The petrology and structure of the Motzfeldt centre, Igaliko, south Greenland.

Abstract

The Motzfeldt Centre is the second of four centres of alkaline igneous activity which form the Igaliko Complex. A vertical exposure of 1700m shows several overlapping intrusive units of syenite and nepheline syenite (units SM1 to SM5), which bear mantle Sr isotope ratios and were emplaced by a combination of 5-10km radius ring fractures and block subsidence. The larvikitic parent to SM5 and the presence of a late alkali gabbro dyke, must attest to the availability of more basic magma at depth. Xenoliths from an earlier supracrustal sequence dominated by lavas, form. rafts up to 200-300m thick and several square kilometres in area. Marginal phonolites of the nepheline syenites (SM2, 3 and 4 in particular), which are thought to 'represent liquid compositions, bear a strong resemblance to phonolites from these lavas and they are quite possibly genetically related, A late sill- like body of lujavrite (eudialyte-nepheline microsyenite = SM6) rich in volatiles and "incompatible" elements, formed beneath some of these rafts. In general, phonolitic magmas were emplaced at temperatures of approximately 1000ºC to form the larger units and were substantially solidified probably within 0.1-0.2 Ma. The considerable variation in rock types found in each major intrusion, is thought to be attributable to varying degrees of crystal fractionation in-situ and differential accumulation (by a variety of mechanisms), from an initially homogeneous magma. Of the extensive mineralogical variations which necessarily occurred, crystallisation of zirconium-rich aegirine depended on low oxygen fugacity, high peralkalinity and the absence of eudialyte. Electron probe microanalysis has shown that REE-bearing minerals are common accessories and may or may not have a negative Eu-anomaly; these may be LREE- or HREE-enriched, Both fractionation of phonolite magma and recycling of stoped nepheline syenites by partial re-melting could give rise to magma of lujavritic composition.