FORMATION OF GARNET IN THE HONEY BROOK ANORTHOSITE, CENTRAL APPALACHIAN PIEDMONT, SE PENNSYLVANIA

The Honeybrook Anorthosite is an elliptically shaped, Mesoproterozoic igneous intrusion located in the Piedmont Upland in Chester County of southeastern Pennsylvania. Published maps (Crawford, et al., 1971) show a concentrically zoned body which increases in mafic content outward from a central anorthosite, through dioritic anorthosite, anorthositic diorite, to banded diorite at the margin. Large (1 – 5 cm) garnet is present in the anorthositic diorite. This garnet occurs in polycrystalline clusters surrounded by leucocratic coronas. The clusters are elongate subparallel to weakly developed foliation and in turn frequently occur in linear arrays on outcrop surfaces. This study seeks to determine the pressure and temperature conditions under which these unusual garnets formed.

Garnet is sub-to-euhedral and is unzoned except for a slight increase in Mn at the rim. Garnet composition, Alm₅₃Prp₂₉Grs₁₆Sps₀₂, indicates formation at high T; calculations using TWEEQU give temperatures of approximately 800°C based on Fe-Mg exchange between garnet (Grt) and hornblende (Hbl). We use mineral assemblage and other diagrams calculated with Theriak-Domino (T-D), the Holland & Powell (2011) database, and Hbl solution model of Green et al. (2016) to test an initial hypothesis that Grt formed from Hbl through a diffusion-controlled reaction mechanism under subsolidus conditions. Published bulk rock compositions, whole rock compositions estimated from EDS scans of thin sections, and model compositions calculated from measured mineral compositions and estimated modes were used in T-D calculations. Diagrams show an increase in modal Grt and corresponding decrease in Hbl abundance on isobaric cooling or with increasing pressure, but at much lower T than indicated by Pyp content of garnet. No single reaction where Grt forms from Hbl has been identified and Grt compositions approximating those in the rock do not appear in models at temperatures below 1000°C. Further, calculations indicate a solidus temperature of ~700°C. Thus, our initial hypothesis is inconsistent with model results to date. Instead, we propose that garnet growth may have been driven by local disequilibrium involving fluid or melt. Additional modelling will be employed to evaluate this new hypothesis.