Browsing by Author "Stowell, Harold H"

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    Evaluating the Metamorphic History of the Coast Mountains Batholith in the Stikine River Area: Alaska and British Columbia
    (University of Alabama Libraries, 2023) Gutapfel, Savanna W; Stowell, Harold H
    The Coast Mountains batholith stretches more than 1,700 km along the western edge of North America and preserves a record of terrane amalgamation and magmatic arc processes. I present garnet Sm-Nd ages and pressure, temperature, time (P-T-t) paths for rocks along a transect crossing the Coast Mountains batholith near the Stikine River and Wrangell, AK. These sample locations straddle the Coast Shear Zone (CSZ): a fundamental, arc-parallel crustal break west of the batholith. The three new garnet Sm-Nd ages for metamorphism are 86.5 ± 3.6 (n=5) and 95.3 ± 4.8 (n=6) Ma in the Western Metamorphic Belt (WMB), and 57.0 ± 0.7 (n=6) Ma in the Eastern Metamorphic Belt (EMB). These ages, coupled with associated P-T-t paths provide new constraints on the plutonic and metamorphic history across the Coast Mountains. The WMB, marked by kyanite and staurolite zone mineral assemblages, was metamorphosed at 7.5 ± 1 kbar and 600-650°C between 86 and 95 Ma. Metamorphic conditions in the EMB are constrained by sillimanite replacing andalusite, and the P-T-t path indicates garnet growth at pressures from 4 to 6 kbar and temperatures from 550 to 650°C at ~57 Ma. These results indicate that garnet growth and cooling below garnet growth temperatures was ~30 million years earlier in the WMB than in the EMB. The metamorphic pressure estimates require > 10 km more exhumation in the WMB, which must have occurred significantly earlier in this area than further east. The new metamorphic data can be explained by exhumation due to focused uplift west of the CSZ and/or west-side-up tilting of the batholith ~80 Ma. This was followed by lesser amounts of exhumation in the central and eastern parts of the batholith.
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    Reconstructing Early Earth and Lunar History: Insights from Terrestrial and Extraterrestrial Zircon
    (University of Alabama Libraries, 2025) Faltys, Jordan Paul; Stowell, Harold H
    Zircon (ZrSiO4) is a common accessory mineral in terrestrial and extraterrestrial rocks, valued for its ability to record magmatic conditions and serve as a geochronometer. Its resilience to weathering, capacity to incorporate U and Th while excluding Pb during growth, and ability to retain REEs, Hf, and Ti make zircon a powerful tool for probing high-temperature events such as meteor impacts. These properties have provided insights into early Earth’s evolution, from the Hadean era to the emergence of an atmosphere, oceans, felsic crust, plate tectonics, and the ingredients for life. However, Hadean zircon are rare, limiting data and necessitating modern analogs to improve our understanding. This dissertation comprises three parts: 1) geochemical characterization of inclusions in impact-formed zircon, 2) mathematical modeling of the size-frequency-distribution of impactors during the Hadean, and 3) tomographic analysis of Pb nano-spheres in lunar zircon. First, mineral inclusions in zircon from Sudbury, Canada, and Morokweng, South Africa, are compared to possible parent rock compositions. Inclusions trace host rock composition, and trends in Total Alkali Silica and Quartz – Alkali Feldspar – Plagioclase – Feldspathoid diagrams suggest Hadean zircon formed in tonalite-trondhjemite-granodiorite-like rocks. This research reconstructs ancient crustal compositions erased from the geologic record. The second part of this treatise models of high-energy impacts revealing that SiO2 enrichment occurred early in Earth’s history. Geochemical fractionation patterns in Sudbury’s differentiated impact melt sheet suggest impacts contributed to felsic proto-continents, with a Late Heavy Bombardment producing magma with ≥58 wt.% SiO2, covering ca.10% of Earth’s surface to a minimum thickness of approximately 2 km. Lastly, the third part of this dissertation uses the isotopic and trace element composition of extraterrestrial zircon in order to evaluate high temperature impact events. Atom Probe Tomography (APT) lattice reconstructions for Apollo 14 lunar zircon, identify Pb nano-clusters linked to high-energy impacts. The APT reconstruction of a Pb nanocluster within a ~4.3 Ga lunar zircon has a 207Pb/206Pb ratio of 1.39 ±1σ indicating Pb accumulation from 4.3–3.9 Ga, reflecting a major lunar impact event at ~3.9 Ga. This suggests the Moon’s surface experienced a high-energy event, or events, at 3.9 Ga.