High‐resolution geochemical record of fluid–rock interaction in a mid‐crustal shear zone: a comparative study of major element and oxygen isotope transport in garnet

Abstract

Garnet grains from an intensely metasomatized mid-crustal shear zone in the Reynolds Range, central Australia, exhibit a diverse assortment of textural and compositional characteristics that provide important insights into the geochemical effects of fluid-rock interaction. Electron microprobe X-ray maps and major element profiles, in situ secondary ion mass spectrometry oxygen isotope analyses, and U-Pb and Sm-Nd geochronology are used to reconstruct their thermal, temporal and fluid evolution. These techniques reveal a detailed sequence of garnet growth, re-equilibration and dissolution during intracontinental reworking associated with the Ordovician-Carboniferous (450-300 Ma) Alice Springs Orogeny. A euhedral garnet porphyroblast displays bell-shaped major element profiles diagnostic of prograde growth zoning during shear zone burial. Coexisting granulitic garnet porphyroclasts inherited from precursor wall rocks show extensive cation re-equilibration assisted by fracturing and fragmentation. Oxygen isotope variations in the former are inversely correlated with the molar proportion of grossular, suggesting that isotopic fractionation is linked to Ca substitution. The latter generally show close correspondence to the isotopic composition of their precursor, indicating slow intergranular diffusion of O relative to Fe 2+ , Mg and Mn. Peak metamorphism associated with shearing ($550°C; 5.0-6.5 kbar) occurred at c. 360 Ma, followed by rapid exhumation and cooling. Progressive Mn enrichment in rim domains indicates that the retrograde evolution caused partial garnet dissolution. Accompanying intra-mineral porosity production then stimulated limited oxygen isotope exchange between relict granulitic garnet grains and adjacent metasomatic biotite, resulting in increased garnet d 18 O values over length scales <200 lm. Spatially restricted oxygen interdiffusion was thus facilitated by increased fluid access to reaction interfaces. The concentration of Ca in channelled fracture networks suggests that its mobility was enhanced by a similar mechanism. In contrast, the intergranular diffusion of Fe 2+ , Mg and Mn was rock-wide under the same P-T regime, as demonstrated by a lack of local spatial variations in the re-equilibration of these components. The extraction of detailed reaction histories from garnet must therefore take into account the variable length-and time-scales of elemental and isotopic exchange, particularly where the involvement of a fluid phase enhances the possibility of measureable resetting profiles being generated for slowly diffusing components such as Ca and O, even at low ambient temperatures and relatively fast cooling rates.

High-strain deformation and fluid infiltration diachronism of the middle crust: New Devonian–Permian Alice Springs ages (365–290 Ma) of shear zones in the Strangways Metamorphic Complex, Central Australia

“…Orogeny with estimations of 2-3 °C Ma −1 , contrasting with the very fast cooling of >50 °C Ma −1 estimated by Raimondo et al (2012) for the Reynolds Range in the northwest of the SMC (see Fig. 1 for location).…”

“…Cartwright and Buick, 1999;Raimondo et al, 2012;2011 Table 3) from mica pairs can provide some information, especially when comparing our data with previous work in the Arunta Region, including the SMC (e.g. .…”

“…The following equations were used (Raimondo et al, 2012;2011): Hensen, 1997) (Fig. 2), equilibration temperatures of 500 °C and 450 °C were also considered for the samples.…”

Ion microprobe survey of the grain-scale oxygen isotope geochemistry of minerals in metamorphic rocks

“…All fourteen analyzed garnet grains from seven samples of regional metamorphic rock in this study are homogeneous in d 18 O within error of measurement (Table 4B). Raimondo et al (2012) similarly reported ion microprobe analyses of garnets in amphibolite facies rocks that are either homogeneous in d 18 O within error of measurement or exhibit significant intracrystalline variations of no more than $0.5&. Larger intracrystalline variations in d 18 O of up to 9-10& in garnet from regional metamorphic rocks documented by other ion microprobe studies (Martin et al, 2011(Martin et al, , 2014Page et al, 2014) appear to be exceptional.…”

LA-ICP-MS U–Pb dating of detrital rutile and zircon from the Reynolds Range: A window into the Palaeoproterozoic tectonosedimentary evolution of the North Australian Craton

“…Williams et al, 1996;Vry and Baker, 2006;Morrissey et al, 2014) and were subsequently reworked during the 450-300 Ma Alice Springs Orogeny. The Alice Springs Orogeny produced steeply dipping shear zones that formed pathways for fluid flow and metasomatism (Raimondo et al, 2011(Raimondo et al, , 2012. Similar to the Chewings Orogeny, temperatures during the Alice Springs Orogeny decrease to the northwest with maximum temperatures of ca.…”