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1.
Intraplutonic Quench Zones in the Kap Edvard Holm Layered Gabbro Complex, East Greenland 总被引:1,自引:2,他引:1
The Kap Edvard Holm Layered Gabbro Complex is a large layeredgabbro intrusion (>300 km2) situated on the opposite sideof the Kangerdlugssuaq fjord from the Skaergaard Intrusion.It was emplaced in a continental margin ophiolite setting duringearly Tertiary rifting of the North Atlantic. Gabbroic cumulates, covering a total stratigraphic thicknessof >5 km, have a typical four-phase tholeiitic cumulus mineralogy:plagioclase, clinopyroxene, olivine, and FeTi oxides.The cryptic variation is restricted (plagioclase An8151,olivine Fo8566, clinopyroxene Wo4341 En4637Fs2011) and there are several reversals in mineral chemistry.Crystallization took place in a low-pressure, continuously fractionatingmagma chamber system which was periodically replenished andtapped. Fine-grained (0204 mm) equigranular, thin(053 m), laterally continuous basaltic zones occurwithin an {small tilde}1000 m thick layered sequence in theTaco Point area. Twelve such zones define the bases of individualmacrorhythmic units with an average thickness of {small tilde}80m. The fine-grained basaltic zones grade upwards, over a fewmetres, into medium-grained (>1 mm) poikilitic, olivine gabbrowith smallscale modal layering. Each fine-grained basaltic zoneis interpreted as an intraplutonic quench zone in which magmachilled against the underlying layered gabbros during influxalong the chamber floor. Supercooling by {small tilde}50C isbelieved to have caused nucleation of plagioclase, olivine,and clinopyroxene in the quench zone. The nucleation rate isbelieved to have been enhanced as the result of in situ crystallizationin a continuously flowing magma. The transition to the overlyingpoikilitic olivine gabbro reflects a decreasing degree of supercooling. Compositional variation in the Taco Point sequence is typicalfor an open magma chamber system: olivine (Fo7768 5)and plagioclase cores (An8072) show a zig-zag crypticvariation pattern with no overall systematic trend. Olivinehas the most primitive compositions in the quench zones andmore evolved compositions in the olivine gabbro; plagioclasecores show the opposite trend. Although plagioclase cores arebelieved to retain their original compositions, olivines re-equilibratedby reaction with trapped liquid. Some plagioclase cores containrelatively sodic patches which retain quench compositions. Whole-rock compositions of nine different quench zones varyover a range from 10 to 18% MgO although the mg-number remainsconstant at {small tilde}078. The average composition(477% SiO2, 133%MgO, 157% Na2O+K2O) is takenas a best estimate of the parental magma composition, and isequivalent to a high-magnesian olivine tholeiite. The compositionalvariation of the quench zones is believed to reflect burstsof nucleation and growth of olivine and plagioclase during quenching. Magma emplacement is believed to have taken place by separatetranquil influxes which flowed along the interface between alargely consolidated cumulus pile and the residual magma. Theresident magma was elevated with little or no mixing. At certainlevels in the layered sequence the magma drained back into thefeeder system; such a mechanism is referred to as a surge-typemagma chamber system. 相似文献
2.
At the same Mg-number, a decrease is observed in the averagecontent of Ni, Co, and Cr in basalts between 2.5 and 2.0 b.y.Such decreases can be accounted for by smaller degrees of meltingin the mantle in response to falling geotherms. In terms ofpartially compatible (e.g. Ti, Zr, Nb, Y) and incompatible elements(e.g. Rb, Sr, Ba), Archean basalts are similar to modern island-arcbasalts although they are relatively enriched in Th and Zr anddepleted in Al. The upper concentration limit of partially compatibleand incompatible elements increases in basalts between 2?5 and1.0 b.y. Basalts with large concentrations of these elementsdo not appear to have been produced during the Archean and becomeimportant in the geologic record only after 1?0 b.y. ago. Varyingdegrees of melting, fractional crystallization, or crustal contaminationare not capable of accounting for these changes, and an increasingimportance of enriched mantle sources seems necessary. A greater availability of enriched mantle sources after theArchean may reflect recycling of ancient enriched mantle orcontinental sediments into the mantle during catastrophic continentalgrowth at 2.7 b.y. 相似文献
3.
KENT M. SYVERSON 《Boreas: An International Journal of Quaternary Research》1995,24(3):232-244
Known changes in ice-flow direction during a 100-year interval have been used to evaluate how well ice-flow indicators record complex deglaciation events. At Burroughs Glacier, nunataks emerging from a thinning Neoglacial ice mass and differential ice-surface lowering caused by calving ice margins have produced major changes in ice-flow direction sincc 1892. Cross-cutting striae with angles of divergence of up to 105' reflect the past range of flow directions in the area. Striae from the oldest flow events are deepest, and striae from some late-stage flow events are missing. This may be caused by overprinting during late-stage reversals in the direction of ice movement. The orientation of flutes and surficial bullet boulders reflects the final ice-flow direction, but boulder orientations are less clustered than flute orientations. Surficial till pebble fabrics are weakly to moderately developed, but till fabrics vary with depth and record ice-flow direction changes with time. 相似文献
4.
Reverse tip jets are strong low-level winds with easterly component that form near the southern tip of Greenland. In the present study, a reverse tip jet case which occurred from 21 to 22 December 2000 was examined to clarify its fine structure using a numerical model with a horizontal resolution of 3 km. The reverse tip jet, showing the supergeostrophic wind speed with a maximum wind speed in excess of 45 m s−1 , extended from the east coast of Greenland to the west of Cape Farewell with anticyclonic curvature. A cloud free region coincided with the jet indicated that there was a mesoscale downdraft. Along the eastern edge of the jet, a banded cloud formed between the upstream easterly wind and the colder northerly wind that is a part of the jet and is located along the east coast. This cloud was associated with large gradients in surface wind speed, temperature, moisture, and heat flux. A maximum surface total heat flux of 300 W m−2 coincided with the location of the jet. It is suggested that the orographic deflection by Greenland's large-scale topography as well as small-scale downslope winds behind mountains with fiords causes the reverse tip jet. 相似文献
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6.
Evolution of the Kap Edvard Holm Complex: a Mafic Intrusion at a Rifted Continental Margin 总被引:1,自引:0,他引:1
The Kap Edvard Holm Layered Series forms part of the East GreenlandTertiary Province, and was emplaced at shallow crustal level(at depths corresponding to a pressure of 12 kbar) duringcontinental break-up. It consists of two suites: a gabbro suitecomprising olivine and oxide gabbros, leucocratic olivine gabbrosand anorthosites, and a suite of wehrlites that formed fromthe intrusion of the gabbros during their solidification bya hydrous, high-MgO magma. Ion microprobe analyses of clinopyroxenereveal chemical contrasts between the parental melt of the wehrlitesuite and that of the gabbro suite. Thin sills (12 mthick) of the wehrlite suite, however, have clinopyroxene compositionssimilar to the gabbro suite, and were formed by interactionwith interstitial melts from the host layered gabbros. All evolvedmembers of the gabbro suite have elevated Nd, Zr and Sr concentrationsand Nd/Yb ratios, relative to the melt parental to the gabbrosuite. These characteristics are attributed to establishmentof a magma chamber at depths corresponding to a pressure of10 kbar, where melts evolved before injection into the low-pressuremagma chamber. Anorthosites of the gabbro suite are believedto have crystallized from such injections. The melts becamesupersaturated in plagioclase by the pressure release that followedtransportation to the low-pressure magma chamber after initialfractionation at 10 kbar. The most evolved gabbros formed bysubsequent fractionation within the low-pressure magma chamber.Our results indicate that high-pressure fractionation may beimportant in generating some of the lithological variationsin layered intrusions. KEY WORDS: fractionation; ion microprobe; layered intrusions; rift processes; trace elements
*Corresponding author. 相似文献
7.
Sediments deposited in two small ice-contact lakes with low rates of sediment input have been studied in subaerial exposures. Sediment characteristics are a function of the water source (glacial meltwater versus non-meltwater), proximity to the glacier margin and lake shore, amount of supraglacial debris, and lake duration. Calving Lake expanded (and later partially drained) as a calving ice margin retreated. Nearshore deltas contain 1 × 105 m3 stratified sand and gravel deposited at rates up to 1 m/yr during a 9-yr interval. Deltaic sediment contains types A and B ripple-drift cross-lamination, draped lamination, and scour surfaces caused by variations in water-flow velocity and the amount of sediment settling from suspension. Most water inflow came from non-subglacial meltwater sources and was sediment-poor, so overflow and interflow sedimentation processes dominated the offshore environment. Offshore sediment generally contains massive silt or silt interbedded with fine-grained sand deposited at rates of 1.3-1.5 cm/yr. Iceberg gravity craters observed on the lake plain were formed when icebergs impacted the lake floor during calving events. In Bruce Hills Lake, proximity to glacier ice and the presence of supraglacial sediment formed coarsening-upward successions when debris fell directly from an ice ledge onto silty lacustrine sediment. 相似文献
8.
KENT M. SYVERSON DAVID M. MICKELSON 《Boreas: An International Journal of Quaternary Research》2009,38(1):132-145
Pleistocene lateral meltwater channels are commonly used as evidence of cold-based or polythermal ice. However, lateral meltwater channel formation has been observed for >40 years along the margins of a rapidly thinning temperate glacier in Glacier Bay, Alaska. Flights of nested linear lateral meltwater channels and in-and-out channels have formed on the sides of emerging nunataks. Nested channels at Burroughs Glacier are up to 200 m long; they are good proxies for the slope of the ice margin along the land surface and are terminated by subglacial chutes. A perched water table associated with precipitation and high ablation rates in the temperate ice causes surface meltwater to flow toward the margin above less permeable ice. The water flows along the margin and erodes lateral meltwater channels until a subglacial chute carries the water into the subglacial water system. Rates of channel formation range from 0 to 8 channels/year. Spacing and rates of channel formation are controlled by the land-surface slope, ablation rate, erodibility of the substrate and stream discharge. Because lateral meltwater channels have been observed forming along a temperate glacier margin, care must be exercised when using the presence of lateral meltwater channels as definitive evidence of cold-based or polythermal ice. 相似文献
9.
PEATE D. W.; BAKER J. A.; BLICHERT-TOFT J.; HILTON D. R.; STOREY M.; KENT A. J. R.; BROOKS C. K.; HANSEN H.; PEDERSEN A. K.; DUNCAN R. A. 《Journal of Petrology》2003,44(2):279-304
We present elemental and isotopic (SrNdPbHfOsHe)data on primitive alkalic lavas from the Prinsen af Wales Bjerge,East Greenland. Stratigraphical, compositional and 40Ar39Ardata indicate that this inland alkalic activity was contemporaneouswith the upper parts of the main tholeiitic plateau basaltsand also post-dated them. The alkalic rocks show a marked crustalinfluence, indicating establishment of new magmatic plumbingsystems distinct from the long-lived coastal systems that fedthe relatively uncontaminated plateau basalts. The least contaminatedlavas have high 3He/4He isotope ratios (R/RA 12·418·5),sub-chondritic 187Os/188Osi (0·1200·126),low 相似文献
10.
40Ar/39Ar Geochronology of the Rajmahal Basalts, India, and their Relationship to the Kerguelen Plateau 总被引:3,自引:0,他引:3
KENT RAY W.; PRINGLE MALCOLM S.; MULLER R. DIETMAR; SAUNDERS ANDREW D.; GHOSE NARESH C. 《Journal of Petrology》2002,43(7):1141-1153
During the mid-Cretaceous, extensive magmatism occurred in theIndian Ocean to form volcanic portions of the southern and centralKerguelen Plateau, Elan Bank and Broken Ridge. Basalt was eruptedalso along the rifted margin of eastern India (Rajmahal). Weinvestigated the ages of these Indian basalts using 40Ar/39Arincremental-heating experiments on whole rocks. Our resultsare consistent with the hypothesis that the lava pile of 相似文献