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1.
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   

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.
New geochemical and 40Ar/39Ar age data are presented from the Neogene volcanic units of the Karaburun Peninsula, the westernmost part of Western Anatolia. The volcanic rocks in the region are associated with Neogene lacustrine deposition and are characterized by (1) olivine-bearing basaltic-andesites to shoshonites (Karaburun volcanics), high-K calc-alkaline andesites, dacites and latites (Yaylaköy, Arma?anda? and Kocada? volcanics) of ~ 16–18 Ma, and (2) mildly-alkaline basalts (Ovac?k basalt) and rhyolites (Urla volcanics) of ~ 11–12 Ma. The first group of rocks is enriched in LILE and LREE with respect to the HREE and HFSE on N-MORB-normalised REE and multi-element spider diagrams. They are comparable geochemically with volcanic rocks in the surrounding regions such as Chios Island and other localities in Western Anatolia. The Ovac?k basalt is geochemically similar to the first stage early–middle Miocene volcanic rocks but differs from NW Anatolian late Miocene alkali basalts.  相似文献   

4.
The Chiang Khong segment of the Chiang Khong–Lampang–Tak Volcanic Belt is composed of three broadly meridional sub‐belts of mafic to felsic volcanic, volcaniclastic, and associated intrusive rocks. Associated sedimentary rocks are largely non‐marine red beds and conglomerates. Three representative Chiang Khong lavas have Late Triassic (223–220 Ma) laser ablation inductively coupled mass‐spectroscopy U–Pb zircon ages. Felsic‐dominated sequences in the Chiang Khong Western and Central Sub‐belts are high‐K calc–alkaline rocks that range from basaltic to dominant felsic lavas with rare mafic dykes. The Western Sub‐belt lavas have slightly lower high field strength element contents at all fractionation levels than equivalent rocks from the Central Sub‐belt. In contrast, the Eastern Sub‐belt is dominated by mafic lavas and dykes with compositions transitional between E‐mid‐oceanic ridge basalt and back‐arc basin basalts. The Eastern Sub‐belt rocks have higher FeO* and TiO2 and less light rare earth element enrichment than basalts in the high‐K sequences. Basaltic and doleritic dykes in the Western and Central sub‐belts match the composition of the Eastern Sub‐belt lavas and dykes. A recent geochemical study of the Chiang Khong rocks concluded that they were erupted in a continental margin volcanic arc setting. However, based on the dominance of felsic lavas and the mainly non‐marine associated sediments, we propose an alternative origin, in a post‐collisional extensional setting. A major late Middle to early Late Triassic collisional orogenic event is well documented in northern Thailand and Yunnan. We believe that the paucity of radiometric dates for arc‐like lavas in the Chiang Khong–Lampang–Tak Volcanic Belt that precede this orogenic event, coupled with the geochemistry of the Chiang Khong rocks, and strong compositional analogies with other post‐collisional magmatic suites, are features that are more typical of volcanic belts formed in a rapidly evolving post‐collisional, basin‐and range‐type extensional setting.  相似文献   

5.
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   

6.
Latest Oligocene and Early Miocene volcanic rocks occur on the Northland Peninsula, New Zealand, and record the inception of Cenozoic subduction-related volcanism in the North Island that eventually evolved to its present manifestation in the Taupo Volcanic Zone. This NW-striking Northland Arc is continuous with the Reinga Ridge and comprises two parallel belts of volcanic centres ca. 60 km apart. A plethora of tectonic models have been proposed for its origins. We acquired new trace element and Sr–Nd isotope data to better constrain such models. All Northland Arc rocks carry an arc-type trace element signature, however distinct differences exist between rocks of the eastern and western belt. Eastern belt rocks are typically andesites and dacites and have relatively evolved isotope ratios indicating assimilated crustal material, and commonly contain hornblende. Additionally some eastern belt rocks with highly evolved isotope compositions show fractionated REE compositions consistent with residual garnet, and some contain garnetiferous inclusions in addition to schistose crustal fragments. In contrast, western belt rocks are mostly basalts or basaltic andesites with relatively primitive Sr–Nd isotope compositions, do not contain hornblende and show no rare earth element evidence for cryptic amphibole fractionation. Eastern and western belt rocks contain comparable slab-derived fractions of fluid-mobile trace elements and invariably possess an arc signature. Therefore the difference between the belts may be best explained as due to variation in crustal thickness across the Northland Peninsula, where western belt centres erupted onto a thinner crustal section than eastern belt rocks.The consistent arc signature throughout the Northland arc favours an origin in response to an actual, if short-lived subduction event, rather than slab detachment as proposed in some models. No Northland Arc rocks possess a convincing adakite-like composition that might reflect the subduction of very young oceanic lithosphere such as that of the Oligocene South Fiji Basin. Therefore we favour a model in which subduction of old (Cretaceous) lithosphere drove subduction.  相似文献   

7.
New geochronological and volcanological data from volcanics of the island group of Milos (South Aegean active volcanic arc), allow four main cycles of volcanic activity to be distinguished, ranging in age between 3.5 and 0.1 m.y. B.P. The oldest volcanic activity consists almost totally of pyroclastics and submarine products, followed by subaerial ones. The eruptive centers migrated in time and concentrated, during the most recent volcanic phase, in the central part of Milos and on the island of Antimilos. The oldest products are dominated by andesites and dacites, while the most recent ones mainly consist of rhyolites. The volcanic evolution and the geochemical characters of the erupted products suggest that the feeding system of the oldest volcanism was located in the deep continental crust, where contamination and fractional crystallization jointly occurred. The younger cycle of volcanic activity was fed from small and relatively shallow magma chambers, where contamination processes played a minor role. These magma bodies are considered to responsible for the shallow thermal anomaly giving rise to the high enthalpy field on Milos.  相似文献   

8.
Talat  Ahmad  Kabita C.  Longjam  Baishali  Fouzdar  Mike J.  Bickle  Hazel J.  Chapman 《Island Arc》2009,18(1):155-174
The Sakoli Mobile Belt comprises bimodal volcanic rocks that include metabasalt, rhyolite, tuffs, and epiclastic rocks with metapelites, quartzite, arkose, conglomerate, and banded iron formation (BIF). Mafic volcanic rocks are tholeiitic to quartz‐tholeiitic with normative quartz and hypersthene. SiO2 shows a large compositional gap between the basic and acidic volcanics, depicting their bimodal nature. Both the volcanics have distinct geochemical trends but display some similarity in terms of enriched light rare earth element–large ion lithophile element characteristics with positive anomalies for U, Pb, and Th and distinct negative anomalies for Nb, P, and Ti. These characteristics are typical of continental rift volcanism. Both the volcanic rocks show strong negative Sr and Eu anomalies indicating fractionation of plagioclases and K‐feldspars, respectively. The high Fe/Mg ratios for the basic rocks indicate their evolved nature. Whole rock Sm–Nd isochrons for the acidic volcanic rocks indicate an age of crystallization for these volcanic rocks at about 1675 ± 180 Ma (initial 143Nd/144Nd = 0.51017 ± 0.00017, mean square weighted deviate [MSWD] = 1.6). The εNdt (t = 2000 Ma) varies between ?0.19 and +2.22 for the basic volcanic rock and between ?2.85 and ?4.29 for the acidic volcanic rocks. Depleted mantle model ages vary from 2000 to 2275 Ma for the basic and from 2426 to 2777 Ma for the acidic volcanic rocks, respectively. These model ages indicate that protoliths for the acidic volcanic rocks probably had a much longer crustal residence time. Predominantly basaltic magma erupted during the deposition of the Dhabetekri Formation and part of it pooled at crustal or shallower subcrustal levels that probably triggered partial melting to generate the acidic magma. The influence of basic magma on the genesis of acidic magma is indicated by the higher Ni and Cr abundance at the observed silica levels of the acidic magma. A subsequent pulse of basic magma, which became crustally contaminated, erupted as minor component along with the dominantly acidic volcanics during the deposition of the Bhiwapur Formation.  相似文献   

9.
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   

10.
Forty-six new K-Ar age determinations are presented on whole rock samples and mineral separates from volcanic and subvolcanic rocks of Gran Canaria. The main subaerial shield building basaltic volcanism with estimated volume of about 1000 km3 was confined to the interval about 13.7 m.y. to 13.5 m.y. ago in the middle Miocene. Substantial volume (～100 km3) of silicic volcanics (trachyte and peralkaline rhyolite) were erupted with no detectable time break following the basaltic volcanism, essentially contemporaneous with formation of a large collapse caldera at 13.4±0.3 m.y. ago. Trachytic to phonolitic volcanism continued intermittently in the waning states of activity until about 9 m.y. ago. Following a long hiatus there was resurgence of volcanism with eruption of about 100 km3 of basanitic to hauyne phonolitic rocks of the Roque Nublo Group between about 4.4 m.y. and 3.4 m.y. ago in the Pliocene. After a hiatus of less than 1.0 m.y., olivine nephelinite magmas were erupted and this activity continued intermittently until relatively recent times, the younger eruptives being mainly basanitic in composition. The volume of volcanic products in this phase probably does not exceed 10 km3. Thus the volume of all the resurgent volcanism comprises less than 10 percent of the subaerially exposed part of Gran Canaria. The results show that the subaerial main shield building phase of volcanism in Gran Canaria, consisting of mildly alkali to transitional basalts, occurred over a time interval that was less than 0.5 m.y. Magmatic evolution on Gran Canaria appears to be similar to that found on other basaltic volcanoes in oceanic regions. Thus volcanoes in the Hawaiian, Marquesas and Society Islands all were built by basaltic lavas in similar short-lived episodes of volcanism. In some Hawaiian volcanoes, a resurgent phase of volcanism of strongly undersaturated basalts of small volume is recognized following a long hiatus, again similar to that found on Gran Canaria. The relatively large volume of silicic lavas erupted in Gran Canaria immediately following the main basaltic shield building phase is, however, not matched in the Pacific volcanoes mentioned.  相似文献   

11.
Batur is an active stratovolcano on the island of Bali, Indonesia, with a large, well-formed caldera whose formation is correlated with the eruption about 23,700 years ago of a thick ignimbrite sheet. Our study of the volcanic stratigraphy and geochemistry of Batur shows the formation of the caldera was signalled by a change in the composition of the erupting material from basaltic and andesitic to dacitic. The dacitic rocks are glassy, possess equilibrium phenocryst assemblages, and display compositional characteristics consistent with an origin by crystal-liquid fractionation from more mafic parent magmas in a shallow chamber, possibly at 1.5 km depth and 1000–1070°C.However, although separated by a gap of 6 wt.% SiO2, the dacitic rocks are clearly related in their minor- and trace-element geochemistry to those basalts and basaltic andesites erupted after the caldera was formed rather than to the andesites erupted immediately before the dacites first appeared. We infer from this and published experimental modelling of the possible crystallization behaviour of basaltic magma chambers that a magmatic cycle involving caldera formation began independently of the previous activity of Batur by formation of a new, closed-system magma chamber beneath the volcano. Fractional crystallization, possibly at the walls of the chamber, led to the early production of derivative siliceous magmas and, consequently, to caldera formation, while most of the magma retained its original composition. The postcaldera Batur basalts represent the largely undifferentiated core liquids of this chamber.This model contrasts with the traditional evolutionary model for stratovolcano calderas but may be applicable to the origins of calderas similar to that of Batur, particularly those in volcanic island arcs.  相似文献   

12.
Optical microscopy and chemical analyses for major and trace elements have been performed on 27 Proterozoic metavolcanics from southern Sweden. The metavolcanics and associated metasediments are part of a large arced structure around a vast region with granitic batholiths.The compositional data show that the rocks are altered basalts and andesites and the Si, Ti, Zr and Cr relations demonstrate that almost half of the metabasalts are in accordance with present day arc volcanics and that another large fraction represents ocean floor tholeiites. These relations suggest that major plate tectonic processes occurred already during the Proterozoic in southern Sweden and that during these processes arc volcanics and ocean floor basalts were mixed at a subduction zone.  相似文献   

13.
The Mesozoic volcanic rocks of the Serra Geral Formation in the Paraná Basin, South America, and of the Etendeka Group in northwestern Namibia were erupted shortly before the opening of the South Atlantic. The major widespread silicic volcanic units in the Etendeka Group are interpreted as rheoignimbrites (Milner et al., 1992) and are interbedded with tholeiitic basalts and basaltic andesites.The southern portion of the Etendeka Group is subdivided into a basal Awahab Formation which is overlain disconformably by the Tafelberg Formation. Both formations contain silicic and mafic units. Bulk composition, initial 87Sr/86Sr ratios, phenocryst assemblages and mineral compositions are used to correlate silicic units of the Awahab Formation with the basal units of the Palmas silicic volcanic rocks in the southern Paraná Basin. Silicic units of the Tafelberg Formation can similarly be correlated with silicic units in the upper portion of the Palmas succession, which are also disconformable on the units below them. Not all silicic units in these successions are present in both the Etendeka and Paraná areas, but where correlation of individual units is possible, then this is found to be consistent with the overall stratigraphic sequence.Silicic units in the Awahab Formation were erupted from the Messum Igneous Complex in Namibia and their correlation into Brazil indicates that individual eruptive units must have travelled over 340 km from their source. Serial changes in the composition of silicic units in the Awahab Formation and their correlatives indicates that they were erupted from a single magma system from which a total of ˜ 8600 km3 of material was erupted.  相似文献   

14.
Selected basalts from a suite of dredged and drilled samples (IPOD sites 525, 527, 528 and 530) from the Walvis Ridge have been analysed to determine their rare earth element (REE) contents in order to investigate the origin and evolution of this major structural feature in the South Atlantic Ocean. All of the samples show a high degree of light rare earth element (LREE) enrichment, quite unlike the flat or depleted patterns normally observed for normal mid-ocean ridge basalts (MORBs). Basalts from Sites 527, 528 and 530 show REE patterns characterised by an arcuate shape and relatively low (Ce/Yb)N ratios (1.46–5.22), and the ratios show a positive linear relationship to Nb content. A different trend is exhibited by the dredged basalts and the basalts from Site 525, and their REE patterns have a fairly constant slope, and higher (Ce/Yb)N ratios (4.31–8.50).These differences are further reflected in the ratios of incompatible trace elements, which also indicate considerable variations within the groups. Mixing hyperbolae for these ratios suggest that simple magma mixing between a “hot spot” type of magma, similar to present-day volcanics of Tristan da Cunha, and a depleted source, possibly similar to that for magmas being erupted at the Mid-Atlantic Ridge, was an important process in the origin of parts of the Walvis Ridge, as exemplified by Sites 527, 528 and 530. Site 525 and dredged basalts cannot be explained by this mixing process, and their incompatible element ratios suggest either a mantle source of a different composition or some complexity to the mixing process. In addition, the occurrence of different types of basalt at the same location suggests there is vertical zonation within the volcanic pile, with the later erupted basalts becoming more alkaline and more enriched in incompatible elements.The model proposed for the origin and evolution of the Walvis Ridge involves an initial stage of eruption in which the magma was essentially a mixture of enriched and depleted end-member sources, with the N-MORB component being small. The dredged basalts and Site 525, which represent either later-stage eruptives or those close to the hot spot plume, probably result from mixing of the enriched mantle source with variable amounts and variable low degrees of partial melting of the depleted mantle source. As the volcano leaves the hot spot, these late-stage eruptives continue for some time. The change from tholeiitic to alkalic volcanism is probably related either to evolution in the plumbing system and magma chamber of the individual volcano, or to changes in the depth of origin of the enriched mantle source melt, similar to processes in Hawaiian volcanoes.  相似文献   

15.
The oldest rocks in the mainland southeastern Australian segment of the (Palaeozoic) Lachlan Fold Belt are Cambrian greenstones which outcrop in three narrow linear belts separated by Lower Palaeozoic marine troughs in which many thousands of meters of predominantly greywacke-shale sediments accumulated. The greenstone belt nearest the Australian craton, the Mt. Stavely Greenstone Belt, is composed of calc-alkalic meta-andesites, metadacites and intermediate and acid pyroclastics. The Heathcote Greenstone Belt, of central Victoria, consists of three segments; the northern and southern segments are very similar and their internal stratigraphy, petrology, and geochemistry suggest they represent an incomplete, disrupted ophiolite. However, the central segment of the Heathcote Greenstone Belt, which is more intensely deformed and metamorphosed than the northern and southern segments, is composed of a calc-alkalic volcanic suite dominated by meta-andesites. The Mt. Wellington Greenstone Belt of eastern Victoria shows remarkable overall similarities to the northern segment of the Heathcote Greenstone Belt and evidence, including the presence of fault slices of gabbro and peridotites, suggests that this belt too is a much disrupted ophiolite. We interpret the ophiolites to have been the crust of a marginal sea which developed by rifting of thin continental-type crust at the leading edge of a palaeo-Australian plate in the early Cambrian. Meta-andesites and associated rocks of the Mt. Stavely Greenstone Belt were probably erupted onto this thin continental crust above a Benioff zone, and a rifted-off fragment of this thin continental crust bearing a cover of calc-alkalic volcanic rocks has been preserved as the central segment of the Heathcote Greenstone Belt during later deformation events.  相似文献   

16.
Analyses of the latest Jurassic Santiago Peak volcanic rocks from the western zone of the Peninsular Ranges batholith reveal the existence of two independent groups; one comprising basalts and andesites of the island arc tholeiitic series, and the second made up of the dacites and rhyolites of the subalkaline (calc-alkaline?) series or silicic series. The basalts and andesites have V, Co and Ni contents similar to those estimated for the residual melts in equilibrium with the Peninsular Ranges gabbros. This fact together with the tholeiitic nature of the gabbros and the intimate spatial and temporal relationship between the SPV and the gabbros suggests that the basalts and andesites had a common origin with the gabbros. The mafic volcanism and plutonism seems to have occurred in a youthful island arc and the silicic volcanism in a mature island arc or a continental margin.  相似文献   

17.
This paper is concerned with the islands of Montserrat Nevis, St. Kitts, St. Eustatius and Saba, which lie on the inner volcanic are at the northern of the Lesser Antilles. Andesites greatly predominate over basalts and dacites in this part of the arc. Generally the lavas from the northern Lesser Antilles contain low abundances of Ni, Cr and residual trace elements but lavas from Saba are enriched in these elements compared with the other islands in the group. The most important petrogenetic process in this part of the Lesser Antilles is probably partial melting of subducted oceanie tholeiite and this process accounts satisfactorily for the chemistry (especially the low Ni, Cr) and large volumes of the erupted andesites. Some andesites have, however, been produced by fractional crystallisation of basaltic magma and magma mixing probably accounts for some of the peculiar chemical and petrographic properties of the Saba andesites. The rocks from the Northern Lesser Antilles are different from those in the central part of the arc (more acid rocks, higher residual trace elements) and the southern islands have much higher proportions of basalt, some of it undersaturated and alkaline. It is thought that partial melting of mantle peridotite may be the predominant petrogenetic process at the southern end of the Lesser Antilles whereas partial melting of subducted oceanic crust is more important in the north.  相似文献   

18.
Abstract Volcanism in the back-arc side region of Central Luzon, Philippines, with respect to the Manila Trench is characterized by fewer and smaller volume volcanic centers compared to the adjacent forearc side-main volcanic arc igneous rocks. The back-arc side volcanic rocks which include basalts, basaltic andesites, andesites and dacites also contain more hydrous minerals (ie, hornblende and biotite). Adakite-like geochemical characteristics of these back-arc lavas, including elevated Sr, depleted heavy rare earth elements and high Sr/Y ratios, are unlikely to have formed by slab melting, be related to incipient subduction, slab window magmatism or plagioclase accumulation. Field and geochemical evidence show that these adakitic lavas were most probably formed by the partial melting of a garnet-bearing amphibolitic lower crust. Adakitic lavas are not necessarily arc–trench gap region slab melts.  相似文献   

19.
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   

20.
Masahiko  Yagi 《Island Arc》1993,2(4):240-261
Abstract Alteration of reservoir rocks in the Yurihara Oil and Gas Field, hereafter referred to as the ‘Yurihara field’, have been examined by using samples from six wells. These rocks are basalts in the lowermost part of the basin-fills (‘green tuff’ Formation). These basalts were produced in many eruptions in a submarine environment during the early to middle Miocene, and they underwent continuous intensive alteration genetically associated with Miocene submarine volcanism. The alteration of the basalts is of two types: low grade metamorphism and hydrothermal. The former belongs to the type of ocean floor metamorphism and comprises two subgroups: zeolite (zone I) and prehnite-pumpellyite (zones IIa: vein and amygdule occurrence, and IIb: replacing plagioclase). The latter is characterized by potassic metasomatism accompanied by adularia, quartz and calcite veins (zones IIIa: center and IIIb: margin of the metasomatism). This overprints the low grade metamorphic alteration. The central zone of hydrothermal alteration coincides with a major estimated fault, so that fluids probably assent along the fault. The basalts erupted during 16.5-15.5 Ma, determined by planktonic foraminifera assemblages of inter-bedded shales, then underwent successive low grade metamorphism. In time, the hydrothermal alteration that overprints low grade metamorphism occurred. Adularia veins of the altered rocks located in the hydrothermal alteration zones (zone IIIa and IIIb) have been dated as 9 Ma determined by the K-Ar method. This fact indicates that the activity of low grade metamorphism had already crossed the peak before hydrothermal alteration occurred at 9 Ma. The shape of isotherms of fluid inclusion homogenization temperatures (Th) and that of isolines of apparent salinity (Tm) almost coincide with each other, and these also coincide with the distribution of hydrothermal alteration (zones IIIa and IIIb). This indicates that the fluid inclusions formed at the same time as ascending fluids produced the potassic metasomatism. The maximum Th of the fluid inclusions is 222°C and Tm indicates trapped fluids of up to 3.3 wt% equivalent NaCl (i.e. almost the same as seawater). A Th versus Tm plot indicates mixing occurred between hydrothermal fluids and formation water that has low salinity. Corrensite and chlorite form veins, and the temperatures of their formation, estimated by the extent of aluminium substitution into the tetrahedral site of chlorite, ranges between 165 and 245°C in the centre of the hydrothermal alteration zone (zone IIIa). This is consistent with the result of Th analyses. The deposition temperature of chlorite associated with prehnite in veins ranges between 190 and 215°C in zones IIa and IIb.  相似文献   

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