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Petrology, geochemistry and low-temperature alteration of extrusive lavas and pyroclastic rocks of the Igwisi Hills kimberlites, Tanzania.

Willcox, Alistair and Buisman, Iris and Sparks, Steve and Brown, Richard and Manya, S. and Schumacher, J. C. and Tuffen, Hugh (2015) Petrology, geochemistry and low-temperature alteration of extrusive lavas and pyroclastic rocks of the Igwisi Hills kimberlites, Tanzania. Chemical Geology, 405. pp. 82-101. ISSN 0009-2541

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Abstract

Geochemical data are presented for the kimberlitic Holocene Igwisi Hills volcanoes (IHV), Tanzania, which preserve extra-crater lavas and pyroclastic rocks. Their young age and exceptional preservation enable investigation of kimberlite magma compositions and alteration pathways of kimberlites. The IHV lavas have a variable matrix assemblage dominated by calcite, olivine and a serpentine-like mineral (termed serpentine-X). Minor primary groundmass phases include apatite, phlogopite, monticellite, perovskite and spinel representing late-stage crystalisation. Secondary phases include hydrogarnet, a mixed-layer chlorite-vermiculite-montmorrilonite, minor brucite and low-temperature oxides and clays such as goethite and jamborite. The matrix of pyroclastic rocks is dominated by calcite with fewer groundmass phases. The parental magmas are inferred to have had ~ 21 wt % SiO2, ~ 22 wt % CaO, ~ 23 wt % MgO and Mg# ~ 70. The IHV are classified as calcite kimberlites. The total volatile concentrations of the primary melt are ~ 14 wt %, which predominantly consists of CO2 although the H2O content is also high. Whole-rock geochemical analyses indicate minor crustal contamination, low-temperature alteration and weathering. Pervasive serpentinisation in both lavas and pyroclastic rocks results from low-temperature alteration induced by the circulation of meteoric waters during cooling. Serpentine-X is potentially a new mineral and is richer in Al2O3 and FeO and poorer in SiO2 than published analyses of serpentine minerals. These compositions are attributed to a 1:2 mixture of serpentine and hydrotalcite. We propose that serpentine-X has replaced a reactive, late stage residual silicate glass, the existence of which helps explain the presence of vesicular scoria (similar to glassy basaltic pyroclasts) and viscous kimberlite lavas.

Item Type: Article
Journal or Publication Title: Chemical Geology
Additional Information: Date of Acceptance: 04/04/2015
Uncontrolled Keywords: Extrusive kimberlite ; Igwisi Hills ; serpentine ; geochemistry ; petrology
Subjects:
Departments: Faculty of Science and Technology > Lancaster Environment Centre
ID Code: 52161
Deposited By: ep_importer_pure
Deposited On: 20 Dec 2011 09:36
Refereed?: Yes
Published?: Published
Last Modified: 18 Jun 2015 05:43
Identification Number:
URI: http://eprints.lancs.ac.uk/id/eprint/52161

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