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1.
The Late Cretaceous Chagai arc outcrops in western Pakistan, southern Afghanistan and eastern Iran. It is in the Tethyan convergence zone, formed by northward subduction of the Arabian oceanic plate beneath the Afghan block. The oldest unit of the Chagai arc is the Late Cretaceous Sinjrani Volcanic Group. This is composed of porphyritic lava flows and volcaniclastic rocks, and subordinate shale, sandstone, limestone and chert. The flows are fractionated low-K tholeiitic basalts, basaltic-andesites, and andesites. Relative enrichment in their LILE and depletion in HFSE, and negative Nb and Ta and positive K, Ba and Sr anomalies point to a subduction-related origin. Compared to MORB, the least fractionated Chagai basalts have low Na2O, Fe2O3T, CaO, Ti, Zr, Y and 87Sr/86Sr. Rather than an Andean setting, these results suggest derivation from a highly depleted mantle in an intraoceanic arc formed by Late Cretaceous convergence in the Ceno-Tethys. The segmented subduction zone formed between Gondwana and a collage of small continental blocks (Iran, Afghan, Karakoram, Lhasa and Burma) was accompanied by a chain of oceanic island arcs and suprasubduction ophiolites including Semail, Zagros, Chagai-Raskoh, Kandahar, Muslim Bagh, Waziristan and Kohistan-Ladakh, Nidar, Nagaland and Manipur. These complexes accreted to the southern margin of Eurasia in the Late Cretaceous.  相似文献   

2.
Three main groups of lavas are exposed on islands of the Lau Ridge: the Lau Volcanic Group (LVG), 14.0–5.4 Ma, are predominantly andesite; Korobasaga Volcanic Group (KVG), 4.4–2.4 Ma, are predominantly basalt and Mago Volcanic Group (MVG), 2.0–0.3 Ma, are basalt-hawaiite. LVG and KVG lavas are mostly medium-K tholeiitic rocks with high LILE/HFSE ratios characteristic of islands ares, while MVG lavas are ne-normative alkalic rocks with high LILE and HFSE, characteristic of ocean island basalts. LVG lavas have high ?Nd (+8.0–+8.4) and low 87Sr/86Sr (0.70273–0.70349) similar to N-MORB, whereas KVG lavas have slightly more radiogenic values (?Nd=+7.5?+8.4; 87Sr/86Sr=0.70323-0.70397). MVG lavas form an isotopically distinct group having lower ?Nd (+4.6–+4.9) and (87Sr/86Sr ranging from 0.70347–0.70375). LVG lavas were erupted in a primary oceanic island arc (Vitiaz arc) during the Miocene. Basaltic lavas were derived by approximately 19% partial melting of mantle wedge peridotite with only minor subduction component. Andesites and dacites were produced by low-pressure plagioclase-pyroxene-titanomagnetite dominated crystal fractionation. KVG lavas were erupted during the period immediately prior to or during the initial stages of rifting in the Lau Basin, and, like LVG lavas, show significant chemical differences at the northern and southern ends of the Lau Ridge. Lavas at the northern end (type (ii)) appear to be derived from a more depleted source than LVG but with a greater amount of subduction component. Those at the southern end (type (i)) probably came from a slightly more enriched source with less subduction component. MVG basalts and hawaiites were derived from an enriched mantle with little or no subduction input. The hawaiites (type (i)) could not have been derived from the basalts (type (ii)), and the two magma types must have come from different sources, indicating mantle heterogeneity. The lack of subduction influence indicates the MVG lavas are tectonically unrelated to the present-day Tonga arc, and the lack of depletion indicators suggests they have tapped a different (new?) part of the mantle wedge. This may reflect introduction of sub-Pacific mantle through the present Tonga-Lau subduction system.  相似文献   

3.
全球幔源岩Pb-Sr-Nd同位素体系   总被引：5，自引：0，他引：5  
朱炳泉 《地学前缘》2007,14(2):24-36
根据各种同位素数据库得到的3万多个晚古生代以来的幔源岩(包括洋中脊玄武岩、洋岛玄武岩、岛弧火山岩、大陆与大洋溢流玄武岩以及大陆板内玄武岩)Pb-Sr-Nd同位素资料和图解分析,对各类火山岩的源区以及地幔的垂向与横向不均一性问题作了进一步讨论。笔者认为不存在具有公共性质的EM1、EM2和HIMU地幔端员,它们的源区可能来自上、下地幔过渡带,只在局部地区出现,独一无二。PREMA(FOZO)则是洋岛玄武岩和溢流玄武岩公共端员。DUAPAL异常现象不只是在洋中脊玄武岩中出现,在洋岛玄武岩、岛弧火山岩和大洋溢流玄武岩中也存在同步的地球化学分区现象。溢流玄武岩的同位素体系特征表明它们的源区涉及再循环地幔的壳幔混合、岩石圈减压熔融、上—下地幔过渡带和似原始-略亏损的下地幔。Pb同位素体系为鉴别俯冲带的存在提供了更严格的证据,这种鉴别表明,安第斯弧火山作用不是洋陆俯冲带产生的。  相似文献   

4.
http://www.sciencedirect.com/science/article/pii/S1674987111001125   总被引：1，自引：1，他引：0  
<正>Greenstone belts of the eastern Dharwar Craton,India are reinterpreted as composite tectonostratigraphic terranes of accreted plume-derived and convergent margin-derived magmatic sequences based on new high-precision elemental data.The former are dominated by a komatiile plus Mg-tholeiitic basalt volcanic association,with deep water siliciclastic and banded iron formation(BIF) sedimentary rocks.Plumes melted at90 km under thin rifted continental lithosphere to preserve inlraoceanic and continental margin aspects.Associated alkaline basalts record subduction-recycling of Mesoarchean oceanic crust,incubated in the asthenosphere.and erupted coevally with Mg basalts from a heterogeneous mantle plume.Together.komaliites-Mg basalts-alkaline basalts plot along the Phanerozoic mantle array in Th/Yb versus Nb/Yb coordinate space,representing zoned plumes,establishing that these reservoirs were present in the Neoarchean mantle. Convergent margin magmatic associations are dominated by tholeiitic to calc-alkaline basalts eompositionally similar to recent intraoceanic arcs.As well,boninitic flows sourced in extremely depleted mantle are present,and the association of arc basalts with Mg-andesites-Nb enriched basalts-adakites documented from Cenozoic arcs characterized by subduction of young(20 Ma),hot,oceanic lithosphere. Consequently.Cenozoic style "hot" subduction was operating in the Neoarchean.These diverse volcanic associations were assembled to give composite terranes in a subduction-accretion orogen at～2.1 Ga,coevally with a global accretionary orogen at ~2.7 Ga,and associated orogenic gold mineralization. Archean lithospheric mantle,distinctive in being thick,refractory,and buoyant,formed complementary to the accreted plume and convergent margin terranes.as migrating arcs captured thick plumeplateaus. and the refractory,low density.residue of plume melting coupled with accreted imbricated plume-arc crust.  相似文献   

5.
The Plio-Quaternary Ayutla and Tapalpa volcanic fields in thevolcanic front of the western Mexican Volcanic Belt (WMVB) containa wide variety of alkaline volcanic rocks, rather than onlycalc-alkaline rocks as found in many continental arcs. Thereare three principal rock series in this region: an intraplatealkaline series (alkali basalts and hawaiites), a potassic series(lamprophyres and trachylavas), and a calc-alkaline series.Phlogopite-clinopyroxenite and hornblende-gabbro cumulate xenolithsfrom an augite minette lava flow have orthocumulate textures.The phlogopite-clinopyroxenite xenoliths also contain apatiteand titanomagnetite and probably formed by accumulation of mineralsfractionated from an augite minette more primitive than thehost. The intraplate alkaline series is probably generated bydecompression melting of asthenospheric mantle as a result ofcorner flow in the mantle wedge beneath the arc. Alkaline magmasmay be common in the WMVB as a result of prior metasomatism(during Tertiary Sierra Madre Occidental magmatism) of the Mexicansub-arc mantle. Generation of the more evolved andesites anddacites of the calc-alkaline series is due to either combinedassimilation and fractional crystallization (AFC) or magma mixing.The preponderance of alkaline and hydrous lavas in this regiondemonstrates that these lava types are the norm, rather thanthe exception in western Mexico, and occur in regions that arenot necessarily associated with active rifting. KEY WORDS: arc basalt; subduction; alkali basalt; minette; hawaiite; metasomatism  相似文献   

6.
The Ordovician Snowdon Volcanic Centre (SVC) of North Walescomprises a bimodal basalt–subalkaline/peralkaline associationemplaced around a caldera within a shallow marine environment.The tectonic setting was associated with closure of the LowerPalaeozoic Iapetus Ocean and cessation of ocean plate subduction.The SVC volcanic products include basaltic lavas and pyroclasticrocks, rhyolitic pyroclastic flow deposits, high-level intrusions,domes, and flows, together with reworked equivalents. A programmeof detailed field mapping, sampling, and chemical analysis hasbeen used to evaluate the structure and magmatic evolution ofthe SVC volcanic system. SVC basalts show a range in chemicalcharacteristics between volcanic arc type and within-plate,ocean island basalt (OIB) type. Subalkaline, silica-oversaturatedintermediate intrusions (icelandites) and five chemically distinctgroups of extrusive and intrusive subalkaline/peralkaline rhyolites(termed A1, A2, B1, B2, and B3) were emplaced during the evolutionof the SVC. This evolution was driven by material and thermalinput from basaltic magma. The SVC basaltic lavas were derivedas partial melts from a heterogeneous volcanic arc to OIB-typespinel lherzolite mantle and experienced up to 60% olivine gabbrofractionation during storage in sill networks in the sub-crustor lower crust. Some magma batches experienced further fractionalcrystallization ({small tilde}70%) and minor crustal contamination({small tilde}10%) to yield the icelandites. Trace element andNd isotope data do not favour an origin for the rhyolites bypartial or total fusion of likely crustal material, and thefive rhyolite groups are regarded as distinct homogeneous batchesof magma derived from varied basaltic magmas. The icelanditesand peralkaline rhyolites (group B3) result, respectively, from{small tilde}50% and {small tilde}80–90% zircon-free fractionalcrystallization of SVC basalts. The subalkaline rhyolites (groupsA1 and B1) result from {small tilde}80–90% fractionalcrystallization of subduction-related basalts similar to thoseof Ordovician basalts which pre-date the Lower Rhyolitic TuffFormation, and groups A2 and B2 were formed by mixing and homogenizationof A1, B1, and B3 magma batches. These data and interpretationsprovide the basis of a model for the complex evolution of asilicic magma system below the SVC caldera around the time ofcessation of Caledonian subduction in North Wales. Rhyolitemagma chambers were short lived and discontinuous; the largestwas probably disc shaped and was almost entirely evacuated duringa >60-km3 ash-flow eruption.  相似文献   

7.
PLANK  T. 《Journal of Petrology》2005,46(5):921-944
Arc magmas and the continental crust share many chemical features,but a major question remains as to whether these features arecreated by subduction or are recycled from subducting sediment.This question is explored here using Th/La, which is low inoceanic basalts (<0·2), elevated in the continents(>0·25) and varies in arc basalts and marine sediments(0·09–0·34). Volcanic arcs form linear mixingarrays between mantle and sediment in plots of Th/La vs Sm/La.The mantle end-member for different arcs varies between highlydepleted and enriched compositions. The sedimentary end-memberis typically the same as local trench sediment. Thus, arc magmasinherit their Th/La from subducting sediment and high Th/Lais not newly created during subduction (or by intraplate, adakiteor Archaean magmatism). Instead, there is a large fractionationin Th/La within the continental crust, caused by the preferentialpartitioning of La over Th in mafic and accessory minerals.These observations suggest a mechanism of ‘fractionation& foundering’, whereby continents differentiate intoa granitic upper crust and restite-cumulate lower crust, whichperiodically founders into the mantle. The bulk continentalcrust can reach its current elevated Th/La if arc crust differentiatesand loses 25–60% of its mafic residues to foundering. KEY WORDS: arc magmatism; continental crust; delamination; thorium; sediment subduction  相似文献   

8.
The Neoarchaean Jonnagiri greenstone terrane (JGT) is located at the centre of the arcuate Hutti–Jonnagiri–Kadiri–Kolar composite greenstone belt in the eastern Dharwar Craton. High MgO (MgO = ~14 wt.%; Nb = 0.2 ppm), low Nb (LNB) (MgO = 7.8–12 wt.%; Nb = 0.1–5.1 ppm) and high Nb basalts (HNB) (MgO = 5.6–10.1 wt.%; Nb = 9.0–10.6 ppm) metamorphosed to lower amphibolite facies are identified based on their geochemical compositions. These metabasalts exhibit depleted HFSE (Nb–Ta, Zr–Hf), pronounced LREE and LILE enrichments suggesting contribution from subduction‐related components during their genesis. Th and U enrichment over Nb–Ta indicates influx of fluids dehydrated from subducted oceanic lithosphere. The high MgO basalts with higher Mg# (51) than that of the associated LNB and HNB (Mg# = 34–47) represent early fractionated melts of subduction‐modified mantle peridotite. The LNB were produced by partial melting of mantle wedge metasomatized by slab‐dehydrated fluids, whereas the HNB represents melts of subducted oceanic crust and hybridized mantle wedge. Lower Dy/Yb and variable La/Yb ratios suggest their generation at shallower depth within spinel peridotite stability field. The low Ce–Yb trend of these metabasalts reflects intraoceanic type subduction which straddles the fields of arc and back‐arc basin basalts, resembling the Mariana‐type arc basalts. The Jonnagiri metabasalts were derived in a paired arc‐back‐arc setting marked by nascent back‐arc rift system that developed in the proximity of an intraoceanic arc. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

9.
田原  陈灵  唐立梅  高鹏  方银霞 《地球科学》2021,46(3):840-852
俯冲带地幔演化与岩浆作用是地球各固体圈层之间发生物质和能量交换的重要地质过程.西太平洋雅浦海沟因其极短的沟-弧距离和洋脊碰撞等独特的地质构造特征成为研究复杂条件下俯冲带演化的理想场所.为了探究雅浦海沟地幔演化与岩浆作用,本文将前人对雅浦海沟火成岩的研究数据进行整合,分析了雅浦海沟火成岩的成因,并根据火成岩形成的制约条件,对卡罗琳板块俯冲到菲律宾海板块的地幔演化与岩浆作用过程进行了讨论.结果显示雅浦海沟火成岩均具有与俯冲相关火成岩的典型特征.橄榄岩地球化学特征指示雅浦海沟地幔熔融程度为20%~25%,地幔在部分熔融过程中受到了流体与熔体的双重交代作用.Re-Os同位素特征指示雅浦海沟地幔中存在约1.16 Ga非常古老的残余地幔,表明地幔可能经历过多期熔融事件,从而导致雅浦海沟地幔非常亏损.雅浦岛弧成因至今仍存争议,主要包括:(1)现今雅浦岛弧为帕里希维拉海盆洋壳的一部分,在中新世因卡罗琳洋脊的碰撞导致帕里希维拉海盆洋壳逆冲到原雅浦岛弧之上.(2)雅浦岛弧在不同构造时期经历过多期岛弧岩浆作用,包括俯冲初始阶段(~52 Ma)的弧前玄武岩、俯冲开始后的岛弧玄武岩(~25 Ma)、与卡罗琳洋脊碰撞(21 Ma)后的岛弧拉斑玄武岩(7~11 Ma).其中7~11 Ma的岛弧拉斑玄武岩指示雅浦岛弧岩浆活动并未因卡罗琳洋脊的碰撞完全停止,很有可能在晚中新世短暂恢复活动.   相似文献   

10.
Volcanic suites from Wawa greenstone belts in the southern Superior Province comprise an association of typical late Archean arc volcanic rocks including adakites, magnesian andesites (MA), niobium-enriched basalts (NEB), and ‘normal’ tholeiitic to calc-alkaline basalts to rhyolites. The adakites represent melts from subducted oceanic crust and all other suites were derived from the mantle wedge above the subducting oceanic lithosphere. The magnesian andesites are interpreted to be the product of hybridization of adakite melts with arc mantle wedge peridotite. The initial ?Hf values of the ∼2.7 Ga Wawa adakites (+3.5 to +5.2), magnesian andesites (+2.6 to +5.1), niobium-enriched basalts (+4.4 to +6.6), and ‘normal’ tholeiitic to calc-alkaline arc basalts (+5.3 to +6.4) are consistent with long-term depleted mantle sources. The niobium-enriched basalts and ‘normal’ arc basalts have more depleted ?Hf values than the adakites and magnesian andesites. The initial ?Nd values in the magnesian andesites (+0.4 to +2.0), niobium-enriched basalts (+1.4 to +2.4), and ‘normal’ arc tholeiitic to calc-alkaline basalts (+1.6 to +2.9) overlap with, but extend to lower values than, the slab-derived adakites (+2.3 to +2.8). The lower initial ?Nd values in the mantle-wedge-derived suites, particularly in the magnesian andesites, are attributed to recycling of an Nd-enriched component with lower ?Nd to the mantle wedge. As a group, the slab-derived adakites plot closest to the 2.7 Ga depleted mantle value in ?Nd versus ?Hf space, additionally suggesting that the Nd-enriched component in the mantle wedge did not originate from the 2.7 Ga slab-derived melts. Accordingly, we suggest that the enriched component had been added to the mantle wedge at variable proportions by recycling of older continental material. This recycling process may have occurred as early as 50-70 Ma before the initiation of the 2.7 Ga subduction zone. The selective enrichment of Nd in the sources of the Superior Province magmas can be explained by experimental studies and geochemical observations in modern subduction systems, indicating that light rare earth elements (e.g., La, Ce, Sm, Nd) are more soluble than high field strength elements (e.g., Zr, Hf, Nb, Ta) in aqueous fluids that are derived from subducted slabs. As a corollary, we suggest that the recycled Nd-enriched component was added to the mantle source of the Wawa arc magmas by dehydration of subducted sediments.  相似文献   

11.
Geochemical data from back-arc volcanic zones in the Manus Basinare used to define five magma types. Closest to the New Britainarc are medium-K lavas of the island arc association and back-arcbasin basalts (BABB). Mid-ocean ridge basalts (MORB), BABB andmildly enriched T-MORB (transitional MORB) occur along the ManusSpreading Center (MSC) and Extensional Transform Zone (ETZ).The MSC also erupted extreme back-arc basin basalts (XBABB),enriched in light rare earth elements, P, and Zr. Compared withnormal MORB, Manus MORB are even more depleted in high fieldstrength elements and slightly enriched in fluid-mobile elements,indicating slight, prior enrichment of their source with subduction-relatedcomponents. Chemical variations and modeling suggest systematic,coupled relationships between extent of mantle melting, priordepletion of the mantle source, and enrichment in subduction-relatedcomponents. Closest to the arc, the greatest addition of subduction-relatedcomponents has occurred in the mantle with the greatest amountof prior depletion, which has melted the most. Variations inK2O/H2O indicate that the subduction-related component is bestdescribed as a phlogopite and/or K-amphibole-bearing hybridizedperidotite. Magmas from the East Manus Rifts are enriched inNa and Zr with radiogenic 87Sr/86Sr, possibly indicating crustalinteraction in a zone of incipient rifting. The source for XBABBand lavas from the Witu Islands requires a mantle componentsimilar to carbonatite melt. KEY WORDS: Manus back-arc basin, mantle metasomatism, magma generation  相似文献   

12.
徐峥  郑永飞 《地球科学》2019,44(12):4135-4143
大陆玄武岩通常具有与洋岛玄武岩相似的地球化学成分,其中含有显著的壳源组分.对于洋岛玄武岩来说,虽然其中的壳源组分归咎于深俯冲大洋板片的再循环,但是对板片俯冲过程中的壳幔相互作用缺乏研究.对于大陆玄武岩来说,由于其形成与特定大洋板片在大陆边缘之下的俯冲有关,可以用来确定古大洋板片俯冲的地壳物质再循环.本文总结了我们对中国东部新生代玄武岩所进行的一系列地球化学研究,结果记录了古太平洋板片俯冲析出流体对地幔楔的化学交代作用.这些大陆玄武岩普遍具有与洋岛玄武岩类似的地球化学成分,在微量元素组成上表现为富集LILE和LREE、亏损HREE,但是不亏损HFSE的分布特点,在放射成因同位素组成上表现为亏损至弱富集的Sr-Nd同位素组成.在排除地壳混染效应之后,这些玄武岩的地球化学特征可以由其地幔源区中壳源组分的性质来解释.俯冲大洋地壳部分熔融产生的熔体提供了地幔源区中的壳源组分,其中包括洋壳镁铁质火成岩、海底沉积物和大陆下地壳三种组分.华北和华南新生代大陆玄武岩在Pb同位素组成上存在显著差异,反映它们地幔源区中的壳源组分有所区别.中国东部新生代玄武岩的地幔源区是古太平洋板片于中生代俯冲至亚欧大陆东部之下时,在>200 km的俯冲带深度发生壳幔相互作用的产物.在新生代期间,随着俯冲太平洋板片的回卷引起的中国东部大陆岩石圈拉张和软流圈地幔上涌,那些交代成因的地幔源区发生部分熔融,形成了现今所见的新生代玄武岩.   相似文献   

13.
Sediment mixing and recycling through a subduction zone canbe detected in lead isotopes and trace elements from basaltsand sediments from the Kermadec-Hikurangi Margin volcanic arcsystem and their coupled back-arc basins. Sr, Nd and Pb isotopesfrom the basalts delineate relatively simple, almost overlapping,arrays between back-arc basin basalts of the Havre Trough-NgatoroBasin (87Sr/86Sr = 0.70255; Nd=+9.3; 206Pb/204Pb = 18.52; 208Pb/204Pb= 38.18), island arc basalts from the Kermadec Arc togetherwith basalts from Taupo Volcanic Zone (87Sr/86Sr 0.7042; Nd= +5; 206Pb/204Pb= 18.81; 208Pb/204Pb = 38.61), and sedimentsderived from New Zealand's Mesozoic (Torlesse) basement (87Sr/86Sr 0.715; Nd —4; 206Pb/204Pb 18.86; 208Pb/204Pb 38.8).Basalts from the arc front volcanoes have high Cs, Rb, Ba, Th,U and K, and generally high but variable Ba/La, Ba/Nb ratios,characteristic of subduction-related magmas, relative to typicaloceanic basalts. These signatures are diluted in the back-arcbasins, which are more like mid-ocean ridge basalts. Strongchemical correlations in plots of SiO2 vs CaO and loss on ignitionfor the sediments (finegrained muds) are consistent with mixingbetween detrital and biogenic (carbonate-rich) components. Otherdata, such as Zr vs CaO, are consistent with the detrital componentcomprising a mixture of arc- and continent-derived fractions.In chondrite-normalized diagrams, most of the sediments havelight rare earth element enriched patterns, and all have negativeEu anomalies. The multielement diagrams have negative spikesat Nb, P and Ti and distinctive enrichments in the large ionlithophile elements and Pb relative to mantle. Isotopic measurementsof Pb, Sr and Nd reveal restricted fields of Pb isotopes butwide variation in Nd and Sr relative to other sediments fromthe Pacific Basin. Rare K-rich basalts from Clark Volcano towardthe southern end of the oceanic Kermadec Island Arc show unusualand primitive characteristics ( 2% K2O at 50% SiO2, Ba 600p.p.m., 9–10% MgO and Ni > 100 p.p.m.) but have highlyradiogenic Sr, Nd and Pb isotopes, similar to those of basaltsfrom the continental Taupo Volcanic Zone. These oceanic islandarc basalts cannot have inherited their isotope signatures throughcrustal contamination or assimilation—fractional crystallizationtype processes, and this leads us to conclude that source processesvia bulk sediment mixing, fluid and/or melt transfer or somecombination of these are responsible. Although our results showclear chemical gradients from oceanic island arc to continentalmargin arc settings (Kermadec Arc to Taupo Volcanic Zone), overlapbetween the data from the oceanic and continental sectors suggeststhat the lithospheric (crustal contamination) effect may beminimal relative to that of sediment subduction. Indeed, itis possible to account for the chemical changes by a decreasenorthward in the sediment flux into the zone of magma genesis.This model receives support from recent sediment dispersal studiesin the Southern Ocean which indicate that a strong bottom current(Deep Western Boundary Current) flows northward along the easterncontinental margin of New Zealand and sweeps continental derivedsediment into the sediment-starved oceanic trench system. Thetrace element and isotopic signatures of the continental derivedcomponent of this sediment are readily distinguished, but alsodiluted in a south to north direction along the plate boundary. KEY WORDS: subduction zone basalts; sediments; Sr-, Nd-, Pb-isotopes; trace elements *Present address: School of Earth Sciences, University of Melbourne, Parkville, Vic. 3052, Australia.  相似文献   

14.
华北板块南缘熊耳群火山岩研究的若干问题   总被引：3，自引：1，他引：3  
赵太平  屠森 《地质论评》1994,40(5):446-455
本文基于大量的区域地质学，岩石学和地球化学资料，陈述熊耳群的一些重要资料，阐明作用者对一些争议问题的看法，同时指出今后值得注意的一些研究内容。研究表明，熊耳群火山岩以橄榄玄粗岩，安粗岩和粗面英安岩，流纹岩为主，其次为高钾玄武安山岩，高钾安山岩，高钾英安岩；火山岩系列主要为橄榄玄粗岩系，其次为高钾钙碱性岩系；熊耳群是扩张应力背景的产物，但火山岩的地球化学特征表明，能耳期或熊耳期以前的陆下岩石圈地幔历  相似文献   

15.
Rates of magma emplacement commonly vary as a function of tectonic setting. The late Caledonian granites of Britain and Ireland are associated with closure of the Iapetus Ocean and were emplaced into a varying regime of transpression and transtension throughout the Silurian and into the early Devonian. Here we evaluate a new approach for examining how magma volumes vary as a function of tectonic setting. Available radiometric ages from the late Caledonian granites are used to calculate probability density functions (age spectra), with each pluton weighted by outcrop area as a proxy for its volume. These spectra confirm an absence of magmatic activity during Iapetus subduction between c. 455 Ma and 425 Ma and a dominance of post-subduction magmas between c. 425 Ma and 380 Ma. We review possible reasons why, despite the widespread outcrop of the late Caledonian granites, magmatism appears absent during Iapetus subduction. These include shallow angle subduction or extensive erosion and tectonic removal of the arc.In contrast to previous work, we find no strong difference in the age or major element chemistry of post-subduction granites across all terranes. We propose a common causal mechanism in which the down-going Iapetus oceanic slab peeled back and detached beneath the suture following final Iapetus closure. The lithospheric mantle was delaminated beneath the suture and for about 100 km back beneath the Avalonian margin. While magma generation is largely a function of gravitationally driven lithosphere delamination, strike slip dominated kinematics in the overlying continental crust is what modulated granitic magma emplacement. Early Devonian (419–404 Ma) transtension permitted large volumes of granite emplacement, whereas the subsequent Acadian (late Early Devonian, 404–394 Ma) transpression reduced and eventually suppressed magma emplacement.  相似文献   

16.
The present day Taupo-Hikurangi subduction system is a southward extension of the Tonga-Kermadec Arc system into a sediment-rich continental margin environment. It consists of a shallow structural trench (the Hikurangi Trough), a 150 km wide, imbricate thrust controlled accretionary borderland (the continental slope, shelf, and coastal hills of eastern North Island), a frontal ridge (the main “greywacke” ranges of North Island), and a volcanic arc and marginal basin (the Taupo Volcanic Zone).Structural elements become progressively more elevated and subduction more oblique towards the south. The whole NNE-trending system is truncated at a largely strike-slip, transform boundary that extends along the southwestern part of the Hikurangi Trough and the Hope fault of South Island to the main Alpine Fault.The volcanic arc is 200–270 km from the structural trench and comprises a NNE trending chain of andesite-dacite volcanoes extending along the eastern side of the Taupo Volcanic Zone. Most of the andesites are olivine-bearing and have been erupted within the last 50,000 years.It is suggested the Taupo-Hikurangi margin has evolved by rotation of accretionary elements, from an original NW-trending subduction system north of New Zealand. The older elements of the prism were associated with subduction of a re-entrant of the Pacific Plate (and perhaps the South Fiji Basin) in Mid Tertiary times. They subsequently became separated from their NW-trending volcanic arc by dextral strike-slip movement along curved faults east of the main “greywacke” ranges. During the Plio-Pleistocene, oblique subduction and accretion intensified as the Taupo-Hikurangi margin rotated into line with the NNE-trending Kermadec system and a marginal basin was developed along a similar trend to form the Taupo Volcanic Zone. Within the last 50,000 years olivine-bearing andesite volcanism has commenced along the eastern side of the Taupo Volcanic Zone.  相似文献   

17.
Fault blocks and inliers of uppermost Silurian to Middle Devonian strata in the Yarrol Province of central coastal Queensland have been interpreted either as island-arc deposits or as a continental-margin sequence. They can be grouped into four assemblages with different age ranges, stratigraphic successions, geophysical signatures, basalt geochemistry, and coral faunas. Basalt compositions from the Middle Devonian Capella Creek Group at Mt Morgan are remarkably similar to analyses from the modern Kermadec Arc, and are most consistent with an intra-oceanic arc associated with a backarc basin. They cannot be matched with basalts from any modern continental arc, including those with a thin crust (Southern Volcanic Zone of the Andes) or those built on recently accreted juvenile oceanic terranes (Eastern Volcanic Front of Kamchatka). Analyses from the other assemblages also suggest island-arc settings, although some backarc basin basalt compositions could be present. Arguments for a continental-margin setting based on structure, provenance, and palaeogeography are not conclusive, and none excludes an oceanic setting for the uppermost Silurian to Middle Devonian rocks. The Mt Morgan gold–copper orebody is associated with a felsic volcanic centre like those of the modern Izu–Bonin Arc, and may have formed within a submarine caldera. The data are most consistent with formation of the Capella Creek Group as an intra-oceanic arc related to an east-dipping subduction zone, with outboard assemblages to the east representing remnant arc or backarc basin sequences. Collision of these exotic terranes with the continent probably coincided with the Middle–Upper Devonian unconformity at Mt Morgan. An Upper Devonian overlap sequence indicates that all four assemblages had reached essentially their present relative positions early in Late Devonian time. Apart from a small number of samples with compositions typical of spreading backarc basins, Upper Devonian basalts and basaltic andesites of the Lochenbar and Mt Hoopbound Formations and the Three Moon Conglomerate are most like tholeiitic or transitional suites from evolved oceanic arcs such as the Lesser Antilles, Marianas, Vanuatu, and the Aleutians. However, they also match some samples from the Eastern Volcanic Front of Kamchatka. Their rare-earth and high field strength element patterns are also remarkably similar to Upper Devonian island arc tholeiites in the ophiolitic Marlborough terrane, supporting a subduction-related origin and a lack of involvement of continental crust in their genesis. Modern basalts from rifted backarc basins do not match the Yarrol Province rocks as well as those from evolved oceanic arcs, and commonly have consistently higher MgO contents at equivalent levels of rare-earth and high field strength elements. One of the most significant points for any tectonic model is that the Upper Devonian basalts become more arc-like from east to west, with all samples that can be matched most readily with backarc basin basalts located along the eastern edge of the outcrop belt. It is difficult to account for all geochemical variations in the Upper Devonian basalts of the Yarrol Province by any simplistic tectonic model using either a west-dipping or an east-dipping subduction zone. On a regional scale, the Upper Devonian rocks represent a transitional phase in the change from an intra-oceanic setting, epitomised by the Middle Devonian Capella Creek Group, to a continental margin setting in the northern New England Orogen in the Carboniferous, but the tectonic evolution must have been more complex than any of the models published to date. Certainly there are many similarities to the southern New England Orogen, where basalt geochemistry indicates rifting of an intra-oceanic arc in Middle to Late Devonian time.  相似文献   

18.
V/Sc systematics in peridotites, mid-ocean ridge basalts andarc basalts are investigated to constrain the variation of fO2in the asthenospheric mantle. V/Sc ratios are used here to ‘seethrough’ those processes that can modify barometric fO2determinations in mantle rocks and/or magmas: early fractionalcrystallization, degassing, crustal assimilation and mantlemetasomatism. Melting models are combined here with a literaturedatabase on peridotites, arc lavas and mid-ocean ridge basalts,along with new, more precise data on peridotites and selectedarc lavas. V/Sc ratios in primitive arc lavas from the Cascadesmagmatic arc are correlated with fluid-mobile elements (e.g.Ba and K), indicating that fluids may subtly influence fO2 duringmelting. However, for the most part, the average V/Sc-inferredfO2s of arc basalts, MORB and peridotites are remarkably similar(–1·25 to +0·5 log units from the FMQ buffer)and disagree with the observation that the barometric fO2s ofarc lavas are several orders of magnitude higher. These observationssuggest that the upper part of the Earth's mantle may be stronglybuffered in terms of fO2. The higher barometric fO2s of arclavas and some arc-related xenoliths may be due respectivelyto magmatic differentiation processes and to exposure to large,time-integrated fluid fluxes incurred during the long-term stabilityof the lithospheric mantle. KEY WORDS: vanadium; scandium; oxygen fugacity; mantle; arcs  相似文献   

19.
The discovery of glaucophane relicts in the high-pressure tectonites of the Yenisei suture zone of the Yenisei Ridge suggests the manifestation of the “Chilean-type” convergent margin on the western Siberian Craton, which was controlled by subduction of oceanic crust beneath the continental margin. These rocks are restricted to the tectonic suture between the craton and the Isakovka ocean-island terrane and experienced two metamorphic stages. Petrogeochemical characteristics of the mafic tectonites indicate that their protoliths are N-MORB and E-MORB basalts. More primitive N-MORB basalts were formed at the initial spreading stages through melting of the upper depleted mantle. Higher Ti basalts were formed by melting of enriched mantle protolith at the later spreading stages. U–Pb zircon age of 701.6 ± 8.4 Ma of the metamorphosed analogues of normal basalts marks the initiation of oceanic crust in the region. Revealed sequence of spreading, subduction (640–620 Ma), and shear deformations (~600 Ma) records the early stages in the evolution of the Paleoasian ocean in its junction zone with the western margin of the Siberian craton: from formation of fragments of oceanic crust to the completion of accretionary–subduction events.  相似文献   

20.
黄开年 《地质科学》1988,(3):289-298
本文对峨眉山玄武岩产于弧后扩张环境的论点提出了质疑。主要论据是:(1)近年来的调查表明,与此相关的金沙江—哀牢山俯冲带(及甘孜—理塘俯冲带)在海西—印支期是向西而非向东消减的;(2)从峨眉山玄武岩的主要和微量元素成分中未探查出可靠的来自消减带的组分;(3)古地磁的观测结果说明,晚二叠世扬子地块位于南半球很靠近赤道的低纬地区,此后扬子地块即迅速向北漂移;(4)世界上绝大多数大陆溢流玄武岩的形成与消减过程无关。因此,作者认为,峨眉山玄武岩的成因与消减作用无明显关系,它的喷发很可能是由于古生代末扬子地块西缘及邻区的地幔区域性上涌,从而导致了有关陆块的破裂和分离。  相似文献   

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