282 Crustal Petrology, Part II

This session explores various igneous and metamorphic processes that have shaped Earth’s crust. We welcome contributions from the micro- to macroscale, from the deep to shallow crust, and from field- to laboratory- to modeling-based.

Wednesday, 22 October 2025
1:30 PM - 5:30 PM
HBGCC, 216AB

Tracking Progressive Crustal Thickening from Orogen Core to Margins (French Massif Central)

Paper number: 282-5
Program: Technical Sessions
Time: 2:40 PM – 2:55 PM

Authors

Abstract

Numerical models predict that orogenic crust flows laterally and vertically over great distances in response to tectonic and gravitational forces. The geologic record of flow is more difficult to detect because overprinting commonly erases evidence at various stages of flow/exhumation. We have investigated the pressure-temperature-time-deformation (P-T-t-d) record of crustal flow using meta-mafic pods in quartzofeldspathic gneiss to evaluate the history of flow. Our premise is that coeval mafic and felsic rocks together provide a P-T-t-d record of crustal flow, although typically only the mafic relics preserve evidence of the deep crustal history.

The Variscan French Massif Central (FMC) contains abundant meta-mafic rocks in quartzofeldspathic gneiss and migmatite. We analyzed the composition, age, and P-T conditions of eclogite and amphibolites along a N-S transect to determine and compare their histories. Whole-rock isotopic and mineral trace-element geochemistry, together with lithological associations and structural setting, distinguishes continental (orogenic) eclogite from oceanic (subduction) eclogite in the FMC, with orogenic eclogites being significantly more common.

The cores of zircons in eclogite record protolith ages of pre-Variscan mafic magmatism in a continental setting (500-440 Ma). Zircon rim ages record high-grade metamorphism and vary from older ages (~340 Ma) in the central FMC to younger (315-310 Ma) in the north and south. Temperatures of high-grade metamorphism also vary systematically from higher T in central FMC eclogite (>800°C) to lower T eclogites in the north and south (750-700°C). Eclogites therefore record a progression from older and hotter in the orogen core to younger and colder on the margins. We interpret these results to indicate progressive crustal thickening from orogen core to margins and/or bivergent crustal flow over 30 million years, with colder eclogites corresponding to deep crustal flow at the orogenic margins (foreland).

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

doi: 10.1130/abs/2025AM-413679

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