Petrogenesis of Quartz-Bearing Syenite Occurring Within Nepheline Syenite of the Elchuru Alkaline Complex, Prakasam Province, Andhra Pradesh

A quartz-bearing syenite (QS) dyke cuts across the nepheline syenite (NS) in the Elchuru alkaline complex. The QS is leucocratic, medium grained and hypidiomorphic in texture. It is composed of K-feldspar, albite, amphibole, biotite and quartz. The QS has higher amounts of SiO₂ Al₂O₃ and Na₂O and, lower CaO and K₂O than the average quartz syenite. The QS is metaluminous and oversaturated-in character. It has higher concentrations of Ga, Zr, Hf, Ta, V, Cr and U and, lower Ba, Rb, Sr, Y, Nb, Zn, Pb, Sc, Co, Ni and Th than fhe NS . The QS exhibits LREE-emiched and HREE-depleted pattern with a conspicuous negative Eu anomaly. The normalised incompatible trace element abundance pattern of the QS shows spikes for Ba, Ta and Y. A petrochemical comparison of the QS and NS suggests that they are derived from a feldspathic residual liquid by liquid immiscibility under variable P_H2O and P_CO2 conditions. This process mimics "branching differentiation" mechanism.