Western New York State is noted for its flat-lying, sandstones, limestones, dolostones, siltstones, and shales. However, some very interesting igneous rocks can be found as dikes* intruding these Paleozoic sedimentary rocks in a narrow band that extends from the vicinity of Ithaca, through the Syracuse area, and beyond to Ogdensburg on the St. Lawrence River. The greatest concentration of known dikes is centered near Ithaca, along the southern portion of the Cayuga Lake valley. This is part of a belt of similar rocks that trends north-south from Tennessee to Quebec.
These unusual rocks occur as narrow dikes that formed as molten rock from the upper portion of the earth's mantle pushed upwards into joints (vertical cracks) in the surrounding rock, then cooled, and solidified. The dikes range in size from an inch (2.5 cm.) or less, to 195 feet (60 m.) in width.
Originally called serpentine or serpentinite, the rocks were first discovered in 1837 within the city of Syracuse. Other occurrences were soon located in West Canada Creek near Little Falls to the east, and in Ludlowville to the south. Then, in 1887, similar rocks were described from the South African diamond fields and named "kimberlite," after Kimberly, South Africa. These kimberlites are well known because their magmas brought diamonds that formed deep in the mantle to the surface. Hence, the South African kimberlite is famous, while the similar "kimberlitic rocks" of New York are little known.
Although the kimberlite from both Kimberly and North America have been largely altered to serpentine, their original composition was largely olivine (peridot) and phlogopite mica; hence, they have been called mica peridotites. Detailed study of thin sections of the rock has revealed its composition as crystals (2-15 mm.) of olivine, phlogophite, garnet, phlogopite, and spinel, which are floating in a fine-grained groundmass of phlogopite, calcite, serpentine, perovskite, and magnetite. Other minor minerals that may be present in some specimens include clinopyroxene, clinoamphibole, epidote, chlorite, barite, celestine, spinels, ilmenite, pyrrhotite, pentlandite, and pyrite. However, one mineral that is found in the South African kimberlites but has never been found in those from New York is diamond. Furthermore, the mineral assemblage in our "kimberlites" indicates that they formed in the uppermost portion of the mantle, and were too cool for diamonds to be stable. Therefore, diamonds are not expected to be present in any of the currently known New York dikes.
Although igneous rocks characteristically contain radioisotopes that make it possible to determine their ages, several factors make it difficult to date these kimberlitic rocks. For example, different crystals in a sample formed at different times, and sometimes chunks of crystalline rock from the lower crust or upper mantle can become incorporated in the kimberlitic magma as it works its way upward. This results in an incorrect date if portions of it are sampled. Furthermore, post-crystallization alteration and weathering makes it difficult to extract suitable material for dating. Because of these difficulties, various researchers have obtained dates ranging from approximately 104 - 439 million years for these rocks. Because the youngest rocks that they intrude are around 370 million, clearly the oldest date is incorrect! General consensus for the intrusion of the dikes is from the late Jurrasic to the early Cretaceous periods, or roughly 160 to 135 million years ago.
*dike - a tabular body of igneous rock that cuts across the structure of adjacent rocks.
Reference:
Bailey, David G. and Lupulescu, Marian, 2007, Kimberlitic Rocks of Central New York, Trip A-3: New York State Geological Association 79th Annual Meeting Field Trip Guidebook, p. 53-81.
The images below illustrate both field occurrences, and collected specimens, of a few of the more than 80 kimberlitic dikes that are currently known to occur in central New York State from Syracuse to Ithaca. There are undoubtedly many more dikes buried below glacial cover, under water, or that just haven't been recognized yet.
Field and specimen images by Paul Dudley; photomicrographs courtesy of David Bailey.
