EVIDENCE FOR MULTIPLE AGES OF ANORTHOSITE EMPLACEMENT, MARCY MASSIF, ADIRONDACKS, NY

Twenty-four samples of anorthosite from the 3000 km2 Marcy massif, Adirondack Highlands, have now been dated by SHRIMP U-Pb geochronology. Traditionally, rocks of the Marcy massif have been informally subdivided into two facies, the Whiteface (an intrusive complex of foliated leucogabbro, gabbro, and diorite) and the Marcy (massive to weakly foliated anorthosite). Contrary to a published age of about 1155 Ma for the entire massif (McLelland et al., 2004), our preliminary results previously suggested multiple episodes of anorthosite emplacement, ranging in age from about 1160-1020 Ma. Our new data confirm this age range, enable definition of the timing of specific intrusive events, and provide additional evidence for the geographic distributions of plutons within the massif.

Because zircon usually forms late in the anorthosite crystallization process, it tends to be anhedral with irregular cathodoluminescence (CL) zoning patterns (in contrast to zircon that forms early in felsic igneous rocks with euhedral crystals and concentric oscillatory zoning in CL). Thus, recognition of zircon of igneous, metamorphic, or xenocrystic origin is critical to understanding the petrogenesis of anorthosite. Using a combination of external zircon morphology, CL zoning, trace element geochemistry, and analytical spot location guided by CL zoning, we can determine the origin of the dated zircon zones.

Our SHRIMP data indicate that the eastern lobe of the Marcy massif is primarily composed of anorthosite emplaced during the interval of about 1170-1125 Ma. Igneous zircon cores frequently have irregular rims that are generally unzoned in CL and yield a wide range of ages (about 1130-990 Ma). In contrast, much of the western lobe of the massif is composed of younger anorthosite with igneous zircon core ages ranging from about 1050-1025 Ma and zircon rims of about 1040-990 Ma. Four samples of coarse garnet from old and young anorthosite have Lu-Hf ages of about 1025 Ma. Older anorthosite (ca. 1150 Ma) is preserved in the westernmost and southernmost border zones of the western lobe. Collectively, the data for zircon cores and rims suggest at least three intrusive episodes spanning the Shawinigan and Ottawan orogenic phases and at least three intervening metamorphic episodes during the Ottawan and Rigolet orogenic phases.