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

Abstract

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 closely associated. The peralkaline, pegmatite rich, silica saturated outer and upper margins of the Motzfeldt Sø Formation and its associated microsyenite sheet sequence, host extensive economic reserves of Nb, Ta, Zr, U, Th and LREE. The evolution of these mineralised zones is discussed and the importance of country rock (+ water) — magma interaction emphasised. Recent works have helped clarify the magmatic development of the Gardar Province. Here emphasis has been placed on the structural evolution of the Gardar with the aim of complementing these works. The Gardar represents a prolonged (c.200 MY), cyclic period of limited, passive intracontinental extension. Crustal thinning facilitated the rise, along deep fracture zones, of magmas generated by higher thermal gradients. In response to regional, sinistral shear stresses, ENE extensional fractures and associated dyking developed. In addition, crustal decoupling occurred along several parallel WNW-ESE sinistral strike-slip faults. Motzfeldt and other ring centres of the Gardar are preferentially located at the intersections of these zones of weakness.