New major element data for coexisting minerals and bulk-rock REE analyses,combined with previously obtained petrologic/geologic information,allow comparison of Archean mafic-ultramafic meta-igneous rocks from adjacent...New major element data for coexisting minerals and bulk-rock REE analyses,combined with previously obtained petrologic/geologic information,allow comparison of Archean mafic-ultramafic meta-igneous rocks from adjacent portions of northeastern Sino-Korean shield .Isotope data published by previous workers document an Early Archean age of formation for metabasaltic rocks now occurring as mafic amphibolite inclusions in tonalitic gneisses west of Bohai Bay(area).Mafic-ultramafic mineral assemblages developed in tonalitic gneisses west of Bohai Bay(areaA).Mafic-ultramafic mineral assemblages developed in amphibolite facies Archean supracrustals east of Bohai Bay(areaB)are chemically similar ,but pre-existing phase associations have been overprinted by the effects of several later metamorphic events.Non0peridotitic area B protoliths appear to be more magnesian ,averaging-12wt.%MgO(versus-7wt.%for area A),and are slightly less fractionated in REE concentrations relative to chondrites(La-Lu=30-8X versus 37-14X for area A),thus perhaps being more mantle-like than area A metabasaltic analogues.The basaltic-komatiitic series east of Bohai Bay probably represents the most ancient crust of that region,being invaded by Archean tonalities,which were subsequently converted to orthogneisses.These observations are compatible with the hypothesis that ,similar to area A metabasaltic rocks now found as amphibolitic inclusions in orthogneisses,are B mafic supracrustals were formed during Early of Middle Archean time ,prior to widespread isofacial amphibolite facies metamorphism(600-700℃,5-7kb,low αH2O).展开更多
Production of voluminous igneous arc rocks,high-pressure/low-temperature(HP/LT) metamafic rocks,westward relative migration of the Klamath Mountains province,and U-Pb ages of deposition,sediment sources,and spatial lo...Production of voluminous igneous arc rocks,high-pressure/low-temperature(HP/LT) metamafic rocks,westward relative migration of the Klamath Mountains province,and U-Pb ages of deposition,sediment sources,and spatial locations of Jurassic and younger,detrital zirconbearing clastic rocks constrain geologic development of the northern California continental edge as follows:(1) At~ 175 Ma,transpressive plate underflow began to generate an Andean-type Klamath-Sierran arc along the margin.(2)Oceanic crustal rocks were metamorphosed under HP/LT conditions in an inboard,east-inclined subduction zone from~ 170-155 Ma.Except for the Red Ant blueschists,such Ethologies remained stored at depth;most HP/LT mafic tectonic blocks returned surfaceward only during mid- and Late Cretaceous time,chiefly entrained in circulating,buoyant Franciscan mud-matrix melange.(3) By ~ 165 Ma and continuing to ~ 150-140 Ma,erosion supplied volcanogenic debris to proximal Mariposa-Galice ± Myrtle overlap strata.(4) At ~ 140,immediately prior to the onset of paired Franciscan and Great Valley Group(GVG) +Hornbrook deposition,the Klamath salient was deformed and displaced ~ 100-150 km westward relative to the Sierran arc,stranding pre-existing oceanic crust on the south as the Coast Range Ophiolite(CRO).(5) After the end-ofJurassic seaward step-out of the Farallon-North American convergent plate junction,terrigineous debris began to be deposited in the outboard Franciscan trench and intervening Great Valley forearc.(6) Voluminous sedimentation and accretion of Franciscan Eastern + Central belts and GVG detritus took place during paroxysmal igneous activity and rapid,nearly orthogonal plate convergence at^125-80 Ma.(7) Sierran arc volcanism-plutonism ceased by^80 Ma in northern California,signaling a transition to shallow,nearly subhorizontal eastward plate underflow attending Laramide orogeny far to the east.(8) Presently exposed Paleogenelower Miocene Franciscan Coastal Belt sedimentary strata were deposited in a tectonic realm unaffected by HP/LT subduction.(9) Grenville-age detrital zircons are absent from the post-120 Ma Franciscan section.(10) Judging from petrofacies and zircon U-Pb data,the Franciscan Eastern Belt contains debris derived principally from the Sierra Nevada and Klamath ranges;detritus from the Idaho Batholith as well as Sierra Nevada Batholith may be present in some Central Belt sandstones,whereas clasts from the Idaho Batholith,Challis volcanics,and Cascade Range appear in progressively younger Paleogene-lower Miocene Coastal Belt sediments.(11) Gradual NW dextral offset of the Franciscan trench deposits of as much as^1,600 km may have occurred relative to the native GVG forearc and basement terranes of the American Southwest.展开更多
The Qinling-Dabie accretionary fold belt in east-central China represents the E-W trending suture zone between the Sino-Korean and Yangtze cratons. A portion of the accretionary complex exposed in northern Hubei Provi...The Qinling-Dabie accretionary fold belt in east-central China represents the E-W trending suture zone between the Sino-Korean and Yangtze cratons. A portion of the accretionary complex exposed in northern Hubei Province contains a high-pressure / low-temperature metamorphic sequence progressively metamorphosed from the blueschist through greenschist to epidote-amphibolite / eclogite facies. The Hongan metamorphic belt can be divided into three metamorphic zones,based on progressive changes in mineral assemblages: Zone Ⅰ,in the south,is characterized by transitional blueschist-greenschist facies; ZoneⅡis characterized by greenschist facies; ZoneⅢ,in the northern most portion of the belt,is characterized by eclogite and epidote-amphibolite facies sequences. Changes in amphibole compositions from south to north as well as the appearance of increasingly higher pressure mineral assemblages toward the north document differences in metamorphic P-T conditions during formation of this belt. Preliminary P-T estimates for Zone I metamorphism are 5 ~ 7 kbar,350 ~ 450℃; estimates for Zone III eclogites are 10 ~ 22 kbar,500 ±50℃. The petrographic,chemical and structural characteristics of this metamorphic belt indicate its evolutionin a northward-dipping subduction zone and subsequent uplift prior to and during the final collisionbetween the Sino-Korean and Yangtze cratons.展开更多
文摘New major element data for coexisting minerals and bulk-rock REE analyses,combined with previously obtained petrologic/geologic information,allow comparison of Archean mafic-ultramafic meta-igneous rocks from adjacent portions of northeastern Sino-Korean shield .Isotope data published by previous workers document an Early Archean age of formation for metabasaltic rocks now occurring as mafic amphibolite inclusions in tonalitic gneisses west of Bohai Bay(area).Mafic-ultramafic mineral assemblages developed in tonalitic gneisses west of Bohai Bay(areaA).Mafic-ultramafic mineral assemblages developed in amphibolite facies Archean supracrustals east of Bohai Bay(areaB)are chemically similar ,but pre-existing phase associations have been overprinted by the effects of several later metamorphic events.Non0peridotitic area B protoliths appear to be more magnesian ,averaging-12wt.%MgO(versus-7wt.%for area A),and are slightly less fractionated in REE concentrations relative to chondrites(La-Lu=30-8X versus 37-14X for area A),thus perhaps being more mantle-like than area A metabasaltic analogues.The basaltic-komatiitic series east of Bohai Bay probably represents the most ancient crust of that region,being invaded by Archean tonalities,which were subsequently converted to orthogneisses.These observations are compatible with the hypothesis that ,similar to area A metabasaltic rocks now found as amphibolitic inclusions in orthogneisses,are B mafic supracrustals were formed during Early of Middle Archean time ,prior to widespread isofacial amphibolite facies metamorphism(600-700℃,5-7kb,low αH2O).
基金The National Science Foundation provided additional aid through grant NSF EAR 0948676 to Marty Grove
文摘Production of voluminous igneous arc rocks,high-pressure/low-temperature(HP/LT) metamafic rocks,westward relative migration of the Klamath Mountains province,and U-Pb ages of deposition,sediment sources,and spatial locations of Jurassic and younger,detrital zirconbearing clastic rocks constrain geologic development of the northern California continental edge as follows:(1) At~ 175 Ma,transpressive plate underflow began to generate an Andean-type Klamath-Sierran arc along the margin.(2)Oceanic crustal rocks were metamorphosed under HP/LT conditions in an inboard,east-inclined subduction zone from~ 170-155 Ma.Except for the Red Ant blueschists,such Ethologies remained stored at depth;most HP/LT mafic tectonic blocks returned surfaceward only during mid- and Late Cretaceous time,chiefly entrained in circulating,buoyant Franciscan mud-matrix melange.(3) By ~ 165 Ma and continuing to ~ 150-140 Ma,erosion supplied volcanogenic debris to proximal Mariposa-Galice ± Myrtle overlap strata.(4) At ~ 140,immediately prior to the onset of paired Franciscan and Great Valley Group(GVG) +Hornbrook deposition,the Klamath salient was deformed and displaced ~ 100-150 km westward relative to the Sierran arc,stranding pre-existing oceanic crust on the south as the Coast Range Ophiolite(CRO).(5) After the end-ofJurassic seaward step-out of the Farallon-North American convergent plate junction,terrigineous debris began to be deposited in the outboard Franciscan trench and intervening Great Valley forearc.(6) Voluminous sedimentation and accretion of Franciscan Eastern + Central belts and GVG detritus took place during paroxysmal igneous activity and rapid,nearly orthogonal plate convergence at^125-80 Ma.(7) Sierran arc volcanism-plutonism ceased by^80 Ma in northern California,signaling a transition to shallow,nearly subhorizontal eastward plate underflow attending Laramide orogeny far to the east.(8) Presently exposed Paleogenelower Miocene Franciscan Coastal Belt sedimentary strata were deposited in a tectonic realm unaffected by HP/LT subduction.(9) Grenville-age detrital zircons are absent from the post-120 Ma Franciscan section.(10) Judging from petrofacies and zircon U-Pb data,the Franciscan Eastern Belt contains debris derived principally from the Sierra Nevada and Klamath ranges;detritus from the Idaho Batholith as well as Sierra Nevada Batholith may be present in some Central Belt sandstones,whereas clasts from the Idaho Batholith,Challis volcanics,and Cascade Range appear in progressively younger Paleogene-lower Miocene Coastal Belt sediments.(11) Gradual NW dextral offset of the Franciscan trench deposits of as much as^1,600 km may have occurred relative to the native GVG forearc and basement terranes of the American Southwest.
基金supported by the Stanford-Hubei Bureau of Geology and Mineral Resources co-operative projectMost of the field and analytical data were collected and supported by the GSA and Mc Gee funds ( E. A. E. ),the Stanford China Industrial Affiliates programme and NSF EAR 89-17319
文摘The Qinling-Dabie accretionary fold belt in east-central China represents the E-W trending suture zone between the Sino-Korean and Yangtze cratons. A portion of the accretionary complex exposed in northern Hubei Province contains a high-pressure / low-temperature metamorphic sequence progressively metamorphosed from the blueschist through greenschist to epidote-amphibolite / eclogite facies. The Hongan metamorphic belt can be divided into three metamorphic zones,based on progressive changes in mineral assemblages: Zone Ⅰ,in the south,is characterized by transitional blueschist-greenschist facies; ZoneⅡis characterized by greenschist facies; ZoneⅢ,in the northern most portion of the belt,is characterized by eclogite and epidote-amphibolite facies sequences. Changes in amphibole compositions from south to north as well as the appearance of increasingly higher pressure mineral assemblages toward the north document differences in metamorphic P-T conditions during formation of this belt. Preliminary P-T estimates for Zone I metamorphism are 5 ~ 7 kbar,350 ~ 450℃; estimates for Zone III eclogites are 10 ~ 22 kbar,500 ±50℃. The petrographic,chemical and structural characteristics of this metamorphic belt indicate its evolutionin a northward-dipping subduction zone and subsequent uplift prior to and during the final collisionbetween the Sino-Korean and Yangtze cratons.
