Abstract

U-Pb SHRIMP zircon and 40Ar/39Ar mineral ages are reported for rocks of the Cape Meredith Complex, West Falkland. Felsic gneisses of the oldest Big Cape Formation give a zircon date of 1118 ± 8 Ma, interpreted as the time of extrusion of the rhyolitic protoliths. The three phases of granitoid which intrude the Big Cape Formation, namely granodiorite orthogneiss (Gl), syntectonic granite gneiss (G2) and post-tectonic granite (G3), gave U–Pb dates of c. 1090, 1067 ± 9 and 1003 ± 16 Ma respectively. Metamorphic overgrowths on zircons from the felsic gneisses were dated at c. 1000 Ma, coeval with the G3 granite, whereas a date of 1135 ± 11 Ma from inherited cores in G2 zircons were probably derived from a slightly older component of the Big Cape Formation. 40Ar/39Ar hornblende ages from amphibo-lites of the Big Cape Formation (1009 ± 14 and 1015 ± 6 Ma), along with muscovite (989 ± 3 Ma) and biotite (989 ± 7 Ma) from G3 pegmatites show that the complex cooled relatively rapidly to below 350°C, with no evidence of any Pan-African (c. 500 Ma) thermal overprinting. These data show that the older rocks of the Cape Meredith Complex are significantly younger than the equivalent rocks of the Natal Metamorphic Province, SE Africa (c. 1200 Ma), though they are comparable in age to those of Western Dronning Maud Land (East Antarctica), areas which Gondwana reconstructions place west and east of the Falkland Microplate respectively. The older rocks of the Cape Meredith Complex may therefore represent younger outboard arc terranes relative to those exposed in Natal. However, the syn- to post-tectonic granites of the Cape Meredith Complex are comparable in age in all three areas (1070–1000 Ma), suggesting similar collisional and post-collisional histories throughout the entire region. The lower temperature history of the Cape Meredith Complex is comparable to data from Natal, but does not show the Pan-African (c. 500 Ma) overprint which is characteristic of much of western Dronning Maud Land. This situation is consistent with the proposed position of the Falkland Microplate between SE Africa and East Antarctica in Gondwana, where a general eastward increase in the intensity of the Pan-African thermal effects has been recorded.

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