In order to understand the melting processes that occur within recycled oceanic crust and mantle in a heterogeneous plume (e.g., that beneath the Hawaiian Islands), a series of high-pressure-high-temperature layered e...In order to understand the melting processes that occur within recycled oceanic crust and mantle in a heterogeneous plume (e.g., that beneath the Hawaiian Islands), a series of high-pressure-high-temperature layered experiments were performed at 2.9 GPa, 5 GPa, and 8 GPa, from 1300°C to 1650°C, using a fertile peridotite KLB-1 and N-MORB. Our experiments at conditions below the dry peridotite solidus produced melt compositions that ranged from basaltic andesite to tholeiite. An Opx reaction band formed between eclogite and peridotite layers, likely via chemical reaction between a silica-rich eclogite-derived partial melt and olivine in the peridotite matrix. At temperatures at or above the dry peridotite solidus, substantial melting occurred in both basalt and peridotite layers, and fully molten basalt melt and melt pockets from the peridotite layer combined. In our layered experiments, major and minor element contents in reacted melts closely matched those of Hawaiian tholeiite and picrite, except for Fe. Partial melts of anhydrous run products had ~55 - 57 wt% SiO2 at low temperature (i.e., were andesitic) and had ~50 - 53 wt% SiO2 at high temperatures, slightly below the dry peridotite solidus (i.e., were tholeiitic, and similar to those that occur during the Hawaii shield-building stage). Based on the Fe- and LREE-enriched signature in Hawaiian tholeiites, we propose that recycled components in the Hawaiian plume are not modern N-MORB, but are Fe-rich tholeiite;a lithology that was common in the Archaean and early Proterozoic. We have demonstrated that the entire compositional spectrum of Hawaiian tholeiites (basalt to picrite) can be formed by basalt-peridotite reactive melting near the dry solidus of peridotite. Based on these results, we propose that the potential temperature of the sub-Hawaiian plume may be much lower than previously estimated.展开更多
The highly depleted N-MORB-type volcanic rocks have been discovered in Mianlue mélange of Qinling orogenic belt, central China. These mafic rocks are associated with the meta-peridotites, showing LREE-depleted pa...The highly depleted N-MORB-type volcanic rocks have been discovered in Mianlue mélange of Qinling orogenic belt, central China. These mafic rocks are associated with the meta-peridotites, showing LREE-depleted pattern, similar to N-MORB and typical ophiolites. It is indicated that an ancient ocean existed in the Mianlue area of Qinling orogenic belt during the late Paleozoic.展开更多
Petrological and geochemical studies of Neo-proterozoic metagabbros were carried out in the Muiswirab area,South Eastern Desert of Egypt.The Muiswirab area comprises of ophiolitic metagabbroic rocks(MOM),which are tec...Petrological and geochemical studies of Neo-proterozoic metagabbros were carried out in the Muiswirab area,South Eastern Desert of Egypt.The Muiswirab area comprises of ophiolitic metagabbroic rocks(MOM),which are tectonically thrusted over a thick pile of metavolcanic rocks and intruded by syn-and post-tectonic granitoid rocks.The whole-rock geochemical variations coupled with chemical compositions of mineral constituents are used to attain the genesis and tectonic evolution of the studied metagabbros.The geothermobarometric investiga-tion of the analyzed amphiboles from(MOM)revealed that these metagabbros underwent regional metamorphism under lower to upper greenschist facies(biotite zone)conditions(at a temperature of 450 to 500℃and pressure of 1-3 kbar).Geochemically,the metagabbros(MOM)show tholeiitic affinity and cxhibiting both arc-and MORB-like characters as evidenced by their clinopyrox-ene compositions and the Ti/V ratios(11.84-31.65),which considered as prominent features of forearc tectonic regime.The geochemical features suggest a probable fractionation of olivine±clinopyroxene±plagioclase as well as insignificant crustal contamination.The parental magma of the investigated MOM rocks seems to be developed in a sub-arc mantle wedge setting due to the enrichments of LILE(e.g..Rb,Ba,Sr,Pb)over HFSE(e.g,Ti,Nb,Y,Zr,Hf,Ta).The studied MOM rocks have lower values of Nb/U relative to MORB and OIB indicating that their geochemical variation produced due to the enrichment of a lithosphere mantle by OIB like components.The ratios of Zn/Fet,La/Sm,Sm/Yb,Th/Yb and Nb/Yb indicate that the MOM rocks represent a fragment of oceanic crust originated at a supra-subduction zone environment and their parental magma developed by 5-30%partial melting of a spinel lherzolite mantle rather than pyroxenite in an island arc setting and conformable with most of the Egyptian ophiolitic metagabbros.展开更多
文摘In order to understand the melting processes that occur within recycled oceanic crust and mantle in a heterogeneous plume (e.g., that beneath the Hawaiian Islands), a series of high-pressure-high-temperature layered experiments were performed at 2.9 GPa, 5 GPa, and 8 GPa, from 1300°C to 1650°C, using a fertile peridotite KLB-1 and N-MORB. Our experiments at conditions below the dry peridotite solidus produced melt compositions that ranged from basaltic andesite to tholeiite. An Opx reaction band formed between eclogite and peridotite layers, likely via chemical reaction between a silica-rich eclogite-derived partial melt and olivine in the peridotite matrix. At temperatures at or above the dry peridotite solidus, substantial melting occurred in both basalt and peridotite layers, and fully molten basalt melt and melt pockets from the peridotite layer combined. In our layered experiments, major and minor element contents in reacted melts closely matched those of Hawaiian tholeiite and picrite, except for Fe. Partial melts of anhydrous run products had ~55 - 57 wt% SiO2 at low temperature (i.e., were andesitic) and had ~50 - 53 wt% SiO2 at high temperatures, slightly below the dry peridotite solidus (i.e., were tholeiitic, and similar to those that occur during the Hawaii shield-building stage). Based on the Fe- and LREE-enriched signature in Hawaiian tholeiites, we propose that recycled components in the Hawaiian plume are not modern N-MORB, but are Fe-rich tholeiite;a lithology that was common in the Archaean and early Proterozoic. We have demonstrated that the entire compositional spectrum of Hawaiian tholeiites (basalt to picrite) can be formed by basalt-peridotite reactive melting near the dry solidus of peridotite. Based on these results, we propose that the potential temperature of the sub-Hawaiian plume may be much lower than previously estimated.
文摘The highly depleted N-MORB-type volcanic rocks have been discovered in Mianlue mélange of Qinling orogenic belt, central China. These mafic rocks are associated with the meta-peridotites, showing LREE-depleted pattern, similar to N-MORB and typical ophiolites. It is indicated that an ancient ocean existed in the Mianlue area of Qinling orogenic belt during the late Paleozoic.
文摘Petrological and geochemical studies of Neo-proterozoic metagabbros were carried out in the Muiswirab area,South Eastern Desert of Egypt.The Muiswirab area comprises of ophiolitic metagabbroic rocks(MOM),which are tectonically thrusted over a thick pile of metavolcanic rocks and intruded by syn-and post-tectonic granitoid rocks.The whole-rock geochemical variations coupled with chemical compositions of mineral constituents are used to attain the genesis and tectonic evolution of the studied metagabbros.The geothermobarometric investiga-tion of the analyzed amphiboles from(MOM)revealed that these metagabbros underwent regional metamorphism under lower to upper greenschist facies(biotite zone)conditions(at a temperature of 450 to 500℃and pressure of 1-3 kbar).Geochemically,the metagabbros(MOM)show tholeiitic affinity and cxhibiting both arc-and MORB-like characters as evidenced by their clinopyrox-ene compositions and the Ti/V ratios(11.84-31.65),which considered as prominent features of forearc tectonic regime.The geochemical features suggest a probable fractionation of olivine±clinopyroxene±plagioclase as well as insignificant crustal contamination.The parental magma of the investigated MOM rocks seems to be developed in a sub-arc mantle wedge setting due to the enrichments of LILE(e.g..Rb,Ba,Sr,Pb)over HFSE(e.g,Ti,Nb,Y,Zr,Hf,Ta).The studied MOM rocks have lower values of Nb/U relative to MORB and OIB indicating that their geochemical variation produced due to the enrichment of a lithosphere mantle by OIB like components.The ratios of Zn/Fet,La/Sm,Sm/Yb,Th/Yb and Nb/Yb indicate that the MOM rocks represent a fragment of oceanic crust originated at a supra-subduction zone environment and their parental magma developed by 5-30%partial melting of a spinel lherzolite mantle rather than pyroxenite in an island arc setting and conformable with most of the Egyptian ophiolitic metagabbros.
