Structural and U/Pb chronology of superimposed folds, Adirondack Mountains: implications for the tectonic evolution of the Grenville Province

The Grenville Province of eastern Laurentia formed during Mesoproterozoic events that led to the formation of the supercontinent of Rodinia. Structural analysis and U-Pb geochronology of the Piseco antiform in the southern Adirondack Highlands part of the Grenville Province has shed new light on the geology and structural history of this orogen. An outcrop positioned on the northern limb of the Piseco antiform near Wells, NY, contains a major F1 fold refolded by regional F2 and F3 axes. This is the first recognized large-scale F1 fold in the Adirondack Highlands. Foliated leucocratic gneiss and deformed metagabbro dikes in the core of the antiform have yielded single-grain U-Pb zircon ages of 1172.4±2.2 and 1165.4±1.3 Ma, respectively. Discordant pegmatitic veins that crosscut the foliation in the metagabbro have yielded an estimated 207Pb/206Pb age of 1052.4 Ma. The geochronological data have several important implications. The 1172 Ma age of the Wells leucocratic gneiss is virtually identical with that of the syntectonic Hyde School gneiss, Rockport granite, and Wellesley granite from the Adirondack Lowlands and Frontenac terrane of the Central Metasedimentary Belt, thus extending the known distribution of magmas of this age to the southern Adirondack Highlands. We relate these syntectonic granitoids and F1 folds to the suturing of the ca. 1350–1250 Ma Adirondack Highlands-Green Mountain arc to the composite arc terrane and Laurentia at 1220–1170 Ma. The 1165 Ma deformed metagabbro contains the S2 but not the S1 foliation, and we correlate the metagabbro with the ca. 1170–1125 Ma AMCG suite, interpreted to have formed in response to orogenic collapse and delamination following the collision. The Wells leucocratic gneiss and gabbro were folded by reclined isoclinal F2 folds and upright F3 folds, and cut by apparently undeformed ca. 1052.4 Ma pegmatitic dikes. Since the discordant pegmatites cut the S1, S2, and S3 foliations which postdate the ca. 1090 Ma Hawkeye granite, the timing of F2 and F3 deformations is constrained between ca. 1090 and 1052.4 Ma. This interval corresponds to the ca. 1090–1040 Ma Ottawan Orogeny in the Adirondacks, related to the continent-continent collision of the Laurentian margin with another continent, possibly Amazonia.