Peraluminous A-type granites formed through synchronous fractionation, magma mixing, mingling, and undercooling: evidence from microgranular enclaves and host Mesoproterozoic Kanigiri granite pluton, Nellore Schist Belt, southeast India

Abstract

The field and microstructural features coupled with mineral chemical data from microgranular enclave (ME) and host Mesoproterozoic Kanigiri granite (KG) pluton of Nellore Schist Belt (NSB), Southeastern India, have been documented in order to infer the likely processes responsible for the origin and evolution of ME and host KG magma. The ME and host KG bear the same mineral assemblages barring the KG which does not contain amphibole; however, they are modally disequilibrated. The ME in KG is originated due to multiple intrusions of ME magmas into the crystallizing host KG magma chamber. Field and textural features indicate the dynamic magma flow, mingling, and undercooling of the ME against a relatively cooler surface of host KG magma. The presence of NSB country rock xenoliths and its diffuse boundaries suggest the intrusive relation and marginal assimilation by the intruding KG magma. The occasional cumulate texture in the ME appears to have formed by the accumulation of early-formed minerals that crystallized rapidly in the ME magma globules. The ME shows the magmatically deform features developed due to the flowage and erosion by the subsequent intrusions of ME magma pulses into the crystallizing host KG magma chamber. The ME amphiboles show unusual composition as ferro-edenitic hornblende to potassian-hastingsitic hornblende, that crystallized in the subalkaline-alkaline transition, low fO₂(reducing to mildly oxidizing) magma. The unusual extremely low Mg/Mg + Fe^t = 0.015 (avg.) of ME amphiboles may be related to the changing physico-chemical (P, T, fO₂, and H₂O) condition of the ME magma or they might have crystallized in equilibrium with more evolved KG magma. The KG (FeO^t/MgO = 37.04, avg.) and ME (FeO^t/MgO = 77.72, avg.) biotites are siderophyllite, and buffered between QFM and NNO syn-crystallizing in the water undersaturated (H₂O ≈ 3.58 wt.% in KG; ≈3.53wt.% in ME), alkaline anorogenic (A-type) host magmas that were emplaced at mid-crustal (4–5 kbar; ~ 17 km) depth. Field, microtextural and mineral chemical evidences suggest that the alkaline KG magma originated from crustal source and evolved through synchronous fractionation, mixing, and mingling with coeval ME magmas in the KG magma chamber.