Composition des plagioclases residuels et des plagioclases d'origine anatectique elabores lors du phenomene de palin- genese basique

Forelesninger holdt ved Advanced Study Institute of Feldspar - Oslo 1962
Lectures held at Advanced Study Institute of Feldspar - Oslo 1962

The geological and petrographical study of the anorthosito-leuconoritic complex of HÃ¥land-Helleren (E gersund-Norway) indicates that the genesis of that complex is connected with a process of basic anatexis resulting in the separation of a liquid phase, the anatectic leuconorite (mobilisat), from a residual solid phase, the para-anatectic anorthosite (restite).

Under the microscope, the residual plagioclases, constituting the anorthosite, are easily distinguished from the plagioclases which belong to the mineral association forming the anatectic leuconorite. However, their composition (anorthite content) is identical in both cases (42% An. approximately).

This homogeneous composition can be explained by means of the weil known diagram of the albite-anorthite system, if we take into account that a process of diffusion occurs during the cristallization of the anatectic liquid.

However, in the case of high pressures acting in the deep zones of the earth crust, it would be possible to conceive a variant of that diagram by introducing a minimum point at 42% An.