167-3 The Late Ediacaran to Late Devonian tectonic history of the Seychelles microcontinent: insights from in situ 40Ar/39Ar geochronology

Session: Integrating Geochronology and Geochemistry to Decipher the Tectonic Evolution of Orogenic Belts

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Abstract:

The Seychelles microcontinent is exposed as a series of granitic and syenitic islands in the western Indian Ocean. The oldest granites were emplaced along the margin of Rodinia during the Neoproterozoic (ca. 760–750 Ma) whereas the youngest (ca. 63-61 Ma) syenites were emplaced during Early Paleogene rifting from India. The Neoproterozoic to Early Paleogene tectonic history of the Seychelles microcontinent is unknown. We investigate the post-emplacement thermal history of a mildly deformed Neoproterozoic granite from the island of Praslin using in situ single-grain laser ablation ⁴⁰Ar/³⁹Ar geochronology. Twenty-one biotite crystals yielded ages ranging from 414 to 598 Ma. The older grouping has a peak age at 553.6 ± 5.4 Ma and an intercept age of 553 ± 26 Ma (initial ⁴⁰Ar/³⁶Ar = 300.0 ± 11, MSWD = 5.5). The intercept age represents the time at which the biotite last degassed through the closure temperature (265–304°C). A younger age group is obtained from chloritized biotite and has a peak age of 436.3 ± 9.6 Ma with an intercept age of 425 ± 15 Ma (initial ⁴⁰Ar/³⁶Ar = 301.0 ± 11, MSWD = 5.5). Thus, the older age represents the cooling age, but the presence of chloritized biotite indicates that some crystals underwent retrograde metamorphism after 553 Ma. Twenty-three K-feldspar grains yielded ages from 345 to 403 Ma, with a weighted-mean age peak at 373.8 ± 4.6 Ma and an intercept age of 374 ± 12 Ma (initial ⁴⁰Ar/³⁶Ar = 299.2 ± 9.3 Ma, MSWD = 7.7). The intercept age is interpreted as the time at which the K-feldspar crystals pass through its closure temperature (226–264°C). The initial crystallization depth of the Neoproterozoic granites is estimated to be ~1 kbar (~3.7 km) based on Al-in-amphibole geobarometry. The new thermal-chronological results indicate that rocks were subsequently burial to 2-2.5 kbar (8–9 km) during the Malagasy Orogeny (600–530 Ma). The granites remained at a similar but slightly shallower depth (7-8 km) until the Late Devonian which may be related to the Hirnantian (460-420 Ma) glaciation. Therefore, the exhumation of the Neoproterozoic granites is constrained to no early that the middle Jurassic (~163 Ma) opening of the West Somali Basin, but no later than the Early Paleogene (~62 Ma) opening of the East Somali Basin.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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