Occurrence of Cretaceous basalts in Fuxin County, Liaoning Province provides us an opportunity to understand Mesozoic mantle processes beneath the northern margin of the North China Craton (NNCC). Fuxin Jianguo basalt...Occurrence of Cretaceous basalts in Fuxin County, Liaoning Province provides us an opportunity to understand Mesozoic mantle processes beneath the northern margin of the North China Craton (NNCC). Fuxin Jianguo basalts occur as volcanic channel phases with well-developed columnar jointings and contain few spinel lherzolite and pyroxenite xenoliths. They are poor in silica and rich in alkalis, Ti and Al, belonging to alkaline basalts. In trace element compositions, Jianguo basalts are moderately enriched in LREE and LILE, but not depleted in HFSE. They have low Sr and high Nd and Pb isotopic ratios. These geochemical characteristics suggest that Jianguo basalts originated from the depleted asthenosphere, representing an undifferentiated and uncontaminated primitive magma. Presence of these basalts indicates that the lithosphere beneath the region had thickness less than 65 km at the time of basalt eruption and was mainly composed of fertile pargasite-bearing spinel lherzolite and plagioclase pyroxenite. The voluminous basaltic-andesitic magmatism during the early Jurassic-late Cretaceous time indicates that the commencement and accomplishment of lithosphere thinning in the NNCC was much earlier than that in the southern margin, since the mafic-intermediate volcanism only occurred at the Cretaceous time in the southern margin and the basalts with an asthenosphere isotopic signature at the Tertiary. This shows that highly spatial and temporal heterogeneity existed in the Mesozoic lithosphere evolution.展开更多
The granulitic xenoliths discovered in the late Cretaceous basaltic rocks from Tuoyun basin, western Xinjiang Province contain the assemblage of Opx + Cpx + Pl ± Grt ± Qtz ± Kfs. Mineral chemistry, petr...The granulitic xenoliths discovered in the late Cretaceous basaltic rocks from Tuoyun basin, western Xinjiang Province contain the assemblage of Opx + Cpx + Pl ± Grt ± Qtz ± Kfs. Mineral chemistry, petrochemistry and zircon U-Pb ages from the xenoliths have been reported in the paper. Petrogenesis, the lower crustal nature and the mantle-crust interaction reflected by the granulites are discussed as well. Tuoyun granulites are mainly mafic with few intermediate components, and represent the magmatic products underwent granulite-facies metamorphism in lower crustal condition. Equilibrium temperature and the maximum pressure estimates of the granulites are 910 ± 35℃ and 13.5 × 108 Pa respectively, indicating that the crust beneath southwestern Tianshan had the thickness of less than 44 km and high geotherm (e.g. > 80 mW/m2) when the host magma erupted. During their formation, the fractional crystallization of olivine, pyroxene, plagioclase and ilmenite, plagioclase accumulation and the contamination of felsic components or fluid metasomatism processes had happened. Metamorphic zircon U-Pb age of 253 Ma may record the crust-mantle interaction caused by the orogenic root delamination beneath the southwestern Tianshan.展开更多
基金This work was supported by the National Natural Science Foundation of China(Grant No.40073004)the Chinese Academy of Sciences(KZCX1-07).
文摘Occurrence of Cretaceous basalts in Fuxin County, Liaoning Province provides us an opportunity to understand Mesozoic mantle processes beneath the northern margin of the North China Craton (NNCC). Fuxin Jianguo basalts occur as volcanic channel phases with well-developed columnar jointings and contain few spinel lherzolite and pyroxenite xenoliths. They are poor in silica and rich in alkalis, Ti and Al, belonging to alkaline basalts. In trace element compositions, Jianguo basalts are moderately enriched in LREE and LILE, but not depleted in HFSE. They have low Sr and high Nd and Pb isotopic ratios. These geochemical characteristics suggest that Jianguo basalts originated from the depleted asthenosphere, representing an undifferentiated and uncontaminated primitive magma. Presence of these basalts indicates that the lithosphere beneath the region had thickness less than 65 km at the time of basalt eruption and was mainly composed of fertile pargasite-bearing spinel lherzolite and plagioclase pyroxenite. The voluminous basaltic-andesitic magmatism during the early Jurassic-late Cretaceous time indicates that the commencement and accomplishment of lithosphere thinning in the NNCC was much earlier than that in the southern margin, since the mafic-intermediate volcanism only occurred at the Cretaceous time in the southern margin and the basalts with an asthenosphere isotopic signature at the Tertiary. This shows that highly spatial and temporal heterogeneity existed in the Mesozoic lithosphere evolution.
基金supported by the National Natural Science Foundation of China(Grant Nos.40273001,40425002)the EYTP(Grant No.2003-119).
文摘The granulitic xenoliths discovered in the late Cretaceous basaltic rocks from Tuoyun basin, western Xinjiang Province contain the assemblage of Opx + Cpx + Pl ± Grt ± Qtz ± Kfs. Mineral chemistry, petrochemistry and zircon U-Pb ages from the xenoliths have been reported in the paper. Petrogenesis, the lower crustal nature and the mantle-crust interaction reflected by the granulites are discussed as well. Tuoyun granulites are mainly mafic with few intermediate components, and represent the magmatic products underwent granulite-facies metamorphism in lower crustal condition. Equilibrium temperature and the maximum pressure estimates of the granulites are 910 ± 35℃ and 13.5 × 108 Pa respectively, indicating that the crust beneath southwestern Tianshan had the thickness of less than 44 km and high geotherm (e.g. > 80 mW/m2) when the host magma erupted. During their formation, the fractional crystallization of olivine, pyroxene, plagioclase and ilmenite, plagioclase accumulation and the contamination of felsic components or fluid metasomatism processes had happened. Metamorphic zircon U-Pb age of 253 Ma may record the crust-mantle interaction caused by the orogenic root delamination beneath the southwestern Tianshan.
