CRETACEOUS MAFIC DYKE SWARM, PEARY LAND, NORTHERNMOST GREENLAND: GEOCHRONOLOGY AND PETROLOGY

Abstract

A group of dyke rocks from the Frederick E. Hyde Fjord area form the southern termination of a prominent dyke-swarm in Peary Land, northernmost Greenland. The melanocratic dykes are NS- to NW–SE-trending, vertically dipping, ≤10–12 m wide, and have narrow chilled margins. Whole-rock 40 Ar/ 39 Ar dating of three dyke rocks indicates emplacement at ca. 85 Ma, some 15 to 20 Ma older than previous K–Ar whole-rock dating indicated. Whole-rock 40 Ar/ 39 Ar dating of three Lower Paleozoic metasedimentary rocks collected hundreds of meters away from the dykes reflects partial resetting during dyke-rock emplacement and suggests that the area experienced a greater thermal disturbance than the outcrop distribution of the dyke rocks would suggest. The dyke rocks contain normally zoned, euhedral microphenocrysts of titaniferous ( ≤4 wt.% TiO2) clinopyroxene in a matrix of olivine (Fo 69–35), plagioclase (An69–35, ≤1.0 wt.% K2O), clinopyroxene and ilmenite; rare matrix biotite [Fe/(Fe + Mg) = 0.65] occurs, and intergranular granophyre is present in the most evolved dykes. Whole-rock geochemistry indicates a basaltic composition, with the least evolved rocks containing 5.7% MgO, 14% FeOT, 4% TiO2, 2.8% Na2O, and 0.9% K2O (by weight). In terms of major elements, the rocks correspond to sodic-type alkali basalts; the normative mineralogy indicates an affinity t o both olivine tholeiites and alkali basalts. Trace-element contents (45 ppm Ni, 45 ppm Co, 80 ppm Cr, 335 ppm V, on average) and mg numbers between 0.42 and 0.45 indicate that the rocks do not represent primitive magmas. Abundances of the rare-earth elements (REE = 170 to 200 ppm, LaN = 100) and strongly fractionated patterns (La N/LuN = 10) are consistent with the alkaline nature of the dyke rocks. The lack of negative Eu anomalies in chondrite-normalized REE profiles is consistent with the absence of plagioclase phenocrysts, suppressed by a high H 2O content of the magma. Normalized extended trace-element diagrams and 87 Sr/ 86 Sr versus 143 Nd/ 144 Nd isotopic plots indicate affinities with OIB. Although the 87 Sr/ 86 Sri values (0.70467 ± 10) do not indicate crustal contamination, 18 O values (+6.0 to +7.9‰) are above typical mantle values and suggest some crustal influence.