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1.
Geochemical and isotopic analyses (Sr–Nd–Pb) of late Miocene to Quaternary plateau lavas from the Pali Aike and Morro Chico areas (52°S) were undertaken to constrain the melting processes and mantle sources that contributed to magma generation and the geodynamic evolution of southernmost Patagonia, South America. The Pali Aike and Morro Chico lavas are alkaline (Pali Aike, 45–49 wt.% SiO2; 4.3–5.9 wt.% Na2O+K2O) and subalkaline (Morro Chico, 50.5–50.8 wt.% SiO2; 4.0–4.4 wt.% Na2O+K2O), relatively primitive (Pali Aike, 9.5–13.7 wt.% MgO; Morro Chico, 7.6–8.8 wt.% MgO) mafic volcanic rocks that have typical intraplate ocean island basalt‐like signatures. Incompatible trace element ratios and isotopic ratios of the Pali Aike and Morro Chico lavas differ from those of the majority of Neogene southern Patagonian slab window lavas in showing more enriched characteristics and are similar to high‐μ (HIMU)‐like basalts. The rare earth element (REE) modeling to constrain mantle melting percentages suggests that these lavas were produced by low degrees of partial melting (1.0–2.0% for Pali Aike lavas and about 2.6–2.7% for Morro Chico lavas) of a garnet lherzolite mantle source. The major systematic variations of Sr–Nd–Pb isotopes in southern Patagonian lavas are related to geographic location. The Pali Aike and Morro Chico lavas from the southernmost part of Patagonia have lower 87Sr/86Sr and higher 143Nd/144Nd and 206Pb/204Pb ratios, relative to most of the southern Patagonian lavas erupted north of 49.5°S, pointing to a HIMU‐like signature. An isotopically depleted and HIMU‐like asthenospheric domain may have been the main source of magmas in the southernmost part of Patagonia (e.g. Pali Aike, Morro Chico, and Camusu Aike volcanic field), suggesting the presence of a major discontinuity in the isotopic composition of the asthenosphere in southern Patagonia. On the basis of geochemical and isotope data and the available geological and geotectonic reconstructions, a link between the HIMU asthenospheric mantle domain beneath southernmost Patagonia and the HIMU mega‐province of the southwestern Pacific Ocean is proposed.  相似文献   

2.
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   

3.
The Sintra igneous complex, Portugal was an important centre of activity in late Cretaceous times. The great proportion of thealkaline rocks are felsic and include five large quartz syenite intrusions and trachyandesite, trachyte and alkali rhyolite lavas and dykes, most of which are oversaturated. Mafic rocks are sparse, but vary widely from alkaline and highly undersaturated types containing high K2O, TiO2 and Ba, similar to the contemporaneous Lisbon lavas, to hypersthene normative trachybasalts and one hypersthene normative basalt. The various magma types are intimately associated and a well-developed netveined complex of alkali gabbro, monzonite and syenite is recognised at Cabo da Roca. A study of the dyke distributions, intersections and orientations suggest a close propinquity of both oversaturated and undersaturated and of both felsic and matic magmas. The basic magmas of Sintra and Lisbon show a continuous range in undersaturation (0 to 16% normative nepheline) and rare hypersthene normative basalts. Derivation of the hypersthene normative and mildly undersaturated basalts from the more undersaturated melts by low pressure fractionation or contamination by siliceous crust is shown to be unlikely. High pressure eclogite fractionation of a hypersthene normative basalt or variations in the percentage partial melting of a mantle under conditions where titanphlogopite is a low melting fraction are both processes compatible with the variations in undersaturation and proportions of TiO2, K2O and Ba. The quartz syenites and over satured felsic lavas of Sintra are thought to be derived from hypersthene nor mative parents.  相似文献   

4.
The Chile Triple Junction is a natural laboratory to study the interactions between magmatism and tectonics during the subduction of an active spreading ridge beneath a continent. The MLBA plateau (Meseta del Lago Buenos Aires) is one of the Neogene alkali basaltic plateaus located in the back-arc region of the Andean Cordillera at the latitude of the current Chile Triple Junction. The genesis of MLBA can be related with successive opening of slabs windows beneath Patagonia: within the subducting Nazca Plate itself and between the Nazca and Antarctic plates. Detailed 40Ar/39Ar dating and geochemical analysis of bimodal magmatism from the western flank of the MLBA show major changes in the back-arc magmatism which occurred between 14.5 Ma and 12.5 Ma with the transition from calc-alkaline lavas (Cerro Plomo) to alkaline lavas (MLBA) in relation with slab window opening. In a second step, at 4–3 Ma, alkaline felsic intrusions were emplaced in the western flank of the MLBA coevally with the MLBA basalts with which they are genetically related. These late OIB-like alkaline to transitional basalts were generated by partial melting of the subslab asthenosphere of the subducting Nazca plate during the opening of the South Chile spreading ridge-related slab window. These basalts differentiated with small amounts of assimilation in shallow magma chambers emplaced along transtensional to extensional zones. The close association of bimodal magmatism with extensional tectonic features in the western MLBA is a strong support to the model of Patagonian collapse event proposed to have taken place between 5 and 3 Ma as a consequence of the presence of the asthenospheric window (SCR-1 segment of South Chile Ridge) below the MLBA area.  相似文献   

5.
Lower Carboniferous lavas from the Midland Valley and adjacent regions of Scotland are mildly alkaline and intraplate in nature. The sequence is dominated by basalt and hawaiite, although mugearite, benmoreite, trachyte and rhyolite are also present. Basic volcanic rocks display the LIL element and LREE enrichment typical of intraplate alkali basalt terrains. Low initial87Sr/86Sr (0.7029–0.7046), high εNd (−0.4 to +5.6) and moderately radiogenic206Pb/204Pb (17.77–18.89) ratios are also comparable with alkali basalts from other continental rifts and oceanic islands.When the Carboniferous lavas are compared with subduction-related lavas of Old Red Sandstone age, erupted in and around the Midland Valley ca. 50 Ma earlier (at 410 Ma) remarkable similarities are apparent. Significant overlap occurs in Nd and Pb isotopic compositions. Sr isotopic compositions are, however, more radiogenic in the older subduction-related lavas. This, combined with high K and Rb concentrations in ORS lavas may be explained by the incorporation of a sediment component derived from the subducted slab, which by Lower Carboniferous times had been lost from the mantle source region by convection. A pronounced negative Nb anomaly in the ORS subduction-related lavas may be explained by the retention of a Nb-bearing phase in the mantle during hydrous melting of the mantle wedge above the subduction zone.Allowing for the effects of the added component from the subducted slab, there appears to be no necessity to invoke separate mantle source regions for the two suites of lavas: both may have been derived from chemically similar portions of mantle. If volcanic arc lavas are derived from the mantle wedge, the implication is that such a source lies at relatively shallow depth within the upper mantle: the same may therefore apply to the Carboniferous continental rift basalts. This evidence, combined with the fact that there is no evident hot-spot trail across the Midland Valley despite a long period of within-plate volcanism and rapid plate movements during the Carboniferous, suggests that the alkali basalt magmatism is not the product of a deep-seated mantle plume. Rather, the volcanism appears to owe more to passive rifting and to diapiric upwelling from a source region within the uppermost mantle.  相似文献   

6.
The roles of subduction of the Pacific plate and the big mantle wedge (BMW) in the evolution of east Asian continental margin have attracted lots of attention in past years. This paper reviews recent progresses regarding the composition and chemical heterogeneity of the BMW beneath eastern Asia and geochemistry of Cenozoic basalts in the region, with attempts to put forward a general model accounting for the generation of intraplate magma in a BMW system. Some key points of this review are summarized in the following. (1) Cenozoic basalts from eastern China are interpreted as a mixture of high-Si melts and low-Si melts. Wherever they are from, northeast, north or south China, Cenozoic basalts share a common low-Si basalt endmember, which is characterized by high alkali, Fe2O3T and TiO2 contents, HIMU-like trace element composition and relatively low 206Pb/204Pb compared to classic HIMU basalts. Their Nd-Hf isotopic compositions resemble that of Pacific Mantle domain and their source is composed of carbonated eclogites and peridotites. The high-Si basalt endmember is characterized by low alkali, Fe2O 3 T and TiO2 contents, Indian Mantle-type Pb-Nd-Hf isotopic compositions, and a predominant garnet pyroxenitic source. High-Si basalts show isotopic provinciality, with those from North China and South China displaying EM1-type and EM2-type components, respectively, while basalts from Northeast China containing both EM1- and EM2-type components. (2) The source of Cenozoic basalts from eastern China contains abundant recycled materials, including oceanic crust and lithospheric mantle components as well as carbonate sediments and water. According to their spatial distribution and deep seismic tomography, it is inferred that the recycled components are mostly from stagnant slabs in the mantle transition zone, whereas EM1 and EM2 components are from the shallow mantle. (3) Comparison of solidi of garnet pyroxenite, carbonated eclogite and peridotite with regional geotherm constrains the initial melting depth of high-Si and low-Si basalts at <100 km and ～300 km, respectively. It is suggested that the BMW under eastern Asia is vertically heterogeneous, with the upper part containing EM1 and EM2 components and isotopically resembling the Indian mantle domain, whereas the lower part containing components derived from the Pacific mantle domain. Contents of H2O and CO2 decrease gradually from bottom to top of the BMW. (4) Melting of the BMW to generate Cenozoic intraplate basalts is triggered by decarbonization and dehydration of the slabs stagnated in the mantle transition zone.  相似文献   

7.
A broad zone of dominantly subaerial silicic volcanism associated with regional extensional faulting developed in southern South America during the Middle Jurassic, contemporaneously with the initiation of plutonism along the present Pacific continental margin. Stratigraphic variations observed in cross sections through the silicic Jurassic volcanics along the Pacific margin of southernmost South America indicate that this region of the rift zone developed as volcanism continued during faulting, subsidence and marine innundation. A deep, fault-bounded submarine trough formed near the Pacific margin of the southern part of the volcano-tectonic rift zone during the Late Jurassic. Tholeiitic magma intruded within the trough formed the mafic portion of the floor of this down-faulted basin. During the Early Cretaceous this basin separated an active calc-alkaline volcanic arc, founded on a sliver of continental crust, from the then volcanically quiescent South American continent. Geochemical data suggest that the Jurassic silicic volcanics along the Pacific margin of the volcano-tectonic rift zone were derived by crustal anatexis. Mafic lavas and sills which occur within the silicic volcanics have geochemical affinities with both the tholeiitic basalts forming the ophiolitic lenses which are the remnants of the mafic part of the back-arc basin floor, and also the calc-alkaline rocks of the adjacent Patagonian batholith and their flanking lavas which represent the eroded late Mesozoic calc-alkaline volcanic arc. The source of these tholeiitic and calc-alkaline igneous rocks was partially melted upper mantle material. The igneous and tectonic processes responsible for the development of the volcano-tectonic rift zone and the subsequent back-arc basin are attributed to diapirism in the upper mantle beneath southern South America. The tectonic setting and sequence of igneous and tectonic events suggest that diapirism may have been initiated in response to subduction.  相似文献   

8.
Oxygen isotope analyses have been made on 27 tholeiitic basalts from the Lau and Mariana marginal ocean basins and from mid-ocean ridges. The 18O values are related to the extent of hydration by submarine weathering as indicated by H2O? and total water content. Extrapolation to zero H2O? content gives a δ18O value of 5.5‰ on the SMOW scale for unaltered marginal basin basalts, in exact agreement with the oxygen isotope “signature” of ocean-ridge tholeiites. Three alkali basalts from seamount provinces also fit the tholeiite relationship. A Lau Basin gabbro has the tholeiitic 18O content, but an Indian Ocean gabbro is unusually light (δ18O = 4.0 for whole rock, plagioclase, and amphibole), and resembles the low -18O Iceland basalts. The basalt data confirm petrologic and chemical evidence for origin of marginal basins by extensional processes with production of basalts from depleted mantle material isotopically identical to the source of ocean-ridge tholeiites.  相似文献   

9.

The Madang Cenozoic sodic alkaline basalt occurred in the eastern margin of the Tibetan Plateau, where is a key tectonic transform region of Tibet, North China, and Yangtze blocks. The basalts are characterized by the variation in SiO2=42%–51%, Na2O/K2O>4, belonging to the sodic alkaline basalt series. The rocks are enriched in Ba, Th, Nb, Ta, relative to a slight depletion in K, Rb in the trace and rare earth element (REE) spider diagrams that are similar to the typical oceanic island alkaline basalt. The Sr-Nd-Pb isotopic compositions suggest that they are derived from a mixed mantle reservoir. The western Qinling-Songpan tectonic region was controlled by Tibet, North China and Yangtze blocks since Cenozoic, therefore, the region was in the stage of the substance converge from the mantle to upper crust, producing a mixed mantle reservoir in the studied area. The Madang basalts occurred in the specific tectonic background, they result from partial melting of a mixed asthenospheric mantle reservoir in the western Qinling-Songpan tectonic node.

  相似文献   

10.
An extensive set of samples representative of the evolution of Etna since the beginning of its activity in the Quaternary has been analyzed for trace element contents by neutron activation on whole rock samples. Major element data were taken fromTanguy (1980). Several petrogenetic processes have played a role in the observed chemical variations:
  • binary trace element plots form linear arrays extrapolating to the origin for most second row and higher transition elements and the rare alkali metals thus indicating their hygromagmaphile (HYG) character;
  • 3d transition elements show two types of behavior when plotted against an HYG element. Concentrations remain relatively constant for the less evolved rocks in the range from tholeiite through transitional basalts to alkali basalts. On the contrary, concentrations drop rapidly in the series evolving from alkali basalts to trachytes;
  • in addition to the latter two general trends, geochemical regularities specific to Etnean lavas are born out. Rare alkali metals, as well as potassium, for instance, show a trend of abnormal enrichment in the most recent historical eruptions.
    • The recurrence in time of rather primary magmas indicates that the petrogenetic evolution of the lavas cannot be explained by the differentiation of a single batch of mantle derived magma. However, the chemical composition of the mantle source has remained relatively homogeneous with a nature very near the chondritic one. Mineralogically, the source must differ from that for oceanic basalts to account for Th/Ta differences. Early tholeiitic primary magma evolving into later alkali basalts is adequately modeled by variations of the degree of partial melting of a closed system source. Highly evolved products have episodically been emitted as a result of extensive crystal fractionation, during which no basalts are erupted. These episodes are abruptly interrupted by the eruption of basalts presumably indicating recharge of the magma plumbing system by pristine mantle derived material. Selective contamination in alkali elements from the sedimentary basement is also indicated by the data. The presence of partially digested sedimentary xenoliths is an additional argument for this process.  相似文献   

    11.
    Comparisons are made between the Lesser Antilles and the South Sandwich Islands, the recent volcanic island chains at the eastern margins of the Caribbean and Scotia arcs. Although situated in similar geological and structural environments there are differences in the type of volcanic activity which prevails in these two arcs and in the petrography and chemistry of the lavas emitted. There is good evidence that the South Sandwich Islands are in general appreciably younger than the islands of the Lesser Antilles. Basaltic rocks predominate in the South Sandwich Islands whereas andesite is the dominant rock-type of the Lesser Antilles. Many of the lavas of the South Sandwich Islands, including the andesites and dacites are aphyric whereas those of the Lesser Antilles are almost invariably porphyritic. The basalts of the South Sandwich Islands are of tholeiitic type and the series shows more pronounced iron enrichment than does that of the Lesser Antilles. Basalts of the South Sandwich Islands have a lower Fe2O3/FeO ratio, contain lower concentrations of K, Sr and Ba and higher Cr, Co and Ni than the basalts of the West Indies. It is thought that the South Sandwich Islands may represent a volcanic island-arc in the early stages of development and the Lesser Antilles a later stage.  相似文献   

    12.
    Miocene to Quaternary large basaltic plateaus occur in the back-arc domain of the Andean chain in Patagonia. They are thought to result from the ascent of subslab asthenospheric magmas through slab windows generated from subducted segments of the South Chile Ridge (SCR). We have investigated three volcanic centres from the Lago General Carrera–Buenos Aires area (46–47°S) located above the inferred position of the slab window corresponding to a segment subducted 6 Ma ago. (1) The Quaternary Río Murta transitional basalts display major, trace elements, and Sr and Nd isotopic features similar to those of oceanic basalts from the SCR and from the Chile Triple Junction near Taitao Peninsula (e.g., (87Sr/86Sr)o = 0.70396–0.70346 and εNd = + 5.5 − + 3.0). We consider them as derived from the melting of a Chile Ridge asthenospheric mantle source containing a weak subduction component. (2) The Plio-Quaternary (< 3.3 Ma) post-plateau basanites from Meseta del Lago Buenos Aires (MLBA), Argentina, likely derive from small degrees of melting of OIB-type mantle sources involving the subslab asthenosphere and the enriched subcontinental lithospheric mantle. (3) The main plateau basaltic volcanism in this region is represented by the 12.4–3.3-Ma-old MLBA basalts and the 8.2–4.4-Ma-old basalts from Meseta Chile Chico (MCC), Chile. Two groups can be distinguished among these main plateau basalts. The first group includes alkali basalts and trachybasalts displaying typical OIB signatures and thought to derive from predominantly asthenospheric mantle sources similar to those of the post-plateau MLBA basalts, but through slightly larger degrees of melting. The second one, although still dominantly alkalic, displays incompatible element signatures intermediate between those of OIB and arc magmas (e.g., La/Nb > 1 and TiO2 < 2 wt.%). These intermediate basalts differ from their strictly alkalic equivalents by having lower High Field Strength Element (HFSE) and higher εNd (up to + 5.4). These features are consistent with their derivation from an enriched mantle source contaminated by ca. 10% rutile-bearing restite of altered oceanic crust. The petrogenesis of the studied Mio-Pliocene basalts from MLBA and MCC is consistent with contributions of the subslab asthenosphere, the South American subcontinental lithospheric mantle and the subducted Pacific oceanic crust to their sources. However, their chronology of emplacement is not consistent with an ascent through an asthenospheric window opened as a consequence of the subduction of segment SCR-1, which entered the trench at 6 Ma. Indeed, magmatic activity was already important between 12 and 8 Ma in MLBA and MCC as well as in southernmost plateaus, i.e., 6 Ma before the subduction of the SCR-1 segment. We propose a geodynamic model in which OIB and intermediate magmas derived from deep subslab asthenospheric mantle did uprise through a tear-in-the-slab, which formed when the southernmost segments of the SCR collided with the Chile Trench around 15 Ma. During their ascent, they interacted with the Patagonian supraslab mantle and, locally, with slivers of subducted Pacific oceanic crust that contributed to the geochemical signature of the intermediate basalts.  相似文献   

    13.
    The Aeolian volcanic arc displays a wide range of magmatic products. Mafic lavas range from hypersthene normative calc-alkaline basalts to silica-undersaturated potassic absarokites, although the former are spatially and temporally dominant, consistent with the subduction-zone tectonic setting. In addition, intermediate and acidic members of the various fractionation series may be recognised. Large variations in trace element and isotope ratios accompany the rapid calc-alkaline to potassic transition, and it is argued that these may be largely explained in terms of subduction-zone mantle enrichment involving components derived from both basaltic ocean crust and subducted sediments. In addition, it seems that the mantle wedge itself was substantially heterogeneous prior to the onset of subduction zone processes. Not only are these subduction components similar to those proposed in a number of island arcs, but they also resemble those recognised in the ultra-potassic lavas of the Roman province, supporting recent subduction-related petrogenetic models of the Roman magmatism. Although subducted sediment plays an important role in the generation of some potassic magmatism, it is not uniquely responsible for K2O-rich lavas, which are also produced without a large sediment contribution.  相似文献   

    14.
    The quaternary Rockeskyllerkopf Volcanic Complex (RVC) comprises three spatially and temporally distinct volcanic centers that can also be distinguished on the basis of their geochemical signatures. All the volcanic products in the complex are olivine basanites whose major and trace element compositions span almost the entire range defined for the West Eifel field as a whole. The RVC lavas have lower Al2O3, Na2O and Y contents and higher TiO2, CaO, K2O, Sc, V, Co, Rb, and Ba than the Tertiary lavas in nearby Hocheifel volcanic field. Within the complex, the oldest South East Lammersdorf Center (SEL) comprises primitive lavas with an average MgO content of ～11 wt.% and LaN/YbN of 29?±?2. The second center, Mäuseberg, has similar MgO to SEL but is distinct in its much higher LaN/YbN of 42?±?2. The Rockeskyllerkopf Center, which was erupted after a break in activity, comprises lavas similar in composition to the SEL Center but with distinctly higher Al2O3 and lower MgO contents. Given the lack of evidence for significant fractionation or assimilation in the RVC lavas, we attribute the compositional variations within and between the centers of the RVC to be due to variations in the composition of the source region in combination with magma mixing. Our preferred model involves 1–5% partial melting of LREE-enriched mantle in the garnet stability field, likely within the thermal boundary layer at the base of the lithospheric mantle. These melts mixed to variable degrees with 2–4% partial melts of phlogopite-spinel peridotite formed at higher levels in the modally metasomatised lithospheric mantle.  相似文献   

    15.
    The properties and tectonic significance of the fault bound zone on the northern margin of the Central Tianshan belt are key issues to understand the tectonic framework and evolutionary history of the Tianshan Orogenic Belt. Based on the geological and geochemical studies in the Tianshan orogenic belt, it is suggested that the ophiolitic slices found in the Bingdaban area represent the remaining oceanic crust of the Early Paleozoic ocean between the Hazakstan and Zhungaer blocks. Mainly composed of basalts, gabbros and diabases, the ophiolites were overthrust onto the boundary fault between the Northern Tianshan and Central Tianshan belts. The major element geochemistry is characterized by high TiO2 (1.50%–2.25%) and MgO (6.64%–9.35%), low K2O (0.06%–0.41%) and P2O5 (0.1%–0.2%), and Na2O>K2O as well. Low ΣREE and depletion in LREE indicate that the original magma was derived from a depleted mantle source. Compared with a primitive mantle, the geochemistry of the basalts from the Bingdaban area is featureded by depletion in Th, U, Nb, La, Ce and Pr, and unfractionated in HFS elements. The ratios of Zr/Nb, Nb/La, Hf/Ta, Th/Yb and Hf/Th are similar to those of the typical N-MORB. It can be interpreted that the basalts in the Bingdaban area were derived from a depleted mantle source, and formed in a matured mid-oceanic ridge setting during the matured evolutionary stage of the Northern Tianshan ocean. In comparison with the basalts, the diabases from the Bingdaban area show higher contents of Al2O3, ΣREE and HFS elements as well as unfractionated incompatible elements except Cs, Rb and Ba, and about 10 times the values of the primitive mantle. Thus, the diabases are thought to be derived from a primitive mantle and similar to the typical E-MORB. The diabases also have slight Nb depletion accompanying no apparent Th enrichment compared with N-MORB. From studies of the regional geology and all above evidence, it can be suggested that the diabases from the Bingdaban area were formed in the mid-oceanic ridge of the Northern Tianshan ocean during the initial spreading stage. Supported by the Major State Research Program of PRC (Grant No. 2001CB409801), the National Natural Science Foundation of China (Grant Nos. 40472115 and 40234041) and the State Research Program of China Geological Survey (Grant No. 2001130000-22)  相似文献   

    16.
    Basalt and diabase from the Cretaceous Dumisseau Formation, southern Haiti have Mg-numbers of 43–63, TiO2 contents of 1.6–3.9% and La abundances of 3.6–15.3 ppm.La/Ta ratios average 10, and indicate that the basalts are oceanic in character, distinct from the arc associations forming the northern part of Haiti. Oldest lavas have low TiO2 (1.6%) and are LREE-depleted, similar to N-MORBs, whereas overlying lavas have higher TiO2 (2–3.9%) and are LREE-enriched, similar to E-MORBs or hotspot basalts.87Sr86Sr ratios vary from 0.70280 to 0.70316,143Nd144Nd from 0.512929 to 0.513121, and206Pb204Pb from 19.00 to 19.27. LREE-depleted lavas have high143Nd144Nd (0.51309–0.51310) typical of MORBs, whereas143Nd144Nd in the LREE-enriched lavas varies widely (0.512929–0.513121).Chemical features of the Dumisseau basalts are equivalent to those of Caribbean seafloor basalts recovered on DSDP Leg 15, and support the contention that the Dumisseau is an uplifted section of Caribbean Sea crust. Oldest lavas are analogous to MORB-like basalts cored at Leg 15 Sites 146, 150, 152 and 153, and the overlying lavas are analogous to incompatible-element-enriched basalts cored at Site 151 on the Beata Ridge. Isotopic compositions of the Dumisseau basalts overlap with those of the eastern Pacific Galapagos and Easter Island hotspots. However, the presence of N-MORB basalts in the lower part of the Dumisseau and at the majority of Leg 15 Sites indicates that the anomalously thick Caribbean crust probably did not originate as a hotspot-related basaltic plateau, but may have been generated by on-ridge or near-ridge hotspot magmatism.  相似文献   

    17.
    The Nuku Hiva Pliocene island (Marquesas, French Polynesia) is composed of a large half-collapsed tholeiitic shield volcano (the Tekao edifice), the caldera of which is filled up by the younger Taiohae volcano. The latter edifice is characterised by a complex magmatic association including minor mafic lavas (olivine tholeiites, alkali basalts and basanites), abundant intermediate lavas (hawaiites with subsidiary mugearites, both covering 47% of the surface of the volcano) and lesser amount of evolved lavas (K-rich and Na-rich trachytes and minor benmoreites, covering 25% of the edifice). Most intermediate and evolved Taiohae lavas are amphibole-rich and crystallised under high oxygen fugacities. The mafic Taiohae lavas originated from lower degree of melting of mantle sources more enriched than that of the shield volcano tholeiites. We show that closed-system fractional crystallisation of the Taiohae basaltic magmas can account for the origin of Taiohae hawaiites and mugearites, provided that separation of substantial amount of amphibole and/or apatite occurred during this process. Similarly, fractionation of benmoreitic magmas involving large amounts of amphibole and mica may account for the genesis of K-rich and Na-rich trachytes, respectively. However, fractional crystallisation cannot account for the derivation of benmoreitic magmas from mugearitic ones: since, this process fails to explain the abrupt increase in K2O from the latter to the former. In addition, the isotopic signature of trachytes and benmoreites is clearly distinct (more EM II-rich) from that of Taiohae basalts, hawaiites and mugearites. Three hypotheses could account for the genesis of benmoreitic magmas: assimilation of oceanic material with a strong EM II signature, differentiation of non-sampled mafic magmas derived from a mantle source having a EM II-rich signature and partial melting at depth of mafic material with a strong EM II signature. The oxidised character of Nuku Hiva lavas, uncommon in oceanic island settings, suggests interaction with water and/or the contribution of an oxidised (altered?) source material to their genesis.  相似文献   

    18.
    The composition of basalts erupted at the earliest stages in the evolution of a back-arc basin permit unique insights into the composition and structure of the sub-arc mantle. We report major and trace element chemical data and O-, Sr-, Nd-, and Pb- isotopic analyses for basalts recovered from four dredge hauls and one ALVIN dive in the northern Mariana Trough near 22°N. The petrography and major element chemistry of these basalts (MTB-22) are similar to tholeiites from the widest part of the Trough, near 18°N (MTB-18), except that MTB-22 have slightly more K2O and slightly less TiO2. The trace element data exhibit a very strong arc signature in MTB-22, including elevated K, Rb, Sr, Ba, and LREE contents; relatively lowK/Ba and highBa/La andSr/Nd. The Sr- and Nd- isotopic data plot in a field displaced from that of MTB-18 towards Mariana arc lavas, and the Pb-isotopic composition of MTB-22 is indistinguishable from Mariana arc lavas and much more homogeneous than MTB-18. Mixing of 50–90% Mariana arc component with a MORB component is hypothesized. We cannot determine whether this resulted from physical mixing of arc mantle and MORB mantle, or whether the arc component is introduced by metasomatism of MORB-like mantle by fluids released from the subducted lithosphere. The strong arc signature in back-arc melts from the Mariana Trough at 22°N, where the back-arc basin is narrow, supports general models for back-arc basin evolution whereby early back-arc basin basalts have a strong arc component which diminishes in importance relative to MORB as the back-arc basin widens.  相似文献   

    19.
    Late Cenozoic alkali basaltic lavas of the Lunar Crater Volcanic Field (LCVF), located in the center of the Great Basin of the Western U.S.A., contain a diverse suite of nodule samples of the lower crust and upper mantle. This paper documents a composite nodule from the Marcath flow in which an amphibole-bearing wehrlite (59% olivine, 30% clinopyroxene, 6% amphibole) is cut by a 6–9 mm wide vein of andesine-amphibolite (80% kaersutite, 15% andesine, 3% ilmenite). Aside from nodule-basalt reaction at the nodule exterior, there is little chemical variation either within or between individual grains of hydrous and anhydrous phases in the vein and host wehrlite. Furthermore, there is no systematic compositional zoning in the wehrlite relative to vein proximity. The whole-rock major and trace element composition of the vein is similar to a primitive (Mg/(Mg+Fe)=0.692) basaltic liquid and has Al, Fe, Mg, Ca, Mn, Na, K, Zr, Y and Sr contents similar to basalts observed in the LCVF. In contrast to the Sr isotopic equilibrium displayed by vein feldspar and vein amphibole, Sr isotopic disequilibrium is exhibited between the vein (0.70318(4)), wehrlite (0.70322(4)), and host basalt (0.70357(5) n=3). However, the Sr isotopic ratios of older LCVF basalts (0.7030–0.7038; n=14) overlap those of the vein and wehrlite, and the magmatic activity leading to vein and wehrlite formation could be related to this older phase of LCVF volcanism. Petrographic and geochemical evidence is not consistent with a metasomatic origin for the vein and instead supports the view that the vein originated by the intrusion into a wehrlite mass and subsequent crystallization of a relatively primitive alkali basaltic magma in the lower crust or upper mantle. The wehrlite contains olivine of FO71 and probably originated by crystal separation and accumulation from a relatively differentiated basaltic magma in the lower crust or upper mantle.  相似文献   

    20.
    Three groups of spinels have been identified in dredged basalts from the Mid-Atlantic Ridge in the Azores region (30–40°N): (1) magnesiochromites with 0.4–0.5 Cr/(Cr + Al) are most common and characteristic of olivine tholeiites of the region; (2) titaniferous magnesiochromites are found in an olivine basalt with alkali affinities, of local occurrence and evolved in relatively high fugacity of oxygen; (3) chromian spinels with 0.23 Cr/(Cr + Al) occur in unusual high-Al picrites of local occurrence and possible high-pressure origin. Spinels are restricted in occurrence to the least fractionated lavas, with FeO*/FeO* + MgO ratio less than 0.575 and with Cr content greater than 350 ppm. A close relationship between Al content of liquidus spinel and Al content of magma has been observed for basaltic types. High-Al spinels deviating from this relationship, such as those found in picritic lavas from the Mid-Atlantic Ridge, may have crystallized at high pressure. The use of spinels as geobarometers in magmas of a restricted compositional range seems a promising prospect. There is no evidence of systematic variation in spinel chemistry of occurrence along the Mid-Atlantic Ridge, such as could be related to different mantle sources of the basalts, plume versus non-plume or binary mantle mixing.  相似文献   

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