242 Crustal Petrology, Part I

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
8:00 AM - 12:00 PM
HBGCC, 216AB

Get the Pb out: High-precision zircon geochronology of the Massif-type Marcy Anorthosite, southern Grenville Province

Paper number: 242-12
Program: Technical Sessions
Time: 11:00 AM – 11:15 AM

Authors

Abstract

Proterozoic massif-type anorthosite complexes are among the most distinctive and enigmatic igneous rocks on Earth. Despite more than a century of study, nearly every aspect of their petrogenesis, tectonic setting, emplacement, and geodynamic significance remains controversial. Discerning between endmember models for anorthosite massif formation requires accurate and precise geochronologic constraints, but their composition and frequent association with high-grade metamorphism have long hindered high precision dating. The Marcy massif in the Adirondack Highlands is one of the best studied and most accessible anorthosite massifs on Earth, and prior U-Pb geochronologic analysis of zircon via SHRIMP suggests a mean age of ca. 1154 ± 6 Ma for the entire ~3,000 km2 body. However, these data cannot resolve the composite history of the Marcy massif evident from field relations. We conducted paired LA-ICP-MS and CA-ID-TIMS analysis of zircon from samples with clear intrusive relationships and alleged genetic relationships to discern the timescale(s) of anorthosite magmatism within the Marcy massif. LA-ICP-MS data reveal a 1011.6 ± 6.1 Ma population of zircon with chondrite normalized Lu/Y ratios ~1 consistent with metamorphic growth in the presence of garnet, while analyses with higher Lu/Y of ~3-5 yield a continuum of dates from ca. 1200-1000 Ma. We interpret the latter as igneous zircon afflicted by Pb loss. CA-ID-TIMS dates from whole grains yield discordant arrays that do not define meaningful intercepts, likely due to mixing of igneous, metamorphic, and isotopically disturbed domains. However, dates from double polished, laser milled, zircon wafers are systematically older than whole grains, and define statistically robust discordia arrays with steeper slopes in concordia space. Zircon wafer fragments from anorthosite and gabbroic anorthosite yield weighted mean 206Pb/238U ages of 1158.41 ± 0.49 Ma (MSWD = 1.27) and 1158.83 ± 0.48 Ma (MSWD = 2.98) respectively. The oldest of six 206Pb/238U dates from a cross-cutting ferrodiorite dike with a clear chilled margin is 1157.20 ± 0.92 Ma, which we tentatively interpret as the minimum age. Our results are broadly consistent with rapid emplacement of anorthosite and residual melts, but also highlight the internal complexity, decoupling of chemical zoning and isotopic systematics, and ubiquity of Pb-loss in zircon that pose major challenges to routine geochronology of these polymetamorphic, non-uniformitarian rocks.

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

doi: 10.1130/abs/2025AM-413714

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