In situ SHRIMP U–Pb dating of monazite integrated with petrology and textures: Does bulk composition control whether monazite forms in low-Ca pelitic rocks during amphibolite facies metamorphism? Birger Rasmussen a, * , Janet R. Muhling b , Ian R. Fletcher a , Michael T.D. Wingate c a School of Earth and Geographical Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia b Centre for Microscopy and Microanalysis, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia c Tectonics Special Research Centre, School of Earth and Geographical Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia Received 11 November 2005; accepted in revised form 20 March 2006 Abstract Bulk composition and specific reaction history among common silicate minerals have been proposed as controls on monazite growth in metapelitic rocks during amphibolite facies metamorphism. It has also been implied that monazite that formed during greenschist facies metamorphism may be preserved unchanged under upper amphibolite facies conditions. If correct, this would make the interpre- tation of monazite ages in polymetamorphic rocks exceedingly difficult, because isotopic dates could vary significantly in rocks that have experienced identical metamorphic conditions but differ only slightly in whole-rock composition. Low-Ca pelitic schists from the Mount Barren Group in southwestern Australia display a range of whole-rock compositions in AFM space and different peak mineral assem- blages resulting from amphibolite facies metamorphism (8 kb, 650 °C). In this study, we test whether bulk composition controls the formation of monazite through geochronology and textural evidence linking monazite growth with deformation and peak metamor- phism. X-ray element mapping of monazite from the metapelitic rocks reveals concentric zoning in many grains with compositionally distinct cores and rims. In situ SHRIMP U–Pb geochronology of monazite yields two 207 Pb/ 206 Pb age populations. The cores, and tex- turally early monazite, give an age of 1209 ± 10 Ma, interpreted to record prograde metamorphism, whereas the rims and ‘‘late’’ mon- azite define a single population of 1186 ± 6 Ma, which is considered the likely age of peak thermal metamorphism. The growth of monazite was widespread in low-Ca pelitic schists representing a broad range of compositions in AFM space, indicating that variations in bulk composition in AFM space did not control the formation of monazite during amphibolite facies metamorphism in the Mount Barren Group. Ó 2006 Elsevier Inc. All rights reserved. 1. Introduction Monazite ([LREE,Ca,Th]PO 4 ) is a common accessory mineral in many igneous and metamorphic rocks (Over- street, 1967; Chang et al., 1998; Bea and Montero, 1999; Spear and Pyle, 2002). It is an important mineral for U–Pb geochronology (Parrish, 1990; Montel et al., 1996; Harrison et al., 2002) and has been mostly used to date amphibolite and granulite facies metamorphism (Bingen and van Breemen, 1998; Zhu and O’Nions, 1999; Hawkins and Bowring, 1999; Stern and Berman, 2000; Rubatto et al., 2001). Monazite is only considered to become abun- dant during amphibolite facies metamorphism, at tempera- tures corresponding with the staurolite isograd (Smith and Barreiro, 1990; Kingsbury et al., 1993). Its formation is commonly linked to the breakdown of other accessory phases, particularly allanite (LREE, Ca, Al silicate), during prograde metamorphism (Overstreet, 1967; Ferry, 2000; Wing et al., 2003). However, it has recently been suggested 0016-7037/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.gca.2006.03.025 * 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 70 (2006) 3040–3058