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Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK
1 Department of Earth Sciences, The Open University, Milton Keynes, Buckinghamshire MK7 6AA, UK
Garnet occurs within rhyolitic to dacitic lavas, ignimbrites and volcaniclastic sediments of the Pyrenean calc-alkaline Permo-Carboniferous volcanic rocks. On the basis of texture and major and trace element composition, the garnet is interpreted as having a phenocrystal as opposed to a xenocrystal or restite origin.
The Pyrenean silicic volcanics are strongly peraluminous, consistent with derivation as a partial melt from a sedimentary source. They were erupted contemporaneously with the Hercynian thermal anomaly, which produced regional granulite facies metamorphism and anatectic granites. The ‘Late Hercynian granodiorites’ were also intruded at about this time. Geochemical data suggest that the petrogenesis of the volcanic rocks differed from that of the granodiorites.
REE garnet/matrix partition coefficients for garnets from a welded rhyolitic ignimbrite are similar to those reported for other phenocrystal garnets in rhyolites. REE abundances imply that the garnets crystallized from a HREE depleted melt and this is considered to reflect the effects of garnet in the restite. Experimental constraints suggest that the garnet crystallized at pressures >4–5kb, implying that the rhyolites originated from > 14–17km depth and were erupted rapidly from these depths in order for the garnet to survive. These figures place constraints on the thermal regime prevailing in the late stages of the Hercynian orogeny.
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