METAMORPHISM OF ANORTHOSITE IN THE ADIRONDACK HIGH PEAKS

The Adirondack High Peaks are home to the Marcy anorthosite massif, a ~3000 sq km body dominated by lithologies composed of >90% plagioclase feldspar. While the origin of massif anorthosite is debated, many workers believe that anorthosite plutons formed when low-density plagioclase cumulates rose from lower-crustal magma chambers during the Proterozoic. Following the intrusion of the Marcy massif at ca. 1.15 Ga, the High Peaks underwent several high-grade events, most notably the ca. 1.07 Ottawan orogeny, and the anorthosite preserves both igneous metamorphic assemblages. Although most rocks in the Adirondacks show evidence for high-grade metamorphism, it is often unclear which event caused observed assemblages. This study focuses on granulite-facies metamorphic minerals and textures in the Marcy massif with the goal of constraining their origin.

Peak metamorphic conditions of ca. 800–850°C and ca. 8 kbar have determined for the Marcy anorthosite and associated rocks (Bohlen et al., J Pet 1985; Spear & Markussen, J Pet 1997). Interestingly, these studies point to a ca. 200°C span of pyroxene and garnet formation temperatures in the anorthosite.

For this study, EDS analysis was used to determine mineral compositions in 5 samples from the Marcy Massif. Mineral compositions of metamorphic Cpx and Opx crystals were used with the thermometer of Brey and Kohler (J Pet 1990) to obtain temperatures of 830±30°C. Among the samples, temperature determinations are consistent with the literature and have a range of about 200°C. In other samples the garnet-Cpx thermometer was used with an average result of 810±10°C using the Powell (J Met Pet 1985) calibration. These temperatures are consistent with previous studies and help constrain the metamorphic conditions of the Adirondack High Peaks.

Zircon rims were found around ilmenite grains associated with coronas in many of the samples. Morriset and Scoates (Can Min 2008) proposed that zircon can form in anorthosite via from Zr in ilmenite in the presence of Si-rich minerals. Preliminary LA-ICPMS analysis of zircon in thin section has yielded low-U metamorphic zircon with ca. 1.0 Ga ages.