Response of detrital zircon and monazite, and their U–Pb isotopic systems, to regional metamorphism and host‐rock partial melting, Cooma Complex, southeastern Australia

Abstract

Progressive Early Silurian low-pressure greenschist to granulite facies regional metamorphism of Ordovician flysch at Cooma, southeastern Australia, had different effects on detrital zircon and monazite and their U-Pb isotopic systems. Monazite began to dissolve at lower amphibolite facies, virtually disappearing by upper amphibolite facies, above which it began to regrow, becoming most coarsely grained in migmatite leucosome and the anatectic Cooma Granodiorite. Detrital monazite U-Pb ages survived through mid-amphibolite facies, but not to higher grade. Monazite in the migmatite and granodiorite records only metamorphism and granite genesis at 432.8 Ϯ 3.5 Ma. Detrital zircon was unaffected by metamorphism until the inception of partial melting, when platelets of new zircon precipitated in preferred orientations on the surface of the grains. These amalgamated to wholly enclose the grains in new growth, characterised by the development of {211} crystal faces, in the migmatite and granodiorite. New growth, although maximum in the leucosome, was best dated in the granodiorite at 435.2 Ϯ 6.3 Ma. The combined best estimate for the age of metamorphism and granite genesis is 433.4 Ϯ 3.1 Ma. Detrital zircon U-Pb ages were preserved unmodified throughout metamorphism and magma genesis and indicate derivation of the Cooma Granodiorite from Lower Palaeozoic source rocks with the same protolith as the Ordovician sediments, not Precambrian basement. Cooling of the metamorphic complex was relatively slow (average ~12°C/10 6 y from ~730 tõ 170°C), more consistent with the unroofing of a regional thermal high than cooling of an igneous intrusion. The ages of detrital zircon and monazite from the Ordovician flysch (dominantly composite populations 600-500 Ma and 1.2-0.9 Ga old) indicate its derivation from a source remote from the Australian craton.

CHIME monazite dating: Pb analysis on an R_R = 100 mm spectrometer and correction of interferences between Th, U, and Pb with natural monazite

“…The linear arrangement of screened data on the PbOThO 2 * diagram strongly suggests that the chemical criteria reasonably discriminates concordant analyses from discordant ones. Each screened single date, 52-70 Ma for Kojaku granite and 383-477 Ma for Cooma Granodiorite, accords with the isotopic ages of 68.5 ± 0.7 Ma (Sueoka et al, 2016) and 432.8 ± 3.5 Ma (Williams, 2001) within the uncertainty of EPMA counting. The single date appears to properly represent the grain or domain age.…”

“…Monazite forms subhedral to anhedral grains of around 50 µm in size, with some exceptionally large grains of over 150 µm. Monazite in the Cooma Granodiorite was dated by SHRIMP as 432.8 ± 3.5 Ma (Williams, 2001) and by CHIME as 436 ± 16 Ma (Suzuki and Dunkley, 2003).…”

Hidden Archaean and Palaeoproterozoic crust in NW Ireland? Evidence from zircon Hf isotopic data from granitoid intrusions

“…Identifying whether or not zircon has dissolved is therefore paramount for any interpretation regarding the melted source. One way of demonstrating that zircon dissolution into the melt has occurred is by examining the texture of overgrown inherited zircon cores (Williams, 2001;Wu et al 2007). The absence of textural evidence for zircon dissolution of the inherited component does not of course preclude zircon dissolution having taken place.…”

Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

“…Roberts and Finger 1997;Vavra et al 1999;Schaltegger et al 1999;Hoskin and Black 2000;Rubatto et al 2001Rubatto et al , 2006Williams 2001;Rubatto 2002;Hermann and Rubatto 2003;Möller et al 2003;Tomaschek et al 2003;McFarlane et al 2005McFarlane et al , 2006Vonlanthen et al 2012;Vorhies et al 2013). Recent studies have improved our understanding of how to interpret metamorphic zircon in the context of the P-T evolution of rocks by modelling zircon growth during metamorphism (e.g.…”