Magma mixing and crustal assimilation in a fossilized shallow magma chamber: the Pariquera-Açu Alkaline Suite, SE Brazil

• Ano de defesa: 2025
• Autor(a) principal: Cetina, Lina
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Biblioteca Digitais de Teses e Dissertações da USP
• Palavras-chave em Português: Alkaline magmatism; Apatite geochemistry; Assimilação crustal; Clinopyroxene and olivine populations; Crustal assimilation; Crystal mush; Geoquímica da apatita; Magma Chamber Simulator; Magma mixing; Magmatismo alcalino; Mistura magmática; Mush cristalino; Open-system processes; Pariquera-Açu Suite; Populações de clinopiroxênio e olivina; Processos de sistema aberto; Suíte Pariquera-Açu
• Link de acesso: https://www.teses.usp.br/teses/disponiveis/44/44145/tde-09022026-084045/

This study investigates the open-system processes controlling the magmatic evolution of the Pariquera-Açu Suite, located in the Ponta Grossa Alkaline Province and emplaced during the Early Cretaceous alkaline magmatism. The suite is a zoned intrusion composed of cumulate shonkinites/malignites at the margins, inequigranular malignites at the center, and local occurrences of pegmatoid malignite, nepheline syenite, monzonite, and tephrite. The investigation integrates petrography, mineral chemistry (EPMA, LA-ICP-MS, LA-MC-ICP-MS), and whole-rock analyses (XRF, ICP-MS, TIMS) to characterize textures, zoning patterns, and chemical and isotopic variations. Four populations of clinopyroxene and olivine were identified, recording four main evolutionary stages: (1) crystallization of antecrystic Mg-rich beige clinopyroxene and primitive olivine cores in a deep reservoir (~5.8 kbar, ~1132 °C, obtained through thermobarometry), reflecting fractional crystallization; (2) crystallization in a shallow reservoir (~2 kbar, ~1110 °C) from alkali mafic magmas carrying antecrysts, forming Ti-Al-rich brown clinopyroxene (titanaugite) and intermediate olivine; (3) mafic recharge events recorded by reverse zoning in clinopyroxene (beige bands) and Ni-rich olivine cores; (4) felsic recharge, marked by green-reddish reaction rims in clinopyroxene and evolved olivine rims. These stages indicate fractional crystallization coupled with mafic and felsic recharges, responsible for disaggregation, remobilization, and hybridization of the mafic mush. Apatite, the dominant accessory mineral, provides additional constraints. Two main groups are distinguished: (i) apatite in cumulate sequence, elongated prismatic to skeletal, enriched in V, Sr, and Cl but depleted in REE, Y, and Th, with weak negative Eu anomalies; and (ii) apatite in inequigranular rocks, short-prismatic to acicular, enriched in REE, Y, and Th but depleted in V, Sr, and Cl, showing stronger negative Eu anomalies. These contrasts reflect crystallization from compositionally distinct magmatic domains, mafic and evolved, respectively. Intra-sample heterogeneity in apatite also indicates varying degrees of hybridization between the mafic mush and felsic recharge melts. Sr-rich apatite in hybrid nepheline syenite suggests episodes of local liquid immiscibility, comparable to alkalinecarbonatitic systems. UPb dating of apatite yields an age of 131.6 ± 9.6 Ma, constraining the timing of suite crystallization. Isotopic data (87Sr/86Sri, 143Nd/144Ndi, 206Pb/204Pbi, 207Pb/204Pbi, 208Pb/204Pbi in whole rock, as well as 87Sr/86Sri in apatite) provide evidence for crustal assimilation, recorded by greater isotopic heterogeneity in inequigranular rocks. This interpretation is supported by isotopic mixing curves between samples with more primitive signatures and the host rocks of the Turvo-Cajati Formation. Thermodynamic modeling using the Magma Chamber Simulator shows that fractional crystallization of a highly primitive parental basanitic melt (represented by a regional basanite dyke, with the addition of 6% olivine antecrysts and 24% clinopyroxene antecrysts) reproduces the composition of beige clinopyroxene cores in the deep reservoir. In the shallow reservoir, fractional crystallization of a relatively volatile-rich basanitic melt (3 wt% HO, 1 wt% CO) explains the brown titanaugites but not the green reaction rims, which require additional processes such as felsic recharge and crustal assimilation. Overall, clinopyroxene, olivine, and apatite, combined with isotopic and thermodynamic modeling, demonstrate that the evolution of the Pariquera-Açu Suite was governed by fractional crystallization within a crystal mush system affected by mafic and felsic recharges, including crustal assimilation.