| In this paper,it is reported that the mineralogical,petrological,whole rock geochemical and Sr-Nd isotopic data for the Early Cretaceous lamprophyres in western Shandong,the Xiaya lamprophyres in Feixian county,and the Jingziyu lamprophyres in Zaozhuang City.In addition,the LA-ICP-MS zircon U-Pb dating and Hf isotopic data were also carried out to determine their formation age,nature of magma source,petrogenesis,and the tectonic setting.This study also further reveals the nature of the Early Cretaceous lithosphere mantle and the deep dynamic process of the eastern North China Craton.1.The forming age of the Early Cretaceous lamprophyres in western ShandongThe LA-ICP-MS zircon U-Pb dating analyses have been determined for Xiaya minettes and Jingziyu minettes and pyroxene lamprophyres in western Shandong.The dating results yield 206Pb/238U ages of 127 Ma123 Ma,135 Ma129 Ma and 134 Ma128 Ma,respectively.The weighted mean 206Pb/238U ages are 126±2 Ma,132±1 Ma and 130±1 Ma,respectively,which represent the forming ages?Early Cretaceous?.2.Geochemisty of the Early Cretaceous lamprophyres in western ShandongBased on the petrographical and geochemical data,the Early Cretaceous lamprophyres in western Shandong can be divided into two types:the low-Ti lamprophyres?TiO2<1.1%,Ti/Y<270?and the high-Ti lamprophyres?TiO2>2.0%,Ti/Y>370?.Xiaya and Jingziyu minettes?11SF01,11SF02,SF1-1 and 11SZ05?are defined as low-Ti lamprophyres and Jingziyu pyroxene lamprophyres?11SZ02?are high-Ti lamprophyres.The low-Ti lamprophyres are characterized by low SiO2?40.22–46.04%?and high MgO?1.71–4.92%?,TFe2O3?5.19–6.03%?and Mg#?42–70?.Moreover,they are enriched in light rare earth elements and large-ion lithophile elements?e.g.,Cs,Rb,and Ba?,and depleted in heavy rare earth elements and high-field strength elements?e.g.,Nd,Ta Zr and Hf?as well as weak negative Eu anomalies??Eu=0.78–0.83?.The high-Ti lamprophyres are characterized by low SiO2?34.39–34.74%?and high MgO?13.76–13.92%?,TFe2O3?10.97–11.03%?,TiO2?2.25%?and Mg#?70–74?.Additionally,they are enriched in ligh rare earth elements and depleted in heavy rare earth elements as well as the weak depletion in high-field strength elements?e.g.,Nd,Ta,Zr and Hf?.3.Sr-Nd-Hf isotopic compositions of the Early Cretaceous lamprophyres in western ShandongThe low-Ti lamprophyres of Western Shandong have initial 87Sr/86Sr ratios of0.712083–0.712260 and?Nd?t?values of-18.8–-18.6.The high-Ti lamprophyres of Western Shandong have 87Rb/86Sr ratios of 0.706629–0.706787 and?Nd?t?values of-5.01–-4.64.Zircons from the low-Ti lamprophyres of Western Shandong have176Hf/177Hf ratios and?Hf?t?values ranging from 0.281151 to 0.282612,and-9.97 to 5.46,respectively.The?Hf?t?values of magmatic zircons in the low-Ti lamprophyres range from-9.94 to-3.36.The high-Ti lamprophyres exhibit 176Hf/177Hf ratios and?Hf?t?values of 0.281209 to 0.282453 and-11.77 to 5.61,respectively.Their?Hf?t?values of magmatic zircons in the high-Ti lamprophyres range from-11.77 to-8.50.4.Petrogenesis and tectonic setting of the Early Cretaceous lamprophyres in western ShandongThe Low-Ti lamprophyres in western Shandong are characterized by low SiO2?40.22–46.04%?,high TFe2O3?5.19–6.03%?,MgO?1.71–6.1%?,TiO2?0.50–0.78%?,Mg#?42–70?,and enrichment of transitional elements such as Cr,Co and Ni(104×10-6–535×10-6,19.6×10-6–27.8×10-6 and 106×10-6–269×10-6).Moreover,the low-Ti lamprophyres in western Shandong have the relatively high initial 87Sr/86Sr ratios?0.712083–0.712260?,low?Nd?t?values?-18.81–-18.6?and low zircon?Hf?t?values?-9.94–-3.36?,implying they could be derived from partial melting of the lithospheric mantle.However,they are enriched in light rare earth elements and depleted in heavy rare earth elements,and weakly depleted in high-field strength elements.The above characteristics,combined with the obvious positive Pb anomaly and the existence of magmatic zircons with late Archean Proterozoic and late Neoproterozoic ages and metamorphic zircons with Late Triassic ages,suggest that the primitive magma of the low-Ti lamprophyre originated from the partial melting of an enriched lithospheric mantle that previously modified by the Yangtze Craton basement and lower crust of North China Craton.The high-Ti lamprophyres in western Shandong are characterized by low SiO2?34.39–34.74%?,and high MgO?13.76–13.92%?,TFe2O3?10.97–11.03%?,TiO2?2.25%?and high Mg#?73–74?,enrichment of transitional elements such as Cr,Co and Ni(322×10-6–629×10-6,55.4×10-6–58.1×10-66 and 311×10-6–325×10-6).They also have relatively low initial 87Sr/86Sr ratios?0.706629–0.706787?,high?Nd?t?values?-5.01–-4.64?and high zircon?Hf?t?values?-11.77–-8.50?.The above characteristics show that the high-Ti lamprophyres could be derived from partial melting of the lithospheric mantle.However,the similar geochemical features,such as weak positive Pb and negative Nb,Ta,Zr and Hf anomalies,suggest that primary magma originated from partial melting of an enriched lithospheric mantle which was weakly modified by the lower crust of the North China Craton.The formation times of two types of lamprophyres,i.e.126 Ma,132Ma and 130Ma,are consistent with the age of lithospheric thinning in the eastern China.They are also consistent with the time of the Mesozoic extensive magmatic event in the North China Craton,even eastern China.Combined with the existence of the Early Cretaceous bimodal volcanic rock in the eastern China,the lamprophyres should form under an intensive extensional environment in the eastern North China Craton. |
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