The Apophysis of the Bjerkreim–Sokndal layered intrusion (Rogaland anorthosite province, SW Norway): A composite pluton build up by tectonically-driven emplacement of magmas along the margin of an AMC igneous complex

Abstract

The Late-Proterozoic Bjerkreim–Sokndal layered intrusion (BKSK) is connected to a foliated, sheet-like igneous body (the Apophysis), that is a potential feeder for the BKSK magma chamber. Field, petrographical, geochemical and structural data are used to demonstrate that the Apophysis is a composite igneous body, constructed by coeval mafic to felsic magmas that were collected in a sub-vertical shear zone. Three liquid lines of descent are distinguished in the main Apophysis component (a felsic series, predominantly quartz mangeritic) and in coeval felsic rocks from the upper part of the BKSK. Minor mineralogical and geochemical discrepancies between these three trends are indicative of distinct sources and crustal contaminants, as well as slight differences in the differentiation mechanisms. Jotunitic to noritic cumulates or crystal-laden magmas, associated with their trapped melts, mingled with the felsic series in two distinct portions of the Apophysis. In one area, this association is dominated by a FTP (Fe–Ti–P-rich) jotunite, interpreted as an accumulation of pyroxenes + Fe – Ti oxides + apatite + plagioclase. In the second area, the melt dominates over the associated cumulate; it is a primitive (MgO-rich and K 2O-poor) jotunite, that was also involved in the genesis of another igneous body in the vicinity of the Apophysis. Magma mixing, in addition to mingling, was also potentially important in the petrogenesis of some jotunite rocks.