98 Precambrian Geology (Posters)

Monday, 19 October 2009
9:00 AM - 6:00 PM
Hall A

EDIACARAN GNEISSES ALONG THE OSSA-MORENA - CENTRAL IBERIAN ZONE BOUNDARY, PORTUGAL; THEIR DISTRIBUTION, CHARACTER, AGE AND METAMORPHIC HISTORY

Paper number: 98-10
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

The studied area is located on the Ossa Morena-Central Iberian Zone boundary, where the following sequence is observed from base to top: the Pouchão Mafic Complex, the Mouriscas-Sardoal Gneisses and the Serie Negra Unit, all bounded by thrust faults. The Pouchão Mafic Complex and the Mouriscas-Sardoal Gneisses are high grade metamorphic rocks. The Serie Negra is a low grade volcano-sedimentary succession intruded by rhyolitic dykes. This paper reports the first radiometric ages obtained by U-Pb ID-TIMS method for the high grade metamorphic rocks. A Mouriscas-Sardoal granitic gneiss contains inherited Proterozoic zircon with ages that range up to 1900 Ma, igneous prismatic zircon with an age of 570 ± 5 Ma and metamorphic monazite that yields an age of 540 ± 5 Ma. The Pouchão Mafic Complex contains zircon with equant multi-faceted metamorphic morphology that also yields an age of 539 ± 3 Ma for the high-grade metamorphic event. Igneous monazite from a rhyolite dyke cutting the Serie Negra succession yields an age of 308 ± 1 Ma. These data outline a sequence of events; the formation of a magma that involved melting of a source that contained mid-Proterozoic material, magma crystallization at 570 Ma, followed by a metamorphic event at 540 Ma during the Cadomian Orogeny, with a second metamorphic overprint in the Variscan event.