Linking growth episodes of zircon and metamorphic textures to zircon chemistry: an example from the ultrahigh-temperature granulites of Rogaland (SW Norway)

Abstract

In-situ U-Th-Pb analyses by ion-microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence imaging and trace element analyses to provide evidence for growth episodes of zircon. This approach helps: (a) to unravel the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the Rogaland anorthosite-norite intrusive complex; and (b) to constrain the age of M2 ultrahigh-temperature (UHT) metamorphism and the subsequent retrograde M3 event. All samples yield magmatic inherited zircon of c. 1035 Ma, some an additional group at c. 1050 Ma. This suggests that loss of Pb by volume diffusion in non-metamict zircon is not an important factor even under extreme crustal conditions. Furthermore, the identical inheritance patterns in aluminous (garnet, cordierite ± osumilite-bearing) migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite. Results for the M1 metamorphic event at c. 1000 Ma BP are consistent in all samples, including those from outside the orthopyroxene-in isograd. The latter do not show evidence for zircon growth during the M2 metamorphic episode.Zircon intergrown with or included within M2 metamorphic minerals (magnetite, spinel, orthopyroxene) give an age of 927 ± 7 Ma (2σ, n = 20). The youngest observed results are found in zircon outside M2 minerals, some overgrown by M3 mineral assemblages (late garnet coronas, garnet + quartz and orthopyroxene + garnet symplectites) and yield a slightly younger pooled age of 908 ± 9 Ma (2σ, n = 6). These textures are relative time markers for the crystallization of zircon overgrowths during discrete stages of the UHT event. These youngest age groups are consistent with the emplacement age of the Rogaland intrusive complex and the last magmatic activity (Tellnes dyke intrusion), respectively. This is direct and conclusive evidence for UHT metamorphism in the regional aureole being caused by the intrusions, and corrects earlier notions that the events are not linked. Trace element behaviour of zircon (Tb/U and Y content) has been tracked through time in the samples and shows variations both within and between samples. This heterogeneous behaviour at all scales appears to be common in metamorphic rocks and precludes the use of ‘rules of thumb’ in the interpretation of zircon chemistry, but chemical tracers are useful for recognition of zircon growth or recrystallization during metamorphism.

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

“…However, in some granulite facies (> 900 °C) and (U) HT terranes (e.g. Rogaland: Möller et al 2003; Napier Complex: Kelly and Harley 2005; Eastern Ghats belt: Korhonen et al 2013), metamorphic zircon with Th/U ratios > 0.1 are well known. The trend towards higher Th/U ratios in zircon with increasing metamorphic grade reflects both an increase in Th and a decrease in U concentrations.…”

“…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.…”

Fe–Ti–V–P ore deposits associated with Proterozoic massif-type anorthosites and related rocks

“…Granulite-facies assemblages with orthopyroxene, pigeonite or osumilite are common (Möller et al, 2003).…”

Two Periods of Skarn Mineralization in theBaizhangyanW–MoDeposit, SouthernAnhui Province, SoutheastChina: Evidence from ZirconU–Pband MolybdeniteRe–Osand Sulfur Isotope Data

“…These zircons have uniform U (205-1049 ppm) and variable Th (221-5833 ppm) concentrations, yielding Th/U ratios between 1.04 and 5.80, suggesting a magmatic origin (Th/U > 0.4) (Rubatto & Gebauer, 2000;Moller et al, 2003 Twenty analyses were conducted on 17 zircons of the sample 10BZ-2-6 from the Baizhangyan fine-grained granite. These zircons have U and Th concentrations of 66-1419 and 180-2123 ppm, respectively, yielding Th/U ratios between 0.58 and 3.19, indicative of a magmatic origin (Th/U > 0.4) (Rubatto & Gebauer, 2000;Moller et al, 2003). All 20 spots yielded 206 Pb/ 238 U ages of 131-139 Ma with a weighted mean age of 135.34 ± 0.92 Ma (MSWD = 0.85; Fig.…”

“…The majority of zircons separated from one of the Baizhangyan diorite dykes (sample 10BZ-2-1) have low U (86-284 ppm) and Th (75-489 ppm) concentrations, yielding Th/U ratios of 0.56-1.83, suggesting a magmatic origin (Th/U > 0.4) (Rubatto & Gebauer, 2000;Moller et al, 2003). Nine spots yielded 206 Pb/ 238 U ages of 141-149 Ma with a weighted mean age of 145.3 ± 1.7 Ma (MSWD = 0.81; Fig.…”