Geochronology And Geochemistry Of The Mesozoic Volcanic Rocks In Western Liaoning

The studies of mantle xenoliths hosted in the Paleozoic kimberlites and the Cenozoic basalts indicate that about 100 km lithosphere has bee removed during Phanerozoic. However, timing and mechanism for the thinning is still controversial. The Mesozoic volcanic rocks with variable ages widely developed in Westem Liaoning. The geochemical variation of these rocks could provide constraints on timing and mechanism for the thinning. Therefore, this work focuses on the geochronology and geochemistry of these rocks.1. Timing of the each Mesozoic magmatisms in Western Liaoning.The samples collected from four formations including two fossil-bearing beds have been selected for geochronological studies. The results are ca. 176 Ma for the Xinglonggou Formation, 166-153 Ma for the Lanqi Formation, 125-122 Ma for the Yixian Formation, ca. 106 Ma for the Zhanglaogongtun Formation, ca. 165 Ma for the Daohugou bed and ca. 125 Ma for the Yixian fossil-bearing bed. Based on these and previously reported data, four Mesozoic magmatic episodes in Western Liaoning have been recognized, e.g. they occurred in ca. 176Ma, 166-148 Ma, 132-120 Ma and ca. 106 Ma, respectively. The age of Jehol Biota in Western Liaoning is the early Cretaceous And the age of Daohugou Biota is the Middle Jurassic.2. Geochemical characteristics and origin of the volcanic rocks.Xinglonggou andesites: this Magnesian adakitic rock has not only the typical adakitic characteristics (such as SiO₂%＞56%, Al₃O₂%＞15%, MgO＜3%, Y＜18ppm, Yb＜1.9ppm, Sr＞400ppm, without negative Eu anomaly), but also have high Mg^# (0.57), and high Cr (156ppm) and Ni (82.2ppm) contents. These characteristics indicate the presence of residual gamet in their magma source and interaction between the adakitic melt and the mantle rocks during ascending of the melt. The moderate ⁸⁷Sr/⁸⁶Sr (0.706597), slightly lowε_Nd(T)(-1.5) and high radiogenic Pb (²⁰⁶pb/²⁰⁷pb=18.211, ²⁰⁷pb/²⁰⁴pb=15.554 and ²⁰⁸Pb/²⁰⁴pb=38.176) suggest that the Xinglonggou high Mg adakitic rocks were formed by partial melting of subducted oceanic crust. Lanqi basalts and basaltic andesites: They have extremely low radiogenic Pb (²⁰⁶pb/²⁰⁷pb＜16.5, ²⁰⁷pb/²⁰⁴pb＜15.3 and ²⁰⁸pb/²⁰⁴pb＜36.5), moderate ⁸⁷Sr/⁸⁶Sr(～0.706) and lowε_Nd(T)(～-12). They are enriched in light rare earth dements (LREE), large ion lithophile elements (LILE), and Pb with low Ce/Pb (＜10), while depleted in high field strength elements (HFSE). These geochemical characteristics suggest significant involvement of low crust materials in their mantle source. They have low MgO, Cr, Ni contents, with depletion of Th and U, indicating granulite wall rock assimilation and large amount olivine fractional crystallization during magma underplating. This basaltic magma underplating may thicken the lower mafic crust.Lanqi andesites: They have high Al₂O₃(＞16%), and low MgO (＜2.5%), Cr (＜1ppm) and Ni(＜7ppm) as well as lower Sr and higher Y, with negative Eu anomaly, showing difference from the Xinglonggou andesites and adakites. They have similar Sr-Nd-Pb isotopic compositions to the Lanqi basalts and basaltic andesites. Their moderate ⁸⁷Sr/⁸⁶Sr (0.704～0.706), lowε_Nd(T)(-13～-10) and low radiogenic Pb (²⁰⁶pb/²⁰⁷pb＜16.6, ²⁰⁷pb/²⁰⁴pb＜15.4 and ²⁰⁸pb/²⁰⁴pb＜36.8) suggest that the Lanqi andesites were derived from lower crust. AFC processes during magmatic underplating may provide the heat needed by partial melting of the lower crust.Yixian basalts and basaltic andesites: They show similar geochemical characteristics to the Lanqi basalts, such as low radiogenic Pb (²⁰⁶pb/²⁰⁷pb＜16.7,²⁰⁷Pb/²⁰⁴Pb＜15.3 and ²⁰⁸pb/²⁰⁴pb＜36.7), moderate ⁸⁷Sr/⁸⁶Sr(～0.706), lowε_Nd(T)(＜-9.7), enrichment of LREE and LILE, depletion of HFSE, and low Ce/Pb, suggesting involvement of low crust materials in their mantle source, which could be hybridized by the SiO₂-rich liquids produced by partial melting of delaminated crust. Because high MgO, Cr, Ni contents without depletion of Th and U distinguish them from the Lanqi basalts, indicating that the basaltic magma may derive directly from the mantle source without AFC process.Yixian andesites: They are Magnesian adakitic rocks, similar to the Xinglonggou andesites, such as SiO₂%＞56%, Al₃O₂%＞15%, MgO＜3%, Y＜18ppm, Yb＜1.9ppm, Sr＞400ppm, no negative Eu anomaly, Mg^#＞0.54, Cr＞200ppm, Ni＞100ppm. However, the Yixian andasites are different from the Xinglonggou andesites in Sr-Nd-Pb isotopic compositions. The Yixian andesites have very low radiogenic Pb (²⁰⁶Pb/²⁰⁷Pb＜16.7, ²⁰⁷Pb/²⁰⁴Pb＜15.3 and ²⁰⁸pb/²⁰⁴Pb＜36.7), moderate ⁸⁷Sr/⁸⁶Sr(～0.706) and lowε_Nd(T)(＜-9.7), indicating that the Yixian adakitic magma was derived from delaminated lower continental crust.Zhanglaogongtun basalts: The Zhanglaogongtun basalts are characterized by positive Nb and Ta anomaly, negative Pb anomaly, enriched in LREEs and LILEs, without depletion of HFSEs, low ⁸⁷Sr/⁸⁶Sr(＜0.704), and highε_Nd(T)(＞4) and Pb isotopic ratios (²⁰⁶Pb/²⁰⁷Pb＞17.8, ²⁰⁷Pb/²⁰⁴pb＞15.4 and ²⁰⁸Pb/²⁰⁴Pb＞37.6), suggesting derivation from the depleted upper mantle. The abruptly going up ofε_Nd(T) values showing by Zhanglaogongtun basalts indicates that an important lithospheric thinning event happened at about 106 Ma in Western Liaoing.3. The Mesozoic lithosphere evolution model for the North margin of the NCC.Based on the geochronological, geochemical data mentioned above, a model for lithospheric evolution of the North margin of the NCC has been proposed here. A) The Triassic-early Jurassic: the Mongolia slab subducted under the NCC. B) Partial melting of the subducted oceanic crust, which was underplating beneath the lithosphere of the NCC, occurred at ca. 176Ma to produce the Xinglonggou adakitic rocks. C) 166 -148 Ma: The delamination of subducted oceanic crust caused the mantle upwelling and its partial melting at 166 Ma. The upwelling mantle and basaltic magma may result in magma underplating and partial melting of the lower crust to produce the Lanqi basalts and andesites. Accordingly, the mafic lower crust in Western Liaoning area was thickened. D) 148-132 Ma: The regional contraction of the Yanshanianâ…¡before the Yixian magmatism further thickened the crust, resulting eclogitic facies trasformation in the thickened mafic lower crust. E) 132-120 Ma: The large scale strike-skip of the Tan-Lu fault may trigger the delamination of thickened mafic lower crust. The adakitic melt (i.e. the Yixian Magnesian andesites) produced by partial melting of the delaminated mafic lower crust may hybridize the upwelling mantle to forme a new enriched lithospheric mantle, which replaced the volume formerly occupied by the delaminated lithospheric mantle, resulting in the "continental" geochemical signatures observed in Yixian basalts. F) ca. 106 Ma: The movement of the Tan-Lu fault zone transferred into extension in the late Cretaceous. This large scale extention resulted in farther lithospheric thinning.