Duration of anatexis in a Neoproterozoic-Cambrian UHT terrane: constraints from prograde melt inclusions in zircon

The duration of anatexis in high-grade metamorphism is essential to understand the crustal melting processes and the tectonic settings. In the case of Rundvågshetta, Lützow-Holm Complex, East Antarctica, the linkage between the U-Pb zircon ages and the metamorphic pressure-temperature (P-T) evolution is still unclear. Only the melt crystallization age of ca. 520 Ma is constrained. In this study, we aim to constrain the duration of anatexis by using petrochronological approaches to an ultrahigh-temperature (UHT) granulite sample from Rundvågshetta.

Garnet in the studied sample consists of the P-poor core, P-rich mantle and P-poor rim. Based on the detailed petrography of inclusion minerals in garnet, we interpret that the garnet core was formed as a peritectic product of biotite dehydration melting during prograde metamorphism, and that the garnet mantle and rim were formed in the peak and retrograde stages, respectively, in a clockwise P-T evolution.

Zircon in the rock matrix shows four microstructural domains; oscillatory-zoned inherited core, dark-CL annulus, slightly bright-CL inner rim and bright-CL outer rim. The inner rim was too thin for the LA-ICP-MS U-Pb zircon dating with 20 µm spot size. The inherited cores are always truncated by the dark-annulus with low Th/U ratios below 0.04. The dark-annulus includes muscovite, biotite, rutile, quartz and melt inclusions and yielded weighted mean age of 564.0 ± 4.9 Ma (2σ error, n = 4, MSWD = 1.8). The dark-annulus is further truncated by the outer rim with higher Th/U ratios (0.08-1.13). The outer rim includes sillimanite, K-feldspar and rutile and yielded weighted mean age of 530.5 ± 4.9 Ma (2σ error, n = 13, MSWD = 1.5).

The microstructures of inclusion zircon vary systematically with the phosphorus zoning of the host garnet. Zircon in the garnet rim show four microstructural domains that are common to the matrix zircon. Meanwhile, zircon in the garnet core always lacks the inner and outer rims. The dark-annulus and outer rim of zircon respectively showed steeply positive-sloping and negative-sloping heavy rare earth elements (HREE) patterns. Meanwhile, the garnet core, mantle and rim showed positive, flat and negative HREE patterns, respectively. Based on these systematic microstructures of inclusion zircon and on the partitioning of HREE between zircon and garnet, it is revealed that the outer rim of zircon grew simultaneously with the garnet rim during the retrograde metamorphism, and that the dark-annulus of zircon grew prior to the garnet core during the prograde metamorphism.

Inclusion minerals in the dark-annulus of zircon suggest the possible occurrence of muscovite dehydration melting at ca. 560 Ma. Therefore, microstructural observations of zircon enabled us to deduce the prograde anatexis prior to the attainment of UHT condition that is not recorded in garnet. Taking the melt crystallization age of ca. 520 Ma into account, the duration of anatexis in Rundvågshetta is constrained to be at least ~40 Myr. Further U-Pb dating of the thin inner rim of zircon may reveal the duration of the UHT itself precisely.