Geochemical characterization, petrogenetic modelling and engineering behaviour of granitic rocks and basic dykes from the northern Indian plate in north-western Pakistan

Abstract

The pre-Himalayan magmatic events along the northern margin of Indian plate in north-western Pakistan have been investigated and correlated with analogous magmatism in other Himalayan and northern Gondwana regions. The samples from Utla and Mansehra regions of NW Pakistan are dominantly megacrystic two mica granites, strongly peraluminous (A/CNK > 1.1) and intruded by aplite dykes and quartz-rich veins. The high precision zircon U-Pb ages (471-479 Ma) show their emplacement in early Paleozoic. These granites are enriched in light rare-earth elements (LREEs) and show similar chondrite normalized REE patterns with negative Eu anomalies (Eu/Eu* = 0.07–0.73). The geochemical signature and REE based modelling indicate that the granites are derived mainly from the partial melting of pelitic source followed by the evolution of melt via fractional crystallization resulting in the formation of aplites. Tourmaline occurrences in distinct modes show post-magmatic alteration of these granites triggered by hydrothermal fluids from different sources. Enrichment of Sn in certain alteration zones and trace elements ratios suggests a strong mineralization potential for these granites. The analogous composition, source rock characteristics and geochronology represent their regional association with other Cambro-Ordovician granitoids from northern Gondwana. Due to these similarities, an early Paleozoic orogenic event has been anticipated for these granitoids initiated due to subduction of Proto-Tethys oceanic lithosphere beneath the northern Gondwana supercontinent. Dykes of basic composition that intrude these granites and other lithologies are divided into dolerites and amphibolites on the basis of their distinct mineralogical and geochemical composition. Major elements composition suggests alkaline to sub alkaline character of both dykes with intraplate tectonic setting, however, amphibolites (>3%) are markedly enriched in TiO2 relative to dolerites (<3%). Trace element ratios designates the origination of dolerites from subcontinental lithosphere with significant crustal contamination. They show analogous geochemical character to Panjal traps which represent a regional scale rift related basic magmatism in Himalayan terrane during Permian. The geochemical signature of amphibolites, however, show similarities to high-Ti Qiangtang dykes which originates from asthenospheric source via deep mantle plume. The sporadic distribution of both dykes in similar aged host rocks represent their evolution from distinct sources in separate but synchronous magmatic pulses during extensional tectonism related to separation of Cimmeria from Gondwana. Granites with distinct petrographic features have been tested to examine the influence of textural characteristics on the variation of their respective strength. Comparison of petrographic observations before and after the strength tests and the relationship of fracture propagation with mineral boundaries specifies vital impact of textural variation in evaluating the mechanical behaviour of granites. The important textural features include average grain size of rock, grain boundary recrystallization, maximum grain size of major rock forming minerals, mean grain size of cleaved minerals, mineral exsolution and variation of grain size within a rock. The petrographic observations, however, are more effective to describe the strength variation of granites having analogous weathering grade as change in degree of weathering has a dominant effect on rock mechanics.