36 Rates, Dates, and Plates: Petrochronological Approaches to Unraveling Tectonometamorphic Histories

This session welcomes contributions  applying modern approaches in metamorphic petrology, geochronology, and structural geology to unravel the dynamics of convergent margin processes from the Appalachians and beyond. We encourage submission from Early Career participants.

Tuesday, 24 March 2026
8:00 AM - 12:00 PM
CCC, Room 22/23

Carboniferous Exhumation of the Sierras Pampeanas, Argentina: Insights from Thermal History Modeling with Monazite (U-Th)/He Thermochronometry

Paper number: 36-10
Program: Technical Sessions
Time: 11:00 AM – 11:20 AM

Authors

Abstract

The Carboniferous-Permian Paganzo Basin (Argentina) archives stratigraphic evidence of alpine glaciation during the Late Paleozoic Ice Age (LPIA), but the timing and extent of unconformity development and the formation of basin-bounding paleotopography prior to the LPIA remain debated. Because previous low-T thermochronology can only resolve Triassic rifting and younger events, a multi-geo-thermochronometry study provides an opportunity to investigate the timing and magnitude of exhumation of the pre-Paganzo Basin landscape, and to illuminate tectonic contributions to glacier development. To address this, we present novel monazite (U-Th)/He stepped-heating and age data, integrated with conventional monazite U-Th-Pb, zircon/apatite (U-Th)/He, and zircon fission-track (ZFT) geo-thermochronology – spanning ~60–270 °C of upper crustal temperature sensitivity. Sample localities include crystalline basement rocks underlying Carboniferous deposits at three locations across the Paganzo Basin (Guandacol, Paganzo, and Valle Fértil).

New monazite U-Th-Pb results yield weighted mean ages of 489–481 Ma at Valle Fértil and 429–421 Ma at Paganzo. Stepped-heating He diffusion measurements of select monazites show linear Arrhenius behavior. Linear fitting of grain-specific diffusion measurements produced Ea values of 210–178 kJ mol-1 and D0/r2 values of 2.40–0.75 x 104 s-1, corresponding to closure temperatures of ~190–270 °C for a 10 °C/myr cooling rate. Monazite (U-Th)/He dates from Guandacol and Valle Fértil range from 330–244 Ma, overlapping with the 303–296 Ma ZFT ages. A single MHe date from Paganzo (444 ± 36 Ma) is older than the monazite U-Th-Pb age but overlaps in uncertainty. Zircon (U-Th)/He dates vary by locality: 260–250 Ma (Guandacol), 336 Ma (Valle Fértil), and 324–169 Ma (Paganzo). Guandacol and Paganzo apatite (U-Th)/He results yield Miocene dates (ca. 24–5 Ma).

Inverse thermal history modeling of Guandacol and Valle Fertil datasets suggests that rapid cooling and exhumation began 340–330 Ma, while the Paganzo site’s exhumation is less constrained, beginning by the Late Devonian. The rates, magnitude, and spatial extent of Ordovician–Silurian basement cooling from the modeling results suggest tectonically driven exhumation. Cooling occurred immediately prior to or concurrent with the Carboniferous onset of regional alpine glaciation in the Paganzo Basin associated with the LPIA. We interpret that rapid exhumation was associated with high tectonic uplift rates, which may have created local structural relief and sufficient elevation to promote alpine glaciation.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411458

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