Petrogenesis of Late Cretaceous lava flows from a Ceno-Tethyan island arc: The Raskoh arc, Balochistan, Pakistan

Abstract

The Raskoh arc is about 250km long, 40km wide and trends in an ENE direction. The oldest rock unit in the Raskoh arc is an accretionary complex (Early to Late Jurassic), which is followed in age by Kuchakki Volcanic Group, the most wide spread unit of the Raskoh arc. The Volcanic Group is mainly composed of basaltic to andesitic lava flows and volcaniclastics, including agglomerate, volcanic conglomerate, breccia and tuff, with subordinate shale, sandstone, limestone and chert. The flows generally form 3–15m thick lenticular bodies but rarely reach up to 300m. They are mainly basaltic–andesites with minor basalts and andesites. The main textures exhibited by these rocks are hypocrystalline porphyritic, subcumulophyric and intergranular. The phenocrysts comprise mainly plagioclase (An30–54 in Nok Chah and An56–64 in Bunap). They are embedded in a micro-cryptocrystalline groundmass having the same minerals. Apatite, magnetite, titanomagnetite and hematite occur as accessory minerals.Major, trace and rare earth elements suggest that the volcanics are oceanic island arc tholeiites. Their low Mg # (42–56) and higher FeO (total)/MgO (1.24–2.67) ratios indicate that the parent magma of these rocks was not directly derived from a mantle source but fractionated in an upper level magma chamber. The trace element patterns show enrichment in LILE and depletion in HFSE relative to N-MORB. Their primordial mantle-normalized trace element patterns show marked negative Nb anomalies with positive spikes on K, Ba and Sr which confirm their island arc signatures. Slightly depleted LREE to flat chondrite normalized REE patterns further support this interpretation. The Zr versus Zr/Y and Cr versus Y studies show that their parent magma was generated by 20–30% melting of a depleted mantle source. The trace elements ratios including Zr/Y (1.73–3.10), Ti/Zr (81.59–101.83), Ti/V (12.39–30.34), La/YbN (0.74–2.69), Ta/Yb (0.02–0.05) and Th/Yb (0.11–0.75) of the volcanics are more consistent with oceanic island arcs rather than continental margin arcs. It is suggested that the Raskoh arc is an oceanic island arc which formed due to the intra-oceanic convergence in the Ceno-Tethys during the Late Cretaceous rather than constructed on the southern continental margin of the Afghan block, as claimed by previous workers. It is further suggested that the Semail, Zagros, Chagai–Raskoh, Muslim Bagh, and Waziristan island arcs were developed in a single but segmented Cretaceous Ceno-Tethyan convergence zone.