NEW U/PB ZIRCON AGES FROM NORTH ISLESBORO, WESTERN PENOBSCOT BAY, MAINE: STRATIGRAPHIC AND TECTONIC IMPLICATIONS

In western Penobscot Bay, dominantly metasedimentary rocks on Islesboro abut the St. Croix terrane (quartz-rich metasedimentary rocks, negative ε_Nd) to the west and the Ellsworth terrane (marine volcanic rocks, positive ε_Nd) to the east. These contrasting continental and oceanic elements are mainly in contact along pre-Middle Devonian high-angle faults, but they were initially juxtaposed on a thrust of post-Tremadoc/pre-Wenlock age (post-Penobscot/pre-Ames Knob). We present here U/Pb ages of zircon determined by laser ablation ICP-MS on three samples collected from different parts of the Islesboro Formation of Stewart (1998).

Quartzite from Hutchins Island (44°21.045ʹN, 68°52.227ʹW), near deformed marble of the Coombs member and minor foliated amphibolite, yields abundant detrital zircon U/Pb ages (n = 120) of ca. 2 Ga and minor ages of 2.8-2.4 Ga, with a gap at 2.4-2.2 Ga and no grains <1.85 Ga. This age spectrum, unique in the region, permits correlation with the >647 Ma Seven Hundred Acre Island Formation and the Ashburn Formation in coastal New Brunswick, which additionally contains numerous Mesoproterozoic grains.

Near Kedears Hill, where deformation is minor (single steep cleavage), the youngest detrital zircon U-Pb age populations in sandstone (44°22.429ʹN, 68°53.652ʹW; n = 112) and nearby felsic tuff (44°22.454ʹN, 68°53.613ʹW; n = 70) are Paleozoic. The sandstone is no older than ca. 514 Ma, has abundant Neoproterozoic (683, 626, 577 Ma) and numerous Meso- and Paleoproterozoic grains, and grains of the same ages as in the quartzite. The tuff is apparently Devonian (youngest igneous grains ca. 380 Ma), and has complex zircon rim and core relationships including Silurian, Ordovician, and Cambrian ages as well as a strong “Grenvillian” signal (ca. 1160-957 Ma) plus Mesoproterozoic and Archean ages.

The quartzite-hosted detrital zircon ages are consistent with an Amazonian origin, conceivably representing the oldest part of a long-lived Ashburn passive margin. These rocks likely departed Gondwana during the Middle Cambrian, were overthrust by Ellsworth and transferred to the upper Exploits plate pre-Wenlock, and then accreted to Laurentia by Devonian time where they were fragmented along steep faults in an upper-plate position. Clearly, field relations on Islesboro need to be re-examined.