Magmatic Processes: Review of Some Concepts and Models

Schedule of Mafic to Hybrid Magma Injections Into Crystallizing Felsic Magma Chambers and Resultant Geometry of Enclaves in Granites: New Field and Petrographic Observations From Ladakh Batholith, Trans-Himalaya, India

“…The diverse nature and origin of enclaves hosted in granites are described in many studies (e.g., Didier, 1984;Vernon, 1984;Furman and Spera, 1985;Castro et al, 1991a;Castro et al, 1991b;Shellnutt et al, 2010;Kumar et al, 2004a;Kumar et al, 2004b;Kumar et al, 2005;Clemens et al, 2016;Kumar et al, 2017), and are considered to represent: 1) xenolith (Sollas, 1894) as solid fragment of country rocks mostly confined to the margins of a pluton or may represent enroute deeper-derived lithology or unmelted source material, 2) surmicaceous enclave (Lacroix, 1933) as segregation of refractory source materials (restite) left after partial melting, 3) cognate or autolith (Pabst, 1928) as earlycrystallized cumulus phases or segregation of mafic clots or fragments of chilled border rock series of cogenetic felsic magma, 4) microgranular (Didier and Roques, 1959) or microgranitoid (Vernon, 1983) enclaves representing felsic, mafic, and mafic-felsic (intermediate) hybridized magmas entrained, mingled, and undercooled into relatively cooler partly crystalline felsic host magma at any stage of its evolution. However, when mafic or hybrid magma is injected into a largely crystallized felsic magma chamber, it is commonly distributed as swarms of microgranular enclave together with synplutonic dykes (Barbarin, 1989;Kumar, 2014). In the present paper, the term enclave refers to the mafic and mafic-felsic (hybrid) varieties of microgranular or microgranitoid enclaves.…”

“…Low viscosity, minimal rheological differences, and thermal equilibrium between the coeval mafic and felsic magmas will produce convective overturn (Huppert et al, 1984;Sparks and Marshall, 1986) forming a hybrid magma zone hidden below the felsic magma at crustal depth. Under the influence of turbulent convection, the hybrid magma may also be injected into the overlying felsic magma, forming either rounded to elongated hybrid enclave globules or disrupted as angular brecciated to dyke-/sheet-like enclave swarms depending upon the schedule of its injection into evolving felsic magma with changing rheology (e.g., Barbarin, 1989;Fernandez and Barbarin, 1991;Barbarin and Didier, 1992;Fernandez and Gasquet, 1994;Kumar, 2014), as shown by a schematic model (Figure 10). A large fine grained circular composite enclave (Figure 4D) lacks zoning.…”

“…SK is the sole author of the paper and the principal investigator, fund manager, and administrator of the research project on FIGURE 10 | Schematic presentation of mafic to hybrid magma injections into the progressively crystallizing felsic magma chamber and the resultant geometry of enclaves (after Barbarin, 1989;Barbarin and Didier, 1992, with the addition of "hybrid" by Kumar, 2014).…”

Geochemistry of Proterozoic and Cambrian granites from Meghalaya Plateau, north‐east India: Implication on petrogenesis of post‐collisional, transitional from I‐type to A‐type felsic magmatism

“…Harker variation diagrams (Figures 10 and 11) clearly suggest a key role of fractional crystallization in the evolution of these granites. However, the same linear trends on Harker plots can also be generated either by magma mixing of contrasting compositions in various proportions or varying degrees of partial melting of a source, that is, gradual restite‐melt separation (e.g., Kumar, 2014). Although observed negative curvilinear trend for MgO against SiO 2 (Figure 10e) may result from typical Rayleigh fractionation, the expansion of the SiO 2 may linearize a curvilinear trend on Harker plots (Clemens & Stevens, 2012).…”

Allanite-britholite bearing migmatite gneiss-Amet Granite and intrusive syenite-monzonite rocks of SW Rajasthan, India, and their REE potential

“…Magma genesis and assimilation (mixing) of deeper crustal rocks is a possible hypothesis, but mingling or comingling of magma is more plausible in this case (Kumar, 2014) as the migmatite gneiss which is a granitic gneiss, is trondhjemitic in nature (Fig. 12); is replaced or intruded by pulses of syenite-alkali feldspar syenite rocks containing epidote dikes and veins, with thoroughly dispersed primary allanite grains in the rocks.…”