Various petrographic features and geochemical characteristics indicative of disequilibrium are preserved in plagioclase phenocrysts from basaltic to andesitic lavas in East Junggar, northwest China. These characterist...Various petrographic features and geochemical characteristics indicative of disequilibrium are preserved in plagioclase phenocrysts from basaltic to andesitic lavas in East Junggar, northwest China. These characteristics indicate that they crystallized in a magma chamber, which was replenished by less differentiated and high-temperature magmas. The petrographic and geochemical features of the plagioclase phenocrysts are interpreted to record responses to changes in temperature, composition and mechanical effect during magma replenishment. Distinct rare earth element(REE) patterns between cores and rims of the same plagioclase crystal suggest derivation from two end-member magmas. From core to rim, plagioclase phenocrysts commonly display sharp fluctuations of anorthite(An) content up to 20, which either correspond to reverse zoning associated with ovoidal cores and resorption surface(PI), or normal zoning with euhedral form and no resorption surface(P2). Plagioclase crystals with diverse textures and remarkably different An content coexist on the scale of a thin-section. Cores of these plagioclases in each sample display a bimodal distribution of An content. From core to rim in PI, concentrations o f FeOT and Sr increase remarkably as An content increases. During magma replenishment, pre-existing plagioclase phenocrysts in the andesitic magma, which were immersed into hotter and less differentiated magmas, were heated and resorbed to form ovoidal cores, and then were overgrown by a thin rim with much higher contents of An, FeO^T and Sr. However, pre-existing plagioclase phenocrysts in the basaltic magma were injected into cooler and more evolved magmas, and were remained as euhedral cores, which were later enclosed by oscillatory zoned rims with much lower contents of An, Sr and Ba.展开更多
Formation and preservation of greigite can indicate the physicochemical characteristics of sedimentary environment. Presence of greigite can be diagnosed in the late Pleistocene fluvio-lacustrine sedimentary layers of...Formation and preservation of greigite can indicate the physicochemical characteristics of sedimentary environment. Presence of greigite can be diagnosed in the late Pleistocene fluvio-lacustrine sedimentary layers of 29.4–29.7 and 26.1–27.1 m in core ZK30 of the Yellow River delta, based on analysis of particle size, magnetic properties, scanning electron microscope(SEM) and X-ray diffraction(XRD) measurements. These layers are the transition zones from shallow marine facies to fluvio- lacustrine facies, and from fluvio-lacustrine facies to salt marsh facies in an ascending order, respectively. They are characterized by higher SIRM and SIRM/χ(>30 kA m-1) values than those of other layers, suggesting the possible existence of greigite. Both SEM and XRD analyses confirm its presence. However, sediment layer of 29.4–29.7 m are coarser, and greigite coexists with pyrite, but sediment layer of 26.1–27.1 m are finer and the occurrence of greigite is not accompanied by pyrite. The different occurrence of greigite in the two layers suggests that different climate condition and sedimentary environment control its formation and preservation.展开更多
基金financial support by the Natural Science Foundation of China (41573030, 41503024)the Geological Survey Program from China Geological Survey (No. DD20190518)
文摘Various petrographic features and geochemical characteristics indicative of disequilibrium are preserved in plagioclase phenocrysts from basaltic to andesitic lavas in East Junggar, northwest China. These characteristics indicate that they crystallized in a magma chamber, which was replenished by less differentiated and high-temperature magmas. The petrographic and geochemical features of the plagioclase phenocrysts are interpreted to record responses to changes in temperature, composition and mechanical effect during magma replenishment. Distinct rare earth element(REE) patterns between cores and rims of the same plagioclase crystal suggest derivation from two end-member magmas. From core to rim, plagioclase phenocrysts commonly display sharp fluctuations of anorthite(An) content up to 20, which either correspond to reverse zoning associated with ovoidal cores and resorption surface(PI), or normal zoning with euhedral form and no resorption surface(P2). Plagioclase crystals with diverse textures and remarkably different An content coexist on the scale of a thin-section. Cores of these plagioclases in each sample display a bimodal distribution of An content. From core to rim in PI, concentrations o f FeOT and Sr increase remarkably as An content increases. During magma replenishment, pre-existing plagioclase phenocrysts in the andesitic magma, which were immersed into hotter and less differentiated magmas, were heated and resorbed to form ovoidal cores, and then were overgrown by a thin rim with much higher contents of An, FeO^T and Sr. However, pre-existing plagioclase phenocrysts in the basaltic magma were injected into cooler and more evolved magmas, and were remained as euhedral cores, which were later enclosed by oscillatory zoned rims with much lower contents of An, Sr and Ba.
基金supported by the National Natural Science Foundation of China(Grant Nos.41030856,41176039,41376054 and 41030856)the Natural Science Foundation of Shandong Province(Grant No.2011ZRE29040)
文摘Formation and preservation of greigite can indicate the physicochemical characteristics of sedimentary environment. Presence of greigite can be diagnosed in the late Pleistocene fluvio-lacustrine sedimentary layers of 29.4–29.7 and 26.1–27.1 m in core ZK30 of the Yellow River delta, based on analysis of particle size, magnetic properties, scanning electron microscope(SEM) and X-ray diffraction(XRD) measurements. These layers are the transition zones from shallow marine facies to fluvio- lacustrine facies, and from fluvio-lacustrine facies to salt marsh facies in an ascending order, respectively. They are characterized by higher SIRM and SIRM/χ(>30 kA m-1) values than those of other layers, suggesting the possible existence of greigite. Both SEM and XRD analyses confirm its presence. However, sediment layer of 29.4–29.7 m are coarser, and greigite coexists with pyrite, but sediment layer of 26.1–27.1 m are finer and the occurrence of greigite is not accompanied by pyrite. The different occurrence of greigite in the two layers suggests that different climate condition and sedimentary environment control its formation and preservation.