A petrographic, structural and geochemical study of the alkaline igneous rocks of the motzfeldt centre, south Greenland

• Main Author: Bradshaw, Colin
• Format: Thesis
• Language: unknown
• Published: 1988
• Subjects: Greenland; Igaliko
• Online Access: http://etheses.dur.ac.uk/6446/; http://etheses.dur.ac.uk/6446/1/6446_3746.PDF

The Motzfeldt Centre (1310 +/- 31 MY) is one of four Gardar alkaline igneous Centres belonging to the Igaliko Nepheline syenite Complex, South Greenland. Motzfeldt is a multiphase, high-level intrusive ring-centre comprised principally of nepheline syenite and emplaced in the Proterozoic Julianehab granite and the overlying Gardar volcano-sedimentary succession. The Centre commenced with the intrusion of three poorly centralised satellitic intrusions of syenite, pulaskite and nepheline syenite, collectively known as the Geologfjeld Formation. These are partly truncated by concentric, multiple intrusions comprising the Motzfeldt Ring Series whose steep-sided contacts dip outwards and individual nepheline syenite units young inwards. On the basis of field relations, petrography and geochemistry the Ring Series is further subdivided into the Motzfeldt Sø and Flinks Dal Formations, and a number of minor intrusions collectively termed the Hypabyssal Series. The results of field surveys, carried out during two summer field seasons, are presented on a 1:50,000 geological map. The petrography and field relations are described for 16 distinct, plutonic and hypabyssal rock units which range in lithology from larvikite to lujavrite. These represent at least 10 separate intrusive episodes and show a remarkable array of rock textures, mineralogical and geochemical features.170 whole-rock (XRF), 33 Rare earth element (INAA) and over 300 mineral (EDS) geochemical analyses are presented. These show that the syenite/nepheline syenite lithologies in Motzfeldt can be subdivided chemically and mineralogically into the three groups: 'hypoalkaline', alkaline and peralkaline. The geochemical features of the various units are evaluated and elemental behaviour discussed. The data is additionally assessed, using non-parametric statistics, as a means of discriminating between the units. A number of units which have proved difficult to separate in the field are established to be geochemically distinct, whilst others are shown to be very ...