The study area is located in the entrance of Wadi Girshah in Ablah area, Assir terrain, southwestern Saudi Arabia. The present study aims to shed light on the field, mega- and microscopic description of the volcanicla...The study area is located in the entrance of Wadi Girshah in Ablah area, Assir terrain, southwestern Saudi Arabia. The present study aims to shed light on the field, mega- and microscopic description of the volcaniclastic red beds and the associated scoriaceous basalts of Wadi Al Roaian. It is based mainly up on the field works augmented by petrographic description. The present study revealed that, the succession of the upper part of Girshah Formation comprises three main horizons: 1) lower unit of epidotized and silicified basaltic and andesitic tuffs;2) middle unit of volcaniclastic red beds-scoriaceous basalts of successive cycles (each of these cycles begins by reddish tuffaceous mudstone and sandstone and is terminated by the calcite-bearing scoriaceous basalt). This unit indicated the deposition in lacustrine environments and the red iron oxyhydroxides minerals were formed either by the direct hematitization of the deposited tuffs or by the diagenetic hematitization of the green clays formed instead of the precursor tuffaceous materials;3) upper scoriaceous basalt unit composed from hematitized and calcitized basalt. The unit was formed by basic volcanic eruptions in subaerial condition which was predominated by the formation of calcium carbonate lakes associated with the progressive and subsequent calcitization of the Ca-plagioclase minerals of the basalt and the associated glassy tuffaceous material. The scoriaceous basalts of Wadi Al Roaian area represent subaerial basic volcanic eruption in continental situation and are associated with subsequent events of mineral alteration and formation of secondary minerals i.e. calcite, hematite and goethite.展开更多
Harrat Al Fatih is located southwest of Ablah area, Assir terrain, southwestern Arabian Shield. It is present as black laterally extended horizontal basaltic sheet just overlying folded strata of the volcano-sedimenta...Harrat Al Fatih is located southwest of Ablah area, Assir terrain, southwestern Arabian Shield. It is present as black laterally extended horizontal basaltic sheet just overlying folded strata of the volcano-sedimentary succession of Ablah Formation (green mudstones, dolostones and green and red volcaniclastic red beds). These basalts are rift-related and represent part of the Oligo-Miocene basic volcanics of the western part of Saudi Arabia. The detailed field and microscopic description of these basalts enable the author to classify it into two main types: 1) Grey tuffaceous glassy basalts that present overlying the Ablah Formation. This basalt type is generally soft, bedded and intercalated with grey, green and red tuffaceous mudstones. Microscopically it composed of minute lath-like plagioclase and pyroxene crystals embedded in glassy groundmass. 2) Black olivine-rich basalts which are present in the topmost part of harrat Al Fatih are generally black, hard and contain remarkable reddish brown oxidized olivine spots and some calcite and amorphous quartz domains. Under the microscope, this basalt type is microcrystalline and composed mainly of lamellar twined Ca-plagioclase and colored olivines and pyroxenes. The olivines show different stages of oxidation and formation of amorphous blood red iron-oxyhydroxides and black hematite. The present study revealed the formation of the grey basalts at the initial stages of the volcanic eruption in ephemeral lakes and the second type of basalt was during the consolidation of proper basic magmas at the final stages of the volcanic eruption.展开更多
文摘The study area is located in the entrance of Wadi Girshah in Ablah area, Assir terrain, southwestern Saudi Arabia. The present study aims to shed light on the field, mega- and microscopic description of the volcaniclastic red beds and the associated scoriaceous basalts of Wadi Al Roaian. It is based mainly up on the field works augmented by petrographic description. The present study revealed that, the succession of the upper part of Girshah Formation comprises three main horizons: 1) lower unit of epidotized and silicified basaltic and andesitic tuffs;2) middle unit of volcaniclastic red beds-scoriaceous basalts of successive cycles (each of these cycles begins by reddish tuffaceous mudstone and sandstone and is terminated by the calcite-bearing scoriaceous basalt). This unit indicated the deposition in lacustrine environments and the red iron oxyhydroxides minerals were formed either by the direct hematitization of the deposited tuffs or by the diagenetic hematitization of the green clays formed instead of the precursor tuffaceous materials;3) upper scoriaceous basalt unit composed from hematitized and calcitized basalt. The unit was formed by basic volcanic eruptions in subaerial condition which was predominated by the formation of calcium carbonate lakes associated with the progressive and subsequent calcitization of the Ca-plagioclase minerals of the basalt and the associated glassy tuffaceous material. The scoriaceous basalts of Wadi Al Roaian area represent subaerial basic volcanic eruption in continental situation and are associated with subsequent events of mineral alteration and formation of secondary minerals i.e. calcite, hematite and goethite.
文摘Harrat Al Fatih is located southwest of Ablah area, Assir terrain, southwestern Arabian Shield. It is present as black laterally extended horizontal basaltic sheet just overlying folded strata of the volcano-sedimentary succession of Ablah Formation (green mudstones, dolostones and green and red volcaniclastic red beds). These basalts are rift-related and represent part of the Oligo-Miocene basic volcanics of the western part of Saudi Arabia. The detailed field and microscopic description of these basalts enable the author to classify it into two main types: 1) Grey tuffaceous glassy basalts that present overlying the Ablah Formation. This basalt type is generally soft, bedded and intercalated with grey, green and red tuffaceous mudstones. Microscopically it composed of minute lath-like plagioclase and pyroxene crystals embedded in glassy groundmass. 2) Black olivine-rich basalts which are present in the topmost part of harrat Al Fatih are generally black, hard and contain remarkable reddish brown oxidized olivine spots and some calcite and amorphous quartz domains. Under the microscope, this basalt type is microcrystalline and composed mainly of lamellar twined Ca-plagioclase and colored olivines and pyroxenes. The olivines show different stages of oxidation and formation of amorphous blood red iron-oxyhydroxides and black hematite. The present study revealed the formation of the grey basalts at the initial stages of the volcanic eruption in ephemeral lakes and the second type of basalt was during the consolidation of proper basic magmas at the final stages of the volcanic eruption.
