The Toarcian Marrat Formation is exposed in outcrops in central Saudi Arabia and displays a variety of clastic and carbonate facies associated with well-preserved depositional geometries. It is unconformably overlies ...The Toarcian Marrat Formation is exposed in outcrops in central Saudi Arabia and displays a variety of clastic and carbonate facies associated with well-preserved depositional geometries. It is unconformably overlies the Triassic Minjur Formation and it in turn is overlaid by the Middle Jurassic Dhruma Formation. Thirteen lithofacies types can be identified that permit the recognition of five lithofacies associations in a mixed clastic/carbonate platform. These lithofacies range from low-energy peritidal, intertidal, and back-shoal to moderate-and high-energy shoal and foreshoal lithofacies associations. The Marrat Formation exhibits three depositional sequences, each sequence is grouped into a transgressive systems tract(TST) and a highstand systems tract(HST) and then bounded by sequence boundary surfaces(SBSs). The TSTs are generally identified in clastic tidal-flat beds and back-shoal wackestones, while the HST is generally recorded in the carbonate tidal-flat and shoal. The vertical succession of facies associations from peritidal to foreshoal depositional environments is indicative of a deepening upward and retrogradational systems tract, from Lower to Upper Toarcian. The correlation between the studied sections reveals a general shallowing towards the south and the similarities between the studied sequences and others in the Arabian Gulf, the northern Neo-Tethys Plate, and Gondwanaland countries.展开更多
When gigantic meteorite impact sites in Southern Africa are plotted on the geological map of the region,they overlap with the SW-NE fault lines and it indicates that meteorite impacts have contributed to the geologica...When gigantic meteorite impact sites in Southern Africa are plotted on the geological map of the region,they overlap with the SW-NE fault lines and it indicates that meteorite impacts have contributed to the geological structure of the region.The Morokweng Meteorite Impact of 145 Ma(Million years ago),at the J/K(Jurassic/Cretaceous)boundary,seems to have broken up the Gondwanaland.It is plausible that,at the time of continental break-up,seawater poured into Morokweng crater and ignited a gigantic volcanic eruption which delivered low viscose magma into the outer space to form the Moon.An extremely flat surface of the Megakalahari extending 3,800,000 sq.km at 800-1,200 m above sea level in the central and southern Africa seems to be the terrestrial residue of fluid magma at the end of the volcanic event.展开更多
SHRIMP U-Pb zircon analyses for a gabbro sample from the Garzê ophiolite block yielded a mean 206Pb/238U age of 292±4 Ma, which indicated that the spread- ing time of the Garzê-Litang Tethys was most li...SHRIMP U-Pb zircon analyses for a gabbro sample from the Garzê ophiolite block yielded a mean 206Pb/238U age of 292±4 Ma, which indicated that the spread- ing time of the Garzê-Litang Tethys was most likely at the earliest Permian. Combined with previous studies, we sug- gest that the opening of the Tethys in southwest China was derived from breakup of the East Gondwanaland in the late Paleozoic.展开更多
Mafic granulites have been found as structural lenses within the huge thrust system outcropping about 10 km west of Nam Co of the northern Lhasa Terrane, Tibetan Plateau. Petrological evidence from these rocks indicat...Mafic granulites have been found as structural lenses within the huge thrust system outcropping about 10 km west of Nam Co of the northern Lhasa Terrane, Tibetan Plateau. Petrological evidence from these rocks indicates four distinct metamorphic assemblages. The early metamorphic assemblage (M1) is preserved only in the granulites and represented by plagioclase+hornblende inclusions within the cores of garnet porphyroblasts. The peak assemblage (M2) consists of garnet+clinopyroxene+hornblende+plagioclase in the mafic granulites. The peak metamorphism was followed by near-isothermal decompression (M3), which resulted in the development of hornblende+plagioclase symplectites surrounding embayed garnet porphyroblasts, and decompression-cooling (M4) is represented by minerals of hornblende+plagioclase recrystallized during mylonization. The peak (M2) P-T conditions of garnet+ clinopyroxene+plagioclase+hornblende were estimated at 769-905℃ and 0.86-1.02 GPa based on the geothermometers and geobarometers. The P-T conditions of plagioclase+hornblende symplectites (M3) were estimated at 720-800℃ and 0.55-0.68 GPa, and recrystallized hornblende+plagioclase (M4) at 594-708℃ and 0.26-0.47 GPa. It is impossible to estimate the P-T conditions of the early metamorphic assemblage (M1) because of the absence of modal minerals. The combination of petrographic textures, metamorphic reaction history, thermobarometric data and corresponding isotopic ages defines a clockwise near-isothermal decompression metamorphic path, suggesting that the mafic granulites had undergone initial crustal thickening, subsequent exhumation, and cooling and retrogression. This tectonothermal path is considered to record two major phases of collision which resulted in both the assemblage of Gondwanaland during the Pan-African orogeny at 531 Ma and the collision of the Qiangtang and Lhasa Terranes at 174 Ma, respectively.展开更多
The Ireviken Event was the first Middle Paleozoic event consisting of synchronised faunal,isotopic and facies change to be recognised.An analysis of the conodont faunas throughout the Boree Creek/Borenore Limestone su...The Ireviken Event was the first Middle Paleozoic event consisting of synchronised faunal,isotopic and facies change to be recognised.An analysis of the conodont faunas throughout the Boree Creek/Borenore Limestone succession in the central western region of the Tasman fold belt of New South Wales(Australia)revealing all five conodont zones that comprise the event is presented.While some zonal boundaries are precise,allowing direct comparison of stratigraphic intervals on other paleo-continents,some can only be approximated.Conodont data from pre-Ireviken Event strata,in contrast,only permit the identification of a broad Telychian chronology.The identification of Wenlock post-Ireviken Event conodont zones is incomplete due to lithological variability,namely the presence of tuffaceous beds near the top of the formation and an unconformity between the Boree Creek and overlying Borenore Limestone.The Boree Creek Formation contains the only example of the Ireviken Event discovered to date from the Tasman fold belt of eastern Gondwanaland.展开更多
基金The authors extend their appreciation to Researchers Supporting Project (No. RSP2023R139)。
文摘The Toarcian Marrat Formation is exposed in outcrops in central Saudi Arabia and displays a variety of clastic and carbonate facies associated with well-preserved depositional geometries. It is unconformably overlies the Triassic Minjur Formation and it in turn is overlaid by the Middle Jurassic Dhruma Formation. Thirteen lithofacies types can be identified that permit the recognition of five lithofacies associations in a mixed clastic/carbonate platform. These lithofacies range from low-energy peritidal, intertidal, and back-shoal to moderate-and high-energy shoal and foreshoal lithofacies associations. The Marrat Formation exhibits three depositional sequences, each sequence is grouped into a transgressive systems tract(TST) and a highstand systems tract(HST) and then bounded by sequence boundary surfaces(SBSs). The TSTs are generally identified in clastic tidal-flat beds and back-shoal wackestones, while the HST is generally recorded in the carbonate tidal-flat and shoal. The vertical succession of facies associations from peritidal to foreshoal depositional environments is indicative of a deepening upward and retrogradational systems tract, from Lower to Upper Toarcian. The correlation between the studied sections reveals a general shallowing towards the south and the similarities between the studied sequences and others in the Arabian Gulf, the northern Neo-Tethys Plate, and Gondwanaland countries.
文摘When gigantic meteorite impact sites in Southern Africa are plotted on the geological map of the region,they overlap with the SW-NE fault lines and it indicates that meteorite impacts have contributed to the geological structure of the region.The Morokweng Meteorite Impact of 145 Ma(Million years ago),at the J/K(Jurassic/Cretaceous)boundary,seems to have broken up the Gondwanaland.It is plausible that,at the time of continental break-up,seawater poured into Morokweng crater and ignited a gigantic volcanic eruption which delivered low viscose magma into the outer space to form the Moon.An extremely flat surface of the Megakalahari extending 3,800,000 sq.km at 800-1,200 m above sea level in the central and southern Africa seems to be the terrestrial residue of fluid magma at the end of the volcanic event.
基金This work was supported by the National Natural Science Foundation of China(Grant Nos.40472119 and 40172071)the Progranme of Excelent Young Scientists of the Ministry of Land and Resources,the Ministry of Science and Technology of the People's Republic of China(Grant No.2002CB412608)the China Geological Survey(Grant No.200313000057).
文摘SHRIMP U-Pb zircon analyses for a gabbro sample from the Garzê ophiolite block yielded a mean 206Pb/238U age of 292±4 Ma, which indicated that the spread- ing time of the Garzê-Litang Tethys was most likely at the earliest Permian. Combined with previous studies, we sug- gest that the opening of the Tethys in southwest China was derived from breakup of the East Gondwanaland in the late Paleozoic.
基金China Geological Survey(Grant No.20013000166) Natural Science Foundation of China(Grant No.49902006).
文摘Mafic granulites have been found as structural lenses within the huge thrust system outcropping about 10 km west of Nam Co of the northern Lhasa Terrane, Tibetan Plateau. Petrological evidence from these rocks indicates four distinct metamorphic assemblages. The early metamorphic assemblage (M1) is preserved only in the granulites and represented by plagioclase+hornblende inclusions within the cores of garnet porphyroblasts. The peak assemblage (M2) consists of garnet+clinopyroxene+hornblende+plagioclase in the mafic granulites. The peak metamorphism was followed by near-isothermal decompression (M3), which resulted in the development of hornblende+plagioclase symplectites surrounding embayed garnet porphyroblasts, and decompression-cooling (M4) is represented by minerals of hornblende+plagioclase recrystallized during mylonization. The peak (M2) P-T conditions of garnet+ clinopyroxene+plagioclase+hornblende were estimated at 769-905℃ and 0.86-1.02 GPa based on the geothermometers and geobarometers. The P-T conditions of plagioclase+hornblende symplectites (M3) were estimated at 720-800℃ and 0.55-0.68 GPa, and recrystallized hornblende+plagioclase (M4) at 594-708℃ and 0.26-0.47 GPa. It is impossible to estimate the P-T conditions of the early metamorphic assemblage (M1) because of the absence of modal minerals. The combination of petrographic textures, metamorphic reaction history, thermobarometric data and corresponding isotopic ages defines a clockwise near-isothermal decompression metamorphic path, suggesting that the mafic granulites had undergone initial crustal thickening, subsequent exhumation, and cooling and retrogression. This tectonothermal path is considered to record two major phases of collision which resulted in both the assemblage of Gondwanaland during the Pan-African orogeny at 531 Ma and the collision of the Qiangtang and Lhasa Terranes at 174 Ma, respectively.
文摘The Ireviken Event was the first Middle Paleozoic event consisting of synchronised faunal,isotopic and facies change to be recognised.An analysis of the conodont faunas throughout the Boree Creek/Borenore Limestone succession in the central western region of the Tasman fold belt of New South Wales(Australia)revealing all five conodont zones that comprise the event is presented.While some zonal boundaries are precise,allowing direct comparison of stratigraphic intervals on other paleo-continents,some can only be approximated.Conodont data from pre-Ireviken Event strata,in contrast,only permit the identification of a broad Telychian chronology.The identification of Wenlock post-Ireviken Event conodont zones is incomplete due to lithological variability,namely the presence of tuffaceous beds near the top of the formation and an unconformity between the Boree Creek and overlying Borenore Limestone.The Boree Creek Formation contains the only example of the Ireviken Event discovered to date from the Tasman fold belt of eastern Gondwanaland.
