Abstract
Among the Precambrian (Gardar) intrusions in the Tugtutôq area, syenitic rocks are abundantly represented. Quartz-bearing alkalic rocks were first intruded after the emplacement of a number of large gabbro dykes. Although early Gardar nepheline syenites occur on Tugtutôq, no post-gabbro undersaturated syenites occur within the area under discussion. Quartz syenites and their faster cooled equivalents occur either as ENE directed dykes or as ring-dykes and stocks within a late Gardar central complex. The earliest quartz syenite occurs as the central component in a composite sector of one of the large gabbro dykes. This was succeeded by multiple swarms of dykes with alkalic types ranging from 'rhomb porphyry' microsyenite to comendite. Dyke intrusion was virtually at an end by the time of formation of the central ring-complex. The total petrographic range of the oversaturated alkalic rocks extends from ferroaugite-fayalite syenites with relatively calcic feldspar (ca. Or25Ab66An9) to highly acid riebeckite-bearing rock types. The various intrusions are considered to represent magma batches supplied, at irregular intervals, from a deep-seated syenite complex which was crystallising throughout the latter part of the Gardar period. The inferred sub-surface complex is taken to be closely comparable to the Gardar complexes exposed at Nunarssuit and Kungnât, and to have differentiated by process of crystal fractionation. By analogy with the Kungnât complex the parental magma of the underlying magma chamber(s) is believed to have passed through a larvikitic stage. The perthosite stock which forms the latest major intrusion on Tugtutoq is thought to indicate a slight divergence from the general larvikite-nordmarkite-alkali granite differentiation sequence. The magmas of this main differentiation sequence changed in composition along a "thermal valley" dose to that of the synthetic system Or-Ab-SiO2-H2O.
Highlights
I n a previous publication (UPTON, 1962), a review was presented of the general geology of the Tugtutoq-Skovfjord district, laying particular emphasis on the Precambrian intrusives of the Gardar volcanic cycle which give rise to the more striking geological features of the area
The earliest group of intrusions in the area generally acknowledged to be of Gardar age are the NW trending dykes of olivine dolerite which traverse the area in discordance with the regional NE to ENE strike of the older granitic terrain. These dykes were followed by a prolonged episode in which a multitude of co-genetic basic and alkaline dykes were emplaced parallel to the old-established ENE structural trends of the granodioritic and migmatitic country rocks
A related nepheline syenite forming the central part of the Hviddal giant dyke preceded the more silicic rocks described in this publication and is discussed separately in part IV of this series of papers, (UPTON, 1964)
Summary
I n a previous publication (UPTON, 1962), a review was presented of the general geology of the Tugtutoq-Skovfjord district, laying particular emphasis on the Precambrian intrusives of the Gardar volcanic cycle which give rise to the more striking geological features of the area. The earliest group of intrusions in the area generally acknowledged to be of Gardar age are the NW trending dykes of olivine dolerite which traverse the area in discordance with the regional NE to ENE strike of the older granitic terrain. These dykes were followed by a prolonged episode in which a multitude of co-genetic basic and alkaline dykes were emplaced parallel to the old-established ENE structural trends of the granodioritic and migmatitic country rocks.
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