Evidence of crustal disequilibrium melting, mingling processes, layering and deformation in UHT conditions: The marginal zone of the Egersund-Ogna massif-type anorthosite (S. Norway)

Abstract

Septa of banded gneisses crop out along the margin of the Proterozoic Sveconorwegian Egersund-Ogna (EGOG) anorthosite pluton. They permit the observation of the effects of a diapiric intrusion of anorthosite on its envelope of gneisses. The rocks are mylonitic and result from a strong deformation in UHT granulite facies conditions due to contact metamorphism. Concordance of the foliations in the septa and in the leuconoritic margin of the anorthosite suggests that the deformation was linked to the ballooning of the anorthosite. The septa lithology is dominated by noritic gneisses interleaved with felsic gneisses and comprises minor amounts of mafic granofels, metaquartzites and metagreywackes. A charnockitic dyke is cutting across the foliated structure. Geochemical data (major and trace elements) indicate that the noritic gneisses did not result from the metamorphism of amphibolites belonging to the high-grade gneiss complex characteristic of the wall rocks of the Rogaland anorthosite province. Instead they have strong similarities with a high-alumina basalt that is the possible parental magma of the EGOG anorthosite. Slight variations from norite to leuconorite compositions were induced by metamorphic differentiation. The felsic rocks encompass a large interval of rock types from granite to tonalite and quartz diorite. They compare with leucogranitic enclaves occurring in the charnockitic upper part of the Bjerkreim-Sokndal layered intrusion. Most felsic lithologies show the signature (including high positive Eu anomalies) of anatectic processes in disequilibrium melting conditions and rapid extraction from the source rocks. Flowage differentiation in the emplacement process explains the various compositions, as commonly observed in migmatite leucosomes. The metaquartzites show a relatively high Ti concentration and the Ti-in-quartz geothermometer records a minimum temperature of 820 °C. The study brings evidence that the high-alumina basalt and the felsic magmas were aggregated but did not interacted to yield hybrid magmas, possibly because the time interval was too short. They mingled in various proportions to form the norito-felsic gneisses. The aggregated magmas were intimately linked to the EGOG intrusion in which they played a role as a lubricating agent. They were injected into the wall rocks and were deformed by ballooning from superheated UHT conditions down to subsolidus conditions where mylonitic structures developed. Finally, a charnockitic magma was still able to intrude at a lower temperature below the ductile-brittle transition.