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1.
Four volcanoes form Terceira, one of the islands of the Azores group; three contain both basaltic and peralkaline and one only peralkaline rocks. A recently active basaltic fissure zone trends NW-SE across the island.The rocks fall into the alkaline olivine basalt suite although some young basalts are of transitional affinity. The geochemistry shows two general basaltic series: 1) undersaturated, found in lavas of the oldest volcano and in some recent fissure zone basalts and hawaiites; 2) saturated, found in the younger basaltic lavas.Since the emergence of Terceira there has been a contemporaneity of basalt and salic peralkaline lavas. The younger rocks show a bimodal composition distribution, the most voluminous compositions being alkali olivine basalt and comendite with negligible volume in the benmoreite-trachyte range. Two processes appear viable for the derivation of voluminous oversaturated peralkaline rocks: 1) partial melting of upper mantle material giving small magma batches of contrasting composition or 2) fractionation from a transitional basaltic parental magma.Now at Department of Geology, Victoria University of Wellington, New Zealand.  相似文献   

2.
Cainozoic igneous activity in eastern australia   总被引：4，自引：0，他引：4  
During the Cainozoic there was widespread basaltic igneous activity in eastern AustraIia along and adjacent to the Eastern Highlands. The activity commenced about 70 m.y. ago, and has continued through the Cainozoic at a nearly constant rate. More than fifty igneous provinces are recognized. Each province consists of similar volumes of volcanic material and crustal intrusives, the volcanism generally lasting less than 5 m.y. and resulting in lavas that cover a region 50–200 km across. Three main types of igneous province are recognized:
1. (1) central volcano provinces, which are composed predominantly of slightly undersaturated to saturated basaltic lavas, but with some felsic flows and intrusions;
2. (2) lava field provinces, which consist of basaltic flows, commonly strongly undersaturated;
3. (3) a single, mafic, strongly undersaturated, high potassium province.
Within each type of province the average potassium content of the lavas has not changed with time. Most provinces are consistent with an apparent migration of volcanism westward at 5 mm/year across the Eastern Highlands. This migration is thought to be caused by crustal processes. The distribution and age of the slightly undersaturated provinces suggest a migration of the centre of volcanism southward at 66 ± 5 mm/year. Magmas for these provinces are thought to originate from a magma source or sources, with a limited latitudinal extent, within the asthenosphere. The migration is considered to be related to the movement of the Indian (Australian) lithospheric plate relative to the underlying asthenosphere.  相似文献   

3.
Volcanic activity has produced Late Tertiary and Quaternary cinder cones and flows between the Snake River Plain, U.S.A. and the Yukon Territory, Canada. The rock types include basanites, alkali olivine basalts, high-iron basalts, hawaiites, ankaramites, nephelinites, and olivine tholeiites. The alkali olivine basalts, basanites and hawaiites sampled are chemically similar to rocks from the mid-Atlantic islands. Associated with the volcanic rocks are xenoliths of ultramafic rocks, gabbros, granites and granulites.Seismic data indicate that the Moho throughout the region dips eastward at a very shallow angle. The low velocity zone has been located beneath southern British Columbia and displays a topographic high trending northwest-southeast. The nephelinite was erupted from near the crest of this high with less undersaturated lavas erupted from along its flanks.The suite of ultramafic xenoliths spans a greater variety of rock types than can be generated by maximum amounts of partial melting of a uniform source material to produce the lavas in the region. Calculated residual olivine compositions in equilibrium with the magmas at low velocity zone depths and liquidii temperatures are more iron-rich than the typical lherzolite xenolith olivine. This suggests that the residua from the partial melting episodes which produced the volcanic rocks are different from the upper mantle lid above the low velocity zone as represented by the ultramafic xenoliths.  相似文献   

4.
The study of a volcanic series from the island of Gran Canaria (Canary Islands) in which alkaline and peralkaline, saturated and undersaturated rocks coexist, is reported here. Materials with high volatile content (ignimbritic trachytes) were first emitted and the series ended with the eruption of phonolitic lavas. The average peralkalinity index in these rocks is typically about 1.0 and, therefore, peralkaline rocks coexist with non-peralkaline ones. However, a maximum in peralkalinity is found in the ignimbritic rocks of the lower part of the series. In spite of the evident acid peralkaline tendencies of these rocks, it does not seem appropriate to classify them as pantellerites or comendites. Nor are they consistent with the genetic processes proposed for rocks of similar composition and oceanic environment.The crystallization of the feldspars controls the variation trends among the different magmas but the fractionation alone does not sufficiently explain the genesis of successive fluids. Various factors seem to point to the important role which a gas-transfer process causing a geochemical stratification inside the magmatic chamber may have played.The occurrence of peralkaline silicics at Gran Canaria, which is located for away from the active Mid-Atlantic ridge, is not related to transitional basalts. These rocks are a deviation from the main undersaturated alkalic trend which characterizes the volcanism of the Canary Islands, their genesis being related to the realization of favourable local volcanic conditions.  相似文献   

5.
The petrology and geochemistry of the Azores Islands   总被引：7，自引：0，他引：7  
Forty lavas from the Azores Islands have been analyzed for 87Sr/86Sr ratios, major elements, first transition series metals, and LIL elements. The samples belong to the alkali basalt magma series but range from transitional hy-normative basalts from Terceira to basanitoids from Santa Maria. Differentiated lavas include both typical trachytes and comenditic trachytes and comendites. Major and trace element concentrations define smooth trends on variation diagrams, and these trends can be related to phases crystallizing in the rocks. Systematic interisland differences are also apparent in these variation diagrams. LIL element concentrations in island basalts are roughly twice as high as those in tholeiites from the adjacent Mid-Atlantic Ridge which transects the Azores Plateau. 87Sr/86Sr ratios in lavas from 6 of the 9 islands range from 0.70332 to 0.70354, a range similar to that found in tholeiites from the Mid-Atlantic Ridge transect of the Azores Plateau. This suggests that lavas from these islands and this portion of the Mid-Atlantic Ridge may be derived from a similar source. However, lavas from the islands of Faial and Pico have 87Sr/86Sr ratios up to 0.70394 and ratios in Sao Miguel lavas range up to 0.70525, suggesting basalts from these islands are derived from a chemically distinct source. Differences in the average LIL element concentrations of the least fractionated ridge tholeiites from the Azores Plateau and alkali basalts from the islands result from differences in extent of partial melting and residual mineralogy. The alkali basalts are derived by roughly half as much melting as are the tholeiites. Trace element concentrations in Azores peralkaline lavas preclude their derivation by partial melting of peridotitic mantle or basaltic crust; rather the data suggest they are produced by fractional crystallization of a basaltic parent.  相似文献   

6.
P. Thy 《Lithos》1991,26(3-4):253-269
The south-eastern propagating rift zone of Iceland shows a progression from tholeiitic, to transitional, and mildly alkalic basalts going toward the front of the propagator. A petrogenetic model has been formulated based on low and high pressure melting experiments. The evolution of the magmas behind the front of the propagating rift is dominated by near surface processes (e.g., Katla and Hekla volcanic systems). Compared with the one atmosphere liquid line of descent, the Vestmannaeyjar lavas, erupted at the front of the propagating rift, have systematically higher Al2O3 and Na2O contents consistent with the experimentally determined effects of high pressure, Labradoritic plagioclase megacrysts, which occur in the early phase of the Surtsey eruption at the front of the propagator, are consistent with the predicted effect of high pressure on plagioclase composition. On the other hand, augites similar to those of the high pressure experiments are unknown among the phenocryst and xenolith assemblages of the Vestmannaeyjar lavas (and other Icelandic lavas). Petrographic evidence points toward a high water content in the evolved lavas of the Vestmannaeyjar. A relatively high water activity and high pressure in the magma chambers at the front of the propagator could have caused a significant suppression of the liquids temperature, in particular for plagioclase. Seismic and magnetotelluric evidence suggest that magma chambers behind the propagating front occur at the depth equivalent to 2–3 kbar of pressure. At the front, magma chambers have been located by geophysical evidence at significantly greater depths equivalent to 3–8 kbar. The lavas erupted at the front of the propagator are located to the alkalic side of the thermal divide and, therefore, can be expected to evolve toward nepheline saturation under slightly hydrous conditions. The most evolved of these lavas are of ferrobasaltic compositions and may be saturated with augite. The lavas erupted behind the front are located to the transitional side of the divide and evolve toward quartz saturation under essentially anhydrous conditions. In contrast to the Vestmannaeyjar lavas, the lavas behind the propagator often contain augite as a phenocryst. It is concluded that the chemical variation observed along the south-eastern propagating rift of Iceland is an effect of primary chemical features and that water plays a significant role on the liquid lines of descent at the front of the propagator.  相似文献   

7.
Unspiked K–Ar ages, petrological, geochemical and isotopic data are reported on samples from southern Libya (Wan an Namous—Al Haruj area). The Wan an Namous intracaldera cone dated at 0.2?Ma consists of unusually undersaturated foidite, representing the most extreme compositions among Libyan and Tibestian lavas. A basanitic and a basaltic lava flow located north-west of Wan an Namous, and probably belonging to the Al Haruj volcanic field, were dated at 5.1?Ma and 8.1?Ma. These data extend the range of ages previously reported for Al Haruj lavas. REE and multi-element patterns are typical of alkaline intraplate magmas. Sr ratios range from 0.70314 to 0.70812, whereas Nd ratios are very homogeneous (0.51290–0.51293). Pb ratios (19.231?<?206Pb/204Pb?<?19.547, 15.607?<?207Pb/204Pb?<?15.641 and 38.859?<?208Pb/204Pb?<?39.242) are typical of HIMU-FOZO compositions. Such isotope characteristics are very similar to those available on two Gharyan (northern Libya) lavas and largely overlap those of Hoggar and Cameroon Line alkaline rocks. These lavas were produced by low and variable degrees of partial melting of a garnet- and amphibole-bearing mantle source, constraining the depth of melting between 80 and 150?km. Crustal contamination was also probably involved for the oldest sample. Mineral compositions of a dunite–harzburgite xenolith clearly indicate that the lithospheric mantle was affected by partial melting and metasomatic processes by magmatic liquids, probably associated with the genesis of Cenozoic lavas. Lithospheric delamination and asthenospheric upwelling, due to the reactivation of lithospheric megastructures induced by the Africa-Europe convergence, could represent a model for the genesis of Libyan lavas, as in Hoggar.  相似文献   

8.
Recent paleomagnetic and K-Ar age dating studies in the Caroline Islands show that the high volcanic islands of Truk, Panape and Kusaie formed at the same shallow latitude and display a progression of ages increasing westward from 1 m.y. (Kusaie), through 6 m.y. (Ponape) to 12 m.y. (Truk). These data suggest that the islands were formed by a young hotspot or melting anomaly. One hundred and fifty volcanic rocks from these three islands have been analysed for major and 15 trace elements and results indicate that shield-building lavas on each island (1) evolved by low-pressure open-system fractional crystallisation of a small range of partial melts and (2) became more alkaline with time from Truk to Kusaie. The secular geochemical variation along the Caroline chain may represent a hotspot trace whose magma production was slowly declining over the past 14 m.y.  相似文献   

9.
The Vulsinian lavas are dominated by a suite of undersaturated leucite-bearing basic to intermediate compositions. The remaining lavas are mainly oversaturated and have shoshonitic affinities. One hundred and thirty-five samples have been analysed for major elements and most for 20 trace elements. Twenty-seven lavas have been analysed for REE. They are all perpotassic (for the undersaturated lavas: K2O/Na2O=2–8) and have very high LIL element concentrations, (e.g. Rb=400–800 ppm, Th=25–150 ppm, REE/REEcho=c.200, (LREE/HREE)cho=c.20) even in the most basic rocks.The undersaturated lavas appear to be interrelated by fractional crystallization of cpx±olivine (from 14 to 11 wt.% CaO), cpx+leu±plg±mica (from 11 to 8 wt.% CaO), cpx+leu+plg+apa+magnetite±mica (from 8 to 5 wt.% CaO), and additional sanidine (or hyalophane)±haüyne (from 5 to 3 wt.% CaO). The saturated lavas and the few slightly undersaturated shoshonite basalts are thought to be evolved from the undersaturated magma(s) by crustal contamination or mixing with silica-rich magmas. The parental Vulsinian magma having: Mg-value=c.73, Cr=300–700 ppm, Ni=100–125 ppm, Sc= 40–50 ppm, Fo89–92, Di77–97 approximates a primary, mantle-derived liquid. Enrichment in LIL elements (incl. REE) and LREE/HREE suggest a small degree of partial melting from fertile mantle; whereas the low concentrations of Na, Ti and P suggest larger degrees of partial melting. This indicates that either the primary magma or the parental mantle was metasomatized by a fluid, which previously equilibrated with subducted continental material. This model agrees with published high 18O, high 87Sr/86Sr and low 143Nd/144Nd.  相似文献   

10.
The volcanic rocks of the South Atlantic Ocean islands of Fernando de Noronha and Trindade comprise a diverse magmatic series ranging from nephelinites and basanites to phonolites and, on Fernando de Noronha, trachytes. All rock types are highly silica undersaturated with the exception of Fernando de Noronha trachytes_, and have high abundances of incompatible trace elements and strongly LREE (light rare earth element)-enriched REE patterns. Crystal fractionation of parental basanitic magmas produced evolved phonolites and trachytes which display severe trace-element fractionation, even among trace elements (Nb, Ta, Zr, Hf) which normally behave highly incompatibly during crystallisation of alkaline magmas. Moderately to highly evolved compositions develop strongly MREE (middle REE)-depleted REE patterns, and become increasingly depleted in elements such as Nb and, in particular, Ta. Ratios of Nb/Ta and Zr/Hf are highly fractionated in phonolites (60–65, 64–77 respectively in Fernando de Noronha phonolites) compared to ratios in basanites (14, 45 respectively). The compatibility of Nb, Ta, and the REE, and the strong fractionation of Nb/Ta and Zr/Hf ratios and the MREE, during crystallisation from basanite to phonolite are attributable to the crystallisation of small amounts (<5%) of sphene. Trace-element behaviour is relatively insensitive to the major phenocryst phases, and is controlled by minor phases in highly undersaturated alkaline suites. Incompatible trace-element ratios (e.g. La/Nb, Th/Ta) in nephelinites and basanites from Fernando de Noronha and Trindade are generally comparable with those in basaltic and hawaiitic OIB (ocean island basalt) lavas from other South Atlantic islands, but are distinct from those of Gough and Tristan da Cunha OIB. The mantle source for the highly undersaturated volcanism on Fernando de Noronha and Trindade is similar in trace-element characteristics to the typical OIB source which produces alkaline lavas with significant relative enrichment in Nb and Ta compared to other trace elements (as expressed by low La/Nb, Ba/Nb and Th/Ta ratios). The highly undersaturated nature of the magmas and the slight fractionation of some incompatible-element ratios (elevated Ba/Nb, Ba/Rb, Ba/Th etc.) is consistent with a smaller degree of melting of a typical OIB source, but with residual phlogopite in the source to account for significant K depletion and LIL-element fractionation.  相似文献   

11.
N.C. Ghose 《Lithos》1976,9(1):65-73
The Deccan basalts are essentially composed of saturated tholeiitic lavas. However, undersaturated basalts, nephelinites, carbonatites, intermediate and acid differentiates have also been encountered in parts of western India (Upper Traps). Such unusual rocks are broadly aligned in two major rift zones in western India, the Narbada-Son and Cambay grabens and the faulted west coast. These rocks are younger than the earliest tholeiitic eruptions of central India (Lower Traps), but there are evidences of renewed eruptions of tholeiitic basalts in parts of western India. The earliest eruptions of Deccan basalts of quartz tholeiite composition have been derived by high degrees of partial melting of peridotite at moderate depth (37–41 km). The undersaturated basalts and nephelinites were possibly generated by low degrees of partial melting of garnet peridotite in the low velocity zone along the two tectonically active belts (rifts) in western India. The undersaturated basalts were formed prior to the break-up of Gondwanaland, when a concentration of pressure developed at the tectonically weak zones. The renewed eruptions of saturated tholeiites are thought to be post-tectonic, resulting from the release of stress when the tectonic events had ceased.  相似文献   

12.
Study of the geochemical fingerprints of four geologically distinct suites of volcanic rocks on Cyprus are used to sketch a tectonic history of the island. Lavas from the Mamonia complex resemble alkalic within-plate basalts; lower pillow lavas and diabases of the Troodos Massif have features both of ocean-floor and island-arc tholeiites and could have been erupted in an interarc basin; the upper pillow lavas of the Troodos Massif resemble primitive tholeiitic basalts from island arcs; lavas from the Kyrenia range resemble transitional to alkalic within-plate basalts. The low TiO2 concentrations from the Troodos Massif may indicate a slow spreading rate. The Sr concentrations in the upper pillow lavas indicate an eruption at a maximum distance of 80 km above a Benioff zone. The results suggest formation of the Troodos Massif in the Campanian by spreading in an interarc basin followed by eruption of island-arc tholeiites. Obduction of continental material and ocean islands may have taken place in the Maestrichtian and Middle Miocene.  相似文献   

13.
The rift-related, seaward-dipping reflector sequence (SDRS)SE of Greenland consists of basaltic lavas that exhibit variabledegrees of magmatic differentiation, derived from a heterogeneousmantle source. Platinum-group elements (PGE) are used to provideinsights into the petrogenetic evolution of the SDRS, and tocharacterize the magma sources. Noble metal concentrations correlatewell with indicators for magmatic differentiation (mg-number,MgO), exhibiting two distinct trends. Concentrations of Ir,Ru and Rh tend to decrease with progressive differentiation,indicating compatible behaviour of these elements during fractionalcrystallization processes. The variation of Pt and Pd showssegmented trends. In primitive magmas, Pt and Pd are incompatibleand become enriched in the melt. The primitive magma is S undersaturated,despite derivation from a depleted mid-ocean ridge basalt sourceat a moderate degree of melting, reflecting enhanced S solubilityin the melt caused by high Fe content and elevated temperature.In the more evolved lavas, Pt and Pd decrease with decreasingMgO and mg-number. This indicates that S saturation had occurredwith Pt and Pd being incorporated in sulphides, which probablysegregated during ascent. Bulk partition coefficients for thePGE during partial melting are calculated based on data froma primitive basaltic unit with MgO  相似文献   

14.
STRONG  D. F. 《Journal of Petrology》1972,13(2):181-218
The four volcanic islands comprising the Comores archipelagoform a linear rise across the northern end of the MozambiqueChannel. Volcanism migrated westward from the now deeply dissectedand partially submerged Mayotte to the active Kartala volcanoon Grande Comore. The volcanic rocks on all the islands consistof silica–undersaturated basalts and their derivatives. Grande Comore is formed of two coaleseing shield volcanoes,La Grille in the north and Kartala in the south. The lavas ofKartala are entirely alkali basalts, with abundant ankaramiticand oceanitic varieties, containing an average of 3.7 wt percent normative nepheline. They represent a series resultingfrom low–pressure (< 8kb) fractional crystallizationof alkali basalt parent magma. The formation of this parentmagma can be explained as resulting from more than 10 per centpartial melting of garnet lherzolite upper mantle at pressuresaround 30 kb, with some high–pressure (> 25 kb) fractionationof harzburgite, garnet harzburgite, and either garnet wehrliteor ecogite, followed by polybaric fractionation of olivine±clinopyroxene during ascent. They La Grille lavas are basanitie, with an average of 11–5wt per cent normative nepheline and some normative leucite,and contain only olivine as an important phenocryst phase. Theyare explicable as originating from less than 10 per cent partialmelting of garnet I herzolite upper mantle, with substantialhigh–pressure (> 25 kb) fractionation of garnet wehrliteor eclogite. Interuption of this high–pressure(> 25kb) fractionation at different stages resulted in one trendof chemical variation, on which was superimposed a second trendby polybaric fractionation of lherzolite, wehrlite, and dunite,now found as inclusions in the lavas.  相似文献   

15.
The petrogenesis of leucite-bearing lavas from the Roman Region, Italy, is considered in the light of the petrochemistry and mineralogy of the Sabatini lavas, believed to be transitional between the mafic Alban Hills leucitites and the differentiated Vico and Vulsini suites.The Sabatini leucitites are a close variant of the Alban Hills analogues and are probably related to the same cycle of partial melting in the mantle, The prevalent compositional variation, from leucite-tephrite to leucite-phonolite compositions, conforms to the well-known variation of the Vesbian lavas and is attributed to crystal-liquid processes under a low-pressure volcanic regime. The differentiation trends of both the Sabatini and Vico lavas are related to Sabatini parental leucitite compositions, which interacted with recurrent pulses of leucitite liquid during differentiation in a shallow-level environment.The mineral chemistry of the Sabatini lavas reveals extensive and complex chemical zoning in both early and late-crystallized phases. Among the latter, poikilitic feldspars with exceptionally high Sr content (SrO=2–3% wt.) are reported. Early-crystallized phases, notably feldspars and pyroxene, are xenocrystal and indicate significant contamination of the lavas with disequilibrated mineral compositions. Consequently, genetic considerations based on isotopic data from these lavas are suspect. A review of these data is made and it is proposed that the mantle source material of the Roman Region lavas evolved as an open system under the influence of local concentrations of volatile and other mobile elements (Bailey, 1964, 1972, 1974).  相似文献   

16.
为开展松辽盆地深部长期观测、流体实验和探索白垩纪火山事件,利用松科二井东孔丰富、齐全的测井资料,对营城组火山岩岩性进行评价。通过测井响应特征分析发现,松科二井东孔营城组凝灰岩具有最强的放射性和导电性,高孔隙度的集块熔岩密度为低值,流纹岩表现出高密度和低导电性。利用测井交会图和成像识别模式,识别出松科二井东孔营城组火山岩以流纹岩、凝灰熔岩和集块熔岩为主,少量的凝灰岩。结合凝灰岩处测井曲线变化特点,证明了火山喷发间断的存在。流纹岩具有高碱、高Si、低Fe和低黏土矿物特征。T_2谱分析认为流纹岩有利于后期深部长期观测和流体实验的开展。研究成果对松科二井东孔后续火石岭组火成岩及整个松辽盆地火山岩研究具有一定的参考价值。  相似文献   

17.
The Austral Islands, a volcanic chain in the South-Central Pacific Ocean (French Polynesia) are composed mainly of alkali basalts and basanites with subordinate amounts of olivine tholeiites and strongly undersaturated rocks (phonolite foidites and phonolite tephrites). The basaltic rocks have geochemical features typical of oceanic island suites. The distribution of incompatible trace elements indicate that the lavas were derived from a heterogeneous mantle source. The chondrite-normalized patterns of the incompatible elements in basaltic rocks of the Austral Islands are complementary to those of island arc tholeiites. As supported by isotope data, the observed trace element heterogeneities of the source are probably due to mixing of the upper mantle with subducted oceanic crust from which island arc tholeiitic magma was previously extracted.  相似文献   

18.
Angelo Peccerillo 《Lithos》1998,43(4):267-279
The Pleistocene intra-Apennine volcanic (IAV) centres occurring east of the potassium-rich Roman comagmatic province show variable petrological and geochemical composition. Some rocks have a strongly undersaturated ultrapotassic kamafugitic affinity with K2O/Na2O=8–20, whereas the rocks from the southern center of Mt. Vulture are still strongly undersaturated in silica but are enriched in both Na2O and K2O with K/Na around unity. Carbonate-rich pyroclastic rocks, believed to represent carbonatitic magmas, are found in the IAV centers. Kamafugites have high abundances of LILE and high LILE/HFSE ratios, and their incompatible element patterns resemble closely those of ultrapotassic rocks from the adjoining Roman province. The Vulture volcanics also display high contents of LILE, but their LILE/HFSE ratios are intermediate between intraplate alkaline rocks and kamafugites. The carbonate-rich rocks exhibit an exotic mineralogy and high enrichments in LILE, which speaks for a carbonatitic affinity. However, they have similar incompatible element patterns but consistently lower abundances of almost all the elements than the associated silicate volcanics. These data favour the hypothesis that the IAV carbonate rocks may represent mixtures of silicate magmas and geochemically depleted carbonate material. The sedimentary carbonates that crop out extensively along the Apennine chain may be the source of barren carbonate material. Overall, geochemical data of IAV centres and of the rocks from the Roman province display strong geochemical and isotopic evidence of being generated in an upper mantle that was modified by addition of upper crustal material brought down by subduction processes. A possible exception is represented by Mt. Vulture which, however, occurs east of the main axis of the Apennines, on the western margin of the foreland Adria plate. The occurrence of strongly undersaturated alkaline rocks requires magma generation at high pressures and . This is in agreement with the hypothesis that subduction processes under the Apennines occurred by consumption of poorly hydrated thinned or delaminated continental crust.  相似文献   

19.
Quaternary and upper Tertiary lavas from the volcanic islands along the north coast of east New Guinea consist of pyroxene andesites, andesitic basalts, and basalts; many contain accessory olivine. The lavas of Mts Lamington, Yelia, and Victory, in southeast Papua and New Guinea, are rather more acid hornblende and lamprobolite andesites. Peralkaline obsidian and rhyolite occur at Fergusson and Dobu Islands, east of the Papuan mainland. Chemical analyses show that most of the lavas are of calc‐alkaline type, although some from Karkar and Long Islands appear to have tholeiitic tendencies.  相似文献   

20.
The 1971 Teneguía eruption is the most recent volcanic event of the Cumbre Vieja rift zone on La Palma. The eruption produced basanite lavas that host xenoliths, which we investigate to provide insight into the processes of differentiation, assimilation and magma storage beneath La Palma. We compare our results to the older volcano magmatic systems of the island with the aim to reconstruct the temporal development of the magma plumbing system beneath La Palma. The 1971 lavas are clinopyroxene-olivine-phyric basanites that contain augite, sodic-augite and aluminium augite. Kaersutite cumulate xenoliths host olivine, clinopyroxene including sodic-diopside, and calcic-amphibole, whereas an analysed leucogabbro xenolith hosts plagioclase, sodic-augite-diopside, calcic-amphibole and hauyne. Mineral thermobarometry and mineral-melt thermobarometry indicate that clinopyroxene and plagioclase in the 1971 Teneguía lavas crystallised at 20–45 km depth, coinciding with clinopyroxene and calcic-amphibole crystallisation in the kaersutite cumulate xenoliths at 25–45 km and clinopyroxene, calcic-amphibole and plagioclase crystallisation in the leucogabbro xenolith at 30–50 km. Combined mineral chemistry and thermobarometry suggest that the magmas had already crystallised, differentiated and formed multiple crystal populations in the oceanic lithospheric mantle. Notably, the magmas that supplied the 1949 and 1971 events appear to have crystallised deeper than the earlier Cumbre Vieja magmas, which suggests progressive underplating beneath the Cumbre Vieja rift zone. In addition, the lavas and xenoliths of the 1971 event crystallised at a common depth, indicating a reused plumbing system and progressive recycling of Ocean Island plutonic complexes during subsequent magmatic activity.  相似文献   

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