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1.
The Tengchong volcanic field comprises numerous Quaternary volcanoes in SW China. The volcanic rocks are grouped into Units 1–4 from the oldest to youngest. Units 1, 3 and 4 are composed of trachybasalt, basaltic trachyandesite and trachyandesite, respectively, and Unit 2 consists of hornblende-bearing dacite. This rock assemblage resembles those of arc volcanic sequences related to oceanic slab subduction. Rocks of Units 1 and 3 contain olivine phenocrysts with Fo contents ranging from 65 to 85 mole%, indicating early fractionation of olivine and chromite prior to the eruption of magma. All the rocks from Units 1, 3 and 4 have very low PGE concentrations, with <0.05 ppb Ru and Rh, <0.2 ppb Pt and Pd, and Ir that is commonly close to, or slightly higher than detection limits (0.001 ppb). The small variations of Pt/Pd ratios (0.4–2.2) are explained by fractionation of silicate and oxide minerals. The 5-fold variations in Cu/Pd ratios (200,000–1,000,000) for the lavas at Tengchong, which do not vary systematically with fractionation, likely reflect retention of variable amounts of residual sulfide in the mantle source. In addition, all the rocks from Units 1, 3 and 4 have primitive mantle-normalized chalcophile element patterns depleted in PGE relative to Cu. Together with very low Cu/Zr ratios (0.06–0.24), these rocks are considered to have undergone variable degrees of sulfide-saturated differentiation in shallow crustal staging magma chambers. Large amounts of olivine and chromite crystallization probably triggered sulfide saturation of magma at depth for Units 1 and 3, whereas crustal contamination was responsible for sulfide saturation during ascent of magma for Unit 4.  相似文献   

2.
Summary Two co-existing plutonic rocks (diorite and granodiorite) were studied from an intrusion of Variscan age in the Raztocna Valley – Nízke Tatry Mountains, Western Carpathians. Geochemical analyses of major and trace elements constrain a volcanic arc as emplacement environment and give the first hints of a mixture of two magmatic end-members: the so-called Prasivá granodiorite and the Raztocna diorite. The 87Sr/86Sr(0) ratios vary between 0.7075 and 0.7118, the ε Nd(0) values range from −1.4 to −5.0. Common Pb isotopes reveal a dominant crustal source with minor influences from a mantle and a lower crustal source. Modelling based on Sr and Nd isotope data and using three component mixing calculations indicates that mixing of 2/3 of upper mantle material with 1/3 upper crustal material can produce the isotopic composition of the Raztocna diorite. Very minor amounts of lower crust were incorporated in the diorite. For the Prasivá granodiorite, the mixing ratio of upper mantle and upper crust is similar, but a lower crustal reservoir contributed about 5–10% of the source material.  相似文献   

3.
The Late Paleozoic volcanic and sedimentary rocks are widespread in the North Tianshan along the north margin of the Yili block. They consist of basalt, basaltic andesite, andesite, trachyandesite, dacite, rhyolite, tuff, and tuffaceous sandstone. According to zircon sensitive high-resolution ion microprobe (SHRIMP) dating, the age of the Late Paleozoic volcanic rocks in Tulasu basin in western part of North Tianshan is constrained to be Early Devonian to Early Carboniferous (417–356 Ma), rather than Early Carboniferous as accepted previously. Geochemical characteristics of the Early Devonian to Early Carboniferous volcanic rocks are similar to those of arc volcanic rocks, which suggest that these volcanic rocks could be the major constituents of a continental arc formed by the southward subduction of North Tianshan Oceanic lithosphere. Geochemical studies indicate that the magma source of the volcanic rocks might be the mantle wedge mixed with subduction fluid, which is geochemically enriched than primitive mantle but depleted than E-MORB. The calculation shows that the basalt could be formed by ∼10% partial melting of subduction fluid modified mantle wedge. Andesites with high initial 87Sr/86Sr (0.7094–0.7104) and negative εNd(t) (−4.45 to −4.79) values reveal the contribution of continental crust to its source. The calculation of assimilation–fractional crystallization (AFC) shows that the fractional crystallization process of the basaltic magma, which was accompanied with assimilation by different degree of continental crust, produced andesite (7–9%), dacite (∼12%) and rhyolite (>20%).  相似文献   

4.
The Afar Depression offers a rare opportunity to study the geodynamic evolution of a rift system from continental rifting to sea floor spreading. This study presents geochemical data for crustal and mantle xenoliths and their alkaline host basalts from the region. The basalts have enriched REE patterns, OIB-like trace element characteristics, and a limited range in isotopic composition (87Sr/86Sr = 0.70336–0.70356, ε Nd = +6.6 to +7.0, and ε Hf = +10.0 to +10.7). In terms of trace elements and Sr–Nd isotopes, they are similar to basalts from the Hanish and Zubair islands in the southern Red Sea and are thus interpreted to be melts from the Afar mantle. The gabbroic crustal xenoliths vary widely in isotope composition (87Sr/86Sr = 0.70437–0.70791, ε Nd = −8.1 to +2.5, and ε Hf = −10.5 to +4.9), and their trace element characteristics match those of Neoproterozoic rocks from the Arabian–Nubian Shield and modern arc rocks, suggesting that the lower crust beneath the Afar Depression contains Neoproterozoic mafic igneous rocks. Ultramafic mantle xenoliths from Assab contain primary assemblages of fresh ol + opx + cpx + sp ± pl, with no alteration or hydrous minerals. They equilibrated at 870–1,040°C and follow a steep geothermal gradient consistent with the tectonic environment of the Afar Depression. The systematic variations in major and trace elements among the Assab mantle xenoliths together with their isotopic compositions suggest that these rocks are not mantle residues but rather series of layered cumulate sills that crystallized from a relatively enriched picritic melt related to the Afar plume that was emplaced before the eruption of the host basalts.  相似文献   

5.
We show here that the Amalaoulaou complex, in the Pan-African belt of West Africa (Gourma, Mali), corresponds to the lower and middle sections of a Neoproterozoic intra-oceanic arc. This complex records a 90–130-Ma-long evolution of magmatic inputs and differentiation above a subducting oceanic slab. Early c. 793 Ma-old metagabbros crystallised at lower crustal or uppermost mantle depths (25–30 km) and have geochemical characteristic of high-alumina basalts extracted from a depleted mantle source slightly enriched by slab-derived sedimentary components ((La/Sm)N < 1; εNd: +5.4–6.2; 87Sr/86Sr: 0.7027–0.7029). In response to crustal thickening, these mafic rocks were recrystallised into garnet-granulites (850–1,000°C; 10–12 kbar) and subject to local dehydration–melting reactions, forming trondhjemititic leucosomes with garnet–clinopyroxene–rutile residues. Slightly after the granulitic event, the arc root was subject to strong HT shearing during partial exhumation (detachment faults/rifting or thrusting), coeval with the emplacement of spinel- and garnet-pyroxenite dykes crystallised from a high-Mg andesitic parental magma. Quartz and hornblende-gabbros (700–660 Ma) with composition typical of hydrous volcanic rocks from mature arcs ((La/Sm)N: 0.9–1.8; εNd: +4.6 to +5.2; 87Sr/86Sr: 0.7028–0.7031) were subsequently emplaced at mid-arc crust levels (~15 km). Trace element and isotopic data indicate that magmas tapped a depleted mantle source significantly more enriched in oceanic sedimentary components (0.2%). Exhumation occurred either in two stages (700–660 and 623 Ma) or in one stage (623 Ma) with a final exhumation of the arc root along cold P-T path (550°C, 6–9 kbar; epidote–amphibolite and greenschist facies conditions) during the main Pan-African collision event (620–580 Ma). The composition of magmas forming the Cryogenian Amalaoulaou arc and the processes leading to intra-arc differentiation are strikingly comparable to those observed in the deep section of exposed Mezosoic oceanic arcs, namely the Kohistan and Talkeetna complex. This evolution of the Amalaoulaou oceanic arc and its accretion towards the West African craton belong to the life and closure of the Pharusian Ocean that eventually led to the formation of the Greater Gondwana supercontinent, a similar story having occurred on the other side of the Sahara with the Mozambique Ocean.  相似文献   

6.
The Mid to Late Miocene intraplate alkaline volcanic suites of western Bohemia are relict of the intensive voluminous volcanism accompanied by large-scale uplift and doming. The association with the uplift of the NE flank of the Cheb–Domažlice Graben (CDG) is uncertain in view of the mostly transpressional tectonics of the graben. The volcanism is most probably of the Ohře/Eger Rift off-rift settings. Two cogenetic volcanic suites have been recognised: (i) silica-saturated to oversaturated consisting of olivine basalt–trachybasalt-(basaltic) trachyandesite–trachyte–rhyolite (13.5 to 10.2 Ma) and (ii) silica-undersaturated (significantly Ne-normative) (melilite-bearing) olivine nephelinite–basanite–tephrite (18.3 to 6.25 Ma). A common mantle source is suggested by similar primitive mantle-normalised incompatible element patterns and Sr–Nd–Pb isotopic compositions for the assumed near-primary mantle-derived compositions of both suites, i.e., olivine basalt and olivine nephelinite. Apparently, they were generated by different degrees of partial melting of a common mantle source, with garnet, olivine and clinopyroxene in the residuum. Negative Rb and K anomalies indicate a residual K-phase (amphibole/phlogopite) and melting of partly metasomatised mantle lithosphere. The evolution of the basanite–olivine basalt–trachybasalt-(basaltic) trachyandesite–trachyte–rhyolite suite suggests the presence of an assimilation–fractional crystallization process (AFC). Substantial fractionation of olivine, clinopyroxene, Fe–Ti oxide, plagioclase/alkali feldspar and apatite accompanied by a significant assimilation of magma en route by crustal material is most evident in evolved member, namely, trachytes and rhyolites. The magmas were probably sourced by both sub-lithospheric and lithospheric partly metasomatised mantle. The evolution of the (melilite-bearing) olivine nephelinite–basanite–tephrite suite is less clear because of its limited extent. Parental magma of both these rock suites is inferred to have originated by low-degree melting of the mantle source initiated at ca. 18 Ma and reflects mixing of asthenosphere-derived melts with isotopically enriched lithospheric melts. The older Oligocene alkaline rocks (29–26 Ma) occur within the Cheb–Domažlice Graben (CDG) locally but are significant in the closely adjacent neighbouring western Ohře Rift. The Sr–Nd–Pb isotopic composition of primitive volcanic rocks of both suites is similar to that of the European Asthenospheric Reservoir (EAR). Initial Pb isotopic data plot partly above the northern hemisphere reference line at radiogenic 206Pb/204Pb ratios of ∼19 to 20, and indicate the presence of a Variscan crustal component in the source.  相似文献   

7.
In the Northern Andes of Ecuador, a broad Quaternary volcanic arc with significant across-arc geochemical changes sits upon continental crust consisting of accreted oceanic and continental terranes. Quaternary volcanic centers occur, from west to east, along the Western Cordillera (frontal arc), in the Inter-Andean Depression and along the Eastern Cordillera (main arc), and in the Sub-Andean Zone (back-arc). The adakite-like signatures of the frontal and main arc volcanoes have been interpreted either as the result of slab melting plus subsequent slab melt–mantle interactions or of lower crustal melting, fractional crystallization, and assimilation processes. In this paper, we present petrographic, geochemical, and isotopic (Sr, Nd, Pb) data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic centers (Pululagua, Pichincha, Ilalo, Chacana, Sumaco) forming a NW–SE transect at about 0° latitude and encompassing the frontal (Pululagua, Pichincha), main (Ilalo, Chacana), and back-arc (Sumaco) chains. All rocks display typical subduction-related geochemical signatures, such as Nb and Ta negative anomalies and LILE enrichment. They show a relative depletion of fluid-mobile elements and a general increase in incompatible elements from the front to the back-arc suggesting derivation from progressively lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab. We observe widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene- and/or amphibole-bearing cumulates and of magma mixing at all frontal and main arc volcanic centers. Within each volcanic center, rocks display correlations between evolution indices and radiogenic isotopes, although absolute variations of radiogenic isotopes are small and their values are overall rather primitive (e.g., εNd = +1.5 to +6, 87Sr/86Sr = 0.7040–0.70435). Rare earth element patterns are characterized by variably fractionated light to heavy REE (La/YbN = 5.7–34) and by the absence of Eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation. We interpret the petrographic, geochemical, and isotopic data as indicating open-system evolution at all volcanic centers characterized by fractional crystallization and magma mixing processes at different lower- to mid-crustal levels as well as by assimilation of mafic lower crust and/or its partial melts. Thus, we propose that the adakite-like signatures of Ecuadorian rocks (e.g., high Sr/Y and La/Yb values) are primarily the result of lower- to mid-crustal processing of mantle-derived melts, rather than of slab melts and slab melt–mantle interactions. The isotopic signatures of the least evolved adakite-like rocks of the active and recent volcanoes are the same as those of Tertiary ”normal” calc-alkaline magmatic rocks of Ecuador suggesting that the source of the magma did not change through time. What changed was the depth of magmatic evolution, probably as a consequence of increased compression induced by the stronger coupling between the subducting and overriding plates associated with subduction of the aseismic Carnegie Ridge.  相似文献   

8.
 Agali–Coimbatore dolerite dykes constitute an important Proterozoic magmatic event that affected the south Indian shield. Rb-Sr whole rock isotope data yield an “errorchron” of 2369±400 Ma (2σ error) which is within error of the reported 2030±65 Ma K-Ar age. The dyke magmas were evolved Fe-rich tholeiitic melts produced by fractionation of clinopyroxene, orthopyroxene and olivine in the initial stages. Plagioclase became a fractionation phase during the latter stages of crystallization. The dykes characteristically have high 87Sr/86Sri (0.703–0.706) and are enriched in large-ion lithophile and light rare earth elements relative to primordial mantle values and show negative Nb anomalies. These compositional characteristics are interpreted as source mantle characteristics whereas some crustal effects are visible in some samples with high initial 87Sr/86Sr. Peridotite with minor hydrous metasomatic phases like amphibole (and phlogopite) within the shallow lithospheric mantle could be a potential source material for the dykes. However, at this stage we cannot convincingly differentiate whether the source of the parent magmas is solely lithospheric or a product of asthenosphere-lithosphere mixing. The δ18O values of the dykes range from +5.2 to +7.2 per mil (vs standard mean oceanic water). Initial Nd isotope values at the time of dyke intrusion (ɛNd at t=2.0 Ga) range from −2.3 to −4.8. Whole rocks define a correlation on an Sm-Nd isochron plot with a slope equivalent to an age of 3.15±0.53 Ga (2σ error); Sm-Nd crustal residence ages average at 2.87 Ga. The isochron age does not appear to be the result of systematic mixing with an older crustal component. These results together with trace element geochemistry suggest that the south Indian mantle lithosphere developed by addition of enriched melts/fluids at about 3.0 Ga synchronously with major crustal gene- ration in the south Indian shield. Received 20 June 1994/Accepted: 17 May 1995  相似文献   

9.
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   

10.
The western Anatolian volcanic province formed during Eocene to Recent times is one of the major volcanic belts in the Aegean–western Anatolian region. We present new chemical (whole-rock major and trace elements, and Sr, Nd, Pb and O isotopes) and new Ar/Ar age data from the Miocene volcanic rocks in the NE–SW-trending Neogene basins that formed on the northern part of the Menderes Massif during its exhumation as a core complex. The early-middle Miocene volcanic rocks are classified as high-K calc-alkaline (HKVR), shoshonitic (SHVR) and ultrapotassic (UKVR), with the Late Miocene basalts being transitional between the early-middle Miocene volcanics and the Na-alkaline Quaternary Kula volcanics (QKV). The early-middle Miocene volcanic rocks are strongly enriched in large ion lithophile elements (LILE), have high 87Sr/86Sr(i) (0.70631–0.71001), low 143Nd/144Nd(i) (0.512145–0.512488) and high Pb isotope ratios (206Pb/204Pb = 18.838–19.148; 207Pb/204Pb = 15.672–15.725; 208Pb/204Pb = 38.904–39.172). The high field strength element (HFSE) ratios of the most primitive early-middle Miocene volcanic rocks indicate that they were derived from a mantle source with a primitive mantle (PM)-like composition. The HFSE ratios of the late Miocene basalts and QKV, on the other hand, indicate an OIB-like mantle origin—a hypothesis that is supported by their trace element patterns and isotopic compositions. The HFSE ratios of the early-middle Miocene volcanic rocks also indicate that their mantle source was distinct from those of the Eocene volcanic rocks located further north, and of the other volcanic provinces in the region. The mantle source of the SHVR and UKVR was influenced by (1) trace element and isotopic enrichment by subduction-related metasomatic events and (2) trace element enrichment by “multi-stage melting and melt percolation” processes in the lithospheric mantle. The contemporaneous SHVR and UKVR show little effect of upper crustal contamination. Trace element ratios of the HKVR indicate that they were derived mainly from lower continental crustal melts which then mixed with mantle-derived lavas (~20–40%). The HKVR then underwent differentiation from andesites to rhyolites via nearly pure fractional crystallization processes in the upper crust, such that have undergone a two-stage petrogenetic evolution.  相似文献   

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