Koolau shield basalt as xenoliths entrained during rejuvenated-stage eruptions: perspectives on magma mixing     

J.?P.?Weinstein  R.?V.?FodorEmail author  G.?R.?Bauer 《Bulletin of Volcanology》2004,66(2):182-199

Rejuvenated-stage tuff cones (Honolulu Volcanics) on Koolau volcano, Oahu, Hawaii, contain xenoliths of Koolau shield basalt. Because Koolau subaerial shield lavas represent a Hawaiian geochemical 'end member', and submarine shield lavas have compositions with some affinities to Mauna Loa and Kilauea, we analyzed 28 xenolithic basalts from Salt Lake and Koko Head cones to determine how these seemingly random samplings of the Koolau profile compare to established Koolau geochemistry. Analyses reveal that 24 are shield tholeiitic basalt—the focus of this study—and 4 are rejuvenated-stage basaltic rocks. The tholeiitic xenoliths represent largely upper Koolau shield lavas, as these samples (8.3 to 5.8 wt% MgO) have, with one exception, overall major- and trace-element compositions that overlap those of Koolau subaerial shield lavas. Secondary processes, however, created some distinctions—namely, enrichments/depletions in K, Ba, Sr, SiO₂, and FeO, and, due to zeolitization (chabazite with attending okenite and apophyllite), elevated CaO. One xenolithic basalt with 8.2 wt% MgO has higher Ti, Zr, Nb, and Sc, and lower Zr/Nb than subaerial lavas, and appears to represent relatively early, deeper shield—thereby reinforcing that the Koolau shield source varied temporally. Olivine, orthopyroxene, and plagioclase are the phenocrysts (clinopyroxene is rare), and their core compositions range widely across the suite—Fo_87.8–72, orthopyroxene Mg#s 85–72, and An_74–60. Several xenolithic basalts have both normally and reversely zoned orthopyroxene and plagioclase with a variety of core compositions (e.g., orthopyroxene-core Mg#s 82, 77, and 72, all in one sample). These compositions and zonations record evidence for wide compositional ranges of replenishment (MgO ~13–8 wt%) and reservoir (MgO ~7 to <5 wt%) magmas mixing in varying proportions; however, extreme MgO lavas (~13 and <5 wt%) are not observed as either subaerial or xenolithic basalt, but are indicated by phenocryst cores of Fo_87.8 and orthopyroxene-Mg# 72. The Koolau magma-mixing history resembles that of Kilauea, and is unlike the 'steady-state' mixing known for Mauna Loa. Finally, these basalt samples show that any xenolithic occurrence of Koolau lava is subject to the zeolitization prevalent in the tuff-cone hosts.Editorial handling: M. Carroll  相似文献   

A Quantitative structural study of late pan-african compressional deformation in the central eastern desert (Egypt) during Gondwana assembly   总被引：2，自引：0，他引：2  

M.M. Abdeen  R.O. Greiling 《Gondwana Research》2005,8(4):457-471

The Arabian-Nubian-Shield (ANS) is composed of a number of island arcs together with fragments of oceanic lithospere and minor continental terranes. The terranes collided with each other until c. 600 Ma ago. Subsequently, they were accreted onto West Gondwana, west of the present River Nile. Apart from widespread ophiolite nappe emplacement, collisional deformation and related lithospheric thickening appear to be relatively weak. Early post-collisional structures comprise not only extensional features such as fault-bounded (molasse) basins and metamorphic core complexes, but also major wrench fault systems, and thrusts and folds. The Hammamat Group was deposited in fault-bounded basins, which formed due to N-S to NW-SE directed extension. Hammamat Group sediments were intruded by late orogenic granites, dated as c. 595 Ma old. A NNW-SSE-oriented compression prevailed after the deposition of the Hammamat Sediments. This is documented by the presence of NW-verging folds and SE-dipping thrusts that were refolded and thrusted in the same direction. Restoration of a NNW-SSE- oriented balanced section across Wadi Queih indicates more than 25% of shortening. Transpressional wrenching related to the Najd Fault System followed this stage. The wrenching produced NW-SE sinistral faults associated with positive flower structures that comprise NE-verging folds and SW-dipping thrusts. Section restoration across these late structures indicates 15 17% shortening in the NE-SW direction. At a regional scale, the two post-Hammamat compressional phases produced an interference pattern with domes and basins. It can be shown that the Najd Fault System splays into a horsetail structure in the Wadi Queih area and loses displacement towards N and NW. The present study shows a distinct space and time relationship between deposition of Hammamat Group/late-Pan-African clastic sediments and late stages of Najd Fault wrench faulting: Hammamat deposition is followed by two episodes of compression, with the second episode being related to Najd Fault transpression. Therefore, the Hammamat sediments do not represent the latest tectonic feature of the Pan-African orogeny in the ANS. The latest orogenic episodes were the two successive phases of compression and transpression, respectively. It is speculated that extension during (Hammamat) basin formation was sufficiently effective to reduce the thickness of the orogenic lithosphere until it became gravitationally stable, and incapable of further gravitational deformation.  相似文献   

Marsh gas in shallow soils and safety measures for tunnel construction   总被引：6，自引：0，他引：6  

Yi-Qun Tang  Wei-Min Ye  Yu Huang 《Engineering Geology》2003,67(3-4):373-378

Marsh gas is a danger for tunnelling through soils in Shanghai. Predrilling can assess the hazard and help to vent the gas. Additional measures include tunneling with an airproof shield, adequate ventilation, special precautions against ignition and education of the workers.  相似文献   

Metallogeny of gold in relation to the evolution of the Nubian Shield in Egypt     

Nagy Shawky Botros 《Ore Geology Reviews》2002,19(3-4)

Gold mineralization in the Eastern Desert of Egypt is confined, almost completely, to the basement rocks of the Nubian Shield that was cratonized during the Panafrican orogeny.Island-arc, orogenic and post-orogenic stages are indicated for the tectonic-magmatic evolution of the Nubian Shield in Late Proterozoic times. Different styles of gold mineralization recognised in the Eastern Desert are inferred to have developed during these stages.In the island-arc stage, which is characterized by volcanic and volcaniclastic rocks in an ensimatic environment, gold mineralization is hosted in stratiform to strata-bound Algoma-type BIF and associated tuffaceous sedimentary rocks. Both types represent exhalative deposits, formed during breaks in sub-marine basaltic and bastalic–andesite volcanic eruptions. The volcanic rocks have a tholeiitic affinity and reflect an immature arc stage. Gold hosted in massive-sulphide deposits within calc-alkaline rhyolites represents another style of gold mineralization connected with mature island arc stage.During the orogenic-stage, ophiolites and island arc volcanic and volcaniclastic rocks were thrust onto the Pre-Panafrican continental margin. Subduction was active beneath the continent while the thrusting was still operative. A phase of calc-alkaline magmatic activity developed during this stage and the compressional deformation event was synchronous with regional metamorphism (greenschist–amphibolite facies). Extensional shear fractures (brittle–ductile shear zones) were broadly contemporaneous with the intense compressional tectonic regime. These fractures opened spaces in which the mineralizing fluids penetrated.Gold mineralization associated with the orogenic-stage is represented by vein-type mineralization that constituted the main target for gold since Pharaonic times. Other styles of gold mineralization during this stage are represented by altered ophiolitic serpentinites (listwaenites), Gold mineralization associated with intrusion related deposits (possibly porphyry copper deposits), as well as, auriferous quartz veins at the contacts of younger gabbros and G-2 granites.The post-orogenic stage is characterized by the dominance of intra-plate magmatism. Small amounts of the element in disseminations, stockworks and quartz veins of Sn–W–Ta–Nb mineralization represent gold mineralization connected with this stage.The link between these tectonic–magmatic stages and gold mineralization can be used as a criterion at any exploration strategy for new targets of gold mineralization in Egypt.  相似文献   

Crustal conductivity anomalies in central Sweden and southwestern Finland     

K. Pajunpää  I. Lahti  B. Olafsdottir  T. Korja  BEAR Working Group 《Geophysical Journal International》2002,150(3):695-705

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Santa Bárbara Formation (Early Paleozoic, Caçapava do Sul, southern Brazil): Petrographic and Sm–Nd isotopic provenance parameters     

Andr Weissheimer de Borba  Ana Maria Pimentel Mizusaki  Anderson Jos Maraschin  Diogo Rodrigues Andrade da Silva 《Journal of South American Earth Sciences》2008,26(4):485-497

Integrated petrographic and Sm–Nd isotopic data were applied in order to constrain the provenance of the Early Paleozoic Santa Bárbara Formation, Sul-rio-grandense Shield, southern Brazil. This unit comprises continental sandstones, conglomerates and siltstones deposited under semi-arid climate in a rift or pull-apart basin. Samples were collected within a stratigraphic framework composed of three sequences, in which the two basal ones present northeastwards paleoflow, and the third one marks the inversion of basin filling. Samples from sequence I show, in the south, a strong influence of intermediate volcanic (Hilário Formation) sources, and a significant increase in quartz and metamorphic fragments upsection. In the northern deposits, there is a possible influence of juvenile units (Cambaí/Vacacaí), and a more significant input of Paleoproterozoic-sourced sedimentary rocks (e.g. Maricá Formation) upsection. Samples collected from the topmost deposits of sequence II present a clear increase in the amount of volcanic fragments (mostly acidic), reflecting denudation of the “Caçapava high”. Data obtained in sequence III (Pedra do Segredo) show a progressive decrease in quartz content and a significant increase in feldspathic, plutonic fragments. A more evolved phase of denudation of the “Caçapava high”, exposing leucogranitoids of the Caçapava do Sul complex, is proposed for this interval.  相似文献   

The subsolidus segregation of layer-parallel quartz-feldspar veins in greenschist to upper amphibolite facies metasediments   总被引：2，自引：0，他引：2  

E. W. SAWYER  P.-Y. F. ROBIN 《Journal of Metamorphic Geology》1986,4(3):237-260

Abstract Layer-parallel (i.e. parallel to foliation or bedding) vein formation in the graywackes and pelites of the Quetico Metasedimentary Belt occurred during synchronous prograde metamorphism and regional (D₂) compression. In a traverse across metasediments which change in metamorphic grade from greenschist to upper amphibolite (migmatite) facies, layer-parallel veins show the following trends: (1) an increase in thickness and internal complexity, the latter due to successive boudinage; (2) low-grade veins are parallel to planes of anisotropy due to the original sedimentary fabric of the host rocks, but at higher grades other sites are also used and (3) a systematic increase in plagioclase/quartz ratio in the veins towards higher grade, adjacent mafic selvedges first exhibit quartz depletion then, in the amphibolite facies, plagioclase depletion. Mineralogical zoning is often preserved in a single vein, older parts are more quartz-rich than younger. Mass balance calculations and whole-rock geochemistry based on veins, mafic selvedges and country rock are consistent with a closedsystem subsolidus segregation origin. The layerparallel veins are syntectonic, and migration of the mobile components required to form their mineralogy is a stress-induced mass transfer. The source of these components appears to be dominantly pressure solution of the same minerals in the host rocks, although metamorphic reactions may also have contributed. Veins nucleated first at those sites where initial sedimentary heterogeneites, such as fine-scale graded bedding, provided gradients of normal stress across grain boundaries, and hence of chemical potential, necessary to drive the subsolidus segregation process. The earliest veins are thus parallel to bedding. Later, nucleation of the veins could also occur along more randomly distributed sites within the metasediments, and these veins grew parallel to the schistosity rather than bedding, if the two were distinct. Once formed, the veins themselves, which are more competent than the surrounding rock, provide the stress heterogeneity required for their further growth. The increasing plagioclase/quartz ratio in the veins may be due to a temperature dependent increase in plagioclase component mobility relative to quartz. Alternatively, the increasing transfer distances for silica, resulting from prior quartz depletion in the inner parts of the mafic selvedge, may increase the relative mobility of plagioclase component.  相似文献   

Late Sveconorwegian (Grenville) high-pressure granulite facies metamorphism in southwest Sweden   总被引：3，自引：0，他引：3  

L. JOHANSSON  A. LINDH  C. MÖLLER 《Journal of Metamorphic Geology》1991,9(3):283-292

Mafic granulite, garnet amphibolite and charnockite occur in the southwest Swedish part of the Baltic Shield. This part is generally considered to be the continuation of the Grenville collisional belt in Canada. The area with granulite facies rocks, the Southwest Swedish Granulite Region (SGR), is considerably larger than previously thought. The SGR is bounded to the east and west by two major tectonic zones. The first quantitative age data and P–T determinations for the high-grade metamorphism in the SGR are presented.
Conventional geothermobarometry was applied to mafic granulites from five localities. The estimated P–T conditions for the peak of metamorphism range from 705°C and 8.1 kbar at Hallandsås in the south, to 770°C and 10.5 kbar at Ullared in the north (medium- to high- P granulite facies conditions). Sm–Nd geochronology on minerals from the mafic granulites at Hallandsås and Ullared give late Sveconorwegian (Grenville) ages of 907 ± 12 and 916 ± 11 Ma for the high-grade metamorphism, which is considerably younger than previously thought.
Our results stress the hitherto underestimated importance of the late Sveconorwegian high-grade metamorphism in the southwestern part of the Baltic Shield.  相似文献   

Petrogenesis of ultramafic and mafic xenoliths from Mesozoic basanites in southern Sweden: constraints from mineral chemistry     

Tatjana Rehfeldt  Karsten Obst  Leif Johansson 《International Journal of Earth Sciences》2007,96(3):433-450

Jurassic basanite necks occurring at the junction of two major fault zones in Scania contain ultramafic (peridotites, pyroxenites) and mafic xenoliths, which together indicate a diversity of upper mantle and lower crustal assemblages beneath this region. The peridotites can be subdivided into lherzolites, dunites and harzburgites. Most lherzolites are porphyroclastic, containing orthopyroxene and olivine porphyroclasts. They consist of Mg-rich silicates (Mg# = Mg/(Mg + Fe_tot) × 100; 88–94) and vermicular spinel. Calculated equilibration temperatures are lower in porphyroclastic lherzolites (975–1,007°C) than in equigranular lherzolite (1,079°C), indicating an origin from different parts of the upper mantle. According to the spinel composition the lherzolites represent residues of 8–13% fractional melting. They are similar in texture, mineralogy and major element composition to mantle xenoliths from Cenozoic Central European volcanic fields. Dunitic and harzburgitic peridotites are equigranular and only slightly deformed. Silicate minerals have lower to similar Mg# (83–92) as lherzolites and lack primary spinel. Resorbed patches in dunite and harzburgite xenoliths might be the remnants of metasomatic processes that changed the upper mantle composition. Pyroxenites are coarse, undeformed and have silicate minerals with partly lower Mg# than peridotites (70–91). Pyroxenitic oxides are pleonaste spinels. According to two-pyroxene thermometry pyroxenites show a large range of equilibration temperatures (919–1,280°C). In contrast, mafic xenoliths, which are mostly layered gabbronorites with pyroxene- and plagioclase-rich layers, have a narrow range of equilibration temperatures (828–890°C). These temperature ranges, together with geochemical evidence, indicate that pyroxenites and gabbroic xenoliths represent mafic intrusions within the Fennoscandian crust.  相似文献   

Did hot,high heat-producing granites determine the location of the Oslo Rift?     

Trond Slagstad 《Tectonophysics》2006,412(1-2):105-119

The Late Carboniferous–Early Permian Oslo Rift formed in apparently cold, stable lithosphere of the Fennoscandian Shield in a tensional stress regime widely documented in Northwest Europe at that time. The Rift formed obliquely to older, crustal structures that display only limited Permian reactivation, and, although numerical modelling suggests that the present-day lithospheric structure would serve to focus tensional stresses in the Oslo region, the assumption that no lithospheric evolution has occurred since the Palaeozoic is by no means obvious. Here, I show that, up to 5 km thick, regional-scale Late- to Post-Sveconorwegian granites in the vicinity of the Oslo Rift, with heat-production rates averaging ca. 5 μW/m³, nearly three times higher than the surrounding Sveconorwegian gneisses, would have increased the temperature in the lower crust and lithospheric mantle by up to 100 °C, resulting in significant thermal weakening of the lithosphere in this area. Given a tensional stress regime, weakening by these high heat-producing element granites would have made the Oslo area a favoured site for passive rifting and may have been a first-order parameter locating rifting to this part of the Fennoscandian Shield. The thermo-rheological effects of such granites must be considered along with other factors in future models of initial rift mechanisms in the Oslo Rift, and probably in other rifts elsewhere.  相似文献