Heavy mineral petrographic and geochemical compositions (major and trace/rare earth elements)?of sandstones obtained from the Oligocene-Miocene Ogwashi-Asaba Formation, Niger Delta were studied to determine their prov...Heavy mineral petrographic and geochemical compositions (major and trace/rare earth elements)?of sandstones obtained from the Oligocene-Miocene Ogwashi-Asaba Formation, Niger Delta were studied to determine their provenance, source area weathering conditions and tectonic setting. The heavy mineral suite (opaque minerals, zircon, tourmaline, and rutile) revealed that the sandstones are mineralogically mature and implied rapid disintegration and chemical decomposition of sediments mostly of recycled orogen. The sandstones were geochemically classified as Fe-sand and partly quartz arenitic. Chemical Index of Alteration and Chemical Index of Weathering values of 89.92% and 91.87% respectively suggest that the source region was predominantly felsic and was subjected to intense chemical weathering probably under tropical palaeoclimatic conditions with abundant rainfall that enhanced sediment recycling. Major element concentration discriminant plots also indicated that the sediments were derived from mixed sources (granitic, gneissic or recycled orogen) under passive margin setting. Chondrite normalized plot of the rare earth element pattern is marked by light rare earth element enrichment and negative Eu anomalies, interpreted to mean that provenance was mainly continental crustal rocks. Trace elemental ratios that are provenance diagnostic (La/Sc, Th/Sc, Cr/Th, La/Co, Th/Co, Th/Cr, Eu/Eu*, and Eu*) all point to sediments derived from felsic source and upper continental crust. The mixed provenance of the sandstones can be traced to the southwestern and southeastern Basement Complex (consisting of granites, gneisses, etc.) and sediments derived from the adjacent sedimentary basins (Anambra and Benue Trough).展开更多
This paper reports the systematic studies of geochemistry on the meta sedimentary rocks, felsic gneisses and amphibolites of the Kongling complex from the Archean high grade terrain of the Yangtze craton. It shows t...This paper reports the systematic studies of geochemistry on the meta sedimentary rocks, felsic gneisses and amphibolites of the Kongling complex from the Archean high grade terrain of the Yangtze craton. It shows that the amphibolites originated from a weakly depleted mantle resource. Nb negative anomaly, negative ε (Nd, t ) and the t DM ages which older than their isochron ages of the TTG gneisses imply the possible existence of the crust older than round about 2.7 Ga in the region. Three types of meta sedimentary rocks are identified from the Kongling complex. The first type originated from the juvenile crust with the features of the first cycle sedimentary rocks. Cratonic sedimentary rocks characterize the second type. Mobilization of REE and other elements resulted from partial melting during the migmatization is found in the third type of meta sedimentary rocks. Apart from the early TTG gneisses, some of the mafic rocks are also expected to be the source rocks of the first type of para rocks. The deposition times of the first and second type paragneisses are closed in the Neoarchean of about 2.7 Ga and the early period of Proterozoic respectively, and the period when cratonization of the Yangtze continental block completed is referred to be earlier than the Mesoproterozoic. Nd isotopic tracing on the magma sources of the Jinning granitic rocks in the region reveals that the major part of the Kongling basement is Neoarchean.展开更多
In the present study, the major and trace element compositions, as well as Sr, Nd isotopic compositions and K-Ar age data in mafic dikes from Hainan Island, China, have been analyzed. Whole-rock K-Ar dating yielded a ...In the present study, the major and trace element compositions, as well as Sr, Nd isotopic compositions and K-Ar age data in mafic dikes from Hainan Island, China, have been analyzed. Whole-rock K-Ar dating yielded a magmatic duration of 61-98 Ma for mafic dikes. Mafic dikes have a very high concentration of incompatible elements, for example, Ba, Rb, Sr, K, rare earth elements, and especially light rare earth elements （LREE）, and negative anomalies of Nb, Ta, and Ti in the normalized trace element patterns. The initial 87Sr/86Sr ratios and εSr（t） of the mafic dikes are 0.70634- 0.71193 and ＋27.7 to ＋112.2, respectively. In the 87Sr/86Sr versus εNd（t） diagram, the Hainan Island mafic dikes plot between fields for depleted mantle and enriched mantle type 2. All these characteristics show that the mantle （source region） of mafic dikes in this area experienced metasomatism by fluids relatively enriched in LREE and large ion lithophUe elements. The genesis of Hainan Island mafic dikes is explained as a result of the mixing of asthenospheric mantle with lithospheric mantle that experienced metasomatism by the subduction of the Pacific Plate. This is different from the Hainan Island Cenozoic basalts mainly derived from depleted asthenospheric mantle, and possibly, minor metasomatised lithospheric mantle. This study suggests that the Mesozoic and Cenozoic lithospheric revolutions in Hainan Island can be divided into three stages： （1） the compression orogenesis stage before 98 Ma. The dominant factor during this stage is the subduction of the ancient Pacific Plate beneath this area. The lithospheric mantle changed into enriched mantle type 2 by metasomatism; （2） the thinning and extension stage during 61-98 Ma. The dominant factor during this stage is that the asthenospheric mantle invaded and corroded the lithospheric mantle; and （3） the large-scale thinning and extension stage after 61 Ma. The large-scale asthenospheric upwelling results in the strong erupting of Cenozoic basalts, large-scale thinning of the lithosphere, the southward translating and counterclockwise rotating of Hainan Island, and the opening of the South China Sea.展开更多
The shoshonite province in eastern China is characterized by extensive distribution (ca. 80000 km2) of Late Jurassic-Early Cretaceous (165-100 Ma) shoshonite series with subordinate high-K calc-alkali series. It was f...The shoshonite province in eastern China is characterized by extensive distribution (ca. 80000 km2) of Late Jurassic-Early Cretaceous (165-100 Ma) shoshonite series with subordinate high-K calc-alkali series. It was formed in a dominantly tensile stress field. In comparison with their analogues in island arcs and active continental margins in other countries, the volcanic rocks in the shoshonite province have their specific characteristics in petrology, mineralogy and geochemistry as well as related mineralization association, which are the comprehensive reflection of the special composition and structure of the mantle and crust of the province and the special Mesozoic regional tectonic setting.展开更多
Systematic geochemical studies have been conducted on the North Küda Pluton, West Kunlun, in order to reveal its petrogenesis and tectonic implications. The North Küda Pluton is a potassium\|rich (K\-2O>5...Systematic geochemical studies have been conducted on the North Küda Pluton, West Kunlun, in order to reveal its petrogenesis and tectonic implications. The North Küda Pluton is a potassium\|rich (K\-2O>5.4wt%) I\|type granitic pluton and does not contain any alkaline ferromagnesian mineral. Its relatively high REE, LILE (e.g. Rb, Cs, U and Th) and HFSE (e.g. Nb, Zr) contents make it very akin to the A\|type granites. Its heterogeneous Sr (\{\{\}\+\{87\}Sr\}/\{\{\}\+\{86\}Sr\-i\}=\{0.7049\}～\{0.7098\}) and Nd (εNd\-T=\{-1.05\}～\{-4.04\}) isotope compositions preclude the possibility of a pure sedimentary or igneous source. Instead, its geochemical compositions suggest that it may be derived from partial melting of a complex source, which consists of igneous and sedimentary rocks. Its intraplate characteristics, together with coeval mafic dykes, indicate an extensional environment at the end of Caledonian. The recognition of the extensional event does not support a continuous subduction\|accretion model for the Paleozoic tectonic evolution of the West Kunlun Mountains. On the contrary, it provides new evidence for the two\|stage island\|arc model.展开更多
文摘Heavy mineral petrographic and geochemical compositions (major and trace/rare earth elements)?of sandstones obtained from the Oligocene-Miocene Ogwashi-Asaba Formation, Niger Delta were studied to determine their provenance, source area weathering conditions and tectonic setting. The heavy mineral suite (opaque minerals, zircon, tourmaline, and rutile) revealed that the sandstones are mineralogically mature and implied rapid disintegration and chemical decomposition of sediments mostly of recycled orogen. The sandstones were geochemically classified as Fe-sand and partly quartz arenitic. Chemical Index of Alteration and Chemical Index of Weathering values of 89.92% and 91.87% respectively suggest that the source region was predominantly felsic and was subjected to intense chemical weathering probably under tropical palaeoclimatic conditions with abundant rainfall that enhanced sediment recycling. Major element concentration discriminant plots also indicated that the sediments were derived from mixed sources (granitic, gneissic or recycled orogen) under passive margin setting. Chondrite normalized plot of the rare earth element pattern is marked by light rare earth element enrichment and negative Eu anomalies, interpreted to mean that provenance was mainly continental crustal rocks. Trace elemental ratios that are provenance diagnostic (La/Sc, Th/Sc, Cr/Th, La/Co, Th/Co, Th/Cr, Eu/Eu*, and Eu*) all point to sediments derived from felsic source and upper continental crust. The mixed provenance of the sandstones can be traced to the southwestern and southeastern Basement Complex (consisting of granites, gneisses, etc.) and sediments derived from the adjacent sedimentary basins (Anambra and Benue Trough).
文摘This paper reports the systematic studies of geochemistry on the meta sedimentary rocks, felsic gneisses and amphibolites of the Kongling complex from the Archean high grade terrain of the Yangtze craton. It shows that the amphibolites originated from a weakly depleted mantle resource. Nb negative anomaly, negative ε (Nd, t ) and the t DM ages which older than their isochron ages of the TTG gneisses imply the possible existence of the crust older than round about 2.7 Ga in the region. Three types of meta sedimentary rocks are identified from the Kongling complex. The first type originated from the juvenile crust with the features of the first cycle sedimentary rocks. Cratonic sedimentary rocks characterize the second type. Mobilization of REE and other elements resulted from partial melting during the migmatization is found in the third type of meta sedimentary rocks. Apart from the early TTG gneisses, some of the mafic rocks are also expected to be the source rocks of the first type of para rocks. The deposition times of the first and second type paragneisses are closed in the Neoarchean of about 2.7 Ga and the early period of Proterozoic respectively, and the period when cratonization of the Yangtze continental block completed is referred to be earlier than the Mesoproterozoic. Nd isotopic tracing on the magma sources of the Jinning granitic rocks in the region reveals that the major part of the Kongling basement is Neoarchean.
基金supported by the State Key Program of National Natural Science of China (Grant no.40634020)
文摘In the present study, the major and trace element compositions, as well as Sr, Nd isotopic compositions and K-Ar age data in mafic dikes from Hainan Island, China, have been analyzed. Whole-rock K-Ar dating yielded a magmatic duration of 61-98 Ma for mafic dikes. Mafic dikes have a very high concentration of incompatible elements, for example, Ba, Rb, Sr, K, rare earth elements, and especially light rare earth elements （LREE）, and negative anomalies of Nb, Ta, and Ti in the normalized trace element patterns. The initial 87Sr/86Sr ratios and εSr（t） of the mafic dikes are 0.70634- 0.71193 and ＋27.7 to ＋112.2, respectively. In the 87Sr/86Sr versus εNd（t） diagram, the Hainan Island mafic dikes plot between fields for depleted mantle and enriched mantle type 2. All these characteristics show that the mantle （source region） of mafic dikes in this area experienced metasomatism by fluids relatively enriched in LREE and large ion lithophUe elements. The genesis of Hainan Island mafic dikes is explained as a result of the mixing of asthenospheric mantle with lithospheric mantle that experienced metasomatism by the subduction of the Pacific Plate. This is different from the Hainan Island Cenozoic basalts mainly derived from depleted asthenospheric mantle, and possibly, minor metasomatised lithospheric mantle. This study suggests that the Mesozoic and Cenozoic lithospheric revolutions in Hainan Island can be divided into three stages： （1） the compression orogenesis stage before 98 Ma. The dominant factor during this stage is the subduction of the ancient Pacific Plate beneath this area. The lithospheric mantle changed into enriched mantle type 2 by metasomatism; （2） the thinning and extension stage during 61-98 Ma. The dominant factor during this stage is that the asthenospheric mantle invaded and corroded the lithospheric mantle; and （3） the large-scale thinning and extension stage after 61 Ma. The large-scale asthenospheric upwelling results in the strong erupting of Cenozoic basalts, large-scale thinning of the lithosphere, the southward translating and counterclockwise rotating of Hainan Island, and the opening of the South China Sea.
基金This research was supported by the National Natural Science Foundation of China Grant No. 9488010
文摘The shoshonite province in eastern China is characterized by extensive distribution (ca. 80000 km2) of Late Jurassic-Early Cretaceous (165-100 Ma) shoshonite series with subordinate high-K calc-alkali series. It was formed in a dominantly tensile stress field. In comparison with their analogues in island arcs and active continental margins in other countries, the volcanic rocks in the shoshonite province have their specific characteristics in petrology, mineralogy and geochemistry as well as related mineralization association, which are the comprehensive reflection of the special composition and structure of the mantle and crust of the province and the special Mesozoic regional tectonic setting.
文摘Systematic geochemical studies have been conducted on the North Küda Pluton, West Kunlun, in order to reveal its petrogenesis and tectonic implications. The North Küda Pluton is a potassium\|rich (K\-2O>5.4wt%) I\|type granitic pluton and does not contain any alkaline ferromagnesian mineral. Its relatively high REE, LILE (e.g. Rb, Cs, U and Th) and HFSE (e.g. Nb, Zr) contents make it very akin to the A\|type granites. Its heterogeneous Sr (\{\{\}\+\{87\}Sr\}/\{\{\}\+\{86\}Sr\-i\}=\{0.7049\}～\{0.7098\}) and Nd (εNd\-T=\{-1.05\}～\{-4.04\}) isotope compositions preclude the possibility of a pure sedimentary or igneous source. Instead, its geochemical compositions suggest that it may be derived from partial melting of a complex source, which consists of igneous and sedimentary rocks. Its intraplate characteristics, together with coeval mafic dykes, indicate an extensional environment at the end of Caledonian. The recognition of the extensional event does not support a continuous subduction\|accretion model for the Paleozoic tectonic evolution of the West Kunlun Mountains. On the contrary, it provides new evidence for the two\|stage island\|arc model.
