Volcanostratigraphy of arc volcanic sequences in the Kohistan arc, North Pakistan: volcanism within island arc, back-arc-basin, and intra-continental tectonic settings

摘要

The Kohistan arc was initiated, offshore of Asia, during the mid-Cretaceous above northward subducting, Tethyan oceanic crust. The arc sutured to Asia c. 90 Ma ago. Subduction of oceanic crust beneath the arc continued until Indian Plate continental rocks began to underthrust the arc c. 50 Ma ago. The arc shows an evolutionary history from the juvenile stages of an intra-oceanic island arc, through a thickened Andean-style volcanic arc accreted to a continental margin, to an arc underplated by older continental crust. Extrusive volcanic activity spanned the mid-Cretaceous to Oligocene. This paper presents new and detailed lithostratigraphic data relating to two volcanic groups. The mid-Cretaceous Chalt Volcanic Group (CVG) documents volcanism in the last stages of the island arc phase. The Eocene-Oligocene Shamran Volcanic Group (SVG) documents Andean margin to post-Himalayan collision volcanism. The CVG comprises two formations, formally defined here. The back-arc Hunza Formation is dominated by subaqueous back-arc effusive basalt, andesite and boninite volcanism with a brief phase of subaerial silicic volcanism. The intra-arc Ghizar Formation comprises basalt and andesite-dominated crystalline and volcaniclastic rocks produced by subaerial and subaqueous calc-alkaline arc stratovolcano and shield eruptions. Two facies are present: a basalt and andesite lava flow-dominated sequence and a volcaniclastic-dominated sequence with characteristics that indicate effusive-explosive volcanism and subsequent volcanic sediment reworking and deposition within both subaqueous and subaerial settings. A stratovolcanic centre in the Ishkoman Valley contains abundant proximal volcanic lithofacies suggestive of Strombolian-Vulcanian explosive eruptive activity. The SVG, which unconformably overlies deformed rocks of the CVG, crops out in relatively small, high-altitude outliers. Previous suggestions that it has a large outcrop area in western Kohistan are unfounded. The SVG is an undeformed sequence of reddened, dominantly silicic volcanic rocks comprising mainly andesitic to dacitic and rhyolitic lavas, parataxitic and eutaxitic welded silicic ignimbrites, poorly sorted volcaniclastic sandstones, conglomerates and tuffs, and well-sorted, very fine-grained vitric tuffs. The SVG records highly evolved explosive and effusive volcanism within a mature Andean continental margin to post-Himalayan collisional environment. Primary magmas were probably generated at c. 40-30 Ma within relict metasomatised Tethyan mantle wedged between the Kohistan arc above and the underplating Indian Plate below.