Dehydration Melting and Water-Saturated Melting of Basaltic and Andesitic Greenstones and Amphibolites at 1, 3, and 6. 9 kb |
| |
| Authors: | BEARD JAMES S; LOFGREN GARY E |
| |
| Institution: | SN-2, NASA/Johnson Space Center Houston, Texas 77058
*Present address: Department of Earth Sciences, Virginia Museum of Natural History, 1001 Douglas Avenue, Martinsville, Virginia 24112 |
| |
| Abstract: | The melting relations of five metamorphosed basalts and andesites(greenstones and amphibolites), collected from the late JurassicSmartville arc complex of California, were investigated experimentallyat 800–1000? C and 1, 3, and 6. 9 kb. Dehydration-melting(no water added) experiments contained only the water structurallybound in metamorphic minerals (largely amphiboles). They yieldedmildly peraluminous to metaluminous granodioritic to trondhjemiticmelts (Na/K is a function of starting composition) similar inmajor element composition to silicic rocks in modern oceanicarcs. The dehydration melts are water-undersaturated, with,and coexist with the anhydrous residual solid (restite) assemblageplagioclase + orthopyroxene + clinopyroxene + magnetite ? ilmen-ite,with plagioclase constituting  50% of the restite mode. In thedehydration-melting experiments at 3 kb the onset of meltingoccurred between 850 and 900 ? C, as amphibole and quartz brokedown to yield pyroxenes plus melt. Total pressure is greaterthan in the dehydration-melting experiments and has little effecton melt composition or phase relations. In the water-saturated (water added, so that experiments, meltsformed at 3 kb and above are strongly peraluminous, rich inCa and poor in Fe, Mg, Ti, and K. Their compositions are unlikethose of most silicic igneous rocks. These melts coexist withthe amphibole-rich, plagioclase-poor restite assemblage amphibole+ magnetite ? clinopyroxene ? plagioclase ? ilmenite. The highlyaluminous nature of the melts and the plagioclase-poor natureof the restite both reflect the substantial contribution ofplagioclase (along with quartz) to melts in high-pressure water-saturatedsystems. Water pressure equals Ptoul in the water-saturatedexperiments and has a profound effect on both melt compositionand phase relations. At 1 kb, the water-saturated experimentsyielded melt and mineral products with some characteristicsof the dehydration-melting experiments (no amphibole at highT), and some characteristics of the 3-kb, water-saturated experiments(amphibole plus melt coexisting at lower T, elevated Al, loweredFe). As pressure is increased from 3 to 6. 9 kb, the stabilityfields of both plagioclase and clinopyroxene decrease relativeto amphibole and the Al contents of the melts increase. These experiments have important implications for the petrogenesisof low-K silicic rocks in arcs. First, dehydration melting isa viable mechanism for the formation of these rocks; water-saturatedmelting is not. Second, because of the influence of rock compositionon melt composition, low-grade metamorphic and hydrothermalprocesses that alter the alkali contents and Na/ K in arc basementterranes may have a direct impact on the petrogenesis of silicicmagmas in arcs, particularly the formation of extremely low-Ktrondhjemites. Third, the experiments predict that anhydrous,pyroxene- and plagioclase-rich ‘granulitic’ restiteassemblages should develop as a result of partial melting inarc terranes. Such assemblages occur in at least two deeplyeroded arc complexes. |
| |
| Keywords: | |
| 本文献已被 Oxford 等数据库收录！ |
|
