The petrology and geochemistry of intrusions at selected nunataks in the Ahlmannryggen and Giaeverryggen, western Dronning Maud Land, Antarctica.

Abstract

The mid-Proterozoic Borgmassivet intrusions of western Dronning Maud Land intrude Archaean granites and a volcano-sedimentary sequence, the Ritscherflya Supergroup. They are SiO2-rich ultramafic to mafic tholeiitic rocks which occur as layered bodies of unknown dimensions and sills up to 400 m thick. This thesis records detailed field, petrographic and whole rock geochemical studies on Borgmassivet intrusions at three widely-spaced localities within an area of approximately 20 000 km in the Ahlmannryggen and Giaeverryggen. Annandagstoppane-Juletoppane-Forstefjell area: The Annandagstoppane intrusions form part of a layered body or bodies, of which only a small part is exposed. They consist of a medium-grained 'main suite ' of gabbronorites and minor anorthosites, and a Iyounger suite ' of quartz diorite pegmatites, basaltic dykes, fine- to medium-grained gabbroic sills and minor albitite veins. The main suite rocks are ortho-cumulates in which plagioclase, orthopyroxene and clinopyroxene were primocrysts which crystallized in situ. The dykes and sills show typical basaltic and doleritic textures. Geochemical evidence suggests that the main and younger suites may be consanguineous. Robertskollen-Krylen area: The layered complex at Robertskollen comprises a lower, rhythmically layered ultramafic unit, overlain by a mafic unit. Olivine, orthopyroxene and clinopyroxene are the dominant cumulus phases in the ultramafic rocks, whereas plagioclase, orthopyroxene and clinopyroxene fractionation occurred during crystallization of the mafic rocks. Metastable co-existence of quartz with olivine and anomalous incompatible trace element characteristics of the Robertskollen complex suggest contamination of the magma(s) by crustal material. The Krylen intrusions show petrographic and geochemical characteristics similar to those of the main suite at Juletoppane. A felsic dyke at Krylen may represent a rheomorphic product derived from Ritscherflya sediments. (iii) Grunehogna-Jekselen area: The Grunehogna sill is a medium-grained diorite of unknown thickness, characterized by varying amounts of plagioclase and amphibole and a high Fe-Ti oxide content. It is overlain by a 50 m-thick quartz diorite pegmatite. The overlying 400 m-thick Kullen sill varies in composition from gabbronorite to gabbro and diorite and shows evidence for contamination by crustal material. Vugs, evidence for fusion, destruction of sedimentary structures and deformation in sedimentary contact zones and xenoliths and the abundance of pegmatites suggest that the sills intruded wet, unconsolidated or partially lithified sediments. The Jekselen complex consists of quartz diorites intruding Ritscherflya sediments. Amygdales in the upper zones of the complex indicate the subvolcanic nature of the intrusion. Major, trace and rare earth element data of the Borgmassivet intrusions and the Straumsnutane basalts (Watters, 1969a, 1969b, 1972, pers. comm., 1985) show a strong regional coherence, indicating that the rocks may be consanguineous. Abundance ratio patterns ('spidergrams') of the intrusions and basalts are identical. They are consistent with crustal contamination, possibly leucotonalite, of the magmas during ascent. The present distribution of the Borgmassivet. intrusions and Straumsnutane basalts reflects emplacement at stratigraphically higher levels within the Ritscherflya Supergroup from west to east. Previous radiometric isotope studies suggested that the intrusions are approximately 1700 to 1800 Ma in age, but recent investigations show that the isotopic data are poorly understood and have to be re-evaluated. Some of the isotope characteristics may result from crustal contamination and alteration effects during intrusion into water-saturated sediments.