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

The Plio-Pleistocene volcanic rocks, namedas Koh-e-Sultan Volcanic Group (Jones, 1960),were previously considered as Quaternary(Pleistocene) in age but recent studies (Siddiquiet al., 2002; Siddiqui, 2004) have assigned a2.39 ± 0.05 Ma (K-Ar whole rock age of theandesite) to 0.09 ± 0.01 Ma (K-Ar whole rockage of pumice) age to them. The main exposuresof the Plio-Pleistocene volcanic rocks occur inKoh-e-Sultan mountain, covering 770 km 2 area(Figs. 3 and 4 A). This mountain forms a northwesttrending series of three volcanoes withtheir discrete calderas, named Kansuri, Abu andMiri after the highest peak adjacent to or insidethe volcanic caldera. The Miri volcano, 2,333 mhigh above mean sea level, occurs at thesouthwestern side of this volcanic series and isconsidered youngest. Ithas well developed craterwalls, whereas other two volcanoes formcollapse caldera and appears to be older in age.Inside the Miri Volcano is another small volcanowith well-developed inner circular crater formeddue to resurgent volcanic eruption (Fig. 2). Thediameter of Miri Volcanic crater is 6.5 km,whereas the inner crater measures about 800 min diameter. In the vicinity of Koh-e-Sultanvolcano, a number of small satellite cone andplugs are found: Koh-e-Dalil, Chhota Dalil, MitKoh, Dam Koh, Batal Koh, Bag Koh, Koh-e-Malik, and several other small and unnamedbodies (Figs. 1 and 2).The Pliocene to Pleistocene volcanics arerepresented by andesitic to dacitic lava flows ( 90 volume%). The Pliocene volcanics are generallydominated by andesites, whereas dacites occur asdominant phase in Pleistocene volcanics.2.1. Andesitic lava flowsAndesitic lava flows are commonly form as 1to 2 m thick beds interstratified with thevolcaniclastics. These flows are grey to greenishgrey, hard, resistant to weathering and generallyform small ridges. At least five eruptive cycles areobserved in the middle horizon of thesouthwestern flank of the Koh-e-Sultan volcano,whereas two more are observed northwest of Miripeak (Fig. 4A). These andesitic lavas aregenerally interrupted with tuffs and agglomeratesequence (Fig. 4A). The two cycles of andesiteeruption within Miri Volcano are light grey incolour and form secondary craters. The lava flow,which occurs in the outer crater, is up to 100 mthick, whereas the inner one is up to 3 m thick.The andesites are porphyritic and showphenocrysts of pyroxene, plagioclase andhornblende in a fine-grained groundmass havingthe same mineral composition.2.2. Dacitic lava flowsThe dacitic lava flows generally occur as lavadomes plugs and small satellite volcanoes insideand around Koh-e-Sultan Volcano. The daciticflows are light grey to pinkish grey in colour andporphyritic in texture. Phenocrysts are mainlyrepresented by hornblende, quartz, plagioclaseand biotite and are embedded in a fine-grainedgroundmass having the same composition.3. Petrography3.1. AndesitesUnder microscope, four types of andesites areidentified, (a) hornblende-andesite, (b)hypersthene-andesite, (c) hornblende-hyperstheneandesite,and (d) lamprobolite-andesite. The maintextures exhibited by the andesites arehypocrystalline and porphyritic. Other texturesinclude cumulophyric, vitrophyric and intersetal.The main primary minerals include plagioclase,clinopyroxene, orthopyroxene, amphibole andminor quartzdescribed as under:Plagioclase: Plagioclase crystals are euhedral tosubhedral, lathlike and columnar in shape, andexhibit polysynthetic twinning (according to thealbite and occasionally to the combined albite andCarlsbad laws) and oscillatory zoning. Theplagioclase generally ranges in composition fromAn 26 to An 48, (oligoclase to andesine).5