KIMBERLITES OF CENTRAL NEW YORK STATE: MAGMATISM RELATED TO MESOZOIC EXTENSION AND REACTIVATION OF LITHOSPHERIC STRUCTURES

Over the past 170 years, approximately 80 distinct kimberlitic dikes have been discovered in central New York State. Most exist in clusters within an elongate NNE – SSW area between Syracuse and Ithaca. The Ogdensburg (Eel Weir) dike in St. Lawrence County is the northernmost and most isolated dike in the state. The easternmost dike, and the only dike observed cutting Grenville age basement rocks, is one at Big Nose on the Mohawk River. The New York dikes are part of a larger north-south belt of kimberlitic intrusions on the western flank of the Appalachian mountain belt that extends from Tennessee to Quebec. These dikes are of particular interest to North American geologists because they provide the only direct information on the nature of the mantle and lower crust in the Appalachian interior, and they are the only expressions of Mesozoic magmatism in the region.

Typical of kimberlitic rocks, the intrusions are petrographically variable. Relatively large (2-15mm) macrocrysts of serpentinized olivine in a very fine-grained matrix of serpentine, calcite and opaque oxides form the dominant texture of these rocks. In addition to olivine, macrocrysts of phlogopite, garnet, clinopyroxene, and spinel are observed in some of the intrusions. The matrix typically contains serpentine, calcite, perovskite, magnetite, and apatite along with minor (and localized) clinopyroxene, chlorite, barite, celestine, ilmenite, pyrrhotite, pentlandite, and pyrite.

Despite extensive alteration and variable crustal contamination, the intrusions can be divided into five distinct groups on the basis of whole-rock chemistry. Ratios of HFS elements vary significantly between groups, and systematically across the district (e.g. Ti/Nb ratios decrease from east to west). Initial Sr and Nd isotope ratios of matrix samples of the least crustally contaminated dikes plot very near bulk earth (e^o_CHUR ~0).

Based upon a preliminary analysis of the data we propose a model of localized melting of metasomatized asthenospheric mantle in response to Mesozoic extension. Migration of the melts was controlled largely by pre-existing structures; one possible structure may be the failed St Lawrence rift, which lies on strike with the kimberlite fields of central New York. Additional geochemical and age analyses are planned to test and refine this model.