Deep-seated crustal xenoliths record multiple Paleoproterozoic tectonothermal events in the northern North China Craton

The Precambrian tectonic evolution and crustal accretion in the North China Craton (NCC) have long been matters of debate. The Huai'an Complex, situated at the conjunction area for different tectonic models of the NCC, undoubtedly can provide some key constraints on the tectonic subdivision and crustal evolution of the craton. Here, we report zircon U-Pb ages and Hf-isotope for five deep-seated crustal xenoliths including one felsic granulite, one biotite-plagioclase gneiss, one hornblende-plagioclase gneiss and two hornblendites. The xenoliths were first found in the Cenozoic basalts in the Nangaoya area nearby the Huai'an Complex. The zircons from these xenoliths suggest a protolith age of 2459 ± 71 Ma for the hornblende-plagioclase gneiss, and generally record three periods of ~2060, ~1950 and ~1850 Ma tectonothermal events. The ~2060 Ma event is recorded by the felsic granulite (2063 ± 22 Ma), hornblende-plagioclase gneiss (2046 ± 56 Ma), and hornblendite (2049 ± 45 Ma). The ~1950 Ma event is represented in zircons from the felsic granulite (1951 ± 22 Ma), biotite-plagioclase gneiss (1950 ± 23 Ma), hornblende-plagioclase gneiss (1963 ± 56 Ma), and two hornblendites (1976 ± 33 Ma and 1955 ± 28 Ma). The ~1850 Ma event is recognized in zircon populations with ages of 1868 ± 22 Ma for the felsic granulite, 1846 ± 24 Ma for the biotite-plagioclase gneiss, 1866 ± 34 Ma and 1840 ± 54 Ma for two hornblendite xenoliths. Combined with published data, it is suggested that the ~2060 Ma event may correspond to the subduction stage of an ancient ocean; the ~1950 Ma event records the collision between the Yinshan and Ordos Blocks along the Khondalite Belt and the ~1850 Ma event marks the final collision of the Eastern and Western Blocks along the Trans-North China Orogen within the craton. Thus, these deep-seated crustal xenoliths recorded the Paleoproterozoic tectonothermal evolution for both the Khondalite Belt and TNCO. In addition, zircon Hf isotopes show that the protoliths of the felsic granulite and biotite-plagioclase gneiss were derived from partial melting of ~2.7 Ga juvenile crust; whereas the hornblendites originated from 2.2-2.1 Ga mantle magmas, a period of magmatic quiescence in many cratons of the world. Therefore, the Huai'an Complex in the northern NCC witnessed at least two episodes of continental-crust growth at ~2.7 Ga and at 2.2-2.1 Ga, with the latter more significant than estimated previously.