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1.
Erciyes stratovolcano, culminating at 3917 m, is located in the Cappadocian region of central Anatolia. During its evolution, this Quaternary volcano produced pyroclastic deposits and lava flows. The great majority of these products are calc-alkaline in character and they constitute Kocdag and Erciyes sequences by repeated activities. Alkaline activity is mainly observed in the first stages of Kocdag and approximately first-middle stages of Erciyes sequences. Generally, Kocdag and Erciyes stages terminate by pyroclastic activities. The composition of lavas ranges from basalt to rhyolite (48.4–70.5 wt.% SiO2). Calc-alkaline rocks are represented mostly by andesites and dacites. Some compositional differences between alkaline basaltic, basaltic and andesitic rocks were found; while the composition of dacites remain unchanged. All these volcanics are generally enriched in LIL and HFS elements relative to the orogenic values except Rb, Ba, Nb depleted alkaline basalt. 87Sr/86Sr and 143Nd/144Nd isotopic composition of the volcanics range between 0.703344–0.703964, 0.512920–0.512780 for alkaline basalts and change between 0.704322–0.705088, 0.512731–0.512630 for alkaline basaltic rocks whereas calc-alkaline rocks have relatively high Sr and Nd isotopic ratios (0.703434–0.705468, 0.512942–0.512600). Low Rb, Ba, Nb content with high Zr/Nb, low Ba/Nb, La/Yb ratio and low Sr isotopic composition suggest an depleted source component, while high Ba, Rb, Nb content with high La/Yb, Ba/Nb, low Zr/Nb and low 87Sr/86Sr ratios indicate an OIB-like mantle source for the generation of Erciyes alkaline magma. These elemental and ratio variations also indicate that the different mantle sources have undergone different degree of partial melting episodes. The depletion in Ba, Rb, Nb content may be explained by the removal of these elements from the source by slab-derived fluids which were released from pre-collisional subduction, modified the asthenospheric mantle. The chemically different mantle sources interacted with crustal materials to produce calc-alkaline magma. The Ba/Nb increase of calc-alkaline samples indicates the increasing input of crustal components to Erciyes volcanics. Sr and Nd isotopic compositions and elevated LIL and HFS element content imply that calc-alkaline magma may be derived from mixing of an OIB-like mantle melts with a subduction-modified asthenospheric mantle and involvement of crustal materials in intraplate environments.  相似文献   

2.
Mariana Trough basalt (MTB) glasses from zones of of active seafloor volcanism have incompatible trace element compositions which are intermediate between normal MORB and basaltic rocks from the active northern Mariana Island Arc (MIAB). The chemical variation is observed in trace elemental abundances and ratios such as LIL/LIL and LIL/HFS. MTB glasses with high LIL/HFS and Ba/Sm ratios, and low K/Rb, K/Ba, and Sm/Nd ratios have more enriched Sr and Nd isotopic compositions.Comparison of the SrNd isotopic compositions of MTB and MIAB suggests that the source region within the mantle wedge is heterogeneous. The diverse trace element and isotopic compositions of MTB glasses both within and between dredge sites near 18°N imply small-scale source heterogeneity. Correlation between Sm/Nd and143Nd/144Nd of the MTB glasses is interpreted as due to recent binary mixing, rather than closed system evolution of a common homogeneous source. Mixing of melts at or near the source region between a mantle component with long-term LREE and LIL element depletion (MORB-like) and a relatively enriched component with lower integrated143Nd/144Nd (Arc-like) is suggested by trends of the MTB data on ratio-ratio, ratio-element and element-element plots.  相似文献   

3.
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   

4.
Diverse87Sr/86Sr and143Nd/144Nd isotopic compositions among basalts from the Lau Basin (LBB), an active backarc basin in the southwest Pacific, indicate heterogeneity in the underlying mantle. Isotopic compositions display bimodal distributions which are related to geographic location. Type I LBB (87/Sr86Sr 0.70366;143Nd/144Nd 0.51297) include tholeiites from the central basin, Peggy Ridge, and Rochambeau Bank, while Type II basaltic and andesitic glasses from the northeastern portion of the basin, near Niua Fo'ou island, have higher87Sr/86Sr ( 0.7038) and lower 143Nd/144Nd ( 0.51288). Both depleted (e.g. N-MORB) and enriched (e.g. E-MORB) trace element abundances occur among Type I and Type II LBB.Covariation between trace element and isotopic ratios among Type I LBB is consistent with mixing between depleted mantle similar to the source for MORB and relatively enriched peridotite similar to the source for E-MORB. Relative to MORB, uniformly high87Sr/86Sr ( +0.0005) among all Type I LBB for given Nd isotopic compositions ( εNd = +8 to +12) may reflect a lithospheric component, such as ancient recycled altered ocean crust. Type II LBB have SrNd isotopic compositions which are gradational between enriched mantle similar to the source of OIB and a component with distinct Sr isotopic composition such as that observed in Samoan post-erosional basalts. Isotopic and geographic discontinuity between Type I and Type II LBB, and isotopic affinity of Type II and Niua Fo`ou island basalts with those from Samoa suggests that volcanism in the northeastern portion of the basin is tapping deeper mantle beneath the adjoining Pacific plate, as well as Indo-Australian mantle overlying the Pacific lithosphere that is subducted into the Tonga Trench.  相似文献   

5.
《Journal of Geodynamics》2007,43(1):87-100
The petrology and geochemistry of Icelandic basalts have been studied for more than a century. The results reveal that the Holocene basalts belong to three magma series: two sub-alkaline series (tholeiitic and transitional alkaline) and an alkali one. The alkali and the transitional basalts, which occupy the off-rift volcanic zones, are enriched in incompatible trace elements compared to the tholeiites, and have more radiogenic Sr, Pb and He isotope compositions. Compared to the tholeiites, they are most likely formed by partial melting of a lithologically heterogeneous mantle with higher proportions of melts derived from recycled oceanic crust in the form of garnet pyroxenites compared to the tholeiites. The tholeiitic basalts characterise the mid-Atlantic rift zone that transects the island, and their most enriched compositions and highest primordial (least radiogenic) He isotope signature are observed close to the centre of the presumed mantle plume. High-MgO basalts are found scattered along the rift zone and probably represent partial melting of refractory mantle already depleted of initial water-rich melts. Higher mantle temperature in the centre of the Iceland mantle plume explains the combination of higher magma productivity and diluted signatures of garnet pyroxenites in basalts from Central Iceland. A crustal component, derived from altered basalts, is evident in evolved tholeiites and indeed in most basalts; however, distinguishing between contamination by the present hydrothermally altered crust, and melting of recycled oceanic crust, remains non-trivial. Constraints from radiogenic isotope ratios suggest the presence of three principal mantle components beneath Iceland: a depleted upper mantle source, enriched mantle plume, and recycled oceanic crust.The study of glass inclusions in primitive phenocrysts is still in its infancy but already shows results unattainable by other methods. Such studies reveal the existence of mantle melts with highly variable compositions, such as calcium-rich melts and a low-18O mantle component, probably recycled oceanic crust. Future high-resolution seismic studies may help to identify and reveal the relative proportions of different lithologies in the mantle.  相似文献   

6.
~~Characteristics of the mantle source region of sodium lamprophyres and petrogenetic tectonic setting in northeastern Hunan,China~~  相似文献   

7.
Basalts from the Marquesas Archipelago display significant variations according to magmatic type in 143Nd/144Nd (0.512710–0.512925) and 87Sr/86Sr (0.70288–0.70561) suggesting heterogeneities at various scales in the mantle source, with respectively the highest and lowest values in tholeiites compared to alkali basalts. This relationship is the reverse from that observed in the Hawaiian islands. Systematic indications of magma mixing are recognized from the relationships between trace element and isotopic ratios. Tholeiites from Ua Pou Island which have unradiogenic Sr (about 0.7028) plot close to basalts from Tubuai and St. Helena, i.e. distinctly below the main mantle trend in the Nd vs. Sr isotopic diagram. It is suggested that the source of these tholeiites is ancient subducted lithosphere which has suffered previous extraction of liquid with island arc tholeiite composition. The trace element and isotopic data of the basalts from the other Marquesas Islands imply the contamination of an equivalent source by an enriched component. This latter has trace element characteristics of the upper crust.  相似文献   

8.
Alkali basalts and nephelinites from the southern end of the East African Rift (EAR) in northern Tanzania have incompatible trace element compositions that are similar to those of ocean island basalts (OIB). They define a considerable range of Sr, Nd and Pb isotopic compositions (87Sr/86Sr= 0.7035−0.7058,εNd = −5to+3, and206Pb/204Pb= 17.5−21.3), each of which partially overlaps the range found in OIB. However, they occupy a unique position in combined Nd, Sr and Pb isotopic compositional space. Nearly all of the lavas have radiogenic Pb, similar to HIMU with high time-integrated238U/204Pb coupled with unradiogenic Nd (+2 to −5) and radiogenic Sr (>0.704), similar to EMI. This combination has not been observed in OIB and provides evidence that these magmas predominantly acquired their Sr, Nd and Pb in the subcontinental lithospheric mantle rather than in the convecting asthenosphere. These data contrast with compositions for lavas from farther north in the EAR. The Pb isotopic compositions of basalts along the EAR are increasingly radiogenic from north to south, indicating a fundamental change to sources with higher time-integratedU/Pb, closer to the older cratons in the south. An ancient underplated OIB melt component, isolated for about 2 Ga as enriched lithospheric mantle and then remelted, could generate both the trace element and isotopic data measured in the Tanzanian samples. Whereas the radiogenic Pb in Tanzanian lavas requires a source with high time-integratedU/Pb, most continental basalts that are thought to have interacted with the continental lithospheric mantle have unradiogenic Pb, requiring a source with a history of lowU/Pb. Such lowU/Pb is readily accomplished with the addition of subduction-derived components, since the lower averageU/Pb of arc basalts (0.15) relative to OIB (0.36) probably reflects addition of Pb from subducted oceanic crust. If the subcontinental lithosphere is normally characterized by low time-integratedU/Pb it would appear that subduction magmatism is more important than OIB additions in supplying the Pb inventory of the lithospheric mantle. However,U/Pb ratios of xenoliths derived from the continental lithospheric mantle suggest that both processes may be important. This apparent discrepancy could be because xenoliths are not volumetrically representative of the subcontinental lithospheric mantle, or, more likely, that continental lithospheric mantle components in basalts are normally only identified as such when the isotopic ratios are dissimilar from MORB or OIB. Lithospheric enrichment from subaccreted OIB components appears to be more significant than generally recognized.  相似文献   

9.
Fine-scale sampling with alvin and by dredging of the axial ridge in the Mariana Trough between 17°40′N and 18°30°N recovered basalts with isotopic compositions that span the range between N-type MORB and Mariana island arc basalts. There is a local tectonic-morphological control on basalt compositions; MORB-like basalts are found on the deeper ridge segment bounded by the Pagan transform and the ridge offset at 17°56′N, while basalts from the shallower ridge to the north are typical Mariana Trough basalts (MTB) having compositions intermediate between the two endmember rock types. Arc-like basalts were recovered from one site on the axial ridge.The discovery of basalts with such diverse isotopic characteristics from a short (100 km) section of this backarc spreading center constrains the chemical characteristics and distribution of mantle source variability in the Mariana Trough. SrNdPb isotopic variability suggests that the MTB source is heterogeneous on the scale of individual melt batches. The principal component in the MTB mantle source region is depleted peridotite similar to the source of MORB. The enriched component, most evident in the arc-like basalts and intimately mixed in MTB, has isotopic characteristics similar to those observed in the Mariana arc basalts. The isotopic data suggest that source variability for Mariana axial ridge basalts can be explained by mixed arc-like and MORB-like mantle. We hypothesize that there are fragments of old oceanic lithosphere in the backarc source region. This lithospheric component may reflect remnants of subducted seafloor or forearc-volcanic arc mantle that predate rifting in the backarc basin.  相似文献   

10.
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   

11.
Lower Carboniferous lavas from the Midland Valley and adjacent regions of Scotland are mildly alkaline and intraplate in nature. The sequence is dominated by basalt and hawaiite, although mugearite, benmoreite, trachyte and rhyolite are also present. Basic volcanic rocks display the LIL element and LREE enrichment typical of intraplate alkali basalt terrains. Low initial87Sr/86Sr (0.7029–0.7046), high εNd (−0.4 to +5.6) and moderately radiogenic206Pb/204Pb (17.77–18.89) ratios are also comparable with alkali basalts from other continental rifts and oceanic islands.When the Carboniferous lavas are compared with subduction-related lavas of Old Red Sandstone age, erupted in and around the Midland Valley ca. 50 Ma earlier (at 410 Ma) remarkable similarities are apparent. Significant overlap occurs in Nd and Pb isotopic compositions. Sr isotopic compositions are, however, more radiogenic in the older subduction-related lavas. This, combined with high K and Rb concentrations in ORS lavas may be explained by the incorporation of a sediment component derived from the subducted slab, which by Lower Carboniferous times had been lost from the mantle source region by convection. A pronounced negative Nb anomaly in the ORS subduction-related lavas may be explained by the retention of a Nb-bearing phase in the mantle during hydrous melting of the mantle wedge above the subduction zone.Allowing for the effects of the added component from the subducted slab, there appears to be no necessity to invoke separate mantle source regions for the two suites of lavas: both may have been derived from chemically similar portions of mantle. If volcanic arc lavas are derived from the mantle wedge, the implication is that such a source lies at relatively shallow depth within the upper mantle: the same may therefore apply to the Carboniferous continental rift basalts. This evidence, combined with the fact that there is no evident hot-spot trail across the Midland Valley despite a long period of within-plate volcanism and rapid plate movements during the Carboniferous, suggests that the alkali basalt magmatism is not the product of a deep-seated mantle plume. Rather, the volcanism appears to owe more to passive rifting and to diapiric upwelling from a source region within the uppermost mantle.  相似文献   

12.
The Cenozoic volcanic rocks of eastern China are subalkalic to alkalic basalts erupted in an early Tertiary back-arc rift environment and from scattered late Tertiary and Quaternary volcanic centers in a continental area crossed by active faults, driven by subduction of the Pacific plate and the collision of India and Eurasia. Immobile trace elements and major elements conform very well to each other in classification of the 59 rocks for which complete data are reported and they correctly identify the tectonic setting. LIL-element enrichments of the basalts lie between those of P-MORB and ocean island alkalic basalts, and show a secular increase.87Sr/86Sr ratios of basalts vary from 0.7029 to 0.7048. Alkalic basalts are systematically less radiogenic than geographically coextensive and contemporaneous tholeiitic basalts. Increase of radiogenic Sr with increasing crustal thickness and crustal age and with silica enrichment of the magmas suggests crustal contamination but this is inadequate to explain the LIL-element enrichment patterns and variable LIL-element enrichments. The preferred hypothesis is that the alkalic magmas come from a deeper source, with long-term LIL-element depletion and low Rb/Sr ratio but relatively recent LIL-element enrichment. Conversely the tholeiitic magmas are melts of subcontinental mantle lithosphere that is more LIL-element depleted than the alkalic source, at the time of magma genesis, but has had an elevated Rb/Sr ratio for much of its post-consolidation history.  相似文献   

13.
High 4He/3He ratios of 100 000 to 160 000 found at HIMU ocean islands (“high μ,” where μ is the U/Pb ratio) are usually attributed to the presence of recycled oceanic crust in the HIMU mantle source. However, significantly higher 4He/3He ratios are expected in recycled crust after residence in the mantle for periods greater than 1 Ga. In order to better understand the helium isotopic signatures in HIMU basalts, we have measured helium and neon isotopic compositions in a suite of geochemically well-characterized basalts from the Cook–Austral Islands. We observe 4He/3He ratios ranging from 56 000 to 141 000, suggesting the involvement of mantle reservoirs both more and less radiogenic than the mantle source for mid-ocean ridge basalts (MORBs). In addition, we find that the neon isotopic compositions of HIMU lavas extend from the MORB range to compositions less nucleogenic than MORBs. The Cook-Austral HIMU He–Ne isotopic compositions, along with Sr, Nd, Pb, and Os isotopic compositions, indicate that in addition to recycled crust, a relatively undegassed mantle end-member (e.g., FOZO) is involved in the genesis of these basalts. The association of relatively undegassed mantle material with recycled crust provides an explanation for the close geographical association between HIMU lavas and EM (enriched mantle)-type lavas from this island chain: EM-type signatures represent a higher mixing proportion of relatively undegassed mantle material. Mixing between recycled material and relatively undegassed mantle material may be a natural result of entrainment processes and convective stirring in deep mantle.  相似文献   

14.
Basement intersected in DSDP holes 525A, 528 and 527 on the Walvis Ridge consists of submarine basalt flows and pillows with minor intercalated sediments. These holes are situated on the crest and mid and lower northwest flank of a NNW-SSE-trending ridge block which would have closely paralleled the paleo mid-ocean ridge [13, 14]. The basalts were erupted approximately 70 m.y. ago, an age equivalent to that of immediately adjacent oceanic crust in the Angola Basin and consistent with formation at the paleo mid-ocean ridge [14]. The basalt types vary from aphyric quartz tholeiites on the ridge crest to highly plagioclase phyric olivine tholeiites on the ridge flank. These show systematic differences in incompatible trace element and isotopic composition. Many element and isotope ratio pairs form systematic trends with the ridge crest basalts at one end and the highly phyric ridge flank basalts at the other.The low 143Nd/144Nd (0.51238), 206Pb/204Pb (17.54), 208Pb/204Pb (15.47), 208Pb/204Pb (38.14) and high87Sr/86Sr (0.70512) ratios of the ridge crest basalts suggest derivation from an old Nd/Sm-, Rb/Sr- and Pb/U-enriched mantle source. This isotopic signature is similar to that of alkaline basalts on Tristan de Cunha but offset to significantly lower Nd and Pb isotopic ratios. The isotopic ratio trends may be extrapolated beyond the ridge flank basalts with higher143Nd/144Nd (0.51270), 206Pb/204Pb (18.32), 207Pb/204Pb (15.52), 208Pb/204Pb (38.77) and lower 87Sr/86Sr (0.70417) ratios in the direction of increasingly Nd/Sm-, Rb/Sr- and Pb/U-depleted source compositions. These isotopic correlations are equally consistent with mixing od depleted and enriched end member melts or partial melting of an inhomogenous, variably enriched mantle source. However, observe ZrBaNbY interelement relationships are inconsistent with any simple two-component model of magma mixing, as might result from the rise of a lower mantle plume through the upper mantle. Incompatible element and Pb isotopic systematics also preclude extensive involvement of depleted (N-type) MORB material or its mantle sources. In our preferred petrogenetic model the Walvis Ridge basalts were derived by partial melting of mantle similar to an enriched (E-type) MORB source which had become heterogeneous on a small scale due to the introduction of small-volume melts and metasomatic fluids.  相似文献   

15.
Sung-Hi  Choi  Won-Hie  Choe  Jong-Ik  Lee 《Island Arc》2008,17(1):172-182
Abstract   We determined the Sr, Nd and Pb isotopic compositions of basalts recovered from the Antarctic–Phoenix Ridge (APR), a fossil spreading center in the Drake Passage, Antarctic Ocean, in order to understand the nature of the subridge mantle source. There are no known hotspots in close proximity to the site. We observe that small-scale isotopic heterogeneity exists at a shallow level in the subaxial mantle of the APR. Enriched (E-type) mid-ocean ridge basalts (MORB) coexist with normal (N-type) MORB in this region. The E-type basalts are: (i) relatively young compared to the N-type samples; (ii) were erupted after the extinction of the APR; and (iii) have been generated by low-degree partial melting of an enriched mantle source. Extinction of the APR likely caused the extent of partial melting in this region to decrease. We interpret that the geochemically enriched materials dispersed in the ambient depleted mantle were the first fraction to melt to form the E-type MORB.  相似文献   

16.
Os isotope systematics in ocean island basalts   总被引：5，自引：0，他引：5  
New ReOs isotopic results for Os-poor basalts from St. Helena, the Comores, Samoa, Pitcairn and Kerguelen dramatically expand the known range of initial 186Os/187Os ratios in OIBs to values as high as 1.7. In contrast to the Os isotopic uniformity of Os-rich basalts from the HIMU islands of Tubuai and Mangaia found by Hauri and Hart [1], our values for St. Helena span most of the known range of Os isotopic variability in oceanic basalts (initial 187Os/186Os ranges from 1.2 to 1.7). Generation of such radiogenic Os in the mantle requires melting of source materials that contain large proportions of recycled oceanic crust. The very low Os concentrations of most of the basalts analyzed here, however, leave them susceptible to modification via interaction with materials containing radiogenic Os in the near-surface environment. Thus the high 186Os/187Os ratios may result from assimilation of radiogenic Os-rich marine sediments, such as Mn oxides, within the volcanic piles traversed by these magmas en route to the surface. Furthermore, the Os isotopic signatures of Os-rich, olivine-laden OIBs may reflect the accumulation of lithospheric olivine, rather than simply their mantle source characteristics. The extent to which these processes alter the view of the mantle obtained via study of ReOs systematics in oceanic basalts is uncertain. These effects must be quantified before ReOs systematics in OIBs can be used with confidence to investigate the nature of mantle heterogeneity and its causes.  相似文献   

17.
Many researchers have focused on the tectonic evolution of North Qilian Mountains (NQM) since the 1970s[1―7]. However, the tectonic affinity of the an- cient oceanic mantle in early Paleozoic remains in de-bate. Three general explanations for it have been pro- posed. The first one suggests that the ancient ocean was a part of Proto-Tethys, and the tectonic evolution of NQM should be regarded as a portion of the562 Science in China: Series D Earth Sciences Tethyan tectonic domain[1]. …  相似文献   

18.
The nature and restricted range of Dupal-type Sr, Nd and Pb isotopic compositions of Cretaceous kimberlites, tuffaceous diatremes of kamafugitic affinity and carbonatite complexes which intrude the southwestern São Francisco craton margin in Brazil, indicate that these magmas either interacted extensively with, or were derived from, a light rare earth element (LREE) enriched homogeneous lithospheric mantle source isotopically similar to the “enriched mantle I” (EMI) component. The shallow-derived alkalic rocks contain a greater proportion of this EMI-like component, whereas the lower time-averaged Rb/Sr, Nd/Sm and Pb/U ratios of the kimberlites compared to the other rock types suggest mixing of the EMI-like mantle material with variable amounts of mantle with a high 238U/204Pb (HIMU-like) component. Systematic variations in rock types and geochemistry on a regional scale are believed to be indicative of vertical geochemical heterogeneities which are translated into lateral heterogeneities by different depths of melting. It is proposed that HIMU- and EMI-like signatures in particular, are concentrated in laterally extensive but vertically distinctive portions of the mantle beneath the São Francisco craton. The EMI-type signatures appear to be restricted to shallow-derived volcanism, whereas the HIMU-type signatures may originate from a source that started melting deeper in the mantle. The Nd signatures of the EMI-type volcanics follow the evolution path defined by the NeoProterozoic crustal sequences which overlie and flank the craton margin. This suggests that the source of the EMI-type mantle signatures might be related to the tectono-thermal processes which led to the formation and evolution of such crustal sequences. The isotopic similarity of the sources of the studied rocks and of the high-Ti basalts of the northern Paraná basin to those of some Ocean Island Basalts with Dupal signatures in the South Atlantic (viz. in Walvis Ridge) is ascribed to processes by which continental lithosphere became firstly delaminated, and then contaminated a zone of South Atlantic asthenosphere from which hotspot islands have been erupting.  相似文献   

19.
Combined elemental, and Sr and Nd isotopic data are presented for Mesozoic dolerite dikes of Liberia (Africa) which are related to the initial stage of opening of the Atlantic Ocean.The large scatter of both trace element and isotopic data allows the identification of five groups of dolerites which cannot be related to each other by simple processes of mineral fractionation from a common source. On the contrary, the observed chemical and isotopic variation within some dolerites (Groups I and II) may result either from variable degrees of melting of an isotopically heterogeneous source or mixing between enriched and depleted oceanic type mantle. For the other dolerites (Groups III–V) mixing with a third mantle source with more radiogenic Sr and with element ratios characteristic of subduction environments is suggested. This third source is probably the subcontinental lithospheric mantle.Finally, no significant modification by interaction with continental crust is apparent in most of the analyzed samples.  相似文献   

20.
Geology and volcanology of the Edd-Bahar Assoli area (Ethiopia)   总被引：1，自引：0，他引：1  
The paper presents geological and petrological data on one of the alkaline ranges developed along the borders of the Afar depression (Ethiopia). These alkaline ranges occur in a position transversal to the dominant NNW trend of the spreading zones of northern and central Afar which are characterized by magmas of tholeiitic affinity. The Edd-Bahar Assoli volcanic range consists of broad fields of basic lavas and numerous spatter cones outcropping in the area extending between 13°25′ and 13°75′ lat. N and 41°38′ and 42°15′ long. E. The mineralogical assemblage and the chemical data point to an alkaline nature for this range consisting mainly of alkali olivine basalts and basalts tending to hawaiites, the most evolved terms being largely subordinate. Petrologic differences between the Assab, Edd-Bahar Assoli and Erta Ale ranges are shown. The Edd-Bahar Assoli alkaline volcanism would be related to tectonic patterns trending both from NNW-SSE to N-S and from NE-SW to E-W. The supposed similarity with the transverse structure of the equatorial Atlantic ocean would thus not completely be ascertained in this zone. In Afar, the coexistence of an axial volcanism of tholeiitic affinity with an alkaline volcanism at the margin can better be explained by models based upon the upper mantle temperature distribution in a zone under oceanization.  相似文献   

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