Mineralogy of the Marepalli lamproite dyke from the Nalgonda district, Telangana, Southern India

Abstract

AbstractThe Marepalli dyke of the Vattikod cluster of the Ramadugu Lamproite Field, Nalgonda district, Telangana, India consists of pseudomorphed leucite, phlogopite (Al-poor, Ti-rich zoned phlogopite micas), pseudomorphed olivine, fluorapatite and Al-poor diopside embedded in groundmass consisting mainly of poikilitic Fe-rich titanian phlogopite and potassic amphibole. Other groundmass minerals are Al-Na-poor diopside, Al-poor spinels (titanian magnesian chromites to titanian chromites), Sr-rich fluorapatite and late-stage interstitial anhedral crystals of titanite and K-feldspar. The late-stage deuteric minerals present are REE-rich allanite, pyrite, magnetite, chalcopyrite, galena, hydro-zircon, carbonates (calcite, witherite and strontianite), baryte and cryptocrystalline SiO2. Apatite is an early crystallising phase and is present as inclusions in phlogopite and pyroxene. Phlogopite and amphibole occur as inclusions in titanite and K-feldspar. The compositional trends of phlogopite are of almost constant Al2O3 content with FeOT and TiO2 enrichment, and are typical of lamproitic micas. The FeOT enrichment is accompanied additionally by MgO depletion (reflecting VIFe2+ enrichment) from core to rim together with a slight increase in the tetraferric iron component. Diopside is characterised by <0.4 wt.% alumina and <0.6 wt.% sodium contents and exhibits typical lamproitic affinity. The spinels are alumina-poor with un-evolved titanian magnesian chromite to titanian chromite compositions. The presence of K-richterite, as an abundant amphibole, indicates a lamproite affinity, and on the basis of the typomorphic mineralogy, this rock is classified as a ‘pseudoleucite-amphibole-phlogopite lamproite’. The Marepalli lamproite shows significant difference in compositional ranges of phlogopite, amphibole, pyroxene and spinel in comparison to those reported from the Vattikod, Gundrapalli, Ramadugu, Somavarigudem and Yacharam lamproites of Ramadugu Lamproite Field. These lamproites are considered to form from a common parent magma under reducing conditions as evidenced from: (1) low tetraferric iron content in phlogopite; (2) low Fe3+# and Ti# in spinels; and (3) high F content in phlogopite and apatite.