The geology of Tugtutôq and neighbouring islands, South Greenland. Part IV: The nepheline syenites of the Hviddal composite dyke

Abstract

A 20 km composite dyke ca. 550 m broad intruded early in the Precambrian Gardar igneous province of south Greenland, consists of a central intrusion of syenite and relatively narrow margins of syeno-gabbro. The syenite is later than the syeno-gabbro. The syenite varies along the dyke from a larvikite-type augite syenite at its WSW extremity to a soda-rich nepheline syenite in the ENE end. The feldspar composition range from WSW to ESE is in excess of Or_38.5Ab₆₁An_.5 - Or₄₉Ab₅₁An₀ and the pyroxene changes from titanaugite towards nearly pure aegirine. Amphiboles vary from barkevikite-type hornblendes to ferrohastingsites and are absent in the more alkalic ESE dyke-rocks. Ferrohortonolite and lepidomelane are present in the syenites of the WSW outcrops. Sodium increases at least from Na₂O 5.84% to 10.37% and the Fe’’’/ F’’ ratio rises towards the ENE. Ca, Mg, Ti and P decrease in this same direction. Six rocks from along the dyke were analysed for major and (by emission spectroscopy) for minor elements. Ba and Sr decrease rapidly in the ENE direction through ranges of at least 6000- 10 ppm and 340-14 ppm respectively. In the six rocks examined the Rb/K ratio increases by a factor of over three and the Ga/Al ratio by a factor of over 2 in the ENE direction. Two pyroxene analyses are presented in addition to six partial analyses of feldspars. The observed longitudinal variation is believed lo he due to cryptic and phase layering in an accumulitic suite of dyke syenites which has been lilted and eroded to reveal a sequence of perhaps 2000 m of layered rock. Neither the original base nor roof of the series is seen. The dyke is believed to have crystallised below a thick cover of volcanics under confining pressures in excess of 840 kg/cm² from a 'parental' larvikitic magma. Crystallisation of this body is believed to have extended from above 750° to the region of 400°C.