FIELD AND LABORATORY GEOCHEMICAL ANALYSIS OF HIGH-AL ORTHOPYROXENE MEGACRYSTS IN ADIRONDACK ANORTHOSITE

High-Al orthopyroxene (opx) megacrysts are a common minor phase in Proterozoic anorthosite massifs. These crystals range from euhedral to anhedral and contain plagioclase (and sometimes garnet) exsolution lamellae, in addition to the clinopyroxene and ilmenite exsolution seen in opx megacrysts with lower Al₂O₃ contents. This study re-examines opx megacrysts from the Marcy anorthosite massif in the Adirondack Highlands using bulk-crystal XRF laboratory analyses to accurately and precisely determine mineral chemistry of a suite of megacrysts, and also uses a hand-held XRF to examine outcrop-scale variability in megacryst compositions. Twenty-two opx megacrysts have a 3 to 8 wt% range in Al₂O₃ that correlates with Mg#s of 73 to 63 (laboratory analyses), similar to results found by other workers (e.g. Jaffe and Schumacher, Can Min 1985). Compositions such as these have been interpreted by many workers as indicating crystallization at high pressures; the Emslie et al. (J Geol 1994) calibration yields a range of 5 to 12 kbar for these samples.

A subset of the crystals above were used for a preliminary calibration of a Thermo Fisher Niton XL3t Ultra hand-held XRF for field measurement of megacryst compositions. The precision of replicate analyses of megacrysts in outcrop for Al₂O₃ was normally between ±5 and 25% relative. Forty-two megacrysts were measured (in triplicate) along 700m field traverse of continuous anorthosite exposure at the Bennies Brook slide, where diverse coarse-grained igneous textures and intrusive relationships (“block structure”) are exposed. Megacrysts range from 2 to 9 wt% Al₂O₃ and most are 2–4%. Typically, a group of texturally-related megacrysts from a small area (an anorthosite “block”) have similar Al₂O₃ contents, but often with a range of values larger than analytical uncertainties. These values could be interpreted as reflecting a polybaric crystallization for each block, but might be more of a reflection of the evolving composition of the crystal mush’s interstitial melt.