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

Published online by Cambridge University Press:  05 July 2018

W. Keankeo ,
W. R. Taylor  and
J. D. FitzGerald
W. Keankeo*
Affiliation:
Department of Geology, Australian National University, Canberra, ACT, 0200, Australia
W. R. Taylor
Affiliation:
Research School of Earth Sciences, Australian National University, Canberra, ACT, 0200, Australia
J. D. FitzGerald
Affiliation:
Research School of Earth Sciences, Australian National University, Canberra, ACT, 0200, Australia
*
* E-mail: watchara@geology.anu.edu.au
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Abstract

Garnet pyroxenite xenoliths from the Delegate nephelinitic breccia pipes, New South Wales, Australia, contain relict garnets (py40 alm39 gr21) which 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 μm in width. Analyses by SEM and ICPMS reveal that the kelyphite has an identical bulk chemical composition to the primary garnet. Kelyphitic rims on garnet are well known from xenoliths and xenocrysts in kimberlite pipes and from tectonically-uplifted mafic and ultramafic rocks in some metamorphic terranes. Orthopyroxene occurs in metamorphic kelyphites and it has been assumed that orthopyroxene is also the breakdown product of garnet transported in basic-ultrabasic magmas. However, TEM study of Delegate kelyphite shows that the ultrafine lamellae do not contain orthopyroxene but are instead composed of magnesian clinoferrosilite (En45Fs55), and lesser ferroan spinel and anorthite. The clinoferrosilite is probably the inversion product of initially-formed magnesian protoferrosilite. The breakdown reaction is believed to result from a sudden change to lower temperature and pressure conditions when the xenoliths were transported in the Delegate magma from ∼40 km depth to the surface.

Keywords

kelyphiteclinoferrosiliteTEMgarnetxenolith
Type
Research Article
Information
Mineralogical Magazine , Volume 64 , Issue 3 , June 2000 , pp. 469 - 479
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2000

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