Mineralogy and Petrology of the Metamorphosed Wabush Iron Formation, Southwestern Labrador     

KLEIN  CORNELIS  JR. 《Journal of Petrology》1966,7(2):246-305

The Wabush Iron Formation, of late Precambrian (Proterozoic)age is part of the Labrador Trough in southwestern Labrador,Canada. It is the regionally metamorphosed equivalent of lowgrade metamorphic (chlorite zone) iron-rich sediments of thecentral part of the Labrador Trough. The metamorphic grade iskyanite-staurolite zone, as concluded from conformably underlyingpelitic schist assemblages. Sedimentary textural features suchas very pronounced banding and a very rare occurrence of relicgranules are still preserved. The iron formation consists mainly of quartz, specularite, magnetite,cummingtonite-grunerite, and ferrodolomite-ankerite. Less commonare actinolite, anthophyllite, riebeckitetremolite, magnesioriebeckite,ferrosalite, orthopyroxene, aegirine-augite, aegirine, rhodonite,garnet (almandine, spessartine, calderite), siderite, rhodochrosite,calcite, and kutnahorite. Conventional wet chemical analyses or electron microprobe analyseshave been made of thirty-four phases belonging to the abovelist. Six additional electron probe analyses have been madeof phases from the underlying pelitic schists. All conventionallyanalyzed phases are characterized by complete optical, unitcell parameter, and density measurments. The analyzed assemblages from the silicate and silicate-carbonateiron formation include grunerite-ferrosalite, grunerite-eulite-siderite,grunerite-actinolite, grunerite-almandine, cummingtonite-spessartine,rhodonite-kutnahorite-calderite, aegirine-augite-riebeckite-tremolite,magnesioriebeckite-cummingtonite-rhodonite, aegirine-augite-rhodonite-rhodo-chrosite,and aegirine-rhodonite-calderite-rhodochrosite. The assemblages are concluded to be equilibrium assemblages.Of the volatile components, O₂, CO₂, and H₂O, O₂, is concludedto have behaved as an inert (buffered) component. Variationsin the activity of CO₂ are concluded to have existed betweensilicate-oxide and carbonate-oxide members of the iron formation.It is not clear, however, whether CO₂ has acted as a perfectlymobile component with strong aco₂ gradients throughout the area,or as an inert component in some parts of the area. H₂O is consideredto have been perfectly mobile. An increase in Mg/(Mg+Fe) ratioin ferromagnesian silicates is correlated with an increase inthe oxidation state of the assemblage. A similar increase in(Mg+Mn)/(Mg+Mn+Fe) is found in manganoan ferromagnesian silicateswith increasing activity of O₂. A number of ferromagnesian silicatescontain large amounts of Na⁺ and Fe³⁺ as a result of the verylow Al₂O₃ content of the iron formation. The P and T conditionsof metamorphism are deduced from experimental studies applicableto the underlying pelitic schists.  相似文献   

Anorthite--Forsterite and Anorthite--Enstatite Reactions and their bearing on the Basalt--Eclogite Transformation   总被引：3，自引：0，他引：3  

KUSHIRO  I.; YODER  H. S.  JR. 《Journal of Petrology》1966,7(3):337-362

The reactions betweeon anorthite and forsterite and betweenanorthite and enstatite have been studied at high pressuresand high temperatures with solid-media and gas-media apparatus.For the 1: 1 (molecular ratio) composition of anorthite andforsterite the following three assemblages have been obtainedwith increasing pressure at a constant temperature: anorthiteforsterite,clinopyroxeneorthopyroxenespinelanorthite, and garnet (pyropegrossulariteseries). Two univariant curves between these three assemblageshave been determined. The amounts of pyroxenes relative to spineland anorthite in the second assemblage increase and the contentsof the Tschermak's components in the pyroxenes increase withincreasing pressure. The 1:2 (molecular ratio) composition of anorthite and forsteritewas also studied, and the univariant curve for the reactionclinopyroxeneorthopyroxenespinel garnet (pyropegrossulariteseries)forsterite has been determined. For the 1: 2 (molecular ratio) composition of anorthite andenstatite the following assemblages have been obtained withincreasing pressure at a constant temperature: anorthiteenstatite,anorthiteorthopyroxeneclinopyroxenequartz, and garnet (pyrope-grossulariteseries)quartz. The univariant curve between the last two assemblageshas been determined. Both the pyroxenes in the second assemblagecontain Tschermak's components, which increase in content withincreasing pressure at the expense of anorthite. These experimental results suggest that basalts, either saturatedor undersaturated with silica, transform into aluminous pyroxene-richassemblages at lower pressures or higher temperatures than thestability field of eclogites, and that pyroxene-spinel symplectitesfound in some metamorphosed gabbro, troctolite, and norite mayhave formed in the stability field of the pyroxene-rich assemblages.The stability field of the pyroxene-rich assemblages is probablyincluded within the pressure-temperature conditions of the granulitefacies. It is also suggested that the garnet-free peridotites,such as the peridotite inclusions in basaltic rocks and theintrusive peridotites in the orogenic belts, are stable at lowerpressures or higher temperatures than garnet peridotites, suchas the inclusions in kimberlite and intrusive garnet peridotitein the orogenic belts.  相似文献   

GEOMORPHOLOGY OF THE LAKE SHEWA LANDSLIDE DAM,BADAKHSHAN, AFGHANISTAN,USING REMOTE SENSING DATA     

JOHN F. SHRODER JR  BRANDON J. WEIHS 《Geografiska Annaler: Series A, Physical Geography》2010,92(4):469-483

Lake Shewa in northeastern Badakhshan, Afghanistan, was dammed sometime in antiquity when a large rock avalanche (sturzstrom) from the fault‐shattered and strongly weathered Archean gneisses of the Zirnokh peaks to the north moved into the Arakht River valley. This rock avalanche dammed up the river and its tributaries to a dam thickness of c. 400 m, producing a 12‐km‐long lake that is as much as 270 m deep, leaving c. 80 m of freeboard to the top of the dam. At least four separate instances of slope failure have been mapped at the site of the landslide dam, as well as a rock glacier, using remotely sensed data, historical maps, and Google Earth?. Spring seepage through the dam face has caused several recent subsidiary debris slides, which if continued at a large enough scale for long enough, or with additional seismicity from the active strike‐slip faults that cross beneath the landslide dam, could threaten its integrity. Otherwise the clean water that emerges from the dam face could be the source of an unvarying mini‐hydroelectric power source, in addition to the agricultural irrigation that it provides at the present time.  相似文献