Highly Heterogeneous Pb Isotope Composition In Cenozoic Lower Crust Of The North China Craton

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. This thesis employed 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. The 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, which indicate that the lower crust beneath Yangyuan had a complicated history of formation and evolution. 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 composition of 9.3 at 2.65±0.14 Ga (all uncertainties are 2?). The isotopic heterogeneity among different types of granulite xenoliths 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/38U=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, thus they may be the fragments of Archean granulites. 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. Such 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 modified by the underplating of basaltic magma during Cenozoic, which means that they are newly-formed lower crust generated from magma underplating. Such extensive magma underplating might have significantly modified the Pb isotope composition of the ancient lower crust, and created younger lower crust at the same time. Individual plagioclase crystals from the garnet-bearing pyroxene granulites show variation in isotope 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. Meanwhile, the petrography shows that most of the garnet grains possess dark kelyphitic rims, many plagioclases and clinopyroxenes have prominent sieve-textured rims around their cores and some clinopyroxenes were partially or completely replaced by amphiboles. These observations imply that the large-scale of lithospehric extension and thinning beneath Trans-North China Orogen existed in both upper mantle as well as lower crust.