TEXTURAL AND MINERALOGICAL VARIATIONS IN A SINGLE EXPOSURE OF METAGABBRO AND META-ANORTHOSITE, INDIAN LAKE, ADIRONDACKS, NY

Mineralogical and textural variations were systematically investigated along a single large road-cut exposure along NY Rt. 30 south of the town of Indian Lake, central Adirondacks, New York state. The exposure includes both metagabbroic rocks and meta-anorthosite. This study was undertaken based on the field observation that the two main rock types in the exposure over a span of ~100 meters do not have clear contacts, yet, at the outcrop level, the rock appears to grade north to south from meta-anorthosite to metagabbro. Although there is only a weak local fabric visible at the outcrop level, fabric analysis shows variable fabric development in the rock, including fabrics defined by all minerals except for plagioclase porphyroclasts. The textures are observed at outcrop, but hand-specimen and thin-section analysis reveals these textures as solid-state fabrics, dominated by aligned ferromagnesian minerals, and with variable orientation and strength. Point counting results show that the northernmost end of the exposure is composed primarily of plagioclase and quartz, with almandine garnet, pyroxene, and opaque minerals. Although plagioclase is the dominant mineral at about 60% in the northernmost and central parts of the exposure, percent plagioclase does not justify a designation of anorthositic composition. However, approximately 25% of the rock is labradoritic plagioclase porphyroclasts that are between 5 and 10 cm in the long dimension, increasing in size with traversing from the north end towards the central part. Therefore we interpret the protolith of the central and northern part of the exposure to have been at least sub-anorthositic in composition. The southernmost exposure is gabbroic in composition with much less plagioclase relative to the other end of the exposure, and the presence of pyroxene and garnet at much higher %, relatively. The south-central and southern parts of the exposure appear to be a mixture of the (literal) two end-members with smaller labradorite porphyroclasts (augen) in a metagabbroic composition matrix. To augment field and thin-section data on fabrics and compositions of the rocks, we use single-crystal XRF and XRD to investigate any plagioclase systematic variation across the transition from gabbroic to sub-anorthositic compositions.