ELECTRON MICROPROBE DATING OF S-TYPE GRANITES FROM THE LACHLAN FOLD BELT, AUSTRALIA

In-situ microprobe dating of monazite, which relies on the observation that monazite excludes common lead during crystallization, holds the promise of being an effective technique for obtaining chronologic data. Our research focuses on I- and S-type granites of the Lachlan Fold Belt (LFB), Australia, for which petrology and zircon chronology have been thoroughly characterized. We are documenting textural relationships, morphology and zoning of monazite in these granitic rocks. Recent SHRIMP studies of inherited zircon have documented the presence of Cambrian zircons in the S-type granites of the LFB. This observation has been used to suggest that the granites were derived from Lower Paleozoic source rocks from the accretionary wedge, not Proterozoic microplates as first proposed by Chappell and White. Monazite dates and patterns of inheritance will be compared to previously published TIMS and SHRIMP zircon studies. These comparisons will allow further evaluation of the role played by basement type and source rocks in the petrogenesis of these granitic rocks. The data collected thus far show that monazite occurs less often in I-type granites, despite high P concentrations. The I-type granites usually contain other high Ce and REE enriched minerals, such as allanite or titanite (sphene). All S-type granites studied thus far contain monazite, occurring as inclusions, usually in biotite, and between grain boundaries. Zoned monazites are most often found as inclusions and may have survived anatexis by being armored by the host mineral. Preliminary Th and U count rates for the zoned crystals indicate a heterogeneous age spectrum, reflecting a combination of inheritance, magmatic ages, and/or post-magmatic re-equilibration of the mineral. Apatite is commonly found mantling or in replacement textures with monazite. This relationship may represent a local high phosphorus environment that would inhibit dissolution and enhance the likelihood of monazite preservation and inheritance. Xenotime/monazite pairs have also been documented, allowing temperature estimates to be calculated.