Eoarchean to Paleoproterozoic crustal evolution in the North China Craton: Evidence from U-Pb and Hf-O isotopes of zircons from deep-crustal xenoliths.

Granulite-facies xenoliths entrained within igneous rocks shed a light on poorly understood yet critical questions about age, origin and history of the lower continental crust. Here, we present the first coupled in-situ U-Pb, Lu-Hf and O isotope data for the Precambrian zircons from fourteen deep-crustal xenoliths from the North China craton. The geochronological data demonstrates that the oldest lower crustal remnants formed at ∼3.82 Ga and related magmatism continued until ∼3.55 Ga. The Eo-Paleoarchean zircons, with exception of one analysis, have subchondritic initial Hf isotopes with negative ε Hf (t) and are characterized by normal mantle to elevated δ18O values (5.37–6.90‰), indicating a development of a low Lu/Hf reservoir and hydrosphere-crust interactions in the early Earth. Magmatic zircons from lower crustal xenoliths of North China define a strongly episodic distribution of ages at 3.82–3.55 Ga, ∼2.7 Ga, ∼2.5 Ga and 1.95–1.85 Ga and a non-linear Hf isotope-age array for nearly 2 Gyr, indicating episodic crustal generation and reworking through time. The lower crustal xenoliths record a change in zircon O isotope compositions from a restricted range at 3.8–2.6 Ga to more variable after ∼2.5 Ga. Both heavier (δ18O up to 10.25‰) and lighter (δ18O low to 2.16‰) O isotope compositions than the normal mantle are observed in ∼2.5 Ga magmatic zircons. The secular change in zircon O isotopes documents an increase in recycling rate of surface-derived materials (including high-δ18O sediments, weathered and altered rocks and low-δ18O altered oceanic crust) into magma sources at the end of Archean, which, in turn, is possibly linked to modern style subduction processes and maturation of the crust at that time. [ABSTRACT FROM AUTHOR]