Evolution of the Archean and Paleoproterozoic lower crust beneath the Trans-North China Orogen and the Western Block of the North China Craton

摘要

We report U-Pb ages and Hf isotopes of zircons in granulites from two continental suture zones in the North China Craton (NCC)-from granulite xenoliths entrained in the Cenozoic Hannuoba basalts within the Trans-North China Orogen (TNCO) that amalgamates the Western and Eastern Blocks, and from a mafic granulite associated with ultrahigh-temperature (UHT) granulites within the Inner Mongolia Suture Zone (IMSZ) that welds the Yinshan and Ordos Blocks. The zircons in these rocks, together with the inherited zircons from Mesozoic subalkalic intrusives of these regions, show several distinct U-Pb age populations and provide constraints on the evolution of the lower crust beneath the NCC. Parts of the older zircons from the TNCO yield discordant data that project to ages of about 2.5 to 2.7Ga. These old zircons show Hf T _(DM) model ages of 2.6-2.9Ga suggesting the existence of Archean lower crust beneath the TNCO, which is coupled with the Neoarchean metamorphic basement and greenstone belt. A major zircon population grew at 1.75-2.0Ga, consistent with the timing of the Paleoproterozoic collision between the various crustal blocks within the NCC, suggesting that this event had a significant effect on the Archean lower crust. The older inherited zircons from the Western Block also yield discordant data that project to younger ages of about 1.8 to 1.9Ga. This age is consistent with the timing of intrusion of the mafic magmatic protoliths of the granulites associated with the UHT rocks in the khondalite belt within the IMSZ. These older zircons yield Hf T _(DM) model ages of 2.0-2.5Ga suggesting that the lower crust beneath the Western Block in the NCC to be dominantly Paleoproterozoic, and decoupled with the Archean metamorphic basement of the region. Yet another population of zircons from the Hannuoba banded granulite xenoliths, garnet-bearing pyroxenite cumulate and dark pyroxenite xenoliths yield a large spread of Phanerozoic concordant ages ranging from 470Ma to 40Ma with peaks at 315Ma, 220-230Ma, 120Ma and 46Ma. The Late Paleozoic zircons predominantly occur in the garnet-bearing pyroxenite xenoliths and yield variable εHf(t) values of -24 to +18. Triassic-Cretaceous zircons mainly occur in the banded granulite xenoliths and yield highly variable εHf(t) values, ranging from -48 to +16. The Tertiary zircons occur only in the dark pyroxenite xenoliths and show a restricted εHf(t) with a dominant range from 0 to +18. These data suggest episodic magmatic underplating in the ancient lower crust of the NCC, lasting continuously throughout Phanerozoic, producing zircons from the underplated magmas or providing the heat source for the recrystallization of zircons from the ancient crust. Our data clearly demonstrate the important events of Phanerozoic magmatic underplating beneath the Neoarchean-Paleoproterozoic crust of the NCC.