Petrogenesis of Plio-Pleistocene volcanic rocks from the chagai arc ...

At places zoned plagioclase crystals containabundant fluid inclusions parallel to zoningtowards their margins. A few of them showresorbed and pitted margins. Larger plagioclasephenocrysts have small prismatic and polygonalinclusion of hornblende. Groundmass plagioclasegenerally occurs as small microlites, crystallites,and tiny columnar laths.Pyroxene: The pyroxene is mainly represented byboth, ortho- and clinohypersthene, occurring assmall euhedral to subhedral prismatic crystals andsquare or equant basal sections. Orthopyroxenephenocrysts occasionally show green to pale brownpleochroism and exhibit polysynthetic twinning andlocally developed reaction rims of hornblende. Thegroundmass hypersthene occurs as tinny prismaticcrystals and globules in the interstices between thesmall plagioclase crystals, imparting to the rock asub-intersertal texture. Amphibole: The amphibole ismainly represented by hornblende with substantialoxy- hornblende. Hornblende shows yellowishgreen to green and lamprobolite light brown toreddish brown pleochroism, whereas both displayswell developed sets of 56 and 124 cleavages inbasal sections. It occurs as large euhedral prismaticgrains and polygonal basal sections, occasionallyshowing polysynthetic twinning (Fig. 4C). Some ofthe amphibole crystals exhibit a rim of abundantfluid inclusions as in the case of plagioclase. Thelamprobolitee crystals generally occur in clustersimparting to the rocks a cumulophyric texture (Fig.4E & F). In the groundmass lamprobolitee occurs assmall prismatic crystals and tinny polygonal basalsections.Quartz: It generally occurs as small subhedral andequant phenocrysts.Volcanic Glass: The volcanic glass is commonlydevitrified and has brown colour, which generallyoccurs in groundmass and as fillings in vesicles.Accessory Minerals: These includeapatite,magnetite,pyrite and chalcopyrite. Apatite occursas small euhedral and prismatic crystals enclosedin larger grains of plagioclase. Magnetite occursas small subhedral to anhedral crystals scatteredthroughout the groundmass. The pyrite andchalcopyrite are found as small anhedral andirregular grains.Secondary Minerals: Secondary minerals aredeveloped as partial or complete replacement ofprimary minerals. At places, pyroxene andhornblende are partially altered into chlorite. Claymineral, sericite, and calcite have developed afterpartial alteration of plagioclase. Magnetite ispartially to completely replaced by hematite, butoccasionally by limonite.3.2. DaciteThe dacite is represented by only one variety:hornblende dacite. These rocks (KS-6 & KS-9)are holocrystalline porphyritic and cumulophyricin texture. Large phenocrysts (0.1 - 7.0 mm) ofplagioclase and amphibole are embedded in amicrocrystalline groundmass having the sameminerals. The phenocryst groundmass ratio is45:55.Plagioclase: Plagioclase crystals are euhedral tosubhedral, lathlike, columnar and equant in shapeand exhibit polysynthetic twinning according tothe albite and occasionally to the combined albiteand Carlsbad laws. The anorthite contents ofplagioclase could not be determined due to thedevelopment of strong oscillatory zoning in allphenocrysts (Fig. 4H). At places zonedplagioclase crystals have abundant fluidinclusions towards their margins parallel tozoning planes. Larger plagioclase phenocrystshave small tabular and lathlike inclusion of earliergeneration of plagiclase itself. Groundmassplagioclase generally occurs as small anhedral andequant crystals and tiny columnar laths.Quartz: It generally occurs as small subhedral andequant phenocrysts and microcrystalline anhedraland equant crystals in the groundmass. Quartz isalso found as anhedral inclusions in hornblende.Amphibole: The amphibole is mainly representedby brownish green hornblende. It shows yellowishgreen to brownish green pleochroism and two setsof well-developed cleavages in basal sections.The hornblende generally occurs as large euhedralprismatic grains and polygonal basal sections,which occasionally show polysynthetic twinning.Some of the hornblende crystals exhibit a rim ofabundant fluid inclusions towards their margin asin the case of plagioclase.8