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1.
Pb and Sr isotopic ratios have been determined for tholeiitic shield-building, alkalic cap, and post-erosional stage lavas from Haleakala Crater. Pb isotopic compositions of the tholeiites overlap those of the alkalic cap lavas, although87Sr/86Sr ratios of these two suites are distinct. Alkalic cap and post-erosional lavas appear to be indistinguishable on the basis of Sr and Pb isotopic composition.Sr and Pb isotopic ratios of Haleakala post-shield-building lavas are positively correlated. Such a trend is previously undocumented for any suite of Hawaiian lavas and contrasts with the general negative correlation observed for data from Hawaiian tholeiites. These relations are consistent with a three-component petrogenetic mixing model. Specifically, it is proposed that magma batches at individual Hawaiian volcanoes formed by: (1) mixing of melts generated from mantle plumes containing two isotopically distinct mantle components (primitive vs. enriched), and (2) subsequent variable degrees of interaction between these plume melts and a third (MORB signature) mantle reservoir prior to their emplacement in a crustal magma chamber. These observations and inferences provide new constraints on physical models of Hawaiian magmatism. Based on observed temporal isotopic variations of Haleakala lavas, it is suggested that the ratio of enriched: primitive mantle components in the Hawaiian plume source decreases during the waning stages of alkalic volcanism. Over the same time interval, both decreasing melt production and protracted residence of ascending melts within the upper mantle contribute to a systematic increase in the ratio of depleted vs. plume component.  相似文献   

2.
Kutch (northwest India) experienced lithospheric thinning due to rifting and tholeiitic and alkalic volcanism related to the Deccan Traps K/T boundary event. Alkalic lavas, containing mantle xenoliths, form plug-like bodies that are aligned along broadly east–west rift faults. The mantle xenoliths are dominantly spinel wehrlite with fewer spinel lherzolite. Wehrlites are inferred to have formed by reaction between transient carbonatite melts and lherzolite forming the lithosphere. The alkalic lavas are primitive (Mg# = 64–72) relative to the tholeiites (Mg# = 38–54), and are enriched in incompatible trace elements. Isotope and trace element compositions of the tholeiites are similar to what are believed to be the crustally contaminated Deccan tholeiites from elsewhere in India. In terms of Hf, Nd, Sr, and Pb isotope ratios, all except two alkalic basalts plot in a tight cluster that largely overlap the Indian Ridge basalts and only slightly overlap the field of Reunion lavas. This suggests that the alkalic magmas came largely from the asthenosphere mixed with Reunion-like source that welled up beneath the rifted lithosphere. The two alkalic outliers have an affinity toward Group I kimberlites and may have come from an old enriched (metasomatized) asthenosphere. We present a new model for the metasomatism and rifting of the Kutch lithosphere, and magma generation from a CO2-rich lherzolite mantle. In this model the earliest melts are carbonatite, which locally metasomatized the lithosphere. Further partial melting of CO2-rich lherzolite at about 2–2.5 GPa from a mixed source of asthenosphere and Reunion-like plume material produced the alkalic melts. Such melts ascended along deep lithospheric rift faults, while devolatilizing and exploding their way up through the lithosphere. Tholeiites may have been generated from the main plume head further south of Kutch.  相似文献   

3.
Trace element relationships of near-primary alkalic lavas from La Grille volcano, Grande Comore, in the Indian Ocean, as well as those of the Honolulu volcanic series, Oahu, Hawaii, show that their sources contain amphibole and/or phlogopite. Small amounts of each mineral (2% amphibole in the source of La Grille and 0.5% phlogopite plus some amphibole in the source of the Honolulu volcanics) and a range of absolute degrees of partial melting from 1 to 5% for both series are consistent with the observed trace element variation. Amphibole and phlogopite are not stable at the temperatures of convecting upper mantle or upwelling thermal plumes from the deep mantle; however, they are stable at pressure-temperature conditions of the oceanic lithospheric mantle. Therefore, the presence of amphibole and/or phlogopite in the magma source region of volcanics is strong evidence for lithospheric melting, and we conclude that the La Grille and the Honolulu series formed by melting of the oceanic lithospheric mantle.

The identification of amphibole ± phlogopite in the source region of both series implies that the metasomatism by fluids or volatile-rich melts occurred prior to melting. The presence of hydrous phases results in a lower solidus temperature of the lithospheric mantle, which can be reached by conductive heating by the thermal plumes. Isotope ratios of the La Grille and the Honolulu series display a restricted range in composition and represent compositional end-members for each island. Larger isotopic variations in shield lavas, represented by the contemporaneous Karthala volcano on Grande Comore and the older Koolau series on Oahu, reflect interaction of the upwelling thermal plumes with the lithospheric mantle rather than the heterogeneity of deep-seated mantle plume sources or entrainment of mantle material in the rising plume. Literature OsSr isotope ratio covariations constrain the process of plume-lithosphere interaction as occurring through mixing of plume melts and low-degree melts from the metasomatized oceanic lithospheric mantle.

The characterization of the lithospheric mantle signature allows the isotopic composition of the deep mantle plume components to be identified, and mixing relationships show that the Karthala and Koolau plume end-members have nearly uniform isotopic compositions. Based on independent arguments, isotopic variations on Heard and Easter islands have been shown to be a result of mixing between deep plume sources having distinct isotopic compositions with lithosphere or shallow asthenospheric mantle. To the extent that these case studies are representative of oceanic island volcanism, they indicate that interaction with oceanic lithospheric mantle plays an important role in the compositions of lavas erupted during the shield-building stage of plume magmatism, and that isotopic compositions of deep mantle plume sources are nearly uniform on the scale that they are sampled by melting.  相似文献   


4.
《Journal of Geodynamics》2007,43(1):87-100
The petrology and geochemistry of Icelandic basalts have been studied for more than a century. The results reveal that the Holocene basalts belong to three magma series: two sub-alkaline series (tholeiitic and transitional alkaline) and an alkali one. The alkali and the transitional basalts, which occupy the off-rift volcanic zones, are enriched in incompatible trace elements compared to the tholeiites, and have more radiogenic Sr, Pb and He isotope compositions. Compared to the tholeiites, they are most likely formed by partial melting of a lithologically heterogeneous mantle with higher proportions of melts derived from recycled oceanic crust in the form of garnet pyroxenites compared to the tholeiites. The tholeiitic basalts characterise the mid-Atlantic rift zone that transects the island, and their most enriched compositions and highest primordial (least radiogenic) He isotope signature are observed close to the centre of the presumed mantle plume. High-MgO basalts are found scattered along the rift zone and probably represent partial melting of refractory mantle already depleted of initial water-rich melts. Higher mantle temperature in the centre of the Iceland mantle plume explains the combination of higher magma productivity and diluted signatures of garnet pyroxenites in basalts from Central Iceland. A crustal component, derived from altered basalts, is evident in evolved tholeiites and indeed in most basalts; however, distinguishing between contamination by the present hydrothermally altered crust, and melting of recycled oceanic crust, remains non-trivial. Constraints from radiogenic isotope ratios suggest the presence of three principal mantle components beneath Iceland: a depleted upper mantle source, enriched mantle plume, and recycled oceanic crust.The study of glass inclusions in primitive phenocrysts is still in its infancy but already shows results unattainable by other methods. Such studies reveal the existence of mantle melts with highly variable compositions, such as calcium-rich melts and a low-18O mantle component, probably recycled oceanic crust. Future high-resolution seismic studies may help to identify and reveal the relative proportions of different lithologies in the mantle.  相似文献   

5.
The study of the geochemical compositions and K-Ar or Ar-Ar ages of ca. 350 Neogene and Quaternary lavas from Baja California, the Gulf of California and Sonora allows us to discuss the nature of their mantle or crustal sources, the conditions of their melting and the tectonic regime prevailing during their genesis and emplacement. Nine petrographic/geochemical groups are distinguished: ??regular?? calc-alkaline lavas; adakites; magnesian andesites and related basalts and basaltic andesites; niobium-enriched basalts; alkali basalts and trachybasalts; oceanic (MORB-type) basalts; tholeiitic/transitional basalts and basaltic andesites; peralkaline rhyolites (comendites); and icelandites. We show that the spatial and temporal distribution of these lava types provides constraints on their sources and the geodynamic setting controlling their partial melting. Three successive stages are distinguished. Between 23 and 13 Ma, calc-alkaline lavas linked to the subduction of the Pacific-Farallon plate formed the Comondú and central coast of the Sonora volcanic arc. In the extensional domain of western Sonora, lithospheric mantle-derived tholeiitic to transitional basalts and basaltic andesites were emplaced within the southern extension of the Basin and Range province. The end of the Farallon subduction was marked by the emplacement of much more complex Middle to Late Miocene volcanic associations, between 13 and 7 Ma. Calc-alkaline activity became sporadic and was replaced by unusual post-subduction magma types including adakites, niobium-enriched basalts, magnesian andesites, comendites and icelandites. The spatial and temporal distribution of these lavas is consistent with the development of a slab tear, evolving into a 200-km-wide slab window sub-parallel to the trench, and extending from the Pacific coast of Baja California to coastal Sonora. Tholeiitic, transitional and alkali basalts of subslab origin ascended through this window, and adakites derived from the partial melting of its upper lip, relatively close to the trench. Calc-alkaline lavas, magnesian andesites and niobium-enriched basalts formed from hydrous melting of the supraslab mantle triggered by the uprise of hot Pacific asthenosphere through the window. During the Plio-Quaternary, the ??no-slab?? regime following the sinking of the old part of the Farallon plate within the deep mantle allowed the emplacement of alkali and tholeiitic/transitional basalts of deep asthenospheric origin in Baja California and Sonora. The lithospheric rupture connected with the opening of the Gulf of California generated a high thermal regime associated to asthenospheric uprise and emplaced Quaternary depleted MORB-type tholeiites. This thermal regime also induced partial melting of the thinned lithospheric mantle of the Gulf area, generating calc-alkaline lavas as well as adakites derived from slivers of oceanic crust incorporated within this mantle.  相似文献   

6.
The mantle xenoliths included in Quaternary alkaline volcanics from the Manzaz-district (Central Hoggar) are proto-granular, anhydrous spinel lherzolites. Major and trace element analyses on bulk rocks and constituent mineral phases show that the primary compositions are widely overprinted by metasomatic processes. Trace element modelling of the metasomatised clinopyroxenes allows the inference that the metasomatic agents that enriched the lithospheric mantle were highly alkaline carbonate-rich melts such as nephelinites/melilitites (or as extreme silico-carbonatites). These metasomatic agents were characterized by a clear HIMU Sr–Nd–Pb isotopic signature, whereas there is no evidence of EM1 components recorded by the Hoggar Oligocene tholeiitic basalts. This can be interpreted as being due to replacement of the older cratonic lithospheric mantle, from which tholeiites generated, by asthenospheric upwelling dominated by the presence of an HIMU signature. Accordingly, this rejuvenated lithosphere (accreted asthenosphere without any EM influence), may represent an appropriate mantle section from which deep alkaline basic melts could have been generated and shallower mantle xenoliths sampled, respectively. The available data on lherzolite xenoliths and alkaline lavas (including He isotopes, Ra < 9) indicate that there is no requirement for a deep plume anchored in the lower mantle, and that sources in the upper mantle may satisfactorily account for all the geochemical/petrological/geophysical evidence that characterizes the Hoggar swell. Therefore the Hoggar volcanism, as well as other volcanic occurrences in the Saharan belt, are likely to be related to passive asthenospheric mantle uprising and decompression melting linked to tensional stresses in the lithosphere during Cenozoic reactivation and rifting of the Pan–African basement. This can be considered a far-field foreland reaction of the Africa–Europe collisional system since the Eocene.  相似文献   

7.
The Cenozoic volcanic rocks of eastern China are subalkalic to alkalic basalts erupted in an early Tertiary back-arc rift environment and from scattered late Tertiary and Quaternary volcanic centers in a continental area crossed by active faults, driven by subduction of the Pacific plate and the collision of India and Eurasia. Immobile trace elements and major elements conform very well to each other in classification of the 59 rocks for which complete data are reported and they correctly identify the tectonic setting. LIL-element enrichments of the basalts lie between those of P-MORB and ocean island alkalic basalts, and show a secular increase.87Sr/86Sr ratios of basalts vary from 0.7029 to 0.7048. Alkalic basalts are systematically less radiogenic than geographically coextensive and contemporaneous tholeiitic basalts. Increase of radiogenic Sr with increasing crustal thickness and crustal age and with silica enrichment of the magmas suggests crustal contamination but this is inadequate to explain the LIL-element enrichment patterns and variable LIL-element enrichments. The preferred hypothesis is that the alkalic magmas come from a deeper source, with long-term LIL-element depletion and low Rb/Sr ratio but relatively recent LIL-element enrichment. Conversely the tholeiitic magmas are melts of subcontinental mantle lithosphere that is more LIL-element depleted than the alkalic source, at the time of magma genesis, but has had an elevated Rb/Sr ratio for much of its post-consolidation history.  相似文献   

8.
Over the last two decades great strides have been made in characterizing the spatial distribution, time sequence,geochemical characteristics, mantle sources, and magma evolution processes for various igneous rocks in the Early Permian Tarim Large Igneous Province(TLIP). This work has laid a solid foundation for revealing the evolutionary processes and genetic models of large igneous provinces(LIPs). This study systematically demonstrates the two-stage melting model for the TLIP based on our previous research work and predecessor achievements, and highlights the two types of magmatic rocks within the TLIP.The two-stage melting model suggests that the formation of the TLIP is mantle plume related. The early hot mantle plume caused the low-degree partial melting of the lithosphere mantle, while in the later stage, the plume partially melted due to adiabatic uplift and decompression. Therefore, this model carries signatures of both the "Parana" and "Deccan" models in terms of mantle plume activity. During the early stage, the mantle plume provided the heat required for partial melting of sub-continental lithosphere mantle(SCLM), similar to the "Parana Model", while later the plume acted as the main avenue for melting, as in the "Deccan Model". Basalts that erupted in the first stage have higher 87Sr/86 Sr, lower 143Nd/144 Nd ratios, and are enriched in large ion lithophile elements and high field strength elements, indicating a possible origin from the enriched continental lithosphere mantle,similar to the Parana type geochemical features. The basic-ultrabasic intrusive rocks in the second stage exhibit lower 87Sr/86 Sr,higher 143Nd/144 Nd ratios relative to the basalts, consistent with the involvement of a more depleted asthenospheric material,such as a mantle plume, similar to the Deccan type geochemical features. The first stage basalts can be further subdivided into two categories, i.e., Group 1 and Group 2 basalts. Group 2 basalts have lower 87Sr/86 Sr and higher 143Nd/144 Nd ratios than Group 1 basalts, and lie between compositions of the Group 1 basalts and second stage magmatism. Group 2 basalts may be the intermediate component of the TLIP, and the whole TLIP is the result of plume and lithosphere interaction. Developing this petrogenetic model for the TLIP aids in comprehensively understanding its magmatism and deep geological and geodynamic processes. Furthermore, this work enriches the theories describing the origin of large igneous province and mantle plume activity.  相似文献   

9.
Hualalai is one of five volcanoes whose eruptions built the island of Hawaii. The historic 1800–1801 flows and the analyzed prehistoric flows exposed at the surface are alkalic basalts except for a trachyte cone and flow at Puu Waawaa and a trachyte maar deposit near Waha Pele. The 1800–1801 eruption produced two flows: the upper Kaupulehu flow and the lower Huehue flow. The analyzed lavas of the two 1800–1801 flows are geochemically identical with the exception of a few samples from the toe of the Huehue flow that appear to be derived from a separate magmatic batch. The analyzed prehistoric basalts are nearly identical to the 1800–1801 flows but include some lavas that have undergone considerable shallow crystal fractionation. The least fractionated alkalic basalts from Hualalai are in equilibrium with mantle olivine (Fo87) indicating that the Hawaiian mantle source region is not unusually iron-rich. The 1800–1801 and analyzed prehistoric basalts can be generated by about 5–10% partial fusion of a garnet-bearing source relatively enriched in the light-rare-earths. The mantle underlying the Hawaiian Islands is chemically and mineralogically heterogeneous before and after extraction of the magmas that make up the volcanoes.  相似文献   

10.
Mahshar  Raza  MohdShamim  Khan  MohdSafdare  Azam 《Island Arc》2007,16(4):536-552
Abstract   The northern part of the Aravalli mountain belt of northwestern Indian shield is broadly composed of three Proterozoic volcano-sedimentary domains, i.e. the Bayana, the Alwar and the Khetri basins, comprising collectively the north Delhi fold belt. Major, trace and rare earth element concentrations of mafic volcanic rocks of the three basins exhibit considerable diversity. Bayana and Alwar volcanics are typical tholeiites showing close similarity with low Ti–continental flood basalts (CFB) with the difference that the former shows enriched and the latter flat incompatible trace element and rare earth element (REE) patterns. However, the Khetri volcanics exhibit a transitional composition between tholeiite and calc-alkaline basalts. It appears that the melts of Bayana and Alwar tholeiites were generated by partial melting of a common source within the spinel stability field possibly in the presence of mantle plume. During ascent to the surface the Bayana tholeiites suffered crustal contamination but the Alwar tholeiites erupted unaffected. Geochemically, the Khetri volcanics are arc-like basalts which were generated in a segment of mantle overlying a Proterozoic subduction zone. It is suggested that at about 1800 Ma the continental lithosphere in northeastern Rajasthan stretched, attenuated and fractured in response to a rising plume. The produced rifts have undergone variable degrees of crustal extension. The extension and attenuation of the crust facilitated shallowing of the asthenosphere which suffered variable degree of melting to produce tholeiitic melts – different batches of which underwent different degrees of lithospheric contamination depending upon the thickness of the crust in different rifted basins. The occurrence of subduction-related basaltic rocks of Khetri Belt suggests that a basin on the western margin of the craton developed into a mature oceanic basin.  相似文献   

11.
This paper presents a simple dynamical model for melting and trace element distribution in the Hawaiian mantle plume. I model the plume as a partially molten stagnation point flow against the oceanic lithosphere, and obtain solutions for the temperature, melt migration rate, and trace element concentration within it. Trace element concentrations in the melt exceed simple batch melting predictions by up to 70%. The magnitude of this effect depends strongly on the solid-melt partition coefficientK. Trace elements with differentK therefore experience a “dynamical fractionation” within the plume, and incompatible trace element ratios such asLa/Ce always exceed the batch melting predictions. I suggest a simple model for plume-lithosphere interaction in which melts from these two sources mix in proportions determined by thermodynamic constraints. The model can explain the composition of basalts from Haleakala if the degree of melting of the lithosphereF1 decreases with time from roughly 10% for tholeiites to 2% for alkalic basalts. These values are considerably higher than previous estimates ofF1 < 1%, and imply correspondingly smaller and more realistic values ( 10 km) for the thickness of the melted part of the lithosphere. Partial melting of additional depleted sources such as the asthenosphere is therefore not required by the Haleakala data. Estimates ofF1 are highly sensitive to the values chosen for the partition coefficients, however, and should therefore be interpreted with caution.  相似文献   

12.
The model of lithospheric thinning and reheating for the origin of the Hawaiian swell assumes that the lower lithosphere (> 60 km) is rapidly reset to an asthenospheric temperature as it passes over the hot spot. It is shown that this heat input induces melting in a few kilometer thick layer of lithosphere just above the thermal anomaly. By solving the appropriate energy equation, the mean degree of melting in the molten layer was estimated to be 1–5% with a total melt thickness of 25–150 m. The minimum width of the thermal anomaly required to account for the observed rate of post-erosional eruptions is of the order of 10–40 km which is probably satisfied. The melt generated by this process matches the petrological and geochemical characteristics of Hawaiian post-erosional lava and their typical MORB-related isotopic signature. Because small degrees of melting are involved, the extraction time scale is long (a few million years) and is consistent with the time span of post-erosional eruptions. Also, the characteristic sequence of Hawaiian volcanism can be explained if the source for Hawaiian lava is considered as a molten layer with melt fraction decreasing upward.  相似文献   

13.
The two parallel loci of recent Hawaiian volcanoes, Kea and Loa, have been regarded as the best targets to interpret the chemical structure of an upwelling mantle plume derived from the lower mantle. Here we show that the Sr–Nd–Hf–Pb isotopic data of the shield-building lavas along the Loa locus form a systematic trend from the main shield stage of Koolau (> 2.9 Ma) to the active Loihi volcanoes. During the growth of the Koolau volcano, the dominant material in the melting region successively changed from the proposed KEA, DMK (depleted Makapuu), to EMK (enriched Makapuu) components. The proportion of EMK, dominated by a recycled mafic component, is typified by some Koolau Makapuu-stage and some Lanai lavas. Subsequently, the EMK component decreased and LOIHI component increased toward the Loihi lavas. The temporal coincidence between the episodically elevated magma production rate and the abrupt appearance of the typical Loa-type lavas that is restricted to the last 3 Myr should be linked to magma genesis. We suggest that the abrupt appearance of Loa-type magmatism should be attributed to the transient incorporation of the relatively dense recycled material and surrounding less degassed lower mantle material that accumulated near the core–mantle boundary into the upwelling plume. This episodic involvement could have been trigged by episodic thermal pulses and buoyancy increases in the plume. The continuous appearance of Kea-type lavas during the long history of Hawaiian-chain magmatism and the larger magma volume of Kea-type lavas relative to that of the Loa-type lavas in the last 3 Myr indicate that the Kea locus is closer to the thermal centre of the Hawaiian plume relative to that of the Loa locus.  相似文献   

14.
The Cenozoic basaltic province of the Vogelsberg area (central Germany) is mainly composed of intercalated olivine to quartz tholeiites and near-primary nephelinites to basanites. The inferred mantle source for the alkaline and tholeiitic rocks is asthenospheric metasomatized garnet peridotite containing some amphibole as the main hydrous phase. Trace element modelling indicates 2 to 3% partial melting for the alkaline rocks and 5 to 7% partial melting for the olivine tholeiites. Incompatible trace element abundances and ratios as well as Nd and Sr radiogenic isotope compositions lie between plume compositions and enriched mantle compositions and are similar to those measured in Ocean Island Basalts (OIB) and the Central European Volcanic Province elsewhere. The mafic olivine tholeiites have similar Ba/Nb, Ba/La and Nd–Sr isotope ratios to the alkaline rocks indicating derivation of both magma types from chemically comparable mantle sources. However, Zr/Nb ratios are slightly higher in olivine tholeiites than in basanites reflecting some fractionation of Zr relative to Nb during partial melting. Quartz tholeiites have higher Ba/Nb, Zr/Nb, La/Nb, but lower Ce/Pb ratios and lower Nd isotope compositions than the alkaline rocks which can be explained by interaction of the basaltic melt with lower (granulite facies) crustal material or partial melts thereof during stagnation within the lower crust. It appears most likely that upwelling of hot, asthenospheric material results in the generation of primitive alkaline rocks at the base of the lithosphere at depths of 75–90 km. Lithospheric extension together with minor plume activity and probably lower lithosphere erosion induced melting of shallower heterogenous upper mantle generating a spectrum of olivine tholeiitic melts. These olivine tholeiitic rocks evolved via crystal fractionation and probably limited contamination to quartz tholeiites.  相似文献   

15.
 The 3.9- to 2.9-Ma Waianae Volcano is the older of two volcanoes making up the island of Oahu, Hawaii. Exposed on the volcanic edifice are tholeiitic shield lavas overlain by transitional and alkalic postshield lavas. The postshield "alkalic cap" consists of aphyric hawaiite of the Palehua Member of the Waianae Volcanics, overlain unconformably by a small volume of alkalic basalt of the Kolekole Volcanics. Kolekole Volcanics mantle erosional topography, including the uppermost slopes of the great Lualualei Valley on the lee side of the Waianae Range. Twenty new K–Ar dates, combined with magnetic polarity data and geologic relationships, constrain the ages of lavas of the Palehua member to 3.06–2.98 Ma and lavas of the Kolekole Volcanics to 2.97–2.90 Ma. The geochemical data and the nearly contemporaneous ages suggest that the Kolekole Volcanics do not represent a completely independent or separate volcanic event from earlier postshield activity; thus, the Kolekole Volcanics are reduced in rank, becoming the Kolekole Member of the Waianae Volcanics. Magmas of the Palehua and Kolekole Members have similar incompatible element ratios, and both suites show evidence for early crystallization of clinopyroxene consistent with evolution at high pressures below the edifice. However, lavas of the Kolekole Member are less fractionated and appear to have evolved at greater depths than the earlier Palehua hawaiites. Postshield primary magma compositions of the Palehua and Kolekole Members are consistent with formation by partial melting of mantle material of less than 5–10% relative to Waianae shield lavas. Within the section of Palehua Member lavas, an increase with respect to time of highly incompatible to moderately incompatible element ratios is consistent with a further decrease in partial melting by approximately 1–2%. This trend is reversed with the onset of eruption of Kolekole Member lavas, where an increase in extent of partial melting is indicated. The relatively short time interval between the eruption of Palehua and Kolekole Member lavas appears to date the initial formation of Lualualei Valley, which was accompanied by a marked change in magmatic conditions. We speculate that the mass-wasting event separating lavas of the Palehua and Kolekole Members may be related to the formation of a large submarine landslide west and southwest of Waianae Volcano. Enhanced decompression melting associated with removal of the equivalent volume of this landslide deposit from the edifice is more than sufficient to produce the modeled increase of 1–2% in extent of melting between the youngest Palehua magmas and the posterosional magmas of the Kolekole Member. The association between magmatic change and a giant landsliding event suggests that there may be a general relationship between large mass-wasting events and subsequent magmatism in Hawaiian volcano evolution. Received: 1 September 1996 / Accepted: 26 November 1996  相似文献   

16.
We report Sr, Nd and Pb isotope ratios and parent and daughter element concentrations in 34 volcanic rocks from Samoa. The highly undersaturated post-erosional volcanics, which have erupted in Recent to Historic time along a 250-km-long fissure, have isotopic compositions that define fields distinct from those of the tholeiitic to alkalic lavas of the older Samoan shield volcanoes. Most shield lavas have206Pb/204Pb of 18.9–19.4,87Sr/86Sr of 0.7045–0.7055 and87Sr/86Sr (to 0.7075). In general, isotopic compositions of the shield lavas are similar to those of the Marquesas and Society Islands. Post-erosional samples have lower206Pb/204Pb and143Nd/144Nd and higher87Sr/86Sr than most shield lavas. The most striking feature of the post-erosional data is a negative correlation between207Pb/204Pb and206Pb/204Pb. This suggests that post-erosional lavas are derived from mixtures of the shield source and a high-207Pb/204Pb,87Sr/86Sr, low-206Pb/204Pb and143Nd/144Nd post-erosional source which may contain recycled ancient sediment. This enriched mantle domain may also underlie the Ontong-Java and Manihiki Plateaus to the north and west. Although both the Samoan shield and post-erosional lavas show chemical characteristics often associated with mantle plumes, only the shield volcanism can plausibly be related to a plume. The post-erosional eruptions appear to be the result of flexure and rifting due to plate bending at the northern termination of the Tonga Trench.  相似文献   

17.
地幔柱是最可能形成大火成岩省的原因之一，同时地幔柱与岩石圈的相互作用也极大的影响着岩石圈的构造演化.本文主要集中研究地幔柱与岩石圈相互作用过程中熔融相关的问题.利用开源程序Ellipsis3D，基于质量守恒方程、动量守恒方程、能量守恒方程和岩石流变本构关系，以及不同的熔融损耗关系，通过有限元数值方法模拟得到地幔柱与岩石圈相互作用过程中熔融程度的动态变化.数值模拟结果显示，地幔柱与岩石圈相互作用的熔融相关过程分为三个阶段：地幔柱的初融阶段，地幔柱自身熔融占主导，减压熔融为主因；地幔柱与岩石圈的纵向作用阶段，岩石圈地幔开始熔融，地幔柱以减压熔融为主，岩石圈地幔以升温熔融为主；地幔柱的横向展平阶段，随着地幔柱的扩展岩石圈地幔熔融范围增加，以升温熔融为主，地幔柱自身熔融程度减小.最后基于数值模拟结果及现场资料对峨嵋山大火成岩省地幔柱的发展演化以及峨眉山大火成岩省的形成进行了讨论.  相似文献   

18.
We examine in this paper the use of helium isotope ratios for the study of hotspot volcanism along age-progressive island volcanic chains. The Hawaiian Islands are the original “high 3He” hotspot, with 3He/4He ratios as high as 32 × the atmospheric ratio; in the Pacific they stand out against the surrounding sea of MORB (rather uniformly 8 × atmospheric) which fills the entire Pacific with the exception of the Macdonald-Mehetia-Samoa axis in the South Pacific. The recent availability of a variety of alkalic and tholeiitic glasses from the U.S. Geological Survey and our own dredge hauls has prompted us to look first at isotopic variability within a single fresh and new volcano which is probably sitting directly atop a mantle plume. Thus we have looked in some detail at the total helium in glass pillow rims, at He in the enclosed vesicles, and at He in the glass itself, in both tholeiitic and alkalic lavas, and also at helium in associated phenocrysts and xenoliths. The measured 3He/4He ratios range from atmospheric to 30 × atmospheric, but we see clear evidence that the highly vesiculated lavas suffer exchange of He between the thin glass walls of vesicles and ambient seawater, so that we observe a post-eruptive isotopic disequilibrium between glass and gas phases. The primary effect is the very large loss of initial He content during eruptive vesiculation, which results in quite large isotopic effects from small additions of ambient He (of the order of 0.02 μcc He per gram of basalt; corresponding to a “water/rock ratio” of 0.5). Phenocrystic He in olivines verifies that the gas-phase He is not affected by vesicularities up to about 5%. Alkali basalt He appears to be independent of vesicularity up to values as high as 35%; this He is somewhat lower in 3He/4He ratio, but matches precisely the associated xenolithic He. However, from the present data we cannot exclude the possibility that diffusive exchange with seawater has affected the He ratio in alkalic vesicles.On the large scale, along the 10% of the Hawaiian chain available for subaerial sampling, we find high 3He/4He ratios (24 × atmospheric) in 5.5 × 106-year-old lavas on Kauai. Maximum values of the ratio so far observed are in the pre-erosional Kula basalts on Maui, confirming the previous results of Kaneoka and Takaoka. Hawaii, where these high values were first observed is now seen to range from MORB ratios at Mauna Loa to only 15 × RA at Kilauea fumaroles. Most xenolithic He so far measured is MORB He, but Loihi xenoliths have high values and are quite different in this respect. Finally, we discuss also the hydrogen and carbon isotope results on Loihi lavas, and show that these elements resemble MORB and appear not to show a distinctive plume signature.  相似文献   

19.
Bimodal tholeiitic and mildly alkalic basalts occur near Bhir, in the central part of Deccan Volcanic Province (DVP). Major and trace element concentrations show that, of the ten flows, nine are tholeiitic and one is an alkalic basalt. The Bhir basalts have a wide range of chemical composition. Geochemical variations in the stratigraphic section define three distinct phases of evolution (zones 1 to 3). Crystal fractionation of plagioclase, clinopyroxene, olivine and Fe–Ti oxide expanded the compositional range. Low Mg#s (39–55), low concentrations of Ni and Cr and high Zr suggest the evolved nature of the Bhir basalts. Fractionation modeling suggests about 42% fractional crystallization.In spite of the dominant role of fractional crystallization in the evolution of Bhir basalts, some other processes must be sought to explain the chemical variations. Crustal contamination, magma mixing and degree of partial melting are suggested to explain the observed chemical variations. Resorption, reverse zoning and compositional bimodality in plagioclase phenocrysts indicate magma mixing. Samples of flows one and four suspected of being contaminated all have enriched SiO2 and LILE (K, Rb, and Ba) contents and depletion in Ti and P, believed to be due to ‘granitic’ crustal contamination.As compared to tholeiitic basalts, the alkalic basalts are characterized by low SiO2 and high TiO2, Na2O, K2O and P2O5. Alkalic basalts are richer in LILE (Rb and Ba), HFSE (Nb, Zr, and Y) and REE than the tholeiitic basalts. The alkalic basalt occurrence is important from a petrogenetic point of view and also suggests that the sources of alkalic basalt magmas may be of variable ages under different parts of the DVP. Based on major, trace and rare earth element distributions it is suggested that asthenospheric mantle having affinities with the source of OIB was the source material of the magmas and the range in the composition of tholeiitic and alkalic basalts was probably controlled by different degrees of melting and/or inhomogeneities in the mantle source.  相似文献   

20.
Petrological and geochemical data on dredged samples from five submarine volcanos northwest of Samoa indicate that three of these volcanos belong to the Samoan volcanic province (Field, Lalla Rookh, and Combe banks), and two belong to separate magmatic zones (Wallis Islands and Alexa Bank). The Samoan volcanic province increases in age westward and both shield-building tholeiitic and alkalic lavas (Combe Bank) and strongly undersaturated (post-erosional?) melilitites or nephelinites and ankaramites (Field and Lalla Rookh banks) are present. The age progression and petrochemical character of these rocks is consistent with a fixed hotspot beneath eastern Samoa. Slightly askew from this trend is Alexa Bank where dredged lavas are ocean-island tholeiites; however, its radiometric age and compositional characteristics apparently preclude its association with Samoa by a fixed-hotspot model. Dredged volcanic rocks from near the Wallis Islands are geochemically, petrologically, and temporally different from Samoan volcanism, but are similar in these respects to Quaternary volcanism in Rotuma and Fiji and may be related to plate reorganization accompanying opening of the North Fiji Basin.  相似文献   

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