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1.
Two mineralogically and chemically distinct rhyolite magmas (T1 and T3) were syn-erupted from the same conduit system during the 21.9 ka basalt intrusion-triggered Okareka eruption from Tarawera volcano, New Zealand. High spatial resolution U–Th disequilibrium dating of zircon crystals at the ~ 3–5 μm scale reveals a protracted yet discontinuous zircon crystallization history within the magmatic system. Both magma types contain zircon whose interiors predate the eruption by up to 200 ka. The dominant age peak in the T1 magma is ~ 30 ka with subordinate peaks at ~ 45, ~ 75, and ~ 100 ka, whereas the T3 magma has a dominant zircon interior age peak at ~ 90 ka with smaller modes at ~ 35 and ~ 150 ka. These patterns are consistent with isolated pockets of crystallization throughout the evolution of the system. Crystal rim analyses yield ages ranging from within error of the eruption age to at least ~ 90 ka prior to eruption, highlighting that zircon crystallization frequently stalled long before the eruption. Continuous depth profiling from crystal rims inward demonstrates protracted growth histories for individual crystals (up to ~ 100 ka) that were punctuated by asynchronous hiatuses of up to 30 ka in duration. Disparate zircon growth histories can result from localized thermal perturbations caused by mafic intrusions into a silicic reservoir. The crystal age heterogeneity at hand-sample scale requires considerable crystal transport and mixing. We propose that crystal mixing was achieved through buoyancy instabilities caused by mafic magma flow through crystal mush. A terminal pre-eruptive rejuvenation event was capable of mobilizing voluminous melts that erupted, but was too short (< 102–103 years) to result in extensive zircon growth. The contrasting, punctuated zircon histories argue against closed-system fractional crystallization models for silicic magmatism that require protracted cooling times following a mostly liquid starting condition.  相似文献   

2.
Abundant dunite and harzbugite xenoliths are preserved in Early Cretaceous high-Mg# [63–67, where Mg# = molar 100 × Mg/(Mg + Fetot)] diorite intrusions from western Shandong in the North China Craton (NCC). Dunite and some harzburgite xenoliths typically preserve areas of orthopyroxenite (sometimes accompanied by phlogopite) either as veins or as zones surrounding chromite grains. Harzburgite is chiefly composed of olivine, orthopyroxene, minor clinopyroxene and chromian-spinel. High Mg#'s (averaging 91.4) and depletions in Al2O3 and CaO (averaging 0.52 wt.% and 0.29 wt.%, respectively) in harzburgite and dunite xenoliths suggest that they are residues formed by large degrees of polybaric melting. However, olivines and orthopyroxenes from dunite xenoliths spatially associated with orthopyroxenite display lower Mg#'s (i.e., 82–87 and 83–89, respectively), suggesting that an adakitic melt–peridotite reaction has taken place. This is consistent with the production of veined orthopyroxene or orthopyroxene + phlogopite in dunite and some harzburgite xenoliths in response to the introduction of adakitic melt into the previously depleted lithospheric mantle (i.e., harzburgite and dunite xenoliths). The presence of orthopyroxene in veins or as a zones surrounding chromite in peridotite xenoliths is thought to be representative of adakitic melt metasomatism. The dunite and harzbugite xenoliths are relatively rich in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), poor in heavy rare earth elements (HREEs) and high field strength elements (HFSEs), and lack Eu anomalies on chondrite normalized trace element diagrams. The initial 87Sr/86Sr ratios and εNd(t) values for the xenoliths range from 0.7058 to 0.7212 and + 0.18 to ? 19.59, respectively. Taken together, these features, combined with the strong depletion in HFSE and the existence of Archean inherited zircons in the host rocks, suggest that the adakitic melt was derived from the partial melting of early Mesozoic delaminated lower continental crust. The interaction of the adakitic melt with peridotite is responsible for the high-Mg# character of the early Cretaceous diorites in western Shandong.  相似文献   

3.
The Late Permian (260 Ma) Emeishan large igneous province of SW China contains numerous magmatic Fe–Ti oxide deposits. The Fe–Ti oxide deposits occur in the lower parts of evolved layered gabbroic intrusions which are spatially and temporally associated with A-type granitic rocks. The 260 Ma Panzhihua layered gabbroic intrusion hosts one of the largest magmatic Fe–Ti oxide deposits in China and is coeval with a peralkaline A-type granitic pluton. The granite has intruded the overlying Emeishan flood basalts and fed at least one dyke which erupted onto the surface producing columnar jointed trachytes. The presence of syenodiorite between the layered gabbro and granite is evidence for compositional evolution from mafic to intermediate to felsic rocks. The syenodiorites have intermediate to felsic composition with SiO2 = 61 to 65 wt.%, MgO = 0.27 to 0.6 wt.% and CaO = 1.0 to 2.5 wt.% as compared to the granite SiO2 = 65 to 72 wt.%, MgO = 0.1 to 0.4 wt.%, CaO = < 1.0 wt.%. Primitive-mantle-normalized incompatible element plots show corresponding reciprocal patterns between the mafic and felsic rocks. The chondrite-normalized REE patterns show Eu anomalies changing from > 1(Eu/Eu? = 1.1 to 2.6) in the gabbroic intrusion, to < 1 in the syenodiorite (Eu/Eu? = 0.75 to 0.83), granites and trachytes (Eu/Eu? = 0.55–0.87). Previously published εNd(T) values from clinopyroxenes (εNd(T) = + 1.1 to + 3.2) of the gabbroic intrusion match the whole-rock values of the syenodiorite (εNd(T) = + 2.1 to + 2.5), granite and trachyte (εNd(T) = + 2.2 to + 2.9), suggesting that all rock types originated from the same mantle source. MELTS and trace element modeling confirm that all rock types can be generated by fractional crystallization of high-Ti Emeishan basalt. The jump in SiO2 from the gabbro to the syenodiorite is attributed to the en masse crystallization of the Fe–Ti oxides. The geological and geochemical data indicate that fractional crystallization of a common parental magma produced the layered gabbroic intrusion and Fe–Ti oxide deposit, the syenodiorite, granites and trachyte of the Panzhihua region, which thus form a genetically related plutonic-hypabyssal-volcanic complex. Other granite–gabbro complexes in the region likely formed in a similar manner.  相似文献   

4.
The lavas produced by the Timanfaya eruption of 1730–1736 (Lanzarote, Canary Islands) contain a great many sedimentary and metamorphic (metasedimentary), and mafic and ultramafic plutonic xenoliths. Among the metamorphosed carbonate rocks (calc-silicate rocks [CSRs]) are monomineral rocks with forsterite or wollastonite, as well as rocks containing olivine ± orthopyroxene ± clinopyroxene ± plagioclase; their mineralogical compositions are identical to those of the mafic (gabbros) and ultramafic (dunite, wherlite and lherzolite) xenoliths. The 87Sr/86Sr (around 0.703) and 143Nd/144Nd (around 0.512) isotope ratios of the ultramafic and metasedimentary xenoliths are similar, while the 147Sm/144Nd ratios show crustal values (0.13–0.16) in the ultramafic xenoliths and mantle values (0.18–0.25) in some CSRs. The apparent isotopic anomaly of the metamorphic xenoliths can be explained in terms of the heat source (basaltic intrusion) inducing strong isotopic exchange (87Sr/86Sr and 143Nd/144Nd) between metasedimentary and basaltic rocks. Petrofabric analysis also showed a possible relationship between the ultramafic and metamorphic xenoliths.  相似文献   

5.
Two-dimensional crustal velocity models are derived from passive seismic observations for the Archean Karelian bedrock of north-eastern Finland. In addition, an updated Moho depth map is constructed by integrating the results of this study with previous data sets. The structural models image a typical three-layer Archean crust, with thickness varying between 40 and 52 km. P wave velocities within the 12–20 km thick upper crust range from 6.1 to 6.4 km/s. The relatively high velocities are related to layered mafic intrusive and volcanic rocks. The middle crust is a fairly homogeneous layer associated with velocities of 6.5–6.8 km/s. The boundary between middle and lower crust is located at depths between 28 and 38 km. The thickness of the lower crust increases from 5–15 km in the Archean part to 15–22 km in the Archean–Proterozoic transition zone. In the lower crust and uppermost mantle, P wave velocities vary between 6.9–7.3 km/s and 7.9–8.2 km/s. The average Vp/Vs ratio increases from 1.71 in the upper crust to 1.76 in the lower crust.The crust attains its maximum thickness in the south-east, where the Archean crust is both over- and underthrust by the Proterozoic crust. A crustal depression bulging out from that zone to the N–NE towards Kuusamo is linked to a collision between major Archean blocks. Further north, crustal thickening under the Salla and Kittilä greenstone belts is tentatively associated with a NW–SE-oriented collision zone or major shear zone. Elevated Moho beneath the Pudasjärvi block is primarily explained with rift-related extension and crustal thinning at ∼2.4–2.1 Ga.The new crustal velocity models and synthetic waveform modelling are used to outline the thickness of the seismogenic layer beneath the temporary Kuusamo seismic network. Lack of seismic activity within the mafic high-velocity body in the uppermost 8 km of crust and relative abundance of mid-crustal, i.e., 14–30 km deep earthquakes are characteristic features of the Kuusamo seismicity. The upper limit of seismicity is attributed to the excess of strong mafic material in the uppermost crust. Comparison with the rheological profiles of the lithosphere, calculated at nearby locations, indicates that the base of the seismogenic layer correlates best with the onset of brittle to ductile transition at about 30 km depth.We found no evidence on microearthquake activity in the lower crust beneath the Archean Karelian craton. However, a data set of relatively well-constrained events extracted from the regional earthquake catalogue implies a deeper cut-off depth for earthquakes in the Norrbotten tectonic province of northern Sweden.  相似文献   

6.
Kutch (northwest India) experienced lithospheric thinning due to rifting and tholeiitic and alkalic volcanism related to the Deccan Traps K/T boundary event. Alkalic lavas, containing mantle xenoliths, form plug-like bodies that are aligned along broadly east–west rift faults. The mantle xenoliths are dominantly spinel wehrlite with fewer spinel lherzolite. Wehrlites are inferred to have formed by reaction between transient carbonatite melts and lherzolite forming the lithosphere. The alkalic lavas are primitive (Mg# = 64–72) relative to the tholeiites (Mg# = 38–54), and are enriched in incompatible trace elements. Isotope and trace element compositions of the tholeiites are similar to what are believed to be the crustally contaminated Deccan tholeiites from elsewhere in India. In terms of Hf, Nd, Sr, and Pb isotope ratios, all except two alkalic basalts plot in a tight cluster that largely overlap the Indian Ridge basalts and only slightly overlap the field of Reunion lavas. This suggests that the alkalic magmas came largely from the asthenosphere mixed with Reunion-like source that welled up beneath the rifted lithosphere. The two alkalic outliers have an affinity toward Group I kimberlites and may have come from an old enriched (metasomatized) asthenosphere. We present a new model for the metasomatism and rifting of the Kutch lithosphere, and magma generation from a CO2-rich lherzolite mantle. In this model the earliest melts are carbonatite, which locally metasomatized the lithosphere. Further partial melting of CO2-rich lherzolite at about 2–2.5 GPa from a mixed source of asthenosphere and Reunion-like plume material produced the alkalic melts. Such melts ascended along deep lithospheric rift faults, while devolatilizing and exploding their way up through the lithosphere. Tholeiites may have been generated from the main plume head further south of Kutch.  相似文献   

7.
Understanding the processes at the origin of explosive events is crucial for volcanic hazard mitigation, especially during long-lasting eruptions at andesitic volcanoes. This work exposes the case of Tungurahua volcano, whose unrest occurred in 1999. Since this date, the eruptive activity was characterized by low-to moderate explosiveness, including phases with stronger canon-like explosions and regional ash fallout. However, in 2006, a sudden increase of the explosiveness led to pyroclastic flow-forming eruptions on July 14th (VEI 2) and August 16–17th (VEI 3). All magmas emitted from 1999 to 2005, as well as the samples from the 2006 eruptions, have homogeneous bulk-rock andesitic compositions (58–59 wt.% SiO2), and contain the same mineral assemblage consisting of pl + cpx + opx + mag ± ol. However, during the August 16–17th event, the erupted tephra comprise two types of magmas: a dominant, brown andesitic scoria; and scarce, light-grey pumice representing a subordinate, silica-rich juvenile component. For the andesitic magma, thermobarometric data point to magmatic temperatures ranging from 950 to 1015 °C and pressures in the range of 200 to 250 MPa, which corresponds to 7.5–9.5 km below the summit. Disequilibrium textures in plagioclase and pyroxene phenocrysts, particularly thin overgrowth rims, indicate the recharge of this magma body by mafic magma. Between 1999 and 2005, repeated injections from depth fed the intermittent eruptive activity observed while silica-rich melts were produced by in-situ crystallization in the peripheral parts of the reservoir. In April 2006, the recharge of a primitive magma produced strong convection and homogenisation in the reservoir, as well as pressure increase and higher magma ascent rate after seven years of only moderately explosive activity. This work emphasizes the importance of petrological studies in constraining the pre-eruptive magmatic conditions and processes, as a tool for understanding the fundamental causes of the changes in the eruptive dynamism, particularly the occurrence of paroxysmal phases in andesitic systems with open-vent behaviour.  相似文献   

8.
Several crystal-rich, intermediate to silicic magmas erupted at arc volcanoes record a reheating event shortly prior to eruption: they provide evidence for remobilization of crystal mushes by mafic magmas. As hybridization between the mush and the mafic magma is often limited, bulk mixing could not be the dominant process in transferring heat. Conductive heating from a basaltic underplate plays a role, but a few characteristics of these rejuvenated mushes suggest that reheating occurs faster than predicted by conduction.In the upper crust, a process that can transport heat faster than conduction, and still remain chemically nearly imperceptible, is the upward migration of a hot volatile phase (“gas sparging”) that originates in underplated mafic magmas. Using numerical simulations, we quantified the thermal effects of two-phase flow (a silicic melt phase and a H2O–CO2 fluid phase) in the pore space of shallow silicic mushes that have reached their rheological lock-up point (i.e., rigid porous medium, crystallinity ≥ 50 vol.%). Results show that the reheating rates are significantly faster than conduction for volatile fluxes > 0.1 m3/m2 yr. Considering that volatiles can be rapidly exsolved from the underplated mafic magma, these high fluxes can be promptly reached, leading to fast reheating; sill-like batches of mushes with volumes similar to the 1995–present eruption of the Soufrière Hills (Montserrat, W.I.) can be reheated by a few tens of degrees and remobilized within days to weeks. At these high fluxes, a considerable volume of volatiles is needed (similar to the volume of mush being reheated). Large silicic systems (> 100–1000 km3) require unrealistic amounts of volatiles to be reheated in a continuous, high-flux sparging event. Rejuvenation of batholithic mushes therefore requires multiple sparging episodes separated by periods dominated by near-conductive heat transfer at low-flux sparging (< 0.1 m3/m2 yr) and may take up to 100–200 ky.  相似文献   

9.
The 1.0 Ma Kidnappers supereruption (~ 1200 km3 DRE) from Mangakino volcanic centre, Taupo Volcanic Zone, New Zealand, produced a large phreatomagmatic fall deposit followed by an exceptionally widespread ignimbrite. Detailed sampling and analysis of glass shards and mineral phases have been undertaken through a proximal 4.0 m section of the fall deposit, representing the first two-thirds of erupted extra-caldera material. Major and trace element chemistries of glass shards define three distinct populations (types A, B and C), which systematically change in proportion through the fall deposit and are inferred to represent three magma types. Type B glass and biotite first appear at the same level (~ 0.95 m above base) in the fall deposit suggesting later tapping of a biotite-bearing magma. Plagioclase and Fe–Ti oxide compositions show bimodal distributions, which are linked to types A and B glass compositions. Temperature and pressure (T–P) estimates from hornblende and Fe–Ti oxide equilibria from each magma type are similar and therefore the three magma bodies were adjacent, not vertically stacked, in the crust. Most hornblende model T–P estimates range from 770 to 840 °C and 90 to 170 MPa corresponding to storage depths of ~ 4.0–6.5 km. Hornblende model T–P estimates coupled with in situ trace element fingerprinting imply that the magma bodies were individually well mixed, and not stratified. Compositional gaps between the three glass compositional types imply that no mixing between these magmas occurred. We interpret these data, coupled with the systematic changes in shard compositional proportions through the fall deposit, to reflect that three independent melt-dominant bodies of magma contributed large (A, ~ 270 km3), medium (B, ~ 90 km3) and small (C, ~ 40 km3) volumes (as reflected in the fall deposits) and were systematically tapped during the eruption. We propose that the systematic evacuation of the three independent magma bodies implies that there was tectonic triggering and linkage of eruptions. Our results show that supereruptions can be generated by near simultaneous multiple eruptions from independent magma chambers rather than the evacuation of a large single unitary magma chamber.  相似文献   

10.
Boa Vista, the easternmost island in the Cape Verde archipelago, consists of volcanic products, minor intrusions and a thin partial sedimentary cover. The first 15 age results from 40Ar/39Ar incremental heating analysis of groundmass separates from volcanic and plutonic rocks from Boa Vista are presented. The combination of age results and field observations demonstrates that the volcanic activity that formed the island occurred in three main stages: (1) > 16 Ma, (2) 15–12.5 Ma and (3) 9.5–4.5 Ma. The first stage, restricted to the north eastern part of the island, is dominated by ankaramitic lavas. The second stage, consisting of evolved lavas of phonolitic–trachytic compositions and nepheline syenites, makes up large central parts of the island. The large volume of evolved rocks and the extended eruption period of several Ma make stage 2 in Boa Vista unique to Cape Verde. Mainly basanites and nephelinites were erupted during the third stage, initially dominated by eruption of subaerial mafic lavas around 9 Ma. Pillow lavas are erupted around 7 Ma whereupon dominantly subaerial mafic lavas were erupted. Stage 3 saw volcanism in many centres distributed mainly along the present coastline and with activity partly overlapping in time. The volcanic evolution of Boa Vista constrains the initiation of volcanic activity in the Cape Verde archipelago to the eastern islands. Major and trace element geochemistry of 160 volcanic and plutonic rocks representing the entire exposed chronological sequence on Boa Vista is presented, revealing an extremely well developed Daly Gap. Only a little was modified from the mafic magmas that rose in small batches from the mantle. Compositional variation distinguishes each volcanic complex and was to a large extent present in the mantle melts. The highly evolved stage 2 phonolites and trachytes are related through the fractional crystallization of three compositionally distinct magmas. Two of these may have been derived by crystal fractionation of primitive Boa Vista melts, whereas the third was not.  相似文献   

11.
We report new high-precision laser fluorination three-isotope oxygen data for lunar materials. Terrestrial silicates with a range of δ18O values (− 0.5 to 22.9‰) were analyzed to independently determine the slope of the terrestrial fractionation line (TFL; λ = 0.5259 ± 0.0008; 95% confidence level). This new TFL determination allows direct comparison of lunar oxygen isotope systematics with those of Earth. Values of Δ17O for Apollo 12, 15, and 17 basalts and Luna 24 soil samples average 0.01‰ and are indistinguishable from the TFL. The δ18O values of high- and low-Ti lunar basalts are distinct. Average whole-rock δ18O values for low-Ti lunar basalts from the Apollo 12 (5.72 ± 0.06‰) and Apollo 15 landing sites (5.65 ± 0.12‰) are identical within error and are markedly higher than Apollo 17 high-Ti basalts (5.46 ± 0.11‰). Evolved low-Ti LaPaz mare-basalt meteorite δ18O values (5.67 ± 0.05‰) are in close agreement with more primitive low-Ti Apollo 12 and 15 mare basalts. Modeling of lunar mare-basalt source composition indicates that the high- and low-Ti mare-basalt mantle reservoirs were in oxygen isotope equilibrium and that variations in δ18O do not result from fractional crystallization. Instead, these differences are consistent with mineralogically heterogeneous mantle sources for mare basalts, and with lunar magma ocean differentiation models that result in a thick feldspathic crust, an olivine–pyroxene-rich mantle, and late-stage ilmenite-rich zones that were convectively mixed into deeper portions of the lunar mantle. Higher average δ18O (WR) values of low-Ti basalts compared to terrestrial mid ocean ridge basalts (Δ=0.18‰) suggest a possible oxygen isotopic difference between the terrestrial and lunar mantles. However, calculations of the δ18O of lunar mantle olivine in this study are only 0.05‰ higher than terrestrial mantle olivine. These observations may have important implications for understanding the formation of the Earth–Moon system.  相似文献   

12.
Coloumbo submarine volcano lies 6.5 km offshore the NE part of the Santorini island complex and exhibits high seismicity along with vigorous hydrothermal activity. This study models the local stress field around Coloumbo's magma chamber and investigates its influence on intrusion emplacement and geometry. The two components of the stress field, hoop and radial stress, are calculated using analytical formulas that take into account the depth and radius of the magma chamber as these are determined from seismological and other observations. These calculations indicate that hoop stress at the chamber walls is maximum at an angle of 74° thus favouring flank intrusions, while the radial stress switches from tensile to compressive at a critical distance of 5.7 km from the center of the magma chamber. Such estimates agree well with neotectonic and seismological observations that describe the local/regional stress field in the area. We analyse in detail the case where a flank intrusion reaches the surface very near the NE coast of Thera as this is the worst-case eruption scenario. The geometrical features of such a feeder dyke point to an average volumetric flow rate of 9.93 m3 s−1 which corresponds to a Volcanic Explosivity Index of 3 if a future eruption lasts about 70 days. Hazards associated with such an eruption include ashfall, ballistic ejecta and base surges due to explosive mixing of magma with seawater. Previous studies have shown that areas near erupting vents are also foci of moderate to large earthquakes that precede or accompany an eruption. Our calculations show that a shallow event (3–5 km) of moment magnitude 5.9 near the eruptive vent may cause Peak Ground Acceleration in the range 122–177 cm s−2 at different locations around Santorini. These values indicate that seismic hazard even due to a moderate earthquake near Coloumbo, is not trivial and may have a significant impact especially on older buildings at Thera island.  相似文献   

13.
The Proterozoic Kerala Khondalite Belt (KKB), southern India preserves a distinct high-grade terrain that is interpreted to have been situated adjacent to Madagascar and Sri Lanka during Gondwana assembly. As such, it has become a major focus for testing models of supercontinent amalgamation and dispersal. The lithounits of KKB have remarkable petrological similarities to the Highland Complex (HC) of Sri Lanka and south-central Madagascar. However, there is no well-constrained petrogenetic model for the KKB that fits explicitly within a supercontinent reconstruction model. We present here results from our on-going studies on the origin and evolution of K-rich (potassic, where K2O/Na2O > 1) gneisses of KKB in relation to Proterozoic supercontinent events. Our results show, in a major departure from earlier metasedimentary origin, that potassic gneisses are metamorphosed granitoids. The metagranitoid samples display high K2O contents and low Al2O3/(FeO + MgO + TiO2) values. They are moderate to strongly peraluminous (ASI values ranging from 1.05 to 1.47) rocks showing mineralogical, petrological, and geochemical characteristics distinctive of the high-K calc-alkaline suites. Typical of igneous suites, the high-K metagranites show minor variation in chemical compositions with most oxides showing negative correlation with SiO2. Geochemistry illustrates distinctive features of arc-related magmas with LILE (K, Rb, and Th) and LREE enriched patterns and considerable depletion of HSFE (Nb, Zr, and Ti). The high-K metagranites are further characterized by strong negative anomalies of Eu (Eu/Eu* = 0.10–0.44) and Sr, suggesting melting in plagioclase stability field and retention of plagioclase in the residual phase. Petrogenetic discrimination for granitoids, using major and trace elements demonstrates that the high-K metagranites of the KKB formed by partial melting of igneous source in lower- to middle-crust levels. Overall the geochemical features are supportive of origin in relation to a convergent margin setting, possibly in a continental magmatic arc system, which can be connected to the amalgamation and dispersal of continental fragments in a supercontinent event. This study, therefore, provides a lead towards more robust comparisons between the Proterozoic supercontinent events and processes.  相似文献   

14.
The Permo-Carboniferous Oslo Rift developed in the foreland of the Variscan orogen over a period of some 50 million years through a process characterized by moderate extension and widespread magmatism. The overall tectonic situation places the Oslo Rift in a post-collisional, dextral transtensional setting related to the convergence between Baltica, Laurentia, Gondwana and Siberia during assembly of Pangea, the location probably reflecting the control by pre-existing lithospheric structures. Although a detailed understanding of these factors and processes relies strongly on having a good age control, the dating of mafic to ultramafic alkalic volcanic units formed during initial rifting has been a very challenging task. In this study we have successfully employed perovskite from melilitic and nephelinitic volcanic rocks, together with magmatic titanite in a more evolved ignimbrite, to obtain ID-TIMS high-precision U–Pb ages. Three samples from various levels of the Brunlanes succession, in the southernmost exposures of the Oslo Graben, yield ages of 300.2 ± 0.9, 300.4 ± 0.7 and 299.9 ± 0.9 Ma. A melililitic tuff at the base of the Skien succession further to the northwest yields a slightly younger age of 298.9 ± 0.7 Ma. The initial Pb compositions derived mainly from coexisting pyroxene, apatite and hornblende are characterized by extremely radiogenic initial 206Pb/204Pb ratios (up to 21.3) that confirm a provenance of these early alkaline basalts from HIMU-type sources. The U–Pb ages coincide with the Gzhelian age inferred from fossils in the upper part of the basal rift sedimentary fill of the Asker Group, and post-date the underlying basal sedimentary sequences by some 10 million years, pointing to a relatively rapid initiation of the rifting process.  相似文献   

15.
The exceptionally well-preserved sedimentary rocks of the Taoudeni basin, NW Africa represent one of the world's most widespread (> 1 M km2) Proterozoic successions. Hitherto, the sedimentary rocks were considered to be Mid Tonian based on Rb–Sr illite and glauconite geochronology of the Atar Group. However, new Re–Os organic-rich sediment (ORS) geochronology from two drill cores indicates that the Proterozoic Atar Group is ～ 200 Ma older (1107 ± 12 Ma, 1109 ± 22 Ma and 1105 ± 37 Ma). The Re–Os geochronology suggests that the Rb–Sr geochronology records the age of diagenetic events possibly associated with the Pan African collision.The new Re–Os geochronology data provide absolute age constraints for recent carbon isotope chemostratigraphy which suggests that the Atar Group is Mesoproterozoic and not Neoproterozoic.The new Re–Os ORS geochronology supports previous studies that suggest that rapid hydrocarbon generation (flash pyrolysis) from contact metamorphism of a dolerite sill does not significantly disturb the Re–Os ORS systematics. Modelled contact conditions suggest that the Re–Os ORS systematics remain undisturbed at ～ 650 °C at the sill/shale contact and ≥ 280 °C 20 m from the sill/shale contact.Moreover, the Re–Os geochronology indicates that the West African craton has a depositional history that predates 1100 Ma and that ORS can be correlated on a basin-wide scale. In addition, the Re–Os depositional ages for the ORS of the Taoudeni basin are comparable to those of ORS from the São Francisco craton, suggesting that these cratons are correlatable. This postulate is further supported by identical Osi values for the Atar Group and the Vazante Group of the São Francisco craton.  相似文献   

16.
In view of an anomalous crust–mantle structure beneath the 2001 Bhuj earthquake region, double-difference relocations of 1402 aftershocks of the 2001 Bhuj earthquake were determined, using an improved 1D velocity model constructed from 3D velocity tomograms based on data from 10 to 58 three-component seismograph stations. This clearly delineated four major tectonic features: (i) south-dipping north Wagad fault (NWF), (ii and iii) south-dipping south Wagad faults 1 and 2 (SWF1, SWF2), and (iv) a northeast dipping transverse fault (ITF), which is a new find. The relocated aftershocks correlate satisfactorily with the geologically mapped and inferred faults in the epicentral region. The relocated focal depths delineate a marked variation to the tune of 12 km in the brittle–ductile transition depths beneath the central aftershock zone that could be attributed to a lateral variation in crustal composition (more or less mafic) or in the level of fracturing across the fault zone. A fault intersection between the NWF and ITF has been clearly mapped in the 10–20 km depth range beneath the central aftershock zone. It is inferred that large intraplate stresses associated with the fault intersection, deepening of the brittle–ductile transition to a depth of 34 km due to the presence of mafic/ultramafic material in the crust–mantle transition zone, and the presence of aqueous fluids (released during the metamorphic process of eclogitisation of lower crustal olivine-rich rocks) and volatile CO2 at the hypocentral depths, might have resulted in generating the 2001 Bhuj earthquake sequence covering the entire lower crust.  相似文献   

17.
The Alisitos arc is an approximately 300 × 30 km oceanic arc terrane that lies in the western wall of the Peninsular Ranges batholith south of the modern Agua Blanca fault zone in Baja California. We have completed detailed mapping and dating of a 50 × 30 km segment of this terrane in the El Rosario to Mission San Fernando areas, as well as reconnaissance mapping and dating in the next 50 × 30 km segment to the north, in the San Quintin area. We recognize two evolutionary phases in this part of the arc terrane: (I) extensional oceanic arc, characterized by intermediate to silicic explosive and effusive volcanism, culminating in caldera-forming silicic ignimbrite eruptions at the onset of arc rifting, and (II) rifted oceanic arc, characterized by mafic effusive and hydroclastic rocks and abundant dike swarms. Two types of units are widespread enough to permit tentative stratigraphic correlation across much of this 100-km-long segment of the arc: a welded dacite ignimbrite (tuff of Aguajito), and a deepwater debris-avalanche deposit. New U–Pb zircon data from the volcanic and plutonic rocks of both phases indicate that the entire 4000-m-thick section accumulated in about 1.5 MY, at 111–110 MY. Southwestern North American sources for two zircon grains with Proterozoic 206Pb / 207Pb ages support the interpretation that the oceanic arc fringed North America rather than representing an exotic terrane.The excellent preservation and exposure of the Alistos arc terrane makes it ideal for three-dimensional study of the structural, stratigraphic and intrusive history of an oceanic arc terrane. The segment mapped and dated in detail has a central major subaerial edifice, flanked by a down-faulted deepwater marine basin to the north, and a volcano-bounded shallow-water marine basin to the south. The rugged down-faulted flank of the edifice produced mass wasting, plumbed large-volume eruptions to the surface, and caused pyroclastic flows to disintegrate into turbulent suspensions that mixed completely with water. In contrast, gentler slopes on the opposite flank allowed pyroclastic flows to enter the sea with integrity, and supported extensive buildups of bioherms. Caldera collapse on the major subaerial edifice ponded the tuff of Aguajito to a thickness of at least 3 km. The outflow ignimbrite forms a marker in nonmarine to shallow marine sections, and in deepwater sections it occurs as blocks up to 150 m long in a debris-avalanche deposit. These welded ignimbrite blocks were deposited hot enough to deform plastically and form peperite with the debris-avalanche matrix. The debris avalanche was likely triggered by injection of feeder dikes along the basin-bounding fault zone during the caldera-forming eruption.Intra-arc extension controlled very high subsidence rates, followed shortly thereafter by accretion through back-arc basin closure by 105 Ma. Accretion of the oceanic arc may have been accomplished by detachment of the upper crust along a still hot, thick middle crustal tonalitic layer, during subduction of mafic–ultramafic substrate.  相似文献   

18.
We report trace element, samarium (Sm)–neodymium (Nd) and lead (Pb) isotopic data for individual micro-and mesobands of the Earth's oldest Banded Iron Formation (BIF) from the ∼ 3.7–3.8 Ga Isua Greenstone Belt (IGB, West Greenland) in an attempt to contribute to the characterization of the depositional environment and to the understanding of depositional mechanisms of these earliest chemical sediments. Rare earth element (REE)-yttrium (Y) patterns of the individual mesobands show features of modern seawater with diagnostic cerium (Ce/Ce⁎), presodymium (Pr/Pr⁎) and Y/holmium (Ho) anomalies. Very low high field strength elements (HFSE) concentrations indicate essentially detritus-free precipitation. Uranogenic Pb isotope data define a correlation line with a slope of 3691 ± 41 Ma, indicating that the uranium (U)–lead (Pb) system remained closed after the formation of this BIF. High 207Pb/204Pb relative to 206Pb/204Pb ratios compared to average mantle growth evolution models are a feature shared by BIF, penecontemporaneous basalts and clastic volcanogenic metasediments and are indicative of the ultimate high-μ (238U/204Pb) character of the source region, an essentially mafic Hadean protocrust. Sm–Nd isotopic relations on a layer-by-layer basis point to two REE sources controlling the back-arc basin depositional environment of the BIF, one being seafloor-vented hydrothermal fluids (εNd (3.7 Ga) ∼ + 3.1), the other being ambient surface seawater which reached its composition by erosion of parts of the protocrustal landmass (εNd(3.7 Ga) ∼ + 1.6). The validity of two different and periodically interacting water masses (an essentially two-component mixing system) in the deposition of alternating iron- and silica-rich layers is also reflected by systematic trends in germanium (Ge)/silicon (Si) ratios. These suggest that significant amounts of silica were derived from unexposed and/or destroyed mafic Hadean landmass, unlike iron which probably originated from oceanic crust following hydrothermal alteration by deep percolating seawater. Ge/Si distributional patterns in the early Archean Isua BIF are similar to those reported from the Paleoproterozoic Hamersley (Western Australia) BIF, but overall Ge concentrations are about one order of magnitude higher in the Archean BIF. This seems consistent with other lines of evidence that the ambient Archean seawater was enriched with iron relative to Proterozoic and recent seawater.  相似文献   

19.
Magmas are transported through pre-existing fractures in many repeatedly erupting volcanoes. The study of this special process of magma transport is fundamentally important to understand the mechanisms and conditions of volcanic eruptions. In this paper, we numerically simulate the magma propagation process through a pre-existing vertical fracture in the crust by using the combined finite difference method (FDM), finite element method (FEM) and discontinuous deformation analysis (DDA) approach. FDM is used to analyze magma flow in the pre-existing fracture, FEM is used to calculate the opening of the fracture during magma intrusion, and DDA is used to deal with the contact of the closed fracture surfaces. Both two-dimensional (2D) and three-dimensional (3D) examples are presented. Parametric studies are carried out to investigate the influence of various physical and geometric parameters on the magma transport in the pre-existing fracture. We have considered magma chamber depth ranging from 7 km to 10 km under the crust surface, magma viscosity ranging from 2 × 10−2 to 2 × 10−7 MPa s, and the density difference between the magma and host rock ranging from 300 to 700 kg/m3. The numerical results indicate that (1) the fluid pressure p varies gradually along the depth, (2) the shape of the magma body during propagation is like a torch bar and its width ranges from 2 m to 4 m approximately in the 3D case and 10 m to 50 m in the 2D case for the same physical parameters used, (3) the crust surface around the pre-existing fracture begins to increase on both sides of the fracture, forms a trough between them, then gradually uplifts during the transport of the magma, and finally takes the shape of a crater when the magma reaches the surface. We have also examined the influence of physical and geometric parameters on the minimum overpressure for magma transport in the 3D case. The numerical results show that our numerical technique presented in this paper is an effective tool for simulating magma transport process through pre-existing fractures in the crust.  相似文献   

20.
Siberian gas venting and the end-Permian environmental crisis   总被引：1，自引：0，他引：1  
The end of the Permian period is marked by global warming and the biggest known mass extinction on Earth. The crisis is commonly attributed to the formation of the Siberian Traps Large Igneous Province although the causal mechanisms remain disputed. We show that heating of Tunguska Basin sediments by the ascending magma played a key role in triggering the crisis. Our conclusions are based on extensive field work in Siberia in 2004 and 2006. Heating of organic-rich shale and petroleum bearing evaporites around sill intrusions led to greenhouse gas and halocarbon generation in sufficient volumes to cause global warming and atmospheric ozone depletion. Basin scale gas production potential estimates show that metamorphism of organic matter and petroleum could have generated > 100,000 Gt CO2. The gases were released to the end-Permian atmosphere partly through spectacular pipe structures with kilometre-sized craters. Dating of a sill intrusion by the U–Pb method shows that the gas release occurred at 252.0 ± 0.4 million years ago, overlapping in time with the end-Permian global warming and mass extinction. Heating experiments to 275 °C on petroleum-bearing rock salt from Siberia suggests that methyl chloride and methyl bromide were significant components of the erupted gases. The results indicate that global warming and ozone depletion were the two main drivers for the end-Permian environmental crisis. We demonstrate that the composition of the heated sedimentary rocks below the flood basalts is the most important factor in controlling whether a Large Igneous Provinces causes an environmental crisis or not. We propose that a similar mechanism could have been responsible for the Triassic-Jurassic (~ 200 Ma) global warming and mass extinction, based on the presence of thick sill intrusions in the evaporite deposits of the Amazon Basin in Brazil.  相似文献   

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