Phlogopite and associated minerals from Permian minettes in Devon, South England

Abstract

Electron-microprobe analyses were made of mica, clinopyroxene, alkali feldspar, Fe‒Ti oxides and apatite in Permian minettes from Hannaborough Quarry, Killerton Park and Loxbeare Farm, south Devon, England. Although these mica-lamprophyres contain various mineral assemblages and have undergone alteration, the compositions of unaltered minerals are similar to those of the Navajo minettes whose peridotite inclusions indicate an origin in the upper mantle. Micas from the Devon minettes evolved from Mg-rich fluorphlogopites (FeO 3 wt.%, TiO2 2, Cr2O3 1.5, NiO 0.3, BaO 0.3, F 4) to micas richer in FeO (10), BaO (0.8‒2) and TiO2 (3‒4). Diopside phenocrysts have cores low in TiO2 (< 0.3), FeO (4‒5) and Na2O (~0.4). Alkali-feldspar laths from Killerton Park and Loxbeare Farm (mostly Ab20An5Or75‒Ab29An7Or64) are rich in BaO (0.4‒1.7 wt.%) and SrO (0.4‒1.1). Hannaborough feldspars are partly albitized. Fluorapatite has light-REE enrichment (0.5‒1.4 wt.% RE2O3) similar to apatite phenocrysts from the Navajo and Pendennis minettes; SrO is unexceptional (0.6‒1.3 wt.%). Fe‒Ti-oxides consist of Ti,Mg,Al-magnetites largely altered to haematite and goethite-like minerals. Rare ilmenite has 6 wt.% MgO which lies in the range of 5‒23 wt.% for ilmenite from kimberlites; however the 2.9 wt.% Al2O3 is greater than the 0.8 wt.% maximum for kimberlitic ilmenites. In spite of the general similarities between the minerals from the Devon and Navajo minettes, the micas and apatites indicate a higher fluorine content in the former rocks.

Keywords

• minette
• mica
• pyroxene
• feldspar
• magnetite
• apatite
• Devon