Heterogeneous Pb isotope composition in the Archean lower crust of the North China Craton induced by Cenozoic basaltic magma underplating

Abstract

Granulite xenoliths entrained in Cenozoic basaltic rocks in several regions of the North China Craton (NCC) provide important constraints to evaluate the evolution of lower crust. Here we employ the Pb isotopic composition of plagioclase in granulite xenoliths from Yangyuan obtained through fLA (femtosecond laser ablation)-MC-ICPMS analyses to probe the Phanerozoic lower crustal evolution in the NCC. The newly-discovered granulite xenoliths from Yangyuan can be classified into felsic granulites, garnet-bearing pyroxene granulites and garnet-free pyroxene granulites. Our results show a large variation in plagioclase Pb isotopes (206Pb/204Pb=15.3–18.8; 207Pb/204Pb=14.9–15.9; 208Pb/204Pb=35.0–39.9) among and within samples. Common Pb in plagioclase and the single stage Pb evolution model indicate that the mantle from which the lower crust of the NCC was extracted had a 238U/204Pb ratio of 9.3 at 2.65±0.14Ga (all uncertainties are 2σ). The isotopic heterogeneity reflects the diverse formation and evolution history of the xenoliths. The felsic granulites show the least radiogenic Pb isotopes (206Pb/204Pb=15.3–15.8; 207Pb/204Pb=14.9–15.2; 208Pb/204Pb=35.0–35.9; 238U/204Pb=11.5 and 232Th/238U=4) among the entire xenolith suite with their Pb isotopes lying close to those of the exposed Archean granulite terranes of the NCC, suggesting that the granulites might have inherited the Pb isotope composition of ancient lower crust. The plagioclase from garnet-bearing pyroxene granulites shows the most radiogenic Pb isotopes (206Pb/204Pb=16.5–18.8; 207Pb/204Pb=15.3–15.9; 208Pb/204Pb=36.5–39.9; 238U/204Pb=18.2; 232Th/238U=4.3) in comparison to those of their garnet-free counterparts (206Pb/204Pb=15.9–17.3; 207Pb/204Pb=15.1–15.5; 208Pb/204Pb=36.1–37.5). The Pb isotopes for the former fall within or even above the fields of the I-MORB (Indian Mid-Ocean Ridge Basalt) and OIB (Ocean Island basalt) in 206Pb/204Pb vs. 207Pb/204Pb or 208Pb/204Pb, whereas the latter fall in the fields between the felsic and garnet-bearing pyroxene granulites. We consider that the linear correlation reflects the mixing of the ancient crust with radiogenic Pb in the melt rather than representing an isochron. The garnet-bearing pyroxene granulites might represent the lower crust which was likely modified by the underplating of basaltic magma during Cenozoic. Such extensive magma underplating might have significantly modified the Pb isotopic composition of the ancient lower crust. Individual plagioclase crystals from the garnet-bearing pyroxene granulites show variation in isotopic composition from less radiogenic Pb in the core to more radiogenic in the rim. Together with electron microprobe data, a decompressional melting of the lower crust is inferred in response to the lithosphere extension, with input of radiogenic Pb component.