TNT Anomalies in Adirondack Mangerites: A Possible Subduction-Zone Signature for the AJM (Anorthosite-Jotunite-Mangerite) Suite

A collection of 35 mangerite samples from the Adirondack Mtns., New York, mainly from the Tupper-Saranac sheet, were analyzed for major, minor, and trace elements by XRF and INA methods. The mangerites are green, gneissic, and typically garnet-bearing, with a mineral assemblage of Plag - Kf - Opx - Cpx - Hbl - Bio - Ilm - Mgt - Qtz. The samples comprise a metaluminous, mildly alkali-calcic, and ferroan (mg ~ 0.08–0.31) suite in which bulk compositions vary continuously over the range 53–71 wt% SiO₂ (monzonite to adamelllite), but concentrations of Fe, Ti, P are higher at a given SiO₂ content than in “common” granitoids. Concentrations of Th, U are very low, but Rb, Ba, Sr, Pb are high. Abundances of Y, Zr, Nb, Hf, Ta are also high. REE patterns are enriched (La_N ~ 97-284× chondrites and Lu_N ~15-55×) and fractionated (La_N/Lu_N ~ 3-7), with variable Eu-anomalies (mainly negative). The samples have high Ga/Al and high Zn, indicating an affinity with A-type granitoids, and on selected discriminant plots, lie in the field of “within-plate granites.” Yet, on multielement plots, the samples display pronounced relative depletions in Ti, Nb, Ta — the so-called “subduction component” — despite absolute enrichments in these elements. Other prominent features of the multielement plots include relative depletions in Sr, P, and in Th, Ba. Such features are shared with mangerites (and jotunites) associated with other Grenville anorthosites like Morin, St. Urbain, and Lac Piché. These new results lend support to the interpretation that massif anorthosites and associated rocks formed in an Andean margin-type setting. However, it is clear that the geochemical signature of the LILE and HFSE in the Adirondack rocks is mixed. As such, these features justify retention of the label C-type (“charnockitic”) granitoids for the distinctive AJM suite of Proterozoic igneous rocks.