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Título

COMPLEX CLINOPYROXENE POPULATIONS OF THE ITAPUVUÇU ALKALINE SUITE (SP): CONNECTIONS WITH THE JACUPIRANGA COMPLEX THROUGH A PLUMBING SYSTEM

Texto do resumo

The Itapuvuçu Alkaline Suite (IAS), a small body located a few kilometers east of the well-known Jacupiranga Complex (JC) (~130 Ma), consists of plutonic ultrabasic/basic alkaline rocks. Main lithotypes are olivine-bearing clinopyroxenite, mela-nepheline monzogabbro, melagabbro, mela-nepheline monzosyenite, kaersutite melagabbro, malignite, nepheline syenite and phonolite. Zoned clinopyroxene crystals are the main mineral phase in all the IAS rocks and their textural and compositional diversity suggest a magmatic plumbing system. Based on petrographic criteria and compositional variations (major and trace elements, obtained by electron microprobe and LA-ICP-MS, respectively), the clinopyroxenes are grouped in different populations. Cores of diopside compositions, with high Mg# (Mg/Mg+FeT, mol. proportions) and Cr and often displaying disequilibrium features (sieve texture, patchy zoning), are classified as antecrysts. They were not crystallized within the resident host magma but from earlier stages of the same magmatic system. Ti-augite, the dominant variety among clinopyroxenes from the IAS rocks, occurs both mantling the diopside cores or as individual crystals, with intermediate Mg# values, low-Cr, and high-Ti contents. Predominantly, Ti-augite presents euhedral terminations in these rocks, and is classified as phenocryst. Thick bands (step zonation) of diopside composition in intermediate portions of Ti-augite mantling may be present. In some lithotypes, greenish-gray clinopyroxene (with low Mg# and Ti and high Na contents) appear in a reactional intergrowth with kaersutite, biotite and opaque minerals in the rims of Ti-augite. Diopside and Ti-augite crystals show similar rare earth element pattern, with higher values for Ti-augite crystals. Greenish-gray reactional clinopyroxene also displays a REE pattern enriched in light REEs and a significant negative Eu anomaly, suggestive of previous feldspar fractionation. By applying the new deep learning-based clinopyroxene-only geothermobarometer GAIA, different stages of crystallization for the IAS clinopyroxenes are recognized. The diopsidic cores record high crystallization pressures (ca. 4.5 kbar), while the Ti-augites record a crystallization history over a larger range of pressures (ca. 2.5 to 0.3 kbar), with a peak of pressure distribution at ca. 0.3 kbar, indicating a shallow chamber environment for the final stages of crystallization. Recharge events of primitive liquids can explain the diopside bands alternated with Ti-augite. These intermediate bands present step zoning patterns and occur at intermediate portions of a magmatic system (ca. 2.5 kbar), as indicated by GAIA thermobarometric data. The IAS clinopyroxene antecryst data, in accordance with pressure estimations for the JC clinopyroxenes compiled from the literature, suggest a genetic correlation between these intrusions, at least at deeper and earlier stages of the magmatic plumbing system.
Acknowledgements: This research was supported by grants from Fundação de Amparo à Pesquisa do Estado de São Paulo – FAPESP (Procs. 2019/22084-8; 2023/11675-0) and grants and studentship from Brazilian National Research Council - CNPq (404020/2021-6, 310055/2021-0, 130153/2023-0).

Palavras Chave

magmatic plumbing system; antecrysts; alkaline magmatism; geothermobarometry.