A TIME SCALE FOR GRANITOID EMPLACEMENT IN THE ARCHEAN MURCHISON PROVINCE, WESTERN AUSTRALIA, BY SINGLE ZIRCON GEOCHRONOLOGY

In the Late Archean Murchison Province of Western Australia four major granitoid suites intruded into two stratigraphically distinct supracrustal sequences: the Luke Creek and Mount Farmer Groups. Single zircon U-Pb ages from five granitoid samples, representing three of the four suites, were measured by ion microprobe. The earliest of the suites, the pegmatite-banded gneiss, intruded Luke Creek Group greenstones at approximately 2919 +/- 12 Ma (1sigma). The Luke Creek Group and the pegmatite-banded gneiss protolith were subsequently deformed and then overlain by Mount Farmer group greenstones. Large volumes of monzogranite then intruded greenstones throughout the province; two samples from this suite gave crystallization ages of 2704 +/- 51 and 2681 +/- 6 Ma. This package of greenstones, gneiss and monzogranite was folded and metamorphosed and then intruded by two suites of post-folding granitoids. Two samples from the more southern of these suites gave ages of 2641 +/- 5 and 2602 +/- 14 Ma. Two additional samples were collected from a tonalite pluton previously assigned to the northern post-folding granitoid suite; they gave crystallization ages of 2760 +/- 8 and 2784 +/- 22 Ma (1sigma). This tonalite is now interpreted as a subvolcanic pluton associated with the andesite and dacite volcanism which produced the nearby Mount Farmer Group greenstones. No xenocrystic zircons with ages significantly in excess of 3.0 Ga were found in any of the seven samples, implying that little or no mid-Archean or older rocks are present in the region. Lower concordia intercept ages and Pb-207/Pb-206 ages from individual analyses indicate the presence of Mid- to Late-Proterozoic activity throughout the Murchison Province. Concordia intercept ages were calculated using a least-squares fit of a plane in three dimensions. This approach obviates both the need to assign a common lead isotopic composition and the assumption that no pre-Recent events have disturbed the population's U-Pb system. A further refinement of the data assessment process involved the use of the internal scan-by-scan scatter for defining the precision of the individual Pb/Pb isotopic ratios. Compared with the observed scatter, ion counting error estimates were found to frequently underestimate the uncertainties in the Pb-207/Pb-206 and Pb-204/Pb-206 ratios.