2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 312-9
Presentation Time: 11:00 AM


CHIARENZELLI, Jeffrey R.1, LUPULESCU, Marian V.2, SELLECK, Bruce W.3, ROBINSON, George1, PECHA, Mark4, HAGEN-PETER, Hillary1 and LOCKWOOD, Thomas D.1, (1)Department of Geology, St. Lawrence University, Canton, NY 13617, (2)Research & Collections, New York State Museum, 3140 CEC, Albany, NY 12230, (3)Department of Geology, Colgate University, 13 Oak Drive, Hamilton, NY 13346, (4)Department of Geosciences, University of Arizona, Tucson, AZ 85721, jchiaren@stlawu.edu

Mineral localities in St. Lawrence County, New York have received attention from collectors and provided fine specimens to the world’s mineralogical museums for more than a century, yet little is known about their geological setting and age. Here we present U-Pb zircon data from three locations known for the size, abundance, and quality of their specimens, and a potentially related occurrence of geological interest. An undeformed pegmatitic dike consisting primarily of black tourmaline and quartz, cross-cutting the foliation in Grenville supracrustal gneisses on the Powers farm, in Pierrepont, yielded a population of large, euhedral, zoned zircon crystals whose age is 1158.3 ± 3 Ma, establishing a late Shawinigan intrusive age, consistent with its location in the Adirondack Lowlands. An undeformed intrusive mass of diopside-calcite at Edwards yielded an age of 1049 ± 15 Ma establishing an Ottawan age of intrusion consistent with its location in the Adirondack Highlands. Xenocrystic zircons ranging in age from 1255-1515 Ma were derived from the underlying country rocks. An undeformed pegmatitic occurrence of calcite-purple diopside-scapolite-k-spar pegmatite at Pitcairn has a complex zircon population yielding Ottawan (1045.6 ± 7.5 Ma), Shawinigan (1169.1 ± 10 Ma), and pre-Grenvillian (1373 ± 20 Ma) ages. This deposit occurs in the Highlands and is undeformed, supporting an Ottawan intrusive age. Diopside-calcite-k-spar pegmatitic pods between clasts in a mega-breccia exposed in the bed of the Raquette River within the Carthage Colton Shear Zone (CCSZ) contains two dominant zircon populations including cores with Pre-Grenville ages of 1262 ± 13 and 1332 ± 7.3 Ma and voluminous rims yielding an age of 1165.2 ± 3.1 Ma. A small population of zircons are of Ottawan age (1042.5 ± 7.2 Ma). Given the association with a post-orogenic tectonic breccia, the age of the pegmatite is considered Ottawan; however, xenocrystic zircon indicates the influence of pre-Grenville crust modified by Shawinigan orogenesis. The xenocrystic zircon documents previously unrecognized pre-Grenvillian crust in the vicinity of the CCSZ. The origin of late calcite-diopside Ottawan-aged pegmatites is poorly understood but likely incorporates post-orogenic mantle and crustal components intruded in the deep roots of the orogenic belt.