Alkaline Rocks and Carbonatites of the World

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Velasco Alkaline Province


Occurrence number: 
Santa Cruz
Longitude: -61, Latitude: -15.83

Covering 440 km2 and extending over 80 km, the Velasco Province consists of an early suite of volcanics, 14 overlapping ring and elliptical plutons and the Cerro Manomo carbonatite, which is some 20 km northeast of the plutons and is described above (022-00-006). The province is emplaced in Precambrian gneisses and granites. Remnants of the early volcanic suite consist of agglomerates, welded and brecciated tuffs and flows. The overlapping plutons extend over 50 km and consist of an oversaturated series of biotite and riebeckite granites, quartz syenite and nordmarkite, and undersaturated pulaskite and foyaite. The oversaturated rocks are generally homogeneous, but the undersaturated have a foliation, defined by feldspar tablets, which is vertical and follows the margins of the circular structures, as well as some banding defined by grain size variations. The granites and quartz syenites contain biotite and hornblende but there is also a riebeckite-aegirine granite, and the same rock type is considered to have been produced in places through fenitization of hornblende-biotite granite by the undersaturated plutons. The pulaskites consist of tabular perthite, 0-5% nepheline, biotite, aegirine-augite and sometimes a little alkali amphibole; monazite has also been identified. The foyaites consist of perthite, more than 5% nepheline, with nepheline more abundant than feldspar in some pegmatites, aegirine, biotite and a little alkali amphibole, melanite, pyrochlore, analcime and sodalite. Numerous dykes and veins occur in and up to 30 km from the plutons; an oversaturated and undersaturated series is represented and includes microgranites, syenites, nordmarkites, pulaskites and foyaites; an eudialyte-bearing litchfieldite has also been identified. Rock and mineral analyses will be found in Fletcher (in press).

Rb-Sr whole rock isochrons for quartz-bearing rocks gave 143±4 Ma and for foyaite and pulaskite 140±6 Ma; K-Ar determinations averaged 138±5 Ma for quartz-bearing rocks and 136±5 Ma for quartz-free rocks (Darbyshire & Fletcher, 1979).

DARBYSHIRE, D.P.F. & FLETCHER, C.J.N. 1979. A Mesozoic alkaline province in eastern Bolivia. Geology, 7, 545-8. Fletcher, in press. FLETCHER, C.J.N. & LITHERLAND, M. 1981. The geology and tectonic setting of the Velasco Alkaline Province, eastern Bolivia. Journal of the Geological Society, London, 138, 541-8

Fig. 1_191 The Velasco Alkaline Province (after Fletcher and Litherland, 1981, Fig. 2).
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