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1.
Magma type and tectonic setting discrimination using immobile elements   总被引：1，自引：0，他引：1  
Five minor and trace elements have been variously combined to produce a set of binary diagrams in addition to total alkali-SiO2 diagrams, that discriminate between fresh tholeiitic and alkali basalts. These diagrams are TiO2-Zr, TiO2-Y/Nb, P2O5-Zr, TiO2-Zr/P2O5, and Nb/Y-Zr/P2O5. A clear discrimination between alkaline and tholeiitic basalts can be obtained, although no meaningful separation can be made between the broad groups of oceanic and continental basalts, of either magma type, on the diagrams. As these elements (Ti, P, Zr, Y and Nb) are generally considered immobile during alteration processes, it should be possible to distinguish the magma type of ancient basic volcanics that have been subjected to submarine weathering, spilitization and low-grade metamorphism.  相似文献   

2.
The whole rock K-Ar ages of basalts from the South China Sea basin vary from 3.8 to 7.9 Ma, which suggest that intra-plate volcanism after the cessation of spreading of the South China Sea (SCS) is comparable to that in adjacent regions around the SCS, i.e., Leiqiong Peninsula, northern margin of the SCS, Indochina block, and so on. Based on detailed petrographic studies, we selected many fresh ba-saltic rocks and measured their major element, trace element, and Sr-Nd-Pb isotope compositions. Geochemical characteristics of major element and trace element show that these basaltic rocks belong to alkali basalt magma series, and are similar to OIB-type basalt. The extent of partial melting of mantle rock in source region is very low, and magma may experience crystallization differentiation and cu-mulation during the ascent to or storing in the high-level magma chamber. Sr-Nd-Pb isotopic data of these basaltic rocks imply an inhomogeneous mantle below the South China Sea. The nature of magma origin has a two end-member mixing model, one is EM2 (Enriched Mantle 2) which may be originated from mantle plume, the other is DMM (Depleted MORB Mantle). Pb isotopic characteristics show the Dupal anomaly in the South China Sea, and combined with newly found Dupal anomaly at Gakkel ridge in Arctic Ocean, this implies that Dupal anomaly is not only limited to South Hemisphere. In variation diagrams among Sr, Nd and Pb, the origin nature of mantle below the SCS is similar to those below Leiqiong peninsula, northern margin of the SCS and Indochina peninsula, and is different from those below north and northeast China. This study provides geochemical constraints on Hainan mantle plume.  相似文献   

3.
Analyses for Ti, Zr, Y, Nb and Sr in over 200 basaltic rocks from different tectonic settings have been used to construct diagrams in which these settings can usually be identified. Basalts erupted within plates (ocean island and continental basalts) can be identified using a Ti-Zr-Y diagram, ocean-floor basalts, and low-potassium tholeiites and calc-alkali basalts from island arcs can be identified using a Ti-Zr diagram (for altered samples) and a Ti-Zr-Sr diagram (for fresh samples). Y/Nb is suggested as a parameter for indicating whether a basalt is of tholeiitic or alkalic nature. Analyses of dykes and pillow lavas from the Troodos Massif of Cyprus are plotted on these diagrams and appear to the tholeiitic ocean-floor rocks.  相似文献   

4.
An analysis of the chemical characteristics of about 200 basaltic rocks of India indicate that the rocks of Pavagarh, Girnar, Rajmahal, Mundwara, Cuddapah and Panjal Trap form independent magmatic series of mixed type having entirely different chemical characteristics relative to the Deccan basalt. The tholeiitic and olivine or alkaline basalts do not seem to form independent magma types. The present study indicates that the rocks belonging to the above magma types coexist together and may represent differentiates of a single magma series.  相似文献   

5.
Alkali basalts of Dariganga contain inclusions of dunite, harzburgite, peridotite, and eclogite-like rock. The dunites and peridotites contain few trace elements, but these are somewhat more abundant in the eclogite-like rock, and the alkali basalts are rather rich in them. The trace elements show a more or less continuous change in abundance that can be related by recognized crystal-chemical laws to the major elements of the minerals that are present in the various rock types. Hypotheses for the origin of basaltic magma are discussed in relation to the trace-element distribution in the Dariganga rocks, but it is concluded that the available evidence does not permit any definite decision as to which is correct.  相似文献   

6.
In the Paleozoic basalts of Nova Scotia (Canada) metamorphosed to a greenschist facies grade, U and Th are closely associated with immobile elements (e.g. Zr and Nb). The coherence of these elements with K, typical of igneous rocks is, however, absent, U and Th are apparently not affected by greenschist facies metamorphism as are alkali and alkali earth elements and their variation thus reflects primary magmatic processes. It seems that during the early stages of metamorphism, U and Th were retained together with several other incompatible elements (Zr, Nb, La and Ce) in stable secondary phases or alternatively these elements may be held in primary accessory minerals such as zircon and apatite.  相似文献   

7.
The Mount Edziza Volcanic Complex in north-central British Columbia includes a group of overlapping basaltic shields, salic composite volcanoes, domes and small calderas that range in age from 7.5 Ma to less than 2000 years B.P. The volcanic assemblage is chemically bimodal, comprising voluminous alkali olivine basalt and hawaiite, a salic suite of mainly peralkaline trachyte and comendite, plus a relatively small volume of intermediate rocks (trachybasalt, tristanite, mugearite, benmoreite). The complex is the product of five cycles of magmatic activity, each of which began with alkali olivine basalt and culminated with the eruption of salic magma. The regular chemical variation shown by almost 100 major- and trace-element analyses suggests a genetic lineage between the basic and salic members of each cycle. Least-squares mathematical modelling, indicates that the salic rocks (trachyte and comendite) have formed by fractionation of observed phenocryst and cumulate nodule mineral phases from a common alkali olivine basalt parent magma.Hawaiite is thought to be a cumulate rock, formed by partial fractionation and feldspar accumulation within rising columns of primary alkali olivine basalt. Fractionation leading from alkali olivine basalt through trachybasalt and trachyte to comendite is believed to have taken place where primary basalt became trapped in large crustal reservoirs. The early removal of olivine, clinopyroxene and plagioclase, leading to a trachytic residuum, and subsequent fractionation of mainly alkali feldspar, leading to the peralkaline end members, is consistent with major- and trace-element variation and with isotopic and REE data.The chemical diversity of the complex is attributed to its location over a zone of crustal extension where mantle-derived basalt, trapped in large high-level reservoirs, underwent prolonged fractionation.  相似文献   

8.
Petrographic and geochemical data are given for some basaltic rocks from the Koynaghat, Ambaghat and Panvel sections of the western Deccan volcanic province. This study confirms geochemical features established earlier for the Deccan basalts but brings out minor additional characters. Mineralogical and major-element compositions of the basaltic flows from the Koyna and Panvel sections indicate tholeiitic affinity; the Ambaghat flows exhibit a slight affinity towards alkali basalt. Rare earth element (REE) distribution patterns and trace-element abundances suggest minor fractionation of olivine and plagioclase during the evolution of the flows. The general similarity of chemical and mineralogical features over a wide area and the lack of conspicuous inter-element relationships suggest that the flows reflect the combined effects of partial melting, minor mineral fractionation and selective crustal contamination.  相似文献   

9.
Rocks from Hakone volcano and the adjacent Hata and Aziro areas were studied to clarify the anomalous partition coefficient of Ce between olivine and groundmass in the basalts of Hakone. In addition, the most primitive basalt is more enriched in REE than the other basalts from this volcano.The REE features are well explained by a mechanical mixing model. The basaltic rocks from the volcano are mixtures of basic andesite magma and fragments of gabbroic rock. Most of minerals of phenocrystic size and glomeroporphyritic crystal aggregates in the basaltic rocks are xenocrysts and were derived from a gabbroic body lying beneath the volcano. This conclusion is consistent with major element, mineralogical and petrographical data.  相似文献   

10.
Mafic granulite xenoliths have been discovered in many volcanoes (especially alkali basalt and kimberlite) all over the world. They formed generally in lower crust, and recorded lots of in- formation on the lithosphere formation and crust-mantle interacti…  相似文献   

11.
Loihi Seamount is the southeasternmost active volcano of the Emperor-Hawaii linear volcanic chain. It comprises a spectrum of basalt compositional varieties including basanite, alkali basalt, transitional basalt and tholeiite. Samples from four dredge collections made on Scripps Institution of Oceanography Benthic Expedition in October 1982 are tholeiite. The samples include highly vesicular, olivine-rich basalt and dense glass-rich pillow fragments containing olivine and augite phenocrysts. Both quartz-normative and olivine-normative tholeiites are present. Minor and trace element data indicate relatively high abundances of low partition coefficient elements (e.g., Ti, K, P. Rb, Ba, Zr) and suggest that the samples were derived by relatively small to moderate extent of partial melting, of an undepleted mantle source. Olivine composition, MgO, Cr and Ni abundances, and Mg/(Mg+Fe), are typical of moderately fractionated to relatively unfractionated “primary” magmas. The variations in chemistry between samples cannot be adequately explained by low-pressure fractional crystallization but can be satisfied by minor variations in extent of melting if a homogeneous source is postulated. Alternatively, a heterogeneous source with variable abundances of certain trace elements, or mixing of liquids, may have been involved. Data for 3He/4He, presented in a separate paper, implies a mantle plume origin for the helium composition of the Loihi samples. There is little variation in the helium isotope ratio for samples having different compositions and textures. The helium data are not distinctive enough to unequivocally separate the magma sources for the tholeiitic rocks from the other rock types such as Loihi alkalic basalts and the whole source region for Loihi may have a nearly uniform helium compositions even though other element abundances may be variable. Complex petrologic processes including variable melting, fractional crystallization and magma mixing may have blurred original helium isotopic signatures.  相似文献   

12.
The interactions of seafloor hydrothermal fluid with igneous rocks can result in leaching elements from the rocks,creating potential ore-forming fluids and influencing the chemical compositions of near-bottom seawater.The hydrothermal alteration of plagioclase microphenocrysts and basaltic glass in the pillow basalts from one dredge station(103°57.62′′W,12°50.55′N,water depth 2480 m)on the East Pacific Rise(EPR)near 13°N were analyzed using a scanning electron microscope(SEM)and energy dispersive X-ray spectrometry(EDS).The results show that the edges of the plagioclase microphenocrysts and the basaltic glass fragments are altered but the pyroxene and olivine microphenocrysts in the interior of the pillow basalts appear to be unaffected by the hydrothermal fluids.In addition,our results show that the chemical alteration at the rims of the plagioclase microphenocrysts and the edges of basaltic glass fragments can be divided into separate types of alteration.The chemical difference in hydrothermal alteration of the plagioclase microphenocrysts and the basaltic glass indicate that different degrees of hydrothermal fluid-solid phase interaction have taken place at the surface of the pillow basalts.If the degree of hydrothermal fluid-solid phase interaction is relatively minor,Si,Al,Ca and Na diffuse from the inside of the solid phase out and as a result these elements have a tendency to accumulate in the edge of the plagioclase microphenocrysts or basaltic glass.If the degree of hydrothermal fluid-solid phase interaction is relatively strong,Si,Al,Ca and Na also diffuse from the inside of solid phase out but these elements will have a relatively low concentration in the edge of the plagioclase microphenocrysts or basaltic glass.Based on the chemical variation observed in the edges of plagioclase microphenocrysts and basaltic glass,we estimate that the content of Si,Al and Fe in the edges of plagioclase microphenocrysts can have a variation of 10.69%,17.59%and 109%,respectively.Similarly,the Si,Al and Fe concentrations in the edges of basaltic glass can have a variation of 9.79%,16.30%and 37.83%,respectively,during the interaction of hydrothermal fluids and seafloor pillow basalt.  相似文献   

13.
The Pb isotopic compositions and U and Pb concentrations of the lava series (alkali basalt to comendite) and of their plutonic xenoliths (gabbro to alkaline granite) of Ascension Island are reported. The data are used to evaluate the source of the xenoliths which formed two differentiation suites: the acidic and intermediate xenoliths together with most of the lavas on the one hand, and the gabbroic xenoliths and a basaltic tuff on the other hand. The Pb isotopic compositions imply a mantle origin for the source magmas of the xenoliths and confirm the possibility of generating granitic rocks in an oceanic environment by fractional crystallization of a mantle-derived magma whose geochemical and isotopic characteristics are comparable to the source magmas of oceanic island basalts.  相似文献   

14.
Volcanism related to subduction of the Philippine Sea (PHS) plate began in Central Kyushu at 5 Ma, after a pause of igneous activity lasting about 10 m.y. It formed a large volcano-tectonic depression, the Hohi volcanic zone (HVZ), and has continued to the present at a decreasing eruption rate. The products are largely andesite and dacite, which became enriched in K with time. The proportion of tholeiitic to calc alkalic rocks also increases with time. Calc-alkalic high-Mg basaltic andesites (YbBs) were erupted in the early stage of the HVZ activity (5–3 Ma), and high-alumina basalts (KjBs) were erupted in the later stage (2–0 Ma). In contrast to the basalts in the HVZ, Northwest Kyushu basalts (NWKBs) have been erupted on the backarc side of the HVZ since 11 Ma, and hence are not related to the PHS plate subduction. They are mainly high-alkali tholeiitic to alkali basalt that shows no notable chemical change with time. NWKB, YbB, and KjB have MORB-normalized incompatible-element spectra that differ from each other, as is well expressed in both Nb and Sr anomalies. The patterns of KjB and NWKB are typical of those for island-arc basalt (IAB) and ocean-island basalt (OIB), respectively. YbB shows a pattern intermediate between the two. We suggest that the magma source beneath the HVZ changed in composition from an OIB-type mantle to an IAB-type mantle as the subduction of PHS plate advanced. However, the magma source remained fertile under Northwest Kyushu. In order to explain the temporal change of source mantle beneath the HVZ, we propose a model for progressive contamination of the mantle wedge, in which three processes (contamination by a slab-derived component, subtraction of magma from the mantle, and mixing of the mantle residue and slab-derived component) are repeated as subduction continues. As long as the progressive contamination of mantle wedge proceeds, its trace-element composition converges at a steady-state value for a short period. This value does not depend on the initial composition of the mantle wedge but instead on the composition of the slab-derived component. The trace-element composition of the magma produced in such a mantle wedge approaches that of the slab-derived component with time, but the major-element composition is determined by the phase relations of mantle peridotite. The slab-derived component may be basaltic liquid that is partially melted from rutile-bearing eclogite.  相似文献   

15.
Toshio  Nozaka 《Island Arc》1997,6(4):404-420
Abstract Basic and ultrabasic xenoliths included in Cenozoic alkali basalts from the Kibi and Sera plateaus, Southwest Japan, can be classified into five groups on the basis of mineral association and texture. Their equilibration P-T conditions estimated from paragenesis and mineral chemistry indicate that the dominant rock type from the lower crust to upper mantle changes with increasing depth as follows: (i) pyroxene granulite (Group V) and meta-sediments; (ii) garnet gabbro (Group 111) and corundum anorthosite (Group IV); (iii) spinel pyroxenite (Group 11); and (iv) spinel peridotite and pyroxenite (Group I). Groups I1 and I11 show a lower degree of recrystallization than Groups I and V, and have similarities in composition and mineral chemistry to host basalts. Based on these facts along with the P-T conditions of equilibration, Groups I1 and I11 are interpreted as formed from basaltic magma that intruded beneath the crust-mantle boundary at an early stage of the magmatism of the alkali basalts, where the lower crust and uppermost mantle had consisted of Group V and metasediments, and Group I, respectively. It follows that the crust has grown downward due to underplating of basaltic magma beneath the bottom of pre-existing crust. Group IV has commonly the same mineral assemblage, corundum + calcic plagioclase + aluminous spinel, and shows locally, nearby kyanite crystals, almost the same texture as fine-grained aggregates in a quartzite xenolith. The aggregates appear to have been formed by reaction between kyanite and host basalt, and accordingly Group IV is interpreted as formed by reaction between metasediments and basaltic magma at the time of the underplating. The Kibi, Sera and Tsuyama areas are distinguished from the areas nearby the Sea of Japan by the occurrence of the garnet gabbro and corundum anorthosite xenoliths, by the absence of the association of olivine + plagioclase in basic and ultrabasic xenoliths, and by the lower temperature of equilibration of basic xenoliths. From these facts it is stressed that in general the crust becomes thinner and geothermal gradient becomes higher towards the back-arc side. Such a regional variation in crustal structure must reflect the tectonic situation of Southwest Japan at the time of the magmatism of the alkali basalts, namely rifting and shallow-level magmatism at the back-arc side.  相似文献   

16.
The Fe/Mg+Fe) ratios (XFe) of the Quaternary basalts (SiO2 < 53 wt.%) in the Japanese arcs were examined. The XXFe of relatively magnesian basalts decreases from the volcanic front toward the Japan Sea across the arcs. Based on the partition coefficient of Mg-Fe2+ between olivine and liquid, it is suggested that all the basalts near the volcanic front, which are mostly tholeiitic basalts, are significantly fractionated, whereas many basalts near the Japan Sea, which are mostly alkali basalts, are little fractionated. The K2 O content in the primary basalt magmas increases toward the Japan Sea. Combining the XFe and K2 O data, it is suggested that relatively large amounts of tholeiitic magmas are produced near the volcanic front, but they fractionate during their ascent, whereas smaller amounts of alkali basalt magmas are formed near the Japan Sea, but they can ascend with less fractionation. The density of primary tholeiite magma is significantly larger than that of primary alkali basalt magmas. It is most likely that primary tholeiite magmas cannot ascend beyond the upper crust and would fractionate to produce less dense tholeiitic magmas near the volcanic front, whereas primary alkali basalt magmas can ascend through the upper crust without fractionation, as far as buoyancy is the principal ascending force. In the Japanese arcs, the stress field may be less compressional near the Japan Sea than near the volcanic front, so that magmas can ascend more rapidly in the latter region than in the former. These two factors may be responsible for the above mentioned chemical variations of basalt magmas across the arcs. The variation in volume of the Quaternary volcanic rocks across the arcs can be explained by the presence of a melt-rich zone above but nearly parallel to the subducted slab.  相似文献   

17.
Two volcanic zones (Bukavu and Kamituga) south of Lake Kivu (southeastern Zaire) are part of the western branch of the Eastern African rift. They were formed during three volcanic cycles, one pre-rift (70-7 Ma old) and the other two syn-rift (7.8-1.9 Ma old and 14,000 y.-sub-Recent, respectively), and evolved from quartz tholeiites of the pre-rift period to alkali basalts of the rift stage. The basaltic rocks, which strongly predominate, are compositionally similar to other rift-related basalts and also to oceanic-island rocks. Most of the basalts have undergone only limited fractional crystallization (5–10%) dominated by olivine and clinopyroxene. The distinct variations of incompatible elements even in rocks of very similar major-element composition imply that the basaltic rocks were derived from a heterogeneous source by variable degrees of melting. The inferred source composition closely resembles that of metasomatized peridotite xenoliths from alkali basalts.  相似文献   

18.
The distribution of Ti, Zr, Y, Sr, K, P and Nb has been investigated at 55 sample sites within a single basaltic flow, the Cliefden Outcrop, which has adjusted to low-grade metamorphism of the prehnite-pumpellyite facies. The cluster of points in the plot of Ti, Zr and Y as proposed by Pearce and Cann shows that the ratio of these elements has remained relatively constant after metamorphism and would therefore have significance in attempts at classification of magma type. Likewise a plot involving Nb, Y, Zr and P as proposed by Floyd and Winchester shows only restricted scatter and appears useful. However, plots involving Sr or K show a scatter that is clearly related to metamorphic processes. Consequently, plots involving these two elements are likely to be of little use in classification in this case.Low-grade metamorphism is common, variable and not always recognized. The study concerned one style of alteration and revealed problems in the application of some classification schemes. Consequently it is apparent that much more rigorous testing is needed before classification schemes can be generally applied to sequences of basaltic rocks.  相似文献   

19.
Sub-solidus, mass transfer experiments along a temperature gradient, in which water reacted with alkali olivine basalt, trachybasalt, and peridotite, have yielded acidic extracts directly from basic material. It is proposed that the process may also occur under magmatic conditions by progressive and efficient enrichment of the upper portions of a magma reservoir in the constituents of petrogeny’s residua via an aqueous fluid. This hypothesis could explain the production of silica-oversaturated rocks from nepheline-normative parent material, and their association with basalts of alkaline affinity but without volumetrically significant compositional intermediates. In the case of the trachybasalt experiment the transfer products were also peralkaline and the hypothesis may thus be extended to peralkaline oversaturated rocks.  相似文献   

20.
The lesser antilles — A discussion of the Island arc magmatism   总被引：1，自引：0，他引：1  
The active island arc of Lesser Antilles marks the junction between the Atlantic and Carribbean lithospheric plates. With the exception of the alkali basalts of Grenada, the volcanics of the arc can be regarded as belonging to the low-K, island arc, calc-alkaline suite. Although compositions ranging from basalt to rhyolite have been described, porphyritic andesite appears to be the dominant rock type on most volcanoes (intermediate centers). Variable amounts of basalt and basaltic andesite occur and rarely predominate over andesite (latter are basic centers), whereas the more silicic members are only occasionally found. The calc-alkaline suite is characterized by relatively high Al2O3 and CaO and low K2O, Rb and Ni. Variations, especially in the alkali elements, occur both with space and time. A characteristic feature of many of the volcanoes is the occurrence in the basalt and basaltic andesite volcanics of plutonic blocks, often showing cumulate textures. The blocks which ware composed of plagioclase — amphibole — olivine — clinopyroxene — magnetite are thought to be the products of fractionation. The differences between basic and intermediate centers is probably due to the frequency that the magma ascended to the surface or remained in high level chambers where fractionation occurred.  相似文献   

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