268-4 Multi-step Chemical Abrasion and Dissolution applied to complex zircon grains from the Proterozoic Mt. Marcy massif anorthosites, New York

Session: Old and the New, Long and the Short: Perspectives on Integration of Timescales of Magmatic Processes: Special Session Related to MGPV Awards to Madison Myers and Anita Grunder (Posters)

Presenting Author:

Authors:

Abstract:

In U-Pb zircon geochronology, there is a trade-off between analytical precision and spatial resolution. Isotope Dilution-Thermal Ionization Mass Spectrometry (ID-TIMS) can offer high-precision analyses, but typically analyzes whole grains or fragments, while microbeam methods provide high spatial resolution at lower precision. To bridge this gap, we present preliminary results using a stepwise partial dissolution technique on zircon from the Mesoproterozoic Mt. Marcy anorthosite (New York), where prior work has demonstrated that zircon grains have complex growth histories, including metamorphic overgrowths, which challenge obtaining primary igneous geochronology using bulk methods. Our partial dissolution method leverages the chemical abrasion pretreatment that suppresses Pb/U fractionation during high-temperature leaching, and enables us to isolate distinct components of individual zircon grains. The initial leaching steps remove Pb-loss-affected domains and target the thin outer metamorphic rims, while subsequent leaching steps progressively isolate the igneous component. When age data from the sequential leaching steps are consistent, which we interpret this as the crystallization age of the igneous zircon.

Using this method, we were able to obtain precise zircon crystallization ages, free from Pb-loss or metamorphic overgrowths, from two distinct lobes of the Mt. Marcy massif anorthosite. Two samples of the main phase yielded igneous crystallization ages of ca. 1155.3 Ma and ca. 1156 Ma. In contrast, the Westport phase has an igneous crystallization age of ca. 1160.3 Ma. Our preliminary results indicate at least two discrete episodes of zircon crystallization at approximately 1160 Ma and 1156 Ma, suggesting that the Mt. Marcy massif comprises two or more anorthosite plutons emplaced roughly 4 million years apart. This suggests that, like the Kunene anorthosite complex in Angola, the Mt Marcy massif may be the result of discrete intrusive events.  However, without additional geochronologic data, we cannot fully rule out the possibility of concentric cooling within a single, large, long-lived magma mush. 

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

doi: 10.1130/abs/2025AM-8496

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