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Silica-rich granites and rhyolites are components of igneous rock suites found in many tectonic environments, both continental and oceanic. Silica-rich magmas may arise by a range of processes including partial melting, magma mixing, melt extraction from a crystal mush, and fractional crystallization. These processes may result in rocks dominated by quartz and feldspars. Even though their mineralogies are similar, silica-rich rocks retain in their major and trace element geochemical compositions evidence of their petrogenesis. In this paper we examine silica-rich rocks from various tectonic settings, and from their geochemical compositions we identify six groups with distinct origins. Three groups form by differentiation: ferroan alkali-calcic magmas arise by differentiation of tholeiite, magnesian calc-alkalic or calcic magmas form by differentiation of high-Al basalt or andesite, and ferroan peralkaline magmas derive from transitional or alkali basalt. Peraluminous leucogranites form by partial melting of pelitic rocks, and ferroan calc-alkalic rocks by partial melting of tonalite or granodiorite. The final group, the trondhjemites, is derived from basaltic rocks. Trondhjemites include Archean trondhjemites, peraluminous trondhjemites, and oceanic plagiogranites, each with distinct geochemical signatures reflecting their different origins. Volcanic and plutonic silica-rich rocks rarely are exposed together in a single magmatic center. Therefore, in relating extrusive complements to intrusive silica-rich rocks and determining whether they are geochemically identical, comparing rocks formed from the same source rocks by the same process is important; this classification aids in that undertaking.

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 presence of recently intruded granites at Earth's surface suggests that their exhumation may have occurred rapidly. The Neogene granites of the Tuscan Magmatic Province (Italy) were emplaced during a period of extensional tectonics and are ideal for determining and quantifying the exhumation process. The peraluminous monzogranite of Giglio Island in the northern Tyrrhenian Sea is characterized by the presence of roof pendants, xenoliths and miarolitic cavities. The petrologic study of metamorphic xenoliths and new zircon U-Pb ages show that the granite was emplaced at 6.4-10 km depth at 5.7 ± 0.4 Ma. Exhumation, constrained by apatite (U-Th)/He ages, was essentially complete in 0.9 Myr at a minimum rate of 6 mm/year. This requires rapid tectonic unroofing, isostatic rebound and thermal softening activity, weakening the upper crust and favouring exhumation at a previously undocumented rate.

The Penouta Sn-Ta deposit, in the northwest of Spain, is a greisenized granitic cupola where Ta minerals occur mainly as disseminations in a leucogranite body intruded in Precambrian-Lower Cambrian gneisses and mica-schists. This leucogranite is a medium- to fine-grained inequigranular rock consisting mainly of quartz, albite, K-feldspar and muscovite. Accessory minerals are mainly of spessartine, zircon, cassiterite, Nb-Ta oxides, monazite, xenotime, native bismuth and pyrite. The alteration processes were mainly albitization, muscovitization and kaolinitization. This leucogranite is peraluminous and P-poor, with 0.03-0.07 wt.% P2O5, 900-1500 ppm Rb, 30-65 ppm Cs, 120-533 ppm Li, 80-140 ppm Ta, 51-81 ppm Nb and up to 569 ppm of Sn. Mineralogical characterization of Nb-Ta oxide minerals was determined by X-ray diffraction, scanning electron microscopy, electron microprobe analysis and mineral liberation analysis. Mn-rich members of the columbite-group minerals (CGM) are the most common Ta-bearing phases, but microlite, wodginite, tapiolite and Ta-rich cassiterite occur also. CGM crystals are commonly zoned concentrically, with a Nb-rich core surrounded by a Ta-rich rim, with a sharp boundary between them. Convoluted zoning occurs also. Dissolution textures resulting from the corrosion of columbite and tantalite rims, in particular, are common. The Mn/(Mn + Fe) ratio varies between 0.33 and 0.97 and the Ta/(Ta + Nb) ratio between 0.07 and 0.93. Wodginite and microlite formed as late replacements of CGM and occur associated with tantalite and cassiterite. Subhedral to anhedral cassiterite crystals, usually up to 200 µm across, occur in two generations: the earlier one is Nb,Ta-poor whereas in the later generation, the Ta content can reach >9 wt.% of Ta2O5 and 1.7 wt.% of Nb. The presence of a fluid phase in the apical zone of the granite, probably related to the separation of a fluid/vapour of the melt, could explain the sponge-like textures, the Ta enrichment associated with these textures, the occurrence of Ta-enriched mineral phases (microlite and wodginite) and their common interstitial character.

Previous studies have shown that both NBO content and modifier cation field strength play important roles in increasing the network cation coordination with increasing pressure. It has been observed in previous studies that the increase in average Al coordination with pressure in alkali aluminosilicates depends on NBO concentration, where large increases in Al coordination with pressure have been observed for compositions containing significant concentrations of NBO and little or no Al coordination increase observed in glasses containing negligible NBO at pressures ranging from 1 to 3 GPa. Similarly, in NBO rich aluminosilicates and aluminoborosilicates containing different modifier cations, it was reported that the increase in average Al coordination followed a steeper rise with increasing pressure in compositions containing higher field strength modifiers. In this study, we look at Ca- and Mg-aluminosilicate glasses across all three compositional regimes (peralkaline, metaluminous, and peraluminous) to study the effect of both oxygen speciation and modifier cation field strength on network cation coordination changes with pressure. Our study shows that in Mg aluminosilicate glasses (both peralkaline and metaluminous), the increase in average Al coordination can be quite large and show no significant impact from differences in oxygen speciation (NBO content). In contrast, in Ca-aluminosilicate glasses, the oxygen speciation has a notable impact with the average Al coordination following a steeper rise with increasing pressure in a peralkaline composition and less steep for a metaluminous composition.

Detrital zircons as old as nearly 4.4 Ga offer insights into the earliest moments of Earth history. Results of geochemical investigations of these grains have been interpreted to indicate their formation in near-H2O saturated meta- and peraluminous magmas under a relatively low (15-30 °C/km) geotherm. A key feature in pursuing a petrotectonic model that explains the full spectrum of these observations is their seeming contrast to most Phanerozoic magmatic zircons, specifically their low Ti-in-zircon crystallization temperatures and inclusion assemblages. The ∼22 Ma Arunachal leucogranites of the eastern Himalaya appear, however, to be a rare exception to this generality. They show large-ion lithophile covariance trends indicative of wet basement melting together with a normal distribution of magmatic crystallization temperatures about an average of 660 °C. In the same fashion as Hadean zircons, Arunachal leucogranite and host gneiss zircons are dominated by muscovite + quartz inclusions that yield formation pressures of 5-15 kbars. We suggest that the Arunachal leucogranites originated in the hanging wall of a megathrust that carried H2O-rich foreland sediments to depths of >20 km whereupon de-watering reactions released fluids that fluxed hanging wall anatexis. Modeling suggests the thermal structure of this continental collision environment may have been broadly similar to a Hadean ocean-continent subduction zone. The similarity of these two environments, separated by over 4 Ga may explain seemingly common features of the Hadean and Arunachal leucogranite zircons. Their key difference is the absence of metaluminous magmas in the continental collision environment, which is shielded from juvenile additions.

In the Variscan fold belt of Morocco, the Jebilet massif is characterized by Palaeozoic metasedimentary rocks intruded by syntectonic magmatism that includes an ultramafic–granitoid bimodal association and peraluminous granodiorites emplaced c. 330 Ma, intruded by younger leucogranites c. 300 Ma. The mafic–ultramafic rocks belong to a tholeiitic series, and display chemical and isotopic signatures consistent with mixing between mantle-derived and crust-derived magmas or assimilation and fractional crystallization. The granites within the bimodal association are mainly metaluminous to weakly peraluminous microgranites that show characteristics of A2-type granites. The peraluminous, calc-alkaline series consists mainly of cordierite-bearing granodiorites enclosing magmatic microgranular enclaves and pelitic xenoliths. Detailed element and isotope data suggest that the alkaline and the peraluminous granitoids were formed in the shallow crust (<30 km) by partial melting of tonalitic sources at high temperatures (up to 900°C) and by partial melting of metasedimentary protoliths at relatively low temperatures (c. 750°C), respectively. Mixing between the coeval mantle-derived and crust-derived magmas contributed to the large variation of initial εNd values and initial Sr isotopic ratios observed in the granitoids. Further contamination occurred by wall-rock assimilation during ascent of the granodioritic plutons to the upper crust. The ultramafic–granitoid association has been intruded by leucogranites that have high initial Sr isotopic ratios and low initial εNd values, indicating a purely crustal origin. The heating events that caused emplacement of the Jebilet magmatism are related to cessation of continental subduction and convective erosion/thinning of the lithospheric mantle during plate convergence.

A comprehensive geochronological and geochemical study was carried out on the gneissic monzogranites, porphyritic granodiorites and charnockites in the Gaozhou complex of the Yunkai massif in the southern part of the South China block to better understand the Early Palaeozoic tectonic regime of the South China block. Laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) U–Pb dating of zircons indicates an age of 453.2 ± 5.1 Ma to the formation of the gneissic monzogranites, whereas the porphyritic granodiorites and charnockites were generated at 437.0 ± 1.5 Ma and 435.2 ± 2.2 Ma, respectively. The gneissic monzogranites show geochemical features consistent with the high-K, calc-alkaline rock series and are strongly peraluminous. They have SiO2 contents ranging from 67.75 to 69.65 wt. % and relatively low CaO contents (1.66–1.94 wt. %). Their REE patterns are fractionated with enriched LREEs and negative Eu anomalies. The samples also show enrichment in LILEs (e.g. Rb and K) and Pb, and depletion in Sr, Ba and HFSEs (e.g. Nb, Ta, Ti and P). They have εNd(t) values of −8.2 to −7.7. Conversely, the porphyritic granodiorites and charnockites are characterized as medium-K, calc-alkaline rock series and weakly to strongly peraluminous. They exhibit pronounced depletions in HFSEs and positive Pb anomalies. Compared to the earlier gneissic monzogranites, these rocks have relatively lower SiO2 (65.50–69.36 wt. %), but higher CaO contents (3.34–4.05 wt. %), and have slightly lower εNd(t) values (−9.1 to −8.4). Petrography and geochemical compositions of the gneissic monzogranites indicate that they are S-type granite and likely formed by partial melting of Neoproterozoic to Early Palaeozoic immature metagreywackes; whereas The porphyritic granodiorites and charnockites are A-type granite and likely derived from low degrees of partial melting of the dry, granulitic residue depleted by prior extraction of granitic melt. The new data for the Caledonian granitoids in the Yunkai massif suggest that they were formed in a post-collisional tectonic setting. They represent the earliest post-collisional alkaline magmatism reported so far in the Yunkai massif, and thus indicate a tectonic regime switch, from compression to extension, as early as the Late Ordovician to Early Silurian (~450–435 Ma).

In this study we report laser ablation-inductively coupled plasma-mass spectrometer U-Pb ages of granitoids from the so-called early granodiorites of the northwest Iberian Variscan belt. The U-Pb results attest to significant magmatic activity in Visean time (ca. 347-337 Ma) that generated a hitherto poorly constrained granitoid suite in the northwest Iberian tract of the western European Variscan belt realm. This early Carboniferous suite (ECS) is mainly composed of peraluminous cold and hot crustal granodiorites and monzogranites with minor associated mafic rocks that attest to minor involvement of mantle melting. Based on the geochronological and geochemical data, we compare the Visean granitoids with younger Variscan granitoids in northwest Iberia and, in view of the tectonothermal scenarios of the Variscan collision in northwest Iberia, propose a model for the genesis of the ECS in northwest Iberia that involves rapid melting upon fast exhumation of the thickened Gondwanan crust in the course of the protracted Variscan collision.

Late Carboniferous magmatism in the Chinese Altai provides an important view of geodynamic processes active during crustal growth in the Central Asian Orogenic Belt (CAOB). In this study, five representative peraluminous granite plutons from the Chinese Altai were selected for systematic geochronological, geochemical and Sr–Nd–Hf isotopic analyses (Table 1). These granites were emplaced between 449 and 327 Ma in an active subduction zone, and have moderate to high SiO2 (66.54–76.13 wt%), moderate Na2O+K2O (6.27–7.66 wt%), and high Al2O3 contents (12.43–16.18 wt%). All granite samples in this study showed significant decoupling of the Nd and Hf isotope systems. Results show negative εNd(t) values (−3.3 to −0.9), and predominantly positive εHf(t) values (+0.24 to +8.01, n=57) except for a few negative εHf(t) values (−7.44 to −0.03, n=9), high Mg# values (28.69–53.33), high Nd/Hf ratios (4.26–43.57), and enrichment of large-ion lithophile elements (LILEs; e.g. Pb, Th, and U), suggesting that the granites were derived from the partial melting of oceanic sediments and the associated mantle wedge, with fractionation of plagioclase, K-feldspar and biotite. In situ zircon Hf isotopic analyses yield negative εHf(t) values from −30.6 to −13.7 for the zircon xenocrysts. The U–Pb ages and Hf isotopic ratios of these zircon xenocrysts were probably inherited from oceanic sediments. Zircon saturation temperatures suggest that these peraluminous granites were emplaced at 537–765°C. We propose that: (1) the Nd isotopic system more faithfully reflects the source of peraluminous magmas in the Chinese Altai than the Hf isotopic system, and (2) the oceanic sediment recycling was an important process during continental growth in the CAOB.