PRELIMINARY GEOLOGIC MAP OF THE ISLESBORO 7.5' QUADRANGLE WITH IMPLICATIONS FOR THE EVOLUTION OF THE ISLESBORO BLOCK, COASTAL MAINE

The Islesboro block is a fault-bounded crustal fragment isolated between the St. Croix and Ellsworth terranes in Penobscot Bay, Maine. We present the results of new geological mapping of the Islesboro 7.5´ quadrangle, which is coupled with new laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb zircon data to support revised unit subdivisions and provide an assessment of the age and provenance of the Islesboro block. Based on these data, the Islesboro block consists of: (1) the Paleoproterozoic–Mesoproterozoic(?) Seven Hundred Acre Island Formation, composed of marble, quartzite, and garnet-mica schist, and intruded by highly retrogressed Neoproterozoic (ca. 670–655 Ma) amphibolite and pegmatite, the latter of which is dated herein by LA-ICPMS U-Pb analyses on zircon; (2) the Neoproterozoic–Cambrian(?) Islesboro Formation, a heterogenous package of metaclastic and metacarbonate rocks subdivided here into seven new distinct members; and (3) the informal Cambrian(?) Turtle Head Cove formation, consisting of metaclastic rocks subdivided here into two distinct units. The stratigraphic relationships among these primary formations are commonly obscured by faults, but repeatable map patterns suggest they were once in depositional contact. The Islesboro block is bounded by two major dextral strike-slip faults (Turtle Head and Penobscot Bay faults), which are linked by N-S- and NE-SW-striking faults that offset and overprint at least one previous phase of contractional or transpressional deformation. All units of the Islesboro block record complex deformation, including abundant subvertical faults, open to isoclinal and predominantly asymmetrical folds, and narrow to diffuse zones of cataclasis and mylonitization. Additionally, the Seven Hundred Acre Island Formation contains a highly retrogressed, peak mineral assemblage of garnet-kyanite-staurolite-rutile, indicating that it was subjected to higher amphibolite-facies P-T conditions prior to ca. 655 Ma. Ongoing petrologic, geochronologic, and structural analyses will help clarify the origin and tectonic evolution of the Islesboro block and its significance in the northern Appalachian orogen, but our preliminary results suggest strong affinities to both the adjacent St. Croix and Ellsworth terranes.