Peraluminous Granitic Melts Research Articles

Quantification of Feldspar and Quartz Nucleation Delay in a Hydrous Peraluminous Granitic Melt

A modified model based on classical nucleation theory was applied to a natural hydrous peraluminous pegmatite composition and tested against crystallization experiments in order to further investigate the quantification of nucleation delay in felsic melts. Crystallization experiments were performed in a piston-cylinder apparatus at 630 MPa and temperatures between 650 and 1000 °C for durations ranging from 0.3 to 211 h. Experimental run products were investigated by scanning electron microscopy paired with energy dispersive spectroscopy analyses of both crystalline and quenched liquid phases, the results of which were compared to an established theoretical nucleation delay model from the literature. The experiments showed good agreement (within a factor of 5) with the model for quartz, while it showed moderate agreement (within a factor of 10) with the model for sodic feldspar. Other crystals also nucleated, demonstrating abundant features of disequilibrium. Our research further demonstrates the potential of the model to predict nucleation delay, showing promising results for the quantification of the nucleation delay of quartz and feldspar in natural felsic melts, thus adding to previously published studies on hydrous, metaluminous, felsic melts and dry basaltic melts.

Solubility of Monazite–Cheralite and Xenotime in Granitic Melts, and Experimental Evidence of Liquid–Liquid Immiscibility in Concentrating REE

Abstract We provide new experimental data on monazite, xenotime and U–Th-bearing cheralite solubility in slightly peralkaline to peraluminous granitic melts using dissolution and reverse (i.e. recrystallization after dissolution) experiments in water-saturated and flux-bearing (P + F + Li) granitic melts, at 800 °C and 200 MPa. Although a positive correlation between rare earth element (REE) solubility and melt peralkalinity is confirmed, monazite solubilities reported here are much lower than the values previously published. We suggest that the presence of elevated phosphorus concentrations in our melts depresses monazite solubility, principally because phosphorus complexes with Al and alkali, which normally depolymerize the melt through the formation of non-bridging oxygens. The new solubility data provide an explanation for the very low REE concentrations generally encountered in phosphorus-bearing peraluminous granites and pegmatites. This accounts for the compatibility of REE in peraluminous systems, as the early crystallization of REE-bearing minerals (mainly monazite and zircon) leads to progressive REE depletion during liquid differentiation. In addition, dissolution and reverse experiments on U–Th-bearing cheralite–monazite display liquid–liquid immiscibility processes in our slightly peralkaline glass. The immiscible liquid forms droplets up to 10 µm in diameter and hosts on average 35 wt% P2O5, 25–30 wt% F, 22 wt% Al2O3, 4 wt% CaO, 5 wt% Na2O, 2 wt% La2O3, and 12 wt% ThO2 + UO2. We believe that the droplets formed during the runs and may have coalesced to larger droplets during quenching. We suggest that liquid–liquid immiscibility is a possible mechanism of REE concentration in highly fluxed melts and should be considered in natural systems where REE are extremely concentrated (up to thousands of µg g–1) in magmatic rocks.

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.

The magmatic–hydrothermal transition in rare-element pegmatites from southeast Ireland: LA-ICP-MS chemical mapping of muscovite and columbite–tantalite

The processes involved in the magmatic–hydrothermal transition in rare-element pegmatite crystallization are obscure, and the role of hydrothermal mechanisms in producing economic concentrations of rare elements such as tantalum remains contentious. To decipher the paragenetic information encoded in zoned minerals crystallized during the magmatic–hydrothermal transition, we applied SEM-EDS and LA-ICP-MS chemical mapping to muscovite- and columbite-group minerals (CGM) from a rare-element pegmatite of the albite-spodumene subtype from Aclare, southeast Ireland.We present a three-stage model for the magmatic–hydrothermal transition based on petrography, imaging and quantification of rare-element (Li, B, Rb, Nb, Sn, Cs, Ba, Ta, W, U) zoning, integrated with geochemical modeling and constraints from published literature. Stage I marks the end of purely magmatic crystallization from a peraluminous granitic melt. In stage II, polymerized silicic melt and depolymerized alkaline aqueous melt coexist as immiscible media, both of which influence muscovite and CGM crystallization. Stage II also marks the onset of phyllic alteration of primary mineral assemblages. Hydrothermal fluid release causes further resorption of primary minerals and eventual precipitation of fine-grained albite in stage III.Muscovite and CGM both exhibit trace-element zoning, while CGM also show major-element zoning. Petrographic relationships and geochemical markers such as Ta# (=Ta/[Ta + Nb]) of individual mineral zones reveal that both mineral species crystallized contemporaneously over all three stages. Furthermore, Rayleigh fractional crystallization of muscovite is efficient in fractionating Ta from Nb. Tantalum and Nb are additionally fractionated by halogen-rich aqueous media, which remobilize both elements, but redeposit preferentially Ta-enriched oxides. Columbite–tantalite mineralization is therefore both magmatic and hydrothermal. Albite associated with stage-III muscovite and CGM was likely precipitated from a hydrothermal fluid when pH changed due to hydrolysis of primary minerals. The complex zoning and resorption of minerals indicates that bulk analyses and conventional LA-ICP-MS spot ablation analyses of muscovite in rare element pegmatites may lead to erroneous modelling of element partitioning and fluid evolution.Combined petrographic and high-resolution geochemical analysis of two mineral species (which co-crystallize and incorporate the same elements of interest) is an effective tool to assess the complex processes of crystal–melt–fluid interaction. Our three-stage model may also be applicable to the still not well understood magmatic–hydrothermal transition from fertile granitic melts to formation of Sn-W veins and greisens or porphyry-type deposits.

Melting Inside the Tibetan Crust? Constraint From Electrical Conductivity of Peraluminous Granitic Melt

AbstractMagnetotelluric and seismological studies suggested the presence of partial melts in the middle to lower Himalaya‐Tibetan crust. However, the melt fractions inferred by previous work were based on presumed electrical conductivity of melts. We performed measurements on the electrical conductivity of peraluminous granitic melts with 0.16–8.4 wt % H2O (the expected compositions in the Tibetan crust) at 600–1,300°C and 0.5–1.0 GPa. Peraluminous melt exhibits lower electrical conductivity than peralkaline melt at dry condition, but this difference diminishes at H2O > 2 wt %. With our data, the observed electrical anomalies in the Tibetan crust could be explained by 2–33 vol % of peraluminous granitic melts with H2O > 6 wt %. Possible reasons for our inferred melt fractions being higher than seismological constraints include the following: (1) The real melts are more Na and H2O rich, (2) the effect of melt reducing seismic velocities was overestimated, and (3) the anomalies at some locations are due to fluids.

Eocene magmatism from the Liemai intrusion in the Eastern Tethyan Himalayan Belt and tectonic implications

AbstractMultistage magmatic thermal events occurred in the Yardoi Dome and contain important information on the tectonomagmatic processes. The dome has played a crucial role in understanding the collisional evolution of the Tethyan Himalayan. We present new geochronological and geochemical data for muscovite-granite exposed in the Liemai area, Eastern Tethyan Himalayan Belt. Liemai muscovite-granite is strongly peraluminous, with A/CNK values characterized by evolved geochemical composition with high contents of SiO2-enriched large-ion lithophile elements, and is depleted of high-field-strength elements. These geochemical features indicate that granites possibly derived from partial melting of metasedimentary rocks and plagioclase fractional crystallization probably played a critical role in production of peraluminous granitic melts. Zircon U–Pb dating from muscovite-granite yielded ages of approximately 48.5 ± 1.1 Ma, representing its crystallization ages. This age is the oldest age of Tethyan Himalayan leucogranite from the Yardoi Dome and adjacent areas. However, the inherited zircon cores have ages of 135.7–3339.2 Ma. The εHf(t) values of all zircons vary from –6.4 to –2.3 and have varying Hf-isotope crustal model ages of 731–839 Ma. The geochemical and isotopic compositions indicate that magma of the Liemai granite can most likely be interpreted as products of the break-off related to thermal perturbation along the break-off window associated with the subduction of Neo-Tethyan slab. These magmas were derived mainly from the anatexis of ancient crustal materials under contraction and thickening conditions due to subduction of the Indian continent beneath southeastern Tibet.

Diversity of Mesozoic tin-bearing granites in the Nanling and adjacent regions, South China: Distinctive mineralogical patterns

The Nanling and adjacent regions of South China host a series of tin deposits related to Mesozoic granites with diverse petrological characteristics. The rocks are amphibole-bearing biotite granites, or (topaz-) albite-lepidolite (zinnwaldite) granites, and geochemically correspond to mealuminous and peraluminous types, respectively. Mineralogical studies demonstrate highly distinctive and critical patterns for each type of granites. In mealuminous tin granites amphibole, biotite and perthite are the typical rock-forming mineral association; titanite and magnetite are typical accessory minerals, indicating high f O2 magmatic conditions; cassiterite, biotite and titanite are the principal Sn-bearing minerals; and pure cassiterite has low trace-element contents. However, in peraluminous tin granites zinnwaldite-lepidolite, K-feldspar and albite are typical rock-forming minerals; topaz is a common accessory phase, indicative of high peraluminity of this type of granites; cassiterite is present as a uniquely important tin mineral, typically rich in Nb and Ta. Mineralogical distinction between the two types of tin granites is largely controlled by redox state, volatile content and differentiation of magmatic melts. In oxidized metaluminous granitic melts, Sn4+ is readily concentrated in Ti-bearing rock-forming and accessory minerals. Such Sn-bearing minerals are typical of oxidized tin granites, and are enriched in granites at the late fractionation stage. In relatively reduced peraluminous granitic melts, Sn2+ is not readily incorporated into rock-forming and accessory minerals, except for cassiterite at fractionation stage of granite magma, which serves as an indicator of tin mineralization associated with this type of granites. The nature of magma and the geochemical behavior of tin in the two types of granites thus result in the formation of different types of tin deposits. Metaluminous granites host disseminated tin mineralization, and are locally related to deposits of the chlorite quartz-vein, greisen, and skarn types. Greisen, skarn, and quartz-vein tin deposits can occur related to peraluminous granites, but disseminated mineralization of cassiterite is more typical.

Insights into post-magmatic metasomatism and Li circulation in granitic systems from phosphate minerals of the Nanping No. 31 pegmatite (SE China)

Phosphate minerals are key indicators for understanding the processes of diagenesis and mineralization in granitic systems. More importantly, these minerals provide constraints on the post-magmatic behaviour of Li in pegmatite systems that remain to be explored, although Li from early-crystallized Li phosphates is known to leach into hydrothermal fluids. In this study, we performed a detailed petrographic and compositional analysis of phosphate minerals from the Nanping No. 31 pegmatite in southeastern of China, which provides new evidence of Li circulation in granitic systems. Primary phosphate minerals evolved from xenotime and monazite to Mn-rich fluorapatite and then to triphylite and montebrasite, with large amounts of montebrasite in the intermediate zones of the pegmatite, reflecting the increasing activities of both Li and P in peraluminous granitic melts. Montebrasite experienced stages of both high-temperature (360–273°C) and low-temperature (273–100°C) hydrothermal alteration. The high-temperature hydrothermal alteration of montebrasite by Fe- and Mg-rich fluids resulted in the formation of a series of Fe-, Mg-bearing phosphates; the replacement of montebrasite under low-temperature alteration generated amounts of Ca-, Sr-, Ba-bearing phosphate minerals, muscovite and akdalaite (Al2O3)4·H2O. The formation of micro-networks of akdalaite from montebrasite indicate the low mobility of Al in hydrothermal fluids. A four-stage scenario of post-magmatic Li transport in the Nanping No. 31 pegmatite is proposed: (1) Li derived from the breakdown of primary montebrasite was locally recrystallized and produced secondary montebrasite (Mtb-1); (2) hydrothermal alteration by Fe- and Mg-bearing fluids leached Li from montebrasite to form secondary triphylite and simferite; (3) Li derived from the alteration of montebrasite by Sr-, Ca-rich fluids was involved in the formation of secondary palermoite and bertossaite; and (4) the replacement of secondary triphylite, montebrasite, palermoite and bertossaite by later phases (apatites, ludlamite, anapaite, augelite and fine-grained muscovite) reflect the leaching of Li back into hydrothermal fluids. The re-enrichment of Li during the post-magmatic stage most likely increased the solubility of Ta in the hydrothermal fluids of the granitic pegmatite.

Variscan metagranitoids in the central Tauern Window (Eastern Alps, Austria) and their role in the formation of the Felbertal scheelite deposit

The W mineralised Early Carboniferous orthogneisses (K1 and K3 orthogneiss) in the Felbertal scheelite deposit represent a chemically evolved metagranitoid series. Some of its characteristics are high concentrations of F (<4438ppm), Nb (<86ppm), Ta (<13ppm), and U (<74ppm) and REE patterns with distinct negative Eu-anomalies (Eu/Eu*=0.24–0.48) and increasing HREE concentrations (LuN/HoN=1.93–2.81). The systematic chemical trends documented for a multitude of elements (e.g., SiO2, TiO2, P2O5, Ba, Nb, Ta) and their respective ratios (e.g., 1/TiO2, Nb/Ta, Zr/Hf) indicate that crystal-melt fractionation controlled the evolution of the granitic melts. The higher differentiated, peraluminous light-coloured K1-K3 variety (ASI=0.99–1.08, Nb/Ta=5–7, Zr/Hf=13–18) evolved from the less differentiated, metaluminous dark-coloured variety (ASI=0.93–1.03; Nb/Ta=6–10, Zr/Hf=18–24). Peraluminous holo-leucocratic aplite gneiss represents the most evolved member of the series (ASI=1.11–1.12, Nb/Ta=4, Zr/Hf=9–10). Modelling of magmatic differentiation assuming Rayleigh fractionation shows that c. 70–90% of the residual granitic magma had crystallised at the time of the emplacement of the aplites. When compared to barren metagranitoids in the central Tauern Window (“Zentralgneis”), the metaluminous dark-coloured K1-K3 orthogneiss shows some geochemical similarities with the peraluminous Felbertauern augengneiss, one of the regional orthogneisses exposed near the W deposit. Elevated concentrations of Nb (<36ppm), Ta (<5.3ppm) and U (<30ppm) distinguish it from other regional Zentralgneis types and illustrate its genetic relation with the K1-K3 orthogneiss. We propose that the Felbertauern augengneiss represents a peraluminous granitic melt, generated by melting of a source assemblage containing hydrous F-bearing minerals (i.e., biotite). Progressive dehydration melting of the same (or a similar source) at higher temperature involving a Ca-phase (i.e., hornblende) produced melt batches of metaluminous composition; i.e., dark-coloured K1-K3 orthogneiss. An alternative model for explaining the unusual chemical characteristics of the latter would be entrainment and separation of a peritectic and restitic accessory mineral assemblage co-existing with the granitic melt. Higher contents of fluxing elements such as F in the melt may have been responsible for lowering the solidus temperature of the granitic melts allowing concentration of W and other elements in the residual melts and later on in the exsolved magmatic hydrothermal fluids. These processes were a pre-requisite for the development of highly specialised W-rich granitic melts and formation of the world-class Felbertal scheelite deposit.

Copper partitioning between granitic silicate melt and coexisting aqueous fluid at 850 °C and 100 MPa

Experiments on the partitioning of Cu between different granitic silicate melts and the respective coexisting aqueous fluids have been performed under conditions of 850 °C, 100 MPa and oxygen fugacity (fO2) buffered at approaching Ni–NiO (NNO). Partition coefficients of Cu (DCu = cfluid/cmelt) were varied with different alumina/alkali mole ratios [Al2O3/(Na2O + K2O), abbreviated as Al/Alk], Na/K mole ratios, and SiO2 mole contents. The DCu increased from 1.28 ± 0.01 to 22.18 ± 0.22 with the increase of Al/Alk mole ratios (ranging from 0.64 to 1.20) and Na/K mole ratios (ranging from 0.58 to 2.56). The experimental results also showed that DCu was positively correlated with the HCl concentration of the starting fluid. The DCu was independent of the SiO2 mole content in the range of SiO2 content considered. No DCu value was less than 1 in our experiments at 850 °C and 100 MPa, indicating that Cu preferred to enter the fluid phase rather than the coexisting melt phase under most conditions in the melt-fluid system, and thus a significant amount of Cu could be transported in the fluid phase in the magmatic-hydrothermal environment. The results indicated that Cu favored partitioning into the aqueous fluid rather than the melt phase if there was a high Na/K ratio, Na-rich, peraluminous granitic melt coexisting with the high Cl− fluid.

Petrology and geochemistry of Jura granite and granite gneiss in the Neelum Valley, Lesser Himalayas (Kashmir, Pakistan)

Late Proterozoic rocks of Tanol Formation in the Lesser Himalayas of Neelum Valley area are largely green schist to amphibolite facies rocks intruded by early Cambrian Jura granite gneiss and Jura granite representing Pan-African orogeny event in the area. These rocks are further intruded by pegmatites of acidic composition, aplites, and dolerite dykes. Based on field observations, texture, and petrographic character, three different categories of granite gneiss (i.e., highly porphyritic, coarse-grained two micas granite gneiss, medium-grained two micas granite gneiss, and leucocratic tourmaline-bearing muscovite granite gneiss), and granites (i.e., highly porphyritic coarse-grained two micas granite, medium-grained two micas granite, and leucocratic tourmaline-bearing coarse-grained muscovite granite) were classified. Thin section studies show that granite gneiss and granite are formed due to fractional crystallization, as revealed by zoning in plagioclase. The Al saturation index indicates that granite gneiss and granite are strongly peraluminous and S-type. Geochemical analysis shows that all granite gneisses are magnesian except one which is ferroan whereas all granites are ferroan except one which is magnesian. The CaO/Na2O ratio (>0.3) indicates that granitic melt of Jura granite gneiss and granite is pelite-psammite derived peraluminous granitic melt formed due to partial melting of Tanol Formation. The rare earth element (REE) patterns of the Jura granite and Jura granite gneiss indicate that granitic magma of Jura granite and Jura granite gneiss is formed due to partial melting of rocks that are similar in composition to that of upper continental crust.

Nb-Ta fractionation in peraluminous granites: A marker of the magmatic-hydrothermal transition

In their late stages of evolution, peraluminous granitic melts exsolve large amounts of fluidswhich can modify the chemical composition of granitic whole-rock samples. The niobium/tantalum (Nb/Ta) ratio is expected to decrease during the magmatic differentiation of graniticmelts, but the behavior of both elements at the magmatic-hydrothermal transition remainsunclear. Using a compilation of whole-rock geochemical data available in the literature, wedemonstrate that fractional crystallization alone is not sufficient to explain the distribution ofNb-Ta in most peraluminous granites. However, we notice that most of the granitic samplesdisplaying evidence of interactions with fluids have Nb/Ta < 5. We propose that the decreaseof the Nb/Ta ratio in evolved melts is the consequence of both fractional crystallization andsub-solidus hydrothermal alteration. We suggest that the Nb/Ta value of ~5 fingerprints themagmatic-hydrothermal transition in peraluminous granites. Furthermore, a Nb/Ta ratio of ~5appears to be a good marker to discriminate mineralized from barren peraluminous granites.

The effect of phosphorus on manganocolumbite and mangaotantalite solubility in peralkaline to peraluminous granitic melts

Solubility experiments of Mn-columbite (MnNb2O6) and Mn-tantalite (MnTa2O6) were conducted under water-saturated conditions in synthetic haplogranitic melts containing different amounts of phosphorus at 800 °C and 100 MPa. All experiments were carried out in cold-seal rapid quenching pressure vessels (RQV) with water as a pressure medium. Experimental results show that: (1) the solubilities of MnNb2O6 and MnTa2O6 in peralkaline melts are higher than those in peraluminous melts; (2) phosphorus has strong influence on the solubilities of MnTa2O6 and MnNb2O6 in peralkaline melts, KSpNb and KSpTa decrease from 104.89 × 10−4 mol2/kg2 and 107.62 × 10−4 mol2/kg2 for melts without P2O5 to 16.11 × 10−4 mol2/kg2 and 7.96 × 10−4 mol2/kg2 for melts containing ~4.00 wt% P2O5, respectively; (3) phosphorus has less influence on the solubilities of MnTa2O6 and MnNb2O6 in peraluminous melt, KSpNb decrease from 4.50 × 10−4 mol2/kg2 for melts without P2O5 to 0.73 × 10−4 mol2/kg2, and KSpTa from 3.57 × 10−4 mol2/kg2 to 0.14 × 10−4 mol2/kg2 for melts contaning ~5.00 wt% P2O5. Taking the structural role of phosphorus into account, P decreases the solubility of Mn-columbite and Mn-tantalite via competing for network modifiers.

An experimental determination of W, Nb, and Ta partition coefficients between P-rich peraluminous granitic melt and coexisting aqueous fluid

The partition coefficients of W, Nb, and Ta between the P-rich peraluminous granitic melt and the coexisting aqueous fluid were determined at 800–850 °C and 0.5–1.5 kbar. The experimental results showed that the partition coefficients DW, DNb, and DTa (Div/m = Civ/Cim, where Civ and Cim denote the concentrations of an element, i, in the aqueous fluid and the melt, respectively) were less than 0.1. All partition coefficients were affected by pressure, but there was no evidence for the complexation of P2O5 with these elements in the granitic melt or aqueous fluid, except for with W in the fluid. The results showed that W, Nb, and Ta tended to partition into the granitic melt and, in the late period of crystallization of P-rich magma, they formed independent minerals.

The hot back-arc zone of the Araçuaí orogen, Eastern Brazil: from sedimentation to granite generation

This article presents new lithochemical and geochronological data obtained from gneisses and granites occurring in the region located to the east of the Rio Doce calc-alkaline arc (630 - 580 Ma), which corresponds to the back-arc basin of the Aracuai orogen. The Nova Venecia Complex, represents the most fertile source of peraluminous granitic melts in the studied back-arc zone. It mostly consists of migmatitic Al-rich paragneisses, ranging from biotite-rich gneisses to biotite-free cordierite-rich granulites, whose main protoliths were graywacky sediments. An EW-oriented section across the northern back-arc region reveals a zone rich in cordierite granulites of the Nova Venecia Complex at the base, followed by migmatites that gradually pass to the Ataleia foliated granites rich in metasedimentary enclaves, which in turn lay beneath the Carlos Chagas batholith. To the south of the Carlos Chagas batholith, orthopyroxene-bearing rocks often occur in both the Nova Venecia Complex and the Ataleia Suite, suggesting a deeper crustal level. Our U-Pb data suggest that melting processes started on the Nova Venecia Complex during the late development of the Rio Doce arc, around 590 Ma, forming autochthonous peraluminous melts related to the Ataleia Suite. Progressive anatexis and melt accumulation attained the climax around 575 Ma, leading to the development of the syn-collisional Carlos Chagas batholith. Around 545 - 530 Ma, a late to post-collisional anatectic episode formed garnet-cordierite leucogranites, mostly from the re-melting of the Ataleia and Carlos Chagas granites. A remarkable post-collisional plutonism caused widesperead re-heating of the back-arc domain from ca. 520 Ma to 480 Ma. This long lasting history (ca. 110 Ma) of granite generation in the back-arc zone requires distinct heat sources, such as asthenosphere ascent under the back-arc region in the pre-collisional stage, thrust stacking of the hot arc onto the back-arc, radiogenic heat release from the collisional thickened crust and, finally, asthenosphere uprising during the gravitational collapse of the Aracuai orogen.

Solubility of molybdenite in hydrous granitic melts at 800 °C, 100–200 MPa

The solubility of molybdenite, MoS2, in fluid-saturated, subaluminous to peraluminous granitic melts was determined experimentally using rapid-quench cold-seal pressure vessels at 800°C and 100–200MPa, and analysis by laser-ablation ICP-MS. Molybdenite solubility seems to be independent of pressure, but it shows strong variations with oxygen and sulfur fugacity. At constant log fS2=−1.3 it increases from 0.1–0.7ppm by weight Mo at the Co–CoO buffer to 29–38ppm by weight Mo near the MnO–Mn3O4 buffer. The solubility isopleths are nearly parallel to the pyrrhotite–magnetite equilibrium, along which the solubility varies only slightly, from 10 ppmw Mo at the quartz–fayalite–magnetite buffer to 29–38ppmw Mo at the MnO–Mn3O4 buffer. The observed solubility variations are consistent with the equilibrium MoS2(s)+3/2O2=MoO3(l)+S2 and thus confirm that molybdenum(VI) oxide is the predominant species in subaluminous silicate melts at log fO2=−16 to −11. In addition, the experimental results are well reproduced by a simple thermodynamic model employing the Burnham eight-oxygen formulation for silicate melt species and assuming ideal mixing of dissolved MoO3. The thermodynamic calibration can be used to estimate the molybdenum solubility in subaluminous silicic melts or, for pyrrhotite- and molybdenite-saturated assemblages, the oxygen and sulfur fugacities during magma crystallization.

THE PUENTEMOCHA BERYL-PHOSPHATE GRANITIC PEGMATITE, SALAMANCA, SPAIN: INTERNAL STRUCTURE, PETROGRAPHY AND MINERALOGY

The Puentemocha pegmatite, located in the Tormes Dome of the Central Iberian Zone (Salamanca, Spain), is a beryl–phosphate granitic pegmatite hosted by a Variscan anatectic leucogranite. It shows a zonal internal structure with a wall zone, an intermediate zone, and a prominent quartz core. The essential minerals of the pegmatite are quartz, K-feldspar, plagioclase, muscovite, biotite and beryl. Phosphates, pyrite, arsenopyrite and rutile occur as accessory minerals associated with greisen zones developed in the intermediate zone. The gradational contact and mineral association attest to the intimate petrogenetic relation between the Puentemocha pegmatite and its host granite. Petrographic, mineralogical and chemical data suggest a crystallization from the border inward at temperatures in the range of 700–500°C, under f (O2) conditions at ΔFMQ = ~−0.5, and H2O contents of 6–7.5 wt% at the level of emplacement. As there is no evidence of residual melt extraction, the in situ fractionation of a peraluminous granitic melt is presumed to be sufficient to attain beryl saturation. The greisen bodies developed under subsolidus conditions, at the expense of the intermediate zone. A later episode of Na-metasomatism affected unevenly the primary zones of the pegmatite and the greisen bodies.

Experimental study on tin partition between granitic silicate melt and coexisting aqueous fluid

The distribution of tin between different granitic silicate melts and coexisting aqueous fluids have been determined at 850°C, 100 MPa and fO2 near NNO. Tin distribution coefficients DSnaq.fl./melt are constrained by the composition of melt. When the starting fluid is 0.1 M HCl, DSnaq.fl./melt decreases rapidly from 0.130 ± 0.090∼0.137 ± 0.016 to 4.22 (±0.47) × 10-3 with the Na2O + K2O mole content and ASI of melt varying from 6.55 to 9.39% and 1.37 to 0.66, respectively, and tin concentrations in aqueous fluids and DSnaq.fl./melt decrease from 9.26 ± 0.56 to 0.220 ± 0.013 μg/g and 3.70 (±0.42) × 10-2 to 1.29 (±0.15) × 10-3, respectively with Na/K mole ratio increasing from 0.43 to 1.57 and nearly constant ASI from 1.04 to 1.10. A series of experiments that use the same K-rich peraluminous haplogranitic melt as starting solid whereas investigate the effects of chlorine and fluorine, the presence of chlorine in aqueous fluids increases DSnaq.fl./melt because of tin complexing, the presence of fluorine in the starting fluid does not significantly influence on DSnaq.fl./melt. For the chlorine experiments, DSnaq.fl./melt shows a positive dependence on the concentration of HCl in the starting fluid with log DSnaq.fl./melt = 2.0247 × log[HCl] + 0.6717 ([HCl] unit is M), and SnCl2 is the dominant tin-bearing complex in aqueous fluid. The results of this experiment showed that the peralkalinity of granitic melt and F-bearing in granite can lead to the enrichment of tin in melt phase, and, therefore, they probably serve as a tin ore reservoir or an important transport medium for tin ore formation, whereas K-rich peraluminous granitic melt and high HCl concentrations in the aqueous fluid phase are favorable for tin partitioning in aqueous fluid.

Biotite stability in peraluminous granitic melts: Compositional dependence and application to the generation of two-mica granites in the South Bohemian batholith (Bohemian Massif, Czech Republic)

Crystallization experiments have been conducted in the system Na 2O–K 2O–MgO–FeO–Al 2O 3–SiO 2–H 2O (with 4% normative corundum) in order to constrain the stability of biotite as a function of water activity and the Mg# of biotite [Mg/(Mg +Fe total)] in equilibrium with peraluminous granitic melts. The temperature at which biotite breakdown starts is strongly dependent on the Mg# of biotite. At 500 MPa, the temperature of biotite breakdown to form orthopyroxene increases from 750 °C to 830 °C, as the Mg# of biotite increases from 0.4 to 0.5. Considering that the system investigated is relevant for Ca-poor peraluminous biotite-bearing rocks (metapelites), the biotite dehydration curves obtained are used to discuss the melting reactions and the temperatures that lead to the formation of two distinct types of two-mica granites found in the South Bohemian batholith (specifically the Eisgarn and Deštná granites). The phase relationships were determined experimentally for the composition of these two granites in order to constrain the composition of the biotite in equilibrium with the melt in the protoliths. We demonstrate that Eisgarn granitic melts may have been generated at temperatures in the range 830–850 °C from melting reactions involving biotite with a Mg# up to 0.5 as a reactant. In contrast, Deštná granitic melts cannot have been generated from dehydration melting reactions involving biotite.

Fluorapatite-monazite relationships in granulite-facies metapelites, Schwarzwald, southwest Germany

Abstract Fluorapatite grains with monazite inclusions and/or rim grains are described in two of four samples from a set of granulite-facies metapelites collected from the Variscan Schwarzwald, southern Germany. Fluorapatite in all four samples appears to have experienced some dissolution in the partial granitic melt formed during granulite-facies metamorphism. Monazite inclusions and rim grains are highly deficient in Th and are presumed to have formed from fluorapatite in association with partial melting during granulite-facies metamorphism. Monazite inclusions range from very small (&lt;1 μm) and very numerous to small (1–2 μm), sometimes elongated, and less numerous; both types are evenly distributed throughout the fluorapatite grain interior. Monazite rim grains tend to be 1–10 μm. The formation of monazite inclusions is proposed to be due to dissolution-reprecipitation of the fluorapatite by the aqueous fluids inherent in the granitic melt. We propose that an increase in inclusion size coupled with a decrease in inclusion number is due to Ostwald ripening (interfacial energy reduction), which is greatly facilitated by the presence of an interconnected, fluid-filled porosity in the metasomatized fluorapatite. We further propose that monazite rim grains formed principally during partial dissolution of the fluorapatite in the granitic melt and to a lesser extent by partial dissolutionreprecipitation of the fluorapatite grain rim area allowing for the partial removal of (Y+REE). We conclude that fluorapatite, with monazite inclusions and rim grains, experienced partial dissolution in a H2O-rich peraluminous granitic melt compared to fluorapatite with monazite rim grains and no inclusions which reacted with a similar, relatively less H2O-rich melt. In contrast, monazite-free fluorapatite experienced partial dissolution in a comparatively H2O-poor, subaluminous, possibly peralkaline melt.