MORE GEOCHRONOLOGIC EVIDENCE FOR POLYPHASE 1250-950 MA THERMOTECTONISM IN THE ADIRONDACK LOWLANDS, NEW YORK

Recent U-Th-Pb dating of polygenetic monazite in high-grade gneisses in the Adirondack Lowlands has revealed Elzevirian (~1250 Ma), Shawinigan (~1150 Ma), and Ottawan (~1075 Ma) orogenic signatures, whereas previously there was no Ottawan signature recognized in the Lowlands. Specifically, monazites from non-mylonitized Popple Hill gneiss (PHG) yield age populations of 1271 ± 4 to 1253 ± 9 Ma, 1185 ± 5 to 1148 ± 5 Ma, and 1083 ± 5 Ma (weighted means reported with 95% confidence). Separating the PHG from an adjacent carbonate unit (Lower Marble) is the laterally extensive Hailesboro Ductile Deformation Zone (HDDZ) mylonite. Monazites from HDDZ samples yield age populations of 1246 ± 7, 1152 ± 1, and 1082 ± 4 Ma, thus indicating no age difference between the mylonitized and non-mylonitized PHG. The ~1250 Ma ages are interpreted as dating the volcanic protolith of the PHG, whereas the ~1150 and ~1080 Ma ages record discrete episodes of younger thermotectonism experienced by the PHG. Likewise, monazites from 1172 Ma plutons of Hyde School gneiss (HSG) yield age populations of 1262 ± 4 Ma (Elzevirian inheritance), 1185 ± 4 Ma (Shawinigan emplacement), and 1043 ± 24 Ma (Ottawan reworking). The meta-intrusive HSG bodies are actually mantled gneiss domes, in which the coring HSG is separated from intruded Lower Marble by the Hyde School Marginal Mylonite (HSMM). Monazite within the HSMM records neither Elzevirian nor Shawinigan ages, instead preserving an array of ~1120-1080 Ma mixed ages, a predominance of 1073 ± 4 Ma (Ottawan) ages, and subordinate 975 ± 10 Ma ages (Rigolet orogeny?). Thus, pre-Ottawan ages were locally reset by gneiss doming that produced the HSMM during Ottawan-Rigolet time, whereas only the coring plutons preserve the older signatures of ~1250 Ma Elzevirian inheritance and ~1170 Ma Shawinigan plutonism. The HSMM gneisses also contain large (1–4 cm diameter) porphyroblasts of helicitic garnet that preserve microtextural evidence for two discrete episodes of syntectonic growth, which probably occurred during Shawinigan and Ottawan thermotectonism. Finally, preservation of ~1070-1080 Ma monazite ages in the Lowlands (this study) suggests that the Adirondack Lowlands and Highlands were adjacent domains during Ottawan time, instead of widely separated as inferred in previous models of Rodinia assembly.