APPLICATION OF MONAZITE GEOCHRONOLOGY TO THE I- AND S-TYPE GRANITES OF THE LACHLAN FOLD BELT, SOUTHEASTERN AUSTRALIA

Microprobe dating of monazite is a fast and easy technique for obtaining chronologic data. Because diffusion of major and trace components in monazite is slow, enabling the mineral to record multiple metamorphic episodes, monazite dating has traditionally been used for metamorphic events. The full potential of applying monazite chronology to igneous systems has yet to be realized. Under certain conditions, monazite may survive anatexis. The resulting igneous rock would contain inherited monazite that could provide information about the source rock. By taking advantage of microprobe X-ray maps and in-situ textural analysis, we will document the textural relationships, morphology and zoning of monazite. Our research will focus on the I- and S-type granites within the Lachlan Fold Belt, southeastern Australia, which have been extensively studied in terms of petrogenesis and zircon chronology. Monazite dates obtained from the granites will be compared to previously published TIMS and SHRIMP dates. Zircons within these granites are known to display inheritance, and we will compare zircon patterns of inheritance with premagmatic monazite. Recent petrologic, chronologic and tectonic studies suggest a three-source mixing model between subduction derived mafic magmas and early Paleozoic turbidites for S-type granites and island arc basement for I-type granites. This model contrasts with the long-accepted argument of Chappell and White, which states that I-type granites represent melt derived from Proterozoic basement rocks, while S-type melts represent similarly sourced sedimentary rocks. Preliminary X-ray mapping within the Dalgety (S-type) monzogranite has documented complex behavior. Within one thin-section, unzoned monazite exists with zoned monazite. The zoned monazite occurs within apatite, in a textural relationship that could represent either replacement or armoring. The coexistence of apatite and monazite may represent a local high phosphorus environment that would inhibit dissolution and enhance the likelihood of preservation and inheritance. Our future plan of study will involve characterization of monazites within a suite of I-type and S-type granites from the Lachlan Fold Belt. Currently, we are calibrating UTEP’s microprobe so that interferences can be minimized and accurate background corrections can be determined.