Fractionation and incipient self-granulitization during deep-crust emplacement of Lower Ordovician Valle Fértil batholith at the Gondwana active margin of South America

Abstract

Large granite batholiths were emplaced at the Gondwana active margin during Lower Ordovician in South America. These have contributed to crustal growth by net addition of silicic rocks to the continental crust. New U–Pb SHRIMP zircon age determinations, together with thermobarometric and geochemical data, yield that batholith magma intrusion is the responsible of heating and self-granulitization of early gabbro pulses. Partially molten granulitic gabbros, which appear as either early intruded into the metasedimentary host or as large inclusions within the batholith-forming Qtz-diorites, contain Opx-bearing, trondhjemitic leucosomes surrounding Hbl+Opx+Pl mafic mesosomes forming typical agmatitic structures. Hornblende–Plagioclase equilibria, applied to mineral pairs of granoblastic aggregates in textural equilibrium of metagabbro mesosomes, yield temperatures in the range 850–910°C for core-to-core pairs and in the range 1000–1075°C for rim-to rim pairs, at pressures of about 0.7GPa. SHRIMP zircon age revealed that the whole batholith was emplaced over a narrow time interval of 20Ma from 465 to 485Ma, with most ages clustered at about 470Ma. The age of metagabbros is 473±7Ma for older zircons and 454±4 for younger zircons. These ages are almost coincident within error with the age of host migmatites (477±5Ma) and those of batholith intrusion of 476±9Ma and 475±3Ma for Qtz-diorites and 475±5Ma for granites. Zircon overgrowths of these intrusive rocks yield sages clustered around 450Ma, revealing a protracted thermal history, more complex than previously believed. The geochemical study reveals that Qtz-diorites, tonalites and granodiorites form a continuous trend produced by magmatic fractionation from a parental dioritic magma. A weak adakitic tendency, with Sr/Y>15 in several samples, implies the presence of Grt in the source or magma chamber at a minimum pressure of 1.0GPa, higher than the depth of emplacement at 0.7GPa. The high temperature of magma emplacement, which induced the incipient self-granulitization of early magmatic pulses, together with the cotectic-like fractionation linking coeval Qtz-diorites, tonalites and granodiorites, is compatible with fractionation at the lower crust of a deep-generated, infracrustal, (sublithospheric?) intermediate magma.