75 Mineralogy, Geochemistry, Petrology, and Volcanology Student Session (Posters)

GSA’s MGPV student representatives invite the division’s student population to present their research (oral or poster). We encourage abstracts from any subdiscipline within our respective fields with hopes of stimulating a multi-disciplinary, early career researcher (ECR) community within MGPV.

Sunday, 19 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Unravelling Magma Mush Dynamics at Agua de Pau, Azores Archipelago: Insights from Geochemistry and U-Th Geochronology of Syenite Clasts and Associated Pumices.

Paper number: 75-26
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Água de Pau (also called Fogo) is the largest central volcano on São Miguel Island in the Azores Archipelago. It is known for the voluminous 4.6 ka Fogo ‘A’ deposit, which forms a prominent marker bed containing syenite xenolith clasts. However, the genetic relationship between these xenoliths and the associated volcanic products remains poorly constrained, and geochronological data on the syenite clasts are limited. In this study, we investigate the relationship between the plutonic rocks observed as syenite clasts and their associated volcanic rocks by integrating whole-rock major and trace element geochemistry with zircon U-Th geochronology on syenite clasts and pumice from the Fogo A fall deposit. Zircon analyses were conducted using the Sensitive High-Resolution Ion Microprobe with Reverse Geometry (SHRIMP-RG) on 17 samples (10 syenites and 4 pumices). Whole-rock major and trace elements contents of both pumice and syenites exhibit similar rare earth element (REEs) patterns, indicating an origin from a shared magma chamber undergoing fractional crystallization. Syenites clasts overlap pumices with similar compositional trends in major oxides such as decreasing TiO₂, MgO, CaO, and P₂O₅, and increasing Na₂O and K₂O with increasing SiO₂, supporting a common trachytic magmatic source. Sr concentrations are lower in the trachytes compared to similar age basalts from São Miguel. This results in high Sr/Y ratios in the basalts and low Sr/Y ratios in the trachytes, suggesting the removal of plagioclase during magmatic evolution.  La/Yb ratios increase from silica-undersaturated to silica-saturated syenites, indicating that the magma chamber is influenced by fractional crystallization. Pumice samples lie between silica-saturated and undersaturated syenites, suggesting varying stages of differentiation from a common parental magma. Zircon U-Th geochronology of syenites yields a weighted mean age of 13.71 ± 1 ka (MSWD = 1.5), which aligns with previously published crystallization ages for the syenite clasts. These findings highlight the connection between volcanic-plutonic systems and provide insights into the emerging magma mush dynamics beneath the Água de Pau Volcano.  

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413466

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