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1.
Early–Middle Jurassic igneous rocks (190–170 Ma) are distributed in an E–W-trending band within the Nanling Tectonic Belt, and have a wide range of compositions but are only present in limited volumes. This scenario contrasts with the uniform but voluminous Middle–Late Jurassic igneous rocks (165–150 Ma) in this area. The Early–Middle Jurassic rocks include oceanic-island basalt (OIB)-type alkali basalts, tholeiitic basalts and gabbros, bimodal volcanic rocks, syenites, A-type granites, and high-K calc–alkaline granodiorites. Geochemical and isotopic data indicate that alkaline and tholeiitic basalts and syenites were derived from melting of the asthenospheric mantle, with asthenosphere-derived magmas mixing with variable amounts of magmas derived from melting of metasomatized lithospheric mantle. In comparison, A-type granites in the study area were probably generated by shallow dehydration-related melting of hornblende-bearing continental crustal rocks that were heated by contemporaneous intrusion of mantle-derived basaltic magmas, and high-K calc-alkaline granodiorites resulted from the interaction between melts from upwelling asthenospheric mantle and the lower crust. The Early–Middle Jurassic magmatic event is spatially variable in terms of lithology, geochemistry, and isotopic systematics. This indicates that the deep mantle sources of the magmas that formed these igneous rocks were significantly heterogeneous, and magmatism had a gradual decrease in the involvement of the asthenospheric mantle from west to east. These variations in composition and sourcing of magmas, in addition to the spatial distribution and the thermal structure of the crust–mantle boundary during this magmatic event, indicates that these igneous rocks formed during a period of rifting after the Indosinian Orogeny rather than during subduction of the paleo-Pacific oceanic crust.  相似文献   

2.
The Sula Mountains greenstone belt is the largest of the late-Archaean greenstone belts in the West African Craton. It comprises a thick (5 km) lower volcanic formation and a thinner (2 km) upper metasedimentary formation. Komatiites and basalts dominate the volcanic formation and komatiites form almost half of the succession. All the volcanic rocks are metamorphosed to amphibolite grade and have been significantly chemically altered. Two stages of alteration are recognised and are tentatively ascribed to hydrothermal alteration and later regional amphibolite facies metamorphism. Ratios of immobile trace elements and REE patterns preserve, for the most part, original igneous signatures and these are used to identify five magma types. These are: low-Ti komatiites – depleted in light REE; low-Ti komatiites – with flat REE patterns; high-Ti komatiitic basalts – with flat REE; low-Ti basalts – depleted in light REE; high-Ti basalts – with flat REE patterns. Much of the variation between the magma types can be explained in terms of different melt fractions of the mantle source, although there were two separate mantle sources one light REE depleted, the other not. The interleaving of the basalts and komatiites produced by this melting indicates that the two mantle sources were melted simultaneously. The simplest model with which to explain these complex melting processes is during melting within a rising mantle plume in which there were two different mantle compositions. The very high proportion of komatiites in the Sula Mountains relative to other greenstone belts suggests either extensive deep melting and/or the absence of a thick pre-existing crust which would have acted as a “filter” to komatiite eruption. Received: 10 February 1998 / Accepted: 28 July 1998  相似文献   

3.
Process identification diagrams based on trace element data show that mafic lavas from Tubuai, including alkali basalts, basanites, analcitites and nephelinites, result from different degrees of partial melting of an isotopically homogeneous mantle source. Our fractionation-corrected data are consistent with a batch melting model or a dynamic melting model involving a threshold value for melt separation close to 1% and degrees of melting ranging from 5–8% (alkali basalts) to 1.5–3% (nephelinites). The relative source concentration pattern, calculated using an inverse numerical method, shows an enrichment in highly incompatible elements. We propose that the Tubuai lava suite was derived from a two-stage partial melting process. Melting first affected the plume material located within the transition zone between garnet and spinel domains, producing alkali basalts and basanites. Then, the melting zone migrated upwards to the base of the overlying spinel-bearing lithospheric mantle, producing highly silica-undersaturated lavas. The lower lithosphere had previously been enriched by intrusion of pyroxenite veins representing plume-derived melts which percolated away from the main magma conduits. Received: 11 June 1996 / Accepted: 8 January 1997  相似文献   

4.
Modal metasomatism in the Kaapvaal craton lithosphere is well documented in upper mantle xenoliths sampled by both group I (mainly late Cretaceous) and group II (mainly early Cretaceous to late Jurassic) kimberlites in the Kimberley area. The metasomatic style is characterized by introduction of K, H and large ion lithophile/high field strength (LIL/HFS) elements into the lithospheric mantle leading to the crystallization of hydrous potassic phases such as phlogopite and/or K-amphibole. Textures indicate that the hydrous phases either replace pre-existing assemblages in peridotites, forming the metasomatized peridotite suite (phlogopite–K-richterite–peridotites: PKPs) or crystallize from K-rich melts, forming the mica–amphibole–rutile–ilmenite–diopside (MARID) suite of xenoliths. These K-rich assemblages become potential low melting source components for alkaline incompatible trace element enriched magmas. The timing of metasomatism and its temporal and possible genetic relation to kimberlite magmatism is poorly constrained because of the rarity of phases in the metasomatic assemblages suitable for precise dating. Here we present precise sensitive high resolution ion microprobe (SHRIMP) U–Pb formation ages of 88 ± 2 (1σ=1 standard deviation) and 82 ± 3 Ma data for zircons from a K-richterite–phlogopite-bearing metasomatized peridotite (PKP) and a MARID xenolith respectively, sampled by a group I kimberlite. Both average PKP and MARID zircon ages are indistinguishable from emplacement ages of group I kimberlites in the Kimberley area dated at 83 ± 4 (2σ) and 84 ± 0.9 Ma. One exceptionally old age spot of 102 ± 5 Ma from a PKP zircon provides evidence for modal metasomatism predating group I kimberlite emplacement by several millions of years with minor resetting of the U–Pb isotopic system of most analyzed PKP zircons to a group I emplacement age. Detailed textural and mineral chemical analysis, including high energy X-ray mapping and analysis of fluid inclusion daughter crystals, indicates a complex reaction history for both PKPs and MARIDs. U–Pb zircon ages from this study combined with literature data and experimentally derived models for MARID formation are used to suggest that MARID-formation is concurrent and genetically related to both group I and II kimberlite magmatism in the Kimberley area. MARID and PKP zircon ages are also consistent with the idea first proposed by Dawson and Smith (Geochim Cosmochim Acta 41: 309–323, 1977) that metasomatized peridotites may form from interaction of hydrous fluids expelled by solidifying MARID-type melts with peridotitic wall rocks. Received: 13 December 1999 / Accepted: 13 April 2000  相似文献   

5.
An isotopic study of igneous and metamorphic rocks has been carried out at the Yermakovsky bertrandite-phenakite-fluorite deposit. It has been established that the model age of the schists pertaining to the Zun-Morino Formation is 1360–1260 Ma. In Nd and Sr isotopic composition, these schists deviate from the isotopic composition of the continental crust and are close in this respect to the enriched mantle reservoir (EM-II). The model age of carbonate rocks of the Zun-Morino Formation is 1330–1020 Ma. The Middle Riphean model age of the Zun-Morino Formation is interpreted as the age of its protolith. According to the Sr and Nd isotopic data, all preore igneous rocks (granitic dikes, gabbroic rocks, and gneissose granite of the Tsagan Complex) were formed with the participation of continental crustal material. Synore basic dikes, alkali leucogranite stock, and syenite intrusion are considered to be mixtures of mantle components (DM+HIMU) and various continental crustal components (Tsagan gneissose granite, crystalline schists, the mean composition of granitoids of the Angara-Vitim batholith as an estimate of average composition of the regional continental crust). Synore igneous rocks are genetically cognate and related to the magmatic activity in the Western Transbaikal Rift Zone presumably formed in the Triassic under effect of a mantle plume.  相似文献   

6.
In the Itsaq Gneiss Complex south of the Isua supracrustal belt (West Greenland) some areas of early Archaean tonalite and quartz-diorite are non-gneissic, free of pegmatite veins, and in rarer cases are undeformed with relict igneous textures and hence were little modified by heterogeneous ductile deformation under amphibolite facies conditions in several Archaean events. Such well-preserved early Archaean rocks are extremely rare. Tonalites are high Al, and have bulk compositions close to experimental liquids. Trace element abundances and modelling suggest that they probably originated as melts derived from basaltic compositions at sufficiently high pressures to require residual garnet + amphibolites ± clinopyroxene in the source. The major element characteristics of the quartz-diorites suggest these were derived from more mafic magmas than the tonalites, and underwent either igneous differentiation or mixing with crustal material. As in modern arc magmas, high relative abundances of Sr, Ba, Pb, and alkali elements cannot be generated simply from a basaltic source formed by large degrees of melting of a depleted mantle. This may indicate an important role for fluids interacting with mafic rocks in generating the earliest preserved continental crust. The high Ba/Th, Ba/Nb, La/Nb and low Nb/Th, Ce/Pb, and Rb/Cs ratios of these tonalites are also observed in modern arc magmas. SHRIMP U-Pb zircon geochronology was undertaken on seven tonalites, one quartz-diorite, a thin pegmatitic vein and a thin diorite dyke. Cathodoluminescence images show the zircon populations of the quartz-diorite and tonalites are dominated by single-component oscillatory-zoned prismatic grains, which gave ages of 3806 ± 5 to 3818 ± 8 Ma (2σ) (quartz-diorite and 5 tonalites) and 3795 ± 3 Ma (1 tonalite). Dating of recrystallised domains cutting oscillatory-zoned zircon indicates disturbance as early as 3800–3780 Ma. There are rare ca. 3600 Ma and 3800–3780 Ma (very high U and low Th/U) ≤ 20 μm wide partial overgrowths on the prismatic grains. Given likely Zr-undersaturation of precursor melts and evidence of zircon recrystallisation and metamorphic regrowth as early as 3800–3780 Ma, the age determinations on the prismatic oscillatory-zoned zircon populations give the igneous crystallisation age of the tonalite and quartz-diorite protoliths. When the coherency of the geochemistry is considered, these samples represent the best preserved suites of ca. 3800 Ma felsic igneous rocks yet documented. Received: 1 December 1998 / Accepted: 23 July 1999  相似文献   

7.
The phanerozoic within-plate magmatism of Siberia is reviewed. The large igneous provinces (LIPs) consecutively arising in the Siberian Craton are outlined: the Altai-Sayan LIP, which operated most actively 400–375 Ma ago, the Vilyui LIP, which was formed from the Middle Devonian to the Early Carboniferous, included; the Barguzin-Vitim LIP (305–275 Ma); the Late Paleozoic Rift System of Central Asia (318–250 Ma); the Siberian flood basalt (trap) province and the West Siberian rift system (250–247 Ma); and the East Mongolian-West Transbaikal LIP (230–195 Ma), as well as a number of Late-Mesozoic and Cenozoic rift zones and autonomous volcanic fields formed over the last 160 Ma. The trace-element and isotopic characteristics of the igneous rocks of the above provinces are reviewed; their mantle origin is substantiated and the prevalence of PREMA, EM2, and EM1 mantle magma sources are shown. The paleogeographic reconstructions based on paleomagnetic data assume that the Iceland hot spot was situated beneath the Siberian flood basalts 250 Ma ago and that the mantle plumes retained a relatively stable position irrespective of the movements of the lithospheric plates. At present, the Iceland hot spot occurs near the northern boundary of the African large low shear velocity province (LLSVP). It is suggested that the within-plate Phanerozoic magmatism of Siberia was related to the drift of the continent above the hot spots of the African LLSVP.  相似文献   

8.
The U-Pb (SHRIMP-II) age of zircons from garnet-spinel peridotite nodules in Cenozoic alkali basalts of the Vitim Plateau, Transbaikal region were determined. Most of the zircons are euhedral and subhedral prismatic crystals with an elongation of 1.5–2.0. Fragments of crystals and nearly equant crystals with rounded edges are present as well. Rounded or irregular cores are observed in some grains. None of the zircons yielded an age that would correspond to the time of basalt eruption (21–2.35 Ma or younger). The youngest dates range from 135.2 ± 2.7 Ma to 141 ± 3 Ma (Early Cretaceous). Both concordant values and the lower intersection of discordia with concordia (138.8 ± 5.7 Ma) are within this age interval. The upper intersection corresponds to 1891 ± 26 Ma. A considerable part of the concordant values are grouped within the intervals (164.6 ± 1.6)–(183.4 ± 2.0) and (264.0 ± 7.3)–(295.7 ± 0.76) Ma (Early-Middle Jurassic and Early Permian, respectively). The older concordant values fall in the interval 1462 ± 19 to 1506 ± 4 Ma (Mesoproterozoic). Proterozoic age was obtained for cores of composite zircon grains. Zircons pertaining to all age intervals are enriched in REE relative to chondrite (except La). The chondrite-normalized REE patterns are positively sloped with an increase in contents from LREE to HREE. The LREE and HREE contents and the depth of the Eu minimum tend to increase with age. In composite zircons of Proterozoic age, cores are somewhat enriched in REE. It has been suggested that crystallization of zircon as a separate phase in peridotites extremely depleted in Zr was related to a low degree of partial melting. The melt that formed in the intergranular space and that was repeatedly enriched in Zr was not extracted from the solid framework of rock and crystallized in situ under the changed thermodynamic conditions in the upper mantle. The occurrence of zircons of several age intervals in peridotites testifies to the multistage evolution of the upper mantle and recurrent partial melting under various physicochemical conditions.  相似文献   

9.
The Abbott Unit (∼508 Ma) and the Vegetation Unit (∼475 Ma) of the Terra Nova Intrusive Complex (northern Victoria Land, Antarctica) represent the latest magmatic events related to the Early Paleozoic Ross Orogeny. They show different emplacement styles and depths, ranging from forcible at 0.4–0.5 GPa for the Abbott Unit to passive at ∼0.2 GPa for the Vegetation Unit. Both units consist of mafic, felsic and intermediate facies which collectively define continuous chemical trends. The most mafic rocks from both units show different enrichment in trace element and Sr-Nd isotopic signatures. Once the possible effects of upper crustal assimilation-fractional crystallisation (AFC) and lower crustal coupled AFC and magma refilling processes have been taken into account the following features are recognised: (1) the modelled primary Abbott Unit magma shows a slightly enriched incompatible element distribution, similar to common continental arc basalts and (2) the modelled primary Vegetation Unit magma displays highly enriched isotope ratios and incompatible element patterns. We interpreted these major changes in magmatic affinity and emplacement style as linked to a major change in the tectonic setting affecting melt generation, rise and emplacement of the magmas. The Abbott Unit mafic melts were derived from a mantle wedge above a subduction zone, with subcontinental lithospheric mantle marginally involved in the melting column. The Vegetation Unit mafic melts are regarded as products of a different source involving an old layer of subcontinental lithospheric mantle. The crustal evolution of both types of mafic melts is marked by significant compositional contrasts in Sr and Nd isotopes between mafic and associated felsic rocks. The crustal isotope signature showed an increase with felsic character. Geochemical variations for both units can be accounted for by a similar two-stage hybridisation process. In the first stage, the most mafic magma evolved mainly by fractional crystallisation coupled with assimilation of metasedimentary rocks having crustal time-integrated Sr and Nd compositions similar to those of locally exposed metamorphic basement. The second stage involves contaminated products mixing with independently generated crustal melts. Petrographic, geochemical and isotope data also provide evidence of significant compositional differences in the felsic end-members, pointing to the involvement of metaigneous and metasedimentary source rocks for the Abbott granite and Vegetation leucogranite, respectively. Received: 31 March 1998 / Accepted: 3 May 1999  相似文献   

10.
This paper addresses the composition, geochemistry, isotopic characteristics, and age of rocks from the Carter Seamount of the Grimaldi seamount group at the eastern margin of the Central Atlantic. The age of the seamount was estimated as 57–58 Ma. Together with other seamounts of the Grimaldi system and the Nadir Seamount, it forms a “hot line” related to the Guinea Fracture Zone, which was formed during the late Paleocene pulse of volcanism. The Carter Seamount is made up of olivine melilitites, ankaramites, and analcime-bearing nepheline tephrites, which are differentiated products of the fractional crystallization of melts similar to an alkaline ultramafic magma. The volcanics contain xenoliths entrained by melt at different depths from the mantle, layer 3 of the oceanic crust, which was formed at 113–115 Ma, and earlier magma chambers. The rocks were altered by low-temperature hydrothermal solutions. The parental melts of the volcanics of the Carter Seamount were derived at very low degrees of mantle melting in the stability field of garnet lherzolite at depths of no less than 105 km. Anomalously high Th, Nb, Ta, and La contents in the volcanics indicate that a metasomatized mantle reservoir contributed to the formation of their primary melts. The Sr, Pb, and Nd isotopic systematics of the rocks show that the composition of the mantle source lies on the mixing line between two mantle components. One of them is a mixture of prevailing HIMU and the depleted mantle, and the other is an enriched EM2-type mantle reservoir. These data suggest that the formation of the Carter Seamount volcanics was caused by extension-related decompression melting in the Guinea Fracture Zone of either (1) hot mantle plume material (HIMU component) affected by carbonate metasomatism or (2) carbonated basic enclaves (eclogites) ubiquitous in the asthenosphere, whose isotopic characteristics corresponded to the HIMU and EM2 components. In the former case, it is assumed that the melt assimilated during ascent the material of the metasomatized subcontinental mantle (EM2 component), which was incorporated into the oceanic lithospheric mantle during rifting and the breakup of Pangea.  相似文献   

11.
 Single zircon U–Pb dating combined with 207Pb/206Pb ages obtained by the evaporation method constrains the emplacement of tonalitic, trondhjemitic, and granodioritic orthogneisses of the Moldanubian zone in the Black Forest between 500 and 510 Ma. Two detrital zircon populations of 1.9 and 1.6 Ga indicate Early-Middle Proterozoic material in the former setting of the basement. The initial eNd values range from –0.1 to –3.4 and mean crustal residence ages of 1.0–1.4 Ga are consistent with involvement of Early-Middle Proterozoic crust, and a subordinate juvenile component probably originating from subduction-related melting of the mantle. The orthogneisses have fractionated REE patterns and slightly higher K2O/Na2O ratios than typical low-K tonalite–trondhjemite–granite suites. The chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust. The emplacement of the orthogneisses was superceded by a high-temperature metamorphic event at ∼480 Ma which we interpret as a result of lithospheric thinning in a marginal basin behind a Cambrian magmatic arc. Received: 29 March 1999 / Accepted: 25 August 1999  相似文献   

12.
Sensitive high-resolution ion microprobe (SHRIMP) U–Pb dating, laser-ablation multi-collector ICPMS Hf isotope and electron microprobe element analyses of inherited/antecrystal and magmatic zircons from five granitoid intrusions of Linxi area, in the southern segment of the Great Xing’an Range of China were integrated to solve continental crustal growth mechanisms. These intrusions were divided into two suites. Suites 1 and 2 are mainly granodiorite and syenogranite and correspond to magnesian and ferroan granites, respectively. SHRIMP dating establishes an Early Cretaceous (135–125 Ma) age for most Linxi granitoids and a time of ∼146 Ma when their source rocks were generated or re-melted. However, some granitoids were generated in Early Triassic (241 Ma) and Late Jurassic (146 Ma), after their source rock experienced precursory melting episodes at 263 Ma and 165 Ma, respectively. All zircon 206Pb/238U ages (<300 Ma, n = 100), and high positive zircon εHf(t) values (n = 175) suggest juvenile source materials with an absence of Precambrian basement. Hf–Nd isotopic decoupling of Linxi granitoids suggests a source component of pelagic sediments, i.e. Paleozoic subduction accretion complexes. Zircon εHf(t) values (t = 263–165 Ma) form a trend sub-parallel to the depleted mantle Hf isotope evolution curve, whilst those with t = 146–125 Ma fall markedly below the latter. The first trend indicates a provenance from essentially subducted oceanic slabs. However, the abrupt εHf(t) decrease, together with extensive Early Cretaceous magmatism, is interpreted as reflecting mantle upwelling and resultant underplating, and exhumation of subducted oceanic slabs. Suite 1 granitoids derive mainly from subducted oceanic slabs or Paleozoic subduction accretion complex, whereas Suite 2 from underplated mafic rock and, subordinately, Paleozoic subduction accretion complex. Compositions of Suites 1 and 2 depend on the hydrous, oxidized or relatively anhydrous, reduced nature of source rocks. Among each of these five intrusions, magmatic zircons have systematically lower 176Hf/177Hf than inherited/antecrystal zircons. Hf isotopic and substituting element profiles through inherited/antecrystal zircons (t = 263 to ∼146 Ma) indicate repeated low melt-fraction melting in the source region. In contrast, profiles through inherited/antecrystal and magmatic zircons (t = 146–125 Ma) reveal melting region expansion with a widening range of source compositions and increasing melt fractions. These results lead to the conclusion that continental growth in this region involved a three-step process. This included subduction accretion and repeated underplating, intermediary differentiation of juvenile rocks, and granitoid production from these differentiated rocks.  相似文献   

13.
Late Mesozoic dioritic and quartz dioritic plutons are widespread in the Daye region, eastern Yangtze craton, eastern China. Detailed geochronological, geochemical, and Sr–Nd isotopic studies have been undertaken for most of these plutons, in an attempt to provide a comprehensive understanding in the age, genesis and geodynamical control of the extensive magmatism. SHRIMP and LA-ICP-MS zircon U–Pb dating indicate that the plutons were emplaced in the range of latest Jurassic (ca. 152 Ma) to early Cretaceous (ca. 132 Ma), which was followed by dyke emplacement between 127 and 121 Ma and volcanism during the 130–113 Ma interval. Both diorites and quartz diorites are sodic, metaluminous, high-K calc-alkaline, and characterized by strongly fractionated, sub-parallel REE patterns without obvious Eu anomalies. The rocks are enriched in highly incompatible elements and large ion lithophile elements, but depleted in high field strength elements. Samples of diorite and quartz diorite have similar Sr–Nd isotopic compositions that are consistent with the early Cretaceous basalts and mafic intrusions throughout the eastern Yangtze craton. The geochemical and isotopic data, together with results of geochemical modeling, indicate an enriched mantle source for the plutonic rocks. The quartz diorites have geochemical signatures resembling adakites, such as high Al2O3 (15–19 wt.%), Sr (630–2,080 ppm), Na2O (>3.5 wt.%), negative Nb–Ta anomalies, low Y (7–19 ppm), Yb (0.5–1.8 ppm), Sc (5–15 ppm), and resultant high Sr/Y (45–200) and La/Yb (31–63) ratios. Genesis of the adakitic quartz diorites is best explained in terms of low-pressure intracrustal fractional crystallization of cumulates consisting of hornblende, plagioclase, K-feldspar, magnetite, and apatite from mantle-derived dioritic magmas. Mantle-derived magmatism broadly coeval with that of the Daye region also is widespread in other regions of the eastern Yangtze craton, reflecting large-scale melting of the lithospheric mantle during the Late Mesozoic. The large-scale magmatism was most likely driven by lithospheric extension associated with thinning of lithospheric mantle beneath the eastern China continent.  相似文献   

14.
汪方跃  高山  牛宝贵  张宏 《地学前缘》2007,14(2):98-108
华北克拉通罕见年龄界于120～100Ma的火山岩。承德盆地大北沟组火山岩下部主要由柱状节理橄榄玄武岩组成,中上部主要由安山岩组成。对紧邻玄武岩的上覆安山岩的火山锆石U-PbLA-ICPMS定年结果表明,形成年龄为(113.6±0.87)Ma,代表了该套火山岩的喷发年龄,表明它们形成于早白垩世晚期。对3件玄武岩样品的分析结果表明,它们亏损高场强元素(Nb、Ta、Zr、Hf),初始87Sr/86Sr同位素比值为0.7059,εNd(114Ma)为-11.04,具有富集型岩石圈地幔的特征。但该套玄武岩的主量和微量元素特征则介于华北克拉通中生代年龄>120Ma具古老富集型地幔特征的玄武岩和年龄<100Ma具亏损软流圈性质的玄武岩之间,表明113.6Ma时华北克拉通岩石圈地幔在元素组成方面已具有由富集地幔向亏损型软流圈地幔转变的特征。大北沟组玄武岩的地球化学特征表明,114Ma时华北克拉通岩石圈地幔已减薄。  相似文献   

15.
Tholeiitic basalts in various stages of alteration were dredged from Late Cretaceous volcanic rocks (60 -67 Ma) in the Hebrides Terrace seamount area in the Atlantic Ocean. These rocks are extrusive olivine basalts, including high- and low-Al basalts. High-Al basalts are depleted in MgO, CaO, Cr,Sc, V, St, Zr and enriched in TiO2, Na2O, Nb, Rb as compared with low-A1 basalts. Petrography and bulk-rock composition (major, trace and rare-earth elements) data defined clear tholeiitic suites displaying possible liquid lines of descent related to different degrees of crystal fractionation and partial melting.Isotopic dating of dredged samples gave the guyot an age of 60 - 67 Ma, in support of the assumption that it was formed during the Late Cretaceous.  相似文献   

16.
The Antucoya porphyry copper deposit (300 Mt at 0.45% total Cu) is one of the largest deposits of a poorly known Early Cretaceous porphyry belt in the Coastal Cordillera of northern Chile. It is related to a succession of granodioritic and tonalitic porphyritic stocks and dikes that were emplaced within Jurassic andesitic rocks of the La Negra Formation immediately west of the N–S trending sinistral strike-slip Atacama Fault Zone. New zircon SHRIMP U–Pb data indicate that the porphyries of Antucoya crystallized within the time span from 142.7 ± 1.6 to 140.6 ± 1.5 Ma (±2 σ), and late, unmineralized, NW–SE trending dacite dikes with potassic alteration and internal deformation crystallized at 141.9 ± 1.4 Ma. The Antucoya porphyry copper system appears to be formed after a change of stress conditions along the magmatic arc from extensional in the Late Jurassic to transpressive during the Early Cretaceous and provides support for an Early Cretaceous metallogenic episode of porphyry-type mineralization along the Coastal Cordillera of northern Chile.  相似文献   

17.
Mafic rocks of a Permian crust to mantle section in Val Malenco (Italy) display a multi-stage evolution: pre-Alpine exhumation to the ocean floor, followed by burial and re-exhumation during Alpine convergence. Four prominent generations of amphiboles were formed during these stages. On the basis of microstructural investigations combined with electron microprobe analyses two amphibole generations can be assigned to the pre-Alpine decompression and two to the Alpine metamorphic P–T evolution. The different amphiboles have distinct NaM4, Ca, K and Cl contents according to different P–T conditions and fluid chemistry. Analysing these mixed amphiboles by the 39Ar−40Ar stepwise heating technique yielded very complex age spectra. However, by correlating amphibole compositions directly obtained from the electron microprobe with the components deduced from the release of Ar isotopes during stepwise heating, obtained ages were consistent with the geological history deduced from field and petrological studies. The two generations of pre-Alpine amphiboles gave distinguishable Triassic to Late Jurassic/Early Cretaceous ages (≈225 and 130–140 Ma respectively). High-NaM4 amphiboles have higher isotopic ages than low-NaM4 ones, in agreement with their decompressional evolution. The exhumation of the Permian crust to mantle section is represented by the former age. The latter age concerns Cl-dominated amphibole related to an Early Cretaceous oceanic stage. For the early Alpine, pressure-dominated metamorphism we obtained a Late Cretaceous age (83–91 Ma). The later, temperature-dominated overprint is significantly younger, as indicated by 39Ar−40Ar ages of 67–73 Ma. These Late Cretaceous ages favour an Adriatic origin for the Malenco unit. Our data show that 39Ar−40Ar dating combined with detailed microprobe analysis can exploit the potential to relate conditions of amphibole formation to their respective ages. Received: 1 March 1999 / Accepted: 18 August 2000  相似文献   

18.
Late Carboniferous (300–290 Ma) calc-alkaline basalts, andesites, and rhyolites typical of volcanic arc settings occur in the intermontane Saar-Nahe basin (SW Germany) within the Variscan orogenic belt. The volcanic rock suite was emplaced under a regime of tensional tectonics during orogenic collapse and its origin has been explained by melting of mantle and crust in the course of limited lithospheric rifting. We report major, trace and rare-earth-element data (REE), and Nd-Pb-Sr-O isotope ratios for a representative sample suite, which are fully consistent with an origin closely related to plate subduction. Major and trace element data define continuous melt differentiation trends from a precursor basaltic magma involving fractional crystallization of olivine, pyroxene, plagioclase, and magnetite typical of magma evolution in a volcanic arc. This finding precludes an origin of the andesitic compositions by mixing of mafic and felsic melts as can be expected in anorogenic settings. The mafic samples have high Mg numbers (Mg# = 65–73), and high Cr (up to 330 ppm) and Ni (up to 200 ppm) contents indicating derivation from a primitive parental melt that was formed in equilibrium with mantle peridotite. We interpret the geochemical characteristics of the near-primary basalts as reflecting their mantle source. The volcanic rocks are characterized by enrichment in the large ion lithophile elements (LILE), negative Nb and Ti, and positive Pb anomalies relative to the neighboring REE, suggesting melting of a subduction-modified mantle. Initial Nd values of −0.7 to −4.6, Pb, and 87Sr/86Sr(t) isotope ratios for mafic and felsic volcanics are similar and indicate partial melting of an isotopically heterogeneous and enriched mantle reservoir. The enrichment in incompatible trace elements and radiogenic isotopes of a precursor depleted mantle may be attributed to addition of an old sedimentary component. The geochemical characteristics of the Saar-Nahe volcanic rocks are distinct from typical post-collisional rock suites and they may be interpreted as geochemical evidence for ongoing plate subduction at the margin of the Variscan orogenic belt not obvious from the regional geologic context. Received: 3 August 1998 / Accepted: 2 January 1999  相似文献   

19.
The major and trace element and Pb–Sr–Nd isotopic compositions of Quaternary mafic lavas from the northern Ryukyu arc provide insights into the nature of the mantle wedge and its tectonic evolution. Beneath the volcanic front in the northern part of the arc, the subducted slab of the Philippine Sea Plate bends sharply and steepens at a depth of ∼80 km. Lavas from the volcanic front have high abundances of large ion lithophile elements and light rare earth elements relative to the high field strength elements, consistent with the result of fluid enrichment processes related to dehydration of the subducting slab. New Pb isotopic data identify two distinct asthenospheric domains in the mantle wedge beneath the south Kyushu and northern Ryukyu arc, which, in a parallel with data from the Lau Basin, appear to reflect mantle with affinities to Indian and Pacific-type mid-ocean ridge basalt (MORB). Indian Ocean MORB-type mantle, contaminated with subducted Ryukyu sediments can account for the variation of lavas erupted on south Kyushu, and probably in the middle Okinawa Trough. In contrast, magmas of the northern Ryukyu volcanic front appear to be derived from sources of Pacific MORB-type mantle contaminated with a sedimentary component. Along-arc variation in the northern Ryukyus reflects increasing involvement of a sedimentary component to the south. Compositions of alkalic basalts from the south Kyushu back-arc resemble intraplate-type basalts erupted in NW Kyushu since ∼12 Ma. We propose that the bending of the subducted slab was either caused by or resulted in lateral migration of asthenospheric mantle, yielding Indian Ocean-type characteristics from a mantle upwelling zone beneath NW Kyushu and the East China Sea. This model also accounts for (1) extensional counter-clockwise crustal rotation (∼4–2 Ma), (2) voluminous andesite volcanism (∼2 Ma), and (3) the recent distinctive felsic magmatism in the south Kyushu region. Received: 30 November 1999 / Accepted: 20 July 2000  相似文献   

20.
Four of the major plutons in the vicinity of the Candelaria mine (470 Mt at 0.95% Cu, 0.22 g/t Au, 3.1 g/t Ag) and a dike–sill system exposed in the Candelaria open pit have been dated with the U–Pb zircon method. The new geochronological data indicate that dacite magmatism around 123 Ma preceded the crystallization of hornblende diorite (Khd) at 118 ± 1 Ma, quartz–monzonite porphyry (Kqm) at 116.3 ± 0.4 Ma, monzodiorite (Kmd) at 115.5 ± 0.4 Ma, and tonalite (Kt) at 110.7 ± 0.4 Ma. The new ages of the plutons are consistent with field relationships regarding the relative timing of emplacement. Plutonism temporally overlaps with the iron oxide Cu–Au mineralization (Re–Os molybdenite ages at ∼115 Ma) and silicate alteration (ages mainly from 114 to 116 and 110 to 112 Ma) in the Candelaria–Punta del Cobre district. The dated dacite porphyry and hornblende diorite intrusions preceded the ore formation. A genetic link of the metallic mineralization with the quartz–monzonite porphyry and/or the monzodiorite is likely. Both of these metaluminous, shoshonitic (high-K) intrusions could have provided energy and contributed fluids, metals, and sulfur to the hydrothermal system that caused the iron oxide Cu–Au mineralization. The age of the tonalite at 110.7 Ma falls in the same range as the late alteration at 110 to 112 Ma. Tonalite emplacement may have sustained existing or driven newly developed hydrothermal cells that caused this late alteration or modified 40Ar/39Ar and K/Ar systematic in some areas.  相似文献   

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