KIMBERLITES FROM THE CENTRAL NEW YORK: MINERALOGY, GEOCHEMISTRY, XENOLITHS AND CONTACT PHENOMENA

Our study focuses on the mineralogy and geochemistry of the micaceous kimberlites from Central New York, their xenoliths and contact phenomena. The centimeters to meters width dikes intrude the Cambrian dolomites and Devonian shales and limestones, in places showing multiple intrusions and fluidal texture at the contact with the host rocks. Olivine megacrysts, kink banded or rounded phlogopite, pink and yellow garnet with a fibrous amphibole and phlogopite kelyphite rim, spinel, perovskite, green Cr-rich diopside in a matrix of calcite, in places with dendritic texture, serpentine and phlogopite (type II micas) define the porphyry texture of the rocks. The opaque minerals point out the oxidized character of the kimberlite (magnetite partially replaced by hematite), megacrysts (ilmenite), sulfurization assemblages (pyrite, pyrrhotite, millerite), and polysulfide globules (pentlandite, pyrrhotite, chalcopyrite). The electron microprobe data for the pink garnet show a composition between chrome-pyrope and low calcium chrome-pyrope, which characterizes the garnet lherzolites. The data for the yellow garnet reveal a grossular composition. Phlogopite megacrysts, as samples from a metasomatized mantle display chemical differences comparing with the groundmass phlogopite, formed as a groundmass liquidus phase. The low Ca (0.05%) olivine (Fo85) megacrysts are completely or partially replaced by serpentine or calcite and serpentine. The crustal xenoliths are pyroxene gneisses, hornblende-pyroxene syenite with strings perthitic feldspars, grossular (anisotropic) skarns, sandstones and shales. A very narrow, amphibole-rich contact reaction zone rims the pyroxene gneisses. Both, the pyroxene gneisses and hornblende-pyroxene syenite display evidences of feldspar melting and hornblende resorbtion. Sandstones and shales do not show any contact phenomena at microscopic scale. The geological features of the type II kimberlites from the Central New York point out that kimberlite magma moved in pulses and the rapid rise through the crust disaggregated the metasomatized mantle derived xenoliths. Their mineralogical, chemical and petrographical study unable us to reconstruct a stratigraphical column from the upper mantle to the upper crust under the Appalachian Basin in the Lower Cretaceous.