblage as those in the Mangush and Orikhovo–Pavlohrad structures. We have conducted geochronological study of metaterrigenous rocks of the Temryuk Formation of the Novoukrainka (Kamyana Mohyla) iron deposit. The iron deposit extends along the western limb of the Korsak synclinorium for ca. 20 km. The prevailing strike of the iron deposit is NW 350°. Metamorphic rocks form separate bodies (lenses) that are continuous along the strike for 1200–1500 m with a maximum thickness of 120– 140 m. Ferruginous quartzites are represented by banded iron formation with interlayers of biotite and biotite–amphibole gneisses (Sachky Formation). Micaceous (biotite, garnet–biotite) quartzites occur in the lower part of the section, where they are interbedded with biotite, muscovite–biotite–sillimanite, biotite–amphibole, and, rarely, pyroxene–garnet–biotite gneisses and schists (Temryuk Formation). Fifty–eight grains of detrital zircon from micaceous quartzite of the Temryuk Formation were dated by LA–ICP–MS. Four crystals were dated at 3.2–3.23 Ga, three more grains have an age of 3.08– 3.11 Ga, thirty–two crystals yielded an age of 2.9–2.97 Ga, fifteen crystals had an age of 2.82–2.87 Ga, and, finally, four grains were dated at 2.76–2.78 Ga. According to the obtained data, metasedimentary rocks of the Central Azov Group, bearing iron–rich units in the Korsak synclinorium of the West Azov area were deposited on the basement with the Mesoarchean (2.9–3.1 Ga) Tonalites–Trondjemites–Granites (TTG). Archean granites with ages of 2.82–2.87 and 2.76– 2.78 Ga as well as the remnants of older TTGs provided a minor contribution from the provenance. The Central Azov Group was intruded by the 2.2 Ga Na–K–rich granites. On the Mesoarchean Azov craton, volcanogenic–sedimentary rocks of the Huliaipole Formation in the Huliaipole brachysyncline and the Vovchansk unit in the Orekhiv–Pavlohrad structure were formed during the same time interval. Deposition of this volcanogenic–sedimentary succession with thick Banded Iron Formations (BIFs) is possibly related to submarine volcanism and marine transgression.

Economic appraisal of iron ore deposit in Buttala, Sri Lanka: Preliminary mineral and chemical evidences

Iron ore deposits have greater industrial significance since they supply raw material for steel based industries and such applications are majorly based on their mineral and chemical characteristics. Therefore, initial clear understandings on mineralogy and geochemistry of these ores would be advantageous. Present study aimed to investigate the mineral and chemical composition of the iron ore deposit exposed in Buttala, Monaragala District, Sri Lanka and to assess its economic potential. Sampling was carried out systematically along the ridge of ore containing mountain and from the host rocks. Textural and weathering characteristics of the ore samples were determined in hand specimen scale. Detail mineralogy was analyzed on X-Ray Diffractrometer (XRD) analyzer while elemental characterization was performed on X-Ray Florescence (XRF) spectrometer. Spatial distributions of iron and gangue elements of the ore were mapped using the Quantum GIS software. The deposit is mostly exposed as fresh ore bodies along the ridge of mountain and is hosted in garnet biotite gneiss. As indicated by the XRD results, common minerals of the ore deposit are magnetite and hematite with minor amounts of spinel, ilmenite and goethite. Average Fe2O3 content of the ore deposit is 78.84 wt. % with 55.14 wt. % of calculated Fe content. The spatial distribution of iron is higher at the northern and southern parts of the ridge than its middle part. The resulted Fe content of the ore is greater than many of the other iron ore deposits in Sri Lanka and comparable to the high grade iron ore deposits found elsewhere in the world. Concentrations of common gangue constituents of Al2O3, SiO2, TiO2 and MgO are 7.54 wt. %, 4.40 wt. %, 3.6 wt. % and 2.54 wt. % respectively. Contents of S and P, which are considered as common deleterious elements are negligible. Therefore, the Buttala iron ore deposit is magnetite and hematite containing, high grade iron ore deposit with greater economic potential.

New geochronological evidences of late Neoarchean and late Paleoproterozoic tectono-metamorphic events in the Miyun area, North China Craton

The Miyun area is located in the western margin of the Eastern Block of the North China Craton (NCC), and granulite- to amphibolite facies tectono-metamorphic events occurred in the Precambrian. In this contribution, high-resolution secondary ion mass spectrometry (SIMS) U-Pb dating of metamorphic zircon from garnet-pyroxene amphibolite yielded the metamorphic age of 2507 ± 4 Ma. SIMS U-Pb dating of monazite from garnet-biotite gneiss yielded the metamorphic ages of 2496 ± 16 Ma and 1834 ± 21 Ma, respectively. Furthermore, 40Ar/39Ar dating of hornblende from amphibolite yielded plateau ages of 1817 ± 41–1795 ± 19 Ma, and the biotite yielded a plateau age of 1744 ± 12 Ma. Combined with previous studies and the closure temperatures of the dating minerals, it is inferred that the Miyun area experienced a late Neoarchean granulite-facies metamorphic event occurred at ~2.50 Ga, followed by a late Paleoproterozoic amphibolite-facies metamorphic event occurred at ~1.83–1.74 Ga. These two tectono-metamorphic events are ascribed to the Precambrian orogenic-related environments.