Discovery of Middle Jurassic bimodal volcanic rocks in the Xuanhua district on the northern edge of the North China Craton and their geological significances
The bimodal volcanic rocks have important significance for studying their tectonic environment and geodynamics.The authors have conducted geological,petrological,petrogeochemical,and isotopic chronological studies of the bimodal volcanic rock combination including basalt and trachyte which were firstly discovered in the lower part of the Tiaojishan Formation in the Mangtoushan-Qijiazhuang-Dishuiya area of the Xuanhua district,in order to provide new geological evidences for studying the mantle state and tectonic environment of the North China Craton during the Middle Jurassic period.The results show that there are two types of basalt including the lower basalt and upper basalt.Samples of the lower basalt have SiO2 contents ranging from 42.91％to 46.63％,K2O contents ranging from 0.28％to 0.37％,TiO2 contents ranging from 1.42％to 1.51％,Mg# values ranging from 40 to 61,and ΣREE contents varying from 56.40×10-6 to 85.85× 10 6,with weak fractionation of light and heavy rare earth elements(LaN/YbN=1.55～2.01)and weakly positive Eu anomalies(8Eu=1.04～1.09).The lower basalt belongs to the tholeiitic basalt series.Samples of the upper basalt have SiO2 contents ranging from 39.49％to 49.79％,K2O contents ranging from 2.08％to 2.74％,TiO2 contents ranging from 2.32％to 3.39％,Mg# values ranging from 28 to 42,and ΣREE contents of 248.60×10-6～329.56×10-6,with strong fractionation of light and heavy rare earth elements(LaN/YbN=7.24～8.22),and positive Eu anomalies(8Eu=1.08～1.20).The upper basalt belongs to the alkaline basalt series.Samples of trachyte have SiO2 contents ranging from 61.45％to 63.79％,K2O contents ranging from 5.22％to 5.81％,Na2O contents ranging from 4.31％to 4.7％,A/CNK ratios ranging from 0.96 to 0.99 ΣREE contents of 262.55× 10-6～323.85× 10-6,LaN/YbN values of 8.59～-9.98,and δEu values of 0.91～0.99,with no obvious Eu anomalies.The trachyte belongs to the alkaline metaaluminous rock series.The zircon U-Pb age of trachyte is(162.5±1.3)Ma.The basalts and trachytes of the bimodal volcanic rock combination in the lower part of the Tiaojishan Formation have zircon U-Pb ages ranging from(162.5±1.3)Ma to(164.3±2.6)Ma.It is believed that they were formed in the continental rift setting during the end of Middle Jurassic.Period It is also believed that the lithospheric mantle in the northern edge of North China Craton was destroyed and transformed into the oceanic lithospheric mantle due to the subduction of Paleo-Pacific Ocean Plate at the end of Middle Jurassic.From the lower basalt to the upper basalt,the degrees of partial melting of mantle source materials for forming those basaltic magmas are decreased,and the depths of mantle source materials are increased.The mantle sources are oceanic mantle which were modified by the subduction of the Paleo-Asian Ocean Plate and the enriched continental crust components.Trachyte was formed in the thickened crust setting by magma derived from the partial melting of basalt mixed with highly differentiated Neoarchean crustal materials.
North China CratonTiaojishan Formationbimodal volcanic rocksbasalt and trachytealkaline seriescontinental riftdepleted mantle
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