Monazite as a monitor for melt‐rock interaction during cooling and exhumation

Abstract

Granulite facies cordierite-garnet-biotite gneisses from the southeastern Reynolds Range, central Australia, contain both orthopyroxene-bearing and orthopyroxenefree quartzofeldspathic leucosomes. Mineral reaction microstructures at the interface of gneiss and leucosome observed in outcrop and petrographically, reflect melt-rock interaction during crystallization. Accessory monazite, susceptible to fluid alteration, dissolution and recrystallization at high temperature, is tested for its applicability to constrain the chemical and P-T-time evolution of melt-rock reactions during crystallization upon cooling. Bulk rock geochemistry and phase equilibria modelling constrain peak pressure and temperature conditions to 6.5-7.5 kbar and~850°C, and U-Pb geochronology constrains the timing of monazite crystallization to 1.55 Ga, coeval with the Chewings Orogeny. Modelling predicts the presence of up to 15 vol.% melt at peak metamorphic conditions. Upon cooling below 800°C, melt extraction and in situ crystallization of melt decrease the melt volume to less than 7%, at which time it becomes entrapped and melt pockets induce replacement reactions in the adjacent host rock. Replacement reactions of garnet, orthopyroxene and K-feldspar liberate Y, REE, Eu and U in addition to Mg, Fe, Al, Si and K.We demonstrate that distinguishing between monazite varieties solely on the basis of U-Pb ages cannot solve the chronological order of events in this study, nor does it tie monazite to the evolution of melt or stability of rock-forming minerals. Rather, we argue that analyses of various internal monazite textures, their composition and overprinting relations allow us to identify the chronology of events following the metamorphic peak. We infer that retrograde reactions involving garnet, orthopyroxene and K-feldspar can be attributed to melt-rock interaction subsequent to partial melting, which is reflected in the development of compositionally distinct monazite textural domains. Internal monazite textures and their composition are consistent with dissolution and precipitation reactions induced by a high-T melt. Monazite rims enriched in Y, HREE, Eu and U indicate an increased availability of these elements, consistent with the breakdown of orthopyroxene, garnet and K-feldspar observed petrographically. Our study indicates that compositional and textural analysis of monazite in relation to major rock-forming minerals can be used to infer the post-peak chemical evolution of partial melts during high-to ultrahigh-temperature metamorphism.

Quantitative elemental mapping of granulite‐facies monazite: Textural insights and implications for petrochronology

“…Following similar signatures in granulite‐facies pelitic monazite reported by Prent et al. (2019), domain 3 growth is interpreted as the result of fluid–rock interaction following melt crystallization, causing alteration of feldspar and liberating Eu. Domain 3 is notable for forming isolated patches within grains (Figures 8d and 11d), as well as forming thin rims.…”

“…The low and consistent Eu/Eu* suggests that the melt effectively buffered the Eu budget, likely due to concomitant crystallization of feldspar‐rich leucosome. High Eu/Eu* monazite rims in sample A27A2 may be a sign of post‐peak alteration of feldspar associated with fluid‐rock interaction (Prent et al., 2019).…”

Constraints on the Palaeoproterozoic tectono‐metamorphic evolution of the Lewisian Gneiss Complex, NW Scotland: Implications for Nuna assembly

“…The Ti-rich core commonly hosts euhedral to subhedral titanite grains and rounded to vermicular quartz inclusions, which indicate equilibrium between these mineral phases at high temperatures. The cuspate-lobate shape of quartz inclusions and interstitial quartz together with the presence of quartz films along amphibole grains are interpreted to indicate hydrous melt-rock interactions during cooling (Prent et al, 2019;Putnis et al, 2021) and incipient partial melting (Ferrero et al, 2012;Tacchetto et al, 2019;Vernon, 2004). The garnet-amphibolite LC1909 contains garnet crystals elongated parallel to the amphiboledefined foliation with Mn content increasing from coreto-rim (Figure S1).…”

Petrochronology of high‐pressure granulite facies rocks from Southern Brasília Orogen, SE Brazil: Combining quantitative compositional mapping, single‐element thermometry and geochronology

“…The application of U‐Pb (Th) geochronology techniques to assess accurate and precise ages of accessory minerals such as monazite and rutile has become particularly useful for deciphering the tectonic history of orogenic belts (Armstrong, 1991; Dickin, 1995; Gebauer & Grünenfelder, 1979), and detailed petrochronology is becoming widespread in the many cases where multiple stages of the P‐T‐ t path are be recorded (Engi et al, 2017; Foster et al, 2004). The ages of prograde heating, thermal peak conditions, and retrograde stages of metamorphism can be recorded in monazite (e.g., Prent et al, 2019; Reno et al, 2012; Rocha et al, 2017; Tiwari & Biswal, 2019), whereas rutile is potentially the most suitable mineral for studying the influence of intragrain and intergrain ion diffusion during cooling (Zack & Kooijman, 2017). Combining ages from coexisting minerals is a powerful tool for uncovering complex metamorphic histories and constraining the cooling rates and thermal evolution of high‐grade rocks.…”