FRACTIONAL CRYSTALLIZATION MODELING OF THE SYENITIC AND BASALTIC DIKE SUITES OF THE GARDAR PROVINCE, SOUTH GREENLAND

The Gardar Province in South Greenland contains multiple dike swarms that intruded their Ketilidian host rocks at ~1.28 Ga. and ~1.18 Ga. The 1.18 Ga. dike swarm contains two suites of dikes that both have average trends towards the ENE. One of the suites is comprised of basaltic dikes whereas the other is comprised of syenitic dikes. There are two known composite dikes in the region that contain basaltic margins and syenitic centers suggesting that the two suites are comagmatic. This study seeks to determine if the suites are related by the fractional crystallization of a common parental magma. To answer this question we utilized an Excel based computer program to model the major and minor element trends displayed by both suites of dikes. The major element data show that the suites cover a MgO range of 6.5 wt.% to 0.17 wt.%. Most major and trace elements show an inflexion towards increasing or decreasing concentrations at ~4.0 wt.% MgO, suggesting a significant change in the fractionating phase assemblage. Additionally, we modeled incompatible trace element data by using the average composition of the basaltic dikes and trying to fit the pattern displayed by the average composition of the syenitic dikes. A comparison of the two suites show that the syenitic dikes are enriched in incompatible elements relative to the basaltic dikes with the exception of Ti, P, Sr, and Ba. Neither of the suites show a negative Nb or Ta anomaly as seen in the Ketilidian host rocks, suggesting that crustal contamination did not have a significant affect on the chemical evolution of either suite.

Modeling results for the major and minor elements show that good fits to the data could be produced by a model that first fractionates olivine (Ol), clinopyroxene (Cpx), plagioclase (Plag), and titanomagnetite (Ti-mt) in proportions of 19:25:53:3, respectively. This is followed by fractionating orthopyroxene (Opx), Cpx, Plag, Ti-mt and apatite (Ap) in proportions of 12:25:48:12:3, respectively. These same proportions also gave a good fit for the incompatible trace element pattern displayed by the average syenite. These results suggest that the basaltic and syenitic dikes of the Gardar Province are comagmatic and that the primary differentiation mechanism was fractional crystallization.