Journal of Petrology, Volume 40, Issue 10: October 1999.

Relationships between Zircon U-Pb SHRIMP Ages and Leucosome Type in Migmatites of the Halls Creek Orogen, Western Australia

NICHOLAS H. S. OLIVER¹, SIMON BODORKOS², ALEXANDER A. NEMCHIN², PETER D. KINNY² AND GORDON R. WATT²

¹SCHOOL OF EARTH SCIENCES, JAMES COOK UNIVERSITY, TOWNSVILLE, 4811 QLD., AUSTRALIA
²TECTONICS SPECIAL RESEARCH CENTRE, SCHOOL OF APPLIED GEOLOGY, CURTIN UNIVERSITY OF TECHNOLOGY, GPO BOX U1987, PERTH, 6845 W.A., AUSTRALIA

Field relations, and zircon zoning and SHRIMP U-Pb patterns from an outcrop of deformed pelitic migmatites reveal relationships between leucosome type and the amount of zircon growth during migmatization. Zircons from the oldest, stromatic migmatites show several ²⁰⁷Pb/²⁰⁶Pb intragrain analyses with 1850-1840 Ma anatectic ages and distinctive cathodoluminescence (CL) patterns, although the sample is dominated by inferred pre-1855 Ma detrital zircons. Subsequent thick `contact-sheath' leucosomes and `back-veins' developed at 1845-1830 Ma in response to intrusion of mafic dykes into the stromatic migmatites. Zircons show CL evidence for substantial anatectic growth and overgrowths with post-1850 Ma ages, and a single population age 20 my younger than the other migmatites. Zircons from the structurally youngest, shear- and vein-hosted leucosomes show few overgrowths and retain a large detrital population. Correlation between shear zones and minimal anatectic zircon growth is inferred to reflect rapid deformation-induced melt extraction from the protolith, and/or rapid quenching. The earlier contact-sheath and stromatic leucosomes probably had a combination of slow melt segregation, larger interacting melt volume and longer residence times at high temperatures. Results suggest that rapid melt extraction by deformation will increase the tendency for preservation of source rock isotopic patterns in some granites.

Keywords: anatexis;partial melting;metapelite; geochronology