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1.
Equilibrium melt trace element contents are calculated from Proterozoic Nain Plutonic Suite (NPS) mafic and anorthositic cumulates, and from plagioclase and orthopyroxene megacrysts. Assumed trapped melt fractions (TMF) <20% generally eliminate all minor phases in most mafic cumulate rocks, reducing them to mixtures of feldspar, pyroxene and olivine, which would represent the high-temperature cumulus assemblage. In anorthosites, TMF <15% generally reduce the mode to a feldspar-only assemblage. All model melts have trace element profiles enriched in highly incompatible elements relative to normal mid-ocean ridge basalt (NMORB); commonly with negative Nb and Th anomalies. Most mafic cumulates yield similar profiles with constant incompatible element ratios, and can be linked through fractional crystallization. High K-La subtypes probably represent crust-contaminated facies. Mafic cumulates are inferred to belong to a tholeiitic differentiation series, variably contaminated by upper and lower crustal components, and probably related to coeval tholeiitic basaltic dyke swarms and lavas in Labrador. Model melts from anorthosites and megacrysts have normalized trace element profiles with steeper slopes than those calculated from mafic cumulates, indicating that mafic cumulates and anorthosites did not crystallize from the same melts. Orthopyroxene megacrysts yield model melts that are more enriched than typical anorthositic model melts, precluding an origin from parental melts. Jotunites have lower K-Rb-Ba-Y-Yb and higher La-Ce than model residues from fractionation of anorthositic model melts, suggesting they are not cosanguineous with them, but provide reasonable fits to evolved mafic cumulate model melts. Incompatible element profiles of anorthositic model melts closely resemble those of crustal melts such as tonalites, with steep Y-Yb-Lu segments that suggest residual garnet in the source. Inversion models yield protoliths similar to depleted lower crustal granulite xenoliths with aluminous compositions, suggesting that the incompatible trace element budget of the anorthosites are derived from remobilization of the lower crust. The similarity of the highly incompatible trace elements and LILE between anorthositic and mafic cumulate model melts suggests that the basalts parental to the mafic cumulates locally assimilated considerable quantities of the same crust that yielded the anorthosites. The reaction between underplating basalt and aluminous lower crust would have forced crystallization of abundant plagioclase, and remobilization of these hybrid plagioclase-rich mushes then produced the anorthosite massifs.  相似文献   

2.
Geochemical compositions of lower crustal and lithospheric mantle xenoliths found in alkali basaltic lavas from the Harrat Ash Shamah volcanic field in southern Syria place constraints on the formation of the Arabian–Nubian Shield in northern Arabia. Compositions of lower crustal granulites are compatible with a cumulate formation from mafic melts and indicate that they are not genetically related to their host rocks. Instead, their depletion in Nb relative to other incompatible elements points to an origin in a Neoproterozoic subduction zone as recorded by an average depleted mantle Sm–Nd model age of 630 Ma.Lithospheric spinel peridotites typically represent relatively low degree (< 10%) partial melting residues of spinel lherzolite with primitive mantle compositions as indicated by major and trace element modelling of clinopyroxene and spinel. The primary compositions of the xenoliths were subsequently altered by metasomatic reactions with low degree silicate melts and possibly carbonatites. Because host lavas lack these signatures any recent reaction of the lherzolites with their host magma can be ruled out. Sm–Nd data of clinopyroxene from Arabian lithospheric mantle lherzolites yield an average age of 640 Ma suggesting that the lithosphere was not replaced since its formation and supporting a common origin of the Arabian lower crustal and lithospheric mantle sections.The new data along with published Arabian mantle xenolith compositions are consistent with a model in which the lithospheric precursor was depleted oceanic lithosphere that was overprinted by metasomatic processes related to subduction and arc accretion during the generation of the Arabian–Nubian Shield. The less refractory nature of the northern Arabian lithosphere as indicated by higher Al, Na and lower Si and Mg contents of clinopyroxenes compared to the more depleted nature of the south Arabian lithospheric mantle, and the comparable low extent of melt extraction suggest that the northern Arabian lithosphere formed in a continental arc system, whereas the lithosphere in the southern part of Arabia appears to be of oceanic arc origin.  相似文献   

3.
Giacomo Corti   《Earth》2009,96(1-2):1-53
The Main Ethiopian Rift is a key sector of the East African Rift System that connects the Afar depression, at Red Sea–Gulf of Aden junction, with the Turkana depression and Kenya Rift to the South. It is a magmatic rift that records all the different stages of rift evolution from rift initiation to break-up and incipient oceanic spreading: it is thus an ideal place to analyse the evolution of continental extension, the rupture of lithospheric plates and the dynamics by which distributed continental deformation is progressively focused at oceanic spreading centres.The first tectono-magmatic event related to the Tertiary rifting was the eruption of voluminous flood basalts that apparently occurred in a rather short time interval at around 30 Ma; strong plateau uplift, which resulted in the development of the Ethiopian and Somalian plateaus now surrounding the rift valley, has been suggested to have initiated contemporaneously or shortly after the extensive flood-basalt volcanism, although its exact timing remains controversial. Voluminous volcanism and uplift started prior to the main rifting phases, suggesting a mantle plume influence on the Tertiary deformation in East Africa. Different plume hypothesis have been suggested, with recent models indicating the existence of deep superplume originating at the core-mantle boundary beneath southern Africa, rising in a north–northeastward direction toward eastern Africa, and feeding multiple plume stems in the upper mantle. However, the existence of this whole-mantle feature and its possible connection with Tertiary rifting are highly debated.The main rifting phases started diachronously along the MER in the Mio-Pliocene; rift propagation was not a smooth process but rather a process with punctuated episodes of extension and relative quiescence. Rift location was most probably controlled by the reactivation of a lithospheric-scale pre-Cambrian weakness; the orientation of this weakness (roughly NE–SW) and the Late Pliocene (post 3.2 Ma)-recent extensional stress field generated by relative motion between Nubia and Somalia plates (roughly ESE–WNW) suggest that oblique rifting conditions have controlled rift evolution. However, it is still unclear if these kinematical boundary conditions have remained steady since the initial stages of rifting or the kinematics has changed during the Late Pliocene or at the Pliocene–Pleistocene boundary.Analysis of geological–geophysical data suggests that continental rifting in the MER evolved in two different phases. An early (Mio-Pliocene) continental rifting stage was characterised by displacement along large boundary faults, subsidence of rift depression with local development of deep (up to 5 km) asymmetric basins and diffuse magmatic activity. In this initial phase, magmatism encompassed the whole rift, with volcanic activity affecting the rift depression, the major boundary faults and limited portions of the rift shoulders (off-axis volcanism). Progressive extension led to the second (Pleistocene) rifting stage, characterised by a riftward narrowing of the volcano-tectonic activity. In this phase, the main boundary faults were deactivated and extensional deformation was accommodated by dense swarms of faults (Wonji segments) in the thinned rift depression. The progressive thinning of the continental lithosphere under constant, prolonged oblique rifting conditions controlled this migration of deformation, possibly in tandem with the weakening related to magmatic processes and/or a change in rift kinematics. Owing to the oblique rifting conditions, the fault swarms obliquely cut the rift floor and were characterised by a typical right-stepping arrangement. Ascending magmas were focused by the Wonji segments, with eruption of magmas at surface preferentially occurring along the oblique faults. As soon as the volcano-tectonic activity was localised within Wonji segments, a strong feedback between deformation and magmatism developed: the thinned lithosphere was strongly modified by the extensive magma intrusion and extension was facilitated and accommodated by a combination of magmatic intrusion, dyking and faulting. In these conditions, focused melt intrusion allows the rupture of the thick continental lithosphere and the magmatic segments act as incipient slow-spreading mid-ocean spreading centres sandwiched by continental lithosphere.Overall the above-described evolution of the MER (at least in its northernmost sector) documents a transition from fault-dominated rift morphology in the early stages of extension toward magma-assisted rifting during the final stages of continental break-up. A strong increase in coupling between deformation and magmatism with extension is documented, with magma intrusion and dyking playing a larger role than faulting in strain accommodation as rifting progresses to seafloor spreading.  相似文献   

4.
The Plio-Quaternary Balhaf–Bir Ali volcanic field (BBAVF) constitutes one of the largest volcanic fields in SE Yemen, covering some 500 km2. It comprises cinder cones complexes associated with vesicular lava flows and scoria–spatter cones. In many places, ultramafic xenoliths are encountered within these volcanics. The explosive volcanism is mainly of alkaline character including alkali olivine basalt, hawaiite and mugearite together with subordinate tuffaceous trachytes. Major, trace and REEs data from the basaltic rocks of the BBAVF are interpreted in terms of a mantle-lithospheric origin in which crustal contribution during the initial stage of rift magmatism has occurred. Magma genesis may have resulted from plume-derived melt introducing into the base of the lithosphere. A mantle plume source is proposed for the Balhaf–Bir Ali basaltic lavas that are here interpreted as having been generated by partial melting of garnet lherzolite in the uppermost part of asthenosphere. The magmatic evolution of Balhaf–Bir Ali volcanic field may be accounted for by the recent models developed for plume structure.  相似文献   

5.
A number of large areas of igneous provinces produced in North Asia in the Late Paleozoic and Early Mesozoic include Siberian and Tarim traps and giant rift systems. Among them, the Central Asian Rift System (CARS) has the most complicated structure, evolved during the longest time, and is a large (3000 × 600 km) latitudinally oriented belt of rift zones extending from Transbaikalia and Mongolia to Middle Asia and including the Tarim traps in western China. CARS was produced in the Late Carboniferous, and its further evolution was associated with the lateral migration of rifting zones; it ended in the Early Jurassic and lasted for approximately 110 Ma. CARS was produced on an active continental margin of the Siberian continent and is noted for largest batholiths, which were emplaced simultaneously with rifting. The batholiths are surrounded by rift zones and compose, together with them, concentrically zoned magmatic areas, with crustal (granitoid) magmatism focused within their central portions, whereas mantle (rift-related) magmatism is predominant in troughs and grabens in peripheral zones. The batholiths show geological and isotopic geochemical evidence that their granitoids were produced by the anatexis of the host rocks at active involvement of mantle magmas. Zonal magmatic areas of the type are viewed as analogues of large igneous provinces formed in the environments characteristic of active continental margins. Large within-plate magmatic provinces in North Asia are thought to have been generated in relation to the overlap of at least two mantle plumes by the Siberian continent during its movement above the hot mantle field. In the continental lithosphere, mantle plumes initiated within-plate magmatic activity and facilitated rifting and the generation of traps and alkaline basite and alkali-salic magmatic associations. Because of the stressed states during collision of various type in the continental margin, the mantle melts did not ascend higher than the lowest crustal levels. The thermal effect of these melts on the crustal rocks induced anatexis and eventually predetermined the generation of the batholiths.  相似文献   

6.
New petrological and isotope–geochemical data are obtained for rocks from the South China Sea shelf zone (Thu, Cu-Lao Re, Hong Jo islands and Katuik–Ile des Cendres island group). These data are correlated with the available published data on the volcanic rocks of the terrestrial part of Vietnam and with data on the basaltoids of the South China Sea, Thailand, and the northern part of Hainan island. Despite the fact that the studied volcanic rocks belong to different structural zones—continental margin, shelf zone, marginal sea—their formation is related to the same type of rift volcanism. Owing to this, the different compositions of the primary magmatic melts indicate, first of all, the heterogeneity of the mantle sources. The uniformity of manifestations of volcanism over the entire studied territory within the age boundaries excludes any zoning at the transition from one structure to another. The leading role of mantle diapirism in the evolution of volcanism throughout the entire Indochina region is shown. It is proposed that the formation of alkaline series and “alkaline” trends of changes in their compositions are consequences of fractionation of melts which originated from an asthenosphere plume chamber. Meanwhile, the tholeiitic magma series are of reactionary origin and “tholeiitic” trends record the processes of mixing of melts generated during melting of the lithospheric mantle with those of a plume chamber. Individual samples show insignificant contamination of melts by crustal material.  相似文献   

7.
Compositionally, high-Nb basalts are similar to HIMU (high U/Pb) ocean island basalts, continental alkaline basalts and alkaline lavas formed above slab windows. Tertiary alkaline basaltic lavas from eastern Jamaica, West Indies, known as the Halberstadt Volcanic Formation have compositions similar to high-Nb basalts (Nb > 20 ppm). The Halberstadt high-Nb basalts are divided into two compositional sub-groups where Group 1 lavas have more enriched incompatible element concentrations relative to Group 2. Both groups are derived from isotopically different spinel peridotite mantle source regions, which both require garnet and amphibole as metasomatic residual phases. The Halberstadt geochemistry demonstrates that the lavas cannot be derived by partial melting of lower crustal ultramafic complexes, metasomatised mantle lithosphere, subducting slabs, continental crust, mantle plume source regions or an upper mantle source region composed of enriched and depleted components. Instead, their composition, particularly the negative Ce anomalies, the high Th/Nb ratios and the similar isotopic ratios to nearby adakite lavas, suggests that the Halberstadt magmas are derived from a compositionally variable spinel peridotite source region(s) metasomatised by slab melts that precipitated garnet, amphibole, apatite and zircon. It is suggested that high-Nb basalts may be classified as a distinct rock type with Nb > 20 ppm, intraplate alkaline basalt compositions, but that are generated in subduction zones by magmatic processes distinct from those that generate other intraplate lavas.  相似文献   

8.
We report a new whole-rock dataset of major and trace element abundances and 87Sr/86Sr–143Nd/144Nd isotope ratios for basaltic to rhyolitic lavas from the Rooiberg continental large igneous province (LIP). The formation of the Paleoproterozoic Rooiberg Group is contemporaneous with and spatially related to the layered intrusion of the Bushveld Complex, which stratigraphically separates the volcanic succession. Our new data confirm the presence of low- and high-Ti mafic and intermediate lavas (basaltic—andesitic compositions) with >?4 wt% MgO, as well as evolved rocks (andesitic—rhyolitic compositions), characterized by MgO contents of <?4 wt%. The high- and low-Ti basaltic lavas have different incompatible trace element ratios (e.g. (La/Sm)N, Nb/Y and Ti/Y), indicating a different petrogenesis. MELTS modelling shows that the evolved lavas are formed by fractional crystallization from the mafic low-Ti lavas at low-to-moderate pressures (~?4 kbar). Primitive mantle-normalized trace element patterns of the Rooiberg rocks show an enrichment of large ion lithophile elements (LILE), rare-earth elements (REE) and pronounced negative anomalies of Nb, Ta, P, Ti and a positive Pb anomaly. Unaltered Rooiberg lavas have negative εNdi (??5.2 to ??9.4) and radiogenic εSri (6.6 to 105) ratios (at 2061 Ma). These data overlap with isotope and trace element compositions of purported parental melts to the Bushveld Complex, especially for the lower zone. We suggest that the Rooiberg suite originated from a source similar to the composition of the B1-magma suggested as parental to the Bushveld Lower Zone, or that the lavas represent eruptive successions of fractional crystallization products related to the ultramafic cumulates that were forming at depth. The Rooiberg magmas may have formed by 10–20% crustal assimilation by the fractionation of a very primitive mantle-derived melt within the upper crust of the Kaapvaal Craton. Alternatively, the magmas represent mixtures of melts from a primitive, sub-lithospheric mantle plume and an enriched sub-continental lithospheric mantle (SCLM) component with harzburgitic composition. Regardless of which of the two scenarios is invoked, the lavas of the Rooiberg Group show geochemical similarities to the Jurassic Karoo flood basalts, implying that the Archean lithosphere strongly affected both of these large-scale melting events.  相似文献   

9.
The Cambrian–lower Ordovician volcanic units of the South Armorican and Occitan domains are analysed in a tectonostratigraphic survey of the French Variscan Belt. The South Armorican lavas consist of continental tholeiites in middle Cambrian–Furongian sequences related to continental break-up. A significant volcanic activity occurred in the Tremadocian, dominated by crustal melted rhyolitic lavas and initial rifting tholeiites. The Occitan lavas are distributed into five volcanic phases: (1) basal Cambrian rhyolites, (2) upper lower Cambrian Mg-rich tholeiites close to N-MORBs but crustal contaminated, (3) upper lower–middle Cambrian continental tholeiites, (4) Tremadocian rhyolites, and (5) upper lower Ordovician initial rift tholeiites. A rifting event linked to asthenosphere upwelling took place in the late early Cambrian but did not evolve. It renewed in the Tremadocian with abundant crustal melting due to underplating of mixed asthenospheric and lithospheric magmas. This main tectono-magmatic continental rift is termed the “Tremadocian Tectonic Belt” underlined by a chain of rhyolitic volcanoes from Occitan and South Armorican domains to Central Iberia. It evolved with the setting of syn-rift coarse siliciclastic deposits overlain by post-rift deep water shales in a suite of sedimentary basins that forecasted the South Armorican–Medio-European Ocean as a part of the Palaeotethys Ocean.  相似文献   

10.
Lavas and pyroclastic products of Nisyros volcano (Aegean arc, Greece) host a wide variety of phenocryst and cumulate assemblages that offer a unique window into the earliest stages of magma differentiation. This study presents a detailed petrographic study of lavas, enclaves and cumulates spanning the entire volcanic history of Nisyros to elucidate at which levels in the crust magmas stall and differentiate. We present a new division for the volcanic products into two suites based on field occurrence and petrographic features: a low-porphyricity andesite and a high-porphyricity (rhyo)dacite (HPRD) suite. Cumulate fragments are exclusively found in the HPRD suite and are predominantly derived from upper crustal reservoirs where they crystallised under hydrous conditions from melts that underwent prior differentiation. Rarer cumulate fragments range from (amphibole-)wehrlites to plagioclase-hornblendites and these appear to be derived from the lower crust (0.5–0.8 GPa). The suppressed stability of plagioclase and early saturation of amphibole in these cumulates are indicative of high-pressure crystallisation from primitive hydrous melts (≥ 3 wt% H2O). Clinopyroxene in these cumulates has Al2O3 contents up to 9 wt% due to the absence of crystallising plagioclase, and is subsequently consumed in a peritectic reaction to form primitive, Al-rich amphibole (Mg# > 73, 12–15 wt% Al2O3). The composition of these peritectic amphiboles is distinct from trace element-enriched interstitial amphibole in shallower cumulates. Phenocryst compositions and assemblages in both suites differ markedly from the cumulates. Phenocrysts, therefore, reflect shallow crystallisation and do not record magma differentiation in the deep arc crust.  相似文献   

11.
An intrinsic feature of Cordillera-style orogenic systems is a spatial trend in the radiogenic isotopic composition of subduction-related magmatism. Magmatism is most isotopically juvenile near the trench and becomes increasingly evolved landward. A compilation of radiogenic isotopic data from the central Andes, U.S. Cordillera, and Tibet (the most well-studied examples of modern and ancient Cordilleran systems) demonstrate such spatial trends are long-lived and persist throughout the life of these continental subduction margins. The consistency of the isotopic trend through time in magmatic products is surprising considering the plethora of orogenic processes that might be expected to alter them. In addition to longevity, spatial isotopic trends encompass a broad spectrum of geochemical compositions that represent diverse petrogenetic and geodynamic processes. The two end-members of the spatial isotopic trends are represented by melts sourced within isotopically juvenile asthenospheric mantle and melts sourced from isotopically evolved continental lithospheric mantle and/or lower crust. Mantle lithosphere generally thins toward the magmatic arc and trench in Cordilleran orogens because sub-lithospheric processes such as delamination, subduction erosion, and subduction ablation, operate to thin or remove the continental mantle lithosphere. With time, magmatic additions may impart the isotopic composition of the mantle source on the lower crust, giving rise to an isotopically homogenous deep lithosphere. The results of this analysis have significant implications for interpreting temporal and spatial shifts in isotopic composition within Cordilleran orogens and suggest that the continental mantle lithosphere may be a significant source of magmatism in orogenic interiors.  相似文献   

12.
The Pb isotope compositions of amphiboles and clinopyroxenesin spinel peridotite and pyroxenite mantle xenoliths from theintra-plate Quaternary volcanic fields of the Eifel province(Germany) are strongly correlated with their Sr–Nd isotopeand trace element compositions. High-temperature anhydrous xenolithsfrom a depth of around 60 km have trace element and Sr–Nd–Pbisotope compositions similar to the depleted source of mid-oceanridge basalts (Depleted MORB Mantle, DMM). Amphibole-bearingxenoliths from shallower depths (<45 km) provide evidencefor three temporally distinct episodes of mantle metasomatismin the subcontinental lithosphere: (1) aqueous fluids from anisotopically enriched (EM-like) mantle reservoir caused amphiboleformation during deformation in the shallow continental lithosphericmantle and may be subduction related, probably associated withthe last major tectonic event that influenced the area (Hercynianorogeny). (2) During a second phase of mantle metasomatism theEM-like lithospheric mantle was affected by melts from an ancient,HIMU-like (high time-integrated µ = 238U/204Pb) mantlesource. The HIMU-like component introduced by these fluids hada much more radiogenic Pb isotope composition than the asthenosphericsource of the widespread Cenozoic magmatism in Europe and maybe linked to reactivation of ancient subducted crustal domainsduring the Hercynian orogeny or to early Cretaceous deep-sourcedmantle plumes. (3) During a brief final stage the heterogeneouslyenriched EM–HIMU subcontinental lithosphere was locallymodified by basaltic melts migrating along fractures and veinsthrough the upper mantle as a consequence of the Cenozoic Eifelvolcanism. Although a DMM component is completely lacking inthe metasomatic fluids of the metasomatic episodes 1 and 2,the vein melts of episode 3 and the Cenozoic Eifel lavas requiremantle sources containing three end-member components (DMM–HIMU–EM).Thus, mobilization of the more depleted mantle material occurredat the earliest in the Tertiary, contemporaneously with thedevelopment of the extensive rift system and main melt generationin Europe. Alternatively, the variety of Sr–Nd–Pbisotope signatures of the metasomatic agents may have been producedby melting of isotopically distinct mantle domains in a heterogeneousuprising mantle plume. KEY WORDS: Eifel; Europe; mantle xenoliths; metasomatism; Pb isotopes  相似文献   

13.
Twelve138Ce/136Ce isotope determinations, 31 Nd isotope analyses, and 31 REE profiles are presented for Tertiary basic to intermediate igneous rocks from the Isle of Skye, NW Scotland. The aim of this work is to precisely identify the contamination mechanisms of basic magmas emplaced through old crust, and to test the effectiveness of Ce isotope analysis as a petrogenetic tool.Combined Ce/Nd isotope analysis enables the modelling of the light REE profiles of the mantle-derived precursors to contaminated lavas, using different crustal end-members, in order to compare these with the magmatic lineage of uncontaminated Skye lavas. The geochemical data support a contamination mechanism involving a granitic melt, produced either by large degree melting of Scourian granulitefacies acid sheets, or (possibly) by melting of intermediate gneiss out of isotopic equilibrium.Basaltic lavas showing strong isotopic contamination effects yield calculated degrees of crustal contamination by large degree granitic melts of ca. 8 or 9% based on Ce and Nd isotopic data respectively. However, for lavas with liquidus temperatures of over 1250° C, the temperature dependence of the degree of contamination is weak.The combination of this evidence with new and published Pb isotope data suggests that the bulk of crustal contamination of the Skye lavas occurred in sill complexes at distinct levels in the crust, rather than during the actual ascent of magma through the crust in dykes. It is suggested on the basis of published fluid dynamic and field evidence that the assimilation of large degree melts of acid gneiss by turbulently flowing magma is more likely than assimilation of small degree disequilibrium melts from more refractory intermediate gneisses.It is concluded that Ce isotope analysis is a viable and useful adjunct to Nd isotope data in petrogenetic studies of continental igneous rocks emplaced through old basement.  相似文献   

14.
The source of hotspot volcanism lies in metasomatized regions of the continental mantle proximal to ancient sutures and failed rifts. Such regions are prone to melting under hotcell conditions on continental rifting, and to erosion into the deeper mantle by asthenospheric flow. In opening basins, rifting parallel to such sutures or failed rifts delaminates and cycles continental mantle into the MORB source. Rifting at some angle to a suture or failed rift generates a hotspot track by preferential melting of the metasomatized mantle as it is cycled toward the rift axis. Continental mantle eroded into the asthenosphere becomes displaced from the continent by net westward drift of the lithosphere relative to the deep mantle to give rise to hotspot volcanism in long-lived ocean basins.  相似文献   

15.
Southern Ethiopian flood basalts erupted in two episodes: the pre-rift Amaro and Gamo transitional tholeiites (45-35 million years) followed by the syn-extensional Getra-Kele alkali basalts (19-11 million years). These two volcanic episodes are distinct in both trace element and isotope ratios (Zr/Nb ratios in Amaro/Gamo lavas fall between 7 and 14, and 3-4.7 in the Getra-Kele lavas whereas 206Pb/204Pb ratios fall between 18-19 and 18.9-20, respectively). The distinctive chemistries of the two eruptive phases record the tapping of two distinct source regions: a mantle plume source for the Amaro/Gamo phase and an enriched continental mantle lithosphere source for the Getra-Kele phase. Isotope and trace element variations within the Amaro/Gamo lavas reflect polybaric fractional crystallisation initiated at high pressures accompanied by limited crustal contamination. We show that clinopyroxene removal at high (0.5 GPa) crustal pressures provides an explanation for the common occurrence of transitional tholeiites in Ethiopia relative to other, typically tholeiitic flood basalt provinces. The mantle plume signature inferred from the most primitive Amaro basalts is isotopically distinct from that contributing to melt generation in central Ethiopian and Afar. This, combined with Early Tertiary plate reconstructions and similarities with Kenyan basalts farther south, lends credence to derivation of these melts from the Kenyan plume rather than the Afar mantle plume. The break in magmatism between 35 and 19 Ma is consistent with the northward movement away from the Kenya plume predicted from plate tectonic reconstructions. In this model the Getra-Kele magmatism is a response to heating of carbonatitically metasomatised lithosphere by the Afar mantle plume beneath southern Ethiopia at this time.  相似文献   

16.
Extrusive and intrusive igneous rocks represent different parts of a magmatic system and ultimately provide complementary information about the processes operating beneath volcanoes. To shed light on such processes, we have examined and quantified the textures and mineral compositions of plutonic and cumulate xenoliths and lavas from Bequia, Lesser Antilles arc. Both suites contain assemblages of iddingsitized olivine, plagioclase, clinopyroxene and spinel with rare orthopyroxene and ilmenite. Mineral zoning is widespread, but more protracted in lavas than xenoliths. Plagioclase cores and olivine have high anorthite (An?≤?98) and low forsterite (Fo?≤?84) compositions respectively, implying crystallisation from a hydrous mafic melt that was already fractionated. Xenolith textures range from adcumulate to orthocumulate with variable mineral crystallisation sequences. Textural criteria are used to organize the xenoliths into six groups. Amphibole, notably absent from lavas, is a common feature of xenoliths, together with minor biotite and apatite. Bulk compositions of xenoliths deviate from the liquid line of descent of lavas supporting a cumulate origin with varying degrees of reactive infiltration by evolved hydrous melts, preserved as melt inclusions in xenolith crystals. Volatile saturation pressures in melt inclusions indicate cumulate crystallization over a 162–571 MPa pressure range under conditions of high dissolved water contents (up to 7.8 wt% H2O), consistent with a variety of other thermobarometric estimates. Phase assemblages of xenoliths are consistent with published experimental data on volatile-saturated low-magnesium and high-alumina basalts and basaltic andesite from the Lesser Antilles at pressures of 200–1000 MPa, temperatures of 950–1050 °C and dissolved H2O contents of 4–7 wt%. Once extracted from mid-crustal mushes, residual melts ascend to higher levels and undergo H2O-saturated crystallization in shallow, pre-eruptive reservoirs to form phenocrysts and glomerocrysts. The absence of amphibole from lavas reflects instability at low pressures, whereas its abundance in xenoliths testifies to its importance in mid-crustal differentiation processes. A complex, vertically extensive (6 to at least 21 km depth) magmatic system is inferred beneath Bequia. Xenoliths represent fragments of the mush incorporated into ascending magmas. The widespread occurrence of evolved melts in the mush, but the absence of erupted evolved magmas, in contrast to islands in the northern Lesser Antilles, may reflect the relative immaturity of the Bequia magmatic system.  相似文献   

17.
Tertiary volcanic rocks from the Westerwald region range frombasanites and alkali basalts to trachytes, whereas lavas fromthe margin of the Vogelsberg volcanic field consist of morealkaline basanites and alkali basalts. Heavy rare earth elementfractionation indicates that the primitive Westerwald magmasprobably represent melts of garnet peridotite. The Vogelsbergmelts formed in the spinel–garnet peridotite transitionregion with residual amphibole for some magmas suggesting meltingof relatively cold mantle. Assimilation of lower-crustal rocksand fractional crystallization altered the composition of lavasfrom the Westerwald and Vogelsberg region significantly. Thecontaminating lower crust beneath the Rhenish Massif has a differentisotopic composition from the lower continental crust beneaththe Hessian Depression and Vogelsberg, implying a compositionalboundary between the two crustal domains. The mantle sourceof the lavas from the Rhenish Massif has higher 206Pb/204Pband 87Sr/86Sr than the mantle source beneath the Vogelsbergand Hessian Depression. The 30–20 Ma volcanism of theWesterwald apparently had the same mantle source as the QuaternaryEifel lavas, suggesting that the magmas probably formed in apulsing mantle plume with a maximum excess temperature of 100°Cbeneath the Rhenish Massif. The relatively shallow melting ofamphibole-bearing peridotite beneath the Vogelsberg and HessianDepression may indicate an origin from a metasomatized portionof the thermal boundary layer. KEY WORDS: continental rift volcanism; basanites; trachytes; assimilation; fractional crystallization; partial melting  相似文献   

18.
High-Mg lavas are characteristic of the mid-Miocene volcanism in Inner Asia.In the Vitim Plateau,small volume high-Mg volcanics erupted at 16-14 Ma.and were followed with voluminous moderate-Mg lavas at 14-13 Ma.In the former unit,we have recorded a sequence of(1) initial basaltic melts,contaminated by crustal material,(2) uncontaminated high-Mg basanites and basalts of transitional(K-Na-K) compositions,and(3) picrobasalts and basalts of K series;in the latter unit a sequence of(1) initial basalts and basaltic andesites of transitional(Na-K-Na) compositions and(2) basalts and trachybasalts of K-Na series.From pressure estimation,we infer that the high-Mg melts were derived from the sublithospheric mantle as deep as 150 km,unlike the moderate-Mg melts that were produced at the shallow mantle.The 14-13 Ma rock sequence shows that initial melts equilibrated in a garnet-free mantle source with subsequently reduced degree of melting garnet-bearing material.No melting of relatively depleted lithospheric material,evidenced by mantle xenoliths,was involved in melting,however.We suggest that the studied transition from high-to moderate-Mg magmatism was due to the mid-Miocene thermal impact on the lithosphere by hot sub-lithospheric mantle material from the Transbaikalian low-velocity(melting) domain that had a potential temperature as high as 1510℃.This thermal impact triggered rifting in the lithosphere of the Baikal Rift Zone.  相似文献   

19.
青山群火山岩是华北克拉通破坏期间最具代表性的地幔或地壳熔融产物,记录了华北深部地质演化的重要信息。本文对胶东青山群基性火山岩进行了40Ar/39Ar定年和岩石地球化学分析,结合前人报道的胶东青山群酸性火山岩资料,发现:(1)基性火山岩喷发年龄为122～113Ma,早于青山群酸性火山岩(110～98Ma);(2)基性和酸性火山岩显示了不同的元素和同位素地球化学特征。岩石成因分析表明,基性火山岩为交代富集地幔部分熔融作用的产物,而酸性火山岩为古老下地壳和中生代底侵岩浆的熔融产物(Ling et al.,2009)。因此,胶东地区青山群火山岩记录了岩浆熔融源区从地幔向下地壳的转变。这与长时间尺度的岩石圈减薄过程中热能由地幔向地壳传递过程相吻合,而不同于地壳拆沉作用所预测的岩浆演化趋势。  相似文献   

20.
The paper presents data on the major-component, trace-element, and mineralogical composition of plutonic rocks, and the composition of their minerals, from the Sierra Leone region in the crest zone of the Mid-Atlantic Ridge between the Strakhov and Bogdanov fracture zones. According to their relations with seafloor structures, the rock associations are subdivided into those of rift valleys and nontransform offset zones. The troctolites and olivine gabbro composing the rift association were produced early in the fractionation course of oceanic tholeiite melt in unstationary and relatively small magmatic chambers. Most rocks beneath the nontransform offset zones crystallized during the long-lasting fractionation of the melt in large chambers hosted in serpentinized peridotites. This part consists of various cumulates, ranging from troctolites to gabbroids. Where deep tectonic detachments entered partly consolidated portions of the chambers, the melt interacted with the wall rocks. Fluid that was generated via the dehydration of serpentine and concentrated hydrophile elements, locally modified the composition of the melt and resulted in amphibole-bearing rocks. Under stress, the intercumulus melts were squeezed into tectonically weakened zones, mixed there, and also interacted with the wall rocks. These mix melts produced (with the participation of fractional crystallization) mineralized Fe-Ti gabbroids. Residual portions of the melts generated most of the diorites and plagiogranites. The high-Na diorites likely crystallized from acid melts that were derived via the partial melting of older gabbroids where aqueous fluids circulated; these fluids were generated by the deserpenitization of the host rocks in tectonized zones cutting through the chambers.  相似文献   

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