MINERAL AND WHOLE-ROCK CHEMISTRY OF KIMBERLITE-LIKE ROCKS FROM NEW YORK

More than 80, poorly exposed, cm to m-wide kimberlite-like dikes were emplaced into the Appalachian Plateau in New York at ca. 145 Ma (Heaman & Kjarsgaard 2000) in response to the progressively northward opening of the Atlantic Ocean. The dikes follow a general N-S trend and have sub-vertical to vertical dips. Narrow cm-scale chilled margins are common. Common macrocrysts include olivine (Fo87-91), Cr-rich diopside, spinel, garnet (chrome-pyrope to low-Ca chrome-pyrope, magnesian almandine, pyrope and high titanium ferro-magnesian grossular), and phlogopite. In addition to serpentine and calcite, observed groundmass phases include perovskite, spinel, phlogopite, apatite, pyrrhotite, pentlandite, barite and celestite. Xenoliths from the middle and upper crust are common in some dikes, and a few reacted with the igneous fluid forming narrow rims of diopsidic hornfelses or grossular skarns. Xenolith gneisses display evidence of partial melting of the feldspar.

Whole-rock compositions are variable, but with generally low SiO2 (15.4 – 39.7%), high MgO (up to 29.5%), high CaO (up to 30.8%) and relatively high K2O (up to 3.2%). Most of the dikes have bulk compositions similar to South African type-II kimberlites (orangeiites). The whole-rock trace element compositions show depletion in the compatible elements, and strong enrichment in the LREE relative to chondritic mantle. The overall strong signature of LIL and incompatible element enrichment is the result of both a metasomatized mantle source and crustal contamination.

Clinopyroxene macrocrysts are relatively enriched in LREE (La/YbN = 10.19 to – 12.16) and depleted in HREE (LuN = 0.15 to 0.45) leading to subchondritic ratios of Sm/NdN = 0.83 to 0.86 and Lu/HfN = 0.02 to 0.14. Pyrope garnet macrocrysts have very high values for HREE (LuN = 9.65 to 11.14) and extremely low value for LREE (La/NbN = 0.03). The Sc/Yb and Ti/Sc ratios of the garnet macrocrysts are close to chondrites (C1), whereas these ratios in clinopyroxene macrocrysts are suprachondritic. The scarcity of garnet macrocrysts and the depletion of HREE in the whole-rock indicate that garnet was left behind in the upper mantle source. The primary mineral macrocrysts and trace element composition of clinopyroxene and garnet suggest a metasomatized garnet lherzolite as the parental source.