Crystallization Of Granite Research Articles

Miocene high-temperature leucogranite magmatism in the Himalayan orogen

AbstractHimalayan leucogranites of Cenozoic age are generally attributed to partial melting of metasedimentary rocks at low temperatures of <770 °C. It is unknown what the spatial distribution and characteristics of high-temperature (>800 °C) leucogranites are in the Himalayan orogen. The present study reports the occurrence of such leucogranites in the collisional orogen. We use the Ti-in-zircon thermometry in combination with the thermodynamically calibrated relationships of T-aSiO2-aTiO2 to retrieve crystallization temperatures of Miocene (ca. 17 Ma) two-mica granites from Yalaxiangbo, in the eastern Himalaya, SE Tibet. The results give the maximum temperature as high as ∼850 °C for granite crystallization, providing a significant constraint on the nature of thermal sources. Phase equilibrium modeling using metasedimentary rocks as the source rocks indicates that felsic melts produced at ∼850 °C and 6–10 kbar can best match the target leucogranites in lithochemistry. In this regard, the anatectic temperatures previously obtained for the production of Himalayan leucogranites would probably be underestimated to some extent. Such high temperatures are difficult to explain purely by the internal heating of the thickened orogenic crust. Instead, they require an extra heat source, which would probably be provided by upwelling of asthenospheric mantle subsequent to thinning of the orogenic lithospheric mantle by foundering along the convergent plate boundary. Therefore, the Himalayan leucogranites of Miocene age would be derived from partial melting of the metasedimentary rocks in the post-collisional stage.

The physico-chemical conditions of crystallization of the Grenvillian arfvedsonite granite of Dimra Pahar, Hazaribagh, India: constraints on possible source regions

The Dimra Pahar Pluton is composed of arfvedsonite ± aegirine - alkali-feldspar granites with homophanous and mylonitic features. The pluton was emplaced along a regional shear zone during the post-collisional stage of the orogeny. Arfvedsonite and aegirine crystallized after alkali feldspar and quartz. The rocks are slightly peraluminous (alkali-index – AI: 0.92) to peralkaline (AI: 1.11). The pluton was emplaced at shallow depth at around 2–3 kbar pressure. The zircon saturation temperature varies between 889 and 1071 °C which is consistent with the liquidus temperature of the granitic melt. However, somewhat lower temperatures of crystallization of zircons (822–836 °C) were obtained by Ti-in-zircon thermometer. Two feldspar thermometry yields crystallization temperature ranging from 647 to 705 °C. F-free arfvedsonite crystallized at around 670 °C. Aegirine also crystallized around 670 °C. Arfvedsonite and aegirine crystallization indicates near solidus temperature of the magma. Zr-poor nature of both arfvedsonite and aegirine and presence of discrete zircon crystals suggest high oxygen fugacity of the magma. During the crystallization of zircon (near liquidus condition of the magma), the oxygen fugacity fluctuated around QFM buffer (ΔQFM = −1.22 to +1.29) as indicated by the Ce-anomalies in zircon. Later on, during the crystallization of arfvedsonite (near solidus condition of the magma), the granitic melt achieved oxygen fugacity condition above QFM buffer as indicated by the high Fe3+/(Fe2++Fe3+) ratios (0.2–0.23) of arfvedsonites. High Si/(Na + Fe + Si) ratios (0.52–0.57) of these arfvedsonites also suggest that oxygen fugacity reached above the NNO buffer during the later stage of crystallization. Pseudosection analysis of the studied rocks suggests that the mineral assemblage was equilibrated within the temperature range from 505 to 515 °C and oxygen fugacity range from −19 to −23 log units (bar), i.e. above NNO-buffer. The nearly Ti-free aegirines crystallized in highly oxidized condition near HM buffer. The development of coronal aegirine around arfvedsonite indicates the crystallization following an oxidizing path, towards the HM buffer. Water content of the granitic melt during crystallization of quartz and feldspar is estimated to be around 6 wt%. During the crystallization of arfvedsonite water content of the melt came down to 0.02 wt%. The high oxidation state and water saturated parental melts of the Dimra Pahar granite require a highly oxidized, hydrated source for the melts. A subduction-modified, metasomatically enriched lithospheric mantle or asthenospheric mantle can be considered as the source for the basaltic parental melt of Dimra Pahar granite.

Source and pressure effects in the genesis of the Late Triassic high Sr/Y granites from the Songpan-Ganzi Fold Belt, eastern Tibetan Plateau

The petrogenesis of granites with high Sr/Y signatures similar to adakitic rocks in continental settings is much debated. It is especially controversial whether these rocks are indicative of high-pressure magmatism (i.e., partial melting or crystallization fractionation) related to a thickened crust, or their high Sr/Y signatures are merely inherited from a high Sr/Y source. To address this, new chronological and geochemical data are presented for high Sr/Y granites from the Riluku batholith in eastern Tibetan Plateau. Zircon U-Pb dating and amphibole barometry suggest a final granite crystallization stage of ca. 207 Ma and an emplacement depth of ~14 km (~ 4 kbar), respectively. They are geochemically characterized by high Sr and low Y contents (481–1195 ppm and 6.62–20.6 ppm, respectively) with high Sr/Y and (La/Yb)N ratios (35–112 and 15–113, respectively). Enriched isotope character of the high Sr/Y granites, in combination with regional tectonics, indicates that these rocks are unlikely to be derived from partial melting of subducting oceanic crust. Low Cr (0.8–1.0 ppm), Ni (0.9–3.2 ppm), and Mg# (24–41) suggest that derivation from the partial melting of delaminated lower crust is also unlikely. Water-fluxed partial melting of crustal rocks at low pressure, which preferentially consumes plagioclase over micas, is inconsistent with the high contents of Rb (mostly >150 ppm) and low Sr/Rb ratios (mostly <5) of the samples. Reverse geochemical modeling, which simulated source composition under different pressures, indicates that evaluation of source compositions is critical to constrain the derivation pressure of the high Sr/Y granites. Our study emphasizes that high Sr/Y granites cannot be simply used to deduce a thickened crust at the time of magmatism without a priori knowledge of the nature of the source.

Tourmaline as a Recorder of Ore-Forming Processes in the Xuebaoding W-Sn-Be Deposit, Sichuan Province, China: Evidence from the Chemical Composition of Tourmaline

The Xuebaoding W-Sn-Be deposit located in the Songpan-Ganze Orogenic Belt (Sichuan Province, China) is a hydrothermal deposit with less developed pegmatite stage. The deposit is famous for the coarse-grained crystals of beryl, scheelite, cassiterite, apatite, fluorite, muscovite, and others. The orebody is spatially associated with the Pankou and Pukouling granites hosted in Triassic marbles and schists. The highly fractionated granites are peraluminous, Li-Rb-Cs-rich, and related to W-Sn-Be mineralization. The mineralization can chiefly be classified based on the wallrock and mineral assemblages as muscovite and beryl in granite (Zone I), then beryl, cassiterite and muscovite at the transition from granite to triassic strata (Zone II), and the main mineralized veins composed of an assemblage of beryl, cassiterite, scheelite, fluorite, and apatite hosted in metasedimentary rock units of marble and schist (Zone III). Due to the stability of tourmaline over a wide range of temperature and pressure conditions, its compositional variability can reflect the evolution of the ore-forming fluids. Tourmaline is an important gangue mineral in the Xuebaoding deposit and occurs in the late-magmatic to early-hydrothermal stage, and can thus be used as a proxy for the fluid evolution. Three types of tourmalines can be distinguished: tourmaline disseminations within the granite (type I), tourmaline clusters at the margin of the granite (type II), and tourmalines occurring in the mineralized veins (type III). Based on their chemical composition, both type I and II tourmalines belong to the alkali group and to the dravite-schorl solid solution. Type III tourmaline which is higher in X-site vacancy corresponds to foitite and schorl. It is proposed that the weakly zoned type I tourmalines result from an immiscible boron-rich aqueous fluid in the latest stage of granite crystallization, that the type II tourmalines showing skeletal texture directly formed from the undercooled melts, and that type III tourmalines occurring in the mineralized veins formed directly from the magmatic hydrothermal fluids. Both type I and type II tourmalines show similar compositional variations reflecting the highly fractionated Pankou and Pukouling granites. The higher Ca, Mg, and Fe contents of type III tourmaline are buffered by the composition of the metasedimentary host rocks. The decreasing Na content (&lt;0.8 atoms per formula unit (apfu)) and increasing Fe3+/Fe2+ ratios of all tourmaline samples suggest that they precipitated from oxidized, low-salinity fluids. The decreasing trend of Al content from type I (5.60–6.36 apfu) and type II (6.01–6.43 apfu) to type III (5.58–5.87 apfu) tourmalines, and associated decrease in Na, may be caused by the crystallization of albite and muscovite. The combined petrographic, mineralogical, and chemical characteristics of the three types of tourmalines thus reflect the late-magmatic to early-hydrothermal evolution of the ore-forming fluids, and could be used as a geochemical fingerprint for prospecting W-Sn-Be mineralization in the Xuebaoding district.

The 4D evolution of the Teutonic Bore Camp VHMS deposits, Yilgarn Craton, Western Australia

The Teutonic Bore Camp, comprised of the Teutonic Bore, Jaguar and Bentley deposits, is one of the most significant volcanic-hosted massive sulphide (VHMS) camps in Western Australia. Despite being extensively studied, only recently there have been advances in the understanding of the mechanism that drove the formation of mineralisation. It has been recognized by recent studies that the volcanic-hosted deposits from the Teutonic Bore Camp represent replacement-type VHMS systems, with significant input of fluids and metals from a magmatic source. This paper tests the existing hypothesis that the nearby Penzance granite acted as the metals source and/or thermal engine driving the development of these ore deposits.New age constraints on the formation of the host volcanic sequence at the Bentley deposit and the crystallization of the Penzance granite allows for the construction of a 4D evolutionary model for the ore system. A new U-Pb SHRIMP monazite age of 2681.9 ± 4.5 Ma indicates that the Penzance granite post-dates the host stratigraphy at Bentley (ca. 2693 Ma) and is probably coeval with mineralisation. All zircons (Penzance, Bentley units I and III) have very similar ƐHf(i), with most values between −1 and + 6, slightly higher than the ƐHf(i) of zircons from other granites and volcanics within the Kurnalpi Terrain, and indicative of juvenile sources. The mean Th/U ratios are ~0.7 and ~0.6 for the Penzance and Bentley zircons, respectively. All zircons have similar Ce/Nd(CN) ratios. The chemical similarities between the zircons from the granite and the volcanic rocks at Bentley support a shared magmatic source between the Penzance and the Teutonic Bore Camp sequence. The Penzance granite is the likely source of heat, and potentially metals, which drove the VHMS mineralisation at the Teutonic Bore Camp.

Proterozoic Granitoid Magmatism at the Southwestern Margin of the Siberian Platform in the Tectonic History of the Angara–Kan Block

The large massifs of ancient granitoids of the South Yenisei Ridge are divided into three complexes that differ in the geological, geochemical, and geochronological features. The Tarak massif consists of two complexes of different ages: the S–A-type porphyritic granitoids (collisional) of the Tarak complex (1.84 Ga) and the A-type subalkaline granites (anorogenic) of the Telkun complex (1.76 Ga). The magmatic crystallization of gneiss granites of the Posolnenskii complex is interrelated with the late stage of metamorphism and falls into the age interval between the above two complexes (1789.8 ± 8.9 Ma). Metamorphic transformations of the enclosing formations are recorded by the presence of small bodies of charnockitoids with dates of 1.84 and 1.73 Ga.

Reconciliation of discrepant U–Pb, Lu–Hf, Sm–Nd, Ar–Ar and U–Th/He dates in an amphibolite from the Cathaysia Block in Southern China

We present a combined garnet Lu–Hf and Sm–Nd, zircon and monazite U–Pb, hornblende Ar–Ar, and zircon U–Th/He geochronology study on a garnet-bearing amphibolite from the Cathaysia Block in Southern China. Pseudosection modeling and garnet–clinopyroxene thermometry indicate that the amphibolite attained peak metamorphic conditions of approximately 0.9 GPa and 780 °C. The Lu–Hf garnet date of 402.7 ± 1.0 Ma and Sm–Nd date of 331.6 ± 3.6 Ma define a ~ 70 Myr difference for the same garnet fractions. Reconciling the large discrepancies between these three geochronological systems with the observed major element diffusion profiles in garnet requires that REEs and Hf were fully re-equilibrated at peak temperatures and that subsequent rapid cooling prevented any disruption of the Lu–Hf or Sm–Nd systems throughout cooling from high temperatures. The Sm–Nd garnet age was partially reset during a thermal pulse associated with intrusion of leucocratic veins, which is inferred to have occurred at ca. 226 Ma based on the coinciding hornblende Ar–Ar (222.3 ± 0.9 Ma) and monazite U–Pb (226.2 ± 2.3 Ma) dates. The T–t path of a reheating event at 226 Ma necessary to produce both the major divalent cation diffusion profiles in garnet and partial resetting of the Sm–Nd garnet age, such that it records an apparent age of ~ 332 Ma, requires an effective diffusion radius of 45–60 μm in garnet during reheating, which is in agreement with the observation of pervasive chlorite-filled fractures within single garnet grains. Diffusion kinetics of Lu and Hf in garnet are sufficiently slow that this reheating episode would not impact the Lu–Hf system, and the 402.7 ± 1.0 Ma garnet age likely reflects the onset of rapid cooling from peak P–T conditions. The zircon U–Pb date of 434.8 ± 2.7 Ma predates the Lu–Hf garnet date by approximately 30 Myr and is interpreted to reflect zircon crystallization during prograde metamorphism. The ~ 226 Ma reheating event is contemporaneous with the intrusion and crystallization of the widespread Triassic A-type granites and alkaline syenites across the Cathaysia Block, indicating an extensional environment. We interpret the zircon U–Th/He date of 143.1 ± 4.2 Ma as reflecting a third thermal pulse based on the concurrence with the early Cretaceous volcanic rocks and granitoids, which overlies the amphibolite.

Enrichment of manganese to spessartine saturation in granite-pegmatite systems

Abstract The enrichment of manganese in peraluminous (S-type) granitic melts beginning with the anatexis of metapelitic rock and ending with the crystallization of highly evolved pegmatites is explained using experimentally derived mineral-melt partition coefficients and solubility data for Mn-rich garnet. Mineral-melt partition coefficients for Fe, Mg, and Mn between garnet, cordierite, tourmaline, and peraluminous, B-bearing hydrous granitic melt were measured between 650 and 850 °C at 200 MPaH2O. The compositions of garnet and tourmaline synthesized in these experiments are similar to those found in nature. Garnets evolve from Sps51Alm23Prp25 to Sps81Alm15Prp4 with decreasing temperature. The Mn content of cordierite increases with decreasing temperature. The composition of tourmaline does not vary with temperature. Partition coefficients, DMα/L, and exchange coefficients, KDα/L=DMα/L/DNα/L where α is a mineral, L is liquid (melt), and M and N are different elements, are presented for mineral-glass pairs. Partition coefficients for Mg, Fe, and Mn increase with decreasing temperature for garnet, tourmaline, and cordierite. The precipitation of garnet alone results in a progressive increase of MgO/FeO and a decrease of MnO/FeO in the melt. Crystallization of cordierite and tourmaline results in a decrease of MgO/FeO and an increase of MnO/FeO in melt. Tourmaline is most efficient at concentrating Mn in residual liquids. The trend toward increasing Mn/Fe in natural garnets in granites and pegmatites is not controlled by garnet itself, but instead by the crystallization of other mafic minerals in which Mg and Fe are more compatible than is Mn. A Rayleigh fractionation model constitutes a test of the partition coefficients reported in this manuscript. The starting composition for the model is that of a liquid (melt inclusions) from an anatectic S-type source. Normative modes of cordierite and biotite are calculated from that composition and are similar to modes of these minerals in natural occurrences. The model consists of crystallization of a cordierite-biotite granite from 850 to 650 °C. The model predicts that ~95% crystallization of the starting composition is required to reach saturation in spessartine-rich garnet at near-solidus temperatures. The model, therefore, is consistent with the occurrence of spessartine as restricted to highly fractionated granite-pegmatite systems at the end stages of magmatism.

Transpressional deformation during Ediacaran accretion of the Paranaguá Terrane, southernmost Ribeira Belt, Western Gondwana

The Paranaguá Terrane is mainly constituted by an Ediacaran arc-related granitic complex, spread in a NE-SW trending elongated stripe, located in the Southern Ribeira Belt, South-Southeastern Brazil. The basement of these granites occurs as a disrupted folded belt composed of metasedimentary and gneissic-migmatitic rocks. The integration of structural and geophysical (gamma spectrometry and magnetometry) data from the Paranaguá Terrane and the adjacent tectonic units, suggests that the deformation in the study area is partially controlled by the geometry of the Luis Alves Cratonic Block, as well as the northern part of Dom Feliciano Belt. The irregular shape of this cratonic unit associated with N-NW tectonic transport direction caused strain partitioning in the area and the development of two structural domains: southern and northern Paranaguá. The southern part is characterized by sinistral transpressional shear zones, while the northern sector is dominated by thrust shear zones that represent a large frontal collision with oblique components. The hierarchy of the structural and microstructural features in the Paranaguá Terrane associated with geochronologic data suggest two deformational phases (D1 and D2) during a transpressive collisional system. The D1 progressive deformational stage is correlated to the main period of granitic crystallization during the Ediacaran and was divided in two moments: i) early D1 (640–610) associated with the regional metamorphism and thrust and folding tectonics with N-NW tectonic transport and; ii) late D1 (610-580 Ma) linked to the strain partitioning and development of the southern (simple-shear dominated) and northern (pure-shear dominated) structural domains with sinistral kinematics, granite emplacement and high-temperature mylonites. The D2 deformation stage (540–500 Ma) is mainly represented by the low-temperature mylonites and deformation under retrograde conditions.

Mica-liquid trace elements partitioning and the granite-pegmatite connection: The St-Sylvestre complex (Western French Massif Central)

We constrain the genetic relation between granite and pegmatite parental melts from the Variscan Saint Sylvestre leucogranite complex and associated pegmatite bodies (Massif Central, France) through compositions of micas. Using mica trace element concentrations and available partition coefficients for Li, Rb, Ba, Cs and F, we calculated the trace element contents of granitic and pegmatitic melts at equilibrium with micas. Biotite in leucogranites and pegmatites ranges mostly from Fe-biotite to siderophyllite. More evolved facies contain protolithionite and zinnwaldite and lepidolite occurs in the most fractionated pegmatite. White micas have homogeneous compositions, from muscovite to Li-phengite. In granites, biotite and muscovite trace element distributions are clustered. In pegmatites, mica trace element contents globally follow differentiation from the least to the most evolved body. The reconstructed pegmatitic and granitic melts present strong compositional similarities such as enrichments in Li, Rb, Cs and depletion in Ba that suggest a similar origin. However, micas are shown to have selectively equilibrated with the last melt (or fluid) in contact and so trace element concentrations of early magmatic liquids are rarely preserved. Inversion of the mica data constrains the composition of parental melts and the trace element evolutions during crystallization. Leucogranite and pegmatite melts show mutual incompatible element evolutions inconsistent with a parent-daughter genetic relation. The data and the modelling suggest that they represent non-cogenetic melts generated by discrete episodes of partial melting. Differentiation is thus inherited from source processes rather than being the consequence of fractional crystallization of a common parental melt/magma. We suggest that both the leucogranites and the pegmatites originate from partial melting of a heterogeneous source rather than pegmatites being the product of granite crystallization.

Badzhal tin magmatic-fluid system (Far east, Russia): the transition from the granite crystallization to the hydrothermal ore deposition

With a view to reveal special characteristics of the transition stage from granite crystallization to rare-metal ore deposition it is studied Badzhal tin-bearing magmatic-fluid system of eponymously-named volcano-plutonic zone of the Middle Priamyrie. For that end the detail research of melt, fluid-melt and fluid inclusions and oxygen isotopes from minerals of granitoids from Verkne-Urmi massif from Badzhal volcano-plutonic zone and also minerals of Sn-W deposits Pravo-Urmi and Blizhnee have been carried out. The formation of greisens and hydrothermal veins were caused by the development of the integrated system associating with establishing of Verkne-Urmi granite massif which is one of a dome fold of Badzhal cryptobatholith. For the first time for tin deposits it has been followed up the transition from the magmatic phase of granite crystallization to the hydrothermal ore formation stage and the evolution of magmatic fluid from its separation from magmatic melt to Sn-W ore deposition. The direct evidence of tin-bearing fluid separation under melt crystallization is combined fluid-melt inclusions. Glass composition in inclusions shows that granites and granite-porphyry were crystallizing from acid and from limited to high-aluminous melts, that is value ASI changes from 0.95 to 1.33 and a content of alkalies varies from 6.02 up to 9.02 mass.%. Cl and F concentrations in glasses are according 0.03–0.14 and 0.14–0.44 mass.% and turned out to be higher of same in the total composition of rocks (0.02 and 0.05–0.13 mass.% in accordance). These differences indicate that Cl and F could be separated from granite melt under its crystallization and degasation. H2O content made from total deficiency electron microprobe analysis is 8–11 mass.%. This evaluation was made inclusive of a probable effect of “Na loss” (Nielsen, Sigurdson, 1981) under aqueous glass crystallization. Considering a high error of a such estimation (Devine et al., 1995), it should take to obtained values as a very approximate evaluation and consider that examined melts contained about 9,5–10,0 mass.% of H2O.&#x0D; The results of melt inclusion examination show that at any rate a part of melt forming magmatic rocks of Badzhal Ore Magmatic System are crystallizing at about T = 650 °C. These melts were acid, limited fluoride and meta- and high aluminous. The reason of low temperatures of its crystallization are likely a high pressure of aqua and also a increased content of F. Most likely that examined inclusions characterize the final stage of establishing of the massif, herewith at the system crystals, residual liquor and magmatic fluid phase coexist.&#x0D; The fluid from which greisens of Pravo-Urmi deposit formed is similar in properties to the supercritical fluid absorbing by magmatic minerals. The salinity of this fluid varying from ~9 to 12 mass.% equiv. NaCl, maximal T = 550 °C (with consideration for the temperature correction of T gom on a pressure ~1 кbar) are similar to such of magmatic fluid, which permit to connect its origin with pluton cooling. The formation of greisens and quartz-topaz veins of Pravo-Urmi deposit is related to fall of temperature of magmatic fluid from 550–450 up to 480–380 °C. The evolution of fluid deposited quartz-cassiterite veins of Blizhnee deposit, which based upon oxygen isotope composition (d18ОН2О ≈ 8.5‰) also separated from magma, was going at more subsurface conditions under much lesser pressure. That led to the gas separation of a fluid with salinity ~13 mass.% equa. NaCl under T = 420–340 °C on thin low salinity vapour and brine with concentration 33.5–37.4 mass.% equiv. NaCl. The research of oxygen isotope system testifies that oxygen isotope composition of ore-forming fluid controlled by equilibrium with granites at wide interval tempera­tures (from ~700 °С up to the beginning of greisen crystallization). Correspondence of measured and calculation data of the offered model indicates that the considerable volume of external fluid with other isotope characteristics which did not reach the isotope equilibrium with Verkhne-Urmi massif did not come into the magmatic isotope system. The discovered differences of physico-chemical conditions for two studied deposits are not “critical” and support an idea about their formation as the single magmatic-fluid system.

Petrogenesis of Maktali fractionated calc-alkaline younger granitoids, Central Eastern Desert, Egypt

The Egyptian calc-alkaline younger granitoids represent part of broadly distributed late collisional high-K calc-alkaline granitoids in the northern Arabian-Nubian Shield. In the Eastern Desert of Egypt, these granitoids have a significant economic value that they are commonly associated with rare metal-bearing granites. Maktali granitoids are located in Central Eastern Desert and they constitute a two-phase pluton, which consists of leucogranites and monzogranites. They intrude the country metavolcanics with sharp contacts. These granitoids are slightly peraluminous and moderately to highly fractionated high-K calc-alkaline granitoids. Leucogranites and monzogranites were emplaced at temperatures 699.8-8.28.8°C and 757.2–909.6°C and under pressure 1.22–4.8 and 2.36–2.98 kbars respectively. Leucogranites and monzogranites were emplaced at depths 3.28–22.96 km and 6.36–8.5 km and they were crystallized under log Oxygen fugacity between − 13.2 to − 16.1 and − 11.6 to − 14.7 respectively. They are magnetite-series granitoids and their crystallization was controlled by NNO fugacity buffer. Leucogranites and monzogranites share the same magmatic source and they were emplaced during late-collisional stage by melting of crustal source rocks may occur as a consequence of decompression following delamination of the lithospheric root and slab breakoff. Maktali leucogranites could be generated by partial melting and subsequent fractional crystallization of mafic lower crust with addition of melts from the mantle. Maktali monzogranites show distinctive characteristics of rare metal-bearing granites that they contain Fe-columbite and transitional micas. Field and textural evidences together with chemistry of columbite, micas, and trace element behavior suggest the magmatic origin of monzogranites. Moreover, REEs in monzogranites show a pronounced Eu anomaly and a well-visible tetrad effect, supporting their generation by fractional crystallization and fluid-rock interaction during the late stage of granite crystallization.

Badzhal Tin Magmatic-Fluid System, Far East, Russia: Transition from Granite Crystallization to Hydrothermal Ore Deposition

The Badzhal tin magmatic-fluid system of the eponymous volcanoplutonic zone in the Middle Amur Region has been studied to reveal special characteristics of the transition from granite crystallization to rare-metal deposition. Therefore, the authors have conducted an in-depth study of melt, fluid-melt, and fluid inclusions and oxygen isotope composition of minerals from granitic rocks of the Verkhneurmiisky pluton within the Badzhal volcanoplutonic zone and minerals of the Pravourmiisky and Blizhnee Sn-W deposits. Greisen alteration and hydrothermal veins at the Pravourmiisky and Blizhnee deposits resulted from a single magmatic-fluid system related to the Verkhneurmiisky granite pluton, which is one of domes of the Badzhal batholith. The transition has been traced from magmatic crystallization of granite to hydrothermal ore formation and evolution of magmatic fluid from its separation to Sn and W deposition. Mixed fluid-melt inclusions directly support tin-bearing fluid separation during melt crystallization. The glass compositions indicate that granite and porphyry granite crystallized from felsic metaluminous to peraluminous melts with an ASI index and alkali content ranging from 0.95 to 1.33 and from 6.02 to 9.02 wt %, respectively. The Cl and F concentrations in glasses are 0.03–0.14 and 0.14–0.44 wt %, respectively, and are higher than those in the bulk rock compositions, 0.02 and 0.05–0.13 wt %, respectively. These differences indicate that Cl and F could have been removed from a granitic melt during its crystallization and degassing. The H2O concentration estimated based on the electron deficiency of summed microprobe analyses is 8–11 wt %. This was estimated taking into account the possible effect of “sodium loss” (Nielsen and Sigurdson, 1981) when analyzing hydrated glasses. Taking into account the high uncertainty of such estimation (Devine et al., 1995), this value is extremely uncertain and the studied melts should be considered as containing 9.5–10.0 wt % H2O. The melt inclusion study shows that some of the magmatic rocks of the Badzhal ore-magmatic system formed at approximately 650°C. The melt from which these rocks crystallized was felsic, moderate in fluorine, and meta- and peraluminous. Low-temperature crystallization is probably caused by high water pressure and elevated fluorine content. These inclusions most likely characterize the final stage of the pluton, at which crystals, residual melt, and magmatic fluid phase coexist. Fluid responsible for the formation of greisens at the Pravourmiisky deposit has features very close to those of supercritical fluid trapped by magmatic minerals. Its salinity, ranging from ∼9 to 12 wt % NaCl equiv., and maximal temperature of 550°C (taking into account pressure correction for ∼1 kbar) are close to those of magmatic fluid. This makes it possible to relate the origin of the fluid to cooling of the pluton. Greisen and quartz-cassiterite-topaz veins of the Pravourmiisky deposit formed from magmatic fluid at a decreased temperature from 550–450 to 480–380°C. The fluid responsible for the formation of quartz-cassiterite veins at the Blizhnee deposit also separated from magma as indicated by the oxygen isotope composition ( $${\delta^{18}}{{\rm{O}}_{{{\rm{H}}_2}{\rm{O}}}}\sim8.5{\%\circ}$$ ), which evolved in a shallower environment under much lower pressure. This resulted in fluid with a salinity of ∼13 wt % NaCl equiv. at 420–340°C being separated into a low-density low-saline vapor and brine with a salinity of 33.5–37.4 wt % NaCl equiv. The oxygen isotope composition of the mineralizing fluid was governed by equilibrium with granite within a wide temperature range (from ∼700°C to the onset of greisen crystallization). The agreement between the measured data and those calculated based on the suggested model indicates that a significant volume of external fluid having different isotopic features and not in equilibrium with the Verkhneurmiisky granite did not enter the magmatic-fluid system. The revealed differences in the physicochemical formation conditions of the two studied deposits are not critical and support their formation within a single magmatic-fluid system.

Geology, igneous geochemistry, mineralization, and fluid inclusion characteristics of the Kougarok tin-tantalum-lithium prospect, Seward Peninsula, Alaska, USA

The Kougarok prospect is situated in a Sn-W (+Ta, Nb, Li, Be) metallogenic belt formed in a post-collisional to within-plate tectonic environment. Crystal fractionation of granitic magma, combined with its mixing/mingling with mantle-derived mafic magma, is proposed as the major process causing the formation of the complex B-rich to Li-F-type granitic suite and associated Sn-rare metal (Ta, Nb, Li) mineralization. An intrusion of alkalic (potassic lamprophyric) mafic magma into a crystallizing biotite-tourmaline granite magma reservoir may have supplied a geochemically distinct assemblage of volatiles (F) and associated metals (Li, Ta, Nb) into the boron- and Sn-rich granitic system. An alternative model considers differentiation-driven unmixing of mafic and granitic silicate melts sequestering different volatile and mineralizing species that could also trigger magma-fluid and fluid-fluid unmixing, with separation of gaseous fluid coexisting with immiscible granitic and mafic magmas in the form of magmatic melt-fluid-crystal “suspension.” Biotite-tourmaline granite was accompanied by early quartz-tourmaline-cassiterite greisen at > 430–380 °C followed by quartz-tourmaline-chlorite-cassiterite stockwork at < 350 °C. The immiscibility of F- and B-rich fluids was followed by preferred ascent of F-rich fluids upward in the granitic magma reservoir, with its strong enrichment in F, Li, Ta, and Nb and subsequent crystallization of zinnwaldite granite at a shallower level, with separation of homogenous high-salinity magmatic fluid at ~ 600 °C. Crystallization and fluid exsolution continued at lower temperatures, followed by formation of topaz-quartz stockscheider at 550–500 °C. Ta-Nb mineralization in the uppermost part of the zinnwaldite granite appears to be associated with final episodes of magmatic crystallization, particularly W-Nb-rutile and columbite-tantalite with a higher Nb content. Some Ta-Nb minerals, such as columbite-tantalite with a higher Ta content, appear to be also stable during the post-magmatic stage, in quartz-tourmaline+topaz greisen and higher-temperature quartz-albite-Li mica alteration that replace zinnwaldite granite at ~ 500 to 400 °C. Nearly contemporaneous, lower-temperature quartz-albite-Li mica alteration and quartz-muscovite, quartz-topaz, and quartz-fluorite greisens formed from boiling fluids at 390–350 °C and caused removal of Ta-Nb minerals. Instead, fluid cooling and neutralization of boiling fluids affected Sn solubility and promoted massive cassiterite deposition in the late greisens. The latest phyllic quartz-sericite-carbonate alteration assemblage, comprising arsenopyrite, pyrrhotite, chalcopyrite and other sulfides, native Bi, and Bi tellurides, formed from boiling fluids at < 310 °C.

Ambiguous isotopic and geochemical signatures resulting from limited melt interactions in a seemingly composite pluton: a case study from the Strzegom\u2013Sob\xf3tka Massif (Sudetes, Poland)

The western part of the late Variscan Strzegom–Sobótka massif exemplifies an intrusion that formed in a multi-component system. The system shows ambiguous isotopic and geochemical signatures and presents a seemingly composite character [biotite granite with negligible amount of hornblende (HBG) and microgranular mafic enclaves (MMEs)]. The melts responsible for forming granitic rock and its MMEs were not derived from contrasting crustal and mantle sources but from different crustal domains. These melts exhibit hybrid characteristics due to subsequent contamination processes. The data suggest two different stages of hybridization of two geochemically distinct magmas corresponding to granite and enclaves. The processes were temporally and spatially distinct. Heterogeneous protoliths and hybridization made these magma isotopic signatures and trace element patterns ambiguous. The main mechanism involved in developing granitic rocks was fractional crystallization (FC); the mechanism responsible for MME composition was mixing, but other local processes were also involved in rock formation. Generally, the FC signature was not obscured by the hybrid mafic melt input. This felsic-mafic melt interaction was a limited one-sided process. Granitic melt was changed by the presence of small mafic magma blobs, but their influence on granite crystallization was negligible. The massif is thus an ideal example of an environment where MMEs coexist with a granitic body, although MMEs do not determine the actual composite character of the pluton. The early stage of Variscan pluton formation within the Bohemian Massif and adjacent areas was shaped by interacting mantle- and crust-derived melts. The pluton described in this paper belongs to the late stage of Variscan magmatism. It is a multi-component system shaped by the participation of melts derived from heterogeneous crustal domains. The presence of enclaves does not explicitly indicate their dominant role in the formation of composite plutons of crust-mantle origin. It is decisive to determine whether the ambiguous signatures appearing in such cases truly indicate both of these sources.

Metamorphic evolution of the Loma Marcelo skarn within the geotectonic context of the crystalline basement of the Ventania System (Argentina)

This study describes the mineralogical and isotopic changes that carbonate xenoliths experienced under multiple metamorphic events and hydrothermal fluid circulation during the evolution of the Ventania System basement. The high reactivity of carbonate xenoliths allowed the preservation of mineral assemblages corresponding to at least three metamorphic events in the resulting Loma Marcelo skarn. The Ventania System basement is composed of Neoproterozoic granites and ignimbrites, Early Cambrian granites, and Middle Cambrian rhyolites. Carbonate xenoliths were incorporated during the intrusion of a calc-alkaline granite with an LA-ICP-MS U-Pb crystallization age of 621.6 ± 2.2 Ma. The intrusion induced pyroxene–hornfels facies metamorphism in the carbonate xenoliths and the associated metasomatism generated calcic and magnesian skarns depending on the protolith composition. Garnet, clinopyroxene, wollastonite, bytownite, and meionite were formed in the calcic skarn (CaS), whereas forsterite and spinel were formed in the magnesian skarn (MgS). Crystallization of Early Cambrian alkaline granites was accompanied by intense hydrothermal activity that was responsible for low temperature (≤300 °C) F-metasomatism in the skarn, as evidenced by the presence of F-rich vesuvianite (CaS) and chondrodite (MgS), among other minerals. Vesuvianite was formed from calc-silicate mineral assemblages of the previous metamorphic event, whereas chondrodite was formed by replacement of forsterite. The low temperature formation of these typical high-grade minerals could be an evidence of mineral formation under disequilibrium conditions favoured by the high reactivity of hydrothermal fluids. Neopalaeozoic basement mylonitization under greenschist facies metamorphism was accompanied by hydrothermal fluid circulation. This event promoted extreme mobility of chemical elements in the basement rocks and epidotization (CaS) and serpentinization (MgS) in the Loma Marcelo skarn. The elongated and boudinaged shape of the skarn bodies, parallel to the mylonitic foliation, is a consequence of dextral shearing that affected the basement rocks. Additionally, almost pure grossular crystallized post-tectonically in the CaS. Carbonates of the Loma Marcelo skarn are depleted in 13C and 18O (δ13CV-PDB = −2.5/−10.1‰; δ18OV-SMOW = +7.3/+14.0‰) relative to carbonate sedimentary rocks. The δ13C and δ18O variations can be attributed to the interaction between large amounts of hydrothermal fluids (W/R = 30–50) and Neoproterozoic carbonate sedimentary rocks.

Formation of monazite-(Ce, La) by fluid-apatite interaction: the Floresta Azul Alkaline Complex, Bahia, Brazil

Monazite is a common accessory mineral in the Floresta Azul Alkaline Complex, occurring in three different rock types, which form this batholith: nepheline syenite, granite and fenite. Two compositional types of monazite can be found: monazite-(Ce) and monazite-(La), the latter being found only in syenites. Monazite occurs in close association with apatite as anhedral crystals ranging in size from 0.1–100 μm, and textures indicate different genetic processes. In nepheline syenite, monazite is closely related to ancylite and apatite, as a late crystallizing phase. Monazite occurs in granites as pore fillings in apatite and was formed by the remobilization of rare earth elements (REE) from apatite by late CO 2 — rich fluids. In fenite, monazite appears as acicular crystals, parallel to apatite’s crystallographic c-axis formed by exsolution from apatite by metamorphism. The data show that the monazite genesis was closely related to the activity of fluids associated either with the final stages of crystallization of nepheline syenite and granite or with fenitization of country rock.

Petrogenetic and Compositional Features of Rare Metal Pan-African Post-Collisional Pegmatites of Southwestern Nigeria; A Status Review

Abstract This research reviews the geology, petrogenesis, compositional trends and geochronology of the rare-metal pegmatite of southwestern Nigeria. The source of these pegmatites is still presently debated which have been explained as either product of highly fractionated molten material or anatexis of the local crust. However, published works of past authors have been compiled to give a detailed understanding of the formation of the mineral deposits. The basement complex of southwestern Nigeria comprises of Precambrian rocks of amphibolite, the hornblende gneiss and the granite gneisses which were formed as a result of the opening and closing of the ensialic basin with significant, extensive subduction during the Pan-African orogeny. The pegmatites in this region have shown internal zoning and a high degree of evolution from the border zone to the core zone during the crystallization and solidification of the felsic granite to pegmatite melt. The rare-metal pegmatites have distinct chemical compositions and mineralogy, containing quartz, biotite, muscovite, microcline, garnet with localized tourmaline, tantalite and columbite. These pegmatites vary significantly by their bulk-rock and mineral chemistry which indicates a more peraluminous attribute and enrichments of lithophile elements of Rb, Cs, Ta and Ba. Previous K/Ar isotopic ages (502.8±13.0 Ma and 514.5±13.2 Ma) suggest that the pegmatites are related to the post-collisional phase of intensive metasomatism. Adopted from previous studies, a five-stage conceptual model of evolution which is widely accepted have been proposed for the origin of the pegmatites.

The timing of high-temperature conditions and ductile shearing in the footwall of the Naxos extensional fault system, Aegean Sea, Greece

We present eight Rb-Sr multi-mineral isochron ages showing that high-temperature metamorphic conditions and partial melting during top-to-the-NNE extensional shearing in the footwall of the Naxos extensional fault system (i.e. Naxos metamorphic core complex) lasted until about 14–12 Ma. One migmatite sample yielded an age of 14.34 ± 0.2 Ma (2σ uncertainty) for crystallization of migmatization-related melt pockets. Four pegmatite samples, which are in part associated with partial melting of their host rocks, provided overlapping ages ranging from 13.81 to 12.23 Ma (age range includes 2σ uncertainty). Additional three samples of amphibolite-facies schist supplied Rb-Sr ages of around 14 Ma. Samples showing fluid- and/or deformation-assisted white mica and biotite reworking gave Rb-Sr mineral apparent ages of 11.1 ± 2.7, 10.16 ± 0.24, 9.7 ± 0.7 and 9.6 ± 0.15 Ma. These ages are interpreted to be associated with late stages of extensional shearing under greenschist-facies metamorphic conditions. Together with published U-Pb zircon ages of migmatite, and S- and I-type granite crystallization, the data indicate that the presence of melt in the footwall of the Naxos extensional fault system lasted for at least 7 Ma (from ~18 to ~11 Ma). This demonstrates that high temperatures and crustal melting resulting from and aiding extensional deformation was a long-lived and not a transient event. We conclude that melt-assisted deformation facilitated large-scale displacement on the Naxos extensional fault system by drastically weakening the extending crust for long periods of time.

Effect of Silicate Matter on Pyrochlore Solubility in Fluoride Solutions at Т = 550–850°C, Р = 50–100 MPa (Experimental Studies)

The experimental results of natural pyrochlore behavior in KF solutions in the presence of quartz at 550–850°C and 50–100 MPa are presented. It is shown that silicate matter (quartz) exerts a significant effect on pyrochlore solubility in aqueous solutions of fluorides of alkaline metals under hydrothermal conditions. This study of the fluid inclusions has revealed the occurrence of reactions of high-temperature hydrolysis of KF under the experimental conditions: KF + H2O = KOH + HF; in which case, the interaction with quartz SiO2 + 2KOH = K2SiO3 + H2O is followed by the formation of a silicate glass phase (an aqueous solution–melt). This phase of alkaline glass is a Nb concentrator (Nb2O5 up to 16 wt %). The coefficient of Nb distribution between the glass and the fluid is ≈500 (in favor of the glass). It is determined that the phase of the silicate solution–melt can serve as an effective concentrator of the ore component (Nb) at the last lowtemperature stages of crystallization of rare-metal granites.