Editorial for Special Issue “Genesis of Calc-Alkaline Granitic Rocks: Evidence from Petrology and Geochemistry”

1. Introduction

Calc-alkaline granitic rocks are fundamental components of continental crust, forming large batholiths in orogenic belts and intrusions across various tectonic regimes [1]. They are recognized in subduction-related magmatic arcs, syn-collisional mountain belts, and even post-orogenic and anorogenic settings. Petrological and geochemical studies of these granites provide crucial insights into crust–mantle interactions and the geodynamic processes driving magma generation [2]. Notably, calc-alkaline granitoids typically involve both crustal and mantle source contributions. Their geochemical signatures (e.g., enrichment in large-ion lithophiles and depletion in high-field-strength elements) reflect processes like subduction-zone fluid fluxes, crustal assimilation, and fractional crystallization, allowing them to be used as a tracer of tectonic setting [3]. Understanding the genesis of these granitic rocks is therefore vital for reconstructing continental evolution and growth processes. This Special Issue assembles six studies that span diverse tectonic contexts (from Neoproterozoic arcs to Mesozoic post-collisional settings), showcasing new petrological and geochemical evidence on the origin and evolution of calc-alkaline granitic magmas.

2. Overview of the Special Issue Contributions

• Sami et al. (2023)—Cryogenian I-Type Granite in the Nubian Shield (NE Sudan)

Sami et al. investigate the Haweit granodiorites of the Gabgaba Terrane in NE Sudan, representing a Cryogenian (~718 Ma) I-type calc-alkaline granite within the Arabian–Nubian Shield. Using zircon U–Pb dating and Lu–Hf isotopes alongside whole-rock geochemistry, they demonstrate that these granodiorites crystallized in a subduction-related environment and have a juvenile crustal character. The rocks are metaluminous and display arc-type geochemical signatures (enriched LILE and LREE and depleted HFSE and HREE, with a pronounced Nb anomaly). Positive εHf(t) values (+7 to +11) and young model ages indicate derivation from newly formed crust. Sami et al. conclude that the granodiorite magma was generated via the partial melting of a low-K mafic lower-crustal source, triggered by upwelling asthenospheric melts during subduction. In other words, the calc-alkaline magma formed as a result of mantle heat input melting juvenile lower crust in an active Neoproterozoic continental margin.

• Xiao et al. (2023)—A-Type Granite Fractionation in the Nanling Range (South China)

Wenzhou Xiao et al. focus on Late Jurassic A-type granites in the Nanling Range of South China, specifically the Jiuyishan complex and Xianghualing stocks. Although A-type granites are not classical calc-alkaline rocks, this study shows magmatic differentiation processes relevant to granite petrogenesis. The authors document two distinct fractional crystallization mechanisms for compositionally similar high-temperature granites (~153–160 Ma) that share a lower-crustal source. Geochemical and isotopic data (zircon U–Pb ages, whole-rock major and trace elements, and Nd–Hf isotopes) reveal that the Jiuyishan pluton evolved through in situ crystal mush fractionation, forming compositionally zoned units, whereas the smaller Xianghualing intrusive stocks underwent flow differentiation. Both bodies have A-type chemical signatures, and εNd(t) as well as εHf(t) are consistent with a dominantly crustal (Proterozoic lower crust) source, with only minor mantle input evidenced by mafic microgranular enclaves. Xiao et al. propose that these granites exemplify how different physical controls on magma differentiation can operate in parallel within an extensional, intraplate felsic magmatic province.

• Xiao et al. (2024)—Early Cretaceous Post-Collisional Magmatism in Tibet

Deng Xiao et al. report on an Early Cretaceous (115–105 Ma) belt of calc-alkaline granitic rocks in the northern Lhasa Block, Tibet, shedding light on post-collisional magmatism following the closure of the Bangong–Nujiang Ocean. This study combines petrology, geochemistry, zircon U–Pb geochronology, and in situ zircon Hf isotopes to characterize the Burshulaling granites and related magmatism along a ~1200 km east–west extent. The granites are high-K, peraluminous A-type rocks in composition, and yield slightly negative εHf(t) values, indicating a significant crustal contribution with some mantle involvement. Xiao et al. demonstrate that these granites were emplaced in a post-collisional setting marked by high temperatures and low pressures, consistent with slab break-off or orogenic root delamination after the continent–continent collision. They argue that the magmatism resulted from the melting and mixing of the thickened lower crust, triggered by the detachment of the subducted slab, and possibly influenced by continued subduction or the collapse of the Bangong–Nujiang oceanic lithosphere. This implies that the final closure of the ocean was accompanied by the generation of an A-type granite belt, linking shallow crustal melting and mantle input to the uplift and growth of the Tibetan Plateau.

• El-Awady et al. (2024)—Transition from Subduction to Post-Collision in the Arabian–Nubian Shield (Egypt)

El-Awady et al. examine two neighboring Neoproterozoic granitoid suites in the Central Eastern Desert of Egypt (part of the northern Arabian–Nubian Shield) to unravel a tectono-magmatic transition from subduction-related to post-collisional regimes. Their study area includes granodiorites of calc-alkaline I-type affinity and slightly younger A-type granites (specifically A2-type, which are post-orogenic granites). The granodiorites are enriched in Sr, K, Rb, Ba (LILE) relative to Nb, Ta, and Ti (HFSE) and have elevated LREE/HREE, consistent with an arc environment. Primary amphibole chemistry confirms a calc-alkaline, water-rich magma derived from a mixed mantle–crust source in a subduction setting. These I-type granodiorites likely formed as an early magmatic phase via the mingling of mantle-derived mafic magma with lower crustal melts, followed by fractional crystallization. In contrast, the syenogranites display high SiO₂ and alkalis, are strongly peraluminous (with muscovite and garnet), and show flat REE patterns with a pronounced Eu anomaly, typical of highly evolved A2-type granites. El-Awady et al. conclude that the syenogranites were generated in a post-collisional extensional setting via the partial melting of a juvenile lower crustal source that had been fertilized by prior mantle underplating. They propose that lithospheric delamination after the Ediacaran collision caused mantle upwelling and the basal heating of the crust, producing these A-type melts. Fractional crystallization then drove the extreme differentiation. The study highlights how crustal thickening and the subsequent extensional collapse of the orogen led to a magmatic switch from subduction-related calc-alkaline granodiorites to voluminous post-collisional A-type granites in the late stages of the East African Orogeny.

• Hui et al. (2025)—Permian Granitoids of the East Kunlun Orogen (NW China)

Hui et al. explore the petrogenesis of Late Permian (~254 Ma) granitoids in the East Kunlun Orogenic Belt of NW China, a region associated with the Paleo-Tethys evolution. Two lithologies are studied: a monzogranite and a quartz porphyry that intruded during the transition from subduction to collision. Through detailed petrology, whole-rock geochemistry, and zircon U–Pb–Lu–Hf isotopic analysis, the authors distinguish the two granitoid types. The monzogranite is metaluminous to weakly peraluminous and low-K calc-alkaline (characteristic of arc I-type granitoids), with high SiO₂ and pronounced depletions in Eu, Ba, Sr, P, and Ti, indicating the significant fractional crystallization of amphibole and feldspars. The quartz porphyry, by contrast, is peraluminous and high-K calc-alkaline I-type, showing enrichment in Rb and LREE, but similarly strong negative anomalies in Nb, Sr, P, and Ti. Despite these differences, both rock types have coherent zircon Hf isotope ranges, suggesting derivation from a common source. Hui et al. infer that both the monzogranite and quartz porphyry were produced via the partial melting of a juvenile mafic lower crustal protolith, metasomatized by subduction components, with the former representing a more evolved magma due to a higher degree of fractional crystallization. The geochemical arc signatures (e.g., LILE enrichment, HFSE depletion, and tectonic discrimination diagrams plotted in the volcanic arc field) confirm an active continental margin setting. The authors outline a five-stage geodynamic model, from oceanic arc initiation, slab rollback, and syn-collisional compression to post-collisional extension, to explain the temporal and chemical evolution of these Permian granitoids in the East Kunlun Belt.

• You et al. (2025)—Highly Fractionated Granite and Rare Earth Mineralization (South China)

You et al. focus on a highly fractionated S-type granite from the Late Jurassic age (Shuitou pluton, South China) and its role in generating ion-adsorption rare earth element deposits. This study stands somewhat apart in emphasis, examining how extreme magmatic differentiation and source characteristics can concentrate heavy REEs. The Shuitou two-mica granite is strongly peraluminous (A/CNK > 1), with muscovite, garnet, and other accessory minerals indicating an evolved melt. Geochemically, it has high silica and alkali contents and a very high differentiation index, consistent with an intensive fractionation of crustal magma. Zircon and monazite U–Pb dating yields 150–145 Ma ages, and their strongly negative εHf(t) values suggest derivation from ancient metasedimentary crust. You et al. show that in the post-orogenic extensional environment of South China, high magmatic temperatures (aided by F-rich compositions) enabled the melting of REE-bearing accessories and the retention of heavy REEs by phases like garnet. This resulted in a HREE-enriched granitic residue. Thus, while not calc-alkaline in the strict sense, this contribution underscores the extreme end of granite differentiation and its metallogenic significance in an extensional setting.

3. Discussion

Collectively, the six studies in this Special Issue illustrate the diverse origins and evolutionary pathways of calc-alkaline and its related granitic magmas under different geodynamic environments. A unifying theme is the interplay between the mantle and crust in granite genesis. In subduction-dominated settings (e.g., Sudan’s Cryogenian arc and the Permian East Kunlun arc), mantle-derived inputs trigger the partial melting of lower crustal sources, yielding I-type calc-alkaline magmas with juvenile isotopic signatures. These arc granites contribute new continental crust, as evidenced by the positive Hf isotopes in the ANS granodiorite and the predominantly juvenile character of the Kunlun magmas. In contrast, post-collisional granites (Egyptian A-type syenogranite or the Tibetan Early Cretaceous A-type belt) reflect crustal reworking after orogenic thickening. Their magmas generated from the melting of previously formed crust are facilitated by tectonic decompression and/or an influx of asthenospheric heat. These magmas carry the geochemical characteristics of crustal sources and also show enrichment in high field strength elements and high alkali contents, marking them as A-type. Importantly, several contributions (El-Awady et al. 2024; Xiao et al. 2024) highlight that the transition from subduction-related calc-alkaline to post-collisional magmatism can occur within the same orogen, as crustal dynamics shift from compression to extension. This underscores how calc-alkaline magmatism is not confined to a single tectonic setting, but rather evolves with the orogenic stage.

Another cross-cutting observation is the role of fractional crystallization and its related differentiation processes in controlling granite diversity. All studies document evidence of magmatic differentiation, from the crystal mush segregation in one A-type system to the extreme incompatible-element enrichment in highly evolved S-type granites. Xiao et al. (2023) demonstrates that even within a single region and time frame, granitic magma bodies can follow different fractionation paths. Likewise, Hui et al. infer that one of the coeval Permian magmas underwent deeper or more prolonged fractionation than the other, explaining their divergent compositions despite a common source. Several papers also discussed magma mixing and assimilation; for instance, the Egyptian granodiorites record mixing between mantle and crustal melts, and the presence of mafic enclaves in South China granites points to injections of mafic magma. These processes, together with varying water contents and oxygen fugacity, contribute to the nuanced geochemical fingerprints exhibited by the granites in this issue.

Crucially, the findings highlight how granite petrogenesis reflects the tectonic context. In thickened crustal regions, calc-alkaline differentiation tends to be pronounced, producing magmas with depleted Fe-Mg and often high silica levels, characteristics that align with the average composition of continental crust. By contrast, in settings of active subduction with continuous mantle input, granites may retain more primitive signatures or show mixing arrays between mantle and crust contributions. By comparing case studies from different periods and locations (Neoproterozoic northeast Africa, Paleozoic Asia, or Mesozoic East Asia), we gain a more holistic understanding of calc-alkaline granite genesis. Common threads include the importance of lower crustal melting, the influence of mantle-derived magmas (either as a heat source or via magma mixing), and the omnipresent filter of fractional crystallization shaping the final rock chemistry. Differences, on the other hand, arise from factors like crustal thickness, the presence or absence of active subduction, and the degree of tectonic extension.

In summary, these studies collectively advance our understanding of how calc-alkaline and its related granitic magmas are generated and evolve in differing tectonic scenarios. From arc systems forming new crust to post-collisional settings recycling existing crust, the granite record encapsulates a wide geodynamic narrative. The petrological and geochemical evidence assembled here not only deciphers the history of individual plutons, but also enriches broader models of continental crust formation and reworking.

4. Conclusions

The contributions to this Special Issue demonstrate that the genesis of calc-alkaline granitic rocks is a multifaceted process controlled by the tectonic environment, source composition, and magmatic differentiation. Calc-alkaline granites act as chronicles of geodynamic transitions, from subduction inception through collisional climax to post-orogenic collapse, each stage leaving an imprint on magma sources and chemical signatures. Together, the studies herein illustrate the continuity between arc magmatism and post-collisional granite generation, emphasizing that crustal growth and crustal reworking via granitic magmatism are integral to the evolution of the continental lithosphere. This collection of research not only provides detailed case studies spanning Neoproterozoic to Mesozoic timescales, but also offers a synthesized perspective that will inform future investigations of granitoid petrogenesis in different tectonic settings.