Raymond P. Fallon

Previous ⁴⁰Ar/³⁹Ar geochronology indicates a complex post-intrusion thermal history for the Port Mouton Pluton (PMP) with most ages lying between ca. 350-310 Ma, suggesting either younger Carboniferous plutonism, slow cooling, or subsequent resetting of an older crystallization age. New U-Pb data for monazite and titanite, and new ⁴⁰Ar/³⁹Ar incremental heating data for muscovite and K-feldspar, coupled with the first reported laserprobe data (total fusion and spot analysis of muscovite) from Meguma Zone plutons, provide clarification of the age and thermal history of the PMP. U-Pb dating of monazite constrains the crystallization age of the main intrusive units to 373 ± 1 Ma. Magmatic muscovite ages of ca. 373 Ma (⁴⁰Ar/³⁹Ar laserprobe) from undeformed samples suggest rapid post-crystallization cooling. Laserprobe muscovite ages of ca. 360 Ma to 345 Ma (with core ages generally older than rim ages) reflect partial loss of accumulated argon from muscovite between ca. 330-300 Ma.. Recrystallized grain margins and core-rim element variation suggest that argon loss from deformed samples occurred by a combination of reaction and deformation mechanisms. Ages of ca. 360-345 Ma from some undeformed samples suggest that other mechanisms may have contributed to argon loss from these samples. Muscovite incremental heating spectra generally yield ages of ca. 360-350 Ma and do not show the apparent age gradients revealed by laserprobe spot fusion analysis. Total fusion ages from all samples are highly variable, possibly reflecting the complex age gradients preserved within PMP muscovite. The oldest ages recorded in the highest-temperature increments of K-feldspar spectra suggest a distinct argon loss event between ca. 280-260 Ma. A U-Pb titanite age of ca. 260 Ma also records this event. Younger ages recorded in low-temperature increments suggest reheating events at ca. 230-210 Ma and ca. 200 Ma. The incremental heating spectra from a sample of pegmatite suggests that argon loss from the least retentive K-feldspar domains occurred fairly recently probably at ambient temperatures. The muscovite laserprobe data presented here are compatible with an argon loss event at ca. 330-300 Ma, a result consistent with previous studies in southwestern Nova Scotia. However, no clear distinction exists between possible separate argon loss events at ca. 330-315 Ma and 300 Ma, a single protracted event lasting ca. 30 Ma, or a single argon loss event at ca. 300 Ma. Argon loss at ca. 330-300 Ma is linked to Alleghanian deformation in the Meguma Zone associated with Laurentian-Gondwanan collision. Although K-feldspar ages ca. 280 Ma could record cooling following thermal resetting at ca. 300 Ma, similar K-feldspar ages from several other plutons in the southwestern Meguma Zone suggest that argon loss was widespread in this part of the Meguma Zone at this time. Feldspar ages of ca. 230-200 Ma correlate well with the timing of mafic magmatism in Nova Scotia associated with North Atlantic opening and elevated temperatures during intrusion could have reset feldspar ages. This study demonstrates the value of integrated geochronological studies involving several techniques on a variety of minerals in elucidating complex thermal histories. In particular, ⁴⁰Ar/³⁹Ar laserprobe studies are more effective than conventional incremental heating analysis in revealing complex age gradients in minerals from complex geological regions.

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Pages: 211
Supervisor: D. Barrie Clarke