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101.
This paper compares the 1.67–1.47 Ga rapakivi granites of Finland and vicinity to the 1.70–1.68 Ga rapakivi granites of the Beijing area in China, the anorogenic 130 Ma granites of western Namibia, and the 20–15 Ma granites of the Colorado River extensional corridor in the Basin and Range Province of southern Nevada. In Finland and China, the tectonic setting was incipient, aborted rifting of Paleoproterozoic or Archean continental crust, in Namibia it was continental rifting and mantle plume activity that led to the opening of southern Atlantic at 130 Ma. The 20–15 Ma granites of southern Nevada were related to rifting that followed the Triassic–Paleogene subduction of the Farallon plate beneath the southwestern United States. In all cases, extension-related magmatism was bimodal and accompanied by swarms of diabase and rhyolite–quartz latite dikes. Rapakivi texture with plagioclase-mantled alkali feldspar megacrysts occurs in varying amounts in the granites, and the latest intrusive phases are commonly topaz-bearing granites or rhyolites that may host tin, tungsten, and beryllium mineralization. The granites are typically ferroan alkali-calcic metaluminous to slightly peraluminous rocks with A-type and within-plate geochemical and mineralogical characteristics. Isotope studies (Nd, Sr) suggest dominant crustal sources for the granites. The preferred genetic model is magmatic underplating involving dehydration melting of intermediate-felsic deep crust. Juvenile mafic magma was incorporated either via magma mingling and mixing, or by remelting of newly hybridized lower crust. In Namibia, partial melting of subcontinental lithospheric mantle was caused by the Tristan mantle plume, in the other cases the origin of the mantle magmatism is controversial. For the Fennoscandian suites, extensive long-time mantle upwelling associated with periodic, migrating melting of the subcontinental lithospheric mantle, governed by heat flow and deep crustal structures, is suggested. 相似文献
102.
Julieta Massaferro Stephen J. Brooks Simon G. Haberle 《Quaternary Science Reviews》2005,24(23-24):2510-2522
We compare high-resolution pollen and chironomid records from the last 15,000 yr in Laguna Facil, southern Chile. Major vegetation and chironomid changes are recorded between ca 14,900 and 14,700 cal. yr BP. During the Lateglacial, changes in the chironomid stratigraphy lag behind changes in the pollen stratigraphy suggesting that the chironomids are responding to changes in the tree canopy or in soil chemistry brought about by vegetational development. At about 7200 cal. yr BP there is a change in the chironomid stratigraphy in advance of changes in the vegetation. This suggests that the response is to regional climatic change. The relatively close correlation of the chironomid and pollen stratigraphies with changes in charcoal concentrations also implicates the importance of fire and/or vulcanism in influencing the dynamics of forest and limnological systems. There is no clear evidence of cooling during the Younger Dryas chronozone in Laguna Facil. 相似文献
103.
Stephen J. Lynton Richard J. Walker Philip A. Candela 《Geochimica et cosmochimica acta》2005,69(13):3337-3347
The fractionation of lithium isotopes among quartz, muscovite, and a chloride-bearing aqueous fluid has been investigated experimentally at 400°-500°C and 50-100 MPa. Experiments were performed for 15-60 days in cold seal vessels with natural mineral specimens. Lithium was introduced primarily through the fluid, which also contained KCl and HCl. In most runs, the fluid was prepared with the L-SVEC standard (δ7Li = 0) and was 1 M in total chloride with K/Li/H = 100/10/1. In two experiments, a 6Li spike was employed. The experiments demonstrate that quartz and muscovite are susceptible to pronounced, rapid shifts in Li isotopic composition by diffusion through interaction with a Li-bearing fluid, particularly at 500°C. At 500°C, fractionation factors were determined to be Δquartz-aqueous ≅ +8 to + 12‰ and Δmuscovite-aqueous ≅ +18 to + 20‰. An intermineral fractionation factor is given by Δmuscovite-quartz ≈ +9‰. At 400°C, the results suggest Δquartz-aqueous ≈ +4 to + 6‰. The study provides evidence of systematic fractionation in lithium isotopes at the temperatures of some magmatic processes, such as those associated with porphyry-type ore systems and pegmatites. 相似文献
104.
The Neogene fossil record of the Antilles is particularly rich and can satisfy the interests of most collectors, from foraminifers to vertebrates. The molluscs are among the commonest and most diverse groups, being found in both terrestrial and marine deposits. The Pliocene Bowden shell beds of southeast Jamaica, best known for its hundreds of species of molluscs, also yields diverse members of other groups, the total exceeding 800 species from a mere 5 m of stratigraphical section. 相似文献
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Stephen R.H. Worthington 《水文研究》2019,33(17):2373-2380
Groundwater often accounts for a substantial fraction of flood hydrographs, but the processes responsible for this have been unclear. However, many aquifers have preferential flow and this explains how aquifers can be so responsive. In bedrock aquifers, weathering enhances the connectivity and apertures along the most efficient flow paths and hence enhances the permeability. This results in celerities and velocities of the preferential flow in these dual‐porosity aquifers that are two to three orders of magnitude higher than if the aquifers behaved as single‐porosity media. The celerities have been determined from artificial and natural flood pulses, from tidal lags, and from pumping tests. Preferential‐flow velocities have been calculated from tests using applied tracers. Celerities in bedrock aquifers are typically one to two orders of magnitude faster than velocities. The ubiquitous preferential flow in aquifers provides an additional explanation, besides groundwater ridging, for the rapid release of groundwater to streams during storm events. 相似文献
107.
Erin R. McMullin Kimberlyn Nelson Charles R. Fisher Stephen W. Schaeffer 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(11):1499-1509
Vestimentiferan tubeworms are a group of large sessile marine polychaete annelids (family Siboglinidae) found in the regions of hydrothermal venting or seepage of the reduced chemical hydrogen sulfide. Hydrocarbon seeps on the Louisiana Slope of the Gulf of Mexico support large communities of the co-occurring vestimentiferan species Lamellibrachia luymesi and Seepiophila jonesi. These sessile species have the opportunity to disperse between the patchy sites of active seepage on the seafloor during a planktonic larval stage. However, it is unclear whether dispersal occurs at a local or global scale. Four (L. luymesi) and seven (S. jonesi) microsatellite loci were used to test for population substructure among ten hydrocarbon seep sites on the Louisiana Slope. Both species showed high levels of allelic diversity, averaging 18.5 (L. luymesi) and 22 (S. jonesi) alleles/locus, respectively, and high observed heterozygosity at all microsatellite loci (0.71–0.9 in L. luymesi, 0.27–0.84 in S. jonesi). The two species showed a significant deficiency in heterozygotes compared to that predicted under the Hardy–Weinberg equilibrium. L. luymesi showed a small but significant amount of population structure, with a positive correlation between genetic and geographic distance among the sample sites spanning 540 km. S. jonesi, in contrast, showed no evidence for isolation by distance, but did show a significant genetic difference between aggregations of different ages. These results suggest that these two species differ in how larvae are able to colonize new seep sites through space (L. luymesi) and though time (S. jonesi). 相似文献
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