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

Laurentia Mesoproterozoic Ferroan (“A-type”) Granites: Isotopic Constraints of Megacrystic K-feldspar Granitoids Across Arizona

Paper number: 242-3
Program: Technical Sessions
Time: 8:35 AM – 8:50 AM

Authors

Abstract

A prominent magmatic episode across Laurentia during the Mesoproterozoic resulted in widespread granitic intrusions commonly referred to as the 1.4 Ga ‘anorogenic’ suite. Both the scale of the event and its significance for the growth of continental crust has sustained long-standing debates surrounding what processes and magma sources were involved in their formation. Here, we characterized 14 granitoids of ca. 1.4 Ga age and two of ca. 1.6 Ga age across three distinct crustal provinces in Arizona for their mineralogy, crystallization ages using U-Pb zircon geochronology (LA-ICP-MS), and source characteristics using a combination of zircon Lu-Hf (initial εHf, LA-MC-ICP-MS) and oxygen isotope values (δ18O ‰ VSMOW, SIMS).

(1) Mojave province: granitoids in NW Arizona yield U-Pb crystallization ages spanning ~1.38 to 1.42 Ga, with initial εHf between ~ -3 to +3 and mantle-like to elevated δ18O up to ~8.5 ‰.

(2) Yavapai province: a single sample from this province in central Arizona yielded an age ca. 1.4 Ga with initial εHf between ~3 to +8 and elevated δ18O values up to ~9 ‰.

(3) Mazatzal province: granitoids in SE Arizona yielded U-Pb crystallization ages spanning ~1.42 to 1.45 Ga. While the initial εHf values of all samples range from approximately +2 to +8, there are two distinct groups in δ18O values; one group shows mantle-like δ18O values around 5 ‰, while the second group has more elevated values up ~10 ‰.

Overall, the isotope compositions of the studied ~1.4 Ga granites are heterogeneous, highlighting the presence of multiple components in their petrogenesis, and are generally consistent with mixing between older crust(s) with varying mass contributions from the mantle throughout the different provinces. Most significantly, the distinct isotopic compositions in granitoids from the Mazatzal Province warrants further investigation to the origin of the diverse ranges in composition for units of relatively close spatial proximity. Nonetheless, this study raises several questions regarding the petrogenesis of these anorogenic/ferroan granitoids, including the nature, duration, and extent of the heat source driving such voluminous magmatism.

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

doi: 10.1130/abs/2025AM-413181

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