AbstractThe geodynamic evolution of the northern Indian continental margin has been extensively studied. The Spinghar Crystalline Complex of NW Pakistan is dominantly comprised of mega-porphyritic granitic gneisses (GG) and leucogranites (LG). Integrated petrographic and geochemical investigation from the granitoids reveal their calc-alkaline, peraluminous (A/CNK > 1) and S-type character. The high precision U-Pb zircon mean ages of 2349 ± 7.3 Ma for GG and 1831 ± 19 Ma for LG show their emplacement in Early Proterozoic. These granitoids show enrichment in light rear-earth elements (LREEs) and exhibit similar chondrite normalized REE patterns with negative Eu anomalies. Major and trace element ratios suggest a derivation from partial melting of metasedimentary psammitic and pelitic sources for the granitic gneisses and leucogranites respectively. The strikingly analogous geochemical characteristics, geological settings and U-Pb systematics between the studied granitoids and adjoining Paleoproterozoic granites from the lesser Himalayan belt suggests that they were formed in a similar syn-collisional to arc related tectono-magmatic environment during the Paleoproterozoic.
Amphibolite dykes of alkaline to subalkaline affinity and basaltic composition intrude these granitoids. They are interpreted as intraplate mafic dykes with a relatively high concentration of TiO2 (~2%). Geochemical characteristics like low Cr, high concentration of incompatible elements (Ba) and LREEs enrichment indicate the role of assimilation and fractional crystallization in the genesis of these dykes. Furthermore, these mafic dykes share geochemical features with back arc basin basalts and other Paleoproterozoic mafic bodies from the lesser Himalaya, suggesting its origin in a back arc setting with a heterogenous mantle source of mixed depleted mantle and recycled components.
The results obtained from this study reveal the generation of these granitoids in a syn-collisional to arc setting with back arc extension leading to injection of basic dykes which further validate the occurrence of a long-lived subduction zone at the northern margin of the Indian plate during the Paleoproterozoic Columbia supercontinent assembly that extends across the Himalayan belt.
|Date of Award
|Sorcha Diskin (Supervisor), Duncan Pirrie (Supervisor) & Muhammad Sajid (Supervisor)