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 μm) 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 μm of grain rims. Secondly, the HREE distribution between zircon inner-core (I) and garnet core is 2 at Gd (GdDZrn/Grt = 2), rising to 8 at Lu (LuDZrn/Grt = 8), whereas those defined from zircon outer-core (II) or rim (III) and garnet core or rim are much lower and generally below 1 for Gd thruogh to Lu (GdDZrn/Grt = 0.8-1.2; LuDZrn/Grt = 0.6-0.7). 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.