Muscovite Pegmatites Research Articles

Pegmatite zonation and the use of muscovite as a geochemical indicator for tin-tantalum-tungsten mineralization: Case studies from the Kalehe and Idjwi areas, Democratic Republic of Congo

The central part of the Karagwe-Ankole Belt (KAB) in the Great Lakes area of Central Africa is rich in various rare and strategic elements. The area north of Kalehe and Idjwi island within the Sud-Kivu Province of eastern Democratic Republic of the Congo (DRC) contains Nb, Ta, Sn and W mineralization hosted in pegmatites and quartz veins that are related to leucogranite intrusions. In this work, two granite-pegmatite fields were studied: the Bugarula field in the northern part of Idjwi island and the Kalehe field, along the west coast of Lake Kivu, with a view to determine the co-genetic link between different pegmatites and leucogranites in these regions. Based on mineralogical assemblages, five pegmatite zones were identified, occurring with some spatial overlap, but with an overall increasing distance from the inferred parental granite: 1) albite pegmatites, 2) two-mica pegmatites, 3) muscovite pegmatites, 4) beryl-bearing Nb–Ta–Sn mineralized pegmatites and 5) tourmaline-bearing Nb–Ta–Sn mineralized pegmatites. Muscovite compositions were analyzed from each zone and used to calculate pegmatite fractionation trends by using Rayleigh fractionation modelling. The model uses published partition coefficients (Kd) and measured contents of K, Rb and Cs in muscovite. Muscovites from the consecutive pegmatite zones in both the Bugarula and Kalehe pegmatite fields record a continuous fractionation trend, indicating a co-genetic magmatic evolution. A parental leucogranitic composition of 3.6 wt% K2O, 8 ppm Cs and 300 ppm Rb was iteratively determined to yield a best fit fractionation trend with the data. This composition is within the range of exposed leucogranites sampled in this study, which average at 4.2 wt% K2O, 7 ppm Cs and 400 ppm Rb (n = 12). It should be noted that individually, none of the sampled leucogranites gave satisfactory fractionation trajectories that would suggest a direct co-genetic link between them and the pegmatites. In the Idjwi granite-pegmatite field, the degree of fractionation with respect to the inferred parental leucogranite ranges from 20% to 99% for the least fractionated and most evolved Nb–Ta–Sn mineralized pegmatites, respectively. In the Kalehe area, fractionation values also range from 22% up to 99% for the least fractionated and most evolved (mineralized) pegmatites, respectively. Muscovites from mineralized pegmatites of zones 4 and 5 show high concentrations of Nb, Ta and Sn. As such, we show that muscovite compositions in these pegmatites provide a powerful tool to track Nb–Ta, Sn (and W) enrichment and potential mineralization. These high degrees of fractionation associated with the enrichment in economically interesting elements are similar to reported values for pegmatite fields elsewhere in the Karagwe-Ankole Belt.

Petrography, mineralogy, and fluid inclusions of Rod El-Biram muscovite pegmatites, Central Eastern Desert, Egypt.

Petrography, mineralogy, and fluid inclusions of Rod El-Biram muscovite pegmatites, Central Eastern Desert, Egypt.

Mineral chemistry and monazite chemical Th–U–total Pb dating of the Wadi Muweilha muscovite pegmatite, Central Eastern Desert of Egypt: constraints on its origin and geodynamic evolution relative to the Arabian Nubian Shield

Mineral chemistry and monazite chemical Th–U–total Pb dating of the Wadi Muweilha muscovite pegmatite, Central Eastern Desert of Egypt: constraints on its origin and geodynamic evolution relative to the Arabian Nubian Shield

Mineral classification of lithium-bearing pegmatites based on laser-induced breakdown spectroscopy: Application of semi-supervised learning to detect known minerals and unknown material

Mineral exploration and active mining relies on extensive drilling campaigns that produce large numbers of drill cores. LIBS is ideally suited for their fast and effective measurement, but matrix effects complicate quantitative geological LIBS applications due to the extensive amount of different minerals, rock types, and lithologies, as well as all textural and optical parameters increasing physical matrix effects. This is challenging for the application of LIBS in geological exploration, since LIBS data processing highly depends on matrix-matched models. The fast acquisition of new data is in conflict with the large amount of existing minerals and lithologies. As a result, new appearances are common during ongoing drilling campaigns, resulting in incomplete train sets for supervised classification and quantification.This paper presents a novel semi-supervised learning (SSL) classification model to resolve related issues by separating known minerals in geological drill cores based on a set of train samples, while also detecting unknown material, i.e. new lithologies and/or minerals not in the train set. Using a combination of supervised Linear Discriminant Analysis (LDA) and semi-supervised One-Class Support Vector Machines (OC-SVM), main minerals and known accessory minerals were effectively separated from unknown material in LIBS mappings of Spodumene and Muscovite pegmatite, as well as from Metagreywacke in drill cores from the Rapasaari lithium deposit in Finland. Self-learning was applied to automatically increase the number of train samples, which effectively decreased the number of unknowns due to physical matrix effects in coherent crystals.Validation with respect to the main minerals revealed an almost perfect classification of albite, spodumene, K-feldspar, quartz, and muscovite. Measurement points of Metagreywacke, which were only included in the test set, were correctly detected as unknown. Transferring the developed model onto LIBS mappings and drill core profile measurements displayed excellent classification results for main and accessory minerals included in the train set. Mixed spectra at mineral borders, as well as accessory minerals not in the train set were correctly identified as unknown.

Petrogenetic characterization of pegmatites and their host rocks in southern Akwanga, North-Central Basement Complex, Nigeria

The pegmatites in southern Akwanga occur within the reactivated belt of the basement complex of Nigeria. The pegmatites consist of dominantly albite–muscovite pegmatites (rare-metal), southern parts of the map areas and biotite-microcline pegmatites (barren) central parts of the map. The pegmatites intruded gneiss-migmatitic complex consist of metasedimentry rocks; granitic gneisses and biotite gneisses and rarely meta-igneous, amphibolites. The rare metal pegmatites are composed of quartz, albite and muscovites and tourmaline. Garnets, ilmenites and minor tin–columbite–tantalite mineralization constitute accessory minerals in contrast to the biotite-microcline pegmatites. The host rocks are composed of quartz, plagioclase (An5–21; albites–oligoclase), microcline and muscovite. Minor constituents include biotites, cordierites and hornblendes. Ilmenite occurs as opaques. The pegmatites and their host rocks are corundum and hypersthene normative, highly peraluminous, exhibiting similar geochemical signatures; however, the rare metal pegmatites are more fractionated than the host rocks and the biotite-microcline pegmatites. The rare metal pegmatites are relatively enriched in Rb, Li, Cs, B, Be, Nb and Ta, low in K/Rb and Al/Ga ratios than the biotite–microcline–pegmatites and their host rocks. The pegmatites are products of crustal anatexis of sedimentary origin. This indicates that the rare metal pegmatites are source rock controlled (product of post-collision activities) rather than fractional crystallization.

Deep Tectonics in the Eastern Hellenides Uncovered: The Record of Variscan Continental Amalgamation, Permo‐Triassic Rifting, and Early Alpine Collision in Pre‐Variscan Continental Crust in the W‐Rhodope (Vertiscos‐Ograzden Complex, N‐Greece)

AbstractThe Vertiskos‐Ograzden metamorphic complex (VOC; W‐Rhodope Metamorphic Domain, RMD) is a key area for reconstructing the geodynamic history of the Hellenides. We distinguish four metamorphic events: (1) an (U)HP eclogite facies metamorphism, succeeded by partial melting during decompression, amphibolite facies reequilibration in the kyanite stability field, and injection of muscovite pegmatites; (2) a LP‐HT event, associated with partial melting within the stability field of sillimanite, cordierite, and K‐feldspar (andalusite at subsolidus conditions) and magmatic intrusions; (3) a prograde moderate high‐pressure (MHP) metamorphism within the kyanite stability field increasing to the NE from epidote amphibolite to upper amphibolite facies; and (4) in the easternmost VOC, a LP amphibolite facies event (new cordierite and andalusite). Considering existing geochronological data, the VOC formed part of successive tectonic settings: (1) the “Variscan” collision zone between “East‐Avalonian” and Armorican continental fragments; (2) the Triassic middle crust of a continental rift zone, (leading to formation of oceanic crust, in the eastern VOC); (3) late Jurassic‐early Cretaceous closure of the oceanic basin and accretion of the VOC to southern European margin and northeastward polarity of subduction/collision; (4) late Cretaceous extension and crustal thinning. Then, subduction of the Neo‐Tethys and Apulian plate underneath Europe occurred, and the VOC plus the upper units of the RMD were part of the upper (European) plate. Large‐scale north dipping low‐angle normal detachments exhumed the VOC from the vicinity of a magmatic arc presently situated in the north RMD and Sredna Gora and Strandja zones and drag it into its present position.

ON ESTIMATION OF VERTICAL AMPLITUDE OF VEIN PEGMATITE SERIES IN THE NORTHWESTERN BELOMORIE

The article is devoted to the problem of the vertical amplitude of the Belomorian vein fields’ pegmatite genesis which has not been characterized so far. This problem became especially important at the initial stage of geological studies and commercial development of pegmatite areas. The theoretically best developed methodological approaches to in-depth prediction of vein pegmatite series are petrogenetic methods which tie the commercial values of granite pegmatite to the thermodynamic conditions of regional metamorphism of host magmatic gneiss complexes. This article is based on the principles of this approach. To address this task the author has determined the vertical residual thickness of pegmatite-bearing layers in all commercial deposits of the Belomorie muscovite pegmatite. On top the thickness is limited by the level of the regional Mz peneplain and from the bottom – mainly by the depth reached by drilling studies. The thickness varies within Nx10 to Nx100 m depending on the kinematics of the vertical displacement of pegmatite-bearing tectonic blocks. Besides, a reconstruction of the pre-erosional Rikolatva structure – the biggest commercial pegmatite deposit in the region, has been carried out to demonstrate that the vertical extent of vein series could be 6–7 km or slightly less (estimating from the altitudinal pressure level of 6 kb).

Geochemistry and U-Pb zircon geochronology of the pegmatites in Ede area, southwestern Nigeria: A newly discovered oldest Pan African rock in southwestern Nigeria

Field and petrographic studies, whole rock geochemistry and in-situ LA-ICP-MS geochemical and isotopic U-Pb measurements on zircons have been performed on granitic pegmatites of Ede area, southwestern Nigeria with a view to characterize them, determining their mineralization potentials, petrogenetic attributes and emplacement age. The pegmatites are hosted by migmatite gneiss complex, biotite-muscovite schist and associated quartzite. The textural and mineralogical characteristics of these pegmatites indicate the occurrence of two main varieties, namely, muscovite pegmatite and garnet pegmatite. Of less importance are inclusions and pods of graphic granite, quartz-microcline aplitic and pegmatitic bodies. At the present level of erosion, the parent igneous rocks of the pegmatites are not exposed. The two dominant pegmatite varieties show slightly different chemical peculiarities but similar peraluminous character. The average K/Rb ratios of 165 and 163, respectively, for muscovite and garnet pegmatites combined with other trace element compositions are indicative of affinity to muscovite class of pegmatite which are generally not promising for rare elements mineralization. However, the unusually high concentration of bismuth in the zircons indicates Bi mineralization in the area which can either be in the pegmatites or host rocks.The Nb/Ta ratios for both muscovite and garnet pegmatites range from 0.7 to 15.2 and 1.0 to 14.8, respectively. These Nb/Ta ratios and Eu anomalies are statistically similar for both pegmatites. These probably indicate the pegmatites crystallized from a common source but separated into crystallization paths that produced different pegmatite varieties through liquid-liquid immiscibity mechanism. In-situ measurements of REE, P, Y, Nb, Hf, Ta, Bi, Th and U of individual zircon grains show the existence of two chemically and texturally different domains which are indicative of alteration that may be due to interface-coupled dissolution-precipitation promoted by microfractures induced by metamictization. Notwithstanding, the (Sm/La)N vs. La plot for zircon, weak positive Ce and variable europium anomaly (Eu*) are suggestive of pegmatites of hydrothermal origin. The pegmatites yielded a discordant U-Pb zircon age with upper concordia intercept of 709 +27/−19 (at 2σ, MSWD = 1.5) Ma which can be attributed to their emplacement. This age represents the oldest Pan African magmatic event reported so far in southwestern Nigeria.

Fluid fractionation of tungsten during granite–pegmatite differentiation and the metal source of peribatholitic W quartz veins: Evidence from the Karagwe-Ankole Belt (Rwanda)

The identification of a magmatic source for granite-associated rare metal (W, Nb, Ta and Sn) mineralisation in metasediment-hosted quartz veins is often obscured by intense fluid–rock interactions which metamorphically overprinted most source signatures in the vein system. In order to address this recurrent metal sourcing problem, we have studied the metasediment-hosted tungsten-bearing quartz veins of the Nyakabingo deposit of the Karagwe-Ankole belt in Central Rwanda. The vein system (992±2Ma) is spatiotemporal related to the well-characterised B-rich, F-poor G4 leucogranite–pegmatite suite (986±10Ma to 975±8Ma) of the Gatumba–Gitarama area which culminated in Nb–Ta–Sn mineralisation. Muscovite in the Nyakabingo veins is significantly enriched in granitophile elements (Rb, Cs, W and Sn) and show alkali metal signatures equivalent to muscovite of less-differentiated pegmatite zones of the Gatumba–Gitarama area. Pegmatitic muscovite records a decrease in W content with increasing differentiation proxies (Rb and Cs), in contrast to the continuous enrichment of other high field strength elements (Nb and Ta) and Sn. This is an indication of a selective redistribution for W by fluid exsolution and fluid fractionation.Primary fluid inclusions in tourmaline of these less-differentiated pegmatites demonstrate the presence of medium to low saline, H2O–NaCl–KCl–MgCl2-complex salt (e.g. Rb, Cs) fluids which started to exsolve at the G4 granite–pegmatite transition stage. Laser ablation inductively coupled plasma mass-spectrometry shows significant tungsten enrichment in these fluid phases (∼5–500ppm). Fractional crystallisation has been identified previously as the driving mechanism for the transition from G4 granites, less-differentiated biotite, biotite–muscovite towards muscovite pegmatites and eventually columbite–tantalite mineralised pegmatites. The general absence of tungsten mineralisation in this magmatic suite, including the most differentiated columbite–tantalite mineralised pegmatites of the Gatumba–Gitarama area, emphasises the efficiency of fluid saturation to extract crystal–melt incompatible tungsten from the differentiating melt phase. Fluid–melt–crystal partitioning calculations support the concept of a magmatic–hydrothermal fluid source for tungsten and constrain the range of permissible crystal–melt and fluid–melt partition coefficients together with realistic values for water solubility in the parental G4 granitic melt. Consequently, we propose that for highly-differentiated B-rich, F-poor granite systems fluid saturation started prior to or at the granite–pegmatite transition stage resulting in apical to peribatholitic tungsten veins systems that are paragenetically older than the final pegmatite stage.

Alkali metal and rare earth element evolution of rock-forming minerals from the Gatumba area pegmatites (Rwanda): Quantitative assessment of crystal-melt fractionation in the regional zonation of pegmatite groups

This study presents a general model for the evaluation of Rayleigh fractional crystallisation as the principal differentiation mechanism in the formation of regionally zoned common and rare-element pegmatites. The magmatic evolution of these systems from a granitic source is reconstructed by means of alkali element and rare earth element (REE) analyses of rock-forming minerals (feldspars, micas and tourmaline), which represent a whole sequence of regional pegmatite zonation. The Gatumba pegmatite field (Rwanda, Central Africa) is chosen as case study area because of its well-developed regional zonation sequence. The pegmatites are spatially and temporally related to peraluminous G4-granites (986±10Ma). The regional zonation is developed around a G4-granite and the proximal pegmatites grade outwardly into biotite, two-mica and muscovite pegmatites. Rare-element (Nb–Ta–Sn) pegmatites occur most distal from the granite.Alkali metal fractionation trends in pegmatitic K-feldspar (Rb 350–6000ppm, Cs 2–160ppm) and muscovite (Rb 670–7000ppm, Cs 10–150ppm) define a single and continuous trend, which is modelled by Rayleigh fractional crystallisation, starting from a parental granite composition (G4-composition: K 3.1wt%, Rb 222ppm, Cs 11ppm). The fractionation model shows, moreover, that the pegmatites adjacent to the parental pluton are the least fractionated, and the distal pegmatites are the most fractionated. Biotite pegmatites form from 0% to 69% crystallisation, two-mica pegmatites from 69% to 92%, and muscovite pegmatites from 92% crystallisation onwards. The extreme Rb- and Cs-enrichment in rare-element pegmatites requires at least 98% fractionation of the initial G4-granite composition. Mathematical derivation of the K/Rb versus Cs relationship in K-feldspars confirms Rayleigh fractional crystallisation as the main differentiation process in the development of regional pegmatite zonation. Moreover (1) it demonstrates the continuity of the fractionation process from biotite pegmatites to rare-element pegmatite and indicates a genetic link among them; and (2) it allows a general evaluation of pegmatite fields in terms of system parameters, i.e. the initial element concentration in the granitic melt and partition coefficients.REE patterns of the rock-forming minerals (feldspars, muscovite, biotite and schorl) show a distinct transition along the regional zonation. They evolve from sloping fractionated to more flat patterns starting from the biotite up to the muscovite pegmatites. Minerals from the rare-element pegmatites (feldspars, muscovite and elbaite) show again more fractionated, heavy REE-depleted patterns. The observed evolution in the REE patterns corresponds to early crystallisation of light REE-enriched monazite and a late crystallisation of mainly cogenetic heavy REE-enriched phases such as apatite, columbite-group minerals and beryl. Modelling of Rayleigh fractionation, starting from an initial parental granite (G4-composition: LaN/YbN 10 and ∑REE 83), shows that this evolution in mineral REE pattern is the result of fractional crystallisation of the pegmatitic melt and precipitation of REE-incorporating minerals such as monazite and apatite.Consequently, trace element modelling indicates that Rayleigh fractional crystallisation governs the mineralogical and geochemical evolution from a granite source to common and eventually rare-element pegmatites. This mechanism shows that granitic pegmatites are extremely fractionated, residual melts which are genetically and directly connected to a main granite body.

URAN A THORIUM V GRANITICKÝCH PEGMATITECH A APLITECH SILEZIKA

Contents of uranium and thorium were measured using a laboratory gamma-ray spectrometer in 575 samples of granitic pegmatites and aplites from Silesicum. Very low contents of the both elements were found in metapegmatites occurring in metagranitoids and blastomylonites of the Desná Group, in pre-Variscan muscovite pegmatites with tourmaline, and in Variscan beryl-columbite pegmatites. Very high uranium and thorium contents are in some pegmatite dykes occurring on the Kluč Hill near Kociánov. The pegmatites are composed mainly of feldspars, quartz and mica (biotite and/or muscovite), and contain accessory minerals of uranium and thorium such as uraninite, coffi nite and thorite. The Kluč Hill pegmatites are spatially and genetically bound to a small I-type Variscan granitoid body (so-called Rudná hora Intrusion). Highest average uranium contents were found in pyroxenic pegmatites occurring in the Žulová Massif mantle (its marbles can be considered the main source of uranium).

Evolution of metallogeny of granitic pegmatites associated with orogens throughout geological time

Abstract Since c. 3.1 Ga, pegmatite mineral deposits in orogenic areas have been formed throughout geological time in pulses, alternating with total absence of generating activity. The higher activity peaks of 2.65–2.60, 1.90–1.85, 1.00–0.95, and 0.30–0.25 Ga suggest a quasi-regular periodicity of 0.8±0.1 Ga. This series is dominated by pegmatites of Laurasian blocks. The lower peaks at 2.85–2.80, 2.10–2.05, 1.20–1.15, and the higher one at 0.55–0.50 make up another series represented by pegmatites in Gondwanan blocks only. Each pegmatite class is characterized by a life cycle of its own, from inception to peak through to decline and eventual extinction. The longest cycle is recorded for the rare-metal class deposits, which first appeared in the Mesoarchaean and persisted through all the later eras, deteriorating gradually after the Early Precambrian. Muscovite pegmatites first appeared in the Palaeoproterozoic and reached the end of their life cycle at the Palaeozoic–Mesozoic boundary. The miarolitic class of pegmatite deposits in orogenic setting first came into being in the terminal Mesoproterozoic and dominated the pegmatite metallogeny of many Phanerozoic belts. The evolution of the pegmatite classes was controlled by the general cooling of the Earth and by associated changes in the tectonics of the lithosphere. Supplementary material: Geochronological data used is available at http://www.geolsoc.org.uk/SUP18435 .

Luminescence of Eu2+ in feldspars from muscovite pegmatites

The data on photoluminescence (PL) that precisely detects Eu2+ centers and X-ray luminescence (XL) were compared for plagioclases and potassium feldspars in 21 samples from muscovite pegmatites of the Mama region. The Eu contents determined in 10 samples vary from 10−4 to 10−6 wt %. Europium occurs mainly as bivalent species that replaces Sr2+, Ca2+, and Ba2+. Eu is gained in the products of early crystallization, and its relative amounts decrease by an order of magnitude in the course of pegmatite formation down to complete disappearance in late generations of feldspars. It is shown that Eu2+ can be detected in XL spectra, and the Eu2+ content can be determined in qualitative terms, for instance, by the intensity of radiation band 400–420 nm in plagioclase.

Petrology of the ultra-high pressure metamorphic Kimi complex in Rhodope (NE Greece). A new insight into the Alpine geodynamic evolution of Rhodope

Structural, penological and geochronological work has revealed that the Rhodope metamorphic province is a synmetamorphic nappe-system of Alpine age. The Kimi complex representing the uppermost entity underwent UHP metamorphism in Lower Cretaceous (>119 Ma). Diamond inclusions in garnet porphyroblasts, exsolutions of quartz rods and rutile needles in garnet from Grt-Ky-Bt-gneisses constrain pressures >4 GPa (probably -7 GPa) and temperatures > 1000 °C, indicating subduction of continental crust into the asthenospheric mantle. The garnet-spinel peridotite of the Kimi area represents a segment of upwelling asthenosphere reequilibrated into the lithospheric mantle wedge at -2.5 GPa and 1235 °C. The spinel-garnet clinopyroxenites, associated with the peridotite, represent HP mantle cumulates crystallized from a melt at similar P-T conditions (i.e. P-2.4 GPa, T~1200°C). Decompression and cooling took place in the mantle wedge within the Cr-Spinel peridotite field up to -1.8 GPa and 900 °C. Subsequent isobaric cooling crossed the stability field of garnet peridotite. At this stage, the peridotite was tectonically emplaced into the educted underlying continental crust. Three stages of exhumation of the crustal assemblage occurred in the Kimi Complex. The first stage, from the maximum depth of -200- 220 Km to -60 Km (P-1.6 GPa, T-800 °C), is characterized by slow cooling rates, indicating rapid exhumation. The second stage, from -60 Km to -38 Km (P-1.05 GPa, T~640°C), is indicated by cooling at slow rates and is characterized by hydration and annealing reequilibration/recrystallization processes. The third stage of exhumation started between 73 and 65 Ma and is characterized by rapid uplift, continuous influx of water, intrusion of muscovite pegmatites at -20 Km depth, and finally by rapid cooling at shallow levels. The Kimi Complex reached the surface before 48-42 Ma.

New constraints for the alpine HP metamorphism of the Ios basement, Cyclades, Greece

The pre-Alpine basement of the Ios Island involves large Variscan granitoid bodies intruded into metasediments that had already been metamorphosed under upper amphibolite facies conditions, as it is indicated by residual migmatitic textures and deformed muscovite pegmatites. The Alpine HP-metamorphism, documented on various cycladic islands, has, also, affected the basement rocks of Ios. Pressures of '25 Kbar and temperatures of '540 °C, are estimated for the Alpine HP event, applying the chloritoid-chlorite and garnetclinopyroxene geothermometers, and the garnet-phengite-omphacite and garnet-rutile-quartz-sphene-clinozoisite geobarometers. Garnet-hornblende geothermometry yielded temperatures of '520 °C and garnet-chlorite geothermobarometry yielded temperatures of 450 °C at 15 Kbar. This suggests that, at least the first stages of decompression were accompanied by cooling, indicating a rapid exhumation, related to tectonic processes.

Post‐depositional history of the Willyama Supergroup in the Broken Hill Block, NSW

Abstract The Broken Hill Block, in far western New South Wales, comprises complexly folded and metamorphosed Proterozoic metasediments and metavolcanics of the Willyama Supergroup, and pre‐ and post‐folding intrusives. The history of the Block is described in four tectonic stages. The precratonic stage commenced with separation of crustal material from the mantle at 2100–2300 Ma. Deposition of the Willyama Supergroup occurred at about 1820 Ma. These rocks were metamorphosed to amphibolite‐granulite facies and deformed twice at about 1660 Ma, in the Olarian Orogeny. A third deformation with accompanying retrograde metamorphism occurred soon after. Before 1570 Ma, retrograde schist zones had formed, some with associated kyanite and staurolite bearing metamorphic assemblages. The transitional tectonic stage commenced at 1490 ± 20 Ma, with the emplacement of Mundi Mundi type granites and minor muscovite pegmatites. Between 1490 Ma and about 1100 Ma, the presently exposed rocks were elevated from 13 to 20 km b...

Fission, track annealing behaviour of uraninite inclusions in muscovite pegmatite of Bhilwara area, Rajasthan. State, India

Fission, track annealing behaviour of uraninite inclusions in muscovite pegmatite of Bhilwara area, Rajasthan. State, India

Potassium–Argon Ages from the Oldest Metamorphic Belt in India

INTENSIVE structural and stratigraphic studies, supplemented by several potassium–argon ages, recently led Sarkar and Saha1–3 to recognize two distinct orogenic belts in the Pre-Cambrian tract of Singhbhum and adjacent areas4,5 in Eastern India (Fig. 1, inset). The orogeny which gave rise to the Iron Ore orogenic belt in the south—which includes the Iron Ore Series of low grade metasediments and metavolcanics—had its culmination in the emplacement of the granitic rocks (about 2,000 × 106 yr) which constitute the northern part of the Singhbhum granitic complex (Fig. 1). The Singhbhum Orogenic belt in the north, which is thought to have a closing age of about 900 × 106 yr and which consists mostly of high grade metasediments and basic rocks, truncates the Iron Ore Orogenic belt along a prominent arcuate thrust zone. Evidence for the existence of an older group—underlying the Iron Ore Series, the Older Metamorphic Series—of moderate to high grade metamorphic argillites, calcmagnesian metasediments and arenites was also recorded2,3. Sarkar and Saha6 also suggested that there was old granitic activity associated with the Older Metamorphic Series about 3,000 × 106 yr ago on the basis of a potassium–argon age of 3,035 × 106 yr (ref. 7) from a coarse muscovite pegmatite near Joropokhar (22° 24′ 45″: 85° 45′ 10″).

The Igneous Complex of Green Island and the Amherst Coast (Lower Burma)

1. The igneous complex shows a remarkably complete series of types from ordinary biotite granite, through contaminated granite, intrusion gneisses, lit-par-lit aplitic intrusions, to aplites and muscovite pegmatites. 2. Pale-coloured reaction zones occur between xenoliths and the granitic masses. The evidence points to an exchange of material between xenoliths and the surrounding magma. 3. All the rocks are mylonized and it is suggested that the micro-structure can only be explained by movement during the final stages of crystallization. The folding is believed to have taken place in late Mesozoic times, as the area forms part of the Indo-Malayan Mountain System.