GEOCHEMISTRY OF THE HONEY BROOK ANORTHOSITE, PENNSYLVANIA PIEDOMNT, AND A COMPARISON WITH OTHER MASSIF ANORTHOSITES OF THE EASTERN USA AND CANADA

The Honey Brook massif is a small, ~10 x 6 km Proterozoic-age pluton (pre-Grenville), located ~50 km NW of Philadelphia.  XRF whole-rock analyses confirm the presence of an anorthosite-leucogabbro suite (~85-97 % normative Fsp) having a considerable compositional range (~An₅₅Or₃ to An₄₁Or₁₁, ~1000-1300 ppm Sr, ~250-500 ppm Ba).  Local examples of TiP-rich leucogabbros also occur (up to ~2 wt% TiO₂ and ~1 wt% P₂O₅, ~77-82 % Fsp, ~An₄₅Or₇).  Unfortunately, the observed compositional variations at Honey Brook show no apparent correlation with position, so pluton-scale zoning – if present – is not revealed by our data.

In terms of An-Or systematics, Honey Brook is distinct from the more-sodic and -potassic pre-Grenville massifs in Virginia (Roseland and Montpelier), as well as from the post-Grenville, more-sodic and -potassic CRUML-belt massifs in Quebec (e.g., St. Urbain and Labrieville).  However, Honey Brook overlaps the compositional range of the pre-Grenville Marcy (New York) and Morin (Quebec) massifs.  In contrast, concentrations of Ba and Sr overlap with those of St. Urbain, but not those of Morin or Marcy (similar Ba but higher Sr), or those of Roseland-Montpelier (similar Sr but higher Ba).  Honey Brook thus has its own distinctive geochemical fingerprint.

Also present in the Honey Brook massif are magnetite-rich, hornblende gabbros interpreted previously as magmatic segregations.  These range from nearly pure anorthosite (~92% Fsp, An₄₀Or₁₅) to melagabbro (~45% Fsp, An₃₀Or₁₀).  Elemental abundances in these rocks lie along linear trends between a felsic component (that extrapolates to An₂₈Or₇ with ~1300 ppm Sr and ~600 ppm Ba), and a mafic component containing >27 wt% Fe₂O₃-T and >3.6 wt% TiO₂, but <0.6 wt% P₂O₅.  The latter component contains high concentrations of both ferromagnesian (>250 ppm V, >50 Ni, >35 Co, >85 Cr, and >300 Zn) and high-field-strength trace elements (>365 Ce, >280 Y, >65 Nb, and >980 Zr).  The juxtaposition of such high concentrations of these two trace-element groups is unlike any material described previously in any anorthosite massif.  If this hornblende-gabbro suite is indeed consanguineous with the other Honey Brook rocks, then a hybrid origin through mixing of earlier- and later-formed materials must be considered. The low P would seem to rule out silicate-oxide melt immiscibility, however.