Sheeted intrusion of granitic magmas in the upper crust - emplacement and thermal evolution of the guandacolinos pluton, NW Argentina

Abstract

The Lower Carboniferous Guandacolinos pluton of northwestern Argentina (Western SierrasPampeanas) preserves field, structural, and petrological evidence of sheet-like transport and assembly ofgranitic magmas in the upper crust. The pluton is a relatively small (∼24 km2) subduction-related graniticbody, elongated in map view, and hosted in Neoproterozoic metamorphic rocks. Exceptional exposurerecords a subparallel array of steep NNE-SSW trending structures, including steep contacts partlyconcordant with host rock structure, numerous sheets of granite separated by host rock rafts, abundantxenoliths, and magmatic and solid-state foliations. Along the eastern half of the pluton, the granite ismassive and host rock inclusions are less abundant. Regional markers of the host rock are deflected alonga concordant bulged contact in the northeastern region of the pluton. Field relations indicateemplacement by multiple material transfer processes including fracture propagation, magma wedging,stoping, and lateral shortening. Contrasting mechanisms imply a changing mechanical response of hostrock and multiple stages of intrusion. Emplacement began with dominant brittle fracturing and intrusionof sheets influenced by host rock anisotropies, followed by a viscoelastic phase were larger batches ofmagma caused downward transfer of stoped blocks, lateral expansion, and minor ductile deformation ofthe host rock. Thermal modelling indicates that the construction of the pluton required lateral accretionrates in the order of dm/years and less than a few tens of thousands of years to form. This case studydocuments the ability of incrementally assembled sheeted intrusions to efficiently heat rocks of theupper crust and trigger conditions favourable for transfer and storage of magma.