Adirondack Apatite Geochemistry as Indicator of Magmatic, Pre-Metamorphic Environment

The Adirondack Mountains, located in Northeastern New York State, represent part of the massif of rocks formed during the Grenville orogeny (1190-980 Ma). During the Ottawan phase of the Grenville Orogeny, a large plutonic suite of rocks intruded into the modern center of the Adirondack highlands, creating the Anorthosite-Mangerite-Charnockite-Granite (AMCG) suite which is made up of several rock types. Common interpretations of how the AMCG was created either invoke two distinct magmas or a single magma which created multiple rock types via fractional crystallization, with no model being definitively accepted. One of the major challenges in unraveling this mystery is that these rocks have been highly metamorphosed subsequently to their intrusion, altering much of their original magmatic geochemistry. Apatite is a phosphate mineral that is uniquely positioned as a petrological indicator since it incorporates high concentrations of rare earth elements (REEs) in its crystal structure, and because metamorphosed apatite commonly contains xenotime and monazite inclusions. LA-ICP-MS geochemistry of apatites from three locations reveal apatites highly enriched in REEs, especially MREE. Apatite REE diagrams all show negative Eu anomalies, consistent with growth in the presence of crystallizing feldspar. Bulk rock geochemistry shows positive Eu anomalies, except the charnockite sample. Regardless of sample location, similar rock types (anorthosite, leucogabbro, charnockite) show similar REE traces in both apatite and bulk rock chemistry. Apatite REEs show anorthosites are enriched in HREE, while leucogabbro and charnockite samples are depleted in HREE. Ratios of Ce/Lu and Sr/Y show similar values for anorthosite samples between locations, with apatites from the leucogabbro and charnockite chemically distinct from other rock types. Modeling of element partition coefficients (KDs) of apatite sample compositions are significantly enriched compared to natural silica compositions, suggesting that these apatites grew in a cumulus phase (growing out of the melt at an early stage). Apatite with a trace element enrichment found in the measured samples would require crystallization from melts significantly higher in silica than the bulk rock. These findings reinforce the idea that apatites are a useful way to fingerprint different melt batches, even in complexly metamorphosed terranes, and can better constrain conditions of AMCG formation.