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1.
A Time Frame for Construction of the Kerguelen Plateau and Broken Ridge   总被引：8，自引：10，他引：8  
A key element in achieving Ocean Drilling Program Leg 183 scienceobjectives is determining the age of volcanism at differentlocations across the Kerguelen Plateau and Broken Ridge. Thispaper reports crystallization ages derived from 40Ar–39Arincremental heating experiments, for whole rocks and feldsparsseparated from basement units recovered at Sites 1136, 1137,1138, 1139, 1140, 1141 and 1142. The subaerial environment oferuption at most sites and the generally evolved, high K contentof these lava flows contributed to precise and reproducibleage determinations. Volcanic activity at southern KerguelenPlateau Site 1136 occurred at 118–119 Ma; at Elan BankSite 1137, 107–108 Ma; at central Kerguelen Plateau Site1138, 100–101 Ma; at Skiff Bank Site 1139, 68–69Ma; at northern Kerguelen Plateau Site 1140, 34–35 Ma;and at Broken Ridge Sites 1141 and 1142, 94–95 Ma. Thenew ages allow calculation of melt production rates throughthe  相似文献   

2.
Roca Redonda volcano is a mostly submarine shield volcano that rises nearly 3 km from the adjacent seafloor. Over twenty lava flows and palagonite tuff are exposed in a 60 meter high oblong outcrop above sea level, and several other flows are exposed in the shallow water surrounding the islet. Thick, slightly alkaline picritic flows form the base of the section. Thinner picrites interbedded with sparsely porphyritic alkali-olivine basaltic pahoehoe toes characterize the upper section. The subaerial section probably records the filling of a palagonite tuff cone with younger lavas. Numerous fumaroles that may have a magmatic component are present in the shallow (<30 m) submarine zone and indicate that the volcano is probably still active. Three lava types are exposed: the basal picrites with 19% > MgO > 14%, high-Mg basalts with MgO of about 9%, and low-Mg basalts with MgO of about 6%. The Sr and Nd isotopic ratios of the three lava types are within analytical uncertainty. Olivine compositions indicate that the picrites are basaltic liquids that have accumulated olivine whose composition is in equilibrium with the host basaltic liquid. Apparently, basaltic magmas percolated through dunite and troctolite that had crystallized from slightly older Roca Redonda basaltic magma. Lavas from Roca Redonda have enriched trace element contents and isotopic ratios relative to nearby Wolf volcano, but they are quite similar to lavas from Cerro Azul and Ecuador volcanoes. The common characteristic of these volcanoes is that they lie on the periphery of the archipelago and are in a stage of subaerial growth. This suggests that Galápagos volcanoes may go through a juvenile alkaline stage before a mature tholeiitic stage, analogous to the Loihi stage of Hawaiian volcanism. A low 3He/4He ratio in olivine from one of the picrites indicates a small contribution by the Galápagos mantle plume. Received: 15 December 1997 / Accepted: 6 May 1998  相似文献   

3.
The uplifted and deeply eroded volcanic succession of Porto Santo (central East-Atlantic) is the product of a wide spectrum of dynamic processes that are active in shoaling to emergent seamounts. Two superimposed lapilli cones marking the base of the exposed section are interpreted as having formed from numerous submarine to subaerial phreatomagmatic explosions, pyroclastic fragmentation being subordinate. The lower basaltic and the upper mugearitic to trachytic sections are dominated by redeposited tephra and are called 'lapilli cone aprons'. Vertical growth due to accumulation of tephra, voluminous intrusions, and minor pillowed lava flows produced ephemeral islands which were subsequently leveled by wave erosion, as shown by conglomerate beds. Periods of volcanic quiescence are represented by abundant biocalcarenite lenses at several stratigraphic levels. The loose tephra piles became stabilized by widespread syn-volcanic intrusions such as dikes and trachytic to rhyolitic domes welding the volcanic and volcaniclastic ensemble into a solid edifice. Shattering of a submarine extrusive trachytic dome by pyroclastic and phreatomagmatic explosions, accentuated by quench fragmentation, resulted in pumice- and crystal-rich deposits emplaced in a prominent submarine erosional channel. The dome must have produced an island as indicated by a collapse breccia comprising surf-rounded boulders of dome material. Subaerial explosive activity is represented by scoria cones and tuff cones. Basaltic lava flows built a resistant cap that protected the island from wave erosion. Some lava flows entered the sea and formed two distinct types of lava delta: 1. closely-packed pillow lava and massive tabular lava flows along the southwestern coast of Porto Santo, and 2. a steeply inclined pillow-hyaloclastite breccia prism composed of foreset-bedded hydroclastic breccia, variably-shaped pillows, and thin sheet flows capped by subhorizontal submarine to subaerial lava flows along the eastern coast of Porto Santo.The facies architectures indicate emplacement: 1. on a gently sloping platform in southwestern Porto Santo, and 2. on steep offshore slopes along high energy shorelines in eastern Porto Santo.Growth of the pillow-hyaloclastite breccia prism is dominated by the formation of foreset beds but various types of syn-volcanic intrusions contributed significantly. Submarine flank eruptions occurred in very shallow water on the flanks of the hyaloclastite prism in eastern Porto Santo. The island became consolidated by intrusion of numerous dikes and by emplacement of prominent intrusions that penetrate the entire volcanic succession. Volcanic sedimentation ended with the emplacement of a debris avalanche that postdates the last subaerial volcanic activity.  相似文献   

4.
Approximately 160 Ma old basaltic lavas obtained from ODP Site 801 in the Pigafetta Basin represent the first Jurassic oceanic crust recovered in the Pacific Ocean and the oldest in situ oceanic crust discovered anywhere. The basement consists of an upper alkali olivine basalt sequence and a lower tholeiitic sequence separated by a yellow Fe-rich hydrothermal sedimentary deposit. The aphyric and sparsely plagiodase-olivine±spinel phyric tholeiites exhibit depleted, open–system fractionated characteristics with trace element abundances and Pb–Nd isotopic compositions similar to normal mid-ocean ridge basalts (N-MORB). The aphyric alkali basalts, although showing some overlap in isotopic composition with MORB, exhibit strong similarities in terms of incompatible element abundances to ocean island basalts (OIB). They could represent either OIB-type off-axis volcanism or an alkalic event possibly associated with the waning stages of spreading axis volcanism in the Pigafetta Basin. All lavas have undergone low-grade anoxic smectite–carbonate alteration, although flows underlying the Fe-rich sediments have suffered hydrothermal alteration and fracturing.  相似文献   

5.
The extensive volcanic activity of Rajmahal occurred during early Cretaceous (~117 Ma). Potentially perspective Gondawana sediments containing coal seam is partially covered by these basalts. Basalts as well as coal are already known to be of immense economic importance over Rajmahal traps. The present study mainly deals with delineation of basaltic lava flows and associated coal layers using the Audio-magneto-telluric (AMT) study in the south central region of Rajmahal traps near Suri, West Bengal, India. Initially, the field situations have been simulated using forward model with top alluvium cover followed by basaltic lava flows, a wedge shaped inter-trappean sedimentary formation with second basaltic lava flows and basement at the bottom. The model was constrained using available litholog. The model study reveals the presence of moderately resistive basalts, relatively conductive inter-trappeans and the basement, except the top alluvium layer and thin inter-trappeans. Subsequently, threeAMT sounding have been carried out over Cholaguria, Hatgacha and Pachami, Suri, West Bengal. The field AMT study reveals three basaltic lava flows with varying thickness from ~40m to ~350m and composite thickness of ~620m to ~640m, which match well with the published borehole litholog. The inter-trappeans have been identified at 500m to 620m, 310m to 400m and 500m to 640m depths over Cholaguria, Hatgacha and Pachami, respectively. It is interesting to mention that the identified inter-trappeans are inferred to be coal/shaly-coal which are matching very well with the published borehole lithologs.  相似文献   

6.
A complete dismembered sequence of ophiolite is well exposed in the south Andaman region that mainly comprises ultramafic cumulates, serpentinite mafic plutonic and dyke rocks, pillow lava, radiolarian chert, and plagiogranite. Pillow lavas of basaltic composition occupy a major part of the Andaman ophiolite suite (AOS). These basalts are well exposed all along the east coast of southern part of the south AOS. Although these basalts are altered due to low-grade metamorphism and late hydrothermal processes, their igneous textures are still preserved. These basalts are mostly either aphyric or phyric in nature. Aphyric type exhibits intersertal or variolitic textures, whereas phyric variety shows porphyritic or sub-ophitic textures. The content of alkalies and silica classify these basalts as sub-alkaline basalts and alkaline basalts. A few samples show basaltic andesite, trachy-basalt, or basanitic chemical composition. High-field strength element (HFSE) geochemistry suggests that studied basalt samples are probably derived from similar parental magmas. Al2O3/TiO2 and CaO/TiO2 ratios classify these basalts as high-Ti type basalt. On the basis of these ratios and many discriminant functions and diagrams, it is suggested that the studied basalts, associated with Andaman ophiolite suite, were derived from magma similar to N-MORB and emplaced in the mid-oceanic ridge tectonic setting.  相似文献   

7.
The least-altered, Permian mafic volcanic rocks from the Pang Mayao area, Phrao District, Chiang Mai Province, part of Chiang Rai–Chiang Mai volcanic belt, have been analyzed and are found to be mid-ocean ridge and ocean–island basalts. The mid-ocean ridge basalts occur as lava flows or dike rocks. They are equigranular, fine- to medium-grained and consist largely of plagioclase, clinopyroxene and olivine. These basalt samples are tholeiitic, and have compositions very similar to T-MORB from the region where the Du Toit Fracture Zone intersects the Southwest Indian Ridge. The ocean–island basalt occurs as pillow breccia, and lava flows or dike rocks. They are slightly to moderately porphyritic, with phenocrysts/microphenocrysts of clinopyroxene, olivine, plagioclase and/or Fe–Ti oxide. The groundmass is very fine-grained, and made up largely of felty plagioclase laths with subordinate clinopyroxene. These basalt samples are alkalic, and chemically analogous to those from Haleakala Volcano, Maui, Hawaiian Chain. These mafic volcanic rocks may have been formed in a major ocean basin rather than in a mature back-arc basin.  相似文献   

8.
Distal pillows occur associated with a sheet flow and megapillows in the me?akoz outcrops of the Basque–Cantabrian Basin (N Spain). Basaltic volcanic rocks are interbedded with Turonian sediments and depict typical features of shallow submarine emissions. An exceptional basaltic flow displays four types of morphology: (1) sheet lava with columnar jointing, (2) welded columnar breccia, (3) megapillows, and (4) pillow lavas with sparse megapillows. The field data from me?akoz combined with experimental and field data from the literature for similar volcanic facies can be integrated into a new propagation model for the transition from sheet flows to pillow lavas in underwater environments. At near vent high emission rates, lava flows develop a thin crust immediately after its emplacement and break at the front under the magma pressure allowing for the massive propagation of lava as a sheet flow. Increased cooling promotes thickening of the lava outer crust far from the vent while continuous supply of fresh magma increases the pressure onto the thick crust until its rupture. The lava emitted in small volumes from the flow front promotes the formation of megapillows and pillow lavas that are later on covered by the advancing sheet flow. The lava flow freezes progressively toward more distal parts, gradually increasing its viscosity until it stops. The crust temporarily holds the residual melt pressure increasing the volume of the flow distal section by inflation. Finally, the internal magma pressure breaks the crust and liberates lava at moderate-to-low flow rates producing pillows, while lava drainage inside the inflated sheet flow produces lava tunnels and gravitational collapse of the roofs by hydrostatic pressure to form breccias nurtured by columnar lava fragments.  相似文献   

9.
Pyroclastic fragments of basaltic composition are obtained in a thick metabasaltic layer from the Singhbhum orogenic belt of Eastern India. Field relations indicate the basalts to be submarine lava flows. Chemically, the metabasalts represent an original dry basaltic magma, showing a differentiation trend of Fe-enrichment much like the abyssal tholeiites and the Skaergaard rocks. The fragments are sharply angular in shape and distinctly richer in FeO, MgO, and MnO relative to the enclosing matrices. It is shown that the fragments are least likely to represent solidified lavas. These are best explained as cumulates which were fragmented and erupted with the fractionated melts.  相似文献   

10.
During Late Carboniferous times a continental magmatic arc developed at the western margin of Gondwana in South America, as several marine sedimentary basins were formed at the same time in the retroarc region. North of 33°S, at Cordón Agua del Jagüel, Precordillera of Mendoza, Argentina, a volcanic sequence crops out which was emplaced in a submarine environment with some subaerial exposures, and it is intercalated in marine sediments of Agua del Jagüel Formation, which fills of one of these retroarc basins. This paper presents, for the first time, a facies analyses together with geochemical and isotopic data of this volcanic suite, suggesting its deposition in an ensialic retroarc marine basin. The volcanic succession comprises debris flows with either sedimentary or volcanic fragments, base surge, resedimented massive and laminated dacitic–andesitic hyaloclastite, pillow lava, basic hyaloclastite and dacitic–andesitic lavas and hyaloclastite facies. Its composition is bimodal, either basaltic or dacitic–andesitic. The geochemistry data indicate a subalkaline, low K calk-alkaline and metaluminous affinity. The geochemistry of the basalts points to an origin of the magmas from a depleted mantle source with some crustal contamination. Conversely, the geochemistry of the dacites–andesites shows an important participation of both crustal components and subduction related fluids. A different magmatic source for the basalts than for the dacites–andesites is also supported by Sr and Nd isotopic initial ratios and Nd model ages. The characteristics of this magmatic suite suggest its emplacement in an extensional setting probably associated with the presence of a steepened subduction zone at this latitude during Upper Carboniferous times.  相似文献   

11.
陆上与水下喷发火山岩在岩性、结构构造、蚀变特征、产状、与下伏地层接触关系、孔隙和裂缝发育特点等6方面有显著区别。陆上喷发火山岩包括各种熔岩、碎屑熔岩、火山碎屑岩和沉火山碎屑岩;熔岩流纹构造发育,火山碎屑岩除发育常见层理外,还可见反丘构造;同生蚀变弱;与下伏地层多呈角度不整合接触,古风化壳常见,常含有陆相植物;主要储集空间为原生孔隙和冷凝收缩节理缝、次生溶蚀孔、矿物解理缝和构造裂缝。水下喷发火山岩多为具玻璃质结构的熔岩和含晶屑玻屑的层/沉凝灰岩、膨润土/伊利石岩/蒙脱石岩/沸石岩;常具枕状、球状构造,水平层理、粒序层理、变形层理;蚀变强烈;水下熔岩呈穹隆状、透镜状,凝灰岩为层状,近火山口的膨润土/伊利石岩/蒙脱石岩/沸石岩呈松散团窝状并夹有火山弹;与下伏地层呈整合、假整合或侵蚀接触;原生气孔、杏仁体内溶蚀孔和炸裂纹,岩球岩枕间孔和粒间孔,基质和斑晶蚀变孔缝,后期构造缝是主要储集空间。松辽盆地营城组陆上、水下喷发火山岩均有发育。其陆上喷发火山岩的典型标志为流纹构造、柱状节理,含炭化木/硅化木,与下伏地层呈角度不整合接触。水下喷发火山岩典型标志为珍珠岩、玻璃质结构、枕状构造、纹层状凝灰岩和膨润土。松辽盆地营城组的储层火山岩以陆上喷发火山岩为主;水下喷发火山岩中侵出相内带亚相珍珠岩为优质储层。  相似文献   

12.
Glasses from Mauna Loa pillow basalts, recent subaerial vents, and inclusions in olivine were analyzed for S, Cl, F, and major elements by electron microprobe. Select submarine glasses were also analyzed for H2O and CO2 by infrared spectroscopy. The compositional variation of these tholeiitic glasses is dominantly controlled by crystal fractionation and they indicate quenching temperatures of 1,115-1,196 °C. Submarine rift zone glasses have higher volatile abundances (except F) than nearly all other submarine and subaerial glasses with the maximum concentrations increasing with water depth. The overwhelming dominance of degassed glasses on the submarine flanks of Mauna Loa implies that much of volcano's recent submarine growth involved subaerially erupted lava that reached great water depths (up to 3.1 km) via lava tubes. Anomalously high F and Cl in some submarine glasses and glass inclusions indicate contamination possibly by fumarolic deposits in ephemeral rift zone magma chambers. The relatively high CO2 but variable H2O/K2O and S/K2O in some submarine rift zone glasses indicates pre-eruptive mixing between degassed and undegassed magma within Mauna Loa's rift system. Volatile compositions for Mauna Loa magmas are similar to other active Hawaiian volcanoes in S and F, but are less Cl-rich than Ll'ihi glasses. However, Cl/K2O ratios are similar. Mauna Loa and Ll'ihi magmas have comparable, but lower H2O than those from Kilauea. Thus, Kilauea's source may be more H2O-rich. The dissimilar volatile distribution in glasses from active Hawaiian volcanoes is inconsistent with predictions for a simple, concentrically zoned plume model.  相似文献   

13.
Trace element mobility during hydrothermal alteration of oceanic basalts   总被引：2，自引：0，他引：2  
Trace element analyses have been carried out on hydrothermally altered pillow basalts of greenschist facies dredged from the median valley of the Mid-Atlantic Ridge. Sr is leached from the rock, and its behavior is apparently controlled by the same reactions as Ca. Cu is also leached from the basalt, but often shows local precipitation in veins as sulfides. Fe, B, Li, Ba, Mn, Ni and Co show sufficient variations in concentration and location within the altered basalts to indicate that some mobilisation occurs, but there may be subsequent uptake or precipitation into the secondary mineral assemblages. V, Y, Zr and Cr do not appear to be affected by hydrothermal alteration.The production of a metal-enriched solution by hydrothermal alteration and subsequent precipitation of metal salts to form metalliferous sediments is indicated, as is precipitation of metal sulfides in the basaltic basement.  相似文献   

14.
The Miocene northeast Honshu magmatic arc, Japan, formed at a terrestrial continental margin via a stage of spreading in a back‐arc basin (23–17 Ma) followed by multiple stages of submarine rifting (19–13 Ma). The Kuroko deposits formed during this period, with most forming during the youngest rifting stage. The mode of magma eruption changed from submarine basalt lava flows during back‐arc basin spreading to submarine bimodal basalt lava flows and abundant rhyolitic effusive rocks during the rifting stage. The basalts produced during the stage of back‐arc basin spreading are geochemically similar to mid‐ocean ridge basalt, with a depleted Sr–Nd mantle source, whereas those produced during the rifting stage possess arc signatures with an enriched mantle source. The Nb/Zr ratios of the volcanic rocks show an increase over time, indicating a temporal increase in the fertility of the source. The Nb/Zr ratios are similar in basalts and rhyolites from a given rift zone, whereas the Nd isotopic compositions of the rhyolites are less radiogenic than those of the basalts. These data suggest that the rhyolites were derived from a basaltic magma via crystal fractionation and crustal assimilation. The rhyolites associated with the Kuroko deposits are aphyric and have higher concentrations of incompatible elements than do post‐Kuroko quartz‐phyric rhyolites. These observations suggest that the aphyric rhyolite magma was derived from a relatively deep magma chamber with strong fractional crystallization. Almost all of the Kuroko deposits formed in close temporal relation to the aphyric rhyolite indicating a genetic link between the Kuroko deposits and highly differentiated rhyolitic magma.  相似文献   

15.
The Khoy ophiolite in northwestern Iran represents a remnant of oceanic lithosphere formed in the Mesozoic Neo-Tethys. This northwest–southeast trending ophiolite complex consists from bottom to top (east to west) of a well-defined basal metamorphic zone, peridotites (dunite, harzburgite) and serpentinized peridotite, gabbros, sheeted dikes, pillow and massive lava flows, and pelagic sedimentary rocks, including radiolarian chert. The rocks of the metamorphic zone have an inverse thermal gradient from amphibolite facies to greenschist facies. The high-grade metamorphic rocks are immediately adjacent to the peridotite and the gabbros and the low-grade rocks are in contact with the Precambrian Kahar Formation. Based on mantle-normalized incompatible trace element diagrams there are two distinct types of basalt flows present at the Khoy ophiolite: (1) massive basalts that have patterns virtually identical to E-MORB, and (2) pillow basalts that have more primitive chemical composition whose trace element patterns plot between E-MORB and N-MORB. The chondrite-normalized REE patterns for the pillow basalts are LREE-depleted [(LaN/SmN)ave=0.70], similar to patterns for the mean diabase composition for the Oman ophiolite and LREE-depleted basalts of the Band-e-Zeyarat ophiolite of southern Iran. The REE patterns for the massive basalts are similar in general REE abundances to the pillow basalt patterns, but they are slightly LREE-enriched [(LaN/SmN)ave=1.09] and their patterns cross those of the pillow basalts. The REE patterns for the gabbros and diorites indicates that the crustal-suite rocks were most likely derived by a process of fractional crystallization from a common basaltic melt. This basaltic melt was most likely generated by approx. 20–25% partial melting of a simple lherzolite source and had REE concentrations of roughly 10× chondrite. A comparison between the results from the Khoy ophiolite and the data from other Iranian ophiolites reveals geochemical evidence to suggest a tectonic link between the Khoy ophiolite and the rest of the Iranian ophiolites. Our results suggest that Khoy ophiolite is equivalent to the inner group of Iranian ophiolites (e.g. Nain, Shahr-Babak, Sabzevar, Tchehel Kureh and Band-e-Zeyarat) and was formed as a result of closure of the northwestern branch of a narrow Mesozoic seaway which once surrounded the Central Iranian microcontinent.  相似文献   

16.
The Late Miocene to Pleistocene evolution of the northwestern Iblean plateau (Sicily) is marked by a complex interplay of subaerial and submarine volcanism, subsidence and uplift, eustatic sea-level changes, and shallow-water carbonate and clay sedimentation. Volcanic activity occurred in distinct phases, differing drastically in volume, chemical composition, eruptive and depositional sites, and eruptive mechanisms. Six of the newly defined formations in the northwestern Iblean plateau are either entirely volcanic or contain significant amounts of volcanics. The eastern part of a shallow marine basin was filled completely by Late Pliocene tholeiitic lava flows (Militello Formation) that had advanced subaerially from the south–southeast. Lava deltas advanced southwestward on top of earlier pillow breccia debris flow deposits intertongued with soft Trubi marls and chalks. True submarine eruptions (Monte Caliella Formation) simultaneously produced densely packed pillow piles up to 250?m thick. Inferred water depths based on volcanologic and paleoecologic criteria of interbedded and overlying calcarenites agree well. Subsequent alkalic, more explosive Pleistocene volcanic eruptions (Poggio Vina Formation) changed from initially submarine to late subaerial indicating growth of edifices above sea level, sea-level rise, or land Subsidence by ca. 50?m. They and the latest Militello volcanics are interlayed with minor shallow-water calcarenites. The Poggio Vina volcanics were submerged during a second sea-level rise amounting to up to 100?m. The sea was generally shallow, i.e., <100?m deep, throughout most of the Late Pliocene and early Pleistocene. The Poggio Vina volcanism took place prior to the Emilian transgression. The sea-level rise might represent a continuation of the subsidence trend that caused the Lower Pliocene Trubi marine basin. Subaerial conditions were reached twice in the approximate time interval 1.9–1.6?Ma during phases of voluminous volcanism that outpaced subsidence. Uplift of approximately 600?m (Palagonia) to 950?m (Monte Lauro) occurred subsequent to emplacement of the Pleistocene alkalic volcanics. Bioclastic carbonates deposited concurrently with uplift drape a major fault scarp east of Palagonia with uplift rates in excess of 0.5?mm/a, provided most uplift occurred during ca. 1?Ma. Basinning continued beneath the half graben of the present Piana di Catania where volcanics several hundreds of meters thick – at least some of them alkalic in composition – occur at a depth of approximately 500–1500?m below the present surface. Quaternary uplift of the northwestern Iblean plateau may have been due to a major phase of underplating or rise of partially melted mantle. Composition of the volcanic rocks, total volume, and mass eruptive rates are well-correlated. The volumetrically very minor highly mafic Messinian nephelinites may have formed in response to Messinian lithosphere unloading following draining of the Mediterranean resulting in very low-degree partial melting. The nephelinitic to basanitic Poggio Inzerillo and Poggio Pizzuto pillow lavas may herald a major mantle decompression event, possibly the rise of a mantle diapir. The remarkably homogeneous bronzite-bearing, relatively SiO2-rich Militello tholeiites, representing a very short-lived but voluminous eruptive phase, resemble E-MORB and reflect a major high-degree partial melting event. The Pleistocene Poggio Vina alkali basalts to nephelinites resemble the late-stage alkalic phase in intraplate magmatic systems. The Iblean cycle of a brief but intense phase of widespread tholeiites followed by alkali basaltic volcanism resembles that of Etna Volcano where widespread basal tholeiites erupted at approximately 0.5?Ma and were followed by (evolved) alkali basaltic lavas. The immediate cause-and-effect relationship between volcanism and tectonism remains speculative.  相似文献   

17.
Special features of linear marine magnetic anomalies and the magnetic measurements made on samples of the ocean crust require that the thickness of the marine magnetic source layer be thicker than the 0.5 km thick pillow lava layer (seismic layer 2A) alone. It is proposed here that the magnetic properties of the samples studied to date indicate a two-layered source to be the most likely: an upper 0.5 km thick pillow lava layer with a natural remanent magnetization of 5 A/m and a lower 3.5 km thick dike and upper gabbro layer with a magnetization of 0.5 A/m.  相似文献   

18.
辽西义县许家屯巨型枕状熔岩产在义县旋回第二亚旋回火山岩沉积夹层中.以其岩枕巨大（5~8m）、形态多样构成独特的火山岩景观.岩枕大体由冷凝边、过渡带和核部3部分组成,三者间在颜色、气孔和杏仁的数量与大小上彼此有别.它们的岩石学、地球化学特征完全可以和下伏火山岩对比,岩性属玄武安山岩.其是在第二亚旋回火山活动间歇期,岩浆再次溢出,涌流进水体（如湖泊）里冷凝而成为陆相环境产物,与海相玄武质枕状熔岩明显不同.  相似文献   

19.
Rubbly pahoehoe lava flows are abundant in many continental flood basalts including the Deccan Traps. However, structures with radial joint columns surrounding cores of flow-top breccia (FTB), reported from some Deccan rubbly pahoehoe flows, are yet unknown from other basaltic provinces. A previous study of these Deccan “breccia-cored columnar rosettes” ruled out explanations such as volcanic vents and lava tubes, and showed that the radial joint columns had grown outwards from cold FTB inclusions incorporated into the hot molten interiors. How the highly vesicular (thus low-density) FTB blocks might have sunk into the flow interiors has remained a puzzle. Here we describe a new example of a Deccan rubbly pahoehoe flow with FTB-cored rosettes, from Elephanta Island in the Mumbai harbor. Noting that (1) thick rubbly pahoehoe flows probably form by rapid inflation (involving many lava injections into a largely molten advancing flow), and (2) such flows are transitional to ‘a’ā flows (which continuously shed their top clinker in front of them as they advance), we propose a model for the FTB-cored rosettes. We suggest that the Deccan flows under study were shedding some of their FTB in front of them as they advanced and, with high-eruption rate lava injection and inflation, frontal breakouts would incorporate this FTB rubble, with thickening of the flow carrying the rubble into the flow interior. This implies that, far from sinking into the molten interior, the FTB blocks may have been rising, until lava supply and inflation stopped, the flow began solidifying, and joint columns developed outward from each cold FTB inclusion as already inferred, forming the FTB-cored rosettes. Those rubbly pahoehoe flows which began recycling most of their FTB became the ‘a’ā flows of the Deccan.  相似文献   

20.
Compositional studies on different forms of magnetite, ulvospinel, ilmenite and hematite mineral phases occurring in 37 lava flows and 6 dykes of the Mandla lobe are presented in this paper. Ilmenite (0001) in equilibrium with titanomanetite show high values of temperature of equilibration, ranging from 1172–974°C, for high alumina quartz normative tholeiitic lava flows of Chemical Type - A; 1129–1229°C for low alumina quartz normative tholeiitic lava flows of Chemical Type - B; 1283–1124°C for tholeiitic lava flows of Chemical Type - F and 1243°C and 99O°C for two diopside olivine normative tholeiite flows of Chemical Type D. High olivine normative flows of Chemical Type - G and H show 1095°C and 1092°C respectively. Whereas, high hypersthene normative tholeiite flow of Chemical me C shows temperature of 1187°C. Data plots disposition over iron-titanium oxide equilibration temperature vs – logfo2, diagram for Mandla lava flows and other parts of the Deccan (Igatpuri, Mahabaleshwer, Nagpur and Sagar areas) revealed that tholeiitic (evolved) basalt of the eastern Deccan volcanic province formed at high temperatures whereas, picritic (primitive) lavas of Igatpuri and tholeiitic basalt of Mahabaleshwar areas were formed at low temperatures. Mahabaleshwer basalts follow FMQ (fayalite-magnetite-quartz) buffer curve but, plots of the Mandla basalts lie above this curve indicating higher temperatures of crystallisation of ilmenite-titanomagnetite than that of the lava flows from other parts of Deccan 'Raps. The eastern Deccan Traps are most evolved types of lava as characterised by its low Mg-number and Ni content whereas, Igatpuri lava flows are picritic (primitive), having high Mg-number and Ni contents. Temperature vs FeO + Fe2O3 / FeO + Fe2O3 + MgO ratio data plots for Mandla and other Deccan lava flows and liquidus data for Hawaiian tholeiites, indicated that Igatpuri basalts lie parallel to the liquidus line of Hawaiian tholeiite but at lower temperatures. Large data plots of Mandla lava flows lie along the liquidus line of the Hawaiian lava. The highly vesicular nature of compound lava flows having large amount of volatile is responsible for low temperature values whereas, lava flows represented by high temperatures show high modal values of glass and opaque minerals.  相似文献   

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