NATURE AND EVOLUTION OF CENOZOIC LITHOSPHERIC MANTLE BENEATH SHANDONGPENINSULA, SINO-KOREAN CRATON, EASTERN CHINA

The Shanwang and Qixia basalts lie within the North China block and we re erupted in Miocene to Pliocene time (18.1 to 4.3 Ma) and Pliocene t ime (6.4 to 5.9 Ma), respectively. The Shanwang area lies astride the Tancheng-Lujiang (Tanlu) fault zone, a major lithospheric fault, where as the Qixia area lies east of the fault zone. The basaltic rocks (alk ali olivine basalts, basanites, nephelinites) carry abundant deep-seat ed xenoliths including spinel Iherzolite (dominant), dunite, and pyrox enite, and a megacryst suite including augite, anorthoclase, phlogopit e, ilmenite, and garnet. Xenoliths with coarse-grained microstructures are common in the Qixia xenolith suite, but are absent in Shanwang. R econstructed bulk compositions of the Iherzolites range from relativel y depleted (<3% modal diopside) to fertile (>12% modal diopside). Equi libration temperatures of 850 degrees to 1020 degrees C indicate entra inment of these Iherzolites from depths less than or equal to 45 km, w ithin the lithosphere; the geotherm may have been higher beneath Shanw ang. The Shanwang suite contains less-depleted Iherzolites, and more p yroxenites, than the Qixia suite. The chondrite-normalized REE pattern s in clinopyroxenes of the Shandong xenoliths vary from LREE depleted, through concave shaped, to LREE enriched; spider-grams for the clinop yroxenes can be divided into depleted, fertile, and metasomatic types. Progressive depletion in Na and Al is accompanied by depletion in mod erately incompatible elements such as Y, Yb, and Zr, and an increase i n Mg#. Ti and Zr in clinopyroxenes have not been affected by the metas omatic process, and MREE have been little disturbed, whereas the light rare-earth elements, Nb, and Sr have been strongly enriched during me tasomatism; this suggests that carbonate-rich fluids/melts were the me tasomatic agent. The mantle beneath the Shandong Peninsula sampled by these basalts is dominantly Phanerozoic in character rather than Arche an or Proterozoic lithospheric mantle. This mantle probably represents a mixture of older lithospheric mantle and newly accreted material th at replaced the Archean lithospheric keel through extension, thermal e rosion, and fluid/melt metasomatism. The differences in microstructure s, chemistry, temperature, and fluid/melt activity between Shanwang an d Qixia are ascribed to their spatial relationships to the Tanlu fault , which is a major translithospheric suture that has played an importa nt role in the Cenozoic replacement of the pre-existing Archean lithos pheric mantle.