首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
The Katla subglacial caldera is one of the most active and hazardous volcanic centres in Iceland as revealed by its historical volcanic activity and recent seismic unrest and magma accumulation. A petrologic and geochemical study was carried out on a suite of mid-Pleistocene to Recent lavas and pyroclastic rocks originated from the caldera. The whole series is characterised by a bimodal composition, including Fe-Ti transitional alkali basalts and mildly alkalic rhyolites. Variations in trace-element composition amongst the basalts and rhyolites show that their chemical differentiation was mainly controlled by fractional crystallisation and possible assimilation. The petrology and chemistry of the few intermediate extrusive rocks show that they were derived from magma mingling or hybridisation. The absence of extrusive rocks of true intermediate magmatic composition and the occurrence of amphibole-bearing felsic xenoliths support the hypothesis of partial melting of the hydrated basalt crust as the main process leading to the generation of rhyolites. The 143Nd/144Nd and 87Sr/86Sr values of Katla volcanic rocks fit the general isotopic array defined by late Quaternary to Recent lavas from Iceland. A few rock specimens are distinguished by low 143Nd/144Nd values suggesting assimilation and mixing of much older crustal material. Despite their similar whole-rock chemical compositions, the postglacial rhyolitic extrusives differ from the felsic xenoliths by their glass composition and the absence of amphibole. This, together with the general chemical trend of volcanic glasses, indicates that the postglacial rhyolitic extrusives were probably derived by a process involving late reheating and partial melting of crustal material by intrusion of basaltic magmas.  相似文献   

2.
Post-glacial tholeiitic basalts from the western Reykjanes Peninsula range from picrite basalts (oldest) to olivine tholeiites to tholeiites (youngest). In this sequence there are large systematic variations in rare earth element (REE) abundances (La/Sm normalized to chondrites ranges from 0.33 in the picrite basalts to 1.25 in the fissure tholeiites) and corresponding variations in 143Nd/144Nd (0.51317 in the picrite basalts to 0.51299 in the fissure tholeiites). The large viaration in 143Nd/144Nd, more than one-third the total range observed in most ocean islands and mid-ocean ridge basalts (MORB), is accompanied by only a small variation in 87Sr/86Sr (0.7031–0.7032). These 87Sr/86Sr ratios are within the range of other Icelandic tholeiites, and distinct from those of MORB.We conclude that the mantle beneath the Reykjanes Peninsula is heterogeneous with respect to relative REE abundances and 143Nd/144Nd ratios. On a time-averaged basis all parts of this mantle show evidence of relative depletion in light REE. Though parts of this mantle have REE abundances and Nd isotope ratios similar to the mantle source of “normal” MORB, 87Sr/86Sr is distinctly higher. Unlike previous studies we find no evidence for chondritic relative REE abundances in the mantle beneath the Reykjanes Peninsula; in fact, the data require significant chemical heterogeneity in the hypothesized mantle plume beneath Iceland, as well as lateral mantle heterogeneity from the Reykjanes Ridge to the Reykjanes Peninsula. The compositional range of the Reykjanes Peninsula basalts is consistent with mixing of magmas produced by different degrees of melting in different parts of the heterogeneous mantle source beneath the Reykjanes Peninsula.  相似文献   

3.
Abstract Whole‐rock chemical and Sr and Nd isotope data are presented for gabbroic and dioritic rocks from a Cretaceous‐Paleogene granitic terrain in Southwest Japan. Age data indicate that they were emplaced in the late Cretaceous during the early stages of a voluminous intermediate‐felsic magmatic episode in Southwest Japan. Although these gabbroic and dioritic rocks have similar major and trace element chemistry, they show regional variations in terms of initial Sr and Nd isotope ratios. Samples from the South Zone have high initial 87Sr/86Sr (0.7063–0.7076) and low initial Nd isotope ratios (?Nd, ?2.5 to ?5.3); whereas those from the North Zone have lower initial 87Sr/86Sr (usually less than 0.7060) and higher Nd isotope ratios (?Nd, ?0.8 to + 3.3). Regional variations in Sr and Nd isotope ratios are similar to those observed in granitic rocks, although gabbroic and dioritic rocks tend to have slightly lower Sr and higher Nd isotope ratios than granitic rocks in the respective zones. Limited variations in Sr and Nd isotope ratios among samples from individual zones may be attributed partly to a combination of upper crustal contamination and heterogeneity of the magma source. Contamination of magmas by upper crustal material cannot, however, explain the observed Sr and Nd isotope variations between samples from the North and South Zones. Between‐zone variations would reflect geochemical difference in magma sources. The gabbroic and dioritic rocks are enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), showing similar normal‐type mid‐ocean ridge basalt (N‐MORB) normalized patterns to arc magmas. Geochronological and isotopic data may suggest that some gabbroic and dioritic rocks are genetically related to high magnesian andesite. Alternatively, mantle‐derived mafic or intermediate rocks which were underplated beneath the crust may be also plausible sources for gabbroic and dioritic rocks. The magma sources (the mantle wedge and lower crust) were isotopically more enriched beneath the South Zone than the North Zone during the Cretaceous‐Paleogene. Sr and Nd isotope ratios of the lower crustal source of the granitic rocks was isotopically affected by mantle‐derived magmas, resulting in similar initial Sr and Nd isotope ratios for gabbroic, dioritic and granitic rocks in each zone.  相似文献   

4.
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   

5.
Neodymium isotope and REE analyses of recent volcanic rocks and spinel lherzolite nodules from the Afar area are reported. The143Nd/144Nd ratios of the volcanic rocks range from 0.51286 to 0.51304, similar to the range recorded from Iceland. However, the87Sr/86Sr ratios display a distinctly greater range (0.70328–0.70410) than those reported from the primitive rocks of Iceland. Whole rock samples and mineral separates from the spinel lherzolite nodules exhibit uniform143Nd/144Nd ratios (ca. 0.5129) but varied87Sr/86Sr ratios in the range 0.70427–0.70528.The SrNd isotope variations suggest that the volcanic rocks may have been produced by mixing between two reservoirs with distinct isotopic compositions. Two possible magma reservoirs in this area are the source which produced the “MORB-type” volcanics in the Red Sea and Gulf of Aden and the anomalous source represented by the nodule suite. The isotopic composition of the volcanics is compatible with mixing between these two reservoirs.It is shown that the anomalous source with a high87Sr/86Sr ratio cannot have been produced by simple processes of partial melting and mixing within normal mantle. Instead the high87Sr/86Sr is equated with a fluid phase. A primitive cognate fluid, subducted seawater or altered oceanic lithosphere may have been responsible for the generation of the source with a high87Sr/86Sr ratio.  相似文献   

6.
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   

7.
Nd and Sr isotopic data on pargasite Iherzolite inclusions, kaersutite megacrysts and their host alkali basalts are presented here to clarify some questions regarding isotopic equilibration during mantle metasomatism and the role of metasomatism in basalt genesis. Five alkali basalts from Nunivak Island within the Aleutian back-arc basin, have87Sr/86Sr ratios of 0.70251–0.70330 and143Nd/144Nd ratios of 0.51289–0.51304. On a Nd versus Sr isotope composition diagram the basalts overlap the fields of MORB and ocean island basalts. Pargasites and mica separated from hydrous nodules found in these basalts have a range in87Sr/86Sr of 0.70256–0.70337 but identical143Nd/144Nd ratios of 0.51302. The metasomatic fluid represented by the pargasite is in isotopic equilibrium, both for Sr and Nd, with the dry mantle as represented by diopside. Eight alkali basalts from the Ataq diatreme, South Yemen, have87Sr/86Sr range of 0.70335–0.70426 and143Nd/144Nd range of 0.51252–0.51305. On a Nd versus Sr isotope composition diagram the basalts from Ataq plot in two distinct fields, (1) within the field of ocean island basalts, and (2) within the range of continental rift basalts but to the left of the Nd-Sr correlation line, somewhat similar to the Skye and Oslo rift basalts. Diopside and pargasite separated from three nodules at Ataq have a more complex history than those at Nunivak. Two nodules contain pargasite and diopside with identical87Sr/86Sr ratios but different143Nd/144Nd ratios. A third nodule contains diopside with a143Nd/144Nd ratio similar to that of other diopsides.The Nunivak basalts are derived from a source with a time-integrated light-REE depletion, in contrast to the light-REE-enriched nature of the basanites. This is best explained by a recent metasomatic event in the source region which increased the LIL element content of the peridotite thus accommodating higher degrees of melting. The Ataq volcanic rocks seem to tap different sources characterized by both light-REE enrichment and depletion, in contrast to the uniform source of the Nunivak basanites. Production of the Ataq basanites is believed to involve anataxis of metasomatically veined continental mantle where local mantle heterogeneities survived the melting event.  相似文献   

8.
Initial87Sr/86Sr ratios have been determined for a representative suite of Upper Cretaceous granodiorites and associated rocks from the Above Rocks composite stock in central Jamaica and the Terre-Neuve pluton in northwestern Haiti. The average initial87Sr/86Sr ratio for severn samples of the Terre-Neuve intrusion is 0.7036, with a range of 0.7026–0.7047. For two samples of the Above Rocks the initial ratios are 0.7033 and 0.7034. A third sample from this intrusive has an initial ratio of 0.7084, which is tenatively attributed to contamination. The initial87Sr/86Sr ratios indicate that neither ancient sialic crust nor sediments carried down a Benioff zone can be the primary source of the granodioritic magma. K/Rb ratios for these rocks range from 178 to 247, which are much lower than the average values (≥1000) for tholeiitic basalts. It is concluded that the magmas originated primarily by melting of downthrust oceanic crust or adjacent mantle material.  相似文献   

9.
87Sr/86Sr and143Nd/144Nd ratios, REE and selected minor and trace elements are presented and compared for present-day volcanic rocks in the Scotia Sea.Tholeiitic basalts from the South Sandwich Islands show widely ranging contents of some lithophile elements, e.g. K2O (0.09–0.55%) and Rb (1.55–14.2 ppm), but fairly constant Na2O and Sr. Total REE contents range from about 4–20 times chondritic abundances with significant light-REE depletion and both positive and negative Eu anomalies. The variations in minor and trace element abundances are consistent with low-pressure fractional crystallization of plagioclase and clinopyroxene but only minor amounts of olivine. The87Sr/86Sr and143Nd/144Nd ratios of the parental magmas are thought be 0.7038–0.7039 and 0.51301–0.51314 respectively, and indicate derivation of at least some87Sr from subducted ocean crust.The back-arc tholeiites in the Scotia Sea have lower87Sr/86Sr ratios (0.7028–0.7033), similar143Nd/144Nd ratios (0.51305) and are variably light-REE-enriched(CeN/YbN= 1.0–1.6). Total REE contents are comparable to those of the South Sandwich Islands tholeiites.  相似文献   

10.
Rosemary  Hickey-Vargas 《Island Arc》2005,14(4):653-665
Abstract Basalts and tonalites dredged from the Amami Plateau in the northern West Philippine Basin have the geochemical characteristics of intraoceanic island arc rocks: low 87Sr/86Sr (0.70297–0.70310), intermediate 143Nd/144Nd (0.51288–0.51292), moderate light rare earth element (LREE) enrichment (La/Yb = 4.1–6.6) and high La/Nb (1.4–4.3). The incremental heating of hornblende from tonalites yielded well‐defined plateaus and 40Ar/39Ar isochron ages of 115.8 ± 0.5 Ma and 117.0 ± 1.1 Ma, while plagioclase yielded disturbed Ar release patterns, with ages ranging from 70 to 112 Ma. Taken together, these results show that the Amami Plateau was formed by subduction‐related magmatism in the Early Cretaceous period, earlier than indicated by prior K/Ar results. The results support tectonic models in which the West Philippine Basin was opened within a complex of Jurassic–Paleocene island arc terranes, which are now scattered in the northern West Philippine Basin, the Philippine Islands and Halmahera. The Amami Plateau tonalites and basalts have higher Sr/Y and lower Y and 87Sr/86Sr compared with younger tonalitic rocks from the northern Kyushu–Palau Ridge and the Tanzawa complex, which were formed by the subduction of the Pacific Plate beneath the Philippine Sea Plate. Based on the geochemical characteristics of the basalts, the Early Cretaceous subduction zone that formed the Amami Plateau may have been the site of slab melting, which suggests that a younger and hotter plate was being subducted at that time. However, the Amami tonalites were probably formed from basaltic magma by fractional crystallization or by partial melting of basaltic arc crust, rather than by melting of the subducted slab.  相似文献   

11.
We have investigated 24 whole rocks and mineral separates of five different rock types from the Cantal shield volcano in France, applying high-precision Rb-Sr techniques. The chemical and isotopic systematics suggest the distinction of two series throughout the different rock classes, one practically uncontaminated, the other seriously influenced by wall rock assimilation. The first group comprises basalts and intermediate rocks with87Sr/86Sr= 0.70340–0.70382. The second group in addition includes rhyolites and the corresponding87Sr/86Sr ratios vary between 0.70421 and 0.71270. The data of mineral separates support the hybridization hypothesis and possibly suggest an original87Sr/86Sr ratio of 0.7028 for the magma source region. Moreover they provide internal isochron ages which place a period of extensive volcanic activity at 8.1–8.8 m.y. ago in accord with K-Ar ages of volcanic rocks from the center of the Cantal volcano.  相似文献   

12.
Andesites from the Peruvian Andes and the Banda arc of Indonesia are characterized by unusually high and variable 87Sr/86Sr ratios. The Banda arc samples, including two cordierite-bearing lavas from Ambon, show a clear positive correlation between 87Sr/86Sr and δ18O. The andesitic rocks have δ18O values that range from 5.6 to 9.2‰. Over that range in δ18O, 87Sr/86Sr increases from 0.7044 to 0.7095. The cordierite-bearing lavas have δ18O values of approximately 15‰ and 87Sr/86Sr ratios of approximately 0.717. The similarity between δ18O values and 87Sr/86Sr ratios in total rocks and separated plagioclase phenocrysts of the Banda arc samples indicates that the measured isotope ratios are primary and have not been affected by secondary, low-temperature post-eruptive alteration. The observed variation between O and Sr isotopic ratios can be modeled by two-component mixing in which one component is of mantle isotopic composition. As the crust beneath the Banda arc is probably oceanic, contamination of the manle component may have resulted from the subduction of either continentally-derived sediments or continental crust. Mixing calculations indicate that the contaminant could have an isotopic composition similar to that observed in the cordierite-bearing lavas.The Andean samples, despite petrographic evidence of freshness, exhibit whole-rock δ18O values significantly higher than those of corresponding plagioclase phenocryst separates, indicating extensive low-temperature post-eruptive alteration. The plagioclase mineral separates show a range of δ18O values between 6.9 and 7.9‰. The 87Sr/86Sr ratios of these same samples are, in most instances, not significantly different from those measured for the whole rock, thus signifying that the phenocrysts and groundmass were in isotopic equilibrium at the time of eruption. Unlike the lavas of the Banda arc, the Andean lavas show no strong positive correlation between 87Sr/86Sr ratios and δ18O values, but instead lower 87Sr/86Sr ratios appear to be associated with higher δ18O values. The δ18O and 87Sr/86Sr values of the Peruvian samples are both slightly higher than those of “normal” island arc volcanics.The small proportions of contaminant implied by the O isotope results seem to preclude continental crustal contamination as a primary cause of high 87Sr/86Sr ratios. The most plausible process that can explain both O and Sr isotope results is one in which sediments of continental origin are partially melted in the subduction zone. These melts rise into overlying mantle material and subsequently participate in the formation of calc-alkaline magmas.If the involvement of a sialic component in the genesis of andesitic magma occurs in the subduction zone, melting of that sialic material signifies temperatures of at least 750–800°C at the top of the subducted lithospheric slab at depths of approximately 150 km. The fact that contamination has apparently occurred in the Banda arc samples without producing any simple widespread correlations between Sr and O isotopic compositions on the one hand and major or trace element abundances on the other, shows that isotopic correlations, possibly including pseudo-isochrons, can be produced by mixing without producing trace element mixing correlations. Because O versus Sr isotope correlations are little affected by processes of partial melting of differentiation, they provide a direct means of testing whether Sr isotopic variations in volcanic rocks are of mantle origin or are due instead to mixing with sialic material.  相似文献   

13.
Seventeen whole-rock samples, generally taken at 25–50 m intervals from 5 to 560 m sub-basement in Hole 504B, drilled in 6.2 m.y. old crust, were analysed for87Sr/86Sr ratios, Sr and Rb concentrations, and18O/16O ratios. Sr isotope ratios for 8 samples from the upper 260 m of the hole range from 0.70287 to 0.70377, with a mean of 0.70320. In the 330–560 m interval, 5 samples have a restricted range of 0.70255–0.70279, with a mean of 0.70266, the average value for fresh mid-ocean ridge basalts (MORB). In the 260–330 m interval, approximately intermediate Sr isotopic ratios are found.δ18O values (‰) range from 6.4 to 7.8 in the upper 260 m, 6.2–6.4 in the 270–320 m interval, and 5.8–6.2 in the 320–560 m interval. The values in the upper 260 m are typical for basalts which have undergone low-temperature seawater alteration, whereas the values for the 320–560 m interval correspond to MORB which have experienced essentially no oxygen isotopic alteration.The higher87Sr/86Sr and18O/16O ratios in the upper part of the hole can be interpreted as the result of a greater overall water/rock ratio in the upper part of the Hole 504B crust than in the lower part. Interaction of basalt with seawater(87Sr/86Sr=0.7091) increased basalt87Sr/86Sr ratios and produced smectitic alteration products which raised whole-rock δ18O values. Seawater circulation in the lower basalts may have been partly restricted by the greater number of relatively impermeable massive lava flows below about 230 m sub-basement. These flows may have helped to seal off lower basalts from through-flowing seawater.  相似文献   

14.
New Sr and Pb isotope data are presented for a selection of lavas and associated coarse-grained blocks from Ascension Island. K-Ar dates for the lavas range up to1.5±0.2Ma. Initial87Sr/86Sr ratios are consistent with earlier measurements and for most rocks are ca. 0.7029, but range up to 0.7135 in the case of the most evolved lavas and blocks. Pb isotope data are also consistent with earlier measurements, but the Pb in two gabbroic blocks is less radiogenic than Pb in the other rocks. It is suggested that these gabbroic blocks crystallized from a magma of tholeiitic composition whose source was similar to that of mid-oceanic ridge basalt whereas the lavas and other blocks crystallized from mildly alkaline magmas derived from a source further from the crest of the Mid-Atlantic Ridge. The high87Sr/86Sr ratios result from contamination of the most silicic magma by radiogenic Sr from pelagic sediments. These data and their interpretation are consistent with the petrological and geochemical observations that the granite blocks are the coarse-grained equivalents of the volcanic suite [11] and not fragments of relict continental material [2,3].  相似文献   

15.
87Sr/86Sr ratios of 15 samples of basalt dredged from Loihi Seamount range from 0.70334 to 0.70368. The basalt types range from tholeiite to basanite in composition and can be divided into six groups on the basis of abundances of K2O, Na2O, Rb and Sr and 87Sr/86Sr ratio. The isotopic data require that the various basalt types be derived from source regions differing in Sr isotopic composition. The Loihi basalts may be produced by mixing of isotopically distinct sources, but the tholeiites and alkalic basalts from Loihi do not show a well-developed inverse trend between Rb/Sr and 87Sr/86Sr that is characteristic of the later stages of Hawaiian volcanoes such as Haleakala and Koolau.  相似文献   

16.
Over the last two decades great strides have been made in characterizing the spatial distribution, time sequence,geochemical characteristics, mantle sources, and magma evolution processes for various igneous rocks in the Early Permian Tarim Large Igneous Province(TLIP). This work has laid a solid foundation for revealing the evolutionary processes and genetic models of large igneous provinces(LIPs). This study systematically demonstrates the two-stage melting model for the TLIP based on our previous research work and predecessor achievements, and highlights the two types of magmatic rocks within the TLIP.The two-stage melting model suggests that the formation of the TLIP is mantle plume related. The early hot mantle plume caused the low-degree partial melting of the lithosphere mantle, while in the later stage, the plume partially melted due to adiabatic uplift and decompression. Therefore, this model carries signatures of both the "Parana" and "Deccan" models in terms of mantle plume activity. During the early stage, the mantle plume provided the heat required for partial melting of sub-continental lithosphere mantle(SCLM), similar to the "Parana Model", while later the plume acted as the main avenue for melting, as in the "Deccan Model". Basalts that erupted in the first stage have higher 87Sr/86 Sr, lower 143Nd/144 Nd ratios, and are enriched in large ion lithophile elements and high field strength elements, indicating a possible origin from the enriched continental lithosphere mantle,similar to the Parana type geochemical features. The basic-ultrabasic intrusive rocks in the second stage exhibit lower 87Sr/86 Sr,higher 143Nd/144 Nd ratios relative to the basalts, consistent with the involvement of a more depleted asthenospheric material,such as a mantle plume, similar to the Deccan type geochemical features. The first stage basalts can be further subdivided into two categories, i.e., Group 1 and Group 2 basalts. Group 2 basalts have lower 87Sr/86 Sr and higher 143Nd/144 Nd ratios than Group 1 basalts, and lie between compositions of the Group 1 basalts and second stage magmatism. Group 2 basalts may be the intermediate component of the TLIP, and the whole TLIP is the result of plume and lithosphere interaction. Developing this petrogenetic model for the TLIP aids in comprehensively understanding its magmatism and deep geological and geodynamic processes. Furthermore, this work enriches the theories describing the origin of large igneous province and mantle plume activity.  相似文献   

17.
Analyses of rim-to-interior samples of fresh tholeiitic pillow basalts, deuterically altered holocrystalline basalts, and older, weathered tholeiitic basalts from the deep sea indicate that 87Sr/86Sr ratios of the older basalts are raised by low temperature interaction with strontium dissolved in sea water. 87Sr/86Sr correlates positively with H2O in these basalts; however, there is little detectable modification of the strontium isotope composition in rocks with H2O contents less than 1%. The isotope changes appear to be a function of relatively long-term, low-temperature weathering, rather than high-temperature or deuteric alteration. Strontium abundance and isotopic data for these rocks suggest that strontium content is only slightly modified by interaction with sea water, and it is a relatively insensitive indicator of marine alteration. Average Rb-Sr parameters for samples of apparently unaltered basalt are: Rb= 1.11ppm; Sr= 132ppm; 87Sr/86Sr= 0.70247.  相似文献   

18.
The Serra Geral (Paraná) continental flood-basalt province of southern Brazil has two main basalt types: low-TiO2 ( 1 wt.%) basalts occupy the southern portion, and high-TiO2 (> 3 wt.%) basalts are largely in the northern part. Low-Ti basalts are less evolved (Mg# 60) and more radiogenic (e.g., 87Sr/86Sr 0.708) than high-Ti basalts (Mg# 35; 87Sr/86Sr 0.705). This is consistent with a model that invokes variable melting of a single mantle source to produce picritic magmas that have relatively lower and higher incompatible element contents. Varying percentages of melting can be related to varying proximity to the early Tristan da Cunha hotspot. The Mg-rich magmas fractionated 60–75% olivine, clinopyroxene, and plagioclase to yield low- or high-Ti flood basalts, assimilating more or less crust in the process. The extent of fractionation and assimilation depended on crustal “warmth” (also tied to location relative to hotspot): (1) above zones of 25% melting, warm crust relatively easily contaminated crystallizing picritic magma that originated by a high degree of melting (i.e., magma with lower incompatible element contents); additionally, high degrees of melting sustained replenishment of magma with low-Ti magma characteristics; (2) above 10% melting zones, cooler crust comparatively restricted assimilation during crystallization (of magma with higher incompatible element contents) and permitted magma evolution to high-Ti derivatives; lesser degrees of melting also limited replenishment magma and thereby allowed greater evolution of existing magma. This model refers all diagnostic geochemical and isotopic features of Serra Geral basalts to percentages of partial melting of an essentially homogeneous mantle material.  相似文献   

19.
The mixing of magmas derived from two major compositional layers in a vertically stratified mantle has been favoured by Zindler et al. [1] in their interpretation of the REE and Sr and Nd isotope data for basalts from the Reykjanes Peninsula. However, a model involving the dynamic partial melting of a regionally homogeneous, veined mantle can also explain the major and trace element data and be reconciled with an alternative interpretation of the time relationships of the lavas to that presented by Jakobsson et al. [2]. Moreover, it is possible to explain the constant87Sr/86Sr but variable143Nd/144Nd ratios of the lavas by this model if the vein and wall rock components of the mantle source have equilibrated for Sr but not for Nd isotopes — a state that has been interpreted for some veined mantle nodules [13]. The model presented also involves more realistic degrees of partial melting than the alternative magma mixing models and satisfactorily explains the erupted volumes of the different magma types found in the area. Interpreting the basalt geochemistry in these terms suggests that Sr isotope ratios of the lavas monitor different scales of heterogeneity in the precursor mantle sources than Nd isotope ratios.  相似文献   

20.
Purico-Chascon is an acid igneous complex less than 1.5 Ma old rising to 5800 m in the North Chilean Andes, and consisting of andesite-dacite cones and dacite domes on an ignimbrite shield. The rocks are subdivided into two groups: those from Chascon appear to exhibit evidence for magma mixing with more basic material now preserved as xenoliths, whereas among those at Purico no xenoliths have been found.87Sr/86Sr=0.7095?0.7081 at Purico, 0.7079?0.7069 at Chascon, and 0.7061-0.7057 in the xenoliths from the Chascon lavas:143Nd/144Nd=0.51222?0.51236 overall. The Purico lavas are characterised by higher SiO2, Rb/Sr,87Sr/86Sr, and REE abundances, and lower Sr/Nd, Sr/Ba and143Nd/144Nd than most Andean igneous suites. There is no indication ofselective crustal contamination of Sr, or any systematic change in isotope ratios during differentiation. Nonetheless the trend of, for example, high Sr/Nd and Sr contents in rocks with low87Sr/86Sr (0.704, Ecuador) to low Sr/Nd and Sr and high SiO2 in rocks with87Sr/86Sr=0.7081?0.7095 at Purico is interpreted as a shift from subduction zone related magmatism to one with greater crustal affinity. The formation of the least evolved Purico lavas (～60%SiO2) is discussed in terms of bulk assimilation of crustal material, mixing between crustal- and mantle-derived magmas, and partial melting of pre-existing crust. Although such models are still extremely primitive, the simplest explanation of the observed chemical variations is that the Purico rocks evolved from parental magmas derived by crustal anatexies. Thermal considerations suggest that such late-stage crustal anatexis is a predictable response to crustal thickening which in the Andes is thought to have taken place during the Cenozoic.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号