Petrography
The phenocryst assemblages of dacite and rhyodacite consist of plagioclase-hornblende-biotite-quartz and locally sanidine set in a glassy to felsity matrix. The predominant textures are microlity porphyry, hyalomicrolity porphyry, glomeroporphyry and felsity.
The plagioclase phenocrysts often show normal or oscillatory zoning. Two types of plagioclases are found together. The first type is quite clear and characterized by normal zoning. The second type is marked by sieve or dusty texture in the core or rim (Fig. 3a, 3b).
Hornblende and biotite are partially to completely oxidized forming opacite pseudomorphs (Fig. 3c) and replacing by Fe-Ti oxides or having a reaction rims indicating disequilibrium crystallization.
Euhedral to subhedral phenocrysts of quartrz are strongly resorbed (rounded grains). Some quartz grains are embayed and corroded (Fig. 3d).
Zircon and apatite are present as inclusions in biotite and amphibole.
The matrix is typically glassy (colorless, variably devitrified), locally flow banded with fine-grain plagioclase crystallites. Devitrification of matrix glasses generated fesitic matrix.
Numerous sub-rounded enclaves are common in dacitic and rhyodacitic rocks. These enclaves with fine grain and granular-texture are similar in mineral assemblage to their host rocks (Fig. 3e).
Figure 3. Disequilibrium textures in rhyodacites and dacites
Photomicrograph of disequilibrium textures in rhyodacites and dacites, all photographs were given in plan polarizad light. (a)Sieved texture of plagioclase crystals; (b) dusty rim in plagioclase; (c) development of reaction rims in amphibole; (d) embayed and corroded quartz; (e) find-grain and grandular-tecture rounded enclave.