Textural and Geochemical Evidence for Multiple, Sheet-like Magma Pulses in the Limeira Intrusion, Paraná Magmatic Province, Brazil

Abstract

Quantitative petrographic, structural, and textural parameters are integrated with geological, geochemical, and Sr-isotope data to examine the emplacement, growth processes, and the magmatic evolution of the high-Ti tholeiitic Limeira Intrusion, in the Paraná Magmatic Province - Southeastern Brazil. Our data strongly support a multiple-stage evolution, due to the nested emplacement of distinct crystal-bearing magma pulses that probably evolved independently, except at their boundaries. A stage of cooling and crystallization between magma injections originates a stepwise T-t path, leading to variations in the plagioclase residence times and effective growth rates inwards, also occasioning sudden changes in crystal shape and size at the boundaries of each magma pulse. The time delay between pulses allows preserving internal “chilled margins” and the development of near-rigid surfaces at their contacts, increasing the alignment and clustering of crystals during magma replenishment. Isotopic and textural data demonstrate a complex assembly history, in which the appearance of mixed plagioclase populations in between magma pulses coincides with the onset of initial Sr isotope ratio increase, which can be attributed to a locally enhanced cooling-rate, and the extraction of residual melts from the previous crystallizing batches and mixing with the younger pulses. Typical C- and S-shaped MgO (wt.%) compositional profiles within individual pulses indicate that the first pulse probably evolved by in situ fractional crystallization followed by melt migration inward, while the younger ones have contributions from both compaction of the lowermost crystallization front and compositional convection. Mafic globular structures are found at the boundaries of magma pulses and constituting the mafic-rich layers in layered rocks. They are interpreted as evidence for chemical disequilibrium, arguably associated with the trigger of silicate liquid immiscibility. The upwards compositional convection of the silica-rich residual liquid and the accumulation of the Fe-Ti-P-rich crystal-bearing end member in the bottom of the latest magma pulses might represent the most significant mechanism of differentiation in the Limeira Intrusion.

The Campinas Jaguariúna Sill, NE Paraná Continental Flood Basalt Province, Brazil: insights on the mechanisms of emplacement and differentiation from geochemical and magnetic data

“…Possibly due to its expressive volume and depth of emplacement, and consequent slow cooling rates, the CJS preserved important structures of melt segregation in form of subhorizontal lenses and eye-shaped pockets of medium-to coarse-grained mesocratic rocks of gabbroic composition that are injected in the host finegrained diabase, mostly at the upper parts of the sill. Similar segregations have been reported in recent works on thick basaltic flows in the PCFB (Oliveira et al, 2020, Gomes et al, 2022 and fractionated, coarse-grained gabbros and monzogabbros are recognized as more differentiated rocks derived from diabase in intrusive bodies occurring near the CJS (Limeira Sill; Oliveira and Dantas, 2008;Farias, 2012, Lino et al, 2018, Lino and Vlach, 2021. In this scenario, the CJS offers an excellent opportunity to study the mechanisms of melt segregation in large basaltic bodies and their geochemical and morphologic relationships with regional analogous PCFB occurrences.…”