Figure 11. Photomicrograph of quartz-feldspar porphyry
(QFXT). This unit forms large masses in the central part of the Aljustrel
area and probably represents a subvolcanic intrusion, although locally
it may have erupted as thin sheets following deposition of the massive
sulfides and rhyolite C. The unit contains feldspar and quartz crystals
up to 2 and 0.5 cm across, respectively. It has the silica contents
of rhyolite (about 70 % SiO2), but higher TiO2 and Fe2O3 contents
that are typical of those in dacites.
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