Other
Scientific paper
Jul 1961
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1961gecoa..24..250w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 24, Issue 3-4, pp.250-259
Other
20
Scientific paper
In high-grade metamorphic rocks a partial anatectic rnelt is formed at a temperature of 700 ± 40°C and 2000 atm of H 2 O-pressure whenever quartz and alkali-feldspar are constituents of the rock. If only plagioclase and quartz are the leucocratic minerals that temperature is a little higher, whereas higher pressures lowers it. Formation of a liquid portion in a high-grade regional metamorphic complex must be reckoned upon, thus giving rise to migmatites and palingenetic granitic rocks. Anatexis always starts with the formation of an aplitic quartz-alkali-feldspar melt; with small rise in temperature, this changes continually to granitic, granodioritic and tonalitic. But it depends on the ratio of the components of albite to potassium-feldspar in the metamorphic rock which of those compositions is attained by the melt in the final anatectic stage. Metamorphosed greywackes with often an extremely high ratio give tonalitic melts; those clays with extremely low ratio give aplitic melts, and rocks with intermediate ratios give granitic or granodioritic melts, as, per example, most shales. Melts formed by anatexis of metamorphosed greywackes and different lime-free and lime-bearing clays (partly with increased Na-content) are represented in Fig. 1 and compared with the frequency of distribution of magmatic rocks which contain quartz as a main constituent, Fig. 2. These are the "granitoid" or granitic rocks in the wider sense. The compositions of the anatectic melts lie well in the field of the granitoid rocks, except the aplitic melts which are not abundant and cannot be expected to be so. Regarding the formation of migmatites, a sedimentary complex of different greywackes only is visualised as an example. By metamorphism a complex of banded gneiss will be formed; when this reaches the stage of anatexis this complex will show that at a same temperature some beds are partially liquified, but the amount of melt is very different, whereas other beds are still completely solid. This has, the consequence of a very different mechanical behaviour of the different beds within the complex. This might be one of the principal causes for the very different structures which migmatites display. If the temperature of anatexis has risen further so that more than about 50% of each of the beds are liquid, then a mingling and an obliteration of the previous boundaries between different beds will take place, thus giving rise to nebulites and even to rather homogenous rocks.
von Platen Hilmar
Winkler Helmut G. F.
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