The analcime-phyric phonolites (blairmorites) and associated analcime kenytes of the Lupata Gorge,Mocambique

The petrology and mineralogy of three lavas from the Lupata Gorge, Mocambique, containing primary euhedral analcime phenocrysts, up to 1.5 cm in diameter, as well as potassium feldspar and nepheline phenocrysts, are described. Electron microprobe analyses of these phases and a whole rock analysis of the blairmorite are given. Reference to published and unpublished experimental work indicates that these rocks must have been generated at water pressures between about 5 and 13 kbars, implying depths of origin of between 20 and 50 km. Very rapid transport to the surface and quenching is implied. It is suggested that these indications of the considerable depth of origin of these rocks, taken together with the absence of associated intermediate and basic rocks, lends credence to the hypothesis of D. K. Bailey that the voluminous associations of salic igneous rocks found in parts of the African continent, unaccompanied by associated basic rocks, are explicable in terms of partial melting, under high water pressures, of the lower part of the crust.     

Resilicification at the top of the foreland bauxite in Surinam and Guyana

In the Northern foreland of Guyana-shield bauxites rest on basement or on Tertiary sediments. They are of Eocene age and represent very porous gibbsite rocks, which are free of quartz and feldspar and which contain only traces of kaolinite. They are covered by Tertiary sediments. The overburden in Guyana consist of the white sands of the Montgomery-Formation and in Surinam of Feldspar bearing sands of the Cosewyne Formation. A late diagenite to epigenetic resilicification which progressed from the bauxite surface into the substratus can be observed at several places in Guyana and Surinam. This resilicification has already been described by Aleva (1965) and by van der Marel (1960 in Montagne 1964). Time and mechanism of this resilification process is of interest. The Eocene foreland bauxite of Surinam and Guyana has been kaolinitizised along joints and fissures upon supply of silicic acid derived from weathering of the sedimentary overburden. At the same time detrial grains of quartz and feldspar have been flushed in from above. The resilicification is thus younger than the sediments on top.     

Microfracturing and faulting in a limestone

Two fundamentally distinct types of microfractures are present in an experimentally deformed limestone: subaxial microfractures and microfaults. Macroscopic faults are composed of coalesced microfaults and are not related to the subaxial microfractures. A high-temperature mechanical instability occurs at temperatures of 200° C and above when the confining pressure is 600 bars or less.     

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