Zircon growth in UHT leucosome: constraints from zircon-garnet rare earth elements (REE) relations in Napier Complex, East Antarctica

Abstract

Feldspathic leucosomes occur in an ultrahigh temperature (UHT) metamorphosed garnet bearing paragneiss/ quartzite in the Napier Complex, East Antarctica. Coarse (200 400 μ m) zircon grains occurring in the leucosomes display 3 textural domains: (I) dark CL (cathodoluminescence) structured inner core, (II) bright CL structured outer core, and (III) dark CL structureless rim. Chemical compositions, especially chondrite normalized REE patterns obtained by SIMS analysis, correlate with these three domains: the inner core (domain I) shows HREE enrichment with Yb(n)/Gd(n) = 3.3, whereas the outer core (II) and rim (III) have flat to relatively depleted HREE patterns with Yb(n)/Gd(n) = 0.7 0.8. Th/U ratios decrease from 3.2 in the zircon inner core (I) to 1.2 in outer core (II) and to 0.3 in the rim (III).Garnets near such zircon grains display two trace element compositional features. Firstly, high core Zr contents (300 ppm) decrease to 100 ppm within 50 100 μ m of grain rims. Secondly, the HREE distribution between zircon inner core (I) and garnet core is 2 at Gd ( . This marked change in the HREE distribution between zircon and garnet must reflect either a change in the minerals with which the zircon was growing or being modified, a change in the physical and chemical conditions of zircon growth, or a combination of the two. Based on comparisons with recent estimates of equilibrium zircon/garnet HREE distribution coefficients we infer that the inner core (I) did not grow with the garnet that occurs in the paragneiss but grew within a garnet absent melt that was then injected into the gneiss. The resulting leucosome then underwent wall rock reaction with the enclosing garnet bearing gneiss, causing a decrease in garnet to Zr contents to values approaching equilibrium with melt, and precipitating the zircon outer core (II). Finally, the zircon rim (III) and later monazite formed in a HREE depleted environment. Melt injection, reaction and crystallization of the leucosomes took place within the time interval 2496 2471 Ma at the end of the UHT history of the Napier Complex.

Zr–LREE rich minerals in residual peraluminous granulites, another factor in the origin of low Zr–LREE granitic melts?

“…Loock et aI., 1990). The flat HREE pattern of the garnets is consistent with their lower crustal derivation; the increase in the Gd/Dy ratio has been previously interpreted as being controlled by load pressure (Bea et aI., 1997) or zircon/garnet partitioning (Whitehouse and Platt� 2003;Hokada and Harley, 2004). It is interesting to note that all analyzed garnets have MREE (pr, N d, Srn) contents higher than chondrite levels, also in agreement with the scarce previous data on felsic granulitic xenoliths (Reid, 1990).…”

U–Pb zircon geochronology in the western part of the Rayner Complex, East Antarctica

“…The Napier Complex contains felsic igneous precursors to orthogneisses as old as~3800 Ma, and most lithologies were formed or metamorphosed during the late Archean, but the timing of peak UHT metamorphism is controversial (Horie et al, 2012, and references therein). Some geochronological works suggested that the UHT metamorphism occurred no earlier than 2840 Ma, and possibly from 2590 to 2550 Ma (e.g., Harley et al, 2001;Crowe et al, 2002;Kelly and Harley, 2005), whereas other researchers interpret the younger~2500-2450 Ma age to represent the UHT event (e.g., Grew, 1998;Carson et al, 2002;Hokada et al, 2003;Hokada and Harley, 2004). The Rayner Complex was affected by amphibolite-to granulite-facies metamorphism between~990 and~900 Ma (e.g., Sheraton et al, 1987; Shiraishi et al, 1997;Harley, 2003).…”

A matter of time: The importance of the duration of UHT metamorphism

“…Color version is available online from http://doi.org/10.2465/jmps.160128. Harley et al, 2001, submitted) and Mt Riiser-Larsen (R23; Hokada and Harley, 2004) and individual concordant zircon age-Ti data from the pelite 49643, described in this study (orange diamonds). The grey curve represents a T-time that suggests the persistence of UHT for 80-90 Myr based on the leucosome data.…”

“…Similarly, zircon U-Pb age data compiled for anatectic leucosomes and migmatitic gneisses from the Napier Complex (Fig. 6: Carson et al, 2002;Hokada and Harley, 2004;Kelly and Harley, 2005;Horie et al, 2012) shows at least four crystallisation 'events' from perhaps 2585 Ma through to 2510 ma and even 2490 Ma. Again, rather than invoking T fluctuations it is conceivable that local melt / wall rock interactions governed zircon crystallisation under UHT conditions.…”

“…10). Hokada and Harley (2004) carried out a similarly integrated in-situ zircon-garnet REE study of a feldspathic leucosome that, from field and textural relationships, was interpreted to have invaded and interacted with a garnet-rich quartz metaquartzites host rock. This interaction resulted in a change in zircon chemistry from HREEenriched cores to flat-HREE outer zones and rims in close REE equilibrium with adjacent garnet rims based on the experimental distribution data of Taylor et al (2015).…”