25 Undergraduate Research – Igneous Petrology (Posters)

Tuesday, 24 April 2001
1:00 PM - 4:30 PM

USING GEOCHEMISTRY TO DISCERN THE PROTOLITHS OF LAYERED ECLOGITES AND DIKES FROM THE DANMARKSHAVN AREA, NORTH-EAST GREENLAND CALEDONIDES

Paper number: 25-10
Program: Technical Sessions
Time: 1:00 PM – 4:30 PM

Authors

Abstract

Metabasic dikes and lenses of layered eclogite facies rocks are common in the Danmarkshavn area (76 46’, 18 40’), in the southeastern part of the North-East Greenland eclogite province. These mafic rocks lie within Archean to Paleoproterozoic quartzofeldspathic gneisses which represent continental crust of the Laurentian margin that was deformed and metamorphosed during the Caledonian orogeny. The aim of this contribution is to characterize the geochemistry of these two suites of mafic rocks in an effort to better understand their protoliths. The dikes are 1-10 km thick; one population is concordant with the dominant gneissosity, while another is clearly discordant with the gneisses. Some of the dikes are folded, while others are tabular for long distances; all contain a Caledonian metamorphic fabric. Lenses of eclogite facies rocks up to 100 m long consist of three main lithologies (kyanite-eclogite, garnet-clinopyroxenites and garnet websterites) that are interlayered on the meter scale. The layered eclogite lenses are boudins within the quartzofeldspathic gneiss, which occupy an along-strike outcrop area of approximately 4 x 1 km; they appear to be dismembered parts of a once more coherent igneous intrusion.

The three main eclogite lithologies may be identified by geochemical differences that generally reflect modal abundances. The garnet clinopyroxenites are characterized by the lowest silica content, higher Na2O and Al2O3 than the websterites, and lower K2O and CaO than both of the other groups. The garnet websterites have the highest silica content, much less Na2O and Al2O3 than the other lithologies, and higher K2O and TiO2 content. The kyanite eclogites have an intermediate silica content, notably less Fe2O3 and MnO than the other lithologies, and the highest Al2O3 content. Dikes are basic in composition (47.8-53% SiO2; 4-7.7% MgO; 36-604 ppm Cr) and show marked increases in incompatible element abundances with Si content, reflecting some differentiation, even over the narrow range of compositions.