Age and composition of granulite and pyroxenite xenoliths in Hannuoba basalts reflect Paleogene underplating beneath the North China Craton

Abstract

In situ U-Pb and Hf-isotope systematics of zircons, mineral chemistry of silicates and whole-rock major-element, trace-element and platinum-group element (PGE) data are reported for five mafic xenoliths (granulites and pyroxenite) in the Hannuoba Cenozoic basalts, North China. Temperature estimates of 859-886 °C suggest that these xenoliths are derived from the lower crust. The whole-rock compositions of the xenoliths show weakly negative Eu anomalies (δEu = 0.67-0.95), high SiO 2/Al 2O 3 (5.80-13.8), low Ir (0.10-0.29 ppb), and wide ranges of Ni (4.5-235 ppm) and La/Nb (1.70-7.64). Most zircons in these samples are structureless and give Paleogene (44.5-47.3 Ma) U-Pb ages; minor populations have Early Mesozoic (210-220 Ma), Late Mesozoic (90 Ma) and Neogene (14 Ma) ages. The fractionation of pyroxene and plagioclase controlled the petrogenesis of the xenoliths, which are interpreted as the products of basaltic underplating and fractionation (cumulates) in Paleogene time (45-47 Ma), with assimilation of some older intermediate crustal components. The Paleogene underplating corresponds in time to lithosphere-scale extension in the North China Craton, leading to the widespread formation of sedimentary basins. Most Paleogene zircons have positive ε Hf (up to + 13.2) with uniform T DM (0.46-0.50 Ga), whereas most Mesozoic zircons have negative ε Hf; this evolution suggests a temporal decrease in the degree of crustal assimilation during the extension of the lithosphere. In addition to complex modification in Early and Late Mesozoic times, the thermal event related to the eruption of the host magmas (ca 14 Ma) also reheated the lower crust beneath the North China Craton. © 2009 Elsevier B.V. All rights reserved.