Clinoferrosilite-bearing kelyphite: a breakdown product of xenolithic garnet, Delegate breccia pipes, New South Wales, Australia

Garnet pyroxenite xenoliths from the Delegate nephelinitic breccia pipes, N ew South Wales, Australia, contain relict garnets (py(40) alm(39) gr(21)) w hich are replaced by dark kelyphitic rims resulting from garnet breakdown. The kelyphite is composed of a lamellar intergrowth of secondary minerals, in which the lamellae are <1 mu m in width. Analyses by SEM and ICPMS revea l that the kelyphite has an identical bulk chemical composition to the prim ary garnet. Kelyphitic rims on garnet are well known from xenoliths and xen ocrysts in kimberlite pipes and from tectonically-uplifted mafic and ultram afic rocks in some metamorphic terranes. Orthopyroxene occurs in metamorphi c kelyphites and it has been assumed that orthopyroxene is also the breakdo wn product of garnet transported in basic-ultrabasic magmas. However, TEM s tudy of Delegate kelyphite shows that the ultrafine lamellae do not contain orthopyroxene but are instead composed of magnesian clinofenosilite (En(45 )Fs(55)), and lesser ferroan spinel and anorthite. The clinoferrosilite is probably the inversion product of initially-formed magnesian protoferrosili te. The breakdown reaction is believed to result from a sudden change to lo wer temperature and pressure conditions when the xenoliths were transported in the Delegate magma from similar to 40 km depth to the surface.