MESOZOIC MAGMATISM IN NORTHERN AND WESTERN NEW YORK STATE: KIMBERLITES AND QUARTZ SYENITES

Mesozoic magmatism was compositionally diverse and geographically widespread across northeastern North America. The westernmost expression of this activity occurred in New York State where it was limited in both extent and volume, and restricted to two distinct styles: syenitic sills in northeastern New York, and small kimberlitic dikes in central and western New York.

The easternmost expression of this activity occurs along the western shore of Lake Champlain at Cannon Point, Essex County. Two 4-6 m thick syenitic sills intrude the Ordovician Canajoharie shale and Trenton Group carbonates. The rocks are porphyritic, with up to 15 modal % alkali feldspar phenocrysts, along with minor quartz and altered amphibole. On the basis of modal mineralogy, these rocks are quartz syenites and alkali feldspar quartz syenites. Whole-rock chemistry reveals two distinct magma compositions, both similar to syenites of the White Mountain Magma Series, and with clear within-plate, A-type granite characteristics. The Cannon Point sills have not yet been dated, but are similar to rocks of the Barber Hill complex in Vermont which has been dated at 111 +/- 2 Ma (Armstrong and Stump, 1971).

There are more than 80 known kimberlitic intrusions in New York State; most are small (<50 cm wide), near-vertical dikes, although three small diatremes are also known. Macrocrysts of olivine and phlogopite are common; pyrope, Cr-diopside, ilmenite, and/or spinel are more restricted in occurrence. The dikes are found in four distinct geographic regions, are mineralogically and chemically diverse, and can be divided into four distinct compositional groups. Available age dates suggest intrusion occurred over a period of more than 30 million years, from ~ 145 Ma to ~ 115 Ma, with a general progression of decreasing age from east to west.

Generation of the kimberlitic melts is believed to be the result of far-field effects of Mesozoic extension and emplacement due to reactivation of ancient crustal structures. The quartz syenites of Cannon Point are interpreted as crustal melts generated following emplacement of mantle-derived melts into the lower crust.