Origin of discordant UPb dates in non-metamict zircons in intrusives deformed at granulite facies: Grain scale processes, and relevance to Cambrian orogeny, Eastern Ghats Belt, India

Abstract

Interpreting the origin of UPb discordance in zircons in intrusives that experience high-T metamorphic-deformation events is challenging. We investigate in depth the origin of UPb dates obtained from garnet-bearing border zone granitoids and a sheared ferrodiorite dyke bordering the Balangir anorthosite pluton, eastern India. The crustal domain is known for discordant ages resulting in disputes regarding the age of intrusion and multiple granulite facies events. We shed light on the origin of the discordant dates utilizing CL imaging, electron backscatter diffraction analysis, trace element mapping in zircons, and Ti-in-zircon thermometry.

Zircons in the foliated border zone granitoids (BZG) comprise (a) embayed cores with varying Y, U and Hf oscillatory zones, and (b) well-faceted chemically-homogeneous mantles crystallographically continuous with the cores, which truncate the oscillatory zones. In the intensely sheared ferrodiorite, cauliflower-shaped zircons possess profuse micropores and micro-fractures. Ti-in-zircon temperatures (700–950 °C) overlap in the texturally-distinct zircons, but are somewhat lower in the mantles/cauliflower-shaped zircons relative to the oscillatory-zoned cores in BZGs.

LA-ICP-MS dates of zircons in four samples constitute a near-unique discordia line with the upper and lower intercepts at ca. 980 Ma and 495 Ma. Interestingly, dates from the mantles are highly variable with no correlation between age decrease and distance to the cores. The lack of concordant dates at the two intercepts, and the increase in discordance away from the intercepts preclude episodic zircon growth, and suggests a single-stage granulite facies event (ca. 495 Ma) that modified the ∼980 Ma zircon xenocrysts entrained from protoliths into the intrusives. We propose the age discordance was caused by melt-mediated high-T interface coupled dissolution-precipitation processes at ∼495 Ma. Variable inheritance of isotopic signatures of ∼980 Ma zircon xenocrysts in the younger zircons contributed to discordance in UPb systematics; the effect of variable inheritance of ∼980 Ma isotope signatures is less pronounced in the cauliflower-shaped zircons from the HFSE enriched ferrodiorite residual melts from high degree of plagioclase fractionation.

Our results are consistent with a distinct Cambrian (∼495 Ma) orogeny involving emplacement of anorthosite pluton and related intrusives and high-T granulite-facies deformation-metamorphism in the ∼980 Ma Eastern Ghats Province, and are a direct record of Cambrian collision between the Eastern Ghats Province and the Archean cratonic nucleus.