Mineralogy and petrology of lamprophyre and dolerite dykes from the end-Cretaceous (~ 66 Ma) Phenaimata alkaline igneous complex, north-western India: evidence for open magma chamber fractionation, mafic recharge, and disaggregation of crystal mush zone in a large igneous province

The end-Cretaceous (ca. 66 Ma) Phenaimata alkaline igneous complex, associated in space and time with the Deccan large igneous province (LIP) in Western India, consists of bimodal (tholeiitic to alkaline) differentiated plutonic to volcanic igneous rocks. Mineralogy and petrology of variably fractionated alkaline lamprophyre and dolerite dykes of the complex are the focus of this study. The two lamprophyre dykes (termed as camptonite-I and camptonite-II) which intrude the host basalt/olivine dolerites and gabbro, differ in their liquidus minerals and crystal size distribution. Their respective rare earth element (REE) and trace element patterns suggest the lamprophyres to be genetically related, with the camptonite-II being relatively more evolved than that of the camptonite-I. Binary mixing model involving trace elements as well as the Sr˗Nd isotopic data in case of the camptonite-I, brings out involvement of crustal contamination in the generation of the lamprophyres. The two dolerites mostly consist of the liquidus phases, with the exception of olivine antecrysts created during early stages of the host gabbro formation but subsequently inherited into their magma. In terms of their trace element composition, the dolerites of this study show a strong similarity with that of the alkali basalts of the complex. The normal compositional zoning of pyroxene and amphibole from the lamprophyres shows that their parental magma initially experienced a closed system fractionation to form a large crystal mush zone and subsequently developed a smaller magmatic chamber where biotites of the camptonite-II first crystallised. Reverse zoning, resorption of crystals, disparity in crystal size distribution and their composition also reveals that a newer batch of magma was introduced from the feeding zone into the initially developed crystal laden magmatic chamber. This replenishment led to the disaggregation of crystals from the mush zone and the resulting magma subsequently evolved to form camptonite-I. Later on, the successive lateral spreading of the newly generated magma increased its buoyancy to rise through the smaller crustal chamber (with biotites) to generate camptonite-II. Our study demonstrates not only the operation of diverse open and closed system processes such as fractionation, replenishment and mush capturing that have occurred within inter-connected plumbing magmatic chambers beneath the alkaline complexes associated with the flood basalt volcanism in large igneous provinces but also their significant role in influencing the ultimate composition of the associated diverse rocks. [ABSTRACT FROM AUTHOR]