Petrogenesis and mineral characteristics of the oldest volcanogenic breccia unit from the Himalayan foreland basin, India

Abstract

Jangalgali breccia unit (JBU), the oldest volcanogenic rock unit in the Himalayan foreland basin, has been studied from five different, however, stratigraphically equivalent localities of Jammu region in NW India. The field and thin section slides of JBU reveal that quartz and plagioclase are the two most common minerals and sanidine, magmatic zircon, rutile, hornblende and biotite are present as accessory phases. Petrographic signatures show that quartz and K-feldspar are set in a fine-grained cryptocrystalline glassy matrix and do not represent any preferred orientation. The euhedral hexagonal dipyramidal quartz phenocrysts, irregularly shaped inclusions having high volatile contents and no signatures of transport/reworking in phenocrysts prior to deposition are the prominent petrographic features of JBU. The mineral grains arrangement varies between tight-fitted fabric geometry to more open-chaotic packing. Overall, the field relationship, texture, mineralogy along with mineral chemistry, and presence of high gas and silica content in the host magma, support the volcanic origin of the JBU litho unit, which has wider implications towards understanding the timing of India-Asia collision as well as geodynamic evolution of the Himalayas.