In situ U–Pb dating and element mapping of three generations of monazite: Unravelling cryptic tectonothermal events in low-grade terranes Birger Rasmussen a, * , Ian R. Fletcher a , Janet R. Muhling b a School of Earth and Geographical Sciences M006, The University of Western Australia, Crawley, WA 6009, Australia b Centre for Microscopy and Microanalysis M010, The University of Western Australia, Crawley, WA 6009, Australia Received 9 March 2006; accepted in revised form 9 October 2006 Abstract In situ U–Pb dating of monazite and xenotime in sedimentary rocks from the mid-Archean Soanesville Group in the Pilbara Craton, yields ages for provenance, diagenesis and multiple low-grade metamorphic events. Detrital monazite and xenotime grains give dates >3250 Ma, whereas diagenetic xenotime provides a new minimum age of 3190 ± 10 Ma for deposition of the basal Soanesville Group, previously constrained between 3235 Ma and 2955 Ma. Metamorphic monazite provides evidence for three episodes of growth: at 2.88, 2.16 and 1.65 Ga. Element mapping of monazite for La, Sm, Y and Th reveals distinct cores and rims in some crystals that were used to guide the placement of analytical spots during in situ U–Pb dating by sensitive high-resolution ion microprobe (SHRIMP). Spe- cifically, La and Sm distributions closely correlate with different generations of monazite. The presence of two generations in single mon- azite crystals highlights the need for characterizing mineral chemistry prior to geochronology. It also shows the importance of using in situ dating techniques rather than methods that rely on the analysis of entire, potentially multi-aged, crystals. The ages recorded by metamorphic monazite span more than one billion years and are interpreted to record cryptic tectonothermal events within the cra- ton. The 2.88 Ga age coincides with a phase of regional deformation, metamorphism and gold mineralization along a major crustal lin- eament, whereas the most common monazite age population (at 2.16 Ga) corresponds with the migration of a foreland fold-and-thrust belt across the craton. The youngest age (1.65 Ga) coincides with an episode of tectonic reworking in the Capricorn Orogen along the southern Pilbara margin. The prolonged history of monazite growth may, in part, relate to channelized fluid flow during reactivation of long-lived N- to NE-trending crustal structures that transect the craton. Despite repeated episodes of metamorphism, the isotopic system in each generation of monazite remained unperturbed, yielding precise dates. The ability of monazite to record three separate events, and in some instances two events in a single crystal, distinguishes it from most other low-temperature mineral chronometers, which are read- ily reset during metamorphic overprinting. Low-temperature monazite geochronology can provide a detailed isotopic history of cryptic thermal events and reveal the temporal and spatial patterns of far-field fluid flow related to tectonic processes. The previously unrecog- nized history of crustal fluid flow in the Pilbara Craton has implications for chemical, mineralogical and isotopic studies seeking to understand conditions on the early Earth. Ó 2006 Elsevier Inc. All rights reserved. 1. Introduction Fluids play an important role in the evolution of the Earth’s crust. The movement of fluids controls the trans- port of chemical constituents and heat through the crust, and governs the formation of petroleum accumulations and hydrothermal ore deposits. Although metamorphic devolatilization reactions and volcanic activity produce flu- ids in the crust, the mechanisms, rates and scale at which these fluids migrate remain unresolved (Halliday et al., 1991). In rocks of the upper crust, little evidence may be preserved to mark the passage of an ancient crustal fluid, and such clues can be destroyed by mineralogical, isotopic 0016-7037/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.gca.2006.10.020 * Corresponding author. Fax: +61 8 6488 1037. E-mail address: brasmuss@cyllene.uwa.edu.au (B. Rasmussen). www.elsevier.com/locate/gca Geochimica et Cosmochimica Acta 71 (2007) 670–690