The enigma of crustal zircons in upper-mantle rocks: Clues from the Tumut ophiolite, southeast Australia

We suggest a new explanation for the presence of crustally derived zircons in the upper-mantle rocks of ophiolitic complexes, as an alternative to subduction-related models. Integrated isotopic (U-Pb, Hf, and O isotopes) and trace-element data for zircons from the Tumut ophiolitic complex (southeast Australia) indicate that these grains are related to granitic magmatism and were introduced into the mantle rocks after their emplacement into the crust. These observations emphasize that a clear understanding of the origin of individual zircon populations and their relationship to the host rock is essential to interpretations of the tectonic history of upper-mantle rocks and the dynamics of crust-mantle interactions. INTRODUCTION Several studies have reported U-Pb ages on zircons recovered from upper-mantle rocks in ophiolitic complexes (e.g., Grieco et al., 2001; Savelieva et al., 2007; Stern et al., 2010; Badanina et al., 2013; Yamamoto et al., 2013; González-Jiménez et al., 2013). Did these zircons crystallize in the mantle from metasomatic fluids (e.g., Grieco et al., 2001; Zaccarini et al., 2004; Zheng et al., 2006), or are they xenocrysts of crustal material recycled during subduction (e.g., Yamamoto et al., 2013; Robinson et al., 2014)? Crucially, what is their significance in the framework of ophiolite evolution? We have studied the geochemical and isotopic signatures of zircons recovered from mantle and crustal rocks of the Coolac serpentinite belt, part of the Tumut ophiolite complex in the Lachlan fold belt of southeastern Australia (Fig. 1). The integrated analysis of U-Pb, Lu-Hf, and O isotopes has provided critical information about the origin of zircons in both mantle and crustal rocks. The zircon data are complemented by field observations and in situ Re-Os isotopic information on laurite grains in the chromitites of the mantle section of this ophiolite. This case study emphasizes that deciphering the origin of zircons requires integration of data sets beyond zircon U-Pb geochronology alone. It reveals that crustally derived zircons can be introduced into mantle rocks after their obduction via microscopic melt networks, and sends a cautionary message about the interpretation of zircon ages from ultramafic rocks in general. GEOLOGICAL SETTING AND SAMPLE BACKGROUND The Coolac serpentinite belt, Wambidgee serpentinite belt, Tumut Ponds serpentinite belt, and smaller bodies form the ~250-km-long Tumut ophiolitic complex in the Lachlan fold belt of southeastern Australia (Fig. 1; Graham et al., 1996a, 1996b; Spaggiari et al., 2003). The Coolac serpentinite belt is ≥63 km long, up to 3.5 km wide, and ≥5 km thick. It was metamorphosed mostly to greenschist facies and consists of variably serpentinized porphyroclastic meta-harzburgites with lenses of dunite that host chromitite bodies. The ultramafic rocks are cut by dikes of gabbro and plagiogranite, and dike-like bodies, veinlets, and pod-like masses of rodingite. On its eastern contact, it is either faulted against or intruded by the Silurian S-type Young Granodiorite; to the west, it is faulted against metavolcanic and metasedimentary units of the Silurian Tumut trough (Stuart-Smith, 1990; Graham et al., 1996b). The age of formation of the ultramafic rocks from the Tumut region is unclear. Previous suggestions range from Cambrian-Ordovician (StuartSmith, 1990) to Devonian (Graham et al., 1996b). Graham et al. (1996b) suggested that a Devonian U-Pb age (ca. 400 Ma), obtained on zircons from plagiogranite dikes, indicated that the ophiolitic rocks and granitic magmas were generated during the same major tectonothermal event in the Lachlan fold belt (430–390 Ma; Chappell, 1994). However, this age is considerably younger than those of the Young Granodiorite (zircon U-Pb age of 428 ± 1.9 Ma; Lyons and Percival, 2002) and the North Mooney Complex (hornGEOLOGY, February 2015; v. 43; no. 2; p. 119–122; Data Repository item 2015049 | doi:10.1130/G36231.1 | Published online 7 January 2015 © 2015 eological Society of A erica. For permission to copy, contact [email protected]. M ur ru m bi dg ee