Metasomatic textural changes in hypabyssal transitional kimberlites: inferences for pyroclastic kimberlites

Abstract

One of the most distinctive features of Kimberley-type pyroclastic kimberlites (KPK) is the occurrence of elliptical serpentinized melt-bearing pyroclasts (also known as pelletal lapilli) with microlitic clinopyroxene rims. We propose a new model for their formation based on petrographic observations in hypabyssal transitional kimberlites (HKt) from Renard 65 and 5034 pipes (Canada). We show that subsolidus metasomatic reactions between the kimberlite and entrained crustal silicate xenoliths create serpentine segregations and pseudoclastic textures around the xenoliths. A preferential serpentinisation of the fine HKt groundmass between coarser olivine grains makes oval “pseudoclasts” resembling pelletal lapilli. Radial outward-oriented microlites of fibrous clinopyroxene form reactive coronas on the xenoliths and on the serpentinized olivine. Petrographic evidence for the metasomatic development of the pseudoclastic texture matches the Perple_X phase equilibria calculations that model the serpentinization in the subsolidus, driven by Si ingress across the xenolith–kimberlite contacts. The serpentine-producing reactions constrained by the observed mineralogy expand the volume by 3–44%. The pseudoclastic texture in HKt forms when silicate xenoliths are abundant, adding enough Si to ensure voluminous serpentine and clinopyroxene production and decomposition of primary kimberlitic calcite. The model of metasomatic development of pseudoclasts may be relevant to KPKs as i) there is a gradual transition from HKt to KPK textures correlated with the silicate xenolith modes, and ii) KPK textures are exclusively associated with pipes in competent crystalline silicate rocks. We propose that Kimberley-type pyroclastic kimberlites include assemblages formed in several intrusive and volcanic emplacement episodes, and some pelletal lapilli in KPK may be recycled from the previous magmatic HKt phases.