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1.
Trace element systematics throughout the cal-calkaline high alumina basalt — basaltic andesite — andesite — dacite — rhyodacite lavas and dyke rocks of the Main Volcanic Series of Santorini volcano, Greece are consistent with the crystal fractionation of observed phenocryst phases from a parental basaltic magma as the dominant mechanism involved in generating the range of magmatic compositions. Marked inflection points in several variation trends correspond to changes in phenocryst mineralogy and divide the Main Series into two distinct crystallisation intervals — an early basalt to andesite stage characterised by calcic plagioclase+augite+olivine separation and a later andesite to rhyodacite stage generated by plagioclase augite+hypersthene+magnetite+apatite crystallisation. Percent solidification values derived from ratios of highly incompatible trace elements agree with previous values derived from major element data using addition-subtraction diagrams and indicate that basaltic andesites represent 47–69%; andesites 70–76%; dacites ca. 80% and rhyodacite ca. 84% crystallisation of the initial basalt magma. Least squares major element mixing calculations also confirm that crystal fractionation of the least fractionated basalts could generate derivative Main Series lavas, though the details of the least squares solutions differ significantly from those derived from highly incompatible element and addition-subtraction techniques. Main Series basalts may result from partial melting of the mantle asthenosphere wedge followed by limited olivine+pyroxene+Cr-spinel crystallisation on ascent through the sub-Aegean mantle and may fractionate to more evolved compositions at pressures close to the base of the Aegean crust. Residual andesitic to rhyodacite magmas may stagnate within the upper regions of the sialic Aegean crust and form relatively high level magma chambers beneath the southern volcanic centres of Santorini. The eruption of large volumes of basic lavas and silicic pyroclastics from Santorini may have a volcanological rather than petrological explanation.  相似文献   

2.
Three linear zones of active andesite volcanism are present in the Andes — a northern zone (5°N–2°S) in Colombia and Ecuador, a central zone (16°S–28°S) largely in south Peru and north Chile and a southern zone (33°S–52°S) largely in south Chile. The northern zone is characterized by basaltic andesites, the central zone by andesite—dacite lavas and ignimbrites and the southern zone by high-alumina basalts, basaltic andesites and andesites. Shoshonites and volcanic rocks of the alkali basalt—trachyte association occur at scattered localities east of the active volcanic chain,The northern and central volcanic zones are 140 km above an eastward-dipping Benioff zone, while the southern zone lies only 90 km above a Benioff zone. Continental crust is ca. 70 km in thickness below the central zone, but is 30–45 km thick below northern and southern volcanic zones. The correlation between volcanic products and their structural setting is supported by trace element and isotope data. The central zone andesite lavas have higher Si, K, Rb, Sr and Ba, and higher initial Sr isotope ratios than the northern or southern zone lavas. The southern zone high-alumina basalts have lower Ce/Yb ratios than volcanics from the other zones. In addition, the central zone andesite lavas show a well-defined eastward increase in K, Rb and Ba and a decrease in Sr.Andean andesite magmas are a result of a complex interplay of partial melting, fractional crystallization and “contamination” processes at mantle depths, and contamination and fractional crystallization in the crust. Variations in andesite composition across the central Andean chain reflect a diminishing degree of partial melting or an increase in fractional crystallization or an increase in “contamination” passing eastwards. Variations along the Andean chain indicate a significant crustal contribution for andesites in the central zone, and indicate that the high-alumina basalts and basaltic andesites of the southern zone are from a shallower mantle source region than other volcanic rocks. The dacite-rhyolite ignimbrites of the central zone share a common source with the andesites and might result from fractional crystallization of andesite magma during uprise through thick continental crust. The occurrence of shoshonites and alkali basalts eat of the active volcanic chain is attributed to partial melting of mantle peridotite distant from the subduction zone.  相似文献   

3.
The Xiong'er volcanic belt, covering an area of more than 60,000 km2 along the southern margin of the North China Craton, has long been considered an intra-continental rift zone and recently interpreted as part of a large igneous province formed by a mantle plume that led to the breakup of the Paleo-Mesoproterozoic supercontinent Columbia. However, such interpretations cannot be accommodated by lithology, mineralogy, geochemistry and geochronology of the volcanic rocks in the belt. Lithologically, the Xiong'er volcanic belt is dominated by basaltic andesite and andesite, with minor dacite and rhyolite, different from rock associations related to continental rifts or mantle plumes, which are generally bimodal and dominated by mafic components. However, they are remarkably similar to those rock associations in modern continental margin arcs. In some of the basaltic andesites and andesites, amphibole is a common phenocryst phase, suggesting the involvement of H2O-rich fluids in the petrogenesis of the Xiong'er volcanic rocks. Geochemically, the Xiong'er volcanic rocks fall in the calc-alkaline series, and in most tectono-magmatic discrimination diagrams, the majority of the Xiong'er volcanic rocks show affinities to magmatic arcs. In the primitive mantle normalized trace-element diagrams, the Xiong'er volcanic rocks show enrichments in the LILE and LREE, and negative Nb–Ta–Ti anomalies, similar to arc-related volcanic rocks produced by the hydrous melting of metasomatized mantle wedge. Nd-isotope compositions of the Xiong'er volcanic rocks suggest that 5–15% older crust has been transferred into the upper lithospheric mantle by subduction-related recycling during Archean to Paleoproterozoic time. Available SHRIMP and LA-ICP-MS U–Pb zircon age data indicate that the Xiong'er volcanic rocks erupted intermittently over a protracted interval from 1.78 Ga, through 1.76–1.75 Ga and 1.65 Ga, to 1.45 Ga, though the major phase of the volcanism occurred at 1.78–1.75 Ga. Such multiple and intermittent volcanism is inconsistent with a mantle plume-driven rifting event, but is not uncommon in ancient and existing continental margin arcs. Taken together, the Xiong'er volcanic belt was most likely a Paleo-Mesoproterozoic continental magmatic arc that formed at the southern margin of the North China Craton. Similar Paleo-Mesoproterozoic continental magmatic arcs were also present at the southern and southeastern margins of Laurentia, the southern margin of Baltica, the northwestern margin of Amonzonia, and the southern and eastern margins of the North Australia Craton, which are considered to represent subduction-related episodic outbuilding on the continental margins of the Paleo-Mesoproterozoic supercontinent Columbia. Therefore, in any configuration of the supercontinent Columbia, the southern margin of the North China Craton could not have been connected to any other continental block as proposed in a recent configuration, but must have faced an open ocean whose lithosphere was subducted beneath the southern margin of the North China Craton.  相似文献   

4.
The Miocene Karamağara volcanics (KMV) crop out in the Saraykent region (Yozgat) of Central Anatolia. The KMV include four principal magmatic components based on their petrography and compositional features: basaltic andesites (KMB); enclaves (KME); andesites (KMA); and dacites (KMD). Rounded and ellipsoidal enclaves occur in the andesites, ranging in diameter from a few millimetres to ten centimetres. A non‐cognate origin for the enclaves is suggested due to their mineralogical dissimilarity to the enclosing andesites. The enclaves range in composition from basaltic andesite to andesite. Major and trace element data and primitive mantle‐normalized rare‐earth element (REE) patterns of the KMV exhibit the effects of fractional crystallization on the evolution of the KME which are the product of mantle‐derived magma. The KMA contain a wide variety of phenocrysts, including plagioclase, clinopyroxene, orthopyroxene, hornblende and opaque minerals. Comparison of textures indicates that many of the hornblende phenocrysts within the KMA were derived from basaltic andesites (KMB) and are not primary crystallization products of the KMA. Evidence of disequilibrium in the hybrid andesite includes the presence of reacted hornblendes, clinopyroxene mantled by orthopyroxene and vice versa, and sieve‐texture and inclusion zones within plagioclase. The KMV exhibit a complex history, including fractional crystallization, magma mixing and mingling processes between mantle and crust‐derived melts. Textural and geochemical characteristics of the enclaves and their hosts require that mantle‐derived basic magma intruded the deep continental crust followed by fractional crystallization and generation of silicic melts from the continental material. Hybridization between basic and silicic melts subsequently occurred in a shallow magma chamber. Modelling of major element geochemistry suggests that the hybrid andesite represents a 62:38 mix of dacite and basaltic andesite. The implication of this process is that calc‐alkaline intermediate volcanic rocks in the Saraykent region represent hybrids resulting from mixing between basic magma derived from the mantle and silicic magma derived from the continental crust. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

5.
The Paraná volcanic province is a window into mantle and crustal processes in the Cretaceous. The variability and complexity of this province can be determined through the study of minerals. An integrated study of zircon from Paraná lavas (one high-Ti basalt, one low-Ti andesite, one high-Ti rhyodacite and one high-Ti andesite sill) was achieved using backscattered electron imaging, sensitive high resolution ion microprobe (SHRIMP-IIe) for U-Pb geochronology, and laser ablation inductively coupled mass spectrometer (LA-ICPMS) to determine the Lu-Hf isotopes and the trace-element compositions. U-Pb-Hf and trace-element data indicate that zircon crystallized from the magma at approximately 134 Ma. This South American large igneous province originated from the mantle and was contaminated by crust either in the mantle or during ascent and crystallization of magma. Contaminant continental crust had Precambrian age. Trace elements point to a new compositional field for zircon, different from other types of provinces. Examination of volcanic zircon improves our understanding of age and contamination of the Paraná volcanics.  相似文献   

6.
The Early Neoproterozoic Beiyixi Formation volcanic rocks of the southern Quruqtagh comprise mainly of a suite of tholeiitic basalts,alkaline andesites,and calc-alkaline rhyolites.The rhyolites are characterized by variably fractionated enrichment in light rare earth elements（LREE） and flat in heavy rare earth elements（HREE）,and strongly negative Eu anomalies.Compared to the rhyolites,the andesites also exhibit enrichment in LREE and flat HREE（chondrite-normalized values of La/Yb,and La/Sm are 13.30-41.09,3.18-6.89 respectively）.Their rare earth element patterns display minor negative Eu anomalies.Both of them exhibit coherent patterns with strongly to moderately negative anomalies of Nb,Zr,Ti,and Hf on spider diagrams.Two rhyolite and one andesite magmatic zircons with defined oscillatory zoning yielded weighted mean 206Pb/238U ages of 743±7 Ma,741±2 Ma,and 727±4 Ma.These ages are interpreted to represent the timing of volcanic eruptions. According to geochemistry and rock type,these volcanic rocks formed within a continental island-arc environment following subduction of the oceanic crust during the Early Neoproterozoic period.  相似文献   

7.
The volcanic rocks of the Xiong'er Group are situated in the southern margin of the North China Craton(NCC).Research on the Xiong er Group is important to understand the tectonic evolution of the NCC and the Columbia supercontinent during the Paleoproterozoic.In this study,to constrain the age of the Xiong'er volcanic rocks and identify its tectonic environment,we report zircon LA-ICP-MS data with Hf isotope,whole-rock major and trace element compositions and Sr-Nd-Pb-Hf isotopes of the volcanic rocks of the Xiong'er Group.The Xiong'er volcanic rocks mainly consist of basaltic andesite,andesite.dacite and rhyolite,with minor basalt.Our new sets of data combined with those from previous studies indicate that Xiong'er volcanism should have lasted from 1827 Ma to 1746 Ma as the major phase of the volcanism.These volcanics have extremely low MgO.Cr and Ni contents,are enriched in LREEs and LILEs but depleted in HFSEs(Nb,Ta,and Ti),similar to arc-related volcanic rocks.They are characterized by negative zircon ε_(Hf)_(t) values of-17.4 to 8.8,whole-rock initial ~(87)Sr/~(86)Sr values of 0.7023 to 0.7177 andε_(Nd)(t) values of-10.9 to 6.4.and Pb isotopes(~(206)Pb/~(204)Pb =14.366-16.431,~(207)Pb/~(204)Pb =15.106-15.371,~(208)Pb/~(204)Pb= 32.455-37.422).The available elemental and Sr-Nd-Pb-Hf isotope data suggest that the Xiong'er volcanic rocks were sourced from a mantle contaminated by continental crust.The volcanic rocks of the Xiong'er Group might have been generated by high-degree partial melting of a lithospheric mantle that was originally modified by oceanic subduction in the Archean.Thus,we suggest that the subduction-modified lithospheric mantle occurred in an extensional setting during the breakup of the Columbia supercontinent in the Late Paleoproterozoic,rather than in an arc setting.  相似文献   

8.
The Izu–Bonin volcanic arc is an excellent example ofan intra-oceanic convergent margin. A total of 1011 chemicalanalyses of 17 Quaternary volcanoes of the arc are reviewedto estimate relative proportions of magmas erupted. Basalt andbasic andesite (SiO2 < 57 wt %) are the predominant eruptiveproducts of the Izu–Bonin arc, and rhyolite (SiO2 >70 wt %) forms another peak in volume. Such rhyolites possesscompositions identical to those of partial melts produced bydehydration-melting of calc-alkaline andesites at low pressure(<7 kbar). Meanwhile, the major element variation of theShirahama Group Mio-Pliocene volcanic arc suite, Izu Peninsula,completely overlaps that of the Quaternary Izu–Bonin arcvolcanoes, and groundmasses of Shirahama Group calc-alkalineandesites have compositions similar to those of Izu–Boninrhyolites. Moreover, phenocryst assemblages of calc-alkalineandesites of the Shirahama Group resemble restite phase assemblagesof dehydration-melting of calc-alkaline andesite. These linesof evidence suggest that the rhyolite magmas may have been producedby dehydration-melting of calc-alkaline andesite in the upperto middle crust. If so, then the presence of large amounts ofcalc-alkaline andesite (3–5 times more abundant than therhyolites) within the oceanic arc crust would be expected, whichis consistent with a recently proposed structural model acrossthe Izu–Bonin arc. The calc-alkaline andesite magmas maybe water saturated, and would crystallize extensively and solidifywithin the crust. The model proposed here suggests that rhyoliteeruptions could be triggered by an influx of hot basalt magmafrom depth, reheating and partially melting the calc-alkalineandesite component of the crust. KEY WORDS: bimodal magmatism; calc-alkaline andesite; oceanic arcs; rhyolite  相似文献   

9.
宁芜中生代火山盆地产出火山岩与侵入岩，火山岩以玄武粗安岩，粗安岩和粗面岩为主，安山岩和响岩少量，火山岩以高钾富碱为特征，已确定为橄榄安粗岩系。侵入岩以辉长闪长玢岩－一辉长闪长岩为主，以高钠低硅为特征，并有辉长岩和花岗岩产生，据地质学和Ｎｄ，Ｓｒ，Ｐｂ同位素资料，侵入岩与火山岩属同一个岩浆系列，是碱性玄武岩浆在下地壳经过轻度ＡＦＣ混合后，侵入上地壳，在轻度混染的情况下，经过以结晶分离为主的岩浆分异形  相似文献   

10.
“三江”构造带中甸弧北部的印支期岩浆活动目前尚无正式的研究报道.本文的岩石学及年代学的研究表明,中甸弧北部的翁水地区存在印支期中酸性岩浆活动,锆石U-Pb定年结果显示这些火山岩在211.1±1.5Ma喷发,与该地区的印支期火山岩与斑岩矿床的成矿斑岩的形成时代一致.翁水安山岩岩石地球化学特征显示其以中性的安山岩为主,具有亏损Nb、Ta、Ti等高场强元素的典型岛弧岩浆成分特征.与同一构造单元内中句弧南部的烂泥塘-浪都安山岩进行主、微量元素的对比研究发现,两者均具有正常岛弧火山岩的成分组成特征.结合其形成时代和构造背景,初步认为中甸弧北部的翁水和南部的烂泥塘-浪都安山岩以及该区域的成矿斑岩很可能形成于同一构造-岩浆事件,都是甘孜-理塘洋晚三叠世俯冲作用的岩浆产物.  相似文献   

11.
吐拉苏盆地大哈拉军山组由两组火山岩组成,一组为玄武安山岩、安山岩,SiO2含量介于54.8%～59.4%之间,另一组为流纹岩,SiO2含量为70.6% ～74.1%,两组岩石具有相似的稀土和微量元素分配型式,均富集U、Th、K、Pb,而亏损Nb、Ta和Ti,同时两组岩石的一些微量元素对比值基本一致,表明流纹岩是本区玄武...  相似文献   

12.
东天山石炭纪企鹅山群火山岩岩石成因   总被引：13，自引：1，他引：12  
土屋矿区南北大沟企鹅山群火山岩的岩石地球化学研究表明:东天山企鹅山群火山岩主要为拉斑系列,少量为钙碱系列;岩石类型为玄武岩、玄武安山岩、英安岩和流纹岩。稀土、微量元素和Sr、Nd同位素特点揭示:该火山岩系形成于大陆裂谷环境;其源区主要为软流圈地幔,同时有岩石圈地幔源组分卷入,酸性岩浆是玄武质岩浆结晶分异的产物。  相似文献   

13.
The volcanic Rooiberg Group represents the earliest phase of Bushveld-related magmatism and comprises, in some areas, the floor and roof rocks of the mafic-ultramafic intrusive units of the Bushveld Complex. The lower to middle Dullstroom Formation is composed of two interbedded series of low Ti and high Ti volcanic strata, which are predominantly basaltic andesites. Volcanic units above these strata range from andesites to dacites in the upper Dullstroom Formation and to predominantly rhyolites in the overlying Damwal and Kwaggasnek Formations. Compositional data suggest that these intermediate to siliceous volcanic rocks are petrogenetically related to the low Ti volcanic suite and suggest that the low Ti magmas resided in a shallow magma chamber where they experienced fractional crystallization and assimilation of crustal material. In contrast, the high Ti volcanic suite is petrogenetically unrelated. These data confirm previous suggestions that Bushveld-related magmas experienced significant amounts of assimilation of continental crust.  相似文献   

14.
The Huerto Andesite is the largest of several andesite sequences interlayered with the large-volume ash-flow tuffs of the San Juan volcanic field, Colorado. Stratigraphically this andesite is between the region's largest tuff (the 27.8 Ma, 3,000 km3 Fish Canyon Tuff) and the evolved product of the Fish Canyon Tuff (the 27.4 Ma, 1,000 km3 Carpenter Ridge Tuff), and eruption was from vents located approximately 20–30 km southwest and southeast of calderas associated with these ashflow tuffs. Olivine phenocrysts are present in the more mafic, SiO2-poor samples of andesite, hence the parent magma was most likely a mantle-derived basaltic magma. The bulk compositions of the olivine-bearing andesites compared to those containing orthopyroxene phenocrysts suggest the phenocryst assemblage equilibrated at 2–5 kbar. Two-pyroxene geothermometry yields equilibrium temperatures consistent with near-peritectic magmas at 2–5 kbar. Fractionation of phenocryst phases (olivine or orthopyroxene + clinopyroxene + plagioclase + Ti-magnetite + apatite) can explain most major and trace element variations of the andesites, although assimilation of some crustal material may explain abundances of some highly incompatible trace elements (Rb, Ba, Nb, Ta, Zr, Hf) in the most evolved lavas. Despite the great distance of the San Juan volcanic field from the inferred Oligocene destructive margin, the Huerto Andesite is similar to typical plate-margin andesites: both have relatively low abundances of Nb and Ta and similar values for trace-element ratios such as La/Yb and La/Nb.Deriving the Fish Canyon and Carpenter Ridge Tuffs by crystal fractionation from the Huerto Andesite cannot be dismissed by major-element models, although limited trace-element data indicate the tuffs may not have been derived by such direct evolution. Alternatively, heat of crystallization released as basaltic magmas evolved to andesitic compositions may have caused melting of crust to produce the felsic-ash flows. Mafic magmas may have been gravitationally trapped below lighter felsic magmas; mafic magmas which ascended to the surface probably migrated upwards around the margins of silicic chambers, as suggested by the present-day outcrops of andesitic units around the margins of recognized ash-flow calderas.  相似文献   

15.
Dikii Greben' Volcano is the largest modern volcano with silicic rocks in the Kurile-Kamchatka island arc. It consists of many domes and lava flows of rhyodacite, dacite and andesite which were erupted in a reverse differentiation sequence. Non-equilibrium phenocryst assemblages (quartz + Mg-rich olivine, An-rich + An-poor plagioclase etc.), abundance of chilled mafic pillows in the dacites and andesites, and linear variations of rock compositions in binary plots are considered as mineralogical, textural and geochemical evidence for mixing. Mafic pillows in volcanics have a lower density (because of high porosity) and contain the same non-equilibrium phenocryst assemblages as the host rocks. Their groundmass contains skeletal microlites of plagioclase and amphibole proving that the groundmass as well as the pillows themselves formed from a water-rich basaltic magma at depth. They are considered as supercooled, vesiculated floating drops of a hot hybrid layer in the magma chamber which formed after refilling. The lower density of the inclusions allows them to float in the host magma and to concentrate at the top of the chamber prior to eruption. Magma mingling was effected by mechanical disintegration of the inclusions in the host magma during eruption. The rhyodacitic and basic end-members of the mixing series cannot be linked by low-P fractionation though high-P, amphibole-rich fractionation is not excluded.  相似文献   

16.
The Pliocene–Pleistocene northern Taiwan volcanic zone (NTVZ) is located within a trench-arc–back-arc basin and oblique arc–continent collision zone. Consequently the origin and tectonic setting of the andesitic rocks within the NTVZ and their relation to other circum-Pacific volcanic island-arc systems is uncertain. Rocks collected from the Tatun volcanic group (TTVG) include basaltic to andesitic rocks. The basalt is compositionally similar to within-plate continental tholeiites whereas the basaltic andesite and andesite are calc-alkaline; however, all rocks show a distinct depletion of Nb-Ta in their normalized incompatible element diagrams. The Sr-Nd isotope compositions of the TTVG rocks are very similar and have a relatively restricted range (i.e. ISr = 0.70417–0.70488; εNd(T) = +2.2 to +3.1), suggesting that they are derived directly or indirectly from the same mantle source. The basalts are likely derived by mixing between melts from the asthenosphere and a subduction-modified subcontinental lithospheric mantle (SCLM) source, whereas the basaltic andesites may be derived by partial melting of pyroxenitic lenses within the SCLM and mixing with asthenospheric melts. MELTS modelling using a starting composition equal to the most primitive basaltic andesite, shallow-pressure (i.e. ≤1 kbar), oxidizing conditions (i.e. FMQ +1), and near water saturation will produce compositions similar to the andesites observed in this study. Petrological modelling and the Sr-Nd isotope results indicate that the volcanic rocks from TTVG, including the spatially and temporally associated Kuanyinshan volcanic rocks, are derived from the same mantle source and that the andesites are the product of fractional crystallization of a parental magma similar in composition to the basaltic andesites. Furthermore, our results indicate that, in some cases, calc-alkaline andesites may be generated by crystal fractionation of mafic magmas derived in an extensional back-arc setting rather than a subduction zone setting.  相似文献   

17.
The Klyuchevskoy group of volcanoes in the Kamchatka arc erupts compositionally diverse magmas (high-Mg basalts to dacites) over small spatial scales. New high-precision Pb isotope data from modern juvenile (1956–present) erupted products and hosted enclaves and xenoliths from Bezymianny volcano reveal that Bezymianny and Klyuchevskoy volcanoes, separated by only 9 km, undergo varying degrees of crustal processing through independent crustal columns. Lead isotope compositions of Klyuchevskoy basalts–basaltic andesites are more radiogenic than Bezymianny andesites (208Pb/204Pb = 37.850–37.903, 207Pb/204Pb = 15.468–15.480, and 206Pb/204Pb = 18.249–18.278 at Bezymianny; 208Pb/204Pb = 37.907–37.949, 207Pb/204Pb = 15.478–15.487, and 206Pb/204Pb = 18.289–18.305 at Klyuchevskoy). A mid-crustal xenolith with a crystallization pressure of 5.2 ± 0.6 kbars inferred from two-pyroxene geobarometry and basaltic andesite enclaves from Bezymianny record less radiogenic Pb isotope compositions than their host magmas. Hence, assimilation of such lithologies in the middle or lower crust can explain the Pb isotope data in Bezymianny andesites, although a component of magma mixing with less radiogenic mafic recharge magmas and possible mantle heterogeneity cannot be excluded. Lead isotope compositions for the Klyuchevskoy Group are less radiogenic than other arc segments (Karymsky—Eastern Volcanic Zone; Shiveluch—Northern Central Kamchatka Depression), which indicate increased lower-crustal assimilation beneath the Klyuchevskoy Group. Decadal timescale Pb isotope variations at Klyuchevskoy demonstrate rapid changes in the magnitude of assimilation at a volcanic center. Lead isotope data coupled with trace element data reflect the influence of crustal processes on magma compositions even in thin mafic volcanic arcs.  相似文献   

18.
The submarine volcanoes, located in the southern part of Andaman Sea, north eastern Indian Ocean, result from the subduction of the Indo-Australian Plate beneath the Southeast Asian Plate and represent one of the less studied submarine volcanism among the global arc systems. The present study provides new petrological and geochemical data for the recovered rocks from the submarine volcanoes and documents the petrogenetic evolution of Andaman arc system. Geochemical attributes classify the studied samples as basaltic andesite, andesite, dacite to rhyodacite reflecting sub-alkaline, intermediate to acidic composition of the magma. Petrographic studies of the basaltic andesites and andesites show plagioclase [An38-An57 in basaltic andesites; An27-An28 in andesites] and clinopyroxene as dominant phenocrystal phase in a cryptocrystalline groundmass. Plagioclase (An25-An45) marks the principal phenocrystal phase in dacite with sub-ordinate proportion of biotite and amphibole of both primary and secondary origin along with minor amount of K-feldspar. The submarine volcanic rocks from Andaman arc system exhibit pronounced LILE, LREE enrichments and HFSE (negative Nb, Ta and Ti anomalies), MREE and HREE depletion thereby endorsing the influence of subduction zone processes in their genesis. Elevated abundances of Th with relatively higher LREE/HFSE than LILE/HFSE, LILE/LREE suggest significant contribution of sediments from the subducting slab over slab-dehydrated aqueous fluids towards mantle wedge metasomatism thereby modifying the sub-arc mantle. Partial melting curves calculated using the non-modal batch melting equation suggest primary magma generated due to ~31–35 % degree of partial melting of spinel lherzolite mantle beneath the arc system. Fractional crystallization model suggests fractionation of 45 % plagioclase, 40 % clinopyroxene, 5–10 % amphibole and 5–10 % biotite which is consistent with the petrographic observations. Further, the assimilation-fractional-crystallization (AFC) model for the studied rocks indicates nominal crustal contamination. Therefore, this study infers that the melt evolution history for the Andaman arc volcanic rocks can be translated in terms of (i) generation of precursor magma by ~31–35 % partial melting of a spinel lherzolite mantle wedge, metasomatized predominantly by subducted slab sediments and (ii) the parent magma generation was ensued by fractionation dominated melt differentiation with nominal input from arc crust.  相似文献   

19.
The volcanics exposed in the northeast Niğde area are characterized by pumiceous pyroclastic rocks present as ash flows and fall deposits and by compositions ranging from dacite to rhyolite. Xenoliths found in the volcanics are basaltic andesite, andesite and dacite in composition. These rocks exhibit linear chemical variations between end‐member compositions and a continuity of trace element behaviour exists through the basaltic andesite–andesite–dacite–rhyolite compositional range. This is consistent with the fractionation of ferromagnesian minerals and plagioclase from a basaltic andesite or andesite parent. These rocks are peraluminous and show typical high‐K calc‐alkaline differentiation trends with total iron content decreasing progressively with increasing silica content. Bulk rock and mineral compositional trends and petrographic data suggest that crustal material was added to the magmas by subducted oceanic crust and is a likely contaminant of the source zone of the Niğde magmas. The chemical variations in these volcanics indicate that crystal liquid fractionation has been a dominant process in controlling the chemistry of the northeast Niğde volcanics. It is also clear, from the petrographic and chemical features, that magma mixing with disequilibrium played a significant role in the evolution of the Niğde volcanic rocks. This is shown by normal and reverse zoning in plagioclase and resorption of most of the observed minerals. The xenoliths found in the Niğde volcanics represent the deeper part of the magma reservoir which equilibrated at the higher pressures. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

20.
中国东北二连盆地周缘分布有三组时代不同的晚中生代火山岩,其中早、中期为两套地球化学性质不同的流纹岩,晚期为玄武质火山岩。本文通过测定火山岩基质Ar-Ar同位素年龄,表明早期查干诺尔组流纹岩形成于142Ma,晚期不拉根哈达组基性火山岩形成于129Ma,可见二连盆地北缘晚中生代火山岩时代均为早白垩世。通过对主、微量元素地球化学特征和Sr-Nd-Pb同位素组成研究,以及与邻区同期满克头鄂博组英安岩和流纹岩、玛尼吐组英安岩、霍林河地区查干诺尔组英安岩、流纹岩对比,认为早期查干诺尔组流纹岩来源于新成下地壳,岩浆演化过程经历了强烈分异作用;中期流纹岩源区为中上地壳或下地壳岩浆经历了上地壳强烈同化混染作用;晚期不拉根哈达组基性火山岩则源于受俯冲洋壳流体交代的富集岩石圈地幔。结合早白垩世区域岩石圈减薄背景,本文认为研究区早白垩世火山岩形成于陆内伸展构造环境。  相似文献   

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