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891.
Sarah K. Appleby Martin R. Gillespie Colin M. Graham Richard W. Hinton Grahame J. H. Oliver Nigel M. Kelly 《Contributions to Mineralogy and Petrology》2010,160(1):115-132
In contrast to I-type granites, which commonly comprise infracrustal and supracrustal sources, S-type granites typically incorporate
predominantly supracrustal sources. The initial aim of this study was to identify the sources of three Scottish Caledonian
(~460 Ma) S-type granites (Kemnay, Cove and Nigg Bay) by conducting oxygen, U–Pb and Hf isotope analyses in zircon in order
to characterise one potential end-member magma involved in the genesis of the voluminous late Caledonian (~430–400 Ma) I-type
granites. Field, whole-rock geochemical and isotopic data are consistent with the generation of the S-type granites by melting
their Dalradian Supergroup country rocks. While Hf isotope compositions of magmatic zircon, U–Pb data of inherited zircons,
and high mean zircon δ18O values of 9.0 ± 2.7‰ (2SD) and 9.8 ± 2.0‰ for the Kemnay and Cove granites support this model, the Nigg Bay Granite contains
zircons with much lower δ18O values (6.8 ± 2.1‰), similar to those found in Scottish I-type granites. This suggests that the Nigg Bay Granite contains
low-δ18O material representing either altered supracrustal material, or more likely, an infracrustal source component with mantle-like
δ18O. Mixing trends in plots of δ18O vs. εHf for S-type granite zircons indicate involvement of at least two sources in all three granites. This pilot study
of Scottish Caledonian S-type granites demonstrates that, while field and whole-rock geochemical data are consistent with
local melting of only supracrustal sources, the oxygen isotopic record stored in zircon reveals a much more complex petrogenetic
evolution involving two or more magma sources. 相似文献
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In respect to the weathering of cave art exposed to the sun, cognizance has yet to be taken of the modified thermal conditions and the potential for endolithic biotic activity where the art is located on a light‐transmissive lithology. Where light penetrates rock, the light‐to‐heat transfer is not solely at the surface, and this leads to a thermal gradient that is different from where the paintings are located (and all transfer is at the surface). Light values of up to 200 W/m2 were recorded at 0.5 mm depth and up to 100 W/m2 at 1mm depth in the dry sandstone; rock moisture data showed that at this site the rock remained dry irrespective of atmospheric conditions. The light penetration means that there can be rapid and large subsurface thermal fluctuations contemporaneous with those at the rock surface, and that the thermal gradient is not as steep (approximately 1°C/mm in the surficial part of the rock) as where light‐to‐heat transfer is solely at the surface. Further, the presence of subsurface photosynthetically active radiation can (potentially) facilitate colonization by endolithic organisms. Here, as part of a study of the weathering of San rock art on sandstone in southern Africa, a first attempt is made to monitor the extent of light penetration and the resulting thermal conditions in the outer few millimeters of the sandstone. © 2009 Wiley Periodicals, Inc. 相似文献
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