have been recognised in the studied samples.
3), and consist principally of fine-grained plagioclase laths and biotite with varying quantities of generally rounded and resorbed xenocrysts
of K-feldspar and quartz; textures include doleritic, ophitic, and microgranular (Fig.
These appear to be true phenocrysts, while types 1 and 2 are clearly mostly xenocrysts
. The core composition for all three types of crystals, as determined mainly by extinction angle methods, is bytownite for units A, C, E and G; labradorite for unit B and oligoclase for unit D.
The KL-2 pipe, meanwhile, is a multiphase diatreme facies complex with at least four distinct phases of tuffistic (volcaniclastic) serpentine kimberlite breccia which, again, are locally rich in mantle xenoliths and xenocrysts
. Sedimentary xenolith content, size and preservation varies considerably between phases, as does the extent of alteration and carbonatisation.
Additionally, limited exchange of xenocrysts
(labradorite, edenitic amphibole, and bronzite in dacites, oligoclase and andesine in andesitic enclaves) might have occurred during this entrainment stage.
We consider the first interpretation to be the best explanation for the Brazil Lake tantalite U-Pb results because tantalite is generally quite rare in crustal rocks and it seems remote that there would be tantalite xenocrysts
originally present in the Brazil Lake magma.
Mantle xenoliths and xenocrysts
, brought to the surface by diamondiferous kimberlite pipes, provide a complex picture of distinct and stratified mantle domains (Kopylova et al., 1998; Griffin et al., 1999; Grutter et al., 1999; Kopylova and Russel, 2000; Carbno and Canil, 2002).
This phenomenon is increasingly recognized in zircon geochronologic studies of airfall volcanic material where some grains are clearly too old to reflect the eruptive event (xenocrysts
), but there are others that are more difficult to distinguish from juvenile zircons (e.g.
The transitional and eclogitic samples also have coronas developed locally around igneous xenocrysts
of plagioclase and olivine.