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

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.