TEXTURAL AND COMPOSITIONAL VARIANCE OF IN-SITU LITHOLOGIES IN THE EASTERN PORTION OF THE MARCY MASSIF

Spatial variations of texture and composition in large igneous intrusions provide insight into magma dynamics, including relative cooling rates, rheology, and chemical reactions as magma cools to form bedrock. The Marcy massif (Mm) is a ca. 3,000 km² exhumed anorthosite batholith in the Adirondack Mountains of northeastern New York. Anorthosite is a coarse-grained leucocratic lithology composed of ≥90% plagioclase feldspar. The Mm is traditionally lumped on geologic maps as one homogeneous unit termed “anorthosite” (e.g., McLelland et al, 2004; Seifert et al, 2010). While this is convenient for maps on large scales, the Mm displays lithological heterogeneity (e.g., garnet-bearing anorthosite and gabbro).

In terms of texture, grain size and shape distributions are considered in this study. Out of five sample locations separated by distances between 5.5 km (shortest) and 39.8 km (longest), two types are present: (1) porphyritic, sub- to euhedral plagioclase megacrysts (3-15 cm) on the central portion of the Mm, and (2) phaneritic, an- to subhedral with fewer plagioclase megacrysts (2-3 cm) near the margin. Through petrographic analysis, composition of Mm lithologies are analyzed. To date, the following minerals have been observed in this study: plagioclase, orthopyroxene, clinopyroxene, hornblende, quartz, and garnet; most samples show opaque minerals. Plagioclase and pyroxene are observed in all samples, and in higher abundance compared to other minerals. Sub- to euhedral garnet is also prominent in most samples; however, its abundance is relatively low (≤5%).

High textural variation in the Mm yields room for interpretation. With respect to texture, particularly in plagioclase, the smaller and less developed grains along the margin are tentatively explained by rapid cooling against country rocks. The presence of weakly-developed sub-planar gneissic fabric (defined by aligned plagioclase or pyroxene) near the margin of the Mm could be explained by magma flow against country rock (i.e., magmatic foliation). The next stage of this study will focus on compositional changes across the massif, including the presence of garnet and opaque minerals as seen in thin section.