The Neogene-Quaternary Harrat Rahat volcanic field is part of the major intercontinental Harrat fields in western Saudi Arabia. It comprises lava flows of olivine basalt and hawaiite, in addition to mugearite, benmori...The Neogene-Quaternary Harrat Rahat volcanic field is part of the major intercontinental Harrat fields in western Saudi Arabia. It comprises lava flows of olivine basalt and hawaiite, in addition to mugearite, benmorite, and trachyte that occur mainly as domes, tuff cones and lava flows. Based on opaque mineralogy and mineral chemistry, the Harrat Rahat volcanic varieties are distinguished into Group I （olivine basalt and hawaiite） and Group II （mugearite, benmorite and trachyte）. The maximum forsterite content （-85） is encountered in zoned forsteritic olivine of Group I, whereas olivine of Group II is characterized by intermediate （Fo=50）, fayalitic （Fo=25） and pure fayalite in the mugearite, benmorite and trachyte, respectively. The more evolved varieties of Group II contain minerals that show enrichment of Fe2＋, Mn2~ and Na~ that indicates normal fractional crystallization. The common occurrence of indicates that pS＋ becomes saturated in this coarse apatite with titanomagnetite in the benmorite rock variety and drops again in trachyte. Cr-spinel is recorded in Group I varieties only and the Cr# （0.5） suggests lherzolite as a possible restite of the Harrat Rahat volcanics. The plots of Cr# vs. the forsterite content （Fo） suggest two distinct trends, which are typical of mixing of two basaltic magmas of different sources and different degrees of partial melting. The bimodality of Harrat Rahat Cr-spinel suggests possible derivation from recycled MORB slab in the mantle as indicated by the presence of high-Al spinel. It is believed that the subcontinental lithospheric mantle was modified by pervious subduction process and played the leading role in the genesis of the Harrat Rahat intraplate volcanics. The trachytes of the Harrat Rahat volcanic field were formed most probably by melting of a lower crust at the mantle-crust boundary. The increase in fo2 causes a decrease in Cr203, and A1203, and a strong increase in the proportion of Fe3＋ and Mg# of spinel crystallizing from the basaltic melt at T -1200~C. The olivine-pyroxene and olivine-spinel geothermometers yielded equilibrium temperature in the range of 935-1025~C, whereas the range of 〈500-850~C from single-pyroxene thermometry indicates either post crystallization re- equilibrium of the clinopyroxene, or the mineral is xenocrystic and re-equilibrated in a cooling basaltic magma.展开更多
Harrat Al-Birk volcanics are products of the Red Sea rift in southwest Saudi Arabia that started in the Tertiary and reached its climax at ~ 5 Ma.This volcanic field is almost monotonous and is dominated by basalts th...Harrat Al-Birk volcanics are products of the Red Sea rift in southwest Saudi Arabia that started in the Tertiary and reached its climax at ~ 5 Ma.This volcanic field is almost monotonous and is dominated by basalts that include mafic-ultramafic mantle xenoliths(gabbro,websterite,and garnet-clinopyroxenite).The present work presents the first detailed petrographic and geochemical notes about the basalts.They comprise vesicular basalt,porphyritic basalt,and flow-textured basalt,in addition to red and black scoria.Geochemically,the volcanic rock varieties of the Harrat Al-Birk are low- to medium-Ti,sodic-alkaline olivine basalts with an enriched oceanic island signature but extruded in a within-plate environment.There is evidence of formation by partial melting with a sort of crystal fractionation dominated by clinopyroxene and Fe-Ti oxides.The latter have abundant titanomagnetite and lesser ilmenite.There is a remarkable enrichment of light rare earth elements and depletion in Ba,Th and K,Ta,and Ti.The geochemical data in this work suggest Harrat Al-Birk basalts represent products of watersaturated melt that was silica undersaturated.This melt was brought to the surface through partial melting of asthenospheric upper mantle that produced enriched oceanic island basalts.Such partial melting is the result of subducted continental mantle lithosphere with considerable mantle metasomatism of subducted oceanic lithosphere that might contain hydrous phases in its peridotites.The fractional crystallization process was controlled by significant separation of clinopyroxene followed by amphiboles and Fe-Ti oxides,particularly ilmenite.Accordingly,the Harrat Al-Birk alkali basalts underwent crystal fractionation that is completely absent in the exotic mantle xenoliths(e.g.Nemeth et al.in The Pleistocene Jabal Akwa A1 Yamaniah maar/tuff ring-scoria cone complex as an analogy for future phreatomagmatic to magmatic explosive eruption scenarios in the Jizan Region,SW Saudi Arabia 2014).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
Rock samples of industrial raw materials that are used in the building, from the West of Saudi Arabia (Arabian Shield), have been investigated using X-ray diffraction to identify the mineral chemical composition. The ...Rock samples of industrial raw materials that are used in the building, from the West of Saudi Arabia (Arabian Shield), have been investigated using X-ray diffraction to identify the mineral chemical composition. The concentrations of Al%, Bi, Pb, Th, U, and K in ppm were measured by atomic absorption analysis. The activity concentrations in Bq/kg dry weight of 226Ra, 232Th and 40K were measured using gamma-ray spectrometry where the range and average were found to be (03.04 - 10.91) 07.90, (03.19 - 13.31) 09.14 and (95.59 - 361.52) 234.81 in Basalt, for Granite (33.81 - 89.13) 80.85, (28.42 - 112.77) 68.50 and (1260.13 - 1629.21) 1376.69, for Gold (00.78 - 11.84) 04.73, (01.48 - 4.69) 02.69 and (13.76 - 445.09) 197.58, for Andesite 05.72, 03.73 and 471.93, and for Marble 01.56, 01.38 and 10.15 respectively. Most results existing within the given values in building materials by UNSCEAR 1993 included Raeq (Bq/kg), D (nGy/h), Deff (mSv/y), Hex and Hin, which meant that it was safe for humans.展开更多
基金funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant No. (7-145-D1432)
文摘The Neogene-Quaternary Harrat Rahat volcanic field is part of the major intercontinental Harrat fields in western Saudi Arabia. It comprises lava flows of olivine basalt and hawaiite, in addition to mugearite, benmorite, and trachyte that occur mainly as domes, tuff cones and lava flows. Based on opaque mineralogy and mineral chemistry, the Harrat Rahat volcanic varieties are distinguished into Group I （olivine basalt and hawaiite） and Group II （mugearite, benmorite and trachyte）. The maximum forsterite content （-85） is encountered in zoned forsteritic olivine of Group I, whereas olivine of Group II is characterized by intermediate （Fo=50）, fayalitic （Fo=25） and pure fayalite in the mugearite, benmorite and trachyte, respectively. The more evolved varieties of Group II contain minerals that show enrichment of Fe2＋, Mn2~ and Na~ that indicates normal fractional crystallization. The common occurrence of indicates that pS＋ becomes saturated in this coarse apatite with titanomagnetite in the benmorite rock variety and drops again in trachyte. Cr-spinel is recorded in Group I varieties only and the Cr# （0.5） suggests lherzolite as a possible restite of the Harrat Rahat volcanics. The plots of Cr# vs. the forsterite content （Fo） suggest two distinct trends, which are typical of mixing of two basaltic magmas of different sources and different degrees of partial melting. The bimodality of Harrat Rahat Cr-spinel suggests possible derivation from recycled MORB slab in the mantle as indicated by the presence of high-Al spinel. It is believed that the subcontinental lithospheric mantle was modified by pervious subduction process and played the leading role in the genesis of the Harrat Rahat intraplate volcanics. The trachytes of the Harrat Rahat volcanic field were formed most probably by melting of a lower crust at the mantle-crust boundary. The increase in fo2 causes a decrease in Cr203, and A1203, and a strong increase in the proportion of Fe3＋ and Mg# of spinel crystallizing from the basaltic melt at T -1200~C. The olivine-pyroxene and olivine-spinel geothermometers yielded equilibrium temperature in the range of 935-1025~C, whereas the range of 〈500-850~C from single-pyroxene thermometry indicates either post crystallization re- equilibrium of the clinopyroxene, or the mineral is xenocrystic and re-equilibrated in a cooling basaltic magma.
基金funded by the Deanship of Scientific Research(DSR),King Abdulaziz University,Jeddah,under Grant No.(D1435-710-145)
文摘Harrat Al-Birk volcanics are products of the Red Sea rift in southwest Saudi Arabia that started in the Tertiary and reached its climax at ~ 5 Ma.This volcanic field is almost monotonous and is dominated by basalts that include mafic-ultramafic mantle xenoliths(gabbro,websterite,and garnet-clinopyroxenite).The present work presents the first detailed petrographic and geochemical notes about the basalts.They comprise vesicular basalt,porphyritic basalt,and flow-textured basalt,in addition to red and black scoria.Geochemically,the volcanic rock varieties of the Harrat Al-Birk are low- to medium-Ti,sodic-alkaline olivine basalts with an enriched oceanic island signature but extruded in a within-plate environment.There is evidence of formation by partial melting with a sort of crystal fractionation dominated by clinopyroxene and Fe-Ti oxides.The latter have abundant titanomagnetite and lesser ilmenite.There is a remarkable enrichment of light rare earth elements and depletion in Ba,Th and K,Ta,and Ti.The geochemical data in this work suggest Harrat Al-Birk basalts represent products of watersaturated melt that was silica undersaturated.This melt was brought to the surface through partial melting of asthenospheric upper mantle that produced enriched oceanic island basalts.Such partial melting is the result of subducted continental mantle lithosphere with considerable mantle metasomatism of subducted oceanic lithosphere that might contain hydrous phases in its peridotites.The fractional crystallization process was controlled by significant separation of clinopyroxene followed by amphiboles and Fe-Ti oxides,particularly ilmenite.Accordingly,the Harrat Al-Birk alkali basalts underwent crystal fractionation that is completely absent in the exotic mantle xenoliths(e.g.Nemeth et al.in The Pleistocene Jabal Akwa A1 Yamaniah maar/tuff ring-scoria cone complex as an analogy for future phreatomagmatic to magmatic explosive eruption scenarios in the Jizan Region,SW Saudi Arabia 2014).
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.
文摘Rock samples of industrial raw materials that are used in the building, from the West of Saudi Arabia (Arabian Shield), have been investigated using X-ray diffraction to identify the mineral chemical composition. The concentrations of Al%, Bi, Pb, Th, U, and K in ppm were measured by atomic absorption analysis. The activity concentrations in Bq/kg dry weight of 226Ra, 232Th and 40K were measured using gamma-ray spectrometry where the range and average were found to be (03.04 - 10.91) 07.90, (03.19 - 13.31) 09.14 and (95.59 - 361.52) 234.81 in Basalt, for Granite (33.81 - 89.13) 80.85, (28.42 - 112.77) 68.50 and (1260.13 - 1629.21) 1376.69, for Gold (00.78 - 11.84) 04.73, (01.48 - 4.69) 02.69 and (13.76 - 445.09) 197.58, for Andesite 05.72, 03.73 and 471.93, and for Marble 01.56, 01.38 and 10.15 respectively. Most results existing within the given values in building materials by UNSCEAR 1993 included Raeq (Bq/kg), D (nGy/h), Deff (mSv/y), Hex and Hin, which meant that it was safe for humans.
