Understanding Archean crustal and mantle evolution hinges upon identification and characterization of oceanic lithosphere. We report and update here more than 10 years work on a complete, yet dismembered and metamorph...Understanding Archean crustal and mantle evolution hinges upon identification and characterization of oceanic lithosphere. We report and update here more than 10 years work on a complete, yet dismembered and metamorphosed Archcan ophiolite sequence in the North China craton, in the Dongwanzi （东湾子）-Zunhua （遵化） structural belt and correlatives in the Wutaishan （五台山） area. Banded iron formation structurally overlies several tens of meters of variably deformed pillow lavas, mafic flows, and picritic amphibolites. These are in structural contact with a 2 km thick mixed gabbro and dike complex with gabbro screens, exposed discontinuously along strike for more than 20 km. The dikes consist of metamorphosed diabase, basalt, Hb-Cpx-gabbro, and pyroxenite. The dike/gabbro complex is underlain by several kilometers of mixed isotropic and foliated gabbro, which preserve compositional layering approximately 2 km below the dike complex, and then over several hundred meters merge into strongly compositionally layered gabbro and olivine-gabbro. The layered gabbro becomes mixed with layered pyroxenite/gabbro marking a transition zone into emulate ultramaflc rocks including serpentinzed dunite, pyroxenite and wehrlite, and finally into strongly deformed and serpentinized olivine and orthopyroxene-bearing ultraInafic rocks interpreted as depleted mantle harzburgite tectoultes. A U/Pb zircon age of 2.505 Ga from gabbro of the Dongwanzi ophioUte makes it one of the world＇s oldest recognized, laterally. extensive complete ophiolite sequences, though older dismembered ophiolites are recognized in South Africa and Greenland, extending back to 3.8 Ga. This age is confirmed by a ca. 2.6 Ga Re-Os isochron from chromites from the belt, and a number of dated 2.5-2.4 Ga cross-cuttlng younger igneous units. The Dongwanzi ophiolite is one of the largest well-preserved greenstone belts in the central orogenic belt that divides the North China craton into eastern and western blocks. More than 1 000 other fragments o展开更多
The Neyriz ophiolite complex is a part of NW-SE thrust belt (Late Cretaceous) of Iran, which is over the Arabian plate margin. The complex is mainly composed of the upper mantle rocks. Our research was focused on the ...The Neyriz ophiolite complex is a part of NW-SE thrust belt (Late Cretaceous) of Iran, which is over the Arabian plate margin. The complex is mainly composed of the upper mantle rocks. Our research was focused on the eastern part of Neyriz ophiolite complex so called as “Dowlat Abad-Tang e Hana”. Mantle sequence of this ophiolitic complex is comprising predominantly of harzburgite and minor lherzolite, dunitc sheaths and chromite pods. Harzburgite is the most abundant ultramafic rock and is associated with the less dunite masses. The chromites are known with Cr# 42 to 76 and Mg# 73 to 89. There is a negative correlation between Cr#-Mg# which is one of the features of podiform chromites. The geochemistry of these chromites is consistent with the overall composition of podiform chromites in terms of Cr#, Mg#, the amounts of Cr2O3 (13.35% - 54.47%), Al2O3 (0.43% - 8%), MgO (13.25% - 38.56%), TiO2 (0.003% - 0.206%) as well as the correlations between various oxides and all of them are high chromium types.展开更多
基金supported by the US National Science Founda-tion (Nos. 02-07886, 01-25925)awarded to Timothy M Kusky, the National Natural Science Foundation of China (Nos. 49832030, 40821061)+1 种基金the Peking University Project (No. 985)the Ministry of Education of China (No. B07039) awarded to Li Jianghai
文摘Understanding Archean crustal and mantle evolution hinges upon identification and characterization of oceanic lithosphere. We report and update here more than 10 years work on a complete, yet dismembered and metamorphosed Archcan ophiolite sequence in the North China craton, in the Dongwanzi （东湾子）-Zunhua （遵化） structural belt and correlatives in the Wutaishan （五台山） area. Banded iron formation structurally overlies several tens of meters of variably deformed pillow lavas, mafic flows, and picritic amphibolites. These are in structural contact with a 2 km thick mixed gabbro and dike complex with gabbro screens, exposed discontinuously along strike for more than 20 km. The dikes consist of metamorphosed diabase, basalt, Hb-Cpx-gabbro, and pyroxenite. The dike/gabbro complex is underlain by several kilometers of mixed isotropic and foliated gabbro, which preserve compositional layering approximately 2 km below the dike complex, and then over several hundred meters merge into strongly compositionally layered gabbro and olivine-gabbro. The layered gabbro becomes mixed with layered pyroxenite/gabbro marking a transition zone into emulate ultramaflc rocks including serpentinzed dunite, pyroxenite and wehrlite, and finally into strongly deformed and serpentinized olivine and orthopyroxene-bearing ultraInafic rocks interpreted as depleted mantle harzburgite tectoultes. A U/Pb zircon age of 2.505 Ga from gabbro of the Dongwanzi ophioUte makes it one of the world＇s oldest recognized, laterally. extensive complete ophiolite sequences, though older dismembered ophiolites are recognized in South Africa and Greenland, extending back to 3.8 Ga. This age is confirmed by a ca. 2.6 Ga Re-Os isochron from chromites from the belt, and a number of dated 2.5-2.4 Ga cross-cuttlng younger igneous units. The Dongwanzi ophiolite is one of the largest well-preserved greenstone belts in the central orogenic belt that divides the North China craton into eastern and western blocks. More than 1 000 other fragments o
文摘The Neyriz ophiolite complex is a part of NW-SE thrust belt (Late Cretaceous) of Iran, which is over the Arabian plate margin. The complex is mainly composed of the upper mantle rocks. Our research was focused on the eastern part of Neyriz ophiolite complex so called as “Dowlat Abad-Tang e Hana”. Mantle sequence of this ophiolitic complex is comprising predominantly of harzburgite and minor lherzolite, dunitc sheaths and chromite pods. Harzburgite is the most abundant ultramafic rock and is associated with the less dunite masses. The chromites are known with Cr# 42 to 76 and Mg# 73 to 89. There is a negative correlation between Cr#-Mg# which is one of the features of podiform chromites. The geochemistry of these chromites is consistent with the overall composition of podiform chromites in terms of Cr#, Mg#, the amounts of Cr2O3 (13.35% - 54.47%), Al2O3 (0.43% - 8%), MgO (13.25% - 38.56%), TiO2 (0.003% - 0.206%) as well as the correlations between various oxides and all of them are high chromium types.
