Origin of reverse compositional and textural zoning in granite plutons by localized thermal overturn of stratified magma chambers

Abstract

This study integrates gravimetry and thermal modelling with petrology, U–Th–Pb monazite and zircon geochronology and whole-rock geochemistry of the early Carboniferous Říčany Pluton, Bohemian Massif, in order to discuss the origin of compositional and textural zoning in granitic plutons and complex histories of horizontally stratified, multiply replenished magma chambers.The pluton consists of two coeval, nested biotite (–muscovite) granite facies: outer one, strongly porphyritic (SPm) and inner one, weakly porphyritic (WPc). Their contact is concealed but is likely gradational over several hundreds of meters. The two facies have nearly identical modal composition, are subaluminous to slightly peraluminous and geochemically evolved. Mafic microgranular enclaves, commonly associated with K-feldspar phenocryst patches, are abundant in the pluton center and indicate a repeated basic magma injection and its multistage interactions with the granitic magma and nearly solidified cumulates. Furthermore, the gravimetric data show that the nested pluton is only a small outcrop of a large anvil-like body reaching the depth of at least 14km, where the pluton root is expected.Trace-element compositions reveal that the pluton is doubly reversely zoned. On the pluton scale, the outer SRG is geochemically more evolved than the inner WPc. On the scale of individual units, outward whole-rock geochemical variations within each facies (SPm, WPc) are compatible with fractional crystallization dominated by feldspars.The proposed genetic model invokes vertical overturn of a deeper, horizontally stratified anvil-shaped magma chamber. The overturn was driven by reactivation of resident felsic magma from the K-feldspar-rich crystal mush. The energy for the melt remobilization, extraction and subsequent ascent is thought to be provided by a long-lived thermal anomaly above the pluton feeding zone, enhanced by the multiple injections of hot basic magmas.In general, it is concluded that the three-dimensional shape of the granitic bodies exerts a first-order control on their cooling histories and thus also on their physico-chemical evolution. Thicker and longer lived portions of magma chambers are the favourable sites for extensive fractionation and/or, potentially vigorous interaction with the basic magmas. These hot domains are then particularly prone to rejuvenation and subsequent extraction of highly mobile magma leading potentially to volcanic eruptions.