Lower Paleozoic rocks are exposed in various regions of Egypt（south central Sinai,north Eastern Desert and southwest Western Desert）,in addition to occurring in the subsurface such as north Western Desert and the Gu...Lower Paleozoic rocks are exposed in various regions of Egypt（south central Sinai,north Eastern Desert and southwest Western Desert）,in addition to occurring in the subsurface such as north Western Desert and the Gulf of Suez.The Lower Paleozoic rocks in Egypt include surface and subsurface rock units of formational status.The surface rock units are the Taba,Araba and Naqus formations. The subsurface rock units include the Shifa,Kohla and Basur formations. The Infracambrian Taba Formation has been discovered recently in the outcrops of the southeast central Sinai in the Taba-Ras El-Naqab area.It is missing and/or not recognized in the subsurface.The Taba Formation consists mainly of reddish brown,unfossiliferous gravelly fine-to medium-grained kaolinitic sandstones and subordinate horizons of paleosols.The Cambrian Araba Formation and its subsurface equivalent（the Shifa Formation） are essentially composed of reddish brown,line-grained laminated sandstone and siltstone with abundant Skolithos and Cruziana sp.In contrast,the Ordovician—Silurian Naqus Formation and its subsurface equivalents（the Kohla Formation and Basur Formation） are mainly composed of white,unfossiliferous,cross-bedded,medium-to coarse-grained sandstones with haphazardly distributed pebbles and cobbles.Sedimentological analysis indicates that the Araba Formation and its subsurface equivalent were deposited in a marginal-marine environment,whereas the Naqus Formation and its subsurface equivalents were laid down in a fluvio-glacial environment. Integrated stratigraphic and sedimentological studies of the Lower Paleozoic rocks permit reconstruction of the paleogeography of Egypt at that time.Egypt has been largely controlled since the Cambrian by the pre-existing structural framework of the pre-Phanerozoic basement rocks inherited from the Late Proterozoic Pan-African event.Additionally,sedimentation processes were controlled during Cambro-Ordovician times by tectonic movements,whereas glacio-eustatic control predominated during the Late Ordovician—Silurian Period.These studies suggest that most areas of Egypt were exposed lands with episodically transgression by epi-continental seas related to the paleo-Tethys.These lands formed a part of a stable subsiding shelf at the northern Gondwana margin.展开更多
The Dabie orogen, Eastern Qinling orogen and Eastern Kunlun orogen are the major components of the Central Mountain Ranges of China and each has distinctively metamorphic processes in their oldest rock units. The Dab...The Dabie orogen, Eastern Qinling orogen and Eastern Kunlun orogen are the major components of the Central Mountain Ranges of China and each has distinctively metamorphic processes in their oldest rock units. The Dabie orogen oldest rock units had experienced an intermediate-higher pressure, upper amphibolite to lower granulite facies metamorphism in the Indosinian intracontinental subduction collision event. The clockwise pt path, synchronous attainment of t max and p max and a segment of high slope retrograde path suggest a tectono driven rapid exhumation of the oldest rock units to upper middle crustal level following the end of the subduction collision process. The oldest rock units, also called Qinling Group, of Eastern Qinling Mountains suffered intensively collisional metamorphism at an immature plate tectonic framework during Jinningian movement about 1 000 Ma ago. The clockwise pt path with reach of t max following several hundreds of MPa decreasing from p max suggests that the denudation of the deeply burial rock units was due essentially to isostatic relaxation. During the Caledonian stage, the highly metamorphosed oldest rock units suffered from a high t thermal event in arc environment and superimposed by contact metamorphism. After thermal peaks, the rock units were exhumed in a short distance and cooled down isobarically. No regional metamorphism higher than upper greenschist facies condition since 300 Ma has been traced within exposed rocks in the eastern portion of Northern Qinling orogen. The oldest rock units in the Eastern Kunlun orogen were metamorphosed to upper amphibolite facies to lower granulite facies rocks early or during the Luliang movement (some 1 800 Ma ago). The high grade metamorphic rock units had a long resident time at the deep crustal level, and were exhumed to middle upper crustal level during the Caledonian to Hercynian tectonometamorphic events. Shallow erosion of the orogenic belt led to good preservation of the low pressure metamorphic belt.展开更多
The Proterozoic pre-Ediacaran metamorphic basement of the southern Tajik （North Afghanistan） con- tinental block and the adjacent Band-e-Bayan zone is exposed in the Ghor Province of Central Afghanistan. It is predo...The Proterozoic pre-Ediacaran metamorphic basement of the southern Tajik （North Afghanistan） con- tinental block and the adjacent Band-e-Bayan zone is exposed in the Ghor Province of Central Afghanistan. It is predominantly composed of the EW-striking supracrustal succession consisting of interbedded felsic schists and gneisses （metapsammites）, amphibolites （metabasalts）, calcite and dolo- mite marbles, The metamorphic facies changes from greenschist in the Band-e-Bayan zone to amphib- olite facies in the Tajik block. The supracrustal rocks of the Band-e-Bayan zone and Tajik block possess common features suggesting that the former represents a tectonized part of the latter. The geochemical characteristics of metapsammites indicate derivation of the clastic material from a continental arc and, partly from a passive continental margin, whereas the composition of metabasalts suggests their possible formation in a continental rift basin. The tectonic setting of supracrustal unit could be interpreted as a back-arc type basin. We presume that the Tajik microcontinent split off the Gondwana supercontinent along an ancient rift zone during the late Paleozoic.展开更多
文摘Lower Paleozoic rocks are exposed in various regions of Egypt（south central Sinai,north Eastern Desert and southwest Western Desert）,in addition to occurring in the subsurface such as north Western Desert and the Gulf of Suez.The Lower Paleozoic rocks in Egypt include surface and subsurface rock units of formational status.The surface rock units are the Taba,Araba and Naqus formations. The subsurface rock units include the Shifa,Kohla and Basur formations. The Infracambrian Taba Formation has been discovered recently in the outcrops of the southeast central Sinai in the Taba-Ras El-Naqab area.It is missing and/or not recognized in the subsurface.The Taba Formation consists mainly of reddish brown,unfossiliferous gravelly fine-to medium-grained kaolinitic sandstones and subordinate horizons of paleosols.The Cambrian Araba Formation and its subsurface equivalent（the Shifa Formation） are essentially composed of reddish brown,line-grained laminated sandstone and siltstone with abundant Skolithos and Cruziana sp.In contrast,the Ordovician—Silurian Naqus Formation and its subsurface equivalents（the Kohla Formation and Basur Formation） are mainly composed of white,unfossiliferous,cross-bedded,medium-to coarse-grained sandstones with haphazardly distributed pebbles and cobbles.Sedimentological analysis indicates that the Araba Formation and its subsurface equivalent were deposited in a marginal-marine environment,whereas the Naqus Formation and its subsurface equivalents were laid down in a fluvio-glacial environment. Integrated stratigraphic and sedimentological studies of the Lower Paleozoic rocks permit reconstruction of the paleogeography of Egypt at that time.Egypt has been largely controlled since the Cambrian by the pre-existing structural framework of the pre-Phanerozoic basement rocks inherited from the Late Proterozoic Pan-African event.Additionally,sedimentation processes were controlled during Cambro-Ordovician times by tectonic movements,whereas glacio-eustatic control predominated during the Late Ordovician—Silurian Period.These studies suggest that most areas of Egypt were exposed lands with episodically transgression by epi-continental seas related to the paleo-Tethys.These lands formed a part of a stable subsiding shelf at the northern Gondwana margin.
文摘The Dabie orogen, Eastern Qinling orogen and Eastern Kunlun orogen are the major components of the Central Mountain Ranges of China and each has distinctively metamorphic processes in their oldest rock units. The Dabie orogen oldest rock units had experienced an intermediate-higher pressure, upper amphibolite to lower granulite facies metamorphism in the Indosinian intracontinental subduction collision event. The clockwise pt path, synchronous attainment of t max and p max and a segment of high slope retrograde path suggest a tectono driven rapid exhumation of the oldest rock units to upper middle crustal level following the end of the subduction collision process. The oldest rock units, also called Qinling Group, of Eastern Qinling Mountains suffered intensively collisional metamorphism at an immature plate tectonic framework during Jinningian movement about 1 000 Ma ago. The clockwise pt path with reach of t max following several hundreds of MPa decreasing from p max suggests that the denudation of the deeply burial rock units was due essentially to isostatic relaxation. During the Caledonian stage, the highly metamorphosed oldest rock units suffered from a high t thermal event in arc environment and superimposed by contact metamorphism. After thermal peaks, the rock units were exhumed in a short distance and cooled down isobarically. No regional metamorphism higher than upper greenschist facies condition since 300 Ma has been traced within exposed rocks in the eastern portion of Northern Qinling orogen. The oldest rock units in the Eastern Kunlun orogen were metamorphosed to upper amphibolite facies to lower granulite facies rocks early or during the Luliang movement (some 1 800 Ma ago). The high grade metamorphic rock units had a long resident time at the deep crustal level, and were exhumed to middle upper crustal level during the Caledonian to Hercynian tectonometamorphic events. Shallow erosion of the orogenic belt led to good preservation of the low pressure metamorphic belt.
文摘The Proterozoic pre-Ediacaran metamorphic basement of the southern Tajik （North Afghanistan） con- tinental block and the adjacent Band-e-Bayan zone is exposed in the Ghor Province of Central Afghanistan. It is predominantly composed of the EW-striking supracrustal succession consisting of interbedded felsic schists and gneisses （metapsammites）, amphibolites （metabasalts）, calcite and dolo- mite marbles, The metamorphic facies changes from greenschist in the Band-e-Bayan zone to amphib- olite facies in the Tajik block. The supracrustal rocks of the Band-e-Bayan zone and Tajik block possess common features suggesting that the former represents a tectonized part of the latter. The geochemical characteristics of metapsammites indicate derivation of the clastic material from a continental arc and, partly from a passive continental margin, whereas the composition of metabasalts suggests their possible formation in a continental rift basin. The tectonic setting of supracrustal unit could be interpreted as a back-arc type basin. We presume that the Tajik microcontinent split off the Gondwana supercontinent along an ancient rift zone during the late Paleozoic.
