NEW U-PB DETRITAL AND IGNEOUS ZIRCON GEOCHRONOLOGICAL RESULTS FROM THE ST. CROIX TERRANE AND ISLESBORO BLOCK, PENOBSCOT BAY, MAINE

The Penobscot Bay region of coastal Maine hosts a diverse, complex and puzzling array of continental trough oceanic peri-Gondwanan elements that for decades has defied a satisfying tectonic explanation. The recent discovery of a potential link to Paleoproterozoic strata of the West African craton adds a piece to the puzzle, but the puzzle remains unsolved. Within the bay, the Turtle Head dextral strike-slip system juxtaposes the Neoproterozoic(?)–Ordovician St. Croix terrane of western Penobscot Bay against the middle Cambrian Ellsworth terrane to the east. The Paleoproterozoic(?)–late Devonian(?) Islesboro block, located between these two terranes, has hypothetical stratigraphic ties to the Rockport belt of the St. Croix terrane, but this connection is based predominantly on broad lithostratigraphic similarities. Moreover, the internal stratigraphy of the Islesboro block itself is complex and poorly understood. In order to test these potential stratigraphic correlations, we performed U-Pb laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) on detrital and igneous zircons from samples collected from several formations within the Islesboro block and St. Croix terrane. Specifically, we targeted the type areas of the Seven Hundred Acre Island and Islesboro formations to perform internal consistency checks on Islesboro stratigraphy and then compare these data to samples collected from the Rockport and lower Cookson groups of the mainland Rockport belt. Together, these data provide critical tests of regional tectono-stratigraphic models and yield new insights into development of the northern margin of the Rheic Ocean.