T318 Pegmatites I Have Known and Loved (Posters)

Wednesday, 22 October 2014
9:00 AM - 6:30 PM
Exhibition Hall C

GEOCHRONOLOGY OF CLASSIC MINERAL DEPOSITS OF ST. LAWRENCE COUNTY, NEW YORK

Paper number: T318-9
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

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.