A pluton-dike system as an incomplete assembled pluton: Field geology, petrography and geochemistry of the Rodríguez dike swarm and the Kranck pluton, Fuegian Andes

Abstract

Dikes extending into the host-rock are a common feature at pluton roofs. Typically, the dikes are considered offshoots from differentiated magma reservoirs within the pluton, i.e. late-stage intrusions of the successive discrete magma pulses that built the pluton. We evaluate this hypothesis on a dike swarm and associated pluton, the Kranck-Rodríguez pluton-dike system, by analyzing its field geology, petrography and geochemistry (whole-rock and mineral). This system was emplaced in the shallow crust (∼1.7 kbar) within the fold-and-thrust belt of the Fuegian Andes. The Kranck pluton is a small (∼4.4 km2), composite body composed of a diverse range of lithologies, from mafic to felsic. Internal contacts are mainly sharp and subvertical, suggesting diking as a mode of assembly. The Rodríguez dike swarm extends up to 6 km from the pluton, mainly exposed adjacent to it. This swarm also displays a wide variety of lithotypes, encompassing a set of dikes affected by ductile deformation, and a younger set without it. Geochemical and petrographic analyses, along with thermobarometry, suggest that the Rodríguez dikes and lithotypes within the pluton were fed from different, deeper magma reservoirs, in an inferred trans-crustal plumbing system. We propose that the dikes represent discrete increments of the growing pluton, in a protracted process of successive injections. This ultimately led to the assembly of the pluton in the sector of the system with greater magma flux. The system developed in two major episodes, pre- and post-ductile deformation, each characterized by multiple, discrete magma injections. We consider this system as an embryonic stage of a broader model that would evolve from sheets separated by host-rock, to bodies with sheeted margins containing host-rock rafts, and eventually to relatively inclusion-free plutons, potentially forming larger bodies if provided with a greater magma supply and/or higher accretion rates.