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1.
Abstract We present chemical and Sr–Nd–Pb isotopic compositions of three Triassic (226–241 Ma) calc‐alkaline granitoids (the Yeongdeok granite, Yeonghae diorite and Cheongsong granodiorite) and basement rocks in the northern Gyeongsang basin, south‐eastern Korea. These plutons exhibit typical geochemical characteristics of I‐type granitoids generated in a continental magmatic arc. The Yeongdeok and Yeonghae plutons have similar initial Sr, Nd and Pb isotope ratios (87Sr/86Srinitial = 0.7041 ~ 0.7050, ?Nd(t) = 2.3 ~ 4.0, 206Pb/204Pbfeldspar = 18.22 ~ 18.34), but distinct rare earth element patterns, suggesting that the two plutons formed from partial melting of a similar source material at different depths. The Cheongsong pluton has slightly more enriched Sr–Nd–Pb isotopic compositions (87Sr/86Srinitial = 0.7047 ~ 0.7065, ?Nd(t) = 3.9 ~ 2.8, 206Pb/204Pbfeldspar = 18.24 ~ 18.37) than the other two plutons. The Nd model ages of the basement rocks (1.1 ~ 1.4 Ga) are slightly older than those of the plutons (0.6 ~ 1.0 Ga). The initial Sr and Nd isotopic ratios of the plutons can be modeled by the mixing between the mid‐oceanic ridge basalt‐like depleted mantle component and the crustal component represented by basement rocks, which is also supported by Pb isotope data. The Sr and Nd isotope data from granitoids and basement rocks suggest that the Gyeongsang basin, the Hida belt and the inner zone of south‐western Japan share relatively young basement histories (middle Proterozoic), compared with those (early Proterozoic to Archean) of the Gyeonggi and Yeongnam massifs and the Okcheon belt. The Nd isotope data of basement rocks suggest that the Hida belt might be better correlated with the basement of the Gyeongsang basin than the Gyeonggi massif, the Okcheon belt or the Yeongnam massif, although it may represent an older continental margin of East Asia than the Gyeongsang basin considering its slightly older Nd model ages.  相似文献   

2.
Twenty-three volcanic rocks from the Setouchi volcanic belt, southwest Japan, were analyzed for Nd and Sr isotopic compositions for the purpose of examining the genetic relationships among the basalt, high-magnesium andesite (HMA) and evolved porphyritic andesite. The andesites have higher87Sr/86Sr (0.70487–0.70537) and lower143Nd/144Nd (0.512509–0.512731) than the basalts, i.e., 0.70408–0.70468 and 0.512691–0.512830, respectively. This result confirms earlier conclusions obtained from petrologic study that the andesites cannot be fractionation products of basaltic magma but that the andesitic and basaltic magmas were generated independently. On the basis of melting experiments for HMA and basalt, it is inferred that there is an isotopically stratified mantle beneath southwest Japan. Evolved porphyritic andesites have essentially identical Sr and Nd isotopic ratios to HMA and can be derived by fractionation of primary andesitic magma. A model to produce orogenic andesite is proposed on petrologic, experimental and isotopic bases.  相似文献   

3.
Mesozoic volcanic rocks are widespread throughout the Great Xing'an Range of northeastern China. However, there has been limited investigation into the age and petrogenesis of the Mesozoic volcanics in the eastern Great Xing'an Range. According to our research, the volcanic rocks of the Dayangshu Basin, eastern Great Xing'an Range are composed mainly of trachybasalt, basaltic andesite, and basaltic trachyandesite, with minor intermediate–basic pyroclastic rocks. In this study, the geochemistry and geochronology of the Mesozoic volcanic rocks are presented in order to discuss the petrogenesis and tectonic setting of the Ganhe Formation in the Dayangshu Basin. Zircon U–Pb dating by laser ablation inductively coupled plasma–mass spectrometry indicates that the Mesozoic lavas formed during the late Early Cretaceous (114.3–108.8 Ma). This suite of rocks exhibits a range of geochemical signatures indicating subduction‐related genesis, including: (i) calc‐alkaline to high‐K calc‐alkaline major element compositions; (ii) enrichment of large ion lithophile elements (e.g. Rb, Ba, K) and light rare earth elements (LREEs/HREEs =7.33–9.85); and (iii) weak depletion in high field strength elements (e.g. Nb, Ta, Ti). Furthermore, Sr–Nd–Pb isotopic data yield initial 87Sr/86Sr values of 0.70450–0.70463, positive εNd(t) values of +1.8 to +3.3, and a mantle‐derived lead isotope composition. Combined with the regional tectonic evolution, the results of this study suggest that the Ganhe Group lavas are derived from decompression melting of a metasomatized (enriched) lithospheric mantle, related to asthenospheric upwelling, which resulted from lithospheric mantle delamination and produced extension of the continental margin following the subduction of the Paleo‐Pacific Plate.  相似文献   

4.
Geochemical and isotopic analyses (Sr–Nd–Pb) of late Miocene to Quaternary plateau lavas from the Pali Aike and Morro Chico areas (52°S) were undertaken to constrain the melting processes and mantle sources that contributed to magma generation and the geodynamic evolution of southernmost Patagonia, South America. The Pali Aike and Morro Chico lavas are alkaline (Pali Aike, 45–49 wt.% SiO2; 4.3–5.9 wt.% Na2O+K2O) and subalkaline (Morro Chico, 50.5–50.8 wt.% SiO2; 4.0–4.4 wt.% Na2O+K2O), relatively primitive (Pali Aike, 9.5–13.7 wt.% MgO; Morro Chico, 7.6–8.8 wt.% MgO) mafic volcanic rocks that have typical intraplate ocean island basalt‐like signatures. Incompatible trace element ratios and isotopic ratios of the Pali Aike and Morro Chico lavas differ from those of the majority of Neogene southern Patagonian slab window lavas in showing more enriched characteristics and are similar to high‐μ (HIMU)‐like basalts. The rare earth element (REE) modeling to constrain mantle melting percentages suggests that these lavas were produced by low degrees of partial melting (1.0–2.0% for Pali Aike lavas and about 2.6–2.7% for Morro Chico lavas) of a garnet lherzolite mantle source. The major systematic variations of Sr–Nd–Pb isotopes in southern Patagonian lavas are related to geographic location. The Pali Aike and Morro Chico lavas from the southernmost part of Patagonia have lower 87Sr/86Sr and higher 143Nd/144Nd and 206Pb/204Pb ratios, relative to most of the southern Patagonian lavas erupted north of 49.5°S, pointing to a HIMU‐like signature. An isotopically depleted and HIMU‐like asthenospheric domain may have been the main source of magmas in the southernmost part of Patagonia (e.g. Pali Aike, Morro Chico, and Camusu Aike volcanic field), suggesting the presence of a major discontinuity in the isotopic composition of the asthenosphere in southern Patagonia. On the basis of geochemical and isotope data and the available geological and geotectonic reconstructions, a link between the HIMU asthenospheric mantle domain beneath southernmost Patagonia and the HIMU mega‐province of the southwestern Pacific Ocean is proposed.  相似文献   

5.
Abstract Rb–Sr and Sm–Nd isochron ages were determined for whole rocks and mineral separates of hornblende‐gabbros and related metadiabases and quartz‐diorite from Shodoshima, Awashima and Kajishima islands in the Ryoke plutono‐metamorphic belt of the Setouchi area, Southwest Japan. The Rb–Sr and Sm–Nd whole‐rock‐mineral isochron ages for six samples range from 75 to 110 Ma and 200–220 Ma, respectively. The former ages are comparable with the Rb–Sr whole‐rock isochron ages reported from neighboring Ryoke granitic rocks and are thus due to thermal metamorphism caused by the granitic intrusions. On the contrary, the older ages suggest the time of formation of the gabbroic and related rocks. The initial 87Sr/86Sr and 143Nd/144Nd ratios of the gabbroic rocks (0.7070–0.7078 and 0.51217–0.51231 at 210 Ma, respectively) are comparable with those of neighboring late Cretaceous granites and lower crustal granulite xenoliths from Cenozoic andesites in this region. Because the gabbroic rocks are considered to be fragments of the lower crustal materials interlayered in the granulitic lower crust, their isotopic signature has been inherited from an enriched mantle source or, less likely, acquired through interaction with the lower crustal materials. The Sr and Nd isotopic and petrologic evidence leads to a plausible conclusion that the gabbroic rocks have formed as cumulates from hydrous mafic magmas of light rare earth element‐rich (Sm/Nd < 0.233) and enriched isotopic (?Sr > 0 and ?Nd < 0) signature, which possibly generated around 220–200 Ma by partial melting of an upper mantle. We further conclude that they are fragments of refractory material from the lower crust caught up as xenoblocks by granitic magmas, the latter having been generated by partial melting of granulitic lower crustal material around 100 Ma.  相似文献   

6.

The trachyte and basaltic trachyte and intruded granite-porphyry of Gazacun formation of Wuyu Group in central Tibet are Neogene shoshonitic rocks. They are rich in LREE, with a weak to significant Eu negative anomalies. The enriched Rb, Th, U, K, negative HFS elements Nb, Ta, Ti and P, and Sr, Nd and Pb isotope geochemistry suggest that the volcanic rocks of Wuyu Group originated from the partial melting of lower crust of the Gangdese belt, with the involvement of the Tethyan oceanic crust. It implies that the north-subducted Tethys ocean crust have arrived to the lower crust of Gangdese belt and recycled in the Neogene magmatism.

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7.
Latest Oligocene and Early Miocene volcanic rocks occur on the Northland Peninsula, New Zealand, and record the inception of Cenozoic subduction-related volcanism in the North Island that eventually evolved to its present manifestation in the Taupo Volcanic Zone. This NW-striking Northland Arc is continuous with the Reinga Ridge and comprises two parallel belts of volcanic centres ca. 60 km apart. A plethora of tectonic models have been proposed for its origins. We acquired new trace element and Sr–Nd isotope data to better constrain such models. All Northland Arc rocks carry an arc-type trace element signature, however distinct differences exist between rocks of the eastern and western belt. Eastern belt rocks are typically andesites and dacites and have relatively evolved isotope ratios indicating assimilated crustal material, and commonly contain hornblende. Additionally some eastern belt rocks with highly evolved isotope compositions show fractionated REE compositions consistent with residual garnet, and some contain garnetiferous inclusions in addition to schistose crustal fragments. In contrast, western belt rocks are mostly basalts or basaltic andesites with relatively primitive Sr–Nd isotope compositions, do not contain hornblende and show no rare earth element evidence for cryptic amphibole fractionation. Eastern and western belt rocks contain comparable slab-derived fractions of fluid-mobile trace elements and invariably possess an arc signature. Therefore the difference between the belts may be best explained as due to variation in crustal thickness across the Northland Peninsula, where western belt centres erupted onto a thinner crustal section than eastern belt rocks.The consistent arc signature throughout the Northland arc favours an origin in response to an actual, if short-lived subduction event, rather than slab detachment as proposed in some models. No Northland Arc rocks possess a convincing adakite-like composition that might reflect the subduction of very young oceanic lithosphere such as that of the Oligocene South Fiji Basin. Therefore we favour a model in which subduction of old (Cretaceous) lithosphere drove subduction.  相似文献   

8.
Late Miocene (7–9 Ma) basaltic rocks from the Monbetsu‐Kamishihoro graben in northeast Hokkaido have chemical affinities to certain back‐arc basin basalts (referred to herein as Hokkaido BABB). Pb‐, Nd‐ and Sr‐isotopic compositions of the Hokkaido BABB and arc‐type volcanic rocks (11–13 Ma and 4–4.5 Ma) from the nearby region indicate mixing between the depleted mantle and an EM II‐like enriched component (e.g. subducted pelagic sediment) in the magma generation. At a given 87Sr/86Sr, Hokkaido BABB have slightly lower 143Nd/144Nd and slightly less radiogenic 206Pb/204Pb compared with associated arc‐type lavas, but both these suites are difficult to distinguish solely on the basis of isotopic compositions. These isotopic data indicate that while generation of the Hokkaido BABB involves smaller amounts of the EM II‐like enriched component than do associated arc lavas, Hokkaido BABB are isotopically distinct from basalts produced at normal back‐arc basin spreading centers. Instead, northeast Hokkaido BABB are more similar to basalts erupted during the initial rifting stage of back‐arc basins. The Monbetsu‐Kamishihoro graben may have developed in association with extension that formed the Kurile Basin, suggesting that opening of the basin continued until late Miocene (7–9 Ma).  相似文献   

9.
It is well known that the destruction of the North China Carton(NCC) is closely related to subduction of the PaleoPacific slab, but materials recording such subduction has not been identified at the peak time of decratonization. This paper presents data of whole-rock major and trace elements and Sr-Nd-Hf isotopes and zircon U-Pb ages and Hf-O isotopes for Mesozoic volcanic rocks from the Liaodong-Jinan region in the northeastern NCC, in order to trace the subduction-related materials in their source and origin. The Mesozoic volcanic rocks in the Liaodong-Jinan region are mainly composed of two series of rocks, including alkaline basaltic trachyandesite, trachyandesite and trachyte, and subalkaline trachyandesite and andesite. Zircon U-Pb dating yields eruption ages of 129–124 Ma for these rocks. The Early Cretaceous volcanic rocks are all enriched in LILEs(such as Rb, Sr, Ba and Th) and LREEs, depleted in HFSEs(such as Nb, Ta and Ti), indicating that they were originated from mantle sources that had been modified by subducted crustal materials. However, they have relatively heterogeneous and variable isotopic compositions. The alkaline basaltic trachyandesite, trachyandesite and trachyte have enriched whole-rock Sr-Nd-Hf and zircon Hf isotopic compositions and mantle-like δ~(18)O values, suggesting that they were derived from low-degree partial melting of an isotopically enriched lithospheric mantle source. In contrast, the subalkaline trachyandesite and andesite have relatively depleted isotopic compositions with zircon ε_(Hf)(t) values up to +5.2 and heavy zircon O isotopic compositions with δ~(18)O values of +8.1‰ to +9.0‰, indicating that they were originated from a lithospheric mantle source that had been metasomatized by melts/fluids derived from the recycled low-T altered oceanic basalt. All of these geochemical features suggest that the Early Cretaceous volcanic rocks in the Liaodong-Jinan region would result from mixing of mafic magmas with different compositions. Such magmas were originated from the enriched lithospheric mantle and the young metasomatized mantle, respectively, with variable extents of enrichment and depletion in trace elements, radiogenic isotopes and O isotopes. Importantly, the identification of the low-T altered oceanic crust component in the origin of Early Cretaceous volcanic rocks by the zircon Hf-O isotopes provides affirmative isotopic evidence and direct material records for Mesozoic subduction of the Paleo-Pacific slab that induced decratonization of the North China Craton.  相似文献   

10.
The trachyte and basaltic trachyte and intruded granite-porphyry of Gazacun formation of Wuyu Group in central Tibet are Neogene shoshonitic rocks. They are rich in LREE, with a weak to significant Eu negative anomalies. The enriched Rb, Th, U, K, negative HFS elements Nb, Ta, Ti and P, and Sr, Nd and Pb isotope geochemistry suggest that the volcanic rocks of Wuyu Group originated from the partial melting of lower crust of the Gangdese belt, with the involvement of the Tethyan oceanic crust. It implies that the north-subducted Tethys ocean crust have arrived to the lower crust of Gangdese belt and recycled in the Neogene magmatism.  相似文献   

11.
The Yixian Formation at Sihetun in western Liao- ning Province has attracted considerable attention over the last two decades due to discovery of a wide range of well-preserved ‘feathered’ dinosaurs and primitive bird fossils[1―4]. This formation is dominated by vol- canic rocks, with fossil-bearing lacustrine sedimentary rocks at the upper part of the section[4]. The sedimen- tary rocks contain thin layers of tuff. According to previous studies[4], the total thickness of the Yixian Form…  相似文献   

12.
Ar–Ar dating, major and trace element analyses, and Sr–Nd–Pb isotope results of two groups of Lower Cretaceous (erupted at 126 and 119 Ma, respectively) intermediate–felsic lava from the northeastern North China Block (NCB) suggest their derivation from melting of mixtures between the heterogeneous lower crust and underplated basalts. Both groups exhibit high‐K calc‐alkaline to shoshonitic affinities, characterized by light rare earth element (LREE) and large ion lithophile element (LILE) enrichment and variable high field strength element (HFSE, e.g. Nb, Ta and Ti) depletion, and moderately radiogenic Sr and unradiogenic Nd and Pb isotopic compositions. Compared with Group 2, Group 1 rocks have relatively higher K2O and Al2O3/(CaO + K2O + Na2O) in molar ratio, higher HFSE concentrations and lower Nb/Ta ratios, and higher Sr–Nd–Pb isotope ratios. Group 1 rocks were derived from a mixture of an enriched mantle‐derived magma and a lower crust that has developed radiogenic Sr and unradiogenic Nd and Pb isotopic compositions, whereas the Group 2 magmas were melts of another mixture between the same mantle‐derived component and another type of lower crust having even lower Sr, Nd, and Pb isotopic ratios. Shift in source region from Group 1 to Group 2 coincided with a change in melting conditions: hydrous melting of both the underplated basalt and the lower crust produced the earlier high‐Nb and low‐Nb/Ta melts with little or no residual Ti‐rich phases; while the younger low‐Nb and high‐Nb/Ta magmas were melted under a water‐deficient system, in which Ti‐rich phases were retained in the source. Generation of the two groups of intermediate–felsic volcanic rocks was genetically linked with the contemporaneous magma underplating event as a result of lithospheric thinning in the eastern NCB.  相似文献   

13.
Whole‐rock geochemical and Sr–Nd isotopic data are presented for late Miocene volcanic rocks associated with the Chah Zard epithermal Au–Ag deposit in the Urumieh‐Dokhtar Magmatic Arc (UDMA), Iran, to investigate the magma source, petrogenesis and the geodynamic evolution of the study area. The Chah Zard andesitic to rhyolitic volcanic rocks are characterized by significant Large Ion Lithophile Element (LILE) and Light Rare Earth Element (LREE) enrichment coupled with High Field Strength Element (HFSE) depletion. Our geochemical data indicate an adakitic‐like signature for the volcanic rocks (e.g. SiO2 > 62 wt%, Al2O3 > 15 wt%, MgO < 1.5 wt%, Sr/Y > 70, La/Yb > 35, Yb < 1 ppm, and Y < 18 ppm, and no significant Eu anomalies), distinguishing them from the other volcanic rocks of the UDMA. The Chah Zard volcanic rocks have similar Sr and Nd isotopic compositions; the 87Sr/86Sr(i) ratios range from 0.704 902 to 0.705 093 and the εNd(i) values are from +2.33 to +2.70. However, the rhyolite porphyry represents the final stage of magmatism in the area and has a relatively high 87Sr/86Sr ratio (0.705 811). Our data suggest that the andesitic magmas are from a heterogeneous source and likely to result from partial melting of a metasomatized mantle wedge associated with a mixture of subducted oceanic crust and sediment. These melts subsequently underwent fractional crystallization along with minor amounts of crustal assimilation. Our study is consistent with the model that the volcanic host rocks to epithermal gold mineralization in the UDMA are genetically related to late Miocene Neo‐Tethyan slab break‐off beneath Central Iran.  相似文献   

14.
In this study, new geochemical, zircon U–Pb, and Lu–Hf isotopic data are presented for volcanics from the Hadataolegai Formation of the central Great Xing'an Range (GXR) in Northeast China. These new data offer insights into the petrogenesis of the volcanics of the Hadataolegai Formation and the tectonic evolution of the Paleo–Asian Ocean (PAO) and Mongol–Okhotsk Ocean (MOO). These volcanics of the Hadataolegai Formation are divided into andesite‐trachyandesites and dacite‐trachydacites. Zircon U–Pb ages show that the volcanics of the Hadataolegai Formation erupted between 230 Ma and 228 Ma during the Late Triassic, which agrees with recently obtained data. The volcanic rocks in this study have low Y (9.9–21.1 ppm) and Yb (0.78–2.02 ppm) contents, high Sr (444–954 ppm) contents, and slight Eu anomalies (δEu = 0.82 to 0.94), similar to ‘adakite‐like’ rocks. The dacites were formed by fractional crystallization of coeval andesitic magmas. The zircons within the andesite and trachyandesite yield higher positive εHf(t) values (+6.3 to +12.0) and model ages (TDM2) between 860 Ma and 453 Ma, which indicates that the magmas were generated by a newly accreted continental crustal source. Moreover, some of the volcanics are relatively high in MgO contents. These characteristics indicate that the volcanic magmas were derived from the partial melting of delaminated lower crust and mixing with mantle materials. Combining these data with previous studies, we suggest that the magmatism in the central GXR was governed by extension due to the closure of the PAO and the back‐arc extension associated with the southward subduction of the MOO plate (western GXR, near the Erguna Block).  相似文献   

15.
Quan-Ru  Geng  Zhi-Ming  Sun  Gui-Tang  Pan  Di-Cheng  Zhu  Li-Quan  Wang 《Island Arc》2009,18(3):467-487
The well‐studied Mesozoic and Cenozoic volcanic rocks of the Gangdise Terrane, southern Tibet, are widely interpreted to have resulted from subduction of the Neotethys; however, Late Paleozoic volcanic rocks and their tectonic setting remain poorly studied. Based on new geological data, we carried out stratigraphical and geochemical analyses of Permian volcano‐stratigraphic sequences within an east–west‐trending, fault‐bounded zone of uplift in the central Gangdise Terrane. Sedimentary rocks in this area consist of platform carbonates and terrigenous clastic rocks that represent widespread shallow‐marine sedimentary basins developed around northern Gondwana. A regression or tectonic uplift event is recorded in Permian sedimentary rocks that show the local development of fluvial environments. The sedimentary succession contains evidence of two volcanic stages: a period of basaltic extrusions and younger explosive felsic magmatism. The first volcanic stage is Early and Middle Permian in age. Tholeiitic basaltic lavas are exposed around Maizhokunggar (Tangjia) and Lhunzhub in central Gangdise. The Lower Permian basalts are relatively enriched in MgO (4.58–12.19%), whereas the Middle Permian basalts are characterized by high Al2O3 contents (11.75–21.22%). Rocks of both ages are enriched in large‐ion lithophile elements (LILE) and light rare earth elements (LREE), and show pronounced negative Nb and Ta anomalies. Total REE contents and light (LREE)/heavy (HREE) ratios increased from the Early to Middle Permian. Observed variations in initial Sr, Nd, and Pb isotopes (87Sr/86Sri = 0.7013–0.7066, 207Pb/204Pbi = 15.53–15.63, and 208Pb/204Pbi = 38.04–38.64 for a given 206Pb/204Pbi; εNd = +0.69 to ?11.55) can be explained by crustal interaction with mantle sources, as is characteristic of metasomatism by slab‐derived fluids or assimilation and fractional crystallization (AFC) processes during magmatic evolution. The observed geochemical signatures, coupled with stratigraphic constraints, support the hypothesis that an initial arc formed during the Permian due to southward subduction of the Paleotethys, predating the well‐known Mesozoic arc preserved in the Gangdise Terrane.  相似文献   

16.
Nd and Sr isotopic study of volcanic rocks from Japan   总被引：1，自引：0，他引：1  
Two older granitic rocks and some selected Quaternary volcanic rocks from the Japanese Islands were analyzed in a reconnaissance study for the purpose of examining the relationships between Nd and Sr isotopic abundances and the megatectonic structure around the Japanese Islands. Model ages of ～0.9 AE were determined by the Nd and Sr methods on a Paleozoic gneiss which confirms that a relatively ancient acidic basement exists in the Japanese Islands. The Nd and Sr isotopic data show that the Cretaceous granodiorite is the result of partial melting of older crust.The Nd of tholeiitic rocks from the Izu arc gives εNd ranging from 8.3 to 9.3 and with the corresponding εSr from ?14.5 to ?18.5. These results are identical to those found for the Mariana arc. These values are distinctly lower than typical MORB by around 1～2 εu. This difference in εNd between arcs and MORB is attributed to the contribution of oceanic sediments to the partial melts produced during subduction of oceanic crust. The Hakone volcano is clearly confirmed as belonging to an oceanic source by Nd isotopic results.εSr-εNd values of the volcanics from a section along the Fossa Magna show a clear indication that they are a blend of oceanic mantle material and continental crustal material. The crustal component clearly increases in going from south to north. Volcanics across the Northeast Japan arc also show a distinct correlation of εSr-εNd related to the position relative to the active subduction zone but with the opposite trend. These relationships of the present isotopic pattern and the zonal arrangement relative to the subduction zone suggest the former existence of a local spreading center in the Japan Sea.In general there appear to be regular isotopic relationships between the Izu-Mariana oceanic island arc and the continental island arc of Japan which indicates that partially melted or assimilated older continental basement is admixed with young rising oceanic arc magmas.  相似文献   

17.
143Nd/144Nd,87Sr/86Sr and trace element results are reported for volcanic and plutonic rocks of the Aleutian island arc. The Nd and Sr isotopic compositions plot within the mantle array with εNd values of from 6.5 to 9.1 and87Sr/86Sr ratios of from 0.70289 to 0.70342. Basalts have mildly enriched light REE abundances but essentially unfractionated heavy REE abundances, while andesites exhibit a greater degree of light to heavy REE fractionation. Both the basalts and andesites have significant large ion lithophile element to light rare earth element (LILE/LREE) enrichments. Variations in the isotopic compositions of Nd and Sr are not related to the spatial distribution of volcanoes in the arc, nor are they related to temporal differences. εNd and87Sr/86Sr do not correlate with major element compositions but do, however, correlate with certain LILE/LREE ratios (e.g. BaN/LaN). Plutonic rocks have isotropic and trace element characteristics identical to some of the volcanic rocks. Rocks that make up the tholeiitic, calc-alkaline and alkaline series in the Aleutians do not come from isotopically distinct sources, but do exhibit some differing LILE characteristics.Given these elemental and isotopic constraints it is shown that the Aleutian arc magmas could not have been derived directly from homogeneous MORB-type mantle, or fresh or altered MORB subducted beneath the arc. Mixtures of partially altered MORB with deep-sea sediment can in principle account for the isotopic characteristics and most of the observed LILE/LREE enrichments. However, some samples have exceedingly high LILE/LREE enrichments which cannot be accounted for by sediment contamination alone. For these samples a more complex scenario is considered whereby dehydration and partial melting of the subducted slab, containing less than 8% sediment, produces a LILE-enriched (relative to REE) metasomatic fluid which interacts with the overlying depleted mantle wedge. The isotopic and LILE characteristics of the mantle are extremely sensitive to metasomatism by small percentages of added fluid, whereas major elements are not substantially effected, Major element compositions of Aleutian magmas are dominantly controlled by the partial melting of this mantle and subsequent crystal fractionation; whereas isotopic and LILE characteristics are determined by localized mantle heterogeneities.  相似文献   

18.
Abstract The Ryoke Belt is one of the important terranes in the South‐west Japan Arc (SJA). It consists mainly of late Cretaceous granitoid rocks, meta‐sedimentary rocks (Jurassic accretionary complexes) and mafic rocks (gabbros, metadiabases; late Permian–early Jurassic). Initial ?Sr (+ 25– + 59) and ?Nd (? 2.1–?5.9) values of the metadiabases cannot be explained by crustal contamination but reflect the values of the source material. These values coincide with those of island arc basalt (IAB), active continental margin basalt (ACMB) and continental flood basalt (CFB). Spiderdiagrams and trace element chemistries of the metadiabases have CFB‐signature, rather than those of either IAB or ACMB. The Sr–Nd isotope data, trace element and rare earth element chemistries of the metadiabases indicate that they result from partial melting of continental‐type lithospheric mantle. Mafic granulite xenoliths in middle Miocene volcanic rocks distributed throughout the Ryoke Belt were probably derived from relatively deep crust. Their geochemical and Sr–Nd isotopic characteristics are similar to the metadiabases. This suggests that rocks, equivalent geochemically to the metadiabases, must be widely distributed at relatively deep crustal levels beneath a part of the Ryoke Belt. The geochemical and isotopic features of the metadiabases and mafic granulites from the Ryoke Belt are quite different from those of mafic rocks from other terranes in the SJA. These results imply that the Ryoke mafic rocks (metadiabase, mafic granulite) were not transported from other terranes by crustal movement but formed in situ. Sr–Nd isotopic features of late Cretaceous granitoid rocks occurring in the western part of the Japanese Islands are coincident with those of the Ryoke mafic rocks. Such an isotopic relation between these two rocks suggests that a continental‐type lithosphere is widely represented beneath the western part of the Japanese Islands.  相似文献   

19.
143Nd/144Nd,87Sr/86Sr and REE results are reported on volcanic rocks from the islands of Dominica and St. Kitts in the Lesser Antilles. Particular attention is given to the lavas and xenoliths of the Foundland (basalt-andesite) and the Plat Pays (andesite-dacite) volcanic centres on Dominica. Combined major and trace element [2] and isotope results suggest that the bulk of the andesites and dacites on Dominica, and by analogy in the rest of the arc, are produced by fractional crystallisation of basaltic magma. The differences in the erupted products of the two volcanoes do not appear to be related to any significant differences in the source rocks of the magmas.Along the arc87Sr/86Sr ratios range from 0.7037 on St. Kitts, to 0.7041–0.7047 on Dominica, and 0.7039–0.7058 on Grenada [5], and these are accompanied by a parallel increase in K, Sr, Ba and the light REE's. Moreover, compared with LIL-element-enriched and -depleted rocks from MOR and intraplate environments, the basic rocks from the Lesser Antilles are preferentially enriched in alkaline elements (K, Ba, Rb, Sr) relative to less mobile elements such as the rare earths.143Nd/144Nd varies from 0.51308 on St. Kitts, to 0.51286 on Dominica, and 0.51264–0.51308 on Grenada [5], and all these samples have relatively high87Sr/86Sr ratios compared with the main trend of Nd and Sr isotopes for most mantle-derived volcanic rocks. Alkaline elements and87Sr appear to have been introduced from the subducted ocean crust, but the results on other, less mobile elements are more ambiguous — island arc tholeiites (as on St. Kitts) do not appear to contain significant amounts of REE's, Zr, Y, etc., from the subducted oceanic crust, but such a contribution may be present in more LIL-element-enriched calc-alkaline rock types.  相似文献   

20.
Talat  Ahmad  Kabita C.  Longjam  Baishali  Fouzdar  Mike J.  Bickle  Hazel J.  Chapman 《Island Arc》2009,18(1):155-174
The Sakoli Mobile Belt comprises bimodal volcanic rocks that include metabasalt, rhyolite, tuffs, and epiclastic rocks with metapelites, quartzite, arkose, conglomerate, and banded iron formation (BIF). Mafic volcanic rocks are tholeiitic to quartz‐tholeiitic with normative quartz and hypersthene. SiO2 shows a large compositional gap between the basic and acidic volcanics, depicting their bimodal nature. Both the volcanics have distinct geochemical trends but display some similarity in terms of enriched light rare earth element–large ion lithophile element characteristics with positive anomalies for U, Pb, and Th and distinct negative anomalies for Nb, P, and Ti. These characteristics are typical of continental rift volcanism. Both the volcanic rocks show strong negative Sr and Eu anomalies indicating fractionation of plagioclases and K‐feldspars, respectively. The high Fe/Mg ratios for the basic rocks indicate their evolved nature. Whole rock Sm–Nd isochrons for the acidic volcanic rocks indicate an age of crystallization for these volcanic rocks at about 1675 ± 180 Ma (initial 143Nd/144Nd = 0.51017 ± 0.00017, mean square weighted deviate [MSWD] = 1.6). The εNdt (t = 2000 Ma) varies between ?0.19 and +2.22 for the basic volcanic rock and between ?2.85 and ?4.29 for the acidic volcanic rocks. Depleted mantle model ages vary from 2000 to 2275 Ma for the basic and from 2426 to 2777 Ma for the acidic volcanic rocks, respectively. These model ages indicate that protoliths for the acidic volcanic rocks probably had a much longer crustal residence time. Predominantly basaltic magma erupted during the deposition of the Dhabetekri Formation and part of it pooled at crustal or shallower subcrustal levels that probably triggered partial melting to generate the acidic magma. The influence of basic magma on the genesis of acidic magma is indicated by the higher Ni and Cr abundance at the observed silica levels of the acidic magma. A subsequent pulse of basic magma, which became crustally contaminated, erupted as minor component along with the dominantly acidic volcanics during the deposition of the Bhiwapur Formation.  相似文献   

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