NATURE AND EVOLUTION OF LATE CRETACEOUS LITHOSPHERIC MANTLE BENEATH THE EASTERN NORTH CHINA CRATON: CONSTRAINTS FROM PETROLOGY AND GEOCHEMISTRY OF PERIDOTITIC XENOLITHS FROM JüNAN, SHANDONG PROVINCE, CHINA

Abstract

Mantle xenoliths entrained in a newly discovered Late Cretaceous basaltic breccia at Jünan, Shandong Province, China provide important constraints on the nature and evolution of the Late Mesozoic lithospheric mantle beneath the eastern North China Craton. The basaltic breccia erupted at 67 Ma contains abundant mantle and lower crustal xenoliths, including peridotites, pyroxenites, and granulites. The peridotitic xenoliths are all spinel-facies lherzolites and two types of lherzolites, high-Mg# and low-Mg#, occur. The low-Mg# lherzolites are dominant peridotitic xenoliths and have coarse-grained texture. These lherzolites are characterized by the occurrence of low Mg# (Fo < 90) olivine, orthopyroxene, and clinopyroxene and the LREE-depleted REE patterns ((La/Yb)N = 0.3-0.8) in cpxs. These mineral features and estimated equilibration temperatures (950-1040 °C) are very similar to those of lherzolite xenoliths entrained in the widespread Cenozoic basalts, eastern China. This similarity suggests that the low-Mg# spinel lherzolites represent the newly accreted lithospheric mantle. The high-Mg# lherzolites are very few and typified with high Mg# olivine (Fo = 92), high Cr# spinel, and convex-upward cpx REE pattern ((La/Yb)N = 1.5 and (La/Nd)N = 0.2). Its refractory geochemical features are completely different from those of low-Mg# lherzolites, but similar to those xenoliths from Archean and Proterozoic lithospheric mantle on the craton, and thus represent the relic of old refractory lithospheric mantle. The convex-upward cpx REE pattern implies an involvement of melt/rock interaction in its origin. No obvious differences in the equilibrium temperature of low-and high-Mg# peridotites suggest that the newly accreted mantle was close contact with the overlying old lithospheric mantle at the time of basaltic eruption. © 2006 Elsevier B.V. All rights reserved.