Muscovite Leucogranite Research Articles

Critical raw material resources in Nugrus-Sikait area, South Eastern Desert, Egypt: Geological and geochemical aspects

The current work aims to assess the critical raw materials (CRMs) in the Arabian-Nubian Shield in accordance with the European Union. CRMs are strategically and economically significant, but they are subject to supply shortages. CRMs, such as Nb, Ta, Zr, U, Th, Rb, Ga, Hf, Zn, Ba, Sr, Sn, Cu, and Pb, are concentrated in post-orogenic highly fractionated granitic plutons, such as Abu Dabbab, Abu Diab, Homrit Waggat, Um Naggat, and Nuweibi. They also occur in the gneiss rocks of the Nugrus-Sikait area. The present work sheds light on the CRM resources in the Nugrus-Sikait area in the South Eastern Desert, Egypt. Gneiss rocks, ophiolitic rocks (ultramafics, metagabbros, schists, and mélange), gneissose biotite granites, muscovite leucogranites, pink biotite granites, and post-granitic dikes and veins are the main exposed rock units in the target area. CRMs are abundant in the examined area and reach up to 1845 ppm Nb, 203 ppm Ta, 3212 ppm Zr, 1789 ppm Y, 278 ppm Hf, 3209 ppm Zn, ˃10000 ppm Pb, 941 ppm Sn, 1458 ppm Cu, 3551 ppm U, 2233 ppm Th, 144 ppm Sr, 1416 ppm Rb, and 121 ppm Ba. Therefore, it's recommended to shed light on CRMs distribution due to their economic importance and strategic uses.

Occurrences and genesis of emerald and other beryls mineralization in Egypt: A review

Beryl and others gem varieties represent the most valuable gemstones due to their good appearance and high hardness (after diamond and ruby). Beryl is a rare mineral due to lack of Be abundance in the upper continental crust. The present work aims to state the occurrences, chemistry, fluid inclusions and genesis of Egyptian emerald and other beryls mineralization. They are recorded in the South Eastern Desert (Zabara-Um Kabu, Homret Akarem- Homret Mikpid) and south Sinai (Ghazala and Sidri) in addition to less significant concentrations in the central Eastern Desert (Igla, Abu Dabbab, Nuweibi and Mueilha granitic rocks). Among all of the Egyptian beryls, Homret Akarem beryl has the highest SiO2 (65.49 wt%), Al2O3 (16.52 wt%), FeO (1.5 wt%) contents. All the recorded beryl and emerald in the Eastern Desert and Sinai are sodic. Emerald and others beryl mineralization of Egypt are found to be formed in tectonic magmatic-related (where beryl is disseminated in tectonic related alkali feldspar granite as a result of Be-bearing fluids from a hidden-granitic intrusion as in Wadi Ghazala, Homret Mikpid and Homret Akarem) and into tectonic metamorphic-related (where emerald and other beryls are formed in mica schists as a result of Be-bearing fluids from tectonic related (Nugrus thrust and related shear zones as in Zabara-Um Kabu district); where muscovite leucogranite and its related pegmatites and quartz veins intrude and interact with Cr, V-bearing mafic-ultramafic rocks).

Multistage magmatism recorded in a single gneiss dome: Insights from the Lhagoi Kangri leucogranites, Himalayan orogen

Determining the geochronological framework and geochemical nature of magmatism associated with orogenic processes is crucial to decipher the evolution of orogenic belts. The Himalayan orogen has recorded considerable information regarding deformation, metamorphism, and magmatism related to orogenic processes. Here, we identify four stages of granitic magmatism on the basis of monazite U–Th–Pb dating of rocks from the Lhagoi Kangri dome, north Himalayan antiform, southern Tibet: middle Eocene (42–40 Ma), late Eocene (ca. 36 Ma), Oligocene (ca. 26 Ma), and Miocene (16–13 Ma). The middle Eocene tourmaline-bearing leucogranite is characterized by (1) relatively high SiO2 (78.7 wt%), Al2O3 (12.6 wt%), and Na2O (6.3 wt%) contents; (2) enrichment in light rare earth elements (LREEs), depletion in heavy REEs, high abundances of REEs, and pronounced negative Eu anomalies; and (3) relatively low values of whole-rock SrNd isotope compositions (87Sr/86Sr(t) = 0.72101 and εNd(t) = −9.8). And the coeval garnet–bearing two–mica granite has a pronounced negative Eu anomaly (Eu/Eu* = 0.07) and a significate lanthanide tetrad effect (TE1,3 = 1.46). These geochemical data suggest that these leucogranites underwent fractional crystallization. They provide a robust evidence for that anatexis of the lower crust of Indian plate began at 42–40 Ma in the central Himalayan orogen. The Oligocene and Miocene two–mica granites share broadly similar geochemical characteristics, such as moderate REE contents and weak negative Eu anomalies, and are distinct from the garnet-bearing muscovite leucogranite. Whole-rock SrNd isotope compositions of the Oligocene and Miocene leucogranites differ from those of the middle Eocene leucogranites, with higher 87Sr/86Sr(t) ratios (0.73524–0.74188) and lower εNd(t) values (−14.7 to −13.7). These geochemical data indicate that the Oligocene and Miocene leucogranites are products of muscovite-dehydration melting of metasedimentary rocks. The garnet-bearing muscovite leucogranite shows the REE tetrad effect, which suggests that it is more evolved than the coeval two–mica granite. We therefore infer that the variations in age and isotopic characteristics of the studied leucogranites were caused by a change in melt source resulting from the compression–extension evolution of the Himalayan collisional orogenic system.

Boron and molybdenum isotopic fractionation during crustal anatexis: Constraints from the Conadong leucogranites in the Himalayan Block, South Tibet

Partial melts derived from crustal anatexis exhibit large variations in field characteristics, composition, and isotope geochemistry. The relative influence of melting process and magmatic differentiation on such heterogeneity is a subject of ongoing debate. Boron and molybdeum isotopes have the potential to shed light on the genesis of crustal melts, but the behaviors of B and Mo isotopes during crustal anatexis and differentiation are unclear. Here, we present geochemical and isotopic data, including whole-rock Sr–Nd–B–Mo and mineral B–Mo data, for the Conadong leucogranites in the Himalayan Block, to provide new insights into these issues. The studied samples include two-mica granites (biotite + muscovite), muscovite leucogranites (muscovite ± garnet), and biotite-rich granites. The two-mica granites were derived from muscovite dehydration melting of metasedimentary rocks. Their initial Sr and Nd isotopic compositions show positive correlation, implying non-modal partial melting associated with dissolution of accessory phases. The B contents (3.44–21.2 ppm) of the two-mica granites decrease but their δ11B (−15.80‰ to −13.25‰) and δ98Mo (−0.15‰ to 0.34‰) values increase with increasing Sr and Ba contents. This may have been controlled by different phases involved in non-modal melting (i.e., muscovite and feldspar). The muscovite leucogranites display features of residual liquids. Their δ11B (−15.38‰ to −11.90‰) and δ98Mo (−0.15‰ to 0.17‰) values negatively correlate with Sr and Ba contents, which can be ascribed to shallow-crustal crystal–liquid separation and melt–fluid interaction. The biotite-rich granites are least evolved, with large variations in their initial Sr–Nd isotopic compositions. They show relatively heavy whole-rock B isotopic compositions of −11.97‰ to −9.00‰. These may indicate that biotite-rich granites may be contaminated by entrained restitic minerals (e.g., biotite, feldspar and accessory minerals). Overall, this study demonstrates that compositional diversity of anatectic melts is controlled primarily by melting processes in the source and secondarily by emplacement-level crystal–liquid separation. Both non-modal partial melting and crystal–liquid separation could result in B–Mo isotopic fractionation in anatectic melts.

Onset of the supercontinent cycle: Evidence for multiple oceanic arc accretion events in the Paleoproterozoic Sefwi Greenstone Belt of the West African Craton

Greenstone belts preserved in the West African Craton (2300–2070 Ma) provide key information for understanding the petrogenetic processes and geodynamic setting for juvenile crust formation during a nascent Paleoproterozoic plate tectonic regime. New geochemical and isotopic data are presented for the magmatic suites exposed in Sefwi Greenstone Belt, SW Ghana. We define five distinct plutonic suites, with extrusive equivalents, including: 1) two generations of high SiO2, Na2O-rich tonalite-trondhjemite-granodiorite (TTG) granitoids; 2) calc-alkaline, LILE-enriched diorites; 3) tholeiitic pyroxenite, gabbro and layered mafic intrusives; 4) high-K quartz monzonites; and, 5) incompatible element-rich, two-mica granites and muscovite leucogranites. Trace element patterns of most magmatic suites display negative Nb-Ta, P and Ti anomalies and weakly positive Pb anomalies, suggesting magma genesis in a subduction setting with minor crustal contamination/assimilation. Complementary zircon U-Pb and Lu-Hf data indicate contemporaneous emplacement of diorites (εHf(t) = +2.5 to +5.4) and low-HREE TTGs (εHf(t) = +4.3 to +7.6) between ca. 2159 and 2153 Ma, derived from contrasting sources, namely the partial melting of the metasomatised mantle wedge and high-pressure partial melting of low-K mafic sources, respectively. High-HREE TTGs are attributed to the shallow, partial melting of mafic material. The emplacement of high-K quartz monzonites (εHf(t) = +2.5 to +5.2) at ca. 2135 Ma reveals mixing of mantle-derived magmas and remelting of existing TTGs in the crustal pile. Consistently positive εHf(t) values indicate predominantly radiogenic, though heterogeneous, magma sources during crustal growth, derived from precursory material extracted from the depleted mantle and an older crustal component with a minimum age of 2650 Ma. Magmatogenesis of two-mica granites and leucogranites (ca. 2092 and 2081 Ma) is interpreted to be the product of crustal anatexis during an oceanic arc-arc collisional event, with a proposed suture parallel to the NW margin of the Sefwi Belt. We hypothesise that the formation and accretion of the West African Craton reflects the rapid amalgamation of individual oceanic arc terranes, providing potential insight into the transitional plate tectonic regime that may have characterised the Earth between 3000 and 2000 Ma.

Mineralogical evidence for fractionation processes in the Himalayan leucogranites of the Ramba Dome, southern Tibet

Himalayan leucogranites provide some of the best examples in the world of highly felsic and strongly peraluminous granites, and they have been paid special attention with regard to investigating the evolution of continental crust. Here, we describe our mineralogical investigations of a late-Miocene leucogranite complex within the Ramba Dome of the Tethyan Himalaya Belt that have helped us decipher the petrogenesis of these high-silica peraluminous granites. The Ramba leucogranites range from two-mica granites (biotite + muscovite) to muscovite leucogranites (muscovite ± garnet ± tourmaline), with the two-mica granites constituting the main part of the pluton and the muscovite leucogranites commonly occurring as late-stage dikes that are located within the pluton or invaded the country rocks. Variations in the mineral assemblages of the two types of leucogranite are associated with systematic changes in bulk-rock geochemistry, and the distinctive mineralogical features demonstrate a genetic relationship between the two types of leucogranite that involved fractional crystallization, with the muscovite leucogranites being more evolved than the two-mica granites. Chemical data for the minerals in the two-mica granites record a long period of differentiation during which a normal granitic melt developed into a highly evolved magmatic system with high flux components (e.g., H2O, F, B). In contrast, the mineralogical data for the muscovite leucogranites record the crystallization of a very late residual melt, i.e., a volatile-rich, highly fluid melt. According to recent studies of differentiation processes in silicic magma reservoirs, we can interpret the Ramba leucogranites as having undergone a high degree of differentiation by in situ fractional crystallization in a crystal mush environment. The two-mica granites represent a congealed crystal mush that was composed of ‘cumulate crystals’ and a trapped interstitial liquid, and the muscovite leucogranites represent the almost pure liquid that was extracted from the crystal mush. Modeling using the trace elements Sr and Ba shows that the extraction probably occurred when the crystallinity of the mush was ~59–64%, at least for the most evolved muscovite leucogranite sample. The two-mica granite corresponds to a residual crystal mush that had a terminal porosity of ~20–26% filled with a trapped interstitial liquid. Collectively, fractional crystallization can account for the geological, mineralogical, petrological, and geochemical variations between the two types of leucogranite. This work highlights the important role of crystal fractionation in the generation of Himalayan leucogranites.

Petrogenesis and tectonic implications of the Yadong leucogranites, southern Himalaya

The leucogranites in the Higher Himalayan Sequence (HHS) provide a probe to elucidate the crustal melting of continental collisional orogen. An integrated geochemical and geochronological study of the Yadong leucogranites, southern Himalaya, shows that these rocks have relatively high SiO2 contents of 69.77 to 75.32wt.% and alumina saturation index (A/CNK) of 1.09–1.40, typical of peraluminous granites. They show moderately fractionated REE patterns with negative Eu anomalies, and are characterized by enriched LILE (Rb and Cs) and depleted HFSE (Zr, Hf, Nb and Ta). LA–ICP–MS U–Pb zircon dating of ten samples yields crystallization ages ranging from 21.0 to 11.7Ma. The zircons have variable εHf(t) values of −26.3 to −3.5 and corresponding Hf two-stage model ages of 2.77–1.33Ga. The present study reveals that the muscovite–biotite leucogranites (2ML) have higher TiO2, MgO, CaO, Sr, Ba and Zr contents, lower Rb/Sr ratios than the tourmaline–muscovite leucogranites (TML). Zircon and monazite saturation thermometry results show that the melt temperatures (681–784°C) of the 2ML are 20–80°C higher than those (663–705°C) of the TML. Combining with previous results, we propose that the TML were derived from the muscovite-dehydration melting, whereas the 2ML dominantly resulted from the biotite-dehydration melting during the prograde metamorphism of the pelitic and felsic granulites of the HHS. Therefore, the Himalayan leucogranites were probably formed during the subduction of the Indian crust following the India and Asia collision.

Highly fractionated Late Eocene (~ 35 Ma) leucogranite in the Xiaru Dome, Tethyan Himalaya, South Tibet

The Xiaru dome is located in the middle section of the North Himalayan Gneiss Domes belt in southern Tibet. The leucogranite, which crops out in the core of the Xiaru dome, is a typical medium-grained garnet+tourmaline+muscovite leucogranite. U–(Th)–Pb dating of zircon and monazite from the leucogranite yielded ages of approximately 35Ma. This finding supports a growing body of evidence indicating that an extensive magmatic event occurred during the late Eocene in the Himalayas. This leucogranite is strongly peraluminous with A/CNK values of 1.08–1.52 and characterized by evolved geochemical composition with high contents of SiO2 and alkali elements; low levels of CaO, MgO, TiO2, and FeOT; enriched large-ion lithophile elements (such as Rb); and depleted of high-field-strength elements (such as Nb, Zr, and Hf). The non-CHARAC (CHarge-And-Radius-Controlled) trace element behaviors, which are typical of a highly fractionated granite system, were recorded in the whole rock and the accessory minerals of the Xiaru leucogranite. Furthermore, the magmatic zircon overgrowths have extremely high content of Hf, consistent with those from the highly fractionated aqueous-like fluid system. In addition, whole-rock geochemical fractionation trends were observed, which can be explained by crystal fractionation of biotite, K-feldspar, zircon, xenotime, and monazite. These geochemical features indicate that the Xiaru leucogranite is a typical highly fractionated granite. The geochronological and geochemical features of the inherited zircons from the Xiaru leucogranite show a close affinity to those of the country rocks, suggesting a certain degree of assimilation from the country rocks during melt ascent and emplacement. Although a restricted range of εHf(t) values from −12.8 to −6.6 with Hf TDM2 model ages of 1.2–1.6Ga was obtained from the late Eocene zircons, it is invalid to constrain the source of the parental magma due to the strong fractionation and assimilation processes.

Age, duration of formation, and geotectonic position of the Zavitaya lithium granite-pegmatite system, Eastern Transbaikalia

The Zavitaya granite-pegmatite system with a lithium deposit is localized in the northern marginal part of the Onon terrane (Aginskii massif) and ajoins to the Ingoda-Shilka branch of the Mongol-Okhotsk suture in the south. This paper presents the first U-Pb (SHRIMP) age of granites and barren and spodumene pegmatites of the Zavitaya field. The Zavitaya polychronous granite-pegmatite system evolved through 40 million years: porphyritic biotite granites (169.0 ± 3 Ma), two mica granites-leucogranites (147.5 ± 3.1 Ma), muscovite leucogranites (140.0 ± 3.0 Ma), barren pegmatites (139.6 ± 3.1 Ma), and lithium spodumem pegmatites (129.6 ± 2.7 Ma). The formation of the system coincides with the change in geodynamic regimes of the region at the Middle Jurassic-Early Cretaceous boundary: the age of the early granites of the system and spodumene pegmatites corresponds to the termination of collision and to the beginning of the Early Cretaceous rifting, respectively.

Chronology (SHRIMP II) of magmatism in the Kalguty rare-metal-tungsten-molybdenum ore-magmatic system, Gorny Altai, Russia

The Kalguty ore-magmatic system (OMS) contains economic greisen and vein rare-metal-tungsten-molybdenum mineralization. The data on U-Pb zircon (SHRIMP II) age of nine samples of igneous rocks from the Kalguty OMS are accompanied by chemical, ICP-MS, and ICP-AES analyses of the main rock varieties. Porphyritic biotite granite of the main phase of the Kalguty pluton is characterized by the concordant age of 207.5 ± 1.7 Ma (MSWD = 0.034). The concordant age of the leucogranitic dikes pertaining to the East Kalguty Complex is 204 ± 2 Ma (MSWD = 0.65) for elvan and 200.8 ± 1.1 Ma (MSWD = 0.72) for ultrapotassic rhyolite porphyry. The two-mica and muscovite leucogranite of the Eastern stock is significantly younger: 195 ± 2.7 Ma (MSWD = 0.076) and 193.1 ± 2.1 Ma (MSWD = 0.0009). Thus, the Chingadatui Complex (the main phase of the Kalguty pluton) and dikes of the East Kalguty Complex are Late Triassic in age. The two-mica leucogranites of the Eastern, Zhumaly, and other stocks are most likely correlated to the Early Jurassic Alacha Complex of rare-metal granites. The superposition of greisen mineralization on elvan and ongonite dikes may be related to the emplacement of younger, Early Jurassic ore-forming two-mica and muscovite granites. Judging from zircon xenocrysts in granites, the Mesoproterozoic igneous rocks dated at 1.5 Ga and products of erosion of the rocks dated at 1.7 and 2.5 Ga occur in the basement of the Kalguty volcanic-tectonic structure. This is sound evidence for the occurrence of ancient continental crustal blocks in the southwestern part of the Altai-Sayan region.

Stanniferrous granites of Vietnam: Rb-Sr and Ar-Ar isotope age, composition, sources, and geodynamic formation conditions

Stanniferrous granite-leucogranite massifs in Vietnam to which the bedrock and placer commercial deposits of cassiterite are associated (Timtuc etc.) are of late Cretaceous age (∼85 Ma by Rb-Sr and Ar-Ar isotope dating). These massifs are presented by stocks and fracture intrusions (to 100 km2 in area) of simple homodromic structure: biotite granites ⇒ two-mica and muscovite leucogranites ⇒ topaz-containing aplites ⇒ rare-metal pegmatites.

Emplacement of ellipsoid-shaped (diapiric?) granite: Structural and gravimetric analysis of the Oulmès granite (Variscan Meseta, Morocco)

The Oulmès granite is a NE–SW elongated stock intruded in Ordovician metasedimentary rocks which crop out at the core of a regional anticlinorium of the Moroccan Variscan Meseta. The stock, dated at around 300 Ma, is made up of peraluminous two-mica granite, with subordinate amounts of muscovite leucogranite. Mineral composition and textural features of both the granite and the thermal aureole enable us to constrain the P–T conditions during magma intrusion in ≈250–300 MPa and ≈600 °C at the contact between the granite and its host rock. Emplacement of the Oulmès granite postdates the main regional structures and is coeval with the late stages of Variscan compression. Prior to granite intrusion, the host rocks were affected by a main deformation phase responsible for the development of a penetrative slaty cleavage and regional-scale folds. Granite emplacement gave way to a 2 km-thick strain aureole, which includes both the uppermost part of the stock and the surrounding host rocks. The strain aureole is characterized by a gently-dipping planar-linear fabric with a generally weak N–S oriented stretching lineation. In the granite, the fabric of the strain aureole consists of a solid-state, locally mylonitic, foliation. In the host rocks, a secondary foliation develops which crenulates the older regional foliation. Magmatic structures, mainly consisting of steeply dipping N–S oriented magmatic foliation, are preserved in the central part of the pluton. The 3D shape of the granite has been modelled from gravity data and, together with the observed magmatic flow along subvertical planes and the strain recorded in the aureole, leads us to envisage a diapiric-dominated mechanism for the emplacement of the Oulmès granite. Drawing on the petrogenesis of peraluminous granites, the diapiric ascent of the granite probably has not exceeded 10 km.

Petrology of high-Mg, low-Ti igneous rocks of the Glenelg River Complex (SE Australia) and the nature of their interaction with crustal melts

Although commonly ascribed a mantle derivation, the nature and origin of the parental magmas of the microgranular enclaves encountered in granitic plutons is typically a matter of deduction. This inevitably imparts uncertainty in petrogenetic models for the host silicic magmas, and, in the extreme, can cast doubt over a mingling origin for the enclaves. In contrast, this paper describes a rare example from southeastern Australia where the near-pristine mafic and felsic end-members involved in magma mingling and microgranular enclave formation are identified in outcrop, and their crustal and mantle heritages, respectively, can be demonstrated. The mafic magmas are represented by unusual, Mg-rich, Ti-depleted quartz melagabbros, quartz meladiorites and melatonalites that have affinity with the boninitic lavas erupted above subduction zones. The distinctive chemical signature of these allows the physiochemical processes culminating in microgranular enclave formation to be traced. The participating felsic end-member is muscovite leucogranite, formed by anatexis of metasedimentary rocks near the current exposure level. Interpenetration between these magmas produced mafic cumulates, evolved, variably hybridised derivatives and dispersed microgranular enclave swarms throughout the leucogranite. Localised mixing generated hornblende tonalite, though the widespread occurrence of dykes of this material indicate that mixing was extensive at depth. These relationships suggest that high-Mg magmas may have a key role in the formation of granitic bodies in subduction environments, and in explaining the geochemical variation shown by these plutons.

Chemical and isotopic (Pb, Sr) zonation in a peraluminous granite pluton: role of fluid fractionation

The Davis Lake pluton (DLP, ~800 km2) of southwestern Nova Scotia, Canada, part of the large peraluminous South Mountain batholith of ca. 380 Ma (U/Pb zircon, Ar/Ar mica), consists of granite and subordinate topaz–muscovite leucogranite that hosts greisen tin-base metal mineralization. A new Pb–Pb isochron age for leucogranite from the most evolved part of the DLP indicates a crystallization age of 378±3.6 Ma, coincident with other radiometric ages of the DLP (Rb–Sr, Re–Os, Pb–Pb). The intrusion displays a compositional zonation defined by lead and strontium isotopic ratios, as well as some major elements (e.g., Si, F), incompatible trace elements (e.g., Li, Rb, Ta, U, Sn), and elemental ratios (e.g., K/Rb and Nb/Ta). The greisens and the leucogranites that host them are characterized by extreme radiogenic compositions for Pb and Sr, and their chemical-isotopic trends are extensions of the trends displayed by the less evolved granites. The covariations of the isotopic ratios with several major and trace elements and elemental ratios as well as the Pb–Pb and Rb–Sr isochrones indicate that all phases of the intrusion originated from a homogeneous parental magma. The granitoid magma underwent extensive fractional crystallization of feldspars, minor biotite and accessory minerals (monazite, apatite and zircon) in a compositionally zoned magma chamber that was subsequently accompanied by fluid fractionation, during which time the internally derived fluorine-rich fluids modified the Rb/Sr, U/Pb and Th/Pb ratios, leading to distinct variations of 87Sr/86Sr, 206Pb/204Pb, 238U/204Pb and 232Th/204Pb isotopic ratios. These data therefore document the evolution of a granitic magma through magmatic (i.e., crystal fractionation), orthomagmatic (i.e., crystal-fluid fractionation) and hydrothermal (i.e., fluid fractionation) stages that culminated in the formation of a tin-base metal deposit. The Pb isotope data also constrain the source region for the DLP as being Avalonian basement that, by inference, must underlie much of the Meguma Terrane.

PERALUMINOUS PLUTONISM: NATURE AND ORIGIN OF THE MOLY MAY LEUCOGRANITE AND ITS COAST PLUTONIC COMPLEX GRANITIC HOST-ROCKS, NORTHWESTERN BRITISH COLUMBIA

The Moly May pluton of northwestern British Columbia is one of several small molybdenite-bearing granitic complexes of Cenozoic age known as the Alice Arm intrusions. These were intruded into the eastern margin of the Coast Plutonic Complex (CPC) and the Bowser Lake Group (greenschist-grade metasedimentary rocks). This pluton consists of three mappable units (aplitic garnet–muscovite leucogranite, medium-grained leucogranite, and coarse-grained to pegmatitic biotite granite), in addition to a molybdenite-mineralized leucogranitic variety. The Moly May leucogranites (MML) and the CPC granite–granodiorite host rocks are chemically distinguished by strikingly different geochemical trends, which indicate that the two granitic suites are not genetically related. The MML is exclusively peraluminous (molar A/CNK in the range 1.00 to 1.78), whereas the CPC rocks are metaluminous to weakly peraluminous (molar A/CNK in the range 0.84 to 1.06). The MML is significantly enriched in K, Rb, Nb, Y, and depleted in Mg, Ca, Na, Ba, Sr, and Zr compared to the CPC. The REE patterns of the MML are fractionated [(La/Yb) N = 23] and show a minor negative Eu anomaly, but those of the garnet-bearing leucogranitic variety lack the REE fractionation [(La/Yb) N = 2], and have a significant negative Eu anomaly. The mineralized leucogranite is strongly depleted in the LREE , K, Rb, and Nb; these elements are interpreted to have been leached out at subsolidus temperatures by mineralizing hydrothermal solutions, which led to deposition of molybdenite. Compared to the REE patterns of the MML, those of the CPC are slightly more fractionated [(La/Yb) N = 27], and lack a Eu anomaly. The MML was developed in association with an Eocene transtensional tectonic regime. Chemical features suggest that the MML is of a sedimentary origin. Results of geochemical modeling show that the leucogranitic magma was produced by a large degree (65%) of batch partial melting of a metasedimentary protolith.

The tectonothermal evolution of the Cadomian belt of northern Brittany, France: a Neoproterozoic volcanic arc

The Neoproterozoic Cadomian belt (northern Brittany, France) results from the accretion of crustal blocks that record distinctive P–T-deformation paths. In addition, abundant gabbro-dioritic to granitic intrusions during and after deformation influence the thermal pattern and are used as time markers. As a whole, the deformation history of the Cadomian belt is characterised by a migration in time of the active zones, which become younger towards the Southeast.Relics of Eocadomian rocks reveal that arc development took place at about 660–640Ma. Abundant calc-alkaline magmatism intruding strongly deformed Palaeoproterozoic orthogneisses characterise the northernmost unit of the Cadomian belt, i.e. the Trégor Unit, which cooled before 600Ma. The Lanvollon basin, a short-lived intra-arc rift containing abundant mafic and felsic volcanics followed by greywackes, opened at about 600–580Ma.The subsequent tectonothermal history of the Cadomian belt is divided into two major episodes. The first episode is related to the shortening of the intra-arc rift, and its southwestward thrusting over the Guingamp Unit. This led to crustal thickening, inducing partial melting in the Guingamp Unit. Exhumation of the latter rocks is accommodated by late extensional shearing along the boundary between the Saint-Brieuc and Guingamp Units. Cooling of the Saint-Brieuc Unit is dated at about 570–560Ma. The second episode is dominated by sinistral wrenching mainly localised in the eastern part of the Cadomian belt, at the boundary between the Domnonean (i.e. the arc domain) and Mancellian (possibly a fore-arc basin) Domains. Several shear zones developed during this episode, some of them being associated with minor granitic intrusions dated around 550Ma. The sinistral shear located Southeast of the arc domain is reworking or crosscutting earlier high-temperature fabrics within the Saint-Brieuc Unit. In addition, the vertical component of this shearing episode contributes significantly to the exhumation of the Saint-Malo migmatites.Near the Proterozoic/Cambrian boundary, cordierite–biotite granitoids intruded at shallow depths the Fougères Unit, their emplacement outlasting deformation of the sedimentary sequences. Minor stocks of muscovite leucogranites of Cambrian age are possible equivalents of the abundant volcanism in an adjacent NW–SE-trending rift. This marks a major change in the kinematic framework, with predominance of extension. Finally, minor reactivation of the Cadomian structures occurred during the Carboniferous, associated with the Variscan orogeny.

Petrology and Inferred Tectonic Setting of the Mountain Pine Ridge Granitoids, Maya Mountains, Belize

The Mountain Pine Ridge batholith is the largest of three major granitoid intrusions that crop out in the Maya Mountains of Belize. These plutons intrude a thick sequence of Upper Paleozoic metasedimentary and metavolcanic rocks known as the Santa Rosa Group. The mineralogy of the granitic rocks at Mountain Pine Ridge varies and the rocks are classified into four petrographic types—biotite leucogranite, muscovite leucogranite, granodiorite, and tonalite. The essential felsic minerals in these rocks include quartz, microcline, plagioclase feldspar ± perthite, and muscovite, whereas the only mafic mineral is biotite. Microprobe analysis shows that significant differences exist between the biotites of the different petrographic types, especially in the levels of Al(vi), Mn, Mg, total Fe, and the Fe/Fe+Mg ratio. Major, minor and trace-element chemistry of the four petrographic types indicates that the Mountain Pine Ridge batholith is composed of three different suites of granitic rocks. Although all three sui...

Fractionation of monazite in the development of V-shaped REE patterns in leucogranite systems: Evidence from a muscovite leucogranite body in central Australia

Geochemical modelling based on K d values for rare earth (REE) and other trace elements suggests that a Proterozoic muscovite leucogranite from the Atnarpa area, southeastern Arunta Inlier, central Australia was derived from a restite-free melt by simple but extensive crystal fractionation involving feldspars, biotite and accessory REE-rich minerals such as monazite and zircon at a pressure of 0.5–1.0 kbars and a temperature below 750°C. This study indicates that REE-rich accessory minerals, particularly monazite, play an important role in the evolution of the REE in high-Si, low temperature, granitic melt systems.

Geología y petrología de los granitoides del sur de la Sierra de Comechingones, Córdoba, Argetina

The petrography and petrology characteristics of a group of granitoid bodies from the crystalline basement of Southern Comechingones Hills, is described here.In the metamorphic Complex biotitic gneisses, anphibolites, marbles and metabasic rocks were recognized. The main metamorphic episode in this area reached high-temperature within medium grade conditions.These granitoid are: biotitic granites, leucogranites and gneissic granites.The biotitic granites, with biotite, apatite sillimanite and garnet, are elongate bodies, concordant with the main penetrative schistosity. Have medium to small grain, with an important foliation, shown by the lineation of micaceous minerals.The muscovite, garnet, tourmaline, dumortierite and biotite leucogranites have tabular shape, and an extension more than one kilometre.The gneissic granites are plutons that intruded concordant to the regional structural trend. Biotite, sillimanite, monacite and apatite minerals are characteristic of these rocks. Radimetric ages obtained in nearby areas, show that granitoids may be of Lower Paleozoic age.Geology and petrology characteristics of granitoids suggest that they may have been produced by partial melt of crustal metasedimentary rocks.

Potassium-argon ages of molybdenum mineralization and associated granites at Bald Hill and correlation with other molybdenum occurrences in the South Island, New Zealand

In the Lyell Stream-Bald Hill area, Northwest Nelson, South Island, New Zealand, molybdenite mineralization occurs within brecciated and hornfels-textured Greenland Group low-grade metasediments in contact with minor quartz trondhjemite porphyry intrusions (Lyell porphyry). Potassium-argon ages of mineralized country rocks with hornfels texture, porphyry intrusions, and several granites of the adjacent Karamea granite batholith at Bald Hill range from 103 to 120 m.y. (mid-Cretaceous) and are grouped as follows. 1) Three biotite ages of the Lyell porphyry, 112 to 116 m.y., and three total rock ages of associated biotite hornfels, 103 to 120 m.y., indicate that the porphyry intrusion and probably the molybdenite mineralization occurred about 116 m.y. ago (Aptian). 2) Phyllitic Greenland Group country rocks (Lower Ordovician) immediately outside the thermal aureole, yield four total rock ages, 112 to 226 m.y., indicating partial argon outgassing during younger thermal events. 3) From the Bald Hill granites (informal name), biotite and muscovite ages of a pink gneissic micro-granite, 106 to 110 m.y., gneissic biotite granite, 102 to 109 m.y., and gneissic muscovite leucogranite, 105 to 110 m.y., are not significantly different; they probably reflect the time of tectonic uplift and cooling of the granites or possibly a later thermal metamorphism associated with the Lyell porphyry (informal name). The geological setting of the Lyell porphyry and Bald Hill granites is similar to that of the Berlins Porphyry and Buckland Granite of the Lower Buller Gorge. The molybdenite mineralization parallels that of other Northwest Nelson occurrences, being associated with minor mid-Cretaceous granitic porphyry intrusions, emplaced in Paleozoic metasediments, close to the margins of the Karamea and Separation Point granite batholiths.