Erupted frothy xenoliths may explain lack of country-rock fragments in plutons

Abstract

Magmatic stoping is discussed to be a main mechanism of magma emplacement. As a consequence of stoping, abundant country-rock fragments should occur within, and at the bottom of, magma reservoirs as “xenolith graveyards”, or become assimilated. However, the common absence of sufficient amounts of both xenoliths and crustal contamination have led to intense controversy about the efficiency of stoping. Here, we present new evidence that may explain the absence of abundant country-rock fragments in plutons. We report on vesiculated crustal xenoliths in volcanic rocks that experienced devolatilisation during heating and partial melting when entrained in magma. We hypothesise that the consequential inflation and density decrease of the xenoliths allowed them to rise and become erupted instead of being preserved in the plutonic record. Our thermomechanical simulations of this process demonstrate that early-stage xenolith sinking can be followed by the rise of a heated, partially-molten xenolith towards the top of the reservoir. There, remnants may disintegrate and mix with resident magma or erupt. Shallow-crustal plutons emplaced into hydrous country rocks may therefore not necessarily contain evidence of the true amount of magmatic stoping during their emplacement. Further studies are needed to quantify the importance of frothy xenolith in removing stoped material.

Magmatic processes during the interaction between alkaline mafic-ultramafic rocks and intruding syenites near the Jabaquara Beach, north of the São Sebastião Island, São Paulo, Brazil

“…Stoping is an intrusion method that fragments and incorporates wall-rock, displacing the magma upwards, however, the relevance of stoping as an ascension mechanism is still an ongoing debate (e.g. Bartley, 2008, 2009;Paterson et al, 2012;Burchardt et al, 2016). While the importance of large xenoliths for magma movement is uncertain, they are still one of the many pieces of evidence that magmatic chambers and plumbing systems are open systems and commonly interact with the crust, proposed to also influence and contaminate the host magma (Farris and Paterson, 2007;Ferreira et al, 2015).…”