首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
The Jurassic Grayback pluton was emplaced in a back-arc settingbehind a contemporaneous oceanic arc. Th\alphae main stage ofthe pluton consists of an early, reversely zoned tonalite togabbro that was intruded by synplutonic noritic and gabbroicmagmas. Late-stage activity was characterized by intrusion oftonalitic and granitic dikes, many of which contain mafic enclavesand hybrid zones. Most mafic rocks in the pluton are calc-alkaline,with characteristic magnesian clinopyroxene, calcic cores inplagioclase, and elemental abundances similar to H2O-rich arcbasalts. However, some mafic rocks contain relatively Fe-richclinopyroxene, lack calcic cores in plagioclase, and are compositionallysimilar to evolved high-alumina tholeiite. Compositional variation in the main stage can be modeled inpart by fractional crystallization and crusted assimilationduring which parental calc-alkaline basalt evolved to graniticcompositions. Cumulates related to this process are representedby modally variable melagabbro and pyroxenite. Mixing of basalticand tonalitic magmas accounts for the compositions of most main-stageintermediate rocks, but mixing of basaltic and granitic magmaswas uncommon until late in the pluton's history. Oxygen, Sr and Nd isotopic data indicate that virtually allmain-stage magmas in the pluton contain a crustal component.Isotopic and trace element data further suggest that late-stagetonalitic dikes represent melts derived from older, metavolcanicarc crust Deep crustal contamination of main-stage rocks tookplace below the level of emplacement, probably in a magma-richzone where basalts ponded and mixed with crustal melts. The Grayback pluton illustrates the diversity of Jurassic back-arcmagmatism in the Klamath province and demonstrates that ancientmagmatism with arc-like features need not be situated in anarc setting. KEY WORDS: Grayback Pluton; Klamath Mountains; Oregon; back arc; crustal contamination *Corresponding author  相似文献   

2.
The Genesis of Intermediate and Silicic Magmas in Deep Crustal Hot Zones   总被引：55，自引：2，他引：55  
A model for the generation of intermediate and silicic igneousrocks is presented, based on experimental data and numericalmodelling. The model is directed at subduction-related magmatism,but has general applicability to magmas generated in other platetectonic settings, including continental rift zones. In themodel mantle-derived hydrous basalts emplaced as a successionof sills into the lower crust generate a deep crustal hot zone.Numerical modelling of the hot zone shows that melts are generatedfrom two distinct sources; partial crystallization of basaltsills to produce residual H2O-rich melts; and partial meltingof pre-existing crustal rocks. Incubation times between theinjection of the first sill and generation of residual meltsfrom basalt crystallization are controlled by the initial geotherm,the magma input rate and the emplacement depth. After this incubationperiod, the melt fraction and composition of residual meltsare controlled by the temperature of the crust into which thebasalt is intruded. Heat and H2O transfer from the crystallizingbasalt promote partial melting of the surrounding crust, whichcan include meta-sedimentary and meta-igneous basement rocksand earlier basalt intrusions. Mixing of residual and crustalpartial melts leads to diversity in isotope and trace elementchemistry. Hot zone melts are H2O-rich. Consequently, they havelow viscosity and density, and can readily detach from theirsource and ascend rapidly. In the case of adiabatic ascent themagma attains a super-liquidus state, because of the relativeslopes of the adiabat and the liquidus. This leads to resorptionof any entrained crystals or country rock xenoliths. Crystallizationbegins only when the ascending magma intersects its H2O-saturatedliquidus at shallow depths. Decompression and degassing arethe driving forces behind crystallization, which takes placeat shallow depth on timescales of decades or less. Degassingand crystallization at shallow depth lead to large increasesin viscosity and stalling of the magma to form volcano-feedingmagma chambers and shallow plutons. It is proposed that chemicaldiversity in arc magmas is largely acquired in the lower crust,whereas textural diversity is related to shallow-level crystallization. KEY WORDS: magma genesis; deep hot zone; residual melt; partial melt; adiabatic ascent  相似文献   

3.
Xenoliths from Engeln–Kempenich in the East Eifel volcanicfield (Germany) comprise gabbroic to ultramafic cumulates, andmeta-igneous and meta-sedimentary granulite- to amphibolite-facieslithologies. They provide evidence for Pleistocene heating andmetasomatism of the lower continental crust by mafic magmas.The metamorphic xenoliths were divided into three types: (1)primitive type P, which are little affected by metasomatic replacementstructures; (2) enriched type E1 defined by metasomatic replacementof primary pyroxene and garnet by pargasitic amphibole and biotite;(3) enriched type E2 defined by breakdown of hydrous phases.Type E rocks are geochemically related to type P and cumulatexenoliths by compositional trends. During modal metasomatism,type E rocks were oxidized. Type E1 rocks were typically enrichedin Rb, Th, U, Nb, K, light rare earth elements (LREE) and Zr,and E2 enriched in Rb, Th, U, Nb, K, REE, Zr, Ti and Y, relativeto type P rocks. Formation of the hydrous, chlorine-bearingphases amphibole and scapolite containing glass and fluid inclusionsin the E1 rocks provides evidence for a water and Cl-bearingfluid phase coexisting with silicate melt. Accordingly, we calculated10 mol % H2O back into the CO2-dominated fluid inclusions, inagreement with experimental data on the composition of a fluidphase coexisting with mafic alkaline melts at elevated pressure.Primary CO2-dominated fluid inclusions coexisting with glassinclusions in metamorphic corona phases and neoblasts, and incumulate xenoliths, have overlapping densities. Fluid inclusionbarometry using the corrected densities indicates that bothcumulates and metamorphic xenoliths originated from the samedepth at 22–25 km (650 ± 50 MPa). This is interpretedas being a main magma reservoir level within the upper partof the lower crust close to the Conrad discontinuity, wherethe xenoliths represent wall-rocks. The Conrad discontinuityseparates an upper-crustal layer, consisting of preferentiallyductile granodioritic and tonalitic gneisses, and more brittlelower-crustal mafic granulites. The brittle–ductile transitionappears to be a preferred level of magma stagnation. KEY WORDS: continental lower crust; fluids; magma chamber; metasomatism; xenoliths  相似文献   

4.
The petrogenesis of calc-alkaline magmatism in the Eocene AbsarokaVolcanic Province (AVP) is investigated at Washburn volcano,a major eruptive center in the low-K western belt of the AVP.New 40Ar/39Ar age determinations indicate that magmatism atthe volcano commenced as early as 55 Ma and continued untilat least 52 Ma. Although mineral and whole-rock compositionaldata reflect near equilibrium crystallization of modal phenocrysts,petrogenetic modeling demonstrates that intermediate compositionmagmas are hybrids formed by mixing variably fractionated andcontaminated mantle-derived melts and heterogeneous siliciccrustal melts. Nd and Sr isotopic compositions along with traceelement data indicate that silicic melts in the Washburn systemare derived from deep-crustal rocks broadly similar in compositionto granulite-facies xenoliths in the Wyoming Province. Our preferredexplanation for these features is that mantle-derived basalticmagma intruded repeatedly in the deep continental crust leadingto fractional crystallization, silicic melt production, andhomogenization of magmas, followed by ascent to shallow reservoirsand crystallization of new plagioclase-rich mineral assemblagesin equilibrium with the intermediate hybrid liquids. The implicationsof this process are that (1) some calc-alkaline magmas may onlybe recognized as hybrids on purely chemical grounds, particularlyin systems where mixing precedes and is widely separated fromcrystallization in space and time, and (2) given the role ascribedto crustal processes at Washburn volcano, the variation betweenrocks that follow calc-alkaline trends in the western AVP andthose that follow shoshonitic trends in the east cannot simplyreflect higher pressures of fractionation to the east in Moho-levelmagma chambers in the absence of crustal interaction. KEY WORDS: petrogenesis; magma mixing; calc-alkaline; Absaroka Volcanic Province; 40Ar/39Ar dates  相似文献   

5.
Fluids or melts derived from a subducting plate are often citedas a mechanism for the oxidation of arc magmas. What remainsunclear is the link between the fluid, oxygen fugacity, andother major and trace components, as well as the spatial distributionof the impact of those fluids. To test the potential effectsof addition of a subduction-derived fluid or melt to the sub-arcmantle, olivine-hosted melt inclusions from primitive basalticlavas sampled from across the central Oregon Cascades (43°–45°N)have been analyzed for major, trace and volatile elements andfO2. Oxygen fugacity was determined in melt inclusions fromsulfur speciation determined by electron microprobe and fromolivine–chromite oxygen geobarometry. The overall rangein fO2 based on sulfur speciation measurements is from <–0·25log units to + 1·9 log units (FMQ, where FMQ is fayalite–magnetite–quartzbuffer). Oxygen fugacity is positively correlated with fluid-mobiletrace element and light rare earth element contents in basaltsgenerated by relatively low-degree partial melting. Establishinga further correlation between fO2 and fluid-mobile trace elementabundances with position along the arc requires the basaltsto be subdivided into shoshonitic, calc-alkaline, low-K tholeiiteand enriched intraplate basalt groups. Melt inclusions fromenriched intraplate and shoshonitic lavas show increasing fO2and trace element abundances closer to the trench, whereas calc-alkalinemelt inclusions exhibit no significant across-arc variations.Low-K tholeiitic melt inclusions record an increase in incompatibletrace elements closer to the trench; however, there is no correlatedincrease in fO2. The correlation observed in enriched intraplateand shoshonitic melt inclusions is interpreted to reflect aprogressively greater proportion of a fluid-rich, oxidized subductioncomponent in magmas generated nearer the subduction zone. Significantly,calc-alkaline melt inclusions with high ratios of large ionlithophile elements to high field strength elements, characteristicof ‘typical’ arc magmas, have oxidation states indistinguishablefrom low-K tholeiite and enriched intraplate basalt melt inclusions.The lack of across-arc geochemical variation in calc-alkalinemelt inclusions may suggest that these basalts are not necessarilythe most appropriate magmas for examining recent addition ofa subduction component to the sub-arc mantle. Flux and batchmelt model results produce a wide range of predicted amountsof melting and subduction component added to the mantle source;however, general trends characterized by increased melting andproportion of the subduction component from enriched intraplate,to low-K tholeiite, to calc-alkaline are robust. The model resultsdo not require enriched intraplate, low-K tholeiite and calc-alkalinemagmas to be produced from the same more fertile mantle source.However, enriched intraplate magmas, in contrast to calc-alkalineand low-K tholeiite magmas, cannot be generated from a depletedmantle source. Flux or batch melting of either the more fertileor depleted mantle sources used to generate the low-K tholeiite,calc-alkaline, and enriched intraplate magmas cannot reproduceshoshonitic compositions, which require a significantly depletedmantle source strongly metasomatized by a subduction component.The potential mantle source for shoshonitic basalts has a predictedfO2 (after oxidation) from + 0·3 to + 2·4 logunits (FMQ) whereas the mantle source for low-K tholeiite, calc-alkaline,and enriched intraplate magmas may range from –1·1to + 0·7 log units (FMQ). KEY WORDS: basalt; Cascades; melt inclusions; oxidation state; volatiles  相似文献   

6.
Major- and trace-element data on the constituent minerals ofgarnet peridotite xenoliths hosted in early Paleozoic (457–500Ma) kimberlites and Neogene (16–18 Ma) volcanic rockswithin the North China Craton are compared with those from thepre-pilot hole of the Chinese Continental Scientific DrillingProject (CCSD-PP1) in the tectonically exhumed Triassic (220Ma) Sulu ultrahigh-pressure (UHP) terrane along its southernmargin. P–T estimates for the Paleozoic and Neogene peridotitexenoliths reflect different model geotherms corresponding tosurface heat flows of 40 mW/m2 (Paleozoic) and 80 mW/m2 (Neogene).Garnet peridotite xenoliths or xenocrysts from the Paleozoickimberlites are strongly depleted, similar to peridotites fromother areas of cratonic mantle, with magnesium olivine (meanFo92.7), Cr-rich garnet and clinopyroxene with high La/Yb. Garnet(and spinel) peridotite xenoliths hosted in Neogene basaltsare derived from fertile mantle; they have high Al2O3 and TiO2contents, low-Mg-number olivine (mean Fo89.5), low-Cr garnetand diopside with flat rare earth element (REE) patterns. Thedifferences between the Paleozoic and Neogene xenoliths suggestthat a buoyant refractory lithospheric keel present beneaththe eastern North China Craton in Paleozoic times was at leastpartly replaced by younger, hotter and more fertile lithosphericmantle during Mesozoic–Cenozoic times. Garnet peridotitesfrom the Sulu UHP terrane have less magnesian olivine (Fo91.5),and lower-Cr garnet than the Paleozoic xenoliths. The diopsideshave low heavy REE (HREE) contents and sinusoidal to light REE(LREE)-enriched REE patterns. These features, and their highMg/Si and low CaO and Al2O3 contents, indicate that the CCSD-PP1peridotites represent a moderately refractory mantle protolith.Details of mineral chemistry indicate that this protolith experiencedcomplex metasomatism by asthenosphere-derived melts or fluidsin Mesoproterozoic, and subsolidus re-equilibration involvingfluids/melts derived from the subducted Yangtze continentalcrust during UHP metamorphism in the early Mesozoic. Tectonicextension of the subcontinental lithospheric mantle of the NorthChina Craton and exhumation of the Sulu UHP rocks in the earlyMesozoic induced upwelling of the asthenosphere. Peridotitessampled by the Neogene basalts represent newly formed lithospherederived by cooling of the upwelling asthenospheric mantle inJurassic–Cretaceous and Paleogene time. KEY WORDS: garnet peridotite xenoliths; North China Craton; lithospheric thinning; Sulu UHP terrane; UHP lithosphere evolution; mantle replacement  相似文献   

7.
The main types of intrusive rocks in the Kelameili-Harlik Hercynian erogenic belt include calc-alkaline granites, diabase dykes, kaligranites and alkaline granites. Investigation in field geology, petrology, mineralogy and geochemistry shows that the calc-alkaline granites belong to the syntexis-type (or I-type) and were formed in a pre-collisional magmatic arc environment. In consideration of the fact that kaligranites have many features of alkaline granites with higher consolidation temperatures than the calc-alkaline granites and show a discontinuity of minor element and REE evolution in respect to the calc-alkaline granites, they could not have been derived by differentiation of magmas for the calc-alkaline granites, but are likely to have been generated in an environment analogous to that for alkaline granites. The triplet of basic dyke swarms, kaligranites and alkaline granites could be regarded as a prominent indication of the initial stage of post-collisional delamination and extension. These ro  相似文献   

8.
K-feldspar megacrysts (Kfm) are used to investigate the magmaticevolution of the 7 Ma Monte Capanne (MC) monzogranite (Elba,Italy). Dissolution and regrowth of Kfm during magma mixingor mingling events produce indented resorption surfaces associatedwith high Ba contents. Diffusion calculations demonstrate thatKfm chemical zoning is primary. Core-to-rim variations in Ba,Rb, Sr, Li and P support magma mixing (i.e. high Ba and P andlow Rb/Sr at rims), but more complex variations require othermechanisms. In particular, we show that disequilibrium growth(related to variations in diffusion rates in the melt) may haveoccurred as a result of thermal disturbance following influxof mafic magma in the magma chamber. Initial 87Sr/86Sr ratios(ISr) (obtained by microdrilling) decrease from core to rim.Inner core analyses define a mixing trend extending towardsa high ISr–Rb/Sr melt component, whereas the outer coresand rims display a more restricted range of ISr, but a largerrange of Rb/Sr. Lower ISr at the rim of one megacryst suggestsmixing with high-K calc-alkaline mantle-derived volcanics ofsimilar age on Capraia. Trace element and isotopic profilessuggest (1) early megacryst growth in magmas contaminated bycrust and refreshed by high ISr silicic melts (as seen in theinner cores) and (2) later recharge with mafic magmas (as seenin the outer cores) followed by (3) crystal fractionation, withpossible interaction with hydrothermal fluids (as seen in therim). The model is compatible with the field occurrence of maficenclaves and xenoliths. KEY WORDS: Elba; monzogranite; K-feldspar megacrysts; zoning; magma mixing; trace element; Sr isotopes; petrogenesis  相似文献   

9.
The problem of mantle metasomatism vs. crustal contamination in the genesis of arc magmas with different potassium contents has been investigated using new trace element and Sr–Nd–Pb isotopic data on the island of Vulcano, Aeolian arc. The analysed rocks range in age from 120 ka to the present day, and cover a compositional range from basalt to rhyolite of the high-K calc-alkaline (HKCA) to shoshonitic (SHO) and potassic (KS) series. Older Vulcano products (>30 ka) consist of HKCA–SHO rocks with SiO2=48–56%. They show lower contents of K2O, Rb and of several other incompatible trace element abundances and ratios than younger rocks with comparable degree of evolution. 87Sr/86Sr ranges from 0.70417 to 0.70504 and increases with decreasing MgO and compatible element contents. 206Pb/204Pb ratios display significant variations (19.31 to 19.76) and are positively correlated with MgO, 143Nd/144Nd (0.512532–0.512768), 207Pb/204Pb (15.66–15.71) and 208Pb/204Pb (39.21–39.49). Overall, geochemical and isotopic data suggest that the evolution of the older series was dominated by assimilation–fractional crystallisation (AFC) with an important role for continuous mixing with mafic liquids. Magmas erupted within the last 30 ka consist mostly of SHO and KS intermediate and acid rocks, with minor mafic products. Except for a few acid rocks, they display moderate isotopic variations (e.g. 87Sr/86Sr=0.70457–0.70484; 206Pb/204Pb=19.28–19.55, but 207Pb/204Pb=15.66–15.82), which suggest an evolution by fractional crystallisation, or in some cases by mixing, with little interaction with crustal material. The higher Sr isotopic ratios (87Sr/86Sr=0.70494–0.70587) of a few, low-volume, intermediate to acid rocks support differentiation by AFC at shallow depths for some magma batches. New radiogenic isotope data on the Aeolian islands of Alicudi and Stromboli, as well as new data for lamproites from central Italy, are also reported in order to discuss along-arc compositional variations and to evaluate the role of mantle metasomatism. Geochemical and petrological data demonstrate that the younger K-rich mafic magmas from Vulcano cannot be related to the older HKCA and SHO ones by intra-crustal evolutionary processes and point to a derivation from different mantle sources. The data from Alicudi and Stromboli suggest that, even though interaction between magma and wall rocks of the Calabrian basement during shallow level magma evolution was an important process locally, a similar interpretation can be extended to the entire Aeolian arc. Received: 27 September 1999 / Accepted: 24 May 2000  相似文献   

10.
Evidence for post‐Archaean crustal growth via magma underplating is largely based on U–Pb dating of zircons from granulite‐facies xenoliths. However, whether the young zircons from such xenoliths are genetically related to magma underplating or to anatexis remains controversial. The lower‐crustal xenoliths carried by igneous rocks in the Chifeng and Ningcheng (North China Craton) have low SiO2 and high MgO, indicating that parental melts of their protoliths were of unambiguous mantle origin. The xenoliths contain abundant magmatic zircons with late‐Palaeozoic ages, and have more radiogenic zircon Hf‐isotope compositions and hence younger model ages than ancient crustal magmas and the “reworking array” of the basement rocks. Our data suggest that the granulites represent episodic magmatic underplating to the lower crust of this craton in Phanerozoic time. Considering the observation that regional lowermost crust (~5 km) is mafic and characterized by Phanerozoic zircons, this work reports an example of post‐Archaean crustal growth via magma underplating.  相似文献   

11.
The Variscan basement of the Central Iberian Zone contains abundantCambro-Ordovician calc-alkaline to peraluminous metagranitesand metavolcanic rocks with two notable features: first, theywere apparently produced with no connection to any major tectonicor metamorphic event; second, they have an unusually high zirconinheritance. U–Pb dating combined with cathodoluminescenceimaging reveals that about 70–80%, in some samples nearer100%, of the zircon grains contain inherited pre-magmatic cores,despite the temperature reached by the magmas (about 900°C,calculated using the Ti-in-zircon thermometer) being high enoughto dissolve all the available zircon (from the rock's zirconsaturation temperature, 770–860°C). The fact thatthe dissolution of zircon was so incomplete can only be attributedto the kinetics of heat transfer to and from the magmas. Three-dimensionalmodeling of zircon dissolution behavior in melts with a compositionsimilar to the Iberian Cambro-Ordovician magmas indicates thatthe survival of zircons from the suggested late Pan-Africanprotolith would be possible only if melt production was rapid,specifically less than 104 years, and probably about 2 x 103years, from the beginning of melting (700°C) to the thermalpeak (900°C). Melt production was followed by fast magmatransfer to upper crustal levels resulting either in surfaceeruption or in the emplacement of small (< 400 m thick) sillsor laccoliths. We suggest that these elevated rates of crustalmelting could only have been caused by intrusion of mantle-derivedmafic magmas, most probably at the base of the crust. This scenariois consistent with a rifting regime in which crust and mantlewere mechanically decoupled; this would explain the scarcityof contemporaneous crustal deformation. Furthermore, fast meltingrates in the lower crust followed by fast melt transportationto the upper crust could also explain the lack of contemporaneousmetamorphism. The speed of the partial melting process resultedin the production of felsic magmas that inherited the geochemicalcharacteristics of their granitoid crustal protolith. This explainsthe apparent contradiction between the calc-alkaline to peraluminousgeochemical characteristics of the magmas and the inferred extensional(i.e. rift-related) tectonic setting. Our model is compatiblewith the hypothesis of fragmentation and dispersal of terranesfrom the northern margin of Gondwana that led to the openingof the Rheic and Galicia–South Brittany oceans and, ultimately,caused the detachment of the Iberian microplate from Armoricaand Gondwana during the early Paleozoic. KEY WORDS: igneous petrology; migmatite; granite; geochemistry; crustal contamination; ICP-MS; laser ablation  相似文献   

12.
The present-day North Chilean Coastal Cordillera between 18°30′S and 22°S records an important part of the magmatic evolution of the Central Andes during the Jurassic. Calc-alkaline to subordinate tholeiitic members from four rock groups with biostratigraphically constrained age display incompatible element pattern characteristic of convergent plate-margin volcanism, whereas alkaline basalts of one group occurring in the Precordillera show OIB-type trace element signatures. The correlation of biostratigraphic ages, regional distribution, and composition of the volcanic rocks provides a basis for the discussion on geochemical evolution and isotope ratios.Major and trace element distributions of the volcanic rocks indicate their derivation from mantle-derived melts. LILE and LREE enrichments in calc-alkaline basaltic andesites to dacites and some of the tholeiites hint at the involvement of hydrous fluids during melting and mobile element transport processes. A part of the Early Bajocian to ?Lower Jurassic and Oxfordian andesites and dacites are adakite-like rocks with a substantial participation of slab melt and are characterized by high Sr/Y ratios and low HREE contents. The Middle Jurassic tholeiitic and calc-alkaline basalts and basaltic andesites have been transported and partly stored within a system of deep-seated feeder fissures and crustal strike-slip faults before eruption.The isotopic composition of Sr (87Sr/86Sri=0.7032-0.7056) and Nd (εNdi=2.2-7.1) of the Jurassic volcanic rocks mostly fall in the range characteristic for mantle melts although some crustal components may have been involved. A few samples show slightly more radiogenic Sr isotopic composition, which is probably due to interaction with ancient sea-water. The Pb isotopic composition of the arc rocks is uncoupled from the isotopic composition of Sr and Nd and is dominated by the crustal component. Since the Cretaceous and Modern arc volcanic rocks show Pb isotopic compositions that can be largely explained by in situ Pb isotope growth of Jurassic arc volcanic rocks, we argue that the various Andean arc systems between 18°30′S and 22°S formed on the same type of basement.Most of the investigated samples have high Ba, Zr, and Th concentrations compared to island arc mafic volcanic rocks. About 20% of the Jurassic arc volcanics comprise of dacitic to rhyolitic rocks. These characteristics combined with the Pb isotopic composition that shows the influence of a Palaeozoic (or partly older) basement point to a continental margin setting for the North Chilean Jurassic arc. The distribution of the magmatic rocks throughout time, their textures, and the character of intercalated sedimentary rocks reflect westward movement of the magma sources and of the arc/back-arc boundary relative to the current coast line during the Early Bajocian on a broad front between 19°30′ and 21°S.  相似文献   

13.
We report the results of a geochemical study of the Jijal andSarangar complexes, which constitute the lower crust of theMesozoic Kohistan paleo-island arc (Northern Pakistan). TheJijal complex is composed of basal peridotites topped by a gabbroicsection made up of mafic garnet granulite with minor lensesof garnet hornblendite and granite, grading up-section to hornblendegabbronorite. The Sarangar complex is composed of metagabbro.The Sarangar gabbro and Jijal hornblende gabbronorite have melt-like,light rare earth element (LREE)-enriched REE patterns similarto those of island arc basalts. Together with the Jijal garnetgranulite, they define negative covariations of LaN, YbN and(La/Sm)N with Eu* [Eu* = 2 x EuN/(SmN + GdN), where N indicateschondrite normalized], and positive covariations of (Yb/Gd)Nwith Eu*. REE modeling indicates that these covariations cannotbe accounted for by high-pressure crystal fractionation of hydrousprimitive or derivative andesites. They are consistent withformation of the garnet granulites as plagioclase–garnetassemblages with variable trapped melt fractions via eitherhigh-pressure crystallization of primitive island arc basaltsor dehydration-melting of hornblende gabbronorite, providedthat the amount of segregated or restitic garnet was low (<5wt %). Field, petrographic, geochemical and experimental evidenceis more consistent with formation of the Jijal garnet granuliteby dehydration-melting of Jijal hornblende gabbronorite. Similarly,the Jijal garnet-bearing hornblendite lenses were probably generatedby coeval dehydration-melting of hornblendites. Melting modelsand geochronological data point to intrusive leucogranites inthe overlying metaplutonic complex as the melts generated bydehydration-melting of the plutonic protoliths of the Jijalgarnet-bearing restites. Consistent with the metamorphic evolutionof the Kohistan lower arc crust, dehydration-melting occurredat the mature stage of this island arc when shallower hornblende-bearingplutonic rocks were buried to depths exceeding 25–30 kmand heated to temperatures above c. 900°C. Available experimentaldata on dehydration-melting of amphibolitic sources imply thatthickening of oceanic arcs to depths >30 km (equivalent toc. 1·0 GPa), together with the hot geotherms now postulatedfor lower island arc crust, should cause dehydration-meltingof amphibole-bearing plutonic rocks generating dense garnetgranulitic roots in island arcs. Dehydration-melting of hornblende-bearingplutonic rocks may, hence, be a common intracrustal chemicaland physical differentiation process in island arcs and a naturalconsequence of their maturation, leading to the addition ofgranitic partial melts to the middle–upper arc crust andformation of dense, unstable garnet granulite roots in the lowerarc crust. Addition of LREE-enriched granitic melts producedby this process to the middle–upper island arc crust maydrive its basaltic composition toward that of andesite, affordinga plausible solution to the ‘arc paradox’ of formationof andesitic continental-like crust in island arc settings. KEY WORDS: island arc crust; Kohistan complex; Jijal complex; amphibole dehydration-melting; garnet granulite; continental crustal growth  相似文献   

14.
The Carpathian–Pannonian Region contains Neogene to Quaternary magmatic rocks of highly diverse composition (calc-alkaline, shoshonitic and mafic alkalic) that were generated in response to complex microplate tectonics including subduction followed by roll-back, collision, subducted slab break-off, rotations and extension. Major element, trace element and isotopic geochemical data of representative parental lavas and mantle xenoliths suggests that subduction components were preserved in the mantle following the cessation of subduction, and were reactivated by asthenosphere uprise via subduction roll-back, slab detachment, slab-break-off or slab-tearing. Changes in the composition of the mantle through time are evident in the geochemistry, supporting established geodynamic models.Magmatism occurred in a back-arc setting in the Western Carpathians and Pannonian Basin (Western Segment), producing felsic volcaniclastic rocks between 21 to 18 Ma ago, followed by younger felsic and intermediate calc-alkaline lavas (18–8 Ma) and finished with alkalic-mafic basaltic volcanism (10–0.1 Ma). Volcanic rocks become younger in this segment towards the north. Geochemical data for the felsic and calc-alkaline rocks suggest a decrease in the subduction component through time and a change in source from a crustal one, through a mixed crustal/mantle source to a mantle source. Block rotation, subducted roll-back and continental collision triggered partial melting by either delamination and/or asthenosphere upwelling that also generated the younger alkalic-mafic magmatism.In the westernmost East Carpathians (Central Segment) calc-alkaline volcanism was simultaneously spread across ca. 100 km in several lineaments, parallel or perpendicular to the plane of continental collision, from 15 to 9 Ma. Geochemical studies indicate a heterogeneous mantle toward the back-arc with a larger degree of fluid-induced metasomatism, source enrichment and assimilation on moving north-eastward toward the presumed trench. Subduction-related roll-back may have triggered melting, although there may have been a role for back-arc extension and asthenosphere uprise related to slab break-off.Calc-alkaline and adakite-like magmas were erupted in the Apuseni Mountains volcanic area (Interior Segment) from15–9 Ma, without any apparent relationship with the coeval roll-back processes in the front of the orogen. Magmatic activity ended with OIB-like alkali basaltic (2.5 Ma) and shoshonitic magmatism (1.6 Ma). Lithosphere breakup may have been an important process during extreme block rotations (60°) between 14 and 12 Ma, leading to decompressional melting of the lithospheric and asthenospheric sources. Eruption of alkali basalts suggests decompressional melting of an OIB-source asthenosphere. Mixing of asthenospheric melts with melts from the metasomatized lithosphere along an east–west reactivated fault-system could be responsible for the generation of shoshonitic magmas during transtension and attenuation of the lithosphere.Voluminous calc-alkaline magmatism occurred in the Cãlimani-Gurghiu-Harghita volcanic area (South-eastern Segment) between 10 and 3.5 Ma. Activity continued south-eastwards into the South Harghita area, in which activity started (ca. 3.0–0.03 Ma, with contemporaneous eruption of calc-alkaline (some with adakite-like characteristics), shoshonitic and alkali basaltic magmas from 2 to 0.3 Ma. Along arc magma generation was related to progressive break-off of the subducted slab and asthenosphere uprise. For South Harghita, decompressional melting of an OIB-like asthenospheric mantle (producing alkali basalt magmas) coupled with fluid-dominated melting close to the subducted slab (generating adakite-like magmas) and mixing between slab-derived melts and asthenospheric melts (generating shoshonites) is suggested. Break-off and tearing of the subducted slab at shallow levels required explaining this situation.  相似文献   

15.
Xenolithic inclusions in calc-alkaline andesite from Mt. Moffettvolcano, Adak Island, Aleutian arc, reveal a nearly continuousrecord of crystallization of basaltic magmas in the crust, andpossibly upper mantle, of the arc. The record is more detailedand continuous than that obtained from study of calc-alkalinevolcanic rocks in the arc. Cumulate xenoliths form a progressiveseries in modal mineralogy from ultramafic, hornblende-bearingolivine clinopyroxenite to both hornblende-bearing and hornblende-freegabbros. The cumulate hornblende gabbro xenoliths are typicalof those found in island arc andesites worldwide. Xenolithicinclusions without cumulate textures, here termed compositexenoliths, are characterized by forsteritic olivine, zoned Cr-diopsideand hornblende, and are interpreted to have resulted from reactionand chilling upon magma mixing at depth. The olivine and clinopyroxene in both cumulate and compositexenoliths show the largest and the most complete variation trendsfor Ni, Cr, and FeO/MgO ratio yet reported in igneous xenolithsfrom island arc volcanic rocks. Variation of Ni in olivine indicatesthat the parent magmas for the xenoliths had minimum MgO contentsof 9 wt. per cent. Variation of Cr in clinopyroxene indicatesthat the magmas were basaltic rather than picritic, probablyin equilibrium with spinel lherzolite at near Moho depths. Successiveinjections of batches of primary melt into a magma chamber fractionatingolivine and clinopyroxene can reproduce observed compatibleelement depletion trends. A steady-state process of cotecticcrystallization in a magma chamber continually replenished withbasaltic magma is a possible mechanism for producing large accumulationsof olivine and clinopyroxene, suggesting that Alaskan-type ultramaficcomplexes are related to hydrous basaltic magmas in island arcs.This steady-state open-system crystallization process can alsoyield the abundant high-alumina basalt type in the Aleutianarc. Continued crystallization of high-alumina basalt in lowercrustal magma chambers, recorded in a mineralogically coherentseries of pyroxenite to hornblende gabbro xenoliths, can yieldbasaltic to andesitic magmas of the calc-alkaline series. No xenoliths with a sedimentary protolith have been found atMt Moffett, evidence that the arc crust is igneous in origin,with the lower crust formed of gabbro crystallized from mantle-derivedmelts. Ultramafic cumulates may reside in both the lower crustor upper mantle beneath the arc. A model is proposed wherebythe cumulate crystallization products of hydrous, mantle beneaththe arc. A model is define the upper mantle and lower crustof the arc over time.The net composition added to the crustof the arc is that of high-alumina basalt.  相似文献   

16.
Late Quaternary andesitic magmas in New Zealand contain complexly zoned antecrysts and glomerocrysts that are not in equilibrium with either the host whole rock compositions or siliceous groundmass glass and glass inclusions. Glass inclusions represent partial melts of mafic to gabbroic cumulates in the lower crust that mix with restite crystals, or cumulates from earlier magma batches. Assimilation of partial melts of mid-crustal rocks, represented by glass in crustal xenoliths, contributes a crustal component to the andesites. Magmas at Egmont are stored at about the brittle/ductile transition at about 10 km depth and variability in the composition of erupted material is a function of the composition of the recharging magma, and which parts of the storage system are tapped during the eruption. At Taranaki recharge occurs on a c. 1400 year cycle while interactions within the storage give rise to shorter period events. A similar process on a less well constrained timescale operates at Ruapehu. Andesites are therefore complex mixtures of fractionated mantle basalts, siliceous partial melts of both the lower crust and underplated cumulates, restite and cumulate crystals. Further modification occurs by interaction with partial melts of lower to middle crustal basement as geotherms increase with time.  相似文献   

17.
TAMURA  Y. 《Journal of Petrology》1995,36(2):417-434
The Mio-Pliocene Shirahama Group, Izu Peninsula, Central Japan,a well-exposed submarine volcanic arc complex of lava flows,pyroclastic rocks and associated shallow intrusives, is characterizedby a tholeiitic series (basalt to dacite) and a calc-alkalineseries (andesite to dacite). Chemical variations in the tholeiiticseries and calc-alkaline series are consistent with crystalfractionation from basalt and magnesian andesite (boninite),respectively. Crystal–liquid phase relations of thesemagmas have been investigated by study of sample suites fromthese two series. Compositions of liquids in equilibrium withphenocrysts were determined by microprobe grid analyses, inwhich 49 points were averaged in 03 mm 03 mm groundmassareas. The liquid compositions, coupled with the phenocrystmineralogy of the same samples, define the liquid lines of descentof these volcanic arc magmas. Major findings include the following:(1) Crystallization of the tholeiitic series magma is consistentwith early stage crystallization in the simple system Fo–Di–Silica–H2O,with olivine having a reaction relation to augite and the tholeiiticliquid. (2) The later stage products of the tholeiitic seriesmagma are, however, crystal-poor (<10%) dacites with no maficminerals, suggesting that tholeiitic liquids, hypersthene andaugite were no longer on the cotectic (3) A characteristic ofthe calc-alkaline series magmas is the development of rhyoliticliquids. Hypersthene, augite, plagioclase and Fe–Ti oxideoccur in most calc-alkaline rocks studied, and hornblende andquartz can be found in about half of these. However, their differentiationpaths show that the cotectic relation between quartz and liquidended at a later stage, resulting in the resorption of quartzphenocrysts and ultimately in the formation of quartz-free magmas.(4) The late-stage liquids of both the tholeiitic and calc-alkalineseries have deviated from their cotectics, which cannot be explainedby fractional crystallization alone. The addition of H2O froman outside system is probably required to explain the differentiationpaths. (5) The formation of chilled margins, the in situ crystallizationof a magma chamber in the solidification zone, and/or the migrationof groundwater into the magma chamber are thought to be likelyprocesses affecting magmas during their migration and intrusioninto the crust. An extreme effect of H2O addition would be tolower the liquidus temperatures of all precipitating silicatephases far below their restorable range before eruption, resultingin the production of aphyric magmas. Even when a temperaturedecrease in the magma chamber causes a liquid to intersect theliquidus of a pre-existing phase, the addition of H2O shiftsthe cotectic toward SiO2, resulting in quartz being the lastphase to crystallize. The resorption of quartz is interpretedto be the result of a liquidus boundary shift caused by theaddition of H2O. The genesis of aphyric rhyolites is thereforeinferred to result from fractional crystallization followingaddition of H20. KEY WORDS: Shirahama Group; Japan; island arc; rhyolite; magma series  相似文献   

18.
A late Mesozoic belt of volcanic–intrusive complexes occursin SE China. Volcanic activity at Xiangshan in the NW of thebelt took place mainly in the Late Jurassic (158–135 Ma).The volcanic rocks from the Xiangshan volcanic complex includerhyolitic crystal tuffs, welded tuffs, rhyolite lavas, porphyriticlavas, and associated subvolcanic rocks. Mineral assemblagesin these magmatic rocks include K-feldspar, plagioclase, quartz,Fe-rich biotite and minor amphibole, orthopyroxene and almandine.Mineral geothermometry indicates a high crystallization temperature(>850°C) for the Xiangshan magmas. The volcanic rocksare generally peraluminous; SiO2 contents are between 65·4%and 76·8% and the samples have high alkalis, rare earthelements (REE), high field strength elements and Ga contentsand high Ga/Al ratios, but are depleted in Ba, Sr and transitionmetals. Trace element geochemistry and Sr–Nd–O isotopesystematics imply that the Xiangshan magmas were probably derivedfrom partial melting of Middle Proterozoic metamorphic lower-crustalrocks that had been dehydrated during an earlier thermal event.These features suggest an A-type affinity. Quenched mafic enclaves,hosted by the subvolcanic rocks, consist mainly of alkali feldspar,plagioclase, clinopyroxene, phlogopite and amphibole. Geothermometrycalculations indicate that the primary magmas that chilled toform the quenched enclaves had anomalously high temperatures(>1200°C). The quenched enclaves have boninitic affinities;for example, intermediate SiO2 contents, high MgO and low TiO2contents, high Mg-numbers and high concentrations of Sc, Ni,Co and V. However, they also have shoshonitic characteristics,e.g. enrichment in alkalis, high K2O contents with high K2O/Na2Oratios, high light REE and large ion lithophile element contents,low initial Nd values (–4·2) and high initial 87Sr/86Srratios (0·7081). We suggest a phlogopite-bearing spinelharzburgitic lithospheric mantle source for these high-Mg potassicmagmas. Underplating of such anomalously high-temperature magmascould have induced granulite-facies lower-crustal rocks to partiallymelt and generate the Xiangshan A-type volcanic suite. A back-arcextensional setting, related to subduction of the Palaeo-Pacificplate, is favoured to explain the petrogenesis of the Xiangshanvolcanic complex and quenched enclaves. KEY WORDS: volcanic complex; quenched enclaves; petrology; geochemistry; back-arc extension setting; Xiangshan; SE China  相似文献   

19.
We report major and trace element abundances and Sr, Nd andPb isotopic data for Miocene (16·5–11 Ma) calc-alkalinevolcanic rocks from the western segment of the Carpathian arc.This volcanic suite consists mostly of andesites and dacites;basalts and basaltic andesites as well as rhyolites are rareand occur only at a late stage. Amphibole fractionation bothat high and low pressure played a significant role in magmaticdifferentiation, accompanied by high-pressure garnet fractionationduring the early stages. Sr–Nd–Pb isotopic dataindicate a major role for crustal materials in the petrogenesisof the magmas. The parental mafic magmas could have been generatedfrom an enriched mid-ocean ridge basalt (E-MORB)-type mantlesource, previously metasomatized by fluids derived from subductedsediment. Initially, the mafic magmas ponded beneath the thickcontinental crust and initiated melting in the lower crust.Mixing of mafic magmas with silicic melts from metasedimentarylower crust resulted in relatively Al-rich hybrid dacitic magmas,from which almandine could crystallize at high pressure. Theamount of crustal involvement in the petrogenesis of the magmasdecreased with time as the continental crust thinned. A strikingchange of mantle source occurred at about 13 Ma. The basalticmagmas generated during the later stages of the calc-alkalinemagmatism were derived from a more enriched mantle source, akinto FOZO. An upwelling mantle plume is unlikely to be presentin this area; therefore this mantle component probably residesin the heterogeneous upper mantle. Following the calc-alkalinemagmatism, alkaline mafic magmas erupted that were also generatedfrom an enriched asthenospheric source. We propose that bothtypes of magmatism were related in some way to lithosphericextension of the Pannonian Basin and that subduction playedonly an indirect role in generation of the calc-alkaline magmatism.The calc-alkaline magmas were formed during the peak phase ofextension by melting of metasomatized, enriched lithosphericmantle and were contaminated by various crustal materials, whereasthe alkaline mafic magmas were generated during the post-extensionalstage by low-degree melting of the shallow asthenosphere. Thewestern Carpathian volcanic areas provide an example of long-lastingmagmatism in which magma compositions changed continuously inresponse to changing geodynamic setting. KEY WORDS: Carpathian–Pannonian region; calc-alkaline magmatism; Sr, Nd and Pb isotopes; subduction; lithospheric extension  相似文献   

20.
  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号