20 Celebrating Allan Ludman’s First 56 Field Seasons in Maine: What Do We Know of Northern Appalachian Geology and What Questions Remain (Posters)

Sunday, 20 March 2022
1:30 PM - 5:30 PM
Freedom Hall A

ZIRCON GEOCHRONOLOGY AND GEOCHEMISTRY OF EARLY TO MIDDLE DEVONIAN GRANITIC AND VOLCANIC ROCKS FROM THE CENTRAL GULF OF MAINE

Paper number: 20-2
Program: Theme Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

New U-Pb laser ablation inductively coupled mass spectrometry (LA-ICPMS) zircon geochronology and geochemistry data are presented for five Early to Middle Devonian granitic and silicic volcanic rocks from the central Gulf of Maine, previously analyzed by U-Pb LA-ICPMS and whole-rock geochemical methods. Coarse-grained alkali-feldspar granite from northwest of the Fundy fault, the interpreted boundary in the offshore between Gondwanan microcontinents Ganderia to the northwest and Avalonia to the southeast, yielded a crystallization age of 415 ± 2 Ma. Southeast of the fault, crystallization ages are 384 ± 2 Ma and 386 ± 2 Ma for two crystal tuff samples near the fault, 400 ± 2 Ma for an alkali-feldspar granite ~50 km southeast of the fault, and 398 ± 2 Ma for syenogranite ~25 km southeast of the fault, which also yielded inherited grains at ~1.3 Ga and between 613 ± 15 Ma and 558 ± 9 Ma.

Zircon Nb/Hf ratios >0.001 and Nb/Th ratios >0.05 indicate a within-plate/anorogenic/rift setting for the samples, consistent with their whole-rock chemistry. U/Yb-Nb/Yb and U/Yb-Hf tectonic setting discrimination diagrams showed juvenile signatures for the alkali-feldspar granite samples, ocean island signatures for the crystal tuff samples, and ocean island to continental arc signature for the syenogranite. Eu/Eu* values are <0.1 for all samples except the alkali-feldspar granite southeast of the Fundy fault, indicating a crustal thickness of ~30 km or less, consistent with an extensional setting and previous interpretations. In addition to zircon with Eu/Eu* values <0.1, the alkali-feldspar granite southeast of the Fundy fault yielded some zircon Eu/Eu* values up to 0.4, suggesting that the crust may have been locally thicker during crystallization, perhaps as a result of Acadian orogenic thickening in Avalonia. Significance of these and similar granite south of Grand Manan Island remain unresolved problems in regional tectonic interpretations.

Geological Society of America Abstracts with Programs. Vol. 54, No. 3

doi: 10.1130/abs/2022NE-375307

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