Litchfield

Litchfield is a pluton of about 3x3 km forming a topographic depression with very little exposure except along the northern and southeastern margins. The foliation of the metamorphic country rocks is deflected around the intrusion. Xenoliths occur along the northern contact and are desilicated with the production of alkali feldspar and sodic amphibole. Five units were recognized in the intrusion by Barker (1965, p. 3), the most northerly being faintly foliated, sugary-textured leucosyenite of about 65% albite and 30% microcline, together with aegirine which is partially enveloped by riebeckite/arfvedsonite. The second and third units are a magnetite-biotite syenite and nepheline syenite which are not distinguished on the map, but nepheline syenite does occur as lenses in the syenite. The syenite consists of perthites up to 1 cm across in a mosaic of albite and microcline, biotite, magnetite and aegirine with arfvedsonite/riebeckite sometimes present. The nepheline syenite, apart from the presence of up to 23% nepheline as crystals up to 5 cm long, cancrinite and some sodalite, is otherwise similar to the syenite. The type litchfieldite belongs to this group and nearly all the constituent minerals have been analysed (Barker, 1965, Table 2). The third unit, a foliated mafic syenite, occurs in the southwest of the intrusion and comprises antiperthite, amphibole, biotite and occasional aegirine-augite; mafics constitute 20-40% of the rock. An albitite pegmatite with corundum, tourmaline, fluorite, garnet etc. occurs adjacent to the southwestern contact.