~(40)AR-~(39)AR RELEASE SPECTRA ON PLAGIOCLASES FROM THE ROGALAND ANORTHOSITE COMPLEX (SW NORWAY)

摘要

Stepwise heating ~(40)Ar-~(39)Ar release spectra were obtained for five plagioclases from different intrusions of the Proterozoic Rogaland igneous complex (Sveconorwegian Province, SW Norway): two megacrysts of the Egersund-Ogna and Helleren massif-type anorthosites, a plagio-clase from the Haland foliated anorthosite-leuconorite and two plagioclases from the Hidra leu-conoritic body. All samples yield staircase type ~(40)Ar-~(39)Ar release spectra with apparent ages of ca 400 Ma for the low temperature release steps and variable apparent ages for high temperature release steps. These release spectra result from a combination of processes. Apparent ages for the high temperature release steps of two spectra (760 ±20 and 750 ±25 Ma) are compatible with Sveconorwegian cooling through the blocking temperature of plagioclase. In the two Hidra samples, excess argon is associated to plagioclase sites outgassing at high temperature. This excess argon was probably introduced in the samples from the nearby K-feldspar rich charnockite intrusion (Farsund char-nockite) during magmatic emplacement. The release spectra of the five samples are characterized by an increase of the Ca/K ratio in the course of the heating experiment and by a positive correlation between the Ca/K ratios and the apparent ages. This trend is a clear indication of a polyphase assemblage, in which a phase with low Ca/K ratio (≤6) outgasses at low temperature whereas a high Ca/K phase (plagioclase) out-gasses at high temperature. Though the content of sericite, which probably results from the low grade Caledonian overprint, does not exceed ～1% in the plagioclase, it has a sufficiently high K content to contribute for a large proportion to the low temperature steps of the argon release spectra.