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1.
Kohala Volcano, the oldest of five shield volcanoes comprising the island of Hawaii, consists of a basalt shield dominated by tholeiitic basalt, Pololu Volcanics, overlain by alkalic lavas, Hawi Volcanics. In the upper Pololu Volcanics the lavas become more enriched in incompatible elements, and there is a transition from tholeiitic to alkalic basalt. In contrast, the Hawi volcanics consist of hawaiites, mugearites, and trachytes. 87Sr/86Sr ratios of 14 Pololu basalts and 5 Hawi lavas range from 0.70366 to 0.70392 and 0.70350 to 0.70355, respectively. This small but distinct difference in Sr isotopic composition of different lava types, especially the lower 87Sr/86Sr in the younger lavas with higher Rb/Sr, has been found at other Hawaiian volcanoes. Our data do not confirm previous data indicating Sr isotopic homogeneity among lavas from Kohala Volcano. Also some abundance trends, such as MgO-P2O5, are not consistent with a simple genetic relationship between Pololu and Hawi lavas. We conclude that all Kohala lavas were not produced by equilibrium partial melting of a compositionally homogeneous source.  相似文献   

2.
The Honolulu Volcanics comprises small volume, late-stage (post-erosional)vents along rifts cutting the older massive Koolau tholeüticshield on Oahu, Hawaii. Most of these lavas and tuff of theHonolulu Volcanics have geochemical features expected of near-primarymagmas derived from a peridotite source containing Fo87–89olivine; e. g. 100 Mg/(Mg + Fe2+) >65, >250 p. p. m. Ni,and presence of ultramafic mantle xenoliths at 18 of the 37vents. Consequently, the geochemistry of the alkali olivinebasalt, basanite, nephelinite and nepheline melilitite lavasand tuff of the Honolulu Volcanics have been used to deducethe composition of their mantle source and the conditions underwhich they were generated by partial melting in the mantle. Compositional trends in 30 samples establish that the magmaswere derived by partial melting of a garnet (<10 per cent)Iherzolite source, which we infer to have been carbon-bearing,from analogy with experimental results. This source was isotopicallyhomogeneous (Sr, Lanphere & Dalrymple, 1980; Pb, Sun, 1980;Nd, Roden et al., 1981), and we infer that the source was compositionallyuniform in all major-element oxides except TiO2, in compatibletrace elements (Sc, V, Cr, Mn, Co and Ni), and in highly incompatibletrace elements (P, Th, La, Ce). However, the source appearsto have been heterogeneous in TiO2, Zr, Hf, Nb, and Ta, elementsthat were not strongly incompatible during partial melting.Some nepheline melilitite samples may be derived from a sourcewith distinct Sc and heavy-rare-earth-elements (REE) abundances,or which had a phase or phases controlling the distributionof these elements. The relatively limited abundance range for several elements,such as Ti, Zr, Nb, is partly a consequence of the low degreesof melting inferred for the series (2 per cent for nephelinemelilitite, 11 per cent for alkali olivine basalt), which failedto exhaust the source in minor residual phases. We infer thatthese residual phases probably included phlogopite, amphibole,and another Ti-rich phase (an oxide?), but not apatite. In comparison with estimates of a primordial mantle compositionand the mantle source of mid-oceanic-ridge basalt the garnetperidotite source of the Honolulu Volcanics was increasinglyenriched in the sequence heavy REEs, Y, Tb, Ti, Sm, Zr, andHf all <P <Nd <Sr Ce <La <Nb Ta. A multi-stagehistory for the source of the Honolulu Volcanics is requiredbecause this enrichment was superimposed on a mantle that hadbeen previously depleted in incompatible elements, as indicatedby the relatively low 87Sr/86Sr ratio, high 143Nd/144Nd ratioand low contents of K, Rb, Ba, and Th. The composition of thesource of the Honolulu Volcanics differs from the source ofthe previously erupted tholeiitic shield. The modal mineralogyof the source of the Honolulu Volcanics is not represented inthe upper-mantle xenoliths, e. g. the garnet pyroxenite andolivine-poor garnet Iherzolite included within the lavas andtuff of the unit.  相似文献   

3.
Hawaiian volcanoes, dominantly shields of tholeiitic basalt,form as the Pacific Plate migrates over a hotspot in the mantle.As these shields migrate away from the hotspot, highly alkaliclavas, forming the rejuvenated stage of volcanism, may eruptafter an interval of erosion lasting for 0·25–2·5Myr. Alkalic lavas with geochemical characteristics similarto rejuvenated- stage lavas erupted on the sea floor north ofOahu along the Hawaiian Arch. The variable Tb/Yb, Sr/Ce, K/Ce,Rb/La, Ba/La, Ti/Eu and Zr/Sm ratios in lavas forming the NorthArch and the rejuvenated-stage Honolulu Volcanics were controlledduring partial melting by residual garnet, clinopyroxene, Fe–Tioxides and phlogopite. However, the distinctively high Ba/Thand Sr/Nd ratios of lava forming the North Arch and HonoluluVolcanics reflect source characteristics. These characteristicsare also associated with shield tholeiitic basalt; hence theyarise from the Hawaiian hotspot, which is interpreted to bea mantle plume. Inversion of the batch melting equation usingabundances of highly incompatible elements, such as Th and La,requires enriched sources with 10–55% clinopyroxene and5–25% garnet for North Arch lavas. The 87Sr/86Sr and 143Nd/144Ndratios in lavas forming the North Arch and Honolulu Volcanicsare consistent with mixing between the Hawaiian plume and adepleted component related to mid-ocean ridge basalts. Specifically,the enrichment of incompatible elements coupled with low 87Sr/86Srand high 143Nd/144Nd relative to bulk Earth ratios is best explainedby derivation from depleted lithosphere recently metasomatizedby incipient melt (<2% melting) from the Hawaiian plume.In this metasomatized source, the incompatible element abundances,as well as Sr and Nd isotopic ratios, are controlled by incipientmelts. In contrast, the large range of published 187Os/188Osdata (0·134–0·176) reflects heterogeneitycaused by various proportions of pyroxenite veins residing ina depleted peridotite matrix. KEY WORDS: Hawaiian plume; Honolulu Volcanics; North Arch; plume–lithosphere interaction; rejuvenated stage; trace element geochemistry; alkalic lavas  相似文献   

4.
Kahoolawe Island, Hawaii (18×11 km), is a basaltic shield volcano with caldera-filling lavas, seven identified postshield vents, and at least two occurrences of apparent rejuvenated-stage eruptive. We examined 42 samples that represent all stages of Kahoolawe volcano stratigraphy for their petrography, whole-rock major-and trace-element contents, mineral compositions, and K–Ar ages. The two oldest shield samples have an average age of 1.34±0.08 Ma, and four postshield samples (3 are alkalic) average 1.15±0.03 Ma; ages of 1.08 and 0.99 Ma for two additional tholeiitic samples probably are minimum ages. Whole-rock major- and trace-element and mineral compositions of Kahoolawe shield and caldera-fill laves are generally similar to the lavas forming Kilauea and Mauna Loa tholeiitic shields, but in detail, Kahoolawe shield lavas have distinctive compositions. An unusual aspect of many postshield Ka-hoolawe lavas is anomalously high REE and Y abundances (up to 200 ppm La and 175 ppm Y) and negative Ce anomalies. These enrichments reflect surficial processes, where weathering and soil development promoted REE-Y transport at the weathering front. Major element abundances (MgO, 10–6 wt.%) for shield and caldera-fill basalts are consistent with fractionation of ol+px+pl in frequently replenished magma reservoirs. In general, tholeiitic basalts erupted from late vents are higher in SiO2 than the shield lavas, and temporal differences in parental magma compositions are the likely explanation. Alkalic basalts that erupted from vents are comparable in composition to those at other Hawaiian volcanoes. Trace-element abundance ratios indicate that alkalic basalts represent either relatively lower degrees of melting of the shield source or a distinct source. Apparent rejuvenated-stage basalts (i.e., emplaced after substantial Kahoolawe erosion) are tholeiitic, unlike the rejuvenated-stages at other Hawaiian volcanoes (alkalic). Kahoolawe, like several other Hawaiian volcanoes, has intercalated tholeiitic and alkalic basalts in the postshield stage, but it is the only volcano that appears to have produced tholeiitic rejuvenated-stage lavas.  相似文献   

5.
Previous studies of alkalic lavas erupted during the waning growth stages (<0.9 Ma to present) of Haleakala volcano identified systematic temporal changes in isotopic and incompatible element abundance ratios. These geochemical trends reflect a mantle mixing process with a systematic change in the proportions of mixing components. We studied lavas from a 250-m-thick stratigraphic sequence in Honomanu Gulch that includes the oldest (1.1 Ma) subaerial basalts exposed at Haleakaka. The lower 200 m of section is intercalated tholeiitic and alkalic basalt with similar isotopic (Sr, Nd, Pb) and incompatible element abundance ratios (e.g., Nb/La, La/Ce, La/Sr, Hf/Sm, Ti/Eu). These lava compositions are consistent with derivation of alkalic and tholeiitic basalt by partial melting of a compositionally homogeneous, clinopyroxene-rich, garnet lherzolite source. The intercalated tholeiitic and alkalic Honomanu lavas may reflect a process which tapped melts generated in different portions of a rising plume, and we infer that the tholeiitic lavas reflect a melting range of 10% to 15%, while the intercalated alkalic lavas reflect a range of 6.5% to 8% melting. However, within the uppermost 50 m of section. 87Sr/86Sr decreases from 0.70371 to 0.70328 as eruption age decreased from 0.97 Ma to 0.78 Ma. We infer that as lava compositions changed from intercalated tholeiitic and alkalic lavas to only alkalic lavas at 0.93 Ma, the mixing proportions of source components changed with a MORB-related mantle component becoming increasingly important as eruption age decreased.  相似文献   

6.
The Koloa Volcanic Suite of Kauai, Hawaii   总被引：3，自引：3，他引：3  
The Koloa lavas of the post-erosional volcanic suite of Kauaivary in composition from melilitite to alkali olivine basalt.87Sr/86Sr varies from {small tilde}0.7030 to 0.7033, the rangebeing similar to that of other Hawaiian post-erosional suites.The chemical composition within single lava flows is variableand forms major-element trends that are different from the mainKoloa trend. The main trend can be related to increased degreesof partial melting during decreasing pressure. The combinedevidence from Sr and Nd isotopes and rare earth elements (REEs)could suggest that an ocean island basalt (OIB) plume was infiltratedby a carbonate-rich melt with a high concentration of Sr andREEs derived from the mid-ocean ridge basalt (MORB) source.However, the Pb-isotope systematics of the Hawaiian islandsdo not suggest that a MORB source was involved during the generationof the posterosional lavas. Instead, we suggest that the Hawaiiantholeiitic and nephelinitic magma suites are derived from azoned plume with a low 87Sr/86Sr core and a high 87Sr/86Sr concentricmargin. The trends of the single flows and their variation inisotope ratios suggest that they formed from magmas accumulatedmainly from the central core of the zoned plume.  相似文献   

7.
Kahoolawe volcano (～10×17 km) forms one of the eight major Hawaiian islands. Access for geologic sampling has long been restricted due to military and preservation policies. However, limited visits to Kahoolawe in the 1980s yielded >200 samples, many of which have since been used to study the volcano within the framework of Hawaiian shield and mantle source geochemistry, petrology, mineralogy, and igneous processes.Kahoolawe is a tholeiitic shield with tholeiitic caldera-filling lavas, and at least seven postshield vents that erupted tholeiitic and (sparse) alkalic lavas. On smaller scales are a gabbro intrusion and ultramafic and gabbroic xenoliths in some postshield lavas. There is no evidence for rejuvenated volcanism. In its structural setting, Kahoolawe lies along the “Loa” trend of Hawaiian shields.Major element compositions of shield and caldera-filling lavas are similar and cluster at ～6–7 wt% MgO, range from ～5.5 to 16 wt% MgO, and include ～9 wt% MgO examples that can be modeled as parental to the evolved lavas. For example, least squares mass balancing demonstrates that from ～15% to 30% crystallization of olivine (±orthopyroxene), clinopyroxene, and plagioclase accounts for the ～5.5–6 wt% MgO range of tholeiitic lavas. Greater differentiation occurred in the gabbro (diabasic) intrusive body as a segregation vein with ～2.9 wt% MgO, and extreme differentiation produced local, small-volume rhyolitic melts that segregated into lava vesicles. Postshield lavas are mainly tholeiitic, have ～5–7 wt% MgO, and overlap shield compositions. Some are alkalic, as low as ～3.9 wt% MgO (hawaiite), and can be modeled as liquids after a ～9 wt% MgO alkalic magma crystallized ～30% olivine, clinopyroxene, plagioclase, and magnetite.Important aspects of Sr, Nd, Hf, and Pb isotopic ratios in Kahoolawe shield and caldera-filling lavas are slightly higher 87Sr/86Sr than in Koolau shield lavas (Oahu island; Makapuu-stage; e.g., Koolau mantle ‘endmember’) when compared at a given 143Nd/144Nd (e.g., ～0.7042 at 0.5128), 206Pb/204Pb largely at the low end of the range for Hawaiian shields (e.g., ～18), and εHf generally comparable to the values of other Hawaiian shields and ocean islands (e.g., εHf 8 at εNd 4). The isotopic ratios overall suggest small-scale source heterogeneity, considering the island size, and that Kahoolawe shield and caldera lavas were derived from a Hawaiian plume source containing recycled oceanic crust of gabbro and sediments. Based on certain geochemical indicators, however, such as Ce/Sr and La/Nb vs. 87Sr/86Sr, the source contained slightly less gabbro component than other shield sources (e.g., Koolau). Isotopic data for Kahoolawe postshield lavas are scarce, but those available generally overlap the shield data. However, ratios among certain alteration-resistant incompatible trace elements (e.g., Zr/Nb) discriminate some postshield alkalic from shield lavas. But because the different ratios for those postshield lavas can be explained by smaller partial-melting percentages of the shield source and by differentiation, neither isotopes nor trace elements identify postshield magmas as originating in a source unlike that for the shield lavas.  相似文献   

8.
Geochemical and Isotopic Evolution of Loihi Volcano, Hawaii   总被引：2，自引：6，他引：2  
A 680m thick section from the deeply dissected east flank ofLoihi Volcano was sampled using the Pisces V submersible toevaluate the volcano's geochemical evolution. Three types oflavas were recovered: tholeiitic, weakly alkalic and stronglyalkalic. The ratio of alkalic to tholeiitic lavas varies systematicallywith depth, from predominantly alkalic at the base of the sectionto tholeiitic at the top. Glasses from these rocks have similarratios of highly incompatible elements and Pb, Sr and Nd isotopes,but distinct ratios of highly to moderately incompatible elements.Partial melting modeling indicates that these tholeiitic andalkalic lavas could be derived by variable degrees of partialmelting of a slightly heterogeneous source. Many distinct parentalmagmas were generated for each rock type during the 100–150k.y. that the east flank section was formed. Crystal fractionationand olivine accumulation were the dominant processes controllingcompositional variation among lavas of the same rock type. Magmamixing features were observed in only a few of the lavas collected. Loihi typifies the preshield stage of Hawaiian volcanism whenthe volcano drifts closer to the focus of the hotspot. The compositionalvariation in Loihi's east flank section, which may represent40% of the volcano's extrusive history, is consistent with thepredicted increase in partial melting during this drift. Thetransition from dominantly alkalic to tholeiitic volcanism onLoihi was fitful but relatively rapid and is now nearly complete.This transition is the opposite of that which occurs duringthe post-shield stage of Hawaiian volcanism as the volcano migratesaway from the hotspot focus. Loihi's tholeiitic lavas overlap in ratios of incompatible traceelements and Pb, Sr and Nd isotopes with lavas from its moreactive neighbor, Kilauea. The small differences in major elementcontents between lavas from these adjacent volcanoes can beexplained by high-pressure orthopyroxene fractionation of Loihimagmas, which may be a consequence of a low magma-supply rate,or by slightly shallower depths of melt segregation for Kilaueamagmas. KEY WORDS: Loihi volcano; Hawaii; geochemistry; Sr-Nd-Pb isotopes  相似文献   

9.
Three main groups of lavas are exposed on islands of the Lau Ridge: the Lau Volcanic Group (LVG), 14.0–5.4 Ma, are predominantly andesite; Korobasaga Volcanic Group (KVG), 4.4–2.4 Ma, are predominantly basalt and Mago Volcanic Group (MVG), 2.0–0.3 Ma, are basalt-hawaiite. LVG and KVG lavas are mostly medium-K tholeiitic rocks with high LILE/HFSE ratios characteristic of islands ares, while MVG lavas are ne-normative alkalic rocks with high LILE and HFSE, characteristic of ocean island basalts. LVG lavas have high ?Nd (+8.0–+8.4) and low 87Sr/86Sr (0.70273–0.70349) similar to N-MORB, whereas KVG lavas have slightly more radiogenic values (?Nd=+7.5?+8.4; 87Sr/86Sr=0.70323-0.70397). MVG lavas form an isotopically distinct group having lower ?Nd (+4.6–+4.9) and (87Sr/86Sr ranging from 0.70347–0.70375). LVG lavas were erupted in a primary oceanic island arc (Vitiaz arc) during the Miocene. Basaltic lavas were derived by approximately 19% partial melting of mantle wedge peridotite with only minor subduction component. Andesites and dacites were produced by low-pressure plagioclase-pyroxene-titanomagnetite dominated crystal fractionation. KVG lavas were erupted during the period immediately prior to or during the initial stages of rifting in the Lau Basin, and, like LVG lavas, show significant chemical differences at the northern and southern ends of the Lau Ridge. Lavas at the northern end (type (ii)) appear to be derived from a more depleted source than LVG but with a greater amount of subduction component. Those at the southern end (type (i)) probably came from a slightly more enriched source with less subduction component. MVG basalts and hawaiites were derived from an enriched mantle with little or no subduction input. The hawaiites (type (i)) could not have been derived from the basalts (type (ii)), and the two magma types must have come from different sources, indicating mantle heterogeneity. The lack of subduction influence indicates the MVG lavas are tectonically unrelated to the present-day Tonga arc, and the lack of depletion indicators suggests they have tapped a different (new?) part of the mantle wedge. This may reflect introduction of sub-Pacific mantle through the present Tonga-Lau subduction system.  相似文献   

10.
The Okete Volcanics form small volume monogenetic volcanoes situated around the flanks of larger tholeiitic cones of the Plio-Pleistocene Alexandra Volcanics, in the back-arc tectonic environment of western North Island, New Zealand. The lavas and tuffs of the Okete Volcanics have compositions which include basanites, alkali olivine basalts, olivine tholeiites, and hawaiites. Most rocks have Mg numbers >66, >250 p.p.m. Ni, >500 p.p.m. Cr, and often contain ultramafic xenoliths, which indicate that they are very close to being primary magmas. The Okete Volcanics show geochemical trends, from basanite to hawaiite, of progressive depletion of both compatible and incompatible trace elements, progressive increase in Al2O3, and heavy REE and Y enrichment with crossingover REE patterns in the hawaiites. These geochemical trends can be accounted for by varying degrees of partial melting of a light REE enriched garnet peridotite with subsequent modification of the melts near source or during ascent by fractional crystallization of olivine and minor clinopyroxene. Mass balance calculations cannot quantitatively constrain the degree of partial melting or fractional crystallization, but nevertheless indicate that the Okete alkali olivine basalts, olivine tholeiites, and hawaiites have been derived by successively larger degrees of partial melting relative to basanites, and have also been progressively more modified by fractional crystallization than have the basanites. Sources of the alkalic melts lay at depths corresponding to >20 kb, and most of the ultramafic xenoliths, apart from some which may be cognate cumulates, are unrelated to the magmas that brought them to the surface. Magmas have changed in composition with time from older smaller-volume volcanoes of basanite or alkali olivine basalt compositions, to younger and more voluminous volcanoes which contain hawaiites. The geochemical trends shown by the Okete Volcanics and their spatial association with voluminous tholeiitic volcanism, are features which are different from those observed elsewhere in the Pliocene to Recent basaltic fields of northern North Island, and may be related to their unique tectonic setting, situated in a distinct structural domain.  相似文献   

11.
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12.
Abstract Basaltic basement has been recovered by deep-sea drilling at seven sites on the linear Ninetyeast Ridge in the eastern Indian Ocean. Studies of the recovered lavas show that this ridge formed from ~ 82 to 38 Ma as a series of subaerial volcanoes that were created by the northward migration of the Indian Plate over a fixed magma source in the mantle. The Sr, Nd and Pb isotopic ratios of lavas from the Ninetyeast Ridge range widely, but they largely overlap with those of lavas from the Kerguelen Archipelago, thereby confirming previous inferences that the Kerguelen plume was an important magma source for the Ninetyeast Ridge. Particularly important are the ~ 81 Ma Ninetyeast Ridge lavas from DSDP Site 216 which has an anomalous subsidence history (Coffin 1992). These lavas are FeTi-rich tholeiitic basalts with isotopic ratios that overlap with those of highly alkalic, Upper Miocene lavas in the Kerguelen Archipelago. The isotopic characteristics of the latter which erupted in an intraplate setting have been proposed to be the purest expression of the Kerguelen plume (Weis et al. 1993a,b). Despite the overlap in isotopic ratios, there are important compositional differences between lavas erupted on the Ninetyeast Ridge and in the Kerguelen Archipelago. The Ninetyeast Ridge lavas are dominantly tholeiitic basalts with incompatible element abundance ratios, such as La/Yb and Zr/Nb, which are intermediate between those of Indian Ocean MORB (mid-ocean ridge basalt) and the transitional to alkalic basalts erupted in the Kerguelen Archipelago. These compositional differences reflect a much larger extent of melting for the Ninetyeast Ridge lavas, and the proximity of the plume to a spreading ridge axis. This tectonic setting contrasts with that of the recent alkalic lavas in the Kerguelen Archipelago which formed beneath the thick lithosphere of the Kerguelen Plateau. From ~ 82 to 38 Ma there was no simple, systematic temporal variation of Sr, Nd and Pb isotopic ratios in Ninetyeast Ridge lavas. Therefore all of the isotopic variability cannot be explained by aging of a compositionally uniform plume. Although Class et al. (1993) propose that some of the isotopic variations reflect such aging, we infer that most of the isotopic heterogeneity in lavas from the Ninetyeast Ridge and Kerguelen Archipelago can be explained by mixing of the Kerguelen plume with a depleted MORB-like mantle component. However, with this interpretation some of the youngest, 42–44 Ma, lavas from the southern Ninetyeast Ridge which have206pb/204Pb ratios exceeding those in Indian Ocean MORB and Kerguelen Archipelago lavas require a component with higher206Pb/204Pb, such as that expressed in lavas from St. Paul Island.  相似文献   

13.
The Hawaiian–Emperor Seamount Chain (ESC), in the northernPacific Ocean, was produced during the passage of the PacificPlate over the Hawaiian hotspot. Major and trace element concentrationsand Sr–Nd–Pb isotopic compositions of shield andpost-shield lavas from nine of the Emperor Seamounts providea 43 Myr record of the chemistry of the oldest preserved Hawaiianmagmatism during the Late Mesozoic and Early Cenozoic (from85 to 42 Ma). These data demonstrate that there were large variationsin the composition of Hawaiian magmatism over this period. Tholeiiticbasalts from Meiji Seamount (85 Ma), at the northernmost endof the ESC, have low concentrations of incompatible trace elements,and unradiogenic Sr isotopic compositions, compared with youngerlavas from the volcanoes of the Hawaiian Chain (<43 Ma).Lavas from Detroit Seamount (81 Ma) have highly depleted incompatibletrace element and Sr–Nd isotopic compositions, which aresimilar to those of Pacific mid-ocean ridge basalts. Lavas fromthe younger Emperor Seamounts (62–42 Ma) have trace elementcompositions similar to those of lavas from the Hawaiian Islands,but initial 87Sr/86Sr ratios extend to lower values. From 81to 42 Ma there was a systematic increase in 87Sr/86Sr of boththoleiitic and alkalic lavas. The age of the oceanic lithosphereat the time of seamount formation decreases northwards alongthe Emperor Seamount Chain, and the oldest Emperor Seamountswere built upon young, thin lithosphere close to a former spreadingcentre. However, the inferred distance of the Hawaiian plumefrom a former spreading centre, and the isotopic compositionsof the oldest Emperor lavas appear to rule out plume–ridgeinteraction as an explanation for their depleted compositions.We suggest that the observed temporal chemical and isotopicvariations may instead be due to variations in the degree ofmelting of a heterogeneous mantle, resulting from differencesin the thickness of the oceanic lithosphere upon which the EmperorSeamounts were constructed. During the Cretaceous, when theHawaiian plume was situated beneath young, thin lithosphere,the degree of melting within the plume was greater, and incompatibletrace element depleted, refractory mantle components contributedmore to melting. KEY WORDS: Emperor Seamounts; Hawaiian plume; lava geochemistry; lithosphere thickness; mantle heterogeneity  相似文献   

14.
The Pleistocene to Holocene Honolulu Volcanic Series was eruptedfrom about 37 vents scattered over the older Koolau tholeiiteshield. The rocks of this series are compositionally zoned withrespect to the shield; near the Koolau caldera the predominantrocks are melilitenepheline basalts, but these give way outwardto nepheline basalts, and ultimately, at the apron of the shield,to alkalic olivine basalts. The xenoliths in these are likewisezoned: most of those in the caldera area consist of dunite,most of those at intermediate distances of lherzolite, and someof those in the apron of the shield consist of garnet pyroxeniteand peridotite. The zoning of the xenoliths, however, does notcoincide with that of the enclosing rocks. We believe that copiouseruption of Koolau tholeiite produced a lateral and verticalheterogeneity in the mantle beneath Oahu, and that the zoningin both Honolulu lavas and their xenoliths is caused by thatheterogeneity. The textures of the xenoliths indicate that thebasalts were mainly produced by fractional melting rather thanfractional crystallization. There is some evidence that thedunite xenoliths are mantle residua produced during the generationof the tholeiite, and that the Honolulu magmas were generatedat greater depths than the Koolau magmas, probably as a resultof elastic unloading.  相似文献   

15.
A comprehensive model is developed to explain the major, trace element and strontium and neodymium isotopic characteristics of alkali basalts from Hawaii. The model is similar to that of Chen and Frey (1983) in that it requires mixing of a small melt fraction of MORB-source material with another component to generate the alkalic suite of a particular Hawaiian volcano. It differs from the Chen and Frey model in that the other end-member must be different from primitive mantle if it is to be consistent with both trace element and isotopic data. Alkali basalts and tholeiites from Kauai analyzed in this study show a nearly complete transition in Sr and Nd isotopes. There is a relatively well-constrained array on a Nd-Sr isotope correlation plot that can be explained by two-component mixing of Kauai tholeiite magma and a small amount of melt of East Pacific Rise source rock. After corrections are made for fractional crystallization (involving primarily clinopyroxene and olivine), the Sr and Ba concentrations of Kauai lavas plot along mixing curves defined by the above sources, providing positive tests of the mixing hypothesis. Implications of this model are: (1) the main source of Hawaiian shield-building tholeiites is a mixture of subducted crust, primitive mantle and depleted asthenosphere that has been homogenized prior to melting, (2) early alkalic volcanism (as at Loihi seamount) will be characterized by greater isotopic heterogeneity than will late-stage alkali basalt production, and (3) there are two fundamentally distinct types of alkalic lavas erupted towards the end of magmatism at a given Hawaiian volcano. One represents smaller degrees of melting of the same source that generated shield-building tholeiites (Kohala-type); the other derives from the mixed source discussed in this paper (Haleakala-, Kauai-type).  相似文献   

16.
碧口群火山岩岩石成因研究   总被引：13，自引：1，他引：12  
新元古代(846~776Ma)碧口群火山岩喷发于大陆板内裂谷环境。该火山岩系以基性火山岩为主,酸性火山岩次之,中性火山岩少见。根据岩石地球化学数据,碧口群裂谷基性熔岩总体上属于低Ti/Y(<500)岩浆类型。元素和同位素数据表明,碧口群基性熔岩的化学变化不是由一个共同的母岩浆的结晶分异作用所产生。它们极有可能是源于地幔柱源(εNd(t)≈+3,87Sr/86Sr(t)≈0.704,La/Nb≈0.7)。地壳混染作用对于碧口群裂谷基性熔岩的形成有重要贡献。我们的研究揭示,碧口群火山岩存在空间上的岩石地球化学变化。东部红岩沟和辛田坝—黑木林地区的碧口群基性熔岩以拉斑玄武岩为主,产生于幔源石榴子石稳定区的高度部分熔融。相反,西部白杨—碧口地区的碧口群基性熔岩的母岩浆则是形成于幔源的尖晶石-石榴子石过渡带:碱性熔岩是产生于部分熔融程度较低的条件下,拉斑玄武质熔岩则是产生于部分熔融条件较高的条件下。它们经受了浅层位辉长岩质(cpx+plag±ol)分离作用,化学变异较大。  相似文献   

17.
Five mafic lava flows located on the southern flank of Mount Baker are among the most primitive in the volcanic field. A comprehensive dataset of whole rock and mineral chemistry reveals the diversity of these mafic lavas that come from distinct sources and have been variably affected by ascent through the crust. Disequilibrium textures present in all of the lavas indicate that crustal processes have affected the magmas. Despite this evidence, mantle source characteristics have been retained and three primitive endmember lava types are represented. These include (1) modified low-K tholeiitic basalt (LKOT-like), (2) typical calc-alkaline (CA) lavas, and (3) high-Mg basaltic andesite and andesite (HMBA and HMA). The Type 1 endmember, the basalt of Park Butte (49.3–50.3 wt% SiO2, Mg# 64–65), has major element chemistry similar to LKOT found elsewhere in the Cascades. Park Butte also has the lowest overall abundances of trace elements (with the exception of the HREE), indicating it is either derived from the most depleted mantle source or has undergone the largest degree of partial melting. The Type 2 endmember is represented by the basalts of Lake Shannon (50.7–52.6 wt% SiO2, Mg# 58–62) and Sulphur Creek (51.2–54.6 wt% SiO2, Mg# 56–57). These two lavas are comparable to calc-alkaline rocks found in arcs worldwide and have similar trace element patterns; however, they differ from each other in abundances of REE, indicating variation in degree of partial melting or fractionation. The Type 3 endmember is represented by the HMBA of Tarn Plateau (51.8–54.0 wt% SiO2, Mg# 68–70) and the HMA of Glacier Creek (58.3–58.7 wt% SiO2, Mg# 63–64). The strongly depleted HREE nature of these Type 3 units and their decreasing Mg# with increasing SiO2 suggests fractionation from a high-Mg basaltic parent derived from a source with residual garnet. Another basaltic andesite unit, Cathedral Crag (52.2–52.6 wt% SiO2, Mg# 55–58), is an Mg-poor differentiate of the Type 3 endmember. The calc-alkaline lavas are least enriched in a subduction component (lowest H2O, Sr/PN, and Ba/Nb), the LKOT-like lavas are intermediate (moderate Sr/PN and Ba/Nb), and the HMBA are most enriched (highest H2O, Sr/PN and Ba/Nb). The generation of the LKOT-like and calc-alkaline lavas can be successfully modeled by partial melting of a spinel lherzolite with variability in composition of slab flux and/or mantle source depletion. The HMBA lavas can be successfully modeled by partial melting of a garnet lherzolite with slab flux compositionally similar to the other lava types, or less likely by partial melting of a spinel lherzolite with a distinctly different, HREE-depleted slab flux.  相似文献   

18.
The major and trace element chemistry of phonolites containing spinel Iherzolite xenoliths from Bokkos (Nigeria), Phonolite Hill (northeastern Australia) and Heldburg (East Germany) is consistent with an origin by fractional crystallization of basanitic magmas at upper mantle pressures (10–15 kbar). At Bokkos, spatially associated lavas ranging from hawaiitic nepheline mugearite to nepheline benmoreite can be modeled very well by fractional crystallization of kaersutitic amphibole + olivine + Fe-Ti-spinel + apatite, a crystal extract consistent with experimentally-determined near-liquidus phase relationships for mugearitic liquids. Further fractional crystallization of aluminous clinopyroxene + mica + apatite will yield the phonolites. A similar model relating the unusual Iherzolite-bearing mafic nepheline benmoreite from Pigroot (New Zealand) to basanitic lavas of the East Otago province is not supported by major and trace element data. The Pigroot lava is possibly the product of melting of a mantle source region previously enriched in Sr and light rare earth elements, with subsequent minor fractional crystallization of olivine + kaersutite. Dynamic flow crystallization processes operating within conduit systems from mantle pressures are capable of yielding large volumes of evolved phonolitic liquids from primary basanitic liquids, if magma flow rates are appropriate. This mechanism may provide an explanation for the volumetric bias towards salic differentiates in some alkalic provinces.  相似文献   

19.
The Quepos, Nicoya and Herradura oceanic igneous terranes in Costa Rica are conspicuous features of a Mid to Late Cretaceous regional magmatic event that encompasses similar terranes in Central America, Colombia, Ecuador and the Caribbean. The Quepos terrane (66?Ma), which consists of ol-cpx phyric, tholeiitic pillow lavas overlain by highly vesicular hyaloclastites, breccias and conglomerates, is interpreted as an uplifted seamount/ocean island complex. The Nicoya (～90?Ma) and Herradura terranes consist of fault-bounded sequences of sediments, tholeiitic volcanics (pillow lavas and massive sheet flows) and plutonic rocks. The volcanic rocks were emplaced at relatively high eruption rates in moderate to deep water, possibly forming part of an oceanic plateau. Major and trace element data from Nicoya/Herradura tholeiites indicate higher melting temperatures than inferred for normal mid-ocean-ridge basalts (MORB) and/or a different source composition. Sr–Nd–Pb isotopic ratios from all three terranes are distinct from MORB but resemble those from the Galápagos hotspot. The volcanological, petrological and geochemical data from Costa Rican volcanic terranes, combined with published age data, paleomagnetic results and plate tectonic reconstructions of this region, provide strong evidence for a Mid Cretaceous (～90Ma) age for the Galápagos hotspot, making it one of the oldest known, active hotspots on Earth. Our results also support an origin of the Caribbean Plate through melting of the head of the Galápagos starting plume.  相似文献   

20.
A convergent margin magma series with characteristic low Nband Ta abundances and enrichments in alkalis and alkaline earthsis intercalated with typical intraplate alkalic basalts in aback-arc setting, 200–250 km above the Wadati-Benioffzone on the North Island, New Zealand. These two contrastingmagma types, together with late-stage K-rich maflc lavas, wereerupted over a short time period (1{dot}60–2{dot}74 Ma)and constitute the Alexandra Volcanics. Field relationshipsindicate that these diverse magma types were contemporaneous,and thus their mantle source regions coexisted, in a singletectonic environment. The convergent margin magma series forms a linear chain of stratovolcanoesaligned at right angles to the present subduction zone. Closed-systempolybaric fractional crystallization models can explain theevolution from ankaramites to transitional olivine basalts toolivine tholeiites to high-Al basalts to medium- and high-Kandesites. The most primitive lavas have geochemical (high LIL/LREEand LIL/HFS element ratios) and Sr, Nd, and Pb isotopic compositionstypical of convergent margin magmas. Calculated source compositionssuggest that three components are involved: a MORB component,a component derived from subducted oceanic crust, and a contributionfrom subducted sediments. The alkalic basalts occur as dispersed monogenetic volcanoesand are intercalated with the larger convergent margin stratovolcanocs.These basalts are enriched in LILE, LREE, Nb, and Ta, and havelow Ba/Nb and Ba/La ratios, all of which are characteristicof ocean island (intraplate) basalts (OIBs). Their relativelyhigh Nd (+5{dot}5 and low 87Sr/86Sr(0{dot}703l–0{dot}7036)are also typical of OIBs. These alkalic magmas were derivedfrom the underlying continental lithospheric mantle that hasbeen enriched by upward-migrating silica-undersaturated melts,probably including volatiles, from the low- velocity zone. Asubducted slab component is not required to account for theirincompatible element enriched character. The K-rich mafic lavas, basanites, and absarokites are volumetricallyminor and cap the largest of the stratovolcanoes, Pirongia.The basanites have geochemical and isotopic compositions whichsuggest they are mixtures of multiple source components, includingthe alkalic and convergent margin region.  相似文献   

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