Mid-Cretaceous lamproite from the Kutch region, Gujarat, India: Genesis and tectonic implications

Abstract

The Phanerozoic tectonic evolution of the Kutch region (NW India) has been attributed to rifting processes. In situ U-Pb perovskite dates (all data=124±4Ma, MSWD=42) obtained on three samples of a newly discovered lamproite dyke emplaced in the Kutch rift setting identify a previously unknown and relatively young (Mid-Cretaceous) magmatic event in this part of India. The dyke was emplaced into the Katrol Formation of Upper Jurassic to very early Cretaceous age. The presence of xenocrystic garnet and fractionated REE distribution patterns (low HREE/LREE) in the lamproite, along with high contents of transition metals and low Al2O3, suggest that it was derived from the garnet stability field in the mantle. Extreme impoverishment in HREE and HFSE suggest a mantle source region that has experienced both depletion and later enrichment. In situ analysis of Nd isotopes in perovskite of three samples gives a mean εNd=0.4±1 and a TDM model age of 598±64Ma, while in situ Sr-isotope analysis gives a mean 87Sr/86Sr=0.70388±2 (2SE), corresponding to εSr=8.6±0.3; both datasets suggest mixing between lithospheric-mantle and depleted-mantle components. The fault-controlled emplacement of this body highlights the role of deep-penetrating fault systems in providing a pathway and initiating decompressional melting of the mantle source rocks. The lamproite represents an earlier phase of rift related magmatism, different from the emplacement of the melanephelinite–basanites and basalts that dominate the igneous activity related to the Deccan volcanism in the Kutch mainland.