Degassing structures and mush hybridization in shallow plutons

WEINBERG, ROBERTO F.; SOLA, ALFONSO; ORTIZ JOYA, GUILLERMO; DE ABREU BARBOSA ARAUJO, LUCAS EDUARDO; MORFULIS, M.; SUZAÑO, NÉSTOR O.; BECCHIO, R. A.

Baveno

Simposio; 10th Hutton Symposium on Granites and Related Rocks; 2023

10th Hutton Symposium on Granites and Related Rocks

Degassing of granitic magmas in the shallow crust is a significant process controlling the nature of volcanism and mineralization. Quoting Barnes et al. (2020, Geosciences) “Effects of the prodigious amount of water carried by arc magmas on the plutonic rocks themselves have generally been underappreciated”. This is possibly because the evidence may be subtle. Here, we use magmatic structures found in rocks of the Early Paleozoic Famatinian Arc, Puna Plateau, Argentina, to speculate on what these structures might be. These magmatic rocks were emplaced in the upper crust (~2 kb) and collectively, their magmatic structures suggest that the volatile-rich liquids enhanced hybridization by increasing the reach of evolved, granite-derived liquids into crystallizing mafic mushes by combining increased element diffusion and mass advection.In levels of increasing subtlety, the magmatic structures related to degassing in heterogeneous diorite bodies emplaced within granitic magma include: a) ovoidal centimetric micropegmatite swarms (Graphic 1), b) decametric pegmatite blobs with hornblende-epidote-titanite, c) a rock dominated by quartz ocelli, d) complex network of narrow seams of evolved granitic/pegmatitic material with titanite invading enclaves (Graphic 2), e) felsic clouds in diorite matrix, f) biotite breakdown to hornblende or titanite, and hornblende and titanite alteration around granite intrusions, and g) “dyke-like” elongated patches of granite xenocrysts isolated inside diorite.We envisage a process whereby local decompression of a H2O-saturated granitic magma triggers the boiling off of volatile-rich liquids, creating a pressure gradient directing these liquids towards low pressure sites, such as mafic mushes. The low density and low viscosity of the pressurized volatile-rich liquid create ideal conditions for magma hybridization. The liquids can either be trapped underneath an impermeable mafic mush or invade the mushes, sometimes energetically, carrying a solid cargo. The hybrids generated are a complex result of the transfer of early-formed granite xenocrysts, and of evolved liquids pervading and reacting with the matrix. The process is powered by diffusion and advection, evidenced by hornblende alteration and hornblende-epidote-titanite pegmatites.