T267 Garnet and Its Use in Unraveling Metamorphic and Tectonic Processes (Poster)

Wednesday, 3 November 2010
8:00 AM - 6:00 PM
Hall D

PRECISE SM-ND AGES FROM LEACHED GARNET: EXAMPLES FROM THE NORTH CASCADES, WASHINGTON AND FIORDLAND, NEW ZEALAND

Paper number: T267-7
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

Garnet Sm-Nd geochronology is a powerful technique for dating metamorphic events because ages can be directly linked to garnet growth and with thermobarometry to P and T. Age precision is increased by >100% with leaching of garnet to remove inclusions and increase measured 147Sm/144Nd ratios. We present data from the amphibolite facies pelitic Chiwaukum Schist and quartz-feldspar Swakane Gneiss, North Cascades, and the granulite facies dioritic Western Fiordland Orthogneiss, Doubtful Sound. Garnet was leached in various combinations of HF, HCl, HNO3, and HClO4 acids at 25 to 180ºC for ≤24 hours. Garnet in 2 Chiwaukum Schist samples contains quartz, muscovite, staurolite, kyanite, plagioclase, zircon, ilmenite, and monazite inclusions. Ages for leached + unleached (90.1 & 90.2 Ma, Labyrinth Mtn.) versus unleached garnet from 2 samples vary by ≤1% - compatible with inclusion-garnet isotopic equilibrium. Age precision increased by 44-57% (to 0.7-1.2 m.y.), largely due to 147Sm/144Nd increases of ≤103%. Garnet in 2 Swakane Gneiss samples contains quartz, biotite, ilmenite, apatite, rutile, plagioclase, epidote, and monazite inclusions. Ages for leached + unleached (65.8 & 73.5 Ma, Wenatchee & Chelan blocks) versus unleached garnet vary by ≤5%, also compatible with inclusion-garnet isotopic equilibrium. Age precision increased by 77-87% (to 1.9-3.0 m.y.), largely due to 147Sm/144Nd increases of up to 817%. Peritectic garnet in 5 Western Fiordland Orthogneiss samples contains plagioclase, amphibole, pyroxene, quartz, potassium feldspar, rutile, ilmenite, titanite, magnetite, zircon, and apatite inclusions. Ages for leached + unleached (113.6, 115.6, 110.6, 113.3, & 111.9 Ma, E to W-Doubtful Sound) versus unleached garnet vary by ≤3%, again compatible with inclusion-garnet isotopic equilibrium. Two samples show age precision increases of 50-59%, largely due to 147Sm/144Nd increases of ≤42%; however, the other 3 samples benefit little from leaching.

The ages calculated from leached plus unleached garnet are the most precise and the age from each sample is inferred to reflect a single Late Cretaceous event that affected all samples from that group. In general, combined HF, HNO3, and HClO4 leaching produces the most precise results; however, H2SO4 produced high-precision ages for monazite-bearing pelites.