Objective The Shiquanhe ophiolite is an important tectonic belt in western Tibet. It has been debated whether the Shiquanhe ophiolite represents an allochthonous nappe derived from the Bangong-Nujiang suture zone to t...Objective The Shiquanhe ophiolite is an important tectonic belt in western Tibet. It has been debated whether the Shiquanhe ophiolite represents an allochthonous nappe derived from the Bangong-Nujiang suture zone to the north or the展开更多
The Zhangguangcai Range in the Xing’an(Hinggan) Mongolian Orogenic Belt, NE China, contains Early Jurassic(c. 188 Ma) Dabaizigou(DBZG) porphyritic dolerite. Compared with other island-arc mafic rocks, the DBZG doleri...The Zhangguangcai Range in the Xing’an(Hinggan) Mongolian Orogenic Belt, NE China, contains Early Jurassic(c. 188 Ma) Dabaizigou(DBZG) porphyritic dolerite. Compared with other island-arc mafic rocks, the DBZG dolerite is characterized by high trace-element contents, relatively weak Nb and Ta enrichments, and no Zr, Hf or Ti depletions, similar to OIB-type rocks. Analysed rocks have(87Sr/86Sr)i ratios of 0.7033–0.7044, relatively uniform positive εNd(t) values of 2.3–3.2 and positive εHf(t) values of 8.5–17.1. Trace-element and isotopic modelling indicates that the DBZG mafic rocks were generated by partial melting of asthenospheric mantle under garnet-to spinel-facies conditions. The occurrence of OIB-like mafic intrusion suggests significant upwelling of the asthenosphere in response to lithospheric attenuation caused by continental rifting. These processes occurred in an incipient continental back-arc environment in the upper plate of a palaeo-Pacific slab subducting W–NW beneath East Asia.展开更多
The Beka volcanic massifs are located northeast of Ngaoundere region, within the Adamawa plateau. It consists mainly of basanites, trachytes and phonolites. The petrographic study shows that all the basanite lavas hav...The Beka volcanic massifs are located northeast of Ngaoundere region, within the Adamawa plateau. It consists mainly of basanites, trachytes and phonolites. The petrographic study shows that all the basanite lavas have porphyritic microlitic textures with a more pronounced magmatic fluidity than the felsic lavas displaying trachytic textures. The lavas are composed of phenocrysts, microlites and microphenocrysts of olivine, clinopyroxene, plagioclase and iron-titanium oxides for the basanites and of greenish clinopyroxene, alkali feldspar, and titanomagnetite for the felsic lavas. Chemical microprobe analysis indicates that the olivine crystals are magnesian (Fo<sub>73-78</sub>). Clinopyroxene crystals have a composition of diopside (Wo<sub>47-</sub>) in the basaltic lavas and diopside near the hedenbergite pole in the trachytes phonoliths and titanomagnetite (TiO<sub>2</sub>: 21.13% - 22.36% and FeO: 68% - 68%). Chemical analyses on whole rocks show that all the lavas belong to the same series and the felsic lavas come from the differentiation of basanite lavas by fractional crystallization of the minerals therein. The basanites originate from a low rate of partial melting of an OIB-type mantle. Contamination and mixing processes are suspected. Lavas of similar composition are found in other volcanic centres of the Adamawa plateau and the continental and oceanic sectors of the Cameroon Volcanic Line, in particular the Kapsiki plateau, Mounts Cameroon and Bamenda.展开更多
基金financially supported by the National Natural Science Foundation (grants No. 41102065 and 41872110)the Sichuan Science and Technology Program (grant No. 2018JY0465)
文摘Objective The Shiquanhe ophiolite is an important tectonic belt in western Tibet. It has been debated whether the Shiquanhe ophiolite represents an allochthonous nappe derived from the Bangong-Nujiang suture zone to the north or the
基金funded by grants from the National Natural Science Foundation of China (41773029,41672063,41573022,41720104009,41373029)the Geological Survey (DD20160023-01)the Foundation of MLR (201511022)
文摘The Zhangguangcai Range in the Xing’an(Hinggan) Mongolian Orogenic Belt, NE China, contains Early Jurassic(c. 188 Ma) Dabaizigou(DBZG) porphyritic dolerite. Compared with other island-arc mafic rocks, the DBZG dolerite is characterized by high trace-element contents, relatively weak Nb and Ta enrichments, and no Zr, Hf or Ti depletions, similar to OIB-type rocks. Analysed rocks have(87Sr/86Sr)i ratios of 0.7033–0.7044, relatively uniform positive εNd(t) values of 2.3–3.2 and positive εHf(t) values of 8.5–17.1. Trace-element and isotopic modelling indicates that the DBZG mafic rocks were generated by partial melting of asthenospheric mantle under garnet-to spinel-facies conditions. The occurrence of OIB-like mafic intrusion suggests significant upwelling of the asthenosphere in response to lithospheric attenuation caused by continental rifting. These processes occurred in an incipient continental back-arc environment in the upper plate of a palaeo-Pacific slab subducting W–NW beneath East Asia.
文摘The Beka volcanic massifs are located northeast of Ngaoundere region, within the Adamawa plateau. It consists mainly of basanites, trachytes and phonolites. The petrographic study shows that all the basanite lavas have porphyritic microlitic textures with a more pronounced magmatic fluidity than the felsic lavas displaying trachytic textures. The lavas are composed of phenocrysts, microlites and microphenocrysts of olivine, clinopyroxene, plagioclase and iron-titanium oxides for the basanites and of greenish clinopyroxene, alkali feldspar, and titanomagnetite for the felsic lavas. Chemical microprobe analysis indicates that the olivine crystals are magnesian (Fo<sub>73-78</sub>). Clinopyroxene crystals have a composition of diopside (Wo<sub>47-</sub>) in the basaltic lavas and diopside near the hedenbergite pole in the trachytes phonoliths and titanomagnetite (TiO<sub>2</sub>: 21.13% - 22.36% and FeO: 68% - 68%). Chemical analyses on whole rocks show that all the lavas belong to the same series and the felsic lavas come from the differentiation of basanite lavas by fractional crystallization of the minerals therein. The basanites originate from a low rate of partial melting of an OIB-type mantle. Contamination and mixing processes are suspected. Lavas of similar composition are found in other volcanic centres of the Adamawa plateau and the continental and oceanic sectors of the Cameroon Volcanic Line, in particular the Kapsiki plateau, Mounts Cameroon and Bamenda.
