Cenozoic Alkaline Research Articles

Formation of the Denchai gem sapphires, northern Thailand: evidence from mineral chemistry and fluid/melt inclusion characteristics

AbstractThe Denchai gem sapphire deposits in Phrae Province, northern Thailand are closely associated with late Cenozoic alkaline basaltic rocks. The sapphires occur in alluvial placer deposits in palaeo-channels at shallow depths. Electron microprobe analysis of minor and trace element contents (Fe, Ti, Cr, Ga and V) of the sapphires indicate the following oxide abundances: Fe2O3 (0.32–1.98 wt.%), TiO2 (0.01–0.23 wt.%), Cr2O3 (<0.01 wt.%), Ga2O3 (0.01–0.03 wt.%) and V2O5 (<0.03 wt.%). Optical studies of sapphires revealed three types of primary fluid/melt inclusions. CO2-rich inclusions (Type I) contain three phases (LH2O + LCO2 + V) with the vapour phase comprising <10–15 vol.%. The presence of CO2 was confirmed by microthermometry and laser Raman analysis. Polyphase inclusions (Type II) (vapour + liquid + solid) contain a fluid bubble (20–30 vol.%), an aqueous phase (10–15 vol.%) and several solid phases. Silicate-melt inclusions (Type III) comprise vapour bubbles, silicate glass and solid phases. Proton-induced X-ray emission (PIXE) analysis revealed high concentrations of K (~;4 wt.%) as well as Ca (~;0.5 wt.%), Ti (~;1 wt.%), Fe (~;2 wt.%), Mn (~;0.1 wt.%), V (<0.03 wt.%), Rb (~;70 ppm) and Zr (~;200 ppm) in the silicate glass. The Ga2O3 abundances and Cr2O3/Ga2O3 values (<1) of the sapphires favour their formation by magmatic processes. The presence of CO2-rich fluids and high K concentrations in the silicate melt inclusions link the origin of the Denchai gem sapphires to CO2-rich alkaline magmatism.

Regional compilation and analysis of aeromagnetic anomalies for the Transantarctic Mountains–Ross Sea sector of the Antarctic

Magnetic observations over the area of the Transantarctic Mountains (TAM) and the Ross Sea have been compiled into a digital database that furnishes a new regional scale view of the magnetic anomaly crustal field in this key sector of the Antarctic continent. This compilation is a component of the ongoing IAGA/SCAR Antarctic Digital Magnetic Anomaly Project (ADMAP). The aeromagnetic surveys total 115 000 line km, and are distributed across the Victoria Land sector of the TAM, the Ross Sea, and Marie Byrd Land. The magnetic campaigns were performed within the framework of the national and international Italian–German–US Antarctic research programs and conducted with differing specifications during nine field seasons from 1971 until 1997. Generally flight line spacing was less than 5 km while survey altitude varied from about 610 to 4000 m above sea level for barometric surveys and was equal to 305 m above topography for the single draped survey. Reprocessing included digitizing the old contour data, improved levelling by means of microlevelling in the frequency domain, and re-reduction to a common reference field based on the DGRF90 model. A multi-frequency grid procedure was then applied to obtain a coherent and merged total intensity magnetic anomaly map. The shaded relief map covers an area of approximately 380 000 km 2. This new compilation provides a regional image of the location and spatial extent of the Cenozoic alkaline magmatism related to the TAM–Ross Sea rift, Jurassic tholeiites, and crustal segments of the Early Palaeozoic magmatic arc. A linear, approximately 100-km wide and 600-km long Jurassic rift-like structure is newly identified. Magnetic fabric in the Ross Sea rift often matches seismically imaged Cenozoic fault arrays. Major buried onshore pre-rift fault zones, likely inherited from the Ross Orogen, are also delineated. These faults may have been reactivated as strike-slip belts that segmented the TAM into various crustal blocks.

B16 韓国, 済州島の新生代アルカリ火山岩類の岩石学

B16 韓国, 済州島の新生代アルカリ火山岩類の岩石学

Existence de dykes doléritiques anciens à composition de tholéiites continentales au sein de la province alcaline de la ligne du Cameroun. Implication sur le contexte géodynamique

Abstract In the Adamawa Plateau (Northern Cameroon), doleritic dykes belong to a magmatic activity which predated the Cenozoic alkaline volcanism of the Cameroon Line. They have the chemical composition of continental tholeiites with Nb–Ta- and Ti-negative anomalies. This magmatism is interpreted as being derived from a sub-continental lithospheric source, which may have been contaminated during a former subduction event, linked to the Pan-African convergence. It is related to an early stage of continental break-up, which was contemporaneous or predated the Cretaceous tholeiitic magmatism of predominantly asthenospheric origin, which indicates a lithospheric thinning.

Eocene and Oligocene volcanism at Mount Petras, Marie Byrd Land: implications for middle Cenozoic ice sheet reconstructions in West Antarctica

Evidence for one late Eocene and four middle Oligocene eruptions of Mount Petras, Marie Byrd Land provides new insights into reconstructions of middle Tertiary ice sheet configurations, surface topography, and volcanism in West Antarctica. The interpretation presented here of the volcanic record at Mount Petras, based on detailed analyses of lithofacies, petrography, 40Ar/39Ar geochronology, and geochemistry, is significantly different from previous interpretations based on reconnaissance studies. A massive, 25 m thick, mugearite lava near the summit of Mount Petras is 40Ar/39Ar dated to 36.11 ± 0.22 Ma (2 σ uncertainty), indicating an onset of Cenozoic alkaline volcanism in the Marie Byrd Land Volcanic Province in latest Eocene time. Middle Oligocene (29-27 Ma) hawaiite volcaniclastic lithofacies at Mount Petras are interpreted as products of mixed magmatic (Strombolian style) and phreatomagmatic (Surtseyan style) subaerial eruptions. The four hawaiite outcrop areas exhibit characteristics of near-vent tuff cone environments. The near-vent deposits are located at different elevations and positions on Mount Petras and suggest four separate eruptive centres, with eruptions dated to between 28.59 ± 0.22 Ma and 27.18 ± 0.23 Ma. The mixed Surtseyan and Strombolian eruptions imply local or intermittent contact with external water, which we infer was derived from melting of a thin, local ice cap or ice and snow on slopes. The 29-27 Ma volcanic deposits at Mount Petras provide the oldest terrestrial evidence for glacial ice in Marie Byrd Land. The 29-27 Ma tuff cone deposits overlie an erosional unconformity, with > 400 m of topographic relief. The relatively high relief pre-volcanic environment is suggestive of ongoing erosion and is inconsistent with previous interpretations of a regional, low relief, early Cenozoic West Antarctic Erosion Surface.

The major- and trace-element and isotope (Sr, Nd, O) geochemistry of Cenozoic alkaline rift-type volcanic rocks from the Rhön area (central Germany): petrology, mantle source characteristics and implications for asthenosphere–lithosphere interactions

Major- and trace-element and radiogenic and oxygen isotope data are presented for an alkaline suite (nephelinites, basanites, hornblende-basalts) from the Rhön area (Germany) that belongs to the European volcanic province. Compositions of some basanites and hornblende-basalts are primitive whereas others show decreasing Cr, Ni, MgO and TiO 2 contents and CaO/Al 2O 3 ratios but increasing Al 2O 3 and incompatible elements (Sr, Zr, Nb, Y, Ce) with increasing SiO 2 indicating fractionation of mainly olivine, clinopyroxene and amphibole. On the other hand, nephelinites are products of increasing degrees of melting as is shown by their decreasing Al 2O 3 and incompatible element content but increasing CaO/TiO 2 ratio with increasing SiO 2. Some chemical features of the primitive members, i.e. high and decreasing CaO/Al 2O 3 ratios and decreasing La but increasing Yb abundances in the sequence nephelinite–basanite–hornblende-basalt can be explained by different degrees of melting of a garnet-bearing source in which garnet is progressively eliminated during melting. Negative anomalies of Rb and K in primitive mantle-normalized diagrams and Rb/K vs. K covariations suggest that amphibole was the major OH-bearing mineral phase. Variations in La/Nb and Zr/Nb ratios demonstrate at least two distinct sources in which low La/Nb–low Zr/Nb ratios are attributed to a HIMU-like asthenospheric source but high La/Nb–high Zr/Nb ratios correspond to an enriched upper mantle reservoir. This source is inferred to be lithospheric. Nephelinites and basanites have a restricted range in 87Sr/ 86Sr and 143Nd/ 144Nd ratios ranging from 0.70325 to 0.70396 and from 0.51279 to 0.51287, respectively. Hornblende-basalts have a larger spread in 87Sr/ 86Sr ratios but a similar spread in 143Nd/ 144Nd ratios which vary between 0.70339 and 0.70420 and 0.51279 and 0.51284, respectively. Sr isotopic composition of the hornblende-basalts are positively correlated with SiO 2 whereas the Nd isotopic compositions are positively correlated with MgO, indicating that interaction with crustal rocks during fractionation has occurred. The range of alteration-corrected oxygen isotope ratios shows δ 18O values between 5.2 and 7.4‰ in which a negative correlation of δ 18O values with Nd isotope composition suggests that the higher δ 18O values are the result of crustal contamination. However, pristine values of 5.2 and 5.8‰ indicate two distinct mantle endmembers with respect to the oxygen isotopic composition. Variations between incompatible trace-element ratios (e.g. La/Nb) and Sr and Nd isotopic compositions suggest that the hornblende-basalts are derived from HIMU sources but experienced crustal contamination whereas the basanites and nephelinites are probably products of HIMU and EM sources. Miocene intraplate basaltic volcanism in the Rhön area occurred probably as a result of minor plume activity coupled with lithospheric extension and some lower lithospheric thermo-mechanical erosion by the underlying plume head. The basalts show a spatial variation in their chemical composition; the early-stage basanites and hornblende-basalts have chemical signatures more compatible with a lower lithospheric source whereas the late-stage nephelinites probably originate from asthenospheric souces. This model, which involves small-scale plume impact followed by continental extension and asthenosphere–lithosphere interaction together with minor crustal contamination, should also be applicable to other intra-continental rift-related areas.

Geochemistry of Late Cenozoic basalts from the Crary Mountains: characterization of mantle sources in Marie Byrd Land, Antarctica

Late Cenozoic (9.34 to 0.04 Ma) alkaline basalts from the Crary Mountains, eastern Marie Byrd Land, Antarctica have low 87 Sr/ 86 Sr (0.70267–0.70283), moderately high 143 Nd/ 144 Nd (0.51286–0.51291) and high 206 Pb/ 204 Pb (19.9–20.9) ratios. The Crary Mountains basalts together with Hobbs Coast basalts from western Marie Byrd Land have the strongest HIMU signature ( 206 Pb/ 204 Pb>20.6 ) in the West Antarctic Rift System (WARS). There is no evidence of crustal contamination and their trace-element patterns are similar to HIMU basalts from oceanic islands. Higher concentrations of some large-ion lithophile (LIL) elements, particularly K, relative to oceanic HIMU, suggest melting of an enriched mantle. Models for small degrees of partial melting between 2% and 3% of a mantle peridotite containing 2.5% amphibole are consistent with trace-element variations in the basalts. The Crary data confirm the presence of a moderately depleted, lower-μ ( μ= 238 U/ 204 Pb ) mantle component with a 206 Pb/ 204 Pb signature between 19.3 and 19.8 in the source regions of West Antarctic basalts. The lower-μ component, defined by the focus of West Antarctic data arrays on 2-D and 3-D isotope plots, is prevalent as a mixing end-member throughout the WARS, whereas the HIMU component, defined by 206 Pb/ 204 Pb ratios greater than 20.5, is present at only a few localities. The positive correlation of many highly incompatible trace-elements with 206 Pb/ 204 Pb ratios indicate that smaller-degree melts preferentially sample the HIMU component, whereas larger degree melts sample the lower-μ component. We also suggest, based on variations in elements moderately incompatible to highly compatible in the mantle phases phlogopite and amphibole, that the lower-μ component is more hydrous than the HIMU source. The lower-μ and HIMU components are also found in alkaline basalts from Tasmania and New Zealand which were adjacent to the Antarctica coast prior to the mid-Cretaceous fragmentation of the proto-Pacific margin of Gondwanaland. We propose that before continental breakup, a HIMU-type mantle plume was trapped and stored beneath a much more extensive pre-existing metasomatised layer within the Gondwanaland lithosphere. Source regions for all known extreme HIMU basalts in the continental borderlands of the southwest Pacific would be encompassed by a modest plume head, 600–800 km in diameter, emplaced around 100 Ma. We suggest that an earlier metasomatic event, possibly related to the Jurassic Bouvet-plume, enriched the originally depleted upper mantle in highly incompatible elements. A fertile lithosphere formed by plume-driven melts and fluids during the Jurassic would allow for the widespread development of the present-day isotopic signature of the lower-μ source.

Lithospheric dynamics and mantle sources of alkaline magmatism of the Cenozoic West Antarctic Rift System

Lithospheric extension in the West Antarctic Rift System and the Ross Sea embayment is related to Cenozoic alkaline volcanism. Basanites and alkali basalts define the two endmembers of primary magmas with distinct petrographic, chemical and isotopic compositions. Basanites are generally more primitive and significantly more enriched in incompatible trace elements than alkali basalts. Parallel incompatible trace-element distribution patterns in both rock types but slightly different isotopic compositions suggest a derivation by different degrees of partial melting from different mantle sources. Sr-, Nd- and Pb-isotopes allow the identification of distinct magma sources: asthenospheric mantle, enriched lithospheric mantle and a HIMU plume component which is widespread beneath the entire area. Spatial and temporal variations in isotopic compositions suggest a relationship between the dynamics of lithospheric extension and changing mantle sources of related magmas. Geothermobarometry on mantle xenoliths documents a pressure–temperature–time evolution of the mantle lithosphere which is related to rifting, uplift and cooling of mantle below the Ross Sea Rift–Transantarctic Mountain transition. This paper reviews existing data and ideas but is biased towards the author's own working area, the Western Ross Sea.

Mantle plumes and Antarctica-New Zealand rifting: evidence from mid-Cretaceous mafic dykes

Ocean floor magnetic anomalies show that New Zealand was the last continental fragment to separate from Antarctica during Gondwana break-up, drifting from Marie Byrd Land, West Antarctica, about 84 Ma ago. Prior to continental drift, a voluminous suite of mafic dykes (dated by Ar–Ar laser stepped heating at 107 ± 5 Ma) and anorogenic silicic rocks, including syenites and peralkaline granitoids (95–102 Ma), were emplaced in Marie Byrd Land during a rifting event. The mafic dyke suite includes both high- and low-Ti basalts. Trace element and Sr and Nd isotope compositions of the mafic dykes may be modelled by mixing between tholeiitic OIB (asthenosphere-derived) and alkaline high- to low-Ti alkaline magmas (lithospheric mantle derived). Pb isotopes indicate that the OIB component had a HIMU composition. We suggest that the rift-related magmatism was generated in the vicinity of a mantle plume. The plume helped to control the position of continental separation within the very wide region of continental extension that developed when the Pacific–Phoenix spreading ridge approached the subduction zone. Separation of New Zealand from Antarctica occurred when the Pacific–Phoenix spreading centre propagated into the Antarctic continent. Sea floor spreading in the region of the mantle plume may have caused an outburst of volcanism along the spreading ridge generating an oceanic plateau, now represented by the 10–15 km thick Hikurangi Plateau situated alongside the Chatham Rise, New Zealand. The plateau consists of tholeiitic OIB-MORB basalt, regarded as Cretaceous in age, and similar in composition to the putative tholeiitic end-member in the Marie Byrd Land dykes. The mantle plume is proposed to now underlie the western Ross Sea, centred beneath Mount Erebus, where it was largely responsible for the very voluminous, intraplate, alkaline McMurdo Volcanic Group. A second mantle plume beneath Marie Byrd Land formed the Cenozoic alkaline volcanic province.

Mantle-derived xenoliths with hotspot type helium in Cenozoic alkali, basalt, northwestern Kyushu, Japan

The 3He/4He ratio of mantle-derived material is not uniform and the distinct 3He/4He signatures are found in association with different tectonic environments. Helium isotopic signature of hot-spot regions such as Hawaii, Iceland, and Rbunion range from 9 to 30 Ra, where Ra is 3He/4He of the atmosphere = 1.4 x 10 -6, which indicates that the hot-spot volcanisms tap a deeper region of the mantle enriched in 3He than MORB reservoir. Cenozoic alkaline basalts from back-arc region of the Japanese island-arc system are less depleted in Ta, Nb and Ti and less enriched in K, Sr, Ba and Rb compared with samples from northeastem Japan. These geochemical features lead to Nakamura et al. (1985) to suggest that the alkali volcanisms of this area were not directly related with subduction processes but were triggered by plumes originated from greater depth. Furthermore, it was suggested that the alkali basalt magmatism may represent the final stage of a mantle diapirism which was most active in the Miocene and caused the opening of the Sea of Japan (Iwamori, 1991). We measured noble gas isotopic compositions of xenoliths in Cenozoic alkali basalt, Takashima, northwestern Kyushu to investigate possible contribution of deep mantle plume.

Chemical Composition of the Primitive Mantle Source of Peridotite Inclusions at Mount Lianshan, Liuhe County, Jiangsu Province

Abstract: This paper presents a study of the major and trace element compositions of fresh mantle‐derived spinel lherzolite and harzburgite inclusions from Cenozoic alkaline basalt in Mount Lianshan and Mount Panshi, Liuhe County, Jiangsu Province. An estimation is made of the contents of the major elements and some of the trace elements in the primitive mantle source region of the area, from which the authors have obtained MgO/Al2O3= 7.86. The contents of MgO and Al2O3 are also obtained as 37.58% and 4.78% respectively based on the correlation of MgO‐Al2O3. Then, the contents of various elements in the primitive mantle are calculated using their regression equations with MgO, and the compositon of the primitive mantle, a basic issue in geochemistry study, is discussed on that basis.

Mantle-derived Xenoliths with Hot-spot Type Helium in Cenozoic Alkali Basalt, Northwestern Kyushu, Japan

The 3He/4He ratio of mantle-derived material is not uniform and the distinct 3He/4He signatures are found in association with different tectonic environments. Helium isotopic signature of hot-spot regions such as Hawaii, Iceland, and Rbunion range from 9 to 30 Ra, where Ra is 3He/4He of the atmosphere = 1.4 x 10 -6, which indicates that the hot-spot volcanisms tap a deeper region of the mantle enriched in 3He than MORB reservoir. Cenozoic alkaline basalts from back-arc region of the Japanese island-arc system are less depleted in Ta, Nb and Ti and less enriched in K, Sr, Ba and Rb compared with samples from northeastem Japan. These geochemical features lead to Nakamura et al. (1985) to suggest that the alkali volcanisms of this area were not directly related with subduction processes but were triggered by plumes originated from greater depth. Furthermore, it was suggested that the alkali basalt magmatism may represent the final stage of a mantle diapirism which was most active in the Miocene and caused the opening of the Sea of Japan (Iwamori, 1991). We measured noble gas isotopic compositions of xenoliths in Cenozoic alkali basalt, Takashima, northwestern Kyushu to investigate possible contribution of deep mantle plume.

Fluid-inclusion studies on lower crustal gabbroic xenoliths from the Mt.Melbourne Volcanic field (Antarctica): Evidence for the post-crystallization uplift history during Cenozoic Ross Sea Rifting

Fluid inclusions in gabbroic xenoliths from Cenozoic alkaline centres of the Mt.Melbourne Volcanic Field at the margin of the Ross Sea Rift System have been studied in order to derive fluid compositions and conditions of entrapment. Most of the inclusions are secondary, and only few are considered primary with respect to igneous crystallization. Textures indicate that the former inclusions formed from complete necking-down of fluid-filled, healed cracks. Most inclusions studied, primary or secondary, contain almost pure CO 2 , as has been tested by heating/freezing experiments. Other fluid components (N 2 , hydrocarbons) have been shown by Raman spectroscopy to be present in some inclusions. Histograms of homogenization temperatures for large (up to 50 mm) secondary inclusions in plagioclase and pyroxene have maxima between ca. 16° and 26°, 12° and 18°. Inclusions in olivine were measured in one rock only and gave lower values from 0° to 10°. We interpret these results as fluid entrapment and crack healing at different P-T-conditions along the steep geotherm previously documented for the lower crust in the Ross Rift region (Berg et al., 1989), which followed metamorphic equilibration of these rocks at conditions of 900-1000°C and 3-5 kbar (Hornig et al., 1992). Our fluid inclusion data then indicate an uplift path for the lower crust from the early high-T conditions to a later stage of crack formation, fluid migration, and fluid entrapment around 300-400°C, estimated from the maximum temperature for brittle behaviour of plagioclase and the 1-2 kbar derived from fluid inclusion measurements. With the limitation of the method in mind, and considering that the pressure estimate is a minimum value due to additional fluid components, this decompression and cooling is interpreted to be related to uplift and cooling of the lower crust in the Mt.Melbourne area during the formation of the Ross Sea Rift and uplift of the Transantarctic Mountains

Preliminary lithofacies assessment and 40Ar/39Ar ages of Cenozoic volcanic sequences in eastern Marie Byrd Land

Because of its remoteness, Marie Byrd Land is among the most inaccessible and least visited parts of Antarctica. However, it contains a very poorly studied, large Cenozoic alkaline volcanic province and an outstanding record of volcanism coeval with glaciation (LeMasurier & Thomson 1990). This short note describes the results of the second of two planned periods of fieldwork, which form part of the West Antarctic Volcano Exploration (WAVE) project. The background to WAVE and preliminary results of the 1989–1990 fieldwork are described in Smellie et al. (1990) and McIntosh et al. (1990). The studies described here took place between 2 November 1990 and 11 January 1991.

Strontium and neodymium isotope study of Bohemian basalts

The Cenozoic alkaline basalts of northern and western Bohemia are part of the Central European Volcanic Province (CEVP) which extends from the Rhineland (Eifel, Germany) to Moravia (Czechoslovakia) and the Lover Silesia (Poland). Seven samlpes from locations within Czechoslovakia have been analyzed isotopically for the Rb-Sr and the Sm-Nd systems. Present-day, normalised 87Sr/86Sr ratios range from 0.7031 to 0.7036, and the corresponding 143Nd/144Nd ratios range from 0.51279 to 0.51286. An eigth sample from the Silurian basalts of the ‘St. Jan’ type (K-Ar age: 420 Ma) occuring in the Barrandian basin in Central Bohemia is also analysed. Its Present-day, normalised, 87Sr/86Sr ratio is 0.7031, and the corresponding 143Nd/144Nd ratio 0.51288.

Physical characteristics of megacrystal high pressure phases

The composition and structural features of megacrystals of garnets, ilmenites, monoclinic pyroxenes, K-Na-feldspars occurring as inclusions in the Cenozoic alkaline basalts of Mongolia have been studied by means of X-ray diffractometry, i.r.-spectroscopy and electrone microscopy. The density of megacrystals has been determined by means of hydrostatic weighing. The high-temperature and deep formation of megacrystals are discussed.

A petrologic geotherm from a continental rift in Antarctica

Most of our knowledge about geothermal gradients in continental-rift zones has been determined indirectly from heat-transfer calculations based on surface heat flow, inferred heat production, inferred heat-transfer mechanisms, etc. We document a petrologically determined crustal geotherm from a continental-rift environment in the Ross Sea and adjacent Transantarctic Mountains. This geotherm is based on thermobarometry of garnet, spinel, and olivine granulites included in Cenozoic alkaline volcanic rocks. The barometry yields pressures that are in good agreement with the geophysically-defined depth to the base of the crust in the Transantarctic Mountains (≈ 40–43 km) and the Ross Embayment (≈ 20–28 km). The thermometry yields very high temperatures (900–1000°C) in the middle and lower crust. These high temperatures and the shape of the geotherm clearly indicate that advection, rather than conduction, was the dominant heat-transfer mechanism in the lower and middle crust of this rift environment. Injection of alkaline basaltic magmas is the likely cause of the advective heat transfer. The data require the upper crust to have an average geothermal gradient of at least 50–100°C/km. Such a high geothermal gradient, without evidence of crustal melting, suggests that the lower crust was free of granitic compositions. It also presents strong support for thermal-expansion models for the uplift of the Transantarctic Mountains.

40B. The nature of primitive magma of Cenozoic alkaline basalts in Kibikogen plateau and Yokota district, Chugoku province.

40B. The nature of primitive magma of Cenozoic alkaline basalts in Kibikogen plateau and Yokota district, Chugoku province.

39B. Temporal and spatial distribution of Cenozoic alkaline basalts in Southwest Japan.

39B. Temporal and spatial distribution of Cenozoic alkaline basalts in Southwest Japan.

28. The tractional crystallizaion of Cenozoic alkaline basalts in Kibi plateau district.(Abstracts of Papers Presented at the Fall Meeting of the Society)

28. The tractional crystallizaion of Cenozoic alkaline basalts in Kibi plateau district.(Abstracts of Papers Presented at the Fall Meeting of the Society)