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1.
This paper describes the chemistry of 33 basaltic rocks dredged from the West Mariana basin and from the Mariana trench during the R/V “Dmitry Mendeleev” 1976 cruise in the western Pacific.The shipboard investigations were carried out by an international working group of 66 earth scientists under the IGCP Project “Ophiolites” and sponsored by the U.S.S.R. Academy of Sciences, Moscow. The purpose of the expedition was to investigate the structure and composition of the oceanic crust of marginal basins, remnant island arcs and deep-sea trenches. Tholeiitic basalts and gabbros as well as ultramafic rocks in various stages of alteration were dredged from the central part of the West Mariana basin demonstrating the presence of oceanic crust.The Pacific slope of the Mariana trench yielded altered basaltic rocks of tholeiitic and alkalic (? trachybasaltic to shoshonitic) composition. The lower part of the island arc slope contains typical tholeiitic basalts, dolerites and gabbros as well as ultramafics associated with flysch-type sediments. This is strong evidence for the formation of an “ophiolite-schuppenzone”, probably due to subduction of Pacific oceanic crust.Associated with these rocks are amygdaloidal, highly magnesian lavas (similar to boninites), which have not been recognized previously in oceanic ridge basalts.These rocks (together with the dolerites) are interpreted as parts of the Mariana island arc and are thought to be the first stage of island arc development (an immature island arc).  相似文献   

2.
Tertiary volcanic rocks of Carriacou occupy two-thirds of the island. The volcanics include volcaniclastics, lava flows and dome lavas and range in composition from basalts to andesites. Carriacou basalts fall into two petrographic types (a) clinopyroxene-plagioclase-phyric basalts and (b) olivine microphyric basalts; the latter having higher MgO and lower Al2O3 than the clinopyroxene basalts. Both types are unusually rich in mafic minerals compared with Lesser Antilles basalts in general, although similar types have been reported from the nearby island of Grenada. The potash to silica ratios are relatively high and confirm the similarity between Carriacou and Grenada basalts and the differences between these basalts and basalts from other islands of the Lesser Antilles. The basaltic andesites and andesites from Carriacou correspond closely in mineralogical and chemical composition with typical andesites found elsewhere in the Lesser Antilles. The geochemistry of the volcanics shows that the olivine microphyric basalts display tholeiitic affinities whereas the clinopyroxeneplagioclase-phyric basalt, basaltic andesites and andesites are calcalkaline. The compositional gradation in both the geochemistry and mineralogy of these volcanics suggests that fractional crystallization played an important role in the derivation of the various magma.  相似文献   

3.
Gabbroic rocks occur only in the west, and are the oldest intrusions in the Peninsular Ranges Cordilleran batholith. They comprise an olivine-pyroxene gabbronorite series and an amphibole gabbro series both of which contain abundant plagioclase and amphibole. They formed by crystal accumulation and in situ differentiation, in multiple intrusive complexes, and are not considered to be related by fractionation to the granitoid rocks of the batholith.Pure mineral separates of plagioclase, olivine, clinopyroxene, orthopyroxene, and amphibole were obtained by magnetic and heavy-liquid methods from a representative suite of gabbroic rocks. Their major- and trace-element contents were determined by X-ray fluorescence, and the data used to test hypotheses on the genesis and fractionation of the gabbros.The plagioclases range from An98 to An65 in composition, olivines, Fo79 to Fo70, occur in rocks where An>36. All clinopyroxenes are augite with Mg #'s varying from 81.1 to 64.7. Orthopyroxene occurs where An<92, and is generally inverted pigeonite or bronzite, and has Mg #'s ranging from 77.9 to 52.1. The amphiboles include tschermakite, tschermakitic hornblende, pargasite, pargasitic hornblende, ferroan pargasite, magnesio-hornblende, and magnesio-taramite, Mg #'s range from 80.4 to 62.5. Systematic chemical and mineralogical changes confirm that differentiation, controlled by mineral assemblages of plagioclase, olivine, spinel, and clinopyroxene initially, and orthopyroxene, amphibole, and magnetite later, took place between intrusive episodes and in situ.The highly clacic plagioclase coexisting with olivine and amphibole suggests that the gabbros were formed from hydrous mafic magmas. The modal mineralogy of the gabbros, and the chemistry of the minerals is very similar to that of the cumulate blocks of the Lesser Antillean volcanoes. These features confirm that the gabbros were derived from a hydrous mafic magma, with high Al2O3 and low TiO2 contents, typical of orogenic environments.Cumulate minerals from the gabbros show little or no zoning and are considered to have formed in equilibrium with the evolving melts. Selected trace-element contents and distribution coefficients are used to calculate the compositions of the melts. The calculations show that the melts in equilibrium with the olivine-pyroxene gabbronorite series contain approximately 100–200 ppm Ba, 200–400 ppm Sr, 30-10 ppm Ni, 20-10 ppm Co, and 300-100 ppm V. K/Rb ratios of the melts, derived from post-cumulus and prismatic amphiboles, are generally in the range 550-250. These values are typical of calc-alkalic basalts and andesites, and it is suggested that they may have erupted at the surface to form a coeval calc-alkalic volcanic sequence.  相似文献   

4.
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   

5.
The Ladakh Mesozoic ophiolite belt (western Himalaya) contains a pile of volcanic thrust sheets (Dras unit) which differ significantly in structure and composition from the ophiolitic mélange zones. The Dras unit is composed of pillow lavas, doleritic sills, very irregular basaltic (?basaltic andesites) and dacitic flows intercalated with pyroclastics, volcanoclastic sediments and radiolarian cherts. According to fossil evidence, this volcanism must have been active between Upper Jurassic and Upper Cretaceous.The presence of relict primary minerals, such as magnesiochromite, clinopyroxene, hastingsitic hornblende and Ti-magnetite as well as distinctive bulk chemistries, suggests that the volcanics belong to island arc tholeiite and to calc-alkaline rock series, typical of present island arcs in the Caribbean and Pacific.Model calculations incorporating probed phenocryst phases indicate that in addition to olivine, clinopyroxene and plagioclase, amphibole and titanomagnetite are crucial fractionating phases in the development of the dacites from a primitive tholeiitic melt. The latter process must have taken place at about 1000°C and at moderate depth of 5–15 km within or underneath the island arc. Today, hornblende-bearing mafic cumulates appear in the vicinity of Kargil within and close to the Dras volcanics.In a Sr-evolution diagram, the Dras volcanics have yielded a “pseudo-isochron” with a low initial ratio of 0.7035 ± 0.0003, which is in the same range as the mean of modern island arc volcanics. However, a geologically unrealistic age of 263 m.y., is obtained from the slope of this isochron.The upper mantle is regarded as the source material for the island arc tholeiitic magmas. Enrichment in K, Ba, Sr and LREE supports the involvement of components derived from dehydration or incipient melting of subducted Tethyan oceanic crust in the mantle.  相似文献   

6.
Petrological evolution of the Tertiary island arc in the Izu-Mariana region has been accompanied by the development of three different volcanic suites: 1) oceanridge basalt now exposed as the metamorphic basement on Yap; 2) island-arc tholeiites of Eocene to early Oligocene age characterized by low contents of incompatible elements at all levels of silica enrichment; and 3) calc-alkalic rocks of late Oligocene to early Miocene age showing higher contents of silica and incompatible elements. All these three suites have primitive, undifferentiated basalts or andesites (boninites) characterized by high Mg/Fe, Cr, and Ni, suggesting that they have been derived from an upper mantle peridotite at relatively high temperatures. The earliest volcanism appears to have occurred at a spreading ridge. Later, as subduction proceeded, the island-arc tholeiite magma may have been produced by the introduction of a smaller amount of water into the locus of fusion from the subducted oceanic crust. An increasingly larger amount of water introduced into the same region could have led to the development of the more siliceous, calc-alkalic magma, as represented typically by the boninite.  相似文献   

7.
A detailed petrological study of the Banda Arc volcanism, documented by extensive microprobe whole-rock and mineral analysis, shows that the apparent geographical eastwards continuity of the Sunda Arc conceals a major geochemical discontinuity adjacent to the southern end of the Weber Deep beneath Serua. The alkali contents and Sr isotope ratios suggest that Nila, Teun and Damar form one volcanic group distinct from Banda and Manuk, and Serua is in the border discontinuity, reflected in its widely fluctuating Fe/Mg partitioning between mineral phases and its variable Sr isotope ratios. With the exception of basalt from S. W. Ambon, all lavas are quartz normative and typical of an ensimatic oceanic island arc. They range from tholeiitic basalt and dacite on S. W. Ambon and Banda, through low-K calc-alkaline andesites on Manuk and Serua, to high-K calc-alkaline andesites on Nila, Teun, Damar, Gunung Api north of Wetar, and Romang which also contains dacite. The higher potassium contents are normally contained in biotite and hornblende, and occasionally in the groundmass glass. Increasing potassium from Banda to Manuk may be related to increasing Benioff Zone depth between the 100 to 200 km contours, but the lavas of Gunung Api north of Wetar have insufficient potassium to be derived from the underlying 400–500 km Benioff Zone. Older cordierite dacites (ambonites) on North Ambon must be derived from a underlying continental crust, but the younger tholeiitic lavas of S.W. Ambon and Banda may be related to a shallow subduction zone dipping southwards from Seram.  相似文献   

8.
Analyses for Ti, Zr, Y, Nb and Sr in over 200 basaltic rocks from different tectonic settings have been used to construct diagrams in which these settings can usually be identified. Basalts erupted within plates (ocean island and continental basalts) can be identified using a Ti-Zr-Y diagram, ocean-floor basalts, and low-potassium tholeiites and calc-alkali basalts from island arcs can be identified using a Ti-Zr diagram (for altered samples) and a Ti-Zr-Sr diagram (for fresh samples). Y/Nb is suggested as a parameter for indicating whether a basalt is of tholeiitic or alkalic nature. Analyses of dykes and pillow lavas from the Troodos Massif of Cyprus are plotted on these diagrams and appear to the tholeiitic ocean-floor rocks.  相似文献   

9.
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   

10.
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11.
Island arc volcanism has contributed and is still contributing to continental growth, but the composition of island arcs differs from that of the upper continental crust in its lower abundance of Si, K, Rb, Ba, Sr and light rare earth elements. In their advanced stage of evolution, island arcs contain more than 80% of tholeiitic and 15% of ‘island arc’ calc-alkaline rocks with varied SiO2 contents. The larger proportion of tholeiitic rocks is in the lower crustal levels. The high stratigraphical levels of the island arcs are composed of tholeiitic plus calc-alkaline and/or high potash (shoshonitic) associations with higher abundances of K, Rb, Sr, and Ba. Stratification of the island arc crust is accentuated by another type of calc-alkaline volcanism (Andean type) originating at a late stage of arc evolution, probably by partial melting at the base of the crust. This causes enrichment of the upper crust in K, Rb, Ba and REE and accounts for upper crustal abundances of these elements as well as of SiO2.  相似文献   

12.
Geochemical and mineralogical characteristics of the Eocene volcanic succession in Tafresh area of the Urumieh–Dokhtar Magmatic Assemblage (UDMA) are unique in the 2000‐km‐length assemblage. Demonstrating rather steep rare earth element (REE) patterns and the widespread presence of amphibole (+biotite) phenocrysts are two distinct characters that dominate the Eocene volcanic succession of mainly andesitic composition. Coincidence of the geochemical and mineralogical characteristics of the whole volcanic succession with adakites, rather amphibole‐ (+biotite) rich dacitic (with 61–64 wt% SiO2) stocks and dykes, is considered as the key in unraveling the role of ‘slab‐derived melt contribution’ in petrogenesis of the volcanic succession. Slab‐derived melting has been an ongoing process that metasomatized some parts of the mantle wedge from which hybrid rocks (andesites) are derived. Basalts with distinct signatures of slab melt metasomatism are yet another support for the occurrence of slab melting. Interlayering of normal, island‐arc‐type calc‐alkaline volcanic rocks with the slab‐melt metasomatized basalts and hybrid andesites suggests that the slab melting has been motivated by the subduction. Formation of the Tafresh Caldera, the likely consequence of an explosive eruption, is compatible with the volatile‐bearing nature of the adakitic volcanism in the study area. It is indicated by the ubiquitous presence of the hydrous minerals. Beneath the Tafresh area, in Eocene time, the subducting slab seems to have reached a critical high depth that is enough for the development of amphibolite–eclogite. The slab deformation, motivated by the geometry of subduction and/or the underlying mantle's steeper geotherms, is suggested to have resulted in the slab melting that helped develop a rock assemblage unique to the UDMA.  相似文献   

13.
Abstract   The geological, geochemical and mineralogical data of dismembered ophiolites of various ages and genesis occurring in accretionary piles of the Eastern Peninsulas of Kamchatka enables us to discriminate three ophiolite complexes: (i) Aptian–Cenomanian complex: a fragment of ancient oceanic crust, composed of tholeiite basalts, pelagic sediments, and gabbroic rocks, presently occurring in a single tectonic slices (Afrika complex) and in olistoplaques in Pikezh complex of the Kamchatsky Mys Peninsula and probably in the mélange of the Kronotsky Peninsula; (ii) Upper Cretaceous complex, composed of highly depleted peridotite, gabbro and plagiogranite, associated with island arc tholeiite, boninite, and high-alumina tholeiitic basalt of supra-subduction origin; and (iii) Paleocene–Early Eocene complex of intra-island arc or back-arc origin, composed of gabbros, dolerites (sheeted dykes) and basalts produced from oceanic tholeiite melts, and back-arc basin-like dolerites. Formation of the various ophiolite complexes is related to the Kronotskaya intra-oceanic volcanic arc evolution. The first ophiolite complex is a fragment of ancient Aptian–Cenomanian oceanic crust on which the Kronotskaya arc originated. Ophiolites of the supra-subduction zone affinity were formed as a result of repeated partial melting of peridotites in the mantle wedge up to the subduction zone. This is accompanied by production of tholeiite basalts and boninites in the Kamchatsky Mys segment and plagioclase-bearing tholeiites in the Kronotsky segment of the Kronotskaya paleoarc. The ophiolite complex with intra-arc and mid-oceanic ridge basalt geochemical characteristics was formed in an extension regime during the last stage of Kronotskaya volcanic arc evolution.  相似文献   

14.
An association of adakite, magnesian andesite (MA), and Nb-enriched basalt (NEB) volcanic flows, which erupted within ‘normal’ intra-oceanic arc tholeiitic to calc-alkaline basalts, has recently been documented in ∼2.7 Ga Wawa greenstone belts. Large, positive initial ?Nd values (+1.95 to +2.45) of the adakites signify that their basaltic precursors, with a short crustal residence, were derived from a long-term depleted mantle source. It is likely that the adakites represent the melts of subducted late Archean oceanic crust. Initial ?Nd values in the MA (+0.14 to +1.68), Nb-enriched basalts and andesites (NEBA) (+1.11 to +2.05), and ‘normal’ intra-oceanic arc tholeiitic to calc-alkaline basalts and andesites (+1.44 to +2.44) overlap with, but extend to lower values than, the adakites. Large, tightly clustered ?Nd values of the adakites, together with Th/Ce and Ce/Yb systematics of the arc basalts that rule out sediment melting, place the enriched source in the sub-arc mantle. Accordingly, isotopic data for the MA, NEBA, and ‘normal’ arc basalts can be explained by melting of an isotopically heterogeneous sub-arc mantle that had been variably enriched by recycling of continental material into the shallow mantle in late Archean subduction zones up to 200 Ma prior to the 2.7 Ga arc. If the late Archean Wawa adakites, MA, and basalts were generated by similar geodynamic processes as their counterparts in Cenozoic arcs, involving subduction of young and/or hot ocean lithosphere, then it is likely that late Archean oceanic crust, and arc crust, were also created and destroyed by modern plate tectonic-like geodynamic processes. This study suggests that crustal recycling through subduction zone processes played an important role for the generation of heterogeneity in the Archean upper mantle. In addition, the results of this study indicate that the Nd-isotope compositions of Archean arc- and plume-derived volcanic rocks are not very distinct, whereas Phanerozoic plumes and intra-oceanic arcs tend to have different Nd-isotopic compositions.  相似文献   

15.
Volcanism in the Taupo Volcanic Zone (TVZ) and the Kermadec arc-Havre Trough (KAHT) is related to westward subduction of the Pacific Plate beneath the Indo-Australian Plate. The tectonic setting of the TVZ is continental whereas in KAHT it is oceanic and in these two settings the relative volumes of basalt differ markedly. In TVZ, basalts form a minor proportion (< 1%) of a dominant rhyolite (97%)-andesite association while in KAHT, basalts and basaltic andesites are the major rock types. Neither the convergence rate between the Pacific and Indo-Australian Plates nor the extension rates in the back-arc region or the dip of the Pacific Plate Wadati-Benioff zone differ appreciably between the oceanic and continental segments. The distance between the volcanic front and the axis of the back-arc basin decreases from the Kermadec arc to TVZ and the distance between trench and volcanic front increases from around 200 km in the Kermadec arc to 280 km in TVZ. These factors may prove significant in determining the extent to which arc and backarc volcanism in subduction settings are coupled.All basalts from the Kermadec arc are porphyritic (up to 60% phenocrysts) with assemblages generally dominated by plagioclase but with olivine, clinopyroxene and orthopyroxene. A single dredge sample from the Havre Trough back arc contains olivine and plagioclase microphenocrysts in glassy pillow rind and is mildly alkaline (< 1% normative nepheline) contrasting with the tholeiitic nature of the other basalts. Basalts from the TVZ contain phenocryst assemblages of olivine + plagioclase ± clinopyroxene; orthopyroxene phenocrysts occur only in the most evolved basalts and basaltic andesites from both TVZ and the Kermadec Arc.Sparsely porphyritic primitive compositions (Mg/(Mg+Fe2) > 70) are high in Al2O3 (>16.5%), and project in the olivine volume of the basalt tetrahedron. They contain olivine (Fo87) phenocrysts and plagioclase (> An60) microphenocrysts. These magmas have ratios of CaO/Al2O3, A12O3/TiO2 and CaO/TiO2 in the range of MORB and MORB picrites and can evolve to the low-pressure MORB cotectic by crystallisation of olivine±plagiociase. Such rocks may be the parents of other magmas whose evolutionary pathways are complicated by interaction of crystal fractionation, crystal accumulation and mixing processes and the filtering action of crust of variable density and thickness. The interplay of these processes likely accounts for the scatter of data about the cotectic. More evolved rocks from both TVZ and KAHT contain clinopyroxene and orthopyroxene phenocrysts and their compositions merge with basaltic andesites and andesites. Stepwise least-squares modelling using phenocryst assemblages in proportions observed in the rocks suggest that crystal fractionation and accumulation processes can account for much of the diversity observed in the major-element compositions of all lavas.We conclude that the parental basaltic magmas for volcanism in the TVZ and KAHT segments are similar thereby implying grossly similar source mineralogy. We attribute the diversity to secondary processes influencing liquids as they ascended through complex plumbing systems in the sub arc mantle and cross.  相似文献   

16.
The Upper Tertiary to Quaternary volcanic complex of Kouh-e-Shahsavaran in southeastern Iran is composed of calc-alkaline rocks of island are type (high-alumina basalts, basic andesites, andesites and dacites) even though it was emplaced on the continental basement. The volcanic rocks of the complex are genetically related and were probably derived by low-pressure fractional crystallization of high-Al basalts. The anomalously high content of Sr in some rocks probably reflects an accumulation of plagioclase. The trace element data are consistent with the origin of the parental magma by partial melting of an “enriched” upper mantle peridotite.  相似文献   

17.
New U–Pb age-data from zircons separated from a Northland ophiolite gabbro yield a mean 206Pb/238U age of 31.6 ± 0.2 Ma, providing support for a recently determined 28.3 ± 0.2 Ma SHRIMP age of an associated plagiogranite and  29–26 Ma 40Ar/39Ar ages (n = 9) of basalts of the ophiolite. Elsewhere, Miocene arc-related calc-alkaline andesite dikes which intrude the ophiolitic rocks contain zircons which yield mean 206Pb/238U ages of 20.1 ± 0.2 and 19.8 ± 0.2 Ma. The ophiolite gabbro and the andesites both contain rare inherited zircons ranging from 122–104 Ma. The Early Cretaceous zircons in the arc andesites are interpreted as xenocrysts from the Mt. Camel basement terrane through which magmas of the Northland Miocene arc lavas erupted. The inherited zircons in the ophiolite gabbros suggest that a small fraction of this basement was introduced into the suboceanic mantle by subduction and mixed with mantle melts during ophiolite formation.

We postulate that the tholeiitic suite of the ophiolite represents the crustal segment of SSZ lithosphere (SSZL) generated in the southern South Fiji Basin (SFB) at a northeast-dipping subduction zone that was initiated at about 35 Ma. The subduction zone nucleated along a pre-existing transform boundary separating circa 45–20 Ma oceanic lithosphere to the north and west of the Northland Peninsula from nascent back arc basin lithosphere of the SFB. Construction of the SSZL propagated southward along the transform boundary as the SFB continued to unzip to the southeast. After subduction of a large portion of oceanic lithosphere by about 26 Ma and collision of the SSZL with New Zealand, compression between the Australian Plate and the Pacific Plate was taken up along a new southwest-dipping subduction zone behind the SSZL. Renewed volcanism began in the oceanic forearc at 25 Ma producing boninitic-like, SSZ and within-plate alkalic and calc-alkaline rocks. Rocks of these types temporally overlap ophiolite emplacement and subsequent Miocene continental arc construction.  相似文献   


18.
Island arc and continental margin (i.e. western Americas) lavas are divided (based on raw data from literature) into basalts (defined by absence of Ca-poor pyroxene, dominated by quartz-normative tholeiites); basaltic andesites and andesites (subdivided on basis of breaks in SiO2 histogram and taken as <56% and 56–63% SiO2; Ca-poor pyroxene present; amphibole and biotite absent); and hornblende (±biotite) lavas, which prove to be mainly relatively silicic andesites. Relative proportions of these types are (576 samples): 23% basalts, 29% basaltic andesites; 30% andesites; 18% hornblende andesites. The compilation emphasizes the dominance of calcic plagioclase (labradorite-anorthite) amongst the phenocryst phases. Pyroxenes are largely augite and hypersthene (En60–75); olivine (Fo65–85) is common through all compositions. There is an overall close similarity in chemistry and mineralogy between continental margin and island arc lavas, although small consistent differences are apparent (e.g. K2O, TiO2, P2O5).Modal data indicate that 70% of lavas are phenocryst-rich (20–60 vol.%), and that phenocryst contents show a bimodal distribution. Statistically and petrologically significant correlations are found between mineralogy and rock chemistry, most notably between total rock Al2O3 and modal phenocrystic plagioclase (found in all data groups, except hornblende andesites). This, and related data and correlations, indicate that the majority of orogenic magmas are modified by crystal fractionation (including crystal accumulation) processes dominated by plagioclase, and interpreted to occur under relatively low pressures. Dominance of plagioclase suggests phenocryst precipitation occurs typically in water-undersaturated magmas.  相似文献   

19.
A geochemical and isotopic study of lavas from Pichincha, Antisana and Sumaco volcanoes in the Northern Volcanic Zone (NVZ) in Ecuador shows their magma genesis to be strongly influenced by slab melts. Pichincha lavas (in fore arc position) display all the characteristics of adakites (or slab melts) and were found in association with magnesian andesites. In the main arc, adakite-like lavas from Antisana volcano could be produced by the destabilization of pargasite in a garnet-rich mantle. In the back arc, high-niobium basalts found at Sumaco volcano could be produced in a phlogopite-rich mantle. The strikingly homogeneous isotopic signatures of all the lavas suggest that continental crust assimilation is limited and confirm that magmas from the three volcanic centers are closely related. The following magma genesis model is proposed in the NVZ in Ecuador: in fore arc position beneath Pichincha volcano, oceanic crust is able to melt and produces adakites. En route to the surface, part of these magmas metasomatize the mantle wedge inducing the crystallization of pargasite, phlogopite and garnet. In counterpart, they are enriched in magnesium and are placed at the surface as magnesian andesites. Dragged down by convection, the modified mantle undergoes a first partial melting event by the destabilization of pargasite and produces the adakite-like lavas from Antisana volcano. Lastly, dragged down deeper beneath the Sumaco volcano, the mantle melts a second time by the destabilization of phlogopite and produces high-niobium basalts. The obvious variation in spatial distribution (and geochemical characteristics) of the volcanism in the NVZ between Colombia and Ecuador clearly indicates that the subduction of the Carnegie Ridge beneath the Ecuadorian margin strongly influences the subduction-related volcanism. It is proposed that the flattening of the subducted slab induced by the recent subduction (<5 Ma?) of the Carnegie Ridge has permitted the progressive warming of the oceanic crust and its partial melting since ca. 1.5 Ma. Since then, the production of adakites in fore arc position has deeply transformed the magma genesis in the overall arc changing from ‘typical’ calc-alkaline magmatism induced by hydrous fluid metasomatism, to the space- and time-associated lithology adakite/high-Mg andesite/adakite-like andesite/high-Nb basalts characteristic of slab melt metasomatism.  相似文献   

20.
Abstract ' In situ basalts' represent the ridge magmatism at and close to the ancient trench-trench-ridge triple junction. Such basalts in the Amami, Mugi, and Setogawa accretionary complexes, Southwest Japan, were described and analysed. The geochemical data show that the ' in situ basalts' include all the types of basalts, ranging from alkali basalts and high-alumina basalts to tholeiites, and the compositions tend towards intermediate and silicic rocks. The data also reveal that the ridge basalts are indistinguishable both from the island arc and intraplate basalts, no affinities with mid-ocean-ridge basalts. The sub-ridge mantle adjacent to the triple junction had a component of sub-arc mantle, and this mantle heterogeneity can be generated by the formation of a slab window.  相似文献   

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