SHRIMP II dating of zircons and monazites: reassessing the timing of high‐grade metamorphism and fluid flow in the Reynolds Range, northern Arunta Block, Australia

Abstract

Key insights into the timing of tectonometamorphic events in a complex high-grade metamorphic terrane can be obtained by combining results from SHRIMP I1 ion microprobe studies of individual monazite grains with SHRIMP I1 studies and scanning electron microscope (SEM)-based cathodoluminescence (CL) imaging of zircons. Results from the Reynolds Range region, Arunta Block, Northern Territory, Australia, show that the final episode of regional metamorphism to high-T and low-P granulite facies conditions is most likely to have occurred at c. 1580 Ma, not at 1785-1775 Ma, as previously accepted. The previous interpretation was based on zircon studies of structurally controlled granitoids, without SEM-based CL imaging. Monazites in a 1806 & 6 Ma megacrystic granitoid preserve rare cores that are interpreted to be inherited magmatic monazite, but record no evidence of another high-T event prior to 1580 Ma. Most monazites from the region record only a single high-T metamorphic event at c. 1580 Ma. Zircon inheritance is very common. Zircons or narrow overgrowths of zircon dated at c. 1580 Ma have only been found in two types of rocks: rocks produced by metasomatic fluid flow at high temperatures (2 750 "C), and rocks that have undergone local partial melting. Previous explanations that attributed these 1580 Ma zircon ages to widespread hydrothermal fluid fluxing associated with post-tectonic pegmatite emplacement at amphibolite facies conditions are not supported by the available evidence including oxygen isotope data.The observed high regional metamorphic temperatures require the involvement of advective heating. However, contrary to a previous tectonic model for the formation of this and other low-P, high-T metamorphic belts, the granites that are exposed at the present structural level do not appear to be the source of that heat, unless some of the granites were emplaced at c. 1580 Ma.

Zircon and monazite response to prograde metamorphism in the Reynolds Range, central Australia

“…8). These data con®rm a Mesoproterozoic age for the regional metamorphism in the RR in accord with previous U±Pb and Pb±Pb age determinations (Hand et al 1995;Vry et al 1996;Williams et al 1996;Buick et al 1999).…”

“…Numerous granitic bodies intruded both the Lander Rock Beds and the RRG, producing local contact metamorphism, before being regionally metamorphosed under low pressure±high temperature conditions (LP-HT). The RR records a metamorphic gradient from NW to SE, which has been extensively documented in both basement and cover (Dirks and Wilson 1990;Clarke and Powell 1991;Dirks et al 1991;Hand et al 1992;Vry et al 1996;Buick et al 1998). A summary of metamorphic facies and conditions is presented in Fig.…”

“…1. The deposition of the RRG post dates the granites intruding the basement at 1805 3 Ma (Vry et al 1996) and pre dates the intrusion of another granitic suite at 1780±1790 Ma (Collins and Williams 1995).…”

“…The ®rst SHRIMP U±Pb dating of zircon from metamorphosed and deformed granitoids of the RR (Collins and Williams 1995) led to the conclusion that the main regional metamorphic event in the RR and the nearby Anmatjira Range occurred between 1780 and 1770 Ma. However, further SHRIMP dating of monazite and zircon from high grade metasedimentary rocks resulted in the interpretation that regional HT metamorphism in the RR occurred at 1570±1590 Ma (Vry et al 1996;Williams et al 1996), much later than the granitic magmatism. Williams et al (1996) suggested that the metamorphism extended over 25 Ma with a peak at 1594 6 Ma and Dirks et al (1991), Clarke and Powell (1991) and Buick et al (1998).…”

Tectono-metamorphic evolution and timing of the melting processes in the Svecofennian Tonalite-Trondhjemite Migmatite Belt: An example from Luopioinen, Tampere area, southern Finland

“…Spear & Parrish 1996). This is consistent with the previous studies which show that monazite may preserve ages corresponding to peak temperatures up to granulite facies conditions (DeWolf et al 1993), and may record prograde growth ages (Vry et al 1996).…”