Gronnedal-Ika

Emplaced in quartzofeldspathic gneisses and metasediments containing some basic dykes, the Gronnedal-Ika complex is 8x2.8 km but intensive faulting has changed an originally relatively simple oval outline (Emeleus, 1964, Fig. 25 and Plate 3) into an irregularly-shaped intrusion. The outer contacts are generally faulted or obscured by drift and weathering, but generally appear to be vertical or steep and outward dipping. Metamorphism and metasomatism of the gneisses is minimal. The igneous rocks can be divided into several groups, the earliest of which is a Lower Series of mainly medium grained, well laminated foyaites which are overlain by a raft-like body of gneiss separating a later Upper Series, also well laminated, of foyaite, pulaskite and pyroxene-rich syenite, the last of which is layered. The lamination generally dips inwards at moderate to steep angles. Xenolithic porphyritic syenite and porphyritic microsyenite complete the nepheline syenite suite, which was then intruded by a central plug of carbonatite. The nepheline syenite suite encompassed a wide range of petrographic types including foyaite, ditroite, ijolite, malignite, nepheline-poor pulaskite and some perthosite. Often rock types grade rapidly from one to another even in the scale of the hand specimen. Alkali feldspar usually forms thin tabular crystals and is perthitic, while nepheline may be fresh or altered to a fibrous, micaceous aggregate. Cancrinite is widespread and some sodalite is present. Aegirine-augite is found in most rocks as prismatic or poikilitic crystals and biotite and amphibole are sometimes present. Apatite, fluorite, calcite and opaques are accessory. There are three major outcrops of carbonatite within the nepheline syenites, and smaller masses throughout the complex and extending into the country rocks, although Emeleus (1964, Plate 3) considered that they once defined a single body. The carbonatite is flow-banded and xenoliths are usually numerous; it extends into the surrounding nepheline syenites, brecciating and altering them. It consists of variable proportions of calcite, siderite and magnetite with minor apatite, sphalerite, pyrite, Sr-rich baryte and accessories. Numerous dykes cut the complex and extend well into the country rocks, the majority pre-dating the major faulting. They include lamprophyres of many types, several generations of feldspar-phyric basalt, dolerite and olivine dolerite, many reaching a considerable size, and several groups of trachytic and phonolitic dykes. The alkaline dykes are generally 1-5 m in thickness and cut all members of the alkaline complex as well as the basic dykes. The majority are phonolites of alkali feldspar, nepheline and aegirine-aegirine-augite, with some containing analcime or natrolite; the chemistry of this group has been investigated by Gill (1972). Trachytes and quartz trachytes comprise perthitic feldspar, biotite and riebeckite with aegirine-augite or aegirine and possibly arfvedsonite. Experimental work on specimens of foyaite and nepheline syenite are described by Sood and Edgar (1970).