10 How are Plutons Made? Physical and Chemical Records of Pluton Construction and Evolution

This session integrates structural and petrologic views of plutonic systems, exploring the crustal response to pluton formation, and processes that control the physical and chemical characteristics of pluton-host rock systems from initial emplacement to solidification.

Sunday, 19 October 2025
8:00 AM - 12:00 PM
HBGCC, 216AB

The Peruvian Coastal Batholith: Along and Across Arc Exploration of Temporal Histories, Compositional Diversity, and Tectonomagmatic Links

Paper number: 10-2
Program: Technical Sessions
Time: 8:20 AM – 8:35 AM

Authors

Abstract

The drivers of spatial and temporal magmatic trends in the Central Andes of Peru have been studied since the 1970s. Early studies of the Peruvian Coastal Batholith (PCB) were crucial for developing models for the Pacific margin of South America. However, the origins of these trends and the mechanisms controlling magma composition remain poorly understood. This research addresses this gap by using an up-to-date geochemical and geochronological database. We aim to 1) extend previous studies of local plutons to the entire PCB, 2) validate proposed chemical trends, 3) examine spatial and temporal changes along and across the PCB over 180 m.y., and 4) link these observations to the tectonic regime, subduction dynamics, magma sources, and crustal assimilation.

We find that the PCB has a clear non-steady-state pattern at variable temporal and spatial scales. The composition of arc magma is highly variable, reflecting a complex interplay of processes, sources, and mechanisms. In the PCB, the flare-ups exhibit a duration of 30-40 m.y. which occur both independently and synchronously across segments. Mantle-derived magma addition rates (MMARs) were calculated using a mantle/crust composition of 80/20. A volume of 967,000 km3 added to the arc crust over 160 m.y., gives an average addition rate of 6000 km3/m.y. Our data confirm that MMARs are episodic and that flare-ups deliver about 8× more volume than periods of quiescence. Additionally, our analysis reveals an eastward arc migration at an average rate of 1 km/m.y. for 200-20 Ma. The similarity of this pattern across segments suggests a larger, externally driven tectonic control.

We propose that the observed diversity in magma composition along and across the arc is due to various factors, such as changes in magma input from the mantle (0-20% crust vs. 80-100% mantle), different basement types, different assimilated materials, changes in crustal thickness, degree of differentiation, arc migration, and transitions from depleted mantle to lithospheric mantle. We suggest that coupling of external (lower plate) and internal (upper plate) processes in complex ways at different spatial and temporal scales form the final arc diversity.

 

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

doi: 10.1130/abs/2025AM-414923

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