Geochemistry Characteristics And Its Geological Significance Of The Late Mesozoic Volcanic Rocks From The Shandong Segment Of The Tan-Lu Fault Zone

Large-scale volcanic rocks were developed in Qingshan formation from the Shandong segment of the Tan-Lu fault zone, belonging to early Cretaceous. Chronological, geochemical and Sr-Nd-Pb isotopic analyses have been carried out on the volcanic rocks along the Shandong segment of the Tan-Lu fault zone. Based on these results, this paper demarcated formation time of these volcanic rocks, summarized nature of their petrology and geochemistry, discussed deep processes of the magmatism,compositions of magma sources, and magmatic evolution features. Based on regional comparsions, this paper also analyzed patterns and characteristics of the magma sources beneath the fault zone.These volcanic rocks were erupted during the Early Cretaceous, the eruption ages varied from 120Ma to 108Ma with a long period of 12Ma. They had SiO2 of 51.8-55.92%, and constitute mainly basalt, latite, trachyte association, belonging to high-K cal-alkaline shoshonite series. These volcanic rocks were notably rich in LILE(Rb,Ba,K) and LREE, depletes in HFSE(Nb,Ta,Ti) and HREE, with slightly negative Eu anomaly. These rocks have high initial 87Sr/86Sr(t) ratios (0.7070 to 0.7110) and lowεNd(t) values (-6.8 to -19.1), 206Pb/204Pb(t) ratios (16.441 to 17.641), 207Pb/204Pb(t) ratios (15.390 to 15.571), 208Pb/204Pb(t) ratios (36.688 to 37.904). Geochemical characteristics show the earlier magma reflect enriched mantle of the North China craton, and have the information of the crust of the NCC and the lower crust of the Yangtze Block, the later together with a small quantity of melt from the isotopic depleted asthenosphere in Guanshuai.During the Early Cretaceous, the volcanic activities on the Tan-Lu fault zone and NCC began and indicated the lithosphere thinning. Becauce of the constantly uplifting of the asthenosphere surface and chemical action became gradually strong, thus appeared the partial melting of the mantle of the lithosphere. The deep-subducted lower crust of the Yangtze Block existed under the lithosphere of the NCC, contributed to melt to earlier magma, this melt was generated from the melting of the subducted lower crust of the Yangtze Craton. The asthenosphere upwelling and lithosphere extension play a key factors for the magmatism. The partial melting of the mantle of the lithosphere mixed with the asthenosphere, the later magma together with a small quantity of melt from the isotopic depleted asthenosphere in Guanshuai.Compared with the volcanic activities inner NCC, the volcanic activities on the Tan-Lu fault zone began earlier and have a comparatively long history, the magma source became shallower. Indicate an intense degree and a long period of of the lithospheric thinning beneath the fault zone, its occurrence and activities provides an routeway and an important role to magma activities.