87 Presentaciones de Geociencias en Español: Continuamos con la experiencia en la GSA 2025/Geoscience Presentations in Spanish: Continuing the Experience at GSA 2025, Part I

This session invites Geosciences presentations in Spanish at all proficiency levels; abstracts can be submitted in English or Spanish and they do not count towards the GSA’s limit of one abstract oral presentation per person.

Monday, 20 October 2025
8:00 AM - 12:00 PM
HBGCC, 217B

Difficulties in dating mafic igneous rocks: the Ediacaran-Cambrian volcanism of western Laurentia.

Paper number: 87-3
Program: Technical Sessions
Time: 8:35 AM – 8:55 AM

Authors

Abstract

Dating igneous mafic rocks using various radiometric methods has proven to be a challenge when they are metamorphosed. These difficulties mainly reside due to the absence of primary minerals to be measured by mass spectrometry, which has led to exploration of new techniques and methodologies to separate, isolate and measure mineral phases related to magma crystallization. For the past 10+ years we have combined several techniques to date late Neoproterozoic mafic-ultramafic volcanic rocks in western Laurentia, most of which are affected by Mesozoic and Tertiary granitoid intrusions. The under-silica saturated characteristics of these alkaline to tholeiitic volcanics with OIB to E-MORB signatures inhibit formation of zircons and baddeleyite is likely absent due to the very-low grade to low grade metamorphism affecting the units. Recent 40Ar/39Ar geochronology in clinopyroxenes, has demonstrated that this metamorphism affects the diopside and augite crystals despite electron microprobe analyses showing the chemically unaffected pheno-/micro-crystals in polished thin sections. The net effect is that dating of these crystals yields ages that are approximately 10% younger than the plausible dates given other geologic constraints for these rocks. For example, comparison of these results to initial phlogopite and amphibole separates suggests an 40Ar/39Ar age of ~ 576 Ma and ~572 Ma respectively for a trachybasalt clast in a mafic agglomerate of the Browns Hole Formation in Utah. [JH1] To resolve these discrepancies, we hope to continue exploring new techniques such as separating rare zircons from mafic rocks using different types of granular feeders and shake tables.

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

doi: 10.1130/abs/2025AM-412034

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