Anatomy of zircon growth in high pressure granulites: SIMS U–Pb geochronology and Lu–Hf isotopes from the Jiaobei Terrane, eastern North China Craton

Abstract

Granulite facies rocks that witnessed high pressure and high temperature metamorphism preserve important records of the growth and recrystallizaton of zircons during the multiple stages of tectonic evolution of orogenic belts developed in complex subduction–collision settings. Here we investigate zircons in the Paleoproterozoic high pressure (HP) granulites from Jiaobei Terrane in the eastern North China Craton that experienced a multi-stage metamorphic evolution. We present SIMS zircon U–Pb ages for zircons from garnet amphibolites, mafic granulites and pelitic granulites. Our data show four age peaks at 2.5Ga, 1.92Ga, 1.87–1.85Ga and 1.83Ga. Zircons belonging to the latter three age groups show similar characteristics of low Th and U contents, low Th/U ratios (<0.1), sector zoning or unzoned internal structures, corresponding to the features of zircon growth during metamorphism. Using zircon Lu–Hf isotopic compositions combined with a careful analysis of the morphology of the zircons, we evaluate the processes of solid-state recrystallization, intergranular melt growth, coupled dissolution–reprecipitation and subsolidus growth. Zircons formed through solid-state recrystallization during peak metamorphic stage inherited the relatively high 176Lu/177Hf ratios and low 176Hf/177Hf ratios of zircons from the protoliths. The youngest U–Pb age of ca. 1899Ma is interpreted as the maximum estimate of the time of peak metamorphism. Zircons formed through intergranular melt growth during low- to medium-pressure retrogression display homogeneous Lu–Hf isotope compositions, low 176Lu/177Hf and high 176Hf/177Hf ratios. The U–Pb age of 1870–1850Ma yielded by zircons crystallized from intergranular melt is interpreted to represent the time of low- to medium-pressure granulite facies retrogression. Some zircon grains with prismatic morphology and low 176Lu/177Hf and 176Hf/177Hf ratios display heterogeneous Lu–Hf isotopic compositions, possibly related to coupled dissolution–reprecipitation. The U–Pb ages of these zircons are identical to those formed from intergranular melt and these two groups of zircons formed during the same metamorphic stage. Zircons formed through subsolidus growth show large variations in Lu–Hf isotopic compositions and their ca.1833Ma age represents the timing of amphibolite facies retrogression. The HP granulite facies metamorphism in Jiaobei is thus constrained between 1870 and 1899Ma, with low- to medium-pressure granulite facies retrogression at ca. 1850–1870Ma followed by amphibolite facies retrogression at ca. 1833Ma.Our data show that several zircon cores and recrystallized grains were entrained from basement rocks during the emplacement of the mafic magmatic protoliths in the case of mafic granulites. For the pelitic granulites, the older zircons are detrital grains of magmatic origin. The Hf crustal model ages of these zircons that show peaks at ca. 2.7Ga and 2.9Ga are consistent with the timing of mafic magmatism in the region as identified in previous studies. The Lu–Hf isotopes of newly grown metamorphic zircons which have equilibrated chemical compositions trace the nature of the protolith. The zircon Hf crustal model age of ca. 2.5–2.7Ga from the pelitic granulites is consistent with that of the basement rocks in the Jiaobei Terrane and the zircon Hf model ages (TDM) of ca.2.0Ga and 2.2–2.3Ga from the mafic granulites might represent the time of magma generation from the mantle with crustal contamination. The geochronology of the basement rocks as revealed by the old zircon cores and the metamorphic evolution of the Jiaobei Terrane are comparable with the tectonic history of the central and western North China Craton and provide insights into the complex tectonothermal events associated with the Neoarchean–Paleoproterozoic crustal evolution history of the craton. Also, the pervasive ~2.5Ga zircons in a variety of rocks seem to suggest that the Jiaobei Terrane was already part of a coherent tectonic unit at such time.