Paleogene NeogeneAgeMaFormationLithologyRecent & SubrecentQuaternary<strong>Pleistocene</strong><strong>Plio</strong>cene0.01171.8065.33Koh-e-SultanVolcanic GroupDalbandin Fm.Unconsolidated gravel, sand, silt and clay.Koh-e-Sultan Volcanic Group: Intercalations <strong>of</strong>dacitic-andesitic lava flows and <strong>volcanic</strong>lastics.Intercalations <strong>of</strong> shale, mudstone, sandstone andconglomerate.MioceneBuze Mashi KohVolcanic GroupBuze Mashi Koh Volcanic Group: Intercalations<strong>of</strong> andesitic-basaltic lava flows and <strong>volcanic</strong>lastics.M e s o z o i c C e n o z o i cT e r t i a r yC r e t a c e o u sMiddleUpperOligoceneEocenePaleoceneMaastrichtianCampanianSantonianConiacianTuronianCenomanianAlbianAptianBerrimian23.0333.955.865.570.683.585.888.693.699.6112.0125.0130.0Juzzak Fm.A m al a f F m .Saindak Fm.RobatLimestoneRakhshani Fm.Humai Fm.SinjraniVolcanic Gr.?DisconformityAmalaf Formation: Intercalations <strong>of</strong> shale, siltstone,sandstone and limestone, with andesitic <strong>volcanic</strong>sin <strong>the</strong> upper part.Saindak Formation: Intercalations <strong>of</strong> shale, siltstone,sandstone, marl and limestone, with andesiticlava flows and <strong>volcanic</strong>lastics in <strong>the</strong> lower part.Robat Limestone: Medium to thick-bedded forameniferaland argillaceous limestone.Tanki Sills: Mainly pyroxene diorites.Juzzak Formation: Intercalations <strong>of</strong> sandstone, shale,mudstone and limestone, with andesitic lava flowsand <strong>volcanic</strong>lastics in <strong>the</strong> lower middle part.Rakhshani Formation: Intercalations <strong>of</strong> sandstone,shale, mudstone and limestone representing a turbiditesequence.Humai Formation: Thick-bedded to massive limestoneon <strong>the</strong> top, intercalations <strong>of</strong> shale, sandstone,siltstone and limestone in <strong>the</strong> middle and conglomerateat <strong>the</strong> basal part.Basaltic-andesitic lava flows and <strong>volcanic</strong>lastics, w-ith minor shale, sandstone, siltstone, lenticular bodies<strong>of</strong> limestone and mudstone.LowerHauterivianValanginianBerriasian133.9140.2145.5Different phases <strong>of</strong> Intrusions =Fig. 2. Generalized stratigraphic sequence in <strong>the</strong> Chagai <strong>arc</strong> (after Jones, 1960; Siddiqui et al., 2005).The ages in <strong>the</strong> time scale are after Ogg et al. (2008).4
The <strong>Plio</strong>-<strong>Pleistocene</strong> <strong>volcanic</strong> <strong>rocks</strong>, namedas Koh-e-Sultan Volcanic Group (Jones, 1960),were previously considered as Quaternary(<strong>Pleistocene</strong>) in age but recent studies (Siddiquiet al., 2002; Siddiqui, 2004) have assigned a2.39 ± 0.05 Ma (K-Ar whole rock age <strong>of</strong> <strong>the</strong>andesite) to 0.09 ± 0.01 Ma (K-Ar whole rockage <strong>of</strong> pumice) age to <strong>the</strong>m. The main exposures<strong>of</strong> <strong>the</strong> <strong>Plio</strong>-<strong>Pleistocene</strong> <strong>volcanic</strong> <strong>rocks</strong> occur inKoh-e-Sultan mountain, covering 770 km 2 area(Figs. 3 and 4 A). This mountain forms a northwesttrending series <strong>of</strong> three volcanoes with<strong>the</strong>ir discrete calderas, named Kansuri, Abu andMiri after <strong>the</strong> highest peak adjacent to or inside<strong>the</strong> <strong>volcanic</strong> caldera. The Miri volcano, 2,333 mhigh above mean sea level, occurs at <strong>the</strong>southwestern side <strong>of</strong> this <strong>volcanic</strong> series and isconsidered youngest. Ithas well developed craterwalls, whereas o<strong>the</strong>r two volcanoes formcollapse caldera and appears to be older in age.Inside <strong>the</strong> Miri Volcano is ano<strong>the</strong>r small volcanowith well-developed inner circular crater formeddue to resurgent <strong>volcanic</strong> eruption (Fig. 2). Thediameter <strong>of</strong> Miri Volcanic crater is 6.5 km,whereas <strong>the</strong> inner crater measures about 800 min diameter. In <strong>the</strong> vicinity <strong>of</strong> Koh-e-Sultanvolcano, a number <strong>of</strong> small satellite cone andplugs are found: Koh-e-Dalil, Chhota Dalil, MitKoh, Dam Koh, Batal Koh, Bag Koh, Koh-e-Malik, and several o<strong>the</strong>r small and unnamedbodies (Figs. 1 and 2).The <strong>Plio</strong>cene to <strong>Pleistocene</strong> <strong>volcanic</strong>s arerepresented by andesitic to dacitic lava flows ( 90 volume%). The <strong>Plio</strong>cene <strong>volcanic</strong>s are generallydominated by andesites, whereas dacites occur asdominant phase in <strong>Pleistocene</strong> <strong>volcanic</strong>s.2.1. Andesitic lava flowsAndesitic lava flows are commonly form as 1to 2 m thick beds interstratified with <strong>the</strong><strong>volcanic</strong>lastics. These flows are grey to greenishgrey, hard, resistant to wea<strong>the</strong>ring and generallyform small ridges. At least five eruptive cycles areobserved in <strong>the</strong> middle horizon <strong>of</strong> <strong>the</strong>southwestern flank <strong>of</strong> <strong>the</strong> Koh-e-Sultan volcano,whereas two more are observed northwest <strong>of</strong> Miripeak (Fig. 4A). These andesitic lavas aregenerally interrupted with tuffs and agglomeratesequence (Fig. 4A). The two cycles <strong>of</strong> andesiteeruption within Miri Volcano are light grey incolour and form secondary craters. The lava flow,which occurs in <strong>the</strong> outer crater, is up to 100 mthick, whereas <strong>the</strong> inner one is up to 3 m thick.The andesites are porphyritic and showphenocrysts <strong>of</strong> pyroxene, plagioclase andhornblende in a fine-grained groundmass having<strong>the</strong> 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 <strong>the</strong> same composition.3. Petrography3.1. AndesitesUnder microscope, four types <strong>of</strong> andesites areidentified, (a) hornblende-andesite, (b)hypers<strong>the</strong>ne-andesite, (c) hornblende-hypers<strong>the</strong>neandesite,and (d) lamprobolite-andesite. The maintextures exhibited by <strong>the</strong> andesites arehypocrystalline and porphyritic. O<strong>the</strong>r 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 polysyn<strong>the</strong>tic twinning (according to <strong>the</strong>albite and occasionally to <strong>the</strong> combined albite andCarlsbad laws) and oscillatory zoning. Theplagioclase generally ranges in composition <strong>from</strong>An 26 to An 48, (oligoclase to andesine).5