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1.
It has been suggested that large areas of the Earth's lithospheric mantle undergo pervasive dehydration melting during the impact of mantle plumes and the Early-Cretaceous Paraná-Etendeka continental flood-basalt (CFB) province has repeatedly been cited as evidence of this phenomenon. During the Cretaceous, however, southern Brazil experienced two phases of mafic magmatism. These igneous events occurred ～50?Ma apart and therefore represent distinct episodes of melt genesis in the underlying mantle. The first phase of magmatism, in the Early Cretaceous, included the emplacement of lava flows associated with the Paraná-Etendeka CFB province and also the intrusion of small-volume mafic alkaline magmas (e.g. Anitápolis, Jacupiranga and Juquiá) in the Dom Feliciano and Ribeira mobile belts. During the Late Cretaceous, both sodic and potassic mafic magmas were emplaced on the margin of the adjacent Luis-Alves craton and intrude the flood-basalts at Lages. On the basis of variations in incompatible trace-element concentrations (e.g. Ba?=?1000 to 2000?ppm), initial 87Sr/86Sr ratios (0.7048–0.7064) and ?Nd values (?3 to ?12), we suggest that all of the Late-Cretaceous mafic potassic magmas were derived from the subcontinental lithospheric mantle (SCLM) which was metasomatically enriched during the Proterozoic. We propose that these relatively low temperature, volatile-rich, mafic melts provide direct evidence that the underlying SCLM did not melt wholesale during the previous Early-Cretaceous Paraná-Etendeka CFB event. Late-Cretaceous melting of the SCLM beneath southern Brazil may have been caused by heat conduction from either: (1) ponded ～132?Ma Tristan plume-head material; or (2) ～85?Ma Trindade plume-head material channelled southwards between the thick cratonic keels of the Amazonas and São Francisco cratons. The Late-Cretaceous magmatism appears to have been contemporaneous with uplift across southern Brazil and Paraguay; we suggest that both of these phenomena represent the widespread effects of the impact of the Trindade mantle plume on the base of the SCLM. Plate margin stresses and lithospheric extension associated with the opening of the South Atlantic may also have changed the geothermal gradient beneath southern Brazil and contributed to mantle melting.  相似文献   

2.
When the subcontinental lithospheric mantle undergoes heatingand/or extension, some of the earliest mafic melts to be generatedare those rich in volatUes and potassium. In some cases, e.g.when a plume impinges on thick cratonic lithosphere or whenthe amount of extension is very small, K-rich mafic igneousrocks may be the only surface expression of mantle melting.The Alto Paranaiba Igneous Province, in SE Brazil, is one ofthe world's most voluminous mafic potassic provinces (>15000km3),which until recently was relatively unknown. The magmas wereemplaced into a narrow Proterozoic mobile belt close to thesurface margin of the Sao Francisco craton, and it is one ofseveral Cretaceous alkaline igneous provinces that are locatedaround the margin of the Parana sedimentary basin in Braziland Paraguay.Detailed geochemical analyses of samples from throughoutthe Alto Paranaiba Igneous Province show that it is composedof a relatively diverse suite of ultrapotassic-potassic, ultramaficmqfic,silica-undersaturated lavas and hypabyssal intrusions, i.e.kimberlites, madupitic olivine lamproites and kamafugitic rocks.These all have very high concentrations of incompatible traceelements and are all strongly enriched in light rare earth relativeto heavy rare earth elements (e.g. La/Yb=50-230). Wide variationsin major element ratios, which are unrelated to the effectsof crystal fractionation in these magmas (e.g. CaO/Al2O3), suggestthat the mafic potassic rocks were derived from a heterogeneousmantle source. They show relatively restricted ranges of initial87Sr/86Sr (070436-070588) and Nd25 values of -4 to -8, intermediatebetween Group I and II South African kimberlites. TDM Nd isotopemodel ages of 900 Ma suggest that the magmas were derived bythe remobilization of subcontinental lithospheric mantle thathad been enriched by small-volume K-rich melt fractions sincethe Late Proterozoic.New K/Ar ages for mica separates show thatthe kimberlites, madupitic olivine lamproites and kamafugiticrocks were emplaced together with large carbonatite-bearingplutonic complexes at 85 Ma. Reconstructions of plate motionsshow that, at this time, the location of the Alto ParanaibaIgneous Province coincided with the postulated position of thepresent-day Trindade(or Martin Vaz) plume. We propose that thewidespread Late Cretaceous alkaline magmatism in SE Brazil mayhave been caused by impingement of this plume on the base ofthe subcontinental lithosphere. Heat penetrating the lithosphere,both by conduction and advection by asthenospheric-source decompressionmelts, may have caused melting of the readily fusible partsof the lithospheric mantle and the genesis of mafic potassicand (after fractionation) carbonatite magmas. The Proterozoicmobile belt (the Brasilia Belt) appears to have acted as a Hhinspofrelative to the adjacent Sao Francisco craton, allowing greaterupwelling and melting of the asthenosphere. Subsequently, asthe craton passed over the plume, volcanism was switched off'until the Early Tertiary when the plume reemerged from beneaththe westward drifting South America continent and was the magmasource for oceanic-islands and seamounts of the Trindade-Vitriachain. Corresponding author  相似文献   

3.
In the western USA calcalkaline magmas were generated hundreds of kilometres from the nearest destructive plate margin, and in some areas during regional extension several Ma after the cessation of subduction. The Mogollon-Datil Volcanic Field (MDVF) in southern New Mexico was a centre of active magmatism in the mid- to late-Tertiary, and a detailed field, petrographic and geochemical study has been undertaken to evaluate the relations between extensional tectonics and calcalkaline magmatism in the period 30–20 Ma. The rocks comprise alkalic to high-K calcalkaline lavas, ranging from basalt to high silica andesitc. Most of the basaltic rocks have relatively low HFSE abundances, elevated 87Sr/86Sr and low 143Nd/144Nd, similar to many Tertiary basalts across the western USA, and they are inferred to have been derived from the continental mantle lithosphere. Two differentiation trends are recognised, with the older magmas having evolved to more calcalkaline compositions by magma mixing between alkalic basaltic andesites and silicic crustal melts, and the younger rocks having undergone 30–40% fractional crystallisation to more alkalic derivatives. The younger basalts also exhibit a shift to relatively higher HSFE abundances, with lower 87Sr/86Sr and higher 143Nd/144Nd, and these have been modelled as mixtures between an average post-5 Ma Basin and Range basalt and the older MDVF lithosphere-derived basalts. It is argued that the presence of subduction-related geochemical signatures and the development of calcalkaline andesites in the 30–20 Ma lavas from the MDVF are not related to the magmatic effects of Tertiary subduction. Rather, basic magmas were generated by partial melting of the lithospheric mantle which had been modified during a previous subduction event. Since these basalts were generated at the time of maximum extension in the upper crust it is inferred that magma generation was in response to lithospheric extension. The association of the 30–20 Ma calcalkaline andesites with the apparently anorogenic tectonism of late mid-Tertiary extension, is the result of crustal contamination, in that fractionated, mildly alkaline, basaltic andesite magmas were mixed with silicic crustal melts, generating hybrid andesite lavas with calcalkaline affinities.  相似文献   

4.
Olivine and clinopyroxene grains have been separated from four large Fe-Ti-V oxide ore-bearing intrusions (Panzhihua, Hongge, Baima and Taihe) in the Panxi area, Emeishan large igneous province, Southwest China, for He and Ar isotope studies. The samples examined revealed extremely low 3He/4He ratios (0.078-4.34 Ra with the mean value 0.78 Ra) for gases extracted by stepwise heating. This feature, combined with low 40Ar/36Ar ratios can be interpreted as due to addition of subduction-related fluids and melts that had been stored in the lithospheric mantle for long periods. Considering the regional geologic history, such addition can be attributed to the paleo subduction that occurred along the western margin of the Yangtze Block during the Neoproterozoic. The subducted oceanic crust beneath the Panxi area underwent eclogite-facies metamorphism and subsequent exhumation. The infiltration of subduction-related melts and fluids into the lithospheric mantle led to enriched isotopic signatures from that of the slightly depleted asthenopheric mantle which has been suggested by the Sr, Nd and Pb isotopic data of the Emeishan basalts and picrites. In addition, considerable amounts of eclogitic melts produced by partial melting of eclogite-facies oceanic crust extensively contaminated the lithospheric mantle. During the late Permian, partial melting of an upwelling mantle plume that contained an eclogite or pyroxenite component generated the parental Fe-rich magma that supplied the ore-bearing intrusions. The combination of these factors may have been the crucial reason that many world-class Fe-Ti-V oxides deposits are clustered in the Panxi area.  相似文献   

5.
Anhydrous spinel peridotite xenoliths from the Ray Pic Quaternary alkali basalt volcano (French Massif Central) show a wide range of mineralogical and geochemical compositions, reflecting significant heterogeneities in the shallow sub-continental lithospheric mantle. Variations in modal mineralogy, mineral chem istry, REE patterns and radiogenic isotope data suggest that depletion by partial melting and enrichment by cryptic metasomatism were the major mantle processes which caused the heterogeneity. The lithospheric mantle beneath Ray Pic contains two contrasting types of peridotite: (i) lherzolites with LREE-depleted compositions, high 143Nd/144Nd, low 87Sr/86Sr and unradiogenic Pb isotope ratios; (ii) lherzolites, harzburgites and a wehrlite with LREE-enriched patterns, low 143Nd/144Nd, high 87Sr/86Sr and radiogenic Pb isotope ratios. The former closely resemble depleted MORB-source mantle. The latter are related to enrichment by recent infiltration of small degree partial melts or fluids from the asthenospheric mantle, possibly related to the “low velocity component” observed by Hoernle et al. (1995) in European Neogene alkaline magmas. Thus, the Ray Pic peridotite xenoliths represent interaction between asthenospheric mantle-derived melts/fluids and depleted lithospheric mantle. This is probably linked to the upwelling mantle plume imaged beneath the Massif Central (Granet et al. 1995). A relationship between textural deformation, equilibration temperature and geochemistry of the xenoliths suggests that the hotter (> 900 °C) undeformed regions are LREE-enriched and tend to have more enriched isotope ratios, whereas the cooler (< 900 °C) regions have undergone more deformation and are more depleted both in LREE and in isotope compositions. Received: 27 July 1996 / Accepted: 25 November 1996  相似文献   

6.
The Denizli region of the Western Anatolia Extensional Province (WAEP) includes a typical example of intra-plate potassic magmatism. Lamproite-like K-rich to shoshonitic alkaline rocks erupted in the Upper Miocene-Pliocene in a tensional tectonic setting. The absence of Nb and Ta depletion, low Th/Zr and high Nb/Zr ratios and distinct isotopic values (i.e. low 87Sr/86Sr, 0.703523–0.703757; high 143Nd/144Nd, 0.512708–0.512784; high 206Pb/204Pb, 19.079–19.227, 207Pb/204Pb, 15.635–15.682, 208Pb/204Pb, 39.144–39.302) mark an anorogenic geochemical signature of the Denizli volcanics. All of the lavas are strongly enriched in large-ion-lithophile elements (e.g. Ba 1,100–2,200 ppm; Sr 1,900–3,100 ppm; Rb 91–295 ppm) and light rare-earth elements (e.g. LaN?=?319–464), with a geochemical affinity to ocean-island basalts and lack of a recognizable subduction signature or any evidence for crustal contamination. The restricted range of isotopic (Sr, Nd, Pb) ratios in both near-primitive (Mg# 66.7–77.2) and more evolved (Mg# 64.6–68.7) members of the Denizli volcanics signify their evolution from an isotopically equilibrated parental mantle source. Their high Dy/Yb and Rb/Sr values also suggest that garnet and phlogopite were present in the mantle source. Their strong EM-II signature, very low Nd model ages (0.44–049 Ga) and isotopic (Sr-Nd-Pb) values analogous to those of the Nyiragongo potassic basanites and kimberlites from the African stable continental settings, suggest that the parental melts that produced the Denizli volcanics are associated with very young and enriched mantle sources, which include both sublithospheric and enriched subcontinental lithospheric mantle melts. Mantle-lithosphere delamination probably played a significant role in the generation of these melts, and could be related to roll-back of the Aegean arc, lithospheric extension and asthenospheric mantle upwelling.  相似文献   

7.
The Aguablanca Ni–(Cu) sulfide deposit is hosted by a breccia pipe within a gabbro–diorite pluton. The deposit probably formed due to the disruption of a partially crystallized layered mafic complex at about 12–19 km depth and the subsequent emplacement of melts and breccias at shallow levels (<2 km). The ore-hosting breccias are interpreted as fragments of an ultramafic cumulate, which were transported to the near surface along with a molten sulfide melt. Phlogopite Ar–Ar ages are 341–332 Ma in the breccia pipe, and 338–334 Ma in the layered mafic complex, and are similar to recently reported U–Pb ages of the host Aguablanca Stock and other nearby calc-alkaline metaluminous intrusions (ca. 350–330 Ma). Ore deposition resulted from the combination of two critical factors, the emplacement of a layered mafic complex deep in the continental crust and the development of small dilational structures along transcrustal strike-slip faults that triggered the forceful intrusion of magmas to shallow levels. The emplacement of basaltic magmas in the lower middle crust was accompanied by major interaction with the host rocks, immiscibility of a sulfide melt, and the formation of a magma chamber with ultramafic cumulates and sulfide melt at the bottom and a vertically zoned mafic to intermediate magmas above. Dismembered bodies of mafic/ultramafic rocks thought to be parts of the complex crop out about 50 km southwest of the deposit in a tectonically uplifted block (Cortegana Igneous Complex, Aracena Massif). Reactivation of Variscan structures that merged at the depth of the mafic complex led to sequential extraction of melts, cumulates, and sulfide magma. Lithogeochemistry and Sr and Nd isotope data of the Aguablanca Stock reflect the mixing from two distinct reservoirs, i.e., an evolved siliciclastic middle-upper continental crust and a primitive tholeiitic melt. Crustal contamination in the deep magma chamber was so intense that orthopyroxene replaced olivine as the main mineral phase controlling the early fractional crystallization of the melt. Geochemical evidence includes enrichment in SiO2 and incompatible elements, and Sr and Nd isotope compositions (87Sr/86Sri 0.708–0.710; 143Nd/144Ndi 0.512–0.513). However, rocks of the Cortegana Igneous Complex have low initial 87Sr/86Sr and high initial 143Nd/144Nd values suggesting contamination by lower crustal rocks. Comparison of the geochemical and geological features of igneous rocks in the Aguablanca deposit and the Cortegana Igneous Complex indicates that, although probably part of the same magmatic system, they are rather different and the rocks of the Cortegana Igneous Complex were not the direct source of the Aguablanca deposit. Crust–magma interaction was a complex process, and the generation of orebodies was controlled by local but highly variable factors. The model for the formation of the Aguablanca deposit presented in this study implies that dense sulfide melts can effectively travel long distances through the continental crust and that dilational zones within compressional belts can effectively focus such melt transport into shallow environments.Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.  相似文献   

8.
The Plio-Pleistocene volcanic rocks of the Bohemian Massif comprise a compositional spectrum involving two series: an older basanitic series (6.0–0.8 Ma) and a younger, melilititic series (1.0–0.26 Ma). The former consists of relatively undifferentiated basaltic rocks, slightly silica-undersaturated, with Mg# ranging from 62 to almost primitive mantle-type values of 74. The major and trace element characteristics correspond to those of primitive intra-plate alkaline volcanic rocks from a common sub-lithospheric mantle source (European Asthenospheric Reservoir – EAR) including positive Nb, and negative K and Pb anomalies. 87Sr/86Sr ratios of 0.7032–0.7034 and 143Nd/144Nd of 0.51285–0.51288 indicate a moderately depleted mantle source as for other mafic rocks of the central European volcanic province with signs of HIMU-like characteristics commonly attributed to recycling of subducted oceanic crust in the upper mantle during the Variscan orogeny. The melilititic series is characterized by higher degrees of silica-undersaturation, and high Mg# of 68–72 values, compatible with primitive-mantle-derived compositions. The high OIB-like Ce/Pb (19–47) and Nb/U (32–53) ratios indicate that assimilation of crustal material was negligible. In both series, concentrations of incompatible elements are mildly elevated and 87Sr/86Sr ratios (0.7034–0.7036) and 143Nd/144Nd ratios (0.51285–0.51288) overlap. Variations in incompatible element concentrations and isotopic compositions in the basanitic series and melilititic series can be explained by a lower degree of mantle melting for the latter with preferential melting of enriched mantle domains. The Sr and Nd isotopic compositions of both rock series are similar to those of the EAR. Minor differences in geochemical characteristics between the two series may be attributed to: (i) to different settings with respect to crust and lithospheric mantle conditions in (a) Western Bohemia (WB) and (b) Northeastern Bohemia (NEB) and the Northern Moravia and Silesia (NMS) areas, (ii) a modally metasomatized mantle lithosphere in WB in contrast to cryptically metasomatized domains in the NEB and NMS, (iii) different degrees of partial melting with very low degrees in WB but higher degrees in NEB and NMS. The geochemical and isotopic similarity between the Plio-Pleistocene volcanic rocks and those of the late Cretaceous and Cenozoic (79–6 Ma) suggests that their magmas came from compositionally similar mantle sources, that underwent low degrees of melting over an interval of ∼80 Ma. The Oligocene to Miocene basanitic series that accompanied the Plio-Pleistoicene basanitic series in the NMS region indicate that they shared a common mantle source. There is no geochemical evidence for thermal erosion of the lithospheric mantle or significant changes in mantle compositions within the time of a weak thermal perturbation in the asthenospheric mantle. These perturbations were caused by a dispersed mantle plume or passively upwelling asthenosphere in zones of lithospheric thinning.  相似文献   

9.
This study reports new geochemical and Sr and Nd isotope data for 11 samples of hynormative late Miocene (～6.5 Ma) basalt, basaltic andesite, and rhyolitic volcanic rocks from Meseta Rio San Juan, located in the states of Hidalgo and Queretaro, Mexico, in the north-central part of the Mexican Volcanic Belt (MVB). The in situ growth-corrected initial isotopic ratios of these rocks are as follows: 87Sr/86Sr 0.703400-0.709431 and 143Nd/144Nd 0.512524-0.512835. For comparison, the isotopic ratios of basaltic rocks from this area show very narrow ranges as follows: 87Sr/86Sr 0.703400-0.703540 and 143Nd/144Nd 0.512794-0.512835. The available geological, geochemical, and isotopic evidence does not support the generation of the basic and intermediate magmas by direct (slab melting), nor by indirect (fluid transport to the mantle) participation of the subducted Cocos plate. The basaltic magmas instead could have been generated by partial melting of the upper mantle. The evolved basaltic andesite magmas could have originated from such basaltic magmas through assimilation coupled with fractional crystallization. Rhyolitic magmas might represent partial melting of different parts of the underlying heterogeneous crust. Their formation and eruption probably was facilitated by extensional tectonics and upwelling of the underlying mantle. The different petrogenetic processes proposed here for basaltic and basaltic andesite magmas on one hand and rhyolitic magmas on the other might explain the bimodal nature of Meseta Rio San Juan volcanism. Finally, predictions by the author about the behavior of Sr and Nd isotopic compositions for subduction-related magmas is confirmed by published data for the Central American Volcanic Arc (CAVA).  相似文献   

10.
The Dexing porphyry Cu–Mo–Au deposit in east China (1,168 Mt at 0.45 % Cu) is located in the interior of the South China Craton (SCC), made up of two lithospheric blocks, the Yangtze and Cathaysia blocks. The Cu–Mo–Au mineralization is associated with mid-Jurassic granodioritic porphyries with three high-level intrusive centers, controlled by a series of lineaments at the southeastern edge of the Yangtze block. Available age data define a short duration (172–170 Ma) of the felsic magmatism and the mineralization (171?±?1 Ma). The deposit shows broad similarities with deposits in volcanoplutonic arcs, although it was formed in an intracontinental setting. Porphyries associated with mineralization are mainly granodiorites, which contain abundant phenocrysts (40–60 %) and carry contemporaneous microgranular mafic enclaves (MMEs). They are mainly high-K calc-alkaline and show geochemical affinities with adakite, characterized by relatively high MgO, Cr, Ni, Th, and Th/Ce ratios. The least-altered porphyries yielded relatively uniform ε Nd(t) values from ?0.9 to +0.6, and wide (87Sr/86Sr)i range between 0.7046 and 0.7058 partially overlapping with the Sr–Nd isotopic compositions of the MMEs and mid-Jurassic mafic rocks in the SCC. Zircons from the porphyries have positive ε Hf(t) values (3.4 to 6.9), and low δ18O values (4.7 to 6.3?‰), generally close to those of depleted mantle. All data suggest an origin by partial melting of a thickened juvenile lower crust involving mantle components (e.g., Neoproterozoic mafic arc magmas), triggered by invasion of contemporaneous mafic melts at Dexing. The MMEs show textural, mineralogical, and chemical evidence for an origin as xenoliths formed by injection of mafic melts into the felsic magmas. These MMEs usually contain magmatic chalcopyrite, and have original, variable contents of Cu (up to 500 ppm). Their geochemical characteristics suggest that they were derived from an enriched mantle source, metasomatized by Proterozoic slab-derived fluids, and supplied a part of Cu, Au, and S for the Dexing porphyry system during their injection into the felsic magmas. The 171?±?1 Ma magmatic-hydrothermal event at Dexing is contemporaneous with the mid-Jurassic extension in the SCC, followed by 160–90 Ma arc-like magmatism in southeastern China. With respect to the tectono-magmatic evolution of the SCC, the emplacement of Cu-bearing porphyries and the associated Cu mineralization occurred in response to the transformation from a tensional regime, related to mid-Jurassic extension, to a transpressional regime, related to the subduction of the Paleo-Pacific oceanic lithosphere.  相似文献   

11.
Basanites and alkali basalts from Mahabad in the West Azerbaijan province of Iran are part of a widespread series of Late Miocene–Quaternary mantle-derived magmas erupted within the Turkish–Iranian orogenic plateau, itself part of the active Arabia–Eurasia collision zone. New elemental and Sr–Nd isotopic results are combined with geophysical and geological constraints to suggest that these lavas formed predominantly by small degrees of partial melting of the thick (≫100 km) Eurasian lithospheric mantle within the garnet facies. Samples are highly enriched in large ion lithophile elements (LILE) and the light rare earth elements (LREE), up to 600 times chondritic values. They mostly possess negative primitive mantle-normalised Rb, K, Nb–Ta, Zr–Hf and Ti anomalies, with an overall signature that indicates a mantle source metasomatised by fluids or melts derived from crust during continental collision or the Tethyan oceanic subduction that preceded it. Sr–Nd isotopic values are similar to other Quaternary centres in NW Iran; 87Sr/86Sr is slightly depleted with respect to Bulk Silicate Earth, at ∼0.7045, and 143Nd/144Nd is slightly enriched, at ∼0.5127. Crustal contamination does not appear to be an important process in the chemistry of these samples. Possible triggers for melting may include: breakdown of hydrous phases during lithospheric thickening; hydration of the mantle lithosphere by underthrusting of the Arabian passive margin; small-scale sub-lithospheric convection due to a significant thickness gradient in the Zagros lithosphere. Such processes may account for small-volume syn-collisional mantle-derived magmatism elsewhere in regions of thick lithosphere where recent slab break-off or lithospheric delamination cannot be proven.  相似文献   

12.
Ischia, one active volcano of the Phlegraean Volcanic District, prone to very high risk, is dominated by a caldera formed 55 ka BP, followed by resurgence of the collapsed area. Over the past 3 ka, the activity extruded evolved potassic magmas; only a few low-energy explosive events were fed by less evolved magmas. A geochemical and Sr–Nd–O isotope investigation has been performed on minerals and glass from products of three of such eruptions, Molara, Vateliero, and Cava Nocelle (<2.6 ka BP). Data document strong mineralogical, geochemical, and isotopic heterogeneities likely resulting from mingling/mixing processes among mafic and felsic magmas that already fed the Ischia volcanism in the past. Detailed study on the most mafic magma has permitted to investigate its origin. The mantle sector below Ischia underwent subduction processes that modified its pristine chemical, isotopic, and redox conditions by addition of ≤1 % of sediment fluids/melts. Similar processes occurred from Southeast to Northwest along the Apennine compressive margin, with addition of up to 2.5 % of sediment-derived material. This is shown by volcanics with poorly variable, typical δ18O mantle values, and 87Sr/86Sr progressively increasing toward typical continental crust values. Multiple partial melting of this modified mantle generated distinct primary magmas that occasionally assimilated continental crust, acquiring more 18O than 87Sr. At Ischia, 7 % of Hercynian granodiorite assimilation produced isotopically distinct, K-basaltic to latitic magmas. A SW–NE regional tectonic structure gave these magmas coming from large depth the opportunity to mingle/mix with felsic magmas stagnating in shallower reservoirs, eventually triggering explosive eruptions.  相似文献   

13.
~(40)Ar/~(39)Ar and zircon U-Pb geochronological and whole-rock geochemical analyses for the Laozanggou intermediate-acidic volcanic rocks from the western Qinling orogenic belt,Central China,constrain their petrogenesis and the nature of the Late Mesozoic lithospheric mantle.These volcanic rocks yield hornblende or whole-rock ~(40)Ar/~(39)Ar plateau ages of 128.3-129.7 Ma and zircon U-Pb age of131.3±1.3 Ma.They exhibit Si02 of 56.86-66.86 wt.%,K_2 O of 0.99-2.46 wt.% and MgO of 1.03-4.47 wt.%,with Mg# of 42-56.They are characterized by arc-like geochemical signatures with significant enrichment in LILE and LREE and depletion in HFSE.All the samples have enriched Sr-Nd isotopic compositions with initial ~(87)Sr/~(86)Sr ratios ranging from 0.7112 to 0.7149 and ε_(Nd)(t) values from 10.2 to 6.3.Such geochemical signatures suggest that these volcanic rocks were derived from enriched lithospherederived magma followed by the assimilation and fractional crystallization(AFC)process.The generation of the enriched lithospheric mantle is likely related to the modification of sediment-derived fluid in response to the Triassic subduction/collision event in Qinling orogenic belt.The early Cretaceous detachment of the lithospheric root provides a reasonable mechanism for understanding the petrogenesis of the Laozanggou volcanic sequence in the western Qinling orogenic belt.  相似文献   

14.
介绍一个产生玄武岩的模型   总被引：1，自引：0，他引：1  
刘新秒 《华北地质》2006,29(2):150-154
地幔柱存在的一个主要证据是大规模高熔玄武岩省的出现,而且多认为玄武岩的来源依赖于地幔柱从下地幔输送。Michele Lustrino研究了造山时下地壳和岩石圈地幔的拆沉和拆离作用,提出了产生玄武岩的一个新模型。该模型认为即使地幔柱不存在,拆沉到地幔的下地壳物质再循环同样可以解释小规模的板内(大洋岛弧和大陆内部)火山岩和大洋、大陆溢流玄武岩及洋中脊玄武岩的生成及其常见的几种地球化学特征。在陆-陆碰撞过程中,下地壳中的变质反应生成石榴石,导致岩石的密度增大,致使过厚岩石圈底部(下地壳和岩石圈地幔)和上地壳分离并沉入上地幔。下地壳发生部分熔融形成富SiO2的熔体,和上涌的软流圈地幔(充填在下沉的岩石圈地幔和下地壳的空间)发生变质交代反应,导致具有强烈的地壳特点的富含斜方辉石层的形成。这个变质交代地幔体可以在拆沉后保持不变长达几个百万年。这种源的部分熔体可以保有下地壳的明显特征,产生类似富集地幔1型玄武岩浆作用。因此,该模型是提供了玄武岩浆来源的一个新选择。  相似文献   

15.
Mesozoic mafic dikes in the Gan-Hang tectonic belt (GHTB) provide an opportunity to explore both the nature of their mantle source(s) and the secular evolution of the underlying Mesozoic lithospheric mantle in the region. The geochronology and primary geochemical and Sr–Nd–Pb isotopic compositions of Group 1 (middle section of GHTB) and Group 2 (the rest of the section) dolerite dikes spanning the GHTB were investigated. K–Ar ages indicate that dikes of both groups were emplaced during the Cretaceous (131–69 Ma). The dikes are doleritic in composition and are enriched in both large ion lithophile elements (LILEs; e.g. Rb, Ba, and Pb) and light rare earth elements (LREEs), with a wide range of Eu anomalies, but are depleted in high field strength elements (HFSEs; e.g. Nb, Ta, and Ti) and heavy rare earth elements (HREEs). Dikes sampled in the middle section of the GHTB (Group 1) show more pronounced REE differentiation and a greater contribution from crustal material than those from the east and west sections (Group 2) and are similar to GHTB volcanic rocks in exhibiting a slight enrichment in LREEs. The dolerites are further characterized by a wide range in 87Sr/86Sr i ?=?0.7041–0.7110, 143Nd/144Nd i ?=?0.511951–0.512758, ?Nd t ?=?–10.4 to?+5.6, and Pb isotopic ratios (206Pb/204Pb i ?=?18.1–18.3, 207Pb/204Pb i ≈ 15.6, and 208Pb/204Pb i ?=?38.2–38.7). The dikes have undergone fractional crystallization of olivine, clinopyroxene, plagioclase, and Ti-bearing phases, except for dikes from the Anding area, which possibly experienced fractionation of plagioclase. Geochemically, all the dike samples originated from mantle sources ranging in composition from depleted to enriched that contained a component of foundered lower crust; crustal contamination during the ascent of these magmas was negligible. In the context of the late Mesozoic lithospheric extension across South China, mafic dike magmatism was likely triggered by the reactivation of deep faults, which promoted foundering of the lower crust and subsequent mantle upwelling in the GHTB.  相似文献   

16.
Lavas from Santiago Island attest to a complex magmatic history, in which heterogeneous mantle source(s) and the interactions of advecting magmas with thick metasomatised oceanic lithosphere played an important role in the observed isotopic and trace element signatures. Young (<3.3 Ma) primitive lavas from Santiago Island are characterised by pronounced negative K anomalies and trace element systematics indicating that during partial melting DK>DCe. These features suggest equilibration with an oceanic lithospheric mantle containing K-rich hydrous mineral assemblages, consistent with the occurrence of amphibole + phlogopite in associated metasomatised lherzolite xenoliths, where orthopyroxene is partially replaced by newly formed olivine + (CO2 + spinel + carbonate inclusion-rich) clinopyroxene. Metasomatism induced a decrease in $ a ^{{{\text{melt}}}}_{{{\text{SiO}}_{{\text{2}}} }} $ and Ti/Eu ratios, as well as an increase in fO 2 , Ca/Sc and Sr/Sm in the Santiago magmas, suggesting a carbonatitic composition for the metasomatic agent. Santiago primitive lavas are highly enriched in incompatible elements and show a moderate range in isotopic compositions (87Sr/86Sr?=?0.70318–0.70391, 143Nd/144Nd?=?0.51261–0.51287, 176Hf/177Hf?=?0.28284–0.28297). Elemental and isotopic signatures suggest the involvement of HIMU and EM1-type mantle end-members, in agreement with the overall isotopic characteristics of the southern Cape Verde Islands. The overall geochemical characteristics of lavas from Santiago Island allow us to consider the EM1-like end-member as resulting from the involvement of subcontinental lithospheric mantle in the genesis of magmas on Santiago.  相似文献   

17.
峨眉山玄武岩Sr、Nd、Pb同位素特征及其物源探讨   总被引：33，自引：3，他引：30  
张招崇  王福生 《地球科学》2003,28(4):431-439
选择峨眉山玄武岩区2个出露最全的云南永胜大迪里剖面和宾川上仓剖面进行了Sr、Nd、Pb同位素地球化学研究.结果表明, 少数样品的Pb同位素与Hanan和Graham定义的C组分相似, 而大多数样品则不在C组分范围之内, 说明除地幔柱物质外, 有岩石圈物质的加入.在多元同位素图解上, 峨眉山玄武岩位于EMⅠ、EMⅡ和DMM三端元之间, 表明其源区可以由地幔柱、富集的岩石圈地幔和地壳不同程度的混合来解释.结合已有的微量元素资料分析, 其中的地壳组分主要为下地壳, 而早期玄武质岩浆在上升过程中由于通道不畅通, 有较多的上地壳组分的混染.岩石圈地幔的富集作用可能与地幔柱释放出的小体积富Na、P而贫K的流体交代作用有关.粗面岩的同位素组成和玄武岩接近, 说明粗面岩是玄武质岩浆分离结晶作用形成的.   相似文献   

18.
青山群火山岩是华北克拉通破坏期间最具代表性的地幔或地壳熔融产物,记录了华北深部地质演化的重要信息。本文对胶东青山群基性火山岩进行了40Ar/39Ar定年和岩石地球化学分析,结合前人报道的胶东青山群酸性火山岩资料,发现:(1)基性火山岩喷发年龄为122～113Ma,早于青山群酸性火山岩(110～98Ma);(2)基性和酸性火山岩显示了不同的元素和同位素地球化学特征。岩石成因分析表明,基性火山岩为交代富集地幔部分熔融作用的产物,而酸性火山岩为古老下地壳和中生代底侵岩浆的熔融产物(Ling et al.,2009)。因此,胶东地区青山群火山岩记录了岩浆熔融源区从地幔向下地壳的转变。这与长时间尺度的岩石圈减薄过程中热能由地幔向地壳传递过程相吻合,而不同于地壳拆沉作用所预测的岩浆演化趋势。  相似文献   

19.
The Middle Miocene Tsushima granite pluton is composed of leucocratic granites, gray granites and numerous mafic microgranular enclaves (MME). The granites have a metaluminous to slightly peraluminous composition and belong to the calc‐alkaline series, as do many other coeval granites of southwestern Japan, all of which formed in relation to the opening of the Sea of Japan. The Tsushima granites are unique in that they occur in the back‐arc area of the innermost Inner Zone of Southwest Japan, contain numerous miarolitic cavities, and show shallow crystallization (2–6 km deep), based on hornblende geobarometry. The leucocratic granite has higher initial 87Sr/86Sr ratios (0.7065–0.7085) and lower εNd(t) (?7.70 to ?4.35) than the MME of basaltic–dacitic composition (0.7044–0.7061 and ?0.53 to ?5.24), whereas most gray granites have intermediate chemical and Sr–Nd isotopic compositions (0.7061–0.7072 and ?3.75 to ?6.17). Field, petrological, and geochemical data demonstrate that the Tsushima granites formed by the mingling and mixing of mafic and felsic magmas. The Sr–Nd–Pb isotope data strongly suggest that the mafic magma was derived from two mantle components with depleted mantle material and enriched mantle I (EMI) compositions, whereas the felsic magma formed by mixing of upper mantle magma of EMI composition with metabasic rocks in the overlying lower crust. Element data points deviating from the simple mixing line of the two magmas may indicate fractional crystallization of the felsic magma or chemical modification by hydrothermal fluid. The miarolitic cavities and enrichment of alkali elements in the MME suggest rapid cooling of the mingled magma accompanied by elemental transport by hydrothermal fluid. The inferred genesis of this magma–fluid system is as follows: (i) the mafic and felsic magmas were generated in the mantle and lower crust, respectively, by a large heat supply and pressure decrease under back‐arc conditions induced by mantle upwelling and crustal thinning; (ii) they mingled and crystallized rapidly at shallow depths in the upper crust without interaction during the ascent of the magmas from the middle to the upper crust, which (iii) led to fluid generation in the shallow crust. The upper mantle in southwest Japan thus has an EMI‐like composition, which plays an important role in the genesis of igneous rocks there.  相似文献   

20.
《International Geology Review》2012,54(13):1630-1657
New geological, geochronological, and geochemical results on volcanic rocks and cobbles from early Mesozoic sedimentary rocks identify two contrasting latest Permian–Triassic volcanic rock suites in the northern North China Craton (NCC). The early rock suite erupted during the latest Permian–Early Triassic at ca. 255–245 Ma and was probably widely distributed in the northern NCC prior to the Early Jurassic. It comprises rhyolitic welded tuff, rhyolite, and tuffaceous sandstone and is characterized by high contents of SiO2 and K2O, moderate initial 87Sr/86Sr, low negative εNd(t) and εHf(t) values, and old Nd-Hf isotopic model ages. It was likely produced by fractional crystallization of lower crustal-derived magmas due to underplating by lithospheric mantle-derived magmas near the crust–mantle boundary in syncollisional to post-collisional/post-orogenic tectonic settings. The late rock suite, erupted during the Middle–Late Triassic at ca. 238–228 Ma, displays adakitic geochemical signatures and consists of intermediate volcanic rocks such as andesite, trachyandesite, and autoclastic trachyandesite breccia, with minor felsic rocks. This suite is characterized by high Al2O3, MgO, Sr, Ba, Cr, V, and Ni concentrations; high Mg# values; low Y and Yb concentrations and high Sr/Y ratios; low initial 87Sr/86Sr; high negative εNd(t) and εHf(t) values; and young Nd-Hf isotopic model ages. The younger suite was generated by mixing of magmas derived from melting of upwelling asthenosphere, with melts of ancient lower crust induced by underplating of basaltic magmas in an intraplate extensional setting. Strong upwelling of asthenospheric mantle and significant involvement of the asthenospheric mantle materials indicate that the lithospheric mantle beneath the northern NCC was partially delaminated during Middle–Late Triassic time, representing the initial destruction and lithospheric thinning of the northern NCC. Lithospheric thinning and delamination are likely the most important reasons for the Triassic tectonic transition and change of magmatism and deformation patterns in the northern NCC.  相似文献   

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