Abstract
Abstract. Dating multiple geological events in single samples using thermochronology and geochronology is relatively common, but it is only with the recent advent of triple quadrupole laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) that in situ rubidium–strontium (Rb–Sr) dating has become a more commonly applied and powerful tool to date K-rich or Rb-bearing minerals. Here, we date two generations of mineral assemblages in individual thin sections using the in situ Rb–Sr method. Two distinct mineral assemblages, both probably associated with Au mineralization, are identified in samples from the Tropicana gold mine in the Albany–Fraser Orogen, Western Australia. For Rb–Sr purposes, the key dateable minerals are two generations of biotite as well as additional phengite associated with the younger assemblage. Our results reveal that the first, coarse-grained generation of biotite grains records a minimum age of 2535±18 Ma, coeval with previous 40Ar∕39Ar biotite, rhenium–osmium (Re–Os) pyrite and uranium–lead (U–Pb) rutile results. The second, fine-grained and recrystallized generation of biotite grains record an age of 1207±12 Ma across all samples. Phengite and muscovite yielded broadly similar results at ca. 1.2 Ga, but data are overdispersed for a single coeval population of phengite and show elevated age uncertainties for muscovite. We propose that the ca. 2530 Ma age recorded by various geochronometers represents cooling and exhumation and that the age of ca. 1210 Ma is related to major shearing associated with the regional deformation as part of Stage II of the Albany–Fraser Orogeny. This is the first time that an age of ca. 1210 Ma has been identified in the Tropicana Zone, which may have ramifications for constraining the timing of mineralization in the region. The in situ Rb–Sr technique is currently the only tool capable of resolving both geological events in these rocks.
Highlights
The ability to date multiple events in individual samples has important consequences for developing a comprehensive understanding of the geological history of complex terranes
Thin sections were subsequently carbon coated and analyzed using a TESCAN integrated mineral analyzer (TIMA) at the John de Laeter Centre (JdLC) at Curtin University to aid in mineral identification
Very fine (< 1 μm) exsolution lamellae of rutile and/or titanite within coarse-grained biotite 1 were observed in samples from the Tropicana and Havana pits (Fig. 4c, d, e, f, g, h), but these were absent from biotite 1 in the Boston Shaker pit (Fig. 4a, b)
Summary
The ability to date multiple events in individual samples has important consequences for developing a comprehensive understanding of the geological history of complex terranes. The uranium–lead (U–Pb) method has long been employed to date crystallization, metamorphism and hydrothermal events, often by targeting cores and rims in individual grains. Many U-bearing minerals have recorded multiple ages due to their ability to participate in metamorphic reaction, hydrothermal episodes or become (partially) reset by events above mineral closure temperatures, including zircon (Liu et al, 2012), monazite (Rasmussen et al, 2007), titanite (Olierook et al, 2019b; Kirkland et al, 2020), rutile (Zack and Kooijman, 2017; Olierook et al, 2019a) and apatite (Kirkland et al, 2018). Not all geological events are associated with (partial) reset or new growth of U-bearing minerals. In these scenarios, it is important to ex-. Olierook et al.: Resolving multiple geological events using in situ Rb–Sr geochronology amine alternative minerals that may provide a more complete record of the geological history
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