Mathematics – Logic
Scientific paper
May 1985
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985esrv...22....1e&link_type=abstract
Earth Science Reviews, Volume 22, Issue 1, p. 1-92.
Mathematics
Logic
5
Scientific paper
Rapakivi granites have puzzled all who have seriously studied them. Typical rapakivi texture is a mixture of variously mantled, non-mantled or partly mantled, concentrically zoned, plastically distorted, fragmented, reaggregated, large and small ovoids. Commonly they are potash feldspar often mantled by, and having a variable content of plagioclase. Some display remarkable sphericity. In form, composition, zoning sequence, and crystallization pattern each ovoid reflects an individual development. Up to five concentric internal plagioclase rims have been observed and some ovoids may be isolated in autoliths and wall-rocks. Anomalies and contradictions arise from any assumption of genesis from a cooling melt. The recorded objective data imply that the “magma” from which rapakivi textures develop had similar diffusive and rheological properties to those of a partly dewatered macromolecular paste or a mixture of gelatinous hydrosilicates. As indicated by deep oil wells this system is found at somewhat elevated temperatures at considerable depths within accumulated sediments. In addition to the very specific diffusive and rheological properties of such partly dewatered sediments, the system has as its major components normal solvated macromolecules of mixed clays, silica gels and hydrous ferromagnesian minerals which are characterised by distinctive particle sizes and geometric shapes (platelets, spheres and rods, respectively). Thixotropic liquefaction and intrusion of such concentrated gelatinous “magma” or sediment paste introduces relative movement between the component macromolecules whereby they can reduce surface energy by interaction to assume a “close-packed” condition and aggregate during laminar flow into macro-accretions comprised essentially of their respective particle shapes. Syneresis of these precursor accretions desorbs ions, including the small montmorillonite particles behaving as a colloidal electrolyte. These diffuse from the illitic cores to form a montmorillonite-rich rim which it is suggested crystallizes together with the illitic cores to form mantled or polymantled feldspar ovoids. Crystallization of the rapakivi massif is associated with strong temperature rise stemming from exothermic crystallization of the close-packed metastable colloids. This follows the development of the characteristic texture. The rounded and rimmed precursor accretions are formed during earlier lower-temperature episodes of thixotropic liquefaction which are isothermal. The fluidity is an earlier event. There is high temperature dependent on the rate of water loss but no molten stage. Forty-six typical features of rapakivi texture are described and illustrated, each of which is directly attributable to specific interactions in an alternately dynamic and static colloidal system. Individual correlation between each observed distinctive feature of the rapakivi texture and the well-documented physico-chemical process is complete. For sediment-derived granites, therefore, the rapakivi texture can confidently be assumed to be an indication of the crystallization of their sedimentary hydrosilicate precursors.
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