Mesoproterozoic Thermotectonic History of the Adirondack Lowlands, New York: Evidence from Monazite Geochronology

Ongoing U-Th-Pb dating of polygenetic monazites from five Adirondack Lowlands lithologies indicates that multiple thermotectonic events amalgamated the Lowlands during the Grenville orogeny between ~1185-950 Ma. The Lowlands consist of three main lithostratigraphic units of Mesoproterozoic age, including the Lower Marble (LM, oldest), Popple Hill Gneiss (PHG), and Upper Marble (youngest). These units are variably intruded by syn-orogenic granitoids, including the Hyde School Gneiss (HSG), Hermon Granite (HG), and Antwerp Granite. The HSG occurs as 14 dome-shaped plutons within the LM and ranges from diorite to alaskite, and the HSG-LM contact is a hybridized mylonite containing garnet and sillimanite or clinopyroxene (Hyde School Marginal Mylonite, HSMM). The PHG is a differentiated metavolcaniclastic unit whose contact with the LM is also a garnet-sillimanite mylonite (Hailsboro Ductile Deformation Zone, HDDZ) representing syntectonically modified PHG, whereas the HG intrudes the LM and PHG. Monazite dating within areally widespread samples of the PHG, HSG, and associated mylonite zones (HDDZ and HSMM) yields inherited Elzevirian-orogenic ages of ~1285-1220 Ma, tectonomagmatic Shawinigan-orogenic ages of ~1185-1145 Ma, and Ottawan-orogenic ages of ~1090-1040 Ma. Additionally, monazites from the HG yield only Shawinigan ages of ~1170-1140 Ma and Ottawan ages of ~1085-1050 Ma, whereas monazites from the PHG, HSG, HSMM, and HG (but apparently not the HDDZ) contain a weak Rigolet-orogenic signature with ages of ~1010-950 Ma. The lack of Elzevirian ages in the HG (250 analyses of five monazite grains) suggests that it was either derived from melting of a different source, or experienced a different assimilation history, than the HSG and PHG. The emerging plate-tectonic implication of this work is that the Adirondack Lowlands and Highlands (which share similar polythermotectonic histories) were adjacent lithotectonic domains during the ~1090-1040 Ma Ottawan orogeny, instead of widely separated at this time as previously inferred in some models of Rodinia assembly.