This suggests that some of the phenocrysts that had formed in the rhyolitic magmas were later enclosed (as xenocrysts
) within the more mafic one (i.e., as enclaves).
Several factors can influence the reproducibility of the chemical signature of the studied samples: a) the percentage of glass and phenocrysts, b) the presence of xenocrysts
and, c) the presence of xenoliths or lithic clasts.
are composed of orthoclase and reach 2-3 cm in size.
Lavas from the northwestern knoll are fresh, slightly ollvine-phyric lavas with abundant vesicles and rare, partially resorbed plagioclase xenocrysts
. The 1970 sample from the northwestern knoll is transitional basalt, plotting on the Hawaiian tholeiite-alkalic basalt boundary, whereas the 1994 lava is tholeiitic basalt with a higher Si[O.sub.2] content (Fig.
Albite and alkali feldspar are absent in the groundmass in which the only feldspar is plagioclase in the andesine-labradorite range, indicating that the megacrysts are xenocrysts
rather than phenocrysts.
Unit D is a relatively fresh, mostly dense but locally scoriaceous, porphyritic trachyandesite with ten to fifteen percent large plagioclase xenocrysts
Pellet-rich tuffistic (volcaniclastic) serpentine kimberlite breccia occupies the southern fissure appendage of the pipe which is extremely rich in mantle xenocrysts
, megacrysts and xenoliths.
Subhedral to anhedral quartz xenocrysts
(<1.0 mm) with irregular cracks and undulated extinction complete the enclave mineral assemblage.
Shimizu, K., Nakamura, E., Kobayashi, K., and Maruyama, S., 2004, Discovery of Archean continental and mantle fragments inferred from xenocrysts
in komatiites, the Belingwe greenstone belt, Zimbabwe: Geology, v.
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.