ISOTOPE AND GEOCHEMICAL ANALYSIS OF ADIRONDACK ANORTHOSITE AND IMPLICATIONS FOR MASSIF EMPLACEMENT HISTORY

A large (3000 km²) composite anorthosite body known as the Marcy massif dominates the Adirondack Mountains of New York, which are a southern extension of the Grenville Province. The anorthosite formed at ca. 1155 Ma during the Grenville Orogenic Cycle, a series of tectonic events which caused regional deformation and igneous and metamorphic activity. The Marcy massif shows typical anorthosite mineralogy; in addition to plagioclase it contains clinopyroxene, orthopyroxene, Fe-Ti oxides, and metamorphic garnet. The depth of emplacement of the Marcy anorthosite is poorly constrained. One control on depth is an area of anomalously low anorthosite δ¹⁸O values around the Tahawus Road, south of the Fe-Ti ore deposits at Sanford Lake, which imply shallow emplacement and interaction with meteoric water (Morrison and Valley, CMP 1989).

This study approaches the emplacement history of the massif by using two separate sample sets, oxygen isotopes and mineral chemistry. First, samples of anorthosite were collected along the Tahawus Road and analyzed for δ¹⁸O by laser fluorination at the University of Wisconsin. Our results range from typical Adirondack anorthosite plagioclase δ¹⁸O of around 8‰ SMOW with an anomalously low zone (as low as 1.2‰) along a 120m series of outcrops. Values of δ¹⁸O in co-existing garnet track plagioclase suggesting high temperature equilibration and a pre-metamorphic origin for the low δ¹⁸O anomaly. The second phase of this project was geochemical analysis of a suite of orthopyroxene megacrysts collected from throughout the Adirondack High Peaks region. The megacrysts were analyzed using bulk XRF and range from 3 to 8 wt% Al₂O₃, which suggests crystallization at pressures of 5 to 12 kbar using the Emslie et al. calibration (J. Geol. 1994). This range of Al₂O₃ contents in the orthopyroxene megacrysts indicates that the anorthosite melt underwent continuous polybaric crystallization as it ascended through the crust. The low δ¹⁸O values in the Tahawus Road rocks suggest eventual shallow emplacement of the anorthosite massif and alteration by heated meteoric waters. Taken together, these results indicate a dynamic process of crystallization as the anorthosite magma made a remarkable ascent from the base of the crust to the shallow depths of meteoric water interaction and emplacement.