Evolution of lithospheric mantle beneath the Tan-Lu fault zone, eastern North China Craton: Evidence from petrology and geochemistry of peridotite xenoliths

摘要

A suite of peridotite xenoliths from Cenozoic Beiyan basalts within the Tancheng-Lujiang (Tan-Lu) wrench fault zone, eastern North China Craton (NCC), has been studied to provide constraints on the nature and evolution of the lithospheric mantle beneath this region. These xenoliths commonly have porphyroclastic, granuloblastic to resorption textures with the absence of coarse-grained texture. They can be subdivided into three types: lherzolite, clinopyroxene (cpx)-rich lherzolite and wehrlite. Lherzolites are characterized by low forsterite contents (Fo) (88-91) in olivines. Whole rock and cpx separates from lherzolites have convex-upward rare earth element (REE) patterns except for one sample which has the highest Fo in olivine and shows a spoon-shaped REE pattern. The Sr-Nd isotopic compositions of cpx separates are depleted, similar to those of mid-ocean ridge basalts (MORE). These geochemical characteristics indicate that the lherzolites represent fragments of newly accreted lithospheric mantle that makes up much of the Late Mesozoic-Cenozoic lithosphere beneath the Tan-Lu fault zone. Cpx-rich lherzolite and wehrlite reflect the interaction of the lithosphere with melt, as evidenced by relatively lower Fo (<87) than the lherzolites (Fo~90), and higher enrichment in cpx and light rare earth elements (LREE). Shallow relics of the Archean cratonic mantle have not been found in this region. Therefore, the abundant cpx-rich lherzolites and wehrlites could be the result of recent modification of lherzolites by asthenospheric melt. The Tan-Lu fault zone facilitated the ascent and migration of asthenospheric melt and enhanced lithospheric mantle-asthenospheric melt reaction. Combined with the data for mantle xenoliths from the adjacent regions, a highly heterogeneous and secular evolution of the lithosphere is inferred beneath the Jiaodong region during Phanerozoic times.