Paleoproterozoic multistage evolution of the lower crust beneath the southern North China Craton

Abstract

It is generally recognized that the North China Craton (NCC) resulted from the amalgamation of several micro-blocks during the Paleoproterozoic period. However, how the lower crust beneath the craton evolved during the amalgamation of these blocks is less well-known. Four garnet-bearing felsic granulite xenoliths entrained by the Mesozoic Xinyang volcanic rocks at the southwestern part of the Eastern Block of the craton were selected for analysis of whole-rock major- and trace elements, and the U–Pb ages, trace-element patterns and Lu–Hf isotopic composition of zircons. These xenoliths are igneous in origin but with distinct sources. The relict cores of magmatic zircon in the xenoliths give ages including ca. 3.61Ga, ca. 2.22Ga and ca. 2.05Ga, and Hadean–Neoarchean Hf model ages (Tcrust=2.4–4.3Ga), implying the existence of the Hadean crustal components and a complex history in Archean–Paleoproterozoic. The metamorphic zircons from the xenoliths define three thermal episodes at ca. 2.33Ga, 1.86–1.91Ga and 1.72–1.77Ga, and exhibit low but flat HREE patterns and negative Eu anomalies, indicating that they formed in garnet-bearing granulite-facies rocks. Combined with the published data, a vertical section of the lower crust can be built: the upper part consists mainly of 3.4–3.6Ga garnet-free felsic granulites and rare pyroxenites, the middle part of 1.8–2.2Ga garnet-bearing felsic granulites and meta-gabbros, and the lowermost part of 1.7–2.0Ga intermediate and mafic granulites and Paleozoic eclogites. These observations suggest that the lower crust beneath the area underwent multistage modification in the Paleoproterozoic, which was linked to the successive geological processes between the Western and Eastern Blocks, such as the subduction, collision and rifting.