The origin of anorthosite and associated igneous gabbronorite and ferrodiorite was investigated through detailed study of a typical massif-type anorthosite complex from Gruber, Central Dronning Maud Land, East Antarc-...The origin of anorthosite and associated igneous gabbronorite and ferrodiorite was investigated through detailed study of a typical massif-type anorthosite complex from Gruber, Central Dronning Maud Land, East Antarc- tica. Field observations showed that the Gruber Complex is made up of gabbronorite-anorthosite pluton which was intruded by ferrodiorite dykes. Systematic samples collected from the Gruber Complex revealed significant geo- chemical variations within the region. Four rock types have been identified, based on modal proportions of mineral phases and their geochemistry data. Clinopyroxene-gabbronorite and plagioclase-gabbronorite are the two types of gabbronorite with the dominance of clinopyroxene and plagioclase, respectively. Anorthosite is represented by rocks having predominance of plagioclase with minor clinopyroxene. Ferrodiorite is characterized by modal abundance of orthopyroxene and Fe-Ti oxide. Major and trace element systematics showed that all the four rock types are co-magmatic and are related through fractional crystallization. Based on this study, it is reported that clinopyroxene was the first phase to crystallize followed by plagioclase and then Fe-Ti oxides. Furthermore, trace element composition of the parental melt was calculated using LA-ICPMS analysis of the most primitive, pure clinopyroxene found in the clinopyroxene gabbronorite. Our analyses suggested that the parental melt was similar to that of continental arc basalt and showed signatures of subduction-related metasomatism. Based on mineral chemical and geochemical data, it is interpreted that the parent melt went through changing sequence of crystallization which led to the forma- tion of massive anorthosite.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
The occurrence of massif-type anorthosite intrusions is a widespread Proterozoic phenomenon.They are usually associated with gabbroic,charnockitic,and granitic rocks,comprising the so-called anorthositemangerite-charn...The occurrence of massif-type anorthosite intrusions is a widespread Proterozoic phenomenon.They are usually associated with gabbroic,charnockitic,and granitic rocks,comprising the so-called anorthositemangerite-charnockite-granite(AMCG)suite.Although these rocks have been extensively studied worldwide,several aspects concerning their formation remain unsettled.Among them,the magma source and the tectonic setting are the most important.To evaluate these issues,we first compiled geochemical and isotopic data of Proterozoic anorthosite massifs and AMCG suites worldwide and stored it in a database named datAMCG.This plethora of data allows us to make some important interpretations.We argue that the wide-ranging multi-isotopic composition of this group of rocks reflects varying proportions of juvenile mantle-derived melts and crustal components.We interpret that the precursor magmas of most massive anorthosite bodies and associated mafic rocks have a mantle-dominated origin.However,we highlight that a crustal component is indispensable to generate these lithologies.Adding variable amounts of this material during succeeding multi-stage assimilation-fractional crystallization(AFC)processes gives these intrusions their typical mantle-crustal hybrid isotopic traits.In contrast,a crustaldominant origin with a complementary mantle component is interpreted for most MCG rocks.In summary,the isotopic information in datAMCG indicates that both sources are necessary to generate AMCG rocks.Therefore,we suggest that hybridized magmas with different mantle-crust proportions originate these rocks.This interpretation might offer a more nuanced and accurate depiction of this phenomenon in future work instead of choosing a single-sourced model as in the past decades.Finally,tectonomagmatic diagrams suggest that the rocks under study were likely generated in a tectonic environment that transitioned between collision and post-collisional extension,sometimes involving subduction-modified mantle sources.This interpretation is supported by geological and geochronological information from most complexes,thus challenging the Andean-type margins as an ideal tectonic setting.展开更多
文摘The origin of anorthosite and associated igneous gabbronorite and ferrodiorite was investigated through detailed study of a typical massif-type anorthosite complex from Gruber, Central Dronning Maud Land, East Antarc- tica. Field observations showed that the Gruber Complex is made up of gabbronorite-anorthosite pluton which was intruded by ferrodiorite dykes. Systematic samples collected from the Gruber Complex revealed significant geo- chemical variations within the region. Four rock types have been identified, based on modal proportions of mineral phases and their geochemistry data. Clinopyroxene-gabbronorite and plagioclase-gabbronorite are the two types of gabbronorite with the dominance of clinopyroxene and plagioclase, respectively. Anorthosite is represented by rocks having predominance of plagioclase with minor clinopyroxene. Ferrodiorite is characterized by modal abundance of orthopyroxene and Fe-Ti oxide. Major and trace element systematics showed that all the four rock types are co-magmatic and are related through fractional crystallization. Based on this study, it is reported that clinopyroxene was the first phase to crystallize followed by plagioclase and then Fe-Ti oxides. Furthermore, trace element composition of the parental melt was calculated using LA-ICPMS analysis of the most primitive, pure clinopyroxene found in the clinopyroxene gabbronorite. Our analyses suggested that the parental melt was similar to that of continental arc basalt and showed signatures of subduction-related metasomatism. Based on mineral chemical and geochemical data, it is interpreted that the parent melt went through changing sequence of crystallization which led to the forma- tion of massive anorthosite.
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.
基金LAEP thanks the Consejo Nacional de Humanidades,Ciencias y Tecnologías(CONAHCYT)for granting him a Ph.D.scholarship.Special thanks to Victor Chashchin,Jean-Clair Duchesne,and Mike Hamilton for sharing valuable articles and published data during the development of datAMCG.This paper contributes to the Conacyt Ciencia de Frontera grant 7351 funded to LAS.HLH is funded by the National Natural Science Foundation of China(Grant No.42272227)。
文摘The occurrence of massif-type anorthosite intrusions is a widespread Proterozoic phenomenon.They are usually associated with gabbroic,charnockitic,and granitic rocks,comprising the so-called anorthositemangerite-charnockite-granite(AMCG)suite.Although these rocks have been extensively studied worldwide,several aspects concerning their formation remain unsettled.Among them,the magma source and the tectonic setting are the most important.To evaluate these issues,we first compiled geochemical and isotopic data of Proterozoic anorthosite massifs and AMCG suites worldwide and stored it in a database named datAMCG.This plethora of data allows us to make some important interpretations.We argue that the wide-ranging multi-isotopic composition of this group of rocks reflects varying proportions of juvenile mantle-derived melts and crustal components.We interpret that the precursor magmas of most massive anorthosite bodies and associated mafic rocks have a mantle-dominated origin.However,we highlight that a crustal component is indispensable to generate these lithologies.Adding variable amounts of this material during succeeding multi-stage assimilation-fractional crystallization(AFC)processes gives these intrusions their typical mantle-crustal hybrid isotopic traits.In contrast,a crustaldominant origin with a complementary mantle component is interpreted for most MCG rocks.In summary,the isotopic information in datAMCG indicates that both sources are necessary to generate AMCG rocks.Therefore,we suggest that hybridized magmas with different mantle-crust proportions originate these rocks.This interpretation might offer a more nuanced and accurate depiction of this phenomenon in future work instead of choosing a single-sourced model as in the past decades.Finally,tectonomagmatic diagrams suggest that the rocks under study were likely generated in a tectonic environment that transitioned between collision and post-collisional extension,sometimes involving subduction-modified mantle sources.This interpretation is supported by geological and geochronological information from most complexes,thus challenging the Andean-type margins as an ideal tectonic setting.
