Geochemical Research On The Early Cretaceous Volcanic Rock Of Sihetun In The Western Liaoning Province

Delamination of lower continental crust and lithospheric mantle occurred within the North China craton during the Mesozoic, which accompanied extensive magmatism formation in western area of Liaoning Province. The Sihetun Yixian Formation has attracted considerable attention over the last two decades, due to a wide range of well preserved 'feathered' dinosaurs and primitive birds fossils have been yielded from the Sihetun Yixian Formation. The 400m thick Yixian Formation mainly consists of volcanic rock, which interlayered with lake facies sedimentary rock that contain fossils on the top of section. Previous researchers developed abundant studies for the sedimentary section containing fossils and made detailed studies of isotope geochronology using Sihetun volcanic and volcaniclastic vocks. The age of Sihetun Yixian Formation is 120 to 130Ma. In contrast, the Yixian volcanic rocks lack systemic geochemical research.85 volcanic rocks have been collected and analyzed from the Sihetun standard section that began at Xinkailing and ended at fossil observation station. The analytic results of major and trace elements indicate that the Sihetun volcanic rocks are high magnesium andesites, besides some basalts on the bottom of section.Integrated detailed petrological and geochemical studies of the Sihetun andesites lead to the following conclusions:1. Sihetun volcanic rocks are mostly high magnesium andesites, whose Mg# is 38 to 69;2. The Sihetun high magnesium andesites (SiO2=52.82~59.31wt%, Al2O3=14.15~16.35wt%) show many characteristics of adakites such as depletion in heavy rare-earth elements(HREE;Yb=1.03~1.88μg/g) and Y(12~2(μg/g) and high Sr(620~1323μg/g) and Sr/Y(32~88), with high LaN/YbN ratio(10~25). Other striking geochemical features of Sihetun lavas are high whole-rock Cr (187~580μg/g) and Ni(89~290μg/g) contents, because of the presence of chromite. The Sihetun lavas also exhibit depletion of Nb, Ta and enrichment of Pb, Sr and without Eu anomaly (Eu/Eu*=0.96~1.05). The initial Sr-Nd isotopic compositions of high-Mg andesites are 143Nd/144Nd (130Ma) = 0.5118-0.5119, εNd(130Ma) =-11.6-13.8 and 87Sr/86Sr(130Ma) =0.7058-0.7064. The high-Mg andesites from the Yixian Formation at Sihetun have similar geochemical characteristics with high-Mg andesites from the Xinglonggou Formation except for more lower Nd isotopic ratio.3. Sr content and Sr/Y ratio indicate significant negative correlation with SiO2 in the Sihetun high-Mg andesites with SiO2 contents > 56%. These indicate Sr content and Sr/Y ratio of Sihetun high-Mg andesites were affected by fractional crystallization of mainly plagioclase. So the affection of fractional crystallization on volcanic Sr and Sr/Y ratio cannot be ignored.4. The lacustrine shale from the fossil observation station is also high-Mg with Mg# being remarkably higher than the average Archaean shale of the North China craton. This suggests that the whole Sihetun lavas are high in magnesium.5. The high-Mg andesites from the Yixian Formation at Sihetun have similar major and trace element characteristics to high-Mg andesites from the Xinglonggou Formation. They were interpreted in a way that eclogite that formed at the base of thicken Archaean lower crust of the North China craton foundered into the convecting mantle and subsequently melted and interacted with peridotite and experienced significant crustl contamination during eruption. Compare with the Xinglonggou lavas (εNd(130Ma)=1.9 to -2.9), the Sihetun andesites have obviously low initial εNd values (-11 to -14) that require the magma source contain more ancient continental crust substance with evolved Nd isotopic compositions.The age of Sihetun Formation was 120 to 130Ma, and this indicated that delamination was lasted to the Early Cretaceousperiod.