Editorial for Special Issue “Mineral Chemistry of Granitoids: Constraints on Crystallization Conditions and Petrological Evolution”

The origin of granitoids has fascinated geologists since the famous meeting of the Geological Society of France in 1847 [1]. This interest stems from the fact that granitic rocks are the most abundant rock type in the upper continental crust, with their source rocks located in the lower crust and/or upper mantle. Moreover, they are important to geologic evolution, as their diversity has been interpreted to result from tectonic processes and the compositional variability of the crust. Granitic rocks are primarily composed of quartz, alkali feldspar, and plagioclase, which together define their fundamental felsic mineralogical framework. Additionally, these rocks may contain smaller and variable amounts of biotite, amphibole, and/or pyroxenes, which contribute to their mineralogical diversity and reflect variations in their magmatic and tectonic histories. Granitoids can include a large number of accessory phases such as tourmaline, garnet, apatite, zircon, apatite, titanite, epidote, topaz, monazite, and metal ore minerals. Deposits of precious and rare metals, such as Au, Mo, Sn, and many others, are directly or indirectly associated with granites.

The mineral chemistry of the major mineral phases constitutes an important tool for assessing the geochemical affinities and magma sources of the host granitoid and estimating the crystallization conditions of the magma from which they originated [2,3,4,5,6,7,8]. The chemical composition of accessory minerals also contributes to the understanding of the crystallization conditions of granitic magmas. The titanite composition is used to estimate pressure and temperature [9]; Fe-Ti oxide compositions are dependent on the redox conditions and magma nature [10] and were used in the process of granite classification [11]; epidote has been used to estimate the crystallization pressure, fO₂, and ascension rate of epidote-bearing magmas [12,13]. It is important to highlight zircon because, in addition to its important geochronologic and isotopic tracer (U-Pb/Lu-Hf), its trace elements composition has been the subject of numerous studies to infer the physicochemical conditions of magma, the components involved in the source, and hydrothermal alterations [14,15,16,17], among other factors.

This Special Issue “Mineral Chemistry of Granitoids: Constraints on Crystallization Condition and Petrological Evolution” includes ten contributions on mineral chemistry, whole-rock geochemistry, isotopic chemistry, tectonic setting, and mineral deposits associated with granitic rocks.

The paper by El-Fatah et al. (Contribution 1) presents a detailed study of the Neoproterozoic post-collisional El Bakriyah granitic ring complex, syenogranite core, and alkali feldspar granite rim, intruded into monzogranites, from the Central Eastern Desert in Egypt. The studied granites are peraluminous, ferroan, and A-type, and they have F, B, Nb, and Ta in considerable concentrations, either in the form of rare metal dissemination or as veins of fluorite and barite. The authors concluded that the granitic magma was generated by high-T dehydration melting of a mixed crust–mantle source, primarily composed of metasediments and amphibolite. The formed magma underwent high fractionation, producing felsic magma emplaced in a within-plate setting. The monzogranitic country rock was emplaced in the transition between the arc and anorogenic settings.

The paper by Zhao et al. (Contribution 2) deals with the study of a highly differentiated I-type composite granitic pluton from Southern Jiangxi Province in China and its relationship with mineralization. Whole-rock geochemistry and the chemistry of biotite and zircon were used to analyze the rock-forming physiochemical and the genetic type of granite. The achieved temperatures (774–777 °C) were lower than those of A-type granites but were close to those reported for highly differentiated I-type granites. The three zircon U-Pb crystallization ages of the two granites were similar: 152 ± 1 Ma, 151 ± 1 Ma, and 149 ± 1 Ma. The authors concluded that the studied granites were formed during Late Jurassic large-scale magmatic activity in an extensional tectonic setting, following the subduction of the Pacific Plate into the Nanling hinterland during the post-orogenic stage. They also found that the high differentiation, high F content, and low oxygen fugacity recorded in the studied granites are conducive to the large-scale mineralization of Sn, Mo, and fluorite.

The paper by Li et al. (Contribution 3) analyzes LA-ICP-MS zircon U-Pb ages and whole-rock geochemistry data from Mo-associated granitoids of the Aolunhua intrusion in Inner Mongolia, NE China, and discusses the Mo-associated granitoids with granite porphyry and the source of the ore-forming rocks of the deposit. The zircon data reveals the crystallization age of the studied granite at 135 ± 1 Ma, correlating it with the widespread Yanshanian intermediate–felsic magmatic activity. Based on the crystallization age, zircon trace element data, and whole-rock geochemistry, the authors concluded that the granite porphyry was formed by the crystallization of crust-derived magmas during a transitional tectonic setting, from compression orogeny to back-arc extension. The Aolunhua ore deposit is within the Cu-Mo metallogenic belt at the northern flank of the Xilamulun River deep fracture. Based on a comparison with other Mo deposits along the banks of the Xilamulun River, the authors proposed that the Tianshan–Linxi constitutes an important Mo metallogenic belt.

Cathelineau et al. (Contribution 4) present a study on two aplite and associated pegmatite dyke swarms from Segura, Portugal. The studied granitoids have high concentrations of fluxing elements (Li, P, F), ranging from 1.5 to 5.0 compared to highly differentiated peraluminous granites, and present a wide variety of Li, Na, Fe-Mn, and Ca-Sr phosphates. Micro-X-ray fluorescence chemical imaging was used to establish the phosphate crystallization sequence. The magmatic differentiation resulted in a P- and Li-rich melt, with the primary crystallization of the amblygonite–montebrasite series and Fe-Mn phosphates. The primary phosphates were replaced by lacroixite during a stage of high Na activity, and external Ca- and Sr-rich hydrothermal fluids replaced the primary Li-Na phosphates with phosphates from the goyazite–crandallite series, followed by apatite formation.

The paper by Jiang et al. (Contribution 5) presents the mineral chemistry of K-feldspar, plagioclase, and biotite, along with Hf zircon isotopic data, Sr-Nd isotopes of the whole rock, and zircon trace element data to discuss the source and physicochemical conditions during the formation of Neoproterozoic Bure adakitic rock in the Western Ethiopian Shield. Based on the results, the authors concluded that the magma was emplaced at a depth of approximately 6.39~10.2 km (1.75~2.81 kbar), under relatively high oxygen fugacity (logfO₂ varying from −18.5 to −4.9), with a crystallization temperature ranging from 659 to 814 °C. They also concluded that the magma was generated by the melting of a Neoproterozoic juvenile crust, coeval with early magmatic stages in the Arabian Nubian Shield.

Ghoneim et al. (Contribution 6) present a systematic study of Neoproterozoic alkali feldspar granite from the Arabian Nubian Shield. This study involved petrography, mineral chemistry, and whole-rock geochemistry. These granites contain significant rare metal mineralization, including thorite, uranothorite, columbite, zircon, monazite, xenotime, pyrite, rutile, and ilmenite. The studied granites are highly fractionated peraluminous, with calc-alkaline affinity and A-type characteristics post-collision, emplaced under an extensional regime of within-plate environments. The authors concluded that the granitic magma evolved through a significant degree of fractionation and that coeval basaltic magmas supplied the necessary heat to melt the crust and provided volatile substances that seeped into the lower crust, resulting in the formation of A-type granite through partial melting of the crust.

Liu et al. (Contribution 7) present a systematic study of A-type Mesoproterozoic porphyritic granites on the northern margin of the North China Craton, including whole-rock geochemistry, zircon Hf isotopes, and zircon U-Pb geochronological data. Two of the studied granites have similar crystallization ages of 1285 ± 3 Ma and 1279 ± 6 Ma. The whole-rock geochemistry classified the studied granites as weakly peraluminous A₂-type granites. The Hf isotopic data suggest that the magmas were derived by partial melting of ancient crustal material. They concluded that the studied granites formed in an intraplate tectonic setting during continental extension and rifting of the north margin of the North China Craton, associated with the late break-up stage of the Columbia supercontinent.

The paper by Cathelineau and Kahou (Contribution 8) used a Modified Quartz–Feldspar Diagram to discriminate muscovitisation processes in the Beauvoir Greisen and compared them to representative series of greisen data from the literature: Cligga Head, Cinovec, Panasqueira, Zhengchong, and Hoggar. Whole-rock geochemical data were obtained by ICP-OES and ICP-MS in unaltered and altered Beauvoir granites from the French Central Massif. Composite and elemental maps using micro-X-ray fluorescence were used to quantitatively determine the relative mineral proportions. They concluded that whole-rock geochemistry provides important information on the main trends of water–rock information, and that substantial muscovite formation, as recorded in the studied area, is explained by a fracture network that channelized fluids in disequilibrium with the mineral assemblage of the granites, particularly albite.

The paper by Engevik et al. (Contribution 9) presents bulk-rock geochemical data, including O-, H-, Sr-, and Nd-isotopic data and zircon and titanite U-Pb geochronological data for Proterozoic and Early Palaeozoic dry gneisses and granitoids in Dronning Maud Land, Antarctica. The granitoid crystallization ages were defined by zircon U-Pb at 520 ± 1.0 Ma, while the U-Pb titanite age of 485 ± 1.4 Ma was interpreted as the alteration age. The gneiss samples were dated by whole-rock Rb-Sr at 517 ± 6 Ma and Sm-Nd at 536 ± 23 Ma. The Sr and Nd isotopic data suggest that the gneiss was derived from a relatively juvenile source but underwent a significant metasomatic effect that introduced radiogenic Sr into the system. The granitoid isotopic data indicate a derivation from Mid-Proterozoic crust, with some additions of mantle components. This paper also highlights the importance of fluids intruding into the studied rocks, causing changes to the rock’s appearance, mineralogy, and chemistry.

The paper by Lima et al. (Contribution 10) presents a review of mineral chemistry and crystallization conditions of Ediacaran–Cambrian (580–525 Ma) A₂-type granites from the central sub-province of Borborema Province, Northeastern Brazil. This study reviews published whole-rock and mineral chemistry data from thirteen Ediacaran–Cambrian A-type intrusions and a related dike swarm, presenting new zircon trace element data for five of the intrusions. The studied granitoids are ferroan, predominantly metaluminous, and mostly alkalic-calcic, crystallized under low ƒO₂ conditions, with temperatures ranging from 990 to 680 °C and pressures of 4 to 7 kbar (crustal depths of 12 to 21 km). The zircon trace elements data suggest post-magmatic hydrothermal processes, which the authors interpreted as being associated with shear zones reactivation.