| Basalts and hosted pyrolites are direct materials that supply us to research the composition and evolution of the mantle. Numerous Cenozoic basalts distribute in Eastern China, many of which classified as alkali basalts and contain varied amounts of mantle xenoliths and high-pressure megacrysts that can provide a good opportunity to reveal chemical compositions and processes in the lithospheric mantle. Here, the study focused on the basalts and hosted peridotites in both Zhejiang and Fujian Provines is implemented via the approaches of petrology, geochronology, elemental and isotopic geochemistry, which reveals petrogenesis and lithospheric mantle properties and limits tectonic evolution of the coastal region of Southeast China.The basalts in this study group into alkali and subalkali basalts following the TAS diagram. Depending on the simplified CIPW norm calculations, the former includes the basanites in Quzhou, Longyou, Taizhou, Xianju and Sanming, the alkali olivine basalts in Shengxian and Xinchang, the olivine tholeiites in Shengxian, Xinchang and Niutoushan, and the olivine basalts in Xinchang. The latter belongs to quartz tholeiites, including sample QZ06 in Quzhou and all ones without peridotite xenoliths in Niutousha. Most of these basalts contain olivine phenocrysts that are characterized by corrosion border and kink band. Modeling of trace elements shows that alkali basalts derived from low degree (2～5%) melting of garnet-lherzolite, in contranst, that tholeiitic melt was the mixed one between high degree (5～10%) melting of garnet-lherzolites and low degree (～5%) melting of spinel-lherzolites with 7:3 scale. The OIB-like style in spider diagrams, higher Ce/Pb and Nb/U values than OIB and MORB as well as enriched Sr-Nd isotopic composition of basalts indicate that the mantle source of these basalts was generated by the mixing of depleted lithospheric mantle and enriched asthenospheric mantle. This character may have resulted from mantle metasomatism due to the oceanic crust sediment into the mantle transition zone after a paleosubduction.Peridotite xenoliths include dunites, spinel-lherzolites and spinel-harzburgites. The dunites have lower Al2O3, CaO weight and total rare earth elements content than other xenoliths, which indicate the dunites are depleted. The Re depletion ages (TRD: 0.62～1.42 Ga) of the dunites are also older than other ones (0.07～0.47 Ga). It is suggested that the dunites had underwent high degree of melting and melt extraction than the others. The Sr-Nd isotopic ratios of peridotite xenoliths, however, have broader range than their host basalts and no depleted mantle character but EM Ⅱ trend. This means that the lithospheric mantle of Southeast China from Ordovician to Pleistocene had underwent varied degree metasomatism which made the lithospheric mantle fertiled. Meanwhile, this study obtains 16～18 Ma mantle Zircons whose U-Pb system was transformed by the host basalt’s high temperature.It is idealized that Cenozoic basalts in Zhejiang, Fujian and Taiwan erupted under the background of extention or rifting. Because their temporal and spatial distribution is not matched with the geological setting. With the reviews on tectonic evolution and reconstruction of Southeast Asia, this study suggest that the coastal region of Southeast China underwent the transition from the left lateral strike-slip with west of due north to the northwest transpression since the Oligocene, which was the result of the combined action among opening of South China Sea, drifting to North and rotation of Philippine Sea Plate, and subduction retreating of pacific slab. |
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