Petrogenesis of a shallow-level kimberlite from Taughannock Creek, New York

The petrogenesis of kimberlite from Taughannock Creek, near Ithaca, New York, can be separated into three distinct stages. Stage 1 comprises the evolution of kimberlitic melt within the upper mantle. Fractionation of olivine, Cr-pyrope, Cr-diopside, enstatite, and Cr-Al-spinel occurred over the temperature interval 950-1350°C in the depth range from 95 km to within the spinel stability field (< 70 km). Spinel peridotite xenoliths were incorporated during this stage. Picroilmenite is absent as a megacryst phase. The geothermobarometric estimates indicate that fractionation within this kimberlite occurred above the low velocity zone, significantly shallower than the typical kimberlite source region. This shallow origin may account for the paucity of picroilmenite, a ubiquitous phase in deeper kimberlites. During its passage through the shallow upper mantle or lower crust (Stage 2) the kimberlite disrupted an eclogitic body incorporating xenocrysts of Cr-poor diopside, almandine, and mineralogically similar eclogite xenoliths. Back-reaction with the more primitive kimberlitic melt produced reverse zoned rims on some xenocrysts. The typical kimberlitic paragenetic sequence Cr-spinel - Ti-magnetite - perovskite is present in the groundmass oxides, reflecting Cr-depletion and Ti-, Fe³⁺-, and Ca-enrichment during the kimberlite's later stages of development. The passive, near surface emplacement (Stage 3) of the kimberlite as narrow dikes along North-South joint planes is probably a function of rift-related tectonic activity associated with the opening of the proto-North Atlantic Ocean.