THE EARTHS OLDEST KNOWN CRUST - A GEOCHRONOLOGICAL AND GEOCHEMICAL STUDY OF 3900-4200 MA OLD DETRITAL ZIRCONS FROM MT NARRYER AND JACK HILLS, WESTERN-AUSTRALIA

Detrital zircons with Pb-207/Pb-206 ages between 3908 and 4270 Ma from the Narryer Gneiss Complex, Western Australia, are the oldest terrestrial minerals found to date. They occur in small proportions (2-3%) together with 3.0-3.75 Ga zircons in quartzites and metaconglomerates of the 3.0 Ga Mt. Narryer (MN) and Jack Hills (JH) metasedimentary belts. We report the results of a geochronological and geochemical study of these zircons which, together with data on grain morphology and inclusion mineralogy, are used in an attempt to place constraints on their source rocks. Pre-3.9 Ga zircons from both localities show variable but typically low degrees of rounding and are optically homogenous to faintly growth-zoned. Among the few grains carrying visible inclusions, two stand out: one has a 40-mu-m K-feldspar-granite inclusion; the other carries apatite, quartz, plagioclase (?), and monazite impurities. The zircons are concordant to near-concordant (some reverse discordancy is observed for MN grains) and have similar U abundances (100-650 ppm, MN; 60-413 ppm, JH) and Th/U ratios (0.3-1.1). Rare exceptions include low-Th/U (presumably metamorphic) overgrowths on two of the MN grains and high-Th/U discordant sites on some JH grains. At least three distinct age groups have been detected, greater-than-or-equal-to 4.3, 4.2, and 4.15 Ga old, respectively, and low-Th/U rims on some of the grains were formed at ca. 3.93 Ga. Zr/Hf ratios (30-57) and low Sc contents (Sc measured for JH only) are consistent with granitic parent rocks. Rare-earth element (REE) patterns measured by ion probe show some variation (up to 15 X) but are generally strongly fractionated (high Lu/La), with low La (< 100 ppb) in a majority of cases, and pronounced negative Eu and positive Ce anomalies. The latter appear to be common in low-LREE (light rare-earth element) zircons (but have rarely been detected in previous studies) and most likely reflect minute amounts of Ce4+ in zircon parent melts. The REE patterns closely resemble those in dioritic-granitic zircons measured by ion probe, although the latter also contain grains with 10(3) X higher LREE contents. Collectively, these morphological, mineralogical, and geochemical characteristics suggest a composite granitoid source for these zircons. This is supported by the striking similarities of zircons from both the pre-3.9 Ga and the post-3.75 Ga age groups in a given sample; based on whole-rock geochemical evidence, a continental provenance dominated by potassic granites has been inferred for the latter. The mixed-source lithologies implied by our data indicate the presence of at least three distinct pre-4 Ga age groups, and the occurrence of ca. 3.9 Ga metamorphic overgrowths indicates a differentiated continental source of substantial thickness rather than a provenance from felsic differentiates within dominantly mafic, oceanic-type crust.