FORMATION OF GARNET IN THE HONEY BROOK ANORTHOSITE - METAMORPHIC OR MAGMATIC?

The Honey Brook Anorthosite (HBA) is a Mesoproterozoic igneous intrusion including garnet-bearing anorthosite, dioritic anorthosite, anorthositic diorite, and Fe-rich hornblende gabbro, with outcrop area ~40 sq. km, located in the Piedmont Upland of southeastern Pennsylvania (Crawford and Hoersch, 1984). Garnet in anorthosite is large (1–5 cm), sub- to euhedral, largely unzoned and of Mg-rich composition, Alm₅₃Prp₂₉Grs₁₆Sps₀₂. Garnet commonly occurs in polycrystalline clusters, surrounded by leucocratic coronas, elongate subparallel to weakly developed foliation and commonly occur in linear arrays on outcrop surfaces (Higgins, et al., 2017). These observations were originally interpreted to suggest a metamorphic paragenesis, with garnet forming from hornblende under subsolidus conditions. However, mineral assemblage and other diagrams calculated with Theriak-Domino do not support this initial hypothesis. While diagrams show an increase in modal Grt and corresponding decrease in Hbl abundance on isobaric cooling or with increasing pressure, no single reaction where Grt forms from Hbl has been identified. Grt compositional isopleths intersect at 950 °C and 700 MPa, conditions well above Hbl stability and the solidus in this rock.

The temperature indicated by garnet composition and the lack of evidence for similarly high metamorphic temperatures in adjacent rock suggest a magmatic origin for Grt. We use rhyolite-MELTS (Asimow and Ghiorso, 1998; Ghiorso, et al., 1995) to test this hypothesis by modeling cooling of anorthositic magma. Using a fractional crystallization model at P = 700 MPa, we find that Grt, with composition nearly identical to that in the rock, begins to crystalize at ~960 °C, with approximately 35 wt.% liquid remaining. At these conditions, Cpx and Opx crystallization has ceased; only Pl and Grt form. We propose that with this volume of liquid remaining, melt was extracted from the Pl + Opx + Cpx crystal mush leaving behind liquid of similar composition in en echelon tension veins. Alternatively, liquid filled the tension veins at 35 wt% liquid remaining with minimal extraction, simply melt migration and dispersal within the mush. The veins ultimately crystalized into the elongate polycrystalline arrays of Grt surrounded by Pl and Grt is a primary igneous mineral in the HBA.