Paper No. 7
Presentation Time: 10:35 AM
SHRIMP U-PB GEOCHRONOLOGY OF ZIRCON AND MONAZITE FROM CA. 1.3 GA ARC-RELATED ROCKS, NEW JERSEY HIGHLANDS
New SHRIMP U-Pb geochronologic data from zircon and monazite define igneous and metamorphic events of Mesoproterozoic age in the New Jersey Highlands involving the arc-related Losee Suite and associated back-arc volcanic and sedimentary rocks. Dating by in situ microanalysis is necessary because zircon and monazite grains from these rocks are complex and contain multiple age components, as revealed by cathodoluminescence (CL) and backscattered electron (BSE) imaging. Ages of volcanic and plutonic rocks of the Losee Suite were determined by analyzing igneous cores of zircon displaying euhedral, concentric, oscillatory zoning in CL. Rocks dated are metatonalite (1282 ± 7 Ma), orthopyroxene (opx)-bearing metadacite (1254 ± 5 Ma), and opx-bearing metadiorite (1248 ± 12 Ma). These data confirm previous interpretations of the Losee Suite as correlative with widespread ca. 1.3 Ga arc-related magmatism in the northern Appalachians, Adirondacks, and Canadian Grenville Province. The Losee Suite is the southernmost recognized calc-alkaline magmatic arc of the eastern margin of Laurentia prior to 1.2 Ga. Felsic gneisses (metarhyolite) are spatially associated with the Franklin Marble and are interpreted to have formed in a back-arc setting. Cores of igneous zircon from two samples of felsic gneiss yield ages of 1299 ± 5 and 1259 ± 7 Ma, dating the times of extrusion of the igneous protoliths. The 1299 Ma age suggests that there may be older (i.e. >1.3 Ga) rocks of the Losee Suite arc sequence. Losee arc and back-arc rocks were intruded by ~1.1-1.2 Ga granitoids of the Byram and Lake Hopatcong Suites.
Overgrowths on igneous zircons (shown by CL as dark and unzoned) from all samples, and monazite from a sample of felsic gneiss, show a range of ages, mostly between about 1025 and 990 Ma, indicating that rocks of the Losee Suite were metamorphosed to high grade during several thermal events broadly related to Ottawan orogenesis. BSE imaging of monazite from opx-bearing metadacite shows well-defined cores, mantles, and rims. These growth zones formed during episodic metamorphic events at 1045 ± 6, 1032 ± 5, and 970 ± 8 Ma. The new geochronologic data from zircon and monazite define a complex, discontinuous sequence of events that, in total, spanned about 330 m.y.