Geology Reference
In-Depth Information
The lamproites from southern Spain, Smoky Butte (Montana, U.S.A.), Shiprock
dyke (New Mexico, U.S.A.), Moon Canyon (Utah, U.S.A.), Sisco (Corsica), Mount
North (West Kimberley, Australia) and Orciatico (Pisa, Italy) mostly plot in the
endiopside
field of the pyroxene quadrilateral (Wagner and Velde 1986). This
mineral constitutes 6
22 wt% in a lamproite. The pyroxenes in lamproites, often
occur as unzoned phenocrysts containing around 1 wt% TiO 2 and 0.1
-
0.3 wt%
-
Cr 2 O 3 .
With the exception of Shiprock lamproite (Smoky Butte), the tetrahedral site of
the pyroxenes is incompletely
filled by Si and Al, average being 1.97
1.93 atom
-
filled by remaining Fe 3+ , when
structural formula is calculated in terms of four cations and six oxygens. This
de
(Wagner and Velde 1986). The vacancy may be
ciency in the tetrahedral site was also noticed in lamproites of Leucite Hills,
Wyoming, U.S.A. (Carmichael 1967) and West Kimberley, Australia (Jaques et al.
1984). Acmite is present in many samples as a thin rim around diopside crystals.
Wagner and Velde noted the presence of a green centre (enriched in Al, Fe and Na)
in diopside phenocrysts of Shiprock lamproites. Similar green centre in pyroxenes
of salite composition, has been described from Leucite Hills by Barton and van
Bergen (1981). Such green-centred pyroxenes are considered to be disaggregation
products of clinopyroxene nodules, and are common features in potassic volanics.
The K-rich lamproites of North Table Mountain at Zirkel Mesa and Spring Boat
(both from Leucite Hills, Wyoming, U.S.A., Kuehner et al. 1981) contain pre-
dominantly diopside, but those from some wyomingites have salite cores, rimmed
by diopsides (Table 2.4 ).
The clinopyroxenes from the minette dykes of Highwood Mountains are mainly
diopsides (Wo 46 En 46 Fs 8 ) containing up to 2 wt% Cr 2 O 3 (O
Brien et al. 1991), but
they also occur in the salite cores of minettes from this area. Salites (Wo 47 En 40 Fs 13 )
from ma
'
c phonolites and syenites of Highwood Mountains grade to ferrosalite and
acmite. The Ti, Al and Na contents in pyroxenes increase with decreasing Cr
contents. All pyroxenes have Fe 3+ /Fe 2+ ratios >1. The mantle and halos of salites
(surrounding cavities in diopside cores) contain abundant inclusions of biotite,
leucite, titanomagnetite and apatite, and all pyroxenes have a fassaitic rim, grown
along different sector zones. Oscillatory zoning and diffusion halos in salites are
common in potassic rocks of the Highwood Mountains region. Such halos are
found as patches in Mg-rich salites. In both types of salites, the reacted margins are
more Mg-rich than the [001] sectors of the unrelated crystals. This type of disso-
lution is commonly restricted to a single zone sector (001). In some cases the
partially dissolved crystals are overgrown by more Fe-rich salite rims (O
'
Brien
et al. 1991).
The K-rich rocks of the Alto Paranaiba Igneous Province are characterised
mainly by diopsidic pyroxenes (Ca 48 Mg 46 Fe 6 ) containing minor amount of enstatite
or hedenbergite in solid solution. The low Al and variable Ti contents of clino-
pyroxenes from Alto Paranaiba are similar to those of clinopyroxenes from lam-
proites of West Kimberley (Australia) and Kapamba (Zambia). Clinopyroxenes
from Brown Leucitic Tuffs of Roccamon
na (Luhr and Giannetti 1987) are green
unzoned salites, which are quite homogeneous in composition. In addition to salite,
 
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