MINERALOGY AND GEOCHEMISTRY OF KIMBERLITE DIKES FROM CENTRAL NEW YORK, USA

More than 80, cm to m-wide ultramafic dikes intrude the Paleozoic sedimentary rocks of central NY (from Ithaca to Manheim). Recent studies have identified these dikes as kimberlites (Foster, 1970; Kay & Foster, 1986), and have yielded Mesozoic (~145Ma) radiometric ages (Basu et al., 1984; Heaman and Kjarsgaard, 2000). The complex mineralogy and chemistry of the dikes, coupled with extensive alteration, makes precise classification and interpretation of these rocks difficult.

The dikes are mineralogically variable. Most contain macrocrysts of olivine and phlogopite; a few contain macrocrysts of garnet, Cr-rich diopside, spinel, and/or ilmenite. The olivine macrocrysts (~Fo85) are partially to totally altered to serpentine and calcite. Two different types of garnet macrocrysts have been found; a pink garnet with compositions between chrome-pyrope and low-Ca chrome-pyrope, and a yellow grossular garnet. The phlogopite macrocrysts are zoned with a Ba-rich core, and are commonly rounded and kink banded. Calcite, serpentine, phlogopite, and hematite comprise the matrix of these dikes. Crustal xenoliths include pyroxene gneisses, hornblende-pyroxene syenites, grossular skarns, sandstones, and shales.

Preliminary XRF whole-rock analyses reveal that the dikes are characterized by low SiO2 (20-42%), high MgO (up to 32%), high CaO (up to 43%), and relatively high K2O (up to 4.7%). The high CaO and MgO reflect the abundance of secondary calcite and serpentine. The high K2O, which primarily reflects the abundance of phlogopite, is atypical of type I kimberlites in that most kimberlites do not exceed 2 wt% K2O (Mitchell, 1995). The high K2O suggests that the rocks may be more accurately classified as type II kimberlites (or orangeiites). ICP-MS trace element analysis, presently underway, will provide additional data that will help clarify the origin and evolution of these unusual rocks.