GREENLAND: GEOCHRONOLOGY AND PETROLOGY
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 40Ar/39Ar dating...
| Main Authors: | , , , , , , |
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| Format: | Text |
| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.556.2392 http://rruff.info/doclib/cm/vol39/CM39_997.pdf |
| Summary: | 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 40Ar/39Ar 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 40Ar/39Ar dating of three Lower Paleozoic metasedimentary rocks collected hundreds of meters away from the dykes reflects partial resetting during dyke-rock emplace-ment 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 (Fo69–35), 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 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 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 (LaN/LuN = 10) are consistent with the alkaline |
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