Petrology, geochemistry and low-temperature alteration of lavas and pyroclastic rocks of the kimberlitic Igwisi Hills volcanoes, Tanzania

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dc.contributor.authorWillcox, Alistair
dc.contributor.authorBuisman, I.
dc.contributor.authorSparks, R. S. J.
dc.contributor.authorBrown, R. J.
dc.contributor.authorManya, Shukrani
dc.contributor.authorSchumacher, J. C.
dc.contributor.authorTuffen, Hugh
dc.date.accessioned2016-03-04T09:11:16Z
dc.date.available2016-03-04T09:11:16Z
dc.date.issued2015-06-05
dc.description.abstractGeochemical 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.en_US
dc.identifier.citationWillcox, A., Buisman, I., Sparks, R.S.J., Brown, R.J., Manya, S., Schumacher, J.C. and Tuffen, H., 2015. Petrology, geochemistry and low-temperature alteration of lavas and pyroclastic rocks of the kimberlitic Igwisi Hills volcanoes, Tanzania. Chemical Geology, 405, pp.82-101.en_US
dc.identifier.other10.1016/j.chemgeo.2015.04.012
dc.identifier.urihttp://hdl.handle.net/123456789/592
dc.language.isoenen_US
dc.publisherElsevieren_US
dc.subjectExtrusive kimberliteen_US
dc.subjectIgwisi Hillsen_US
dc.subjectSerpentineen_US
dc.subjectGeochemistryen_US
dc.subjectPetrologyen_US
dc.titlePetrology, geochemistry and low-temperature alteration of lavas and pyroclastic rocks of the kimberlitic Igwisi Hills volcanoes, Tanzaniaen_US
dc.typeJournal Articleen_US
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