APATITE TRACE GEOCHEMISTRY INDICATES HETEROGENEOUS SOURCE OF THE ADIRONDACK AMCG SUITE, NY

Apatite is a phosphate mineral that is uniquely positioned as a petrological indicator due to high concentrations of a wide range of rare earth elements (REEs). This study uses apatite trace element geochemistry to probe the origins of the AMCG (Anorthosite, Mangerite, Charnokite, Granite) suite in the Adirondack highlands. A co-eval but bi-modal theory of formation has been hypothesized to have created the different associated rocks of the AMCG, and is widely accepted today. Three samples, from the AMCG suite, were analyzed, an anorthosite adjacent to the Woolen Mill Gabbro near Elizabethtown, NY, a garnet bearing leucogabbro from Ore-bed slide on Mt. Gothics, and a gt opx bearing gabbro from the same area. Garnets necklace primary pyroxenes, and the gabbroic sample contains abundant large (>100 µm) apatite grains.

Apatite textures analyzed by scanning electron microscopy do not show typical metamorphic reaction minerals, such as xenotime, suggesting that apatite REEs reflect magmatic conditions. Trace element chemistry of bulk rock powders, digested in acid at high pressure, were measured by ICP-MS. Bulk rock REE patterns are enriched 8-80x compared to chondrite values, and are overall depleted in HREE compared to LREE. The woolen mill associated anorthosite shows a large positive Eu anomaly, suggesting it accumulated plagioclase grown in an oxidizing environment, while samples from the ore-bed slide contain no anomaly.

Apatite REE chemistry using LA-ICPMS reveals especially enriched MREE for all samples. All apatites have negative Eu Anomalies, suggesting they grew in the presence of crystallizing feldspar. Overall, the woolen mill sample shows a flatter apatite REE profile, while the ore-bed samples are relatively depleted in HREE. KDs calculated for apatite/bulk rock pairs, are significantly (10-100x) above experimental values for natural silicate melts of similar compositions. Apatites with a trace element enrichment found in the measured samples would require crystallization from melts higher in silica than that of the bulk rock. The shared apatite trace element signature for rocks from the orebed suite, in contrast with those from elsewhere in the AMCG suite, reinforces the idea that apatites are a useful way to fingerprint different melt batches, even in complexly metamorphosed terranes.