Petrogenesis of Eocene lava flows from the Chagai Arc, Balochistan, Pakistan and its Tectonic Implications
Abstract
The Eocene Lava flows occur in the northwestern part of an EW trending subduction related-magmatic belt known as Chagai arc in the western part of Pakistan. The volcanism in this arc was initiated during the Late Cretaceous, which intermittently continued up to the Quaternary period. In the regional geotectonic context this arc belongs to the Tethyan convergence zone and was believed to have formed due to the northward subduction of Arabian oceanic plate below the southern margin of Afghan micro plate and hence considered as an Andean type arc. Although Eocene volcaniclastic rock occurs throughout the Chagai arc but the lava flows only crop out in a NW-SE elongated (1 km x 6 km) area in the northwestern part of the Chagai arc. These lava flows are represented by two discrete cycles of eruptions found towards the top of the lower pyroclastic sequence of Saindak Formation of Eocene age. The older flow is about 100 m thick and extends for 2 km whereas the younger flow is 700-metre-thick and extends for more than 6 km.
The lava flows are mainly represented by amigdaloidal basaltic-andesites (55.5054.53 wt. % Si02) and andesites (57.40-62.79 wt. % SiO2) with minor basalt (51.88 wt. % SiO2) and dacite (67.81 wt. % SiO2). The main textures exhibited by these flows are hypocrystalline, porphyritic, cummulophyric, vitrophyric and sub pilotaxitic. Large phenocrysts (<1mm - 4mm) of plagioclase (An38-58) and pyroxene are embedded in a micro to criptocrystalline groundmass having the same minerals with devitrified volcanic glass. The phenocrysts groundmass ratio is 35•.65. Apatite, hematite, ilmenite and magnetite are common accessory mineral.
Petrochemical studies reveal that these volcanics belong to medium. to low K-calcalkaline series. They have low Mg # (39-50), and higher FeO (total)/MgO (1.81-2.78) ratios, which suggest that parent magma of these rock suites was not directly derived from a partially melted mantle source but fractionated in an upper leval magma chamber before eruption. The primordial mantle normalized trace element patterns exhibit marked negative Nb anomalies with positive spikes generally on K and Sr, which strongly confirm their island arc signatures. The moderate LREE enriched chondrite normalized REE patterns with negative Eu anomalies confirm their calcalkaline character and suggests fractionation of plagioclase during differentiation. A number of plots in various tectonomagmatic discrimination diagrams together with moderate LILE/HFSE, LREE/HREE) and 87Sr/86Sr ratios depict transitional character of these volcanics between oceanic and continental margin type arcs. This is quite logical as older volcanic rocks (i.e. Late Cretaceous to Paleocene) of the Chagai arc were developed in oceanic arc environment and younger volcanics including Oligocene to Quaternary did form in continental margin setting, which suggests that after the collision of Chagai arc with Afghan block in the Latest Paleocene to Early Eocene the former converted into a continental margin. The Cr versus Y studies, moderate Zr/Y (3.68-4.91) and 87Sr/86Sr (0.70401-0.70402) ratio ratios suggests that these lava flows. are fractionated from about 10 % partially melted transitional (slightly depleted to slightly enriched) sub-arc mantle source. A comparison of Eocene calc-alkaline rocks of the Chagai arc with their Oligocene, Miocene and Quaternary counterparts of the same arc shows a remarkable depletion in LILE and lower LILE/HFSE ratios in the former.
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