Cretaceous mafic dyke swarm, Peary Land, northernmost Greenland: Geochronology and petrology

A group of dyke rocks from the Frederick E. Hyde Fjord area form the southe rn termination of a prominent dyke-swarm in Peary Land, northernmost Greenl and. The melanocratic dykes are NS- to NW-SE-trending, vertically dipping, less than or equal to 10- 12 m wide, and have narrow chilled margins. Whole -rock Ar-40/Ar-39 dating of three dyke rocks indicates emplacement at ca. 8 5 Ma, some 15 to 20 Ma older than previous K-Ar whole-rock dating indicated . Whole-rock Ar-40/Ar-39 dating of three Lower Paleozoic metasedimentary ro cks collected hundreds of meters away from the dykes reflects partial reset ting 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 microphenoc rysts of titaniferous (less than or equal to4 wt.% TiO2) clinopyroxene in a matrix of olivine (Fo(69-35)), plagioclase (An(69-35), less than or equal to1.0 wt.% K2O), clinopyroxene and ilmenite; rare matrix biotite [Fe/(Fe Mg) = 0.65] occurs, and intergranular granophyre is present in the most evo lved 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 affinit y to 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 betwee n 0.42 and 0.45 indicate that the rocks do not represent primitive magmas. Abundances of the rare-earth elements (Sigma REE = 170 to 200 ppm, La-N = 1 00) and strongly fractionated patterns (La-N/Lu-N = 10) are consistent with the alkaline nature of the dyke rocks. The lack of negative Eu anomalies i n chondrite-normalized REE profiles is consistent with the absence of plagi oclase phenocrysts, suppressed by a high H2O content of the magma. Normaliz ed extended trace-element diagrams and Sr-87/Sr-86 versus Nd-143/Nd-144 iso topic plots indicate affinities with OIB. Although the Sr-87/Sr-86(i) value s (0.70467 +/- 10) do not indicate crustal contamination, delta O-18 values (+6.0 to +7.9 parts per thousand) are above typical mantle values and sugg est some crustal influence.