U-PB GEOCHRONOLOGY OF PEGMATITIC GRANITE AT STREAKED MOUNTAIN, WESTERN MAINE

In this study we focus on deciphering U-Pb systematics of zircon from a highly fractionated granitic pluton exposed at Streaked Mountain, Hebron, ME. One of the hypotheses we aim to test is if a temporal link exists with the Mt. Mica gem pegmatite, c. 3.6 km NW of Streaked Mountain. In addition to this proximity, the highly fractionated nature of the Streaked Mountain granites make the pluton an attractive potential pegmatite source. Although trends in rare element abundance between granites and spatially related pegmatite fields are common, this does not provide an unambiguous petrogenetic link. One component of the granite-pegmatite link that is frequently not satisfied is overlap in crystallization age.

The major rock types exposed on the mountain (c. 0.2 km²) include coarse-grained granite, aplite, graphic granite, and crudely zoned pegmatite segregations. The pegmatite bodies occur as meter-sized pods at lower elevations, while larger masses of pegmatite occur at higher elevations. All facies contain both garnet and tourmaline; in places these make up modally significant proportions. Zircon concentrates were extracted by conventional methods from a sample of coarse-grained granite taken at the crest of Streaked Mountain and single grains were hand-picked. Individual pale brown, euhedral, prismatic crystals (c. 0.10-0.40 mm x 0.05-0.10 mm; aspect ratios typically c. 4:1) lack obvious cores in both visible light and when viewed with back-scattered electrons.

Grains mounted in epoxy will be analyzed by laser ablation MC-ICP-MS at the FIRST lab at Stony Brook University. The potential for inheritance makes a spatially-resolved analytical method critical when dealing with zircon in evolved crustal magmatic systems. High U zircon typical of evolved granites such as these (> 2,000 ppm U) is a common victim of metamictization and concomitant Pb loss. The technique of chemical abrasion, involving high temperature annealing followed by etching with hydrofluoric acid, has been effective in single grain TIMS studies, demonstrably improving concordance of Pb/U isotopic ratios and yielding more precise and reliable dates. We have separated a texturally identical population of zircon crystals from the same original rock and will explore the impact of chemical abrasion on age systematics with laser ablation analysis.