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

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.