The petrology, chemistry and crystallization history of the Velasco alkaline province, eastern Bolivia

Abstract

S U M M A R Y: The Velasco alkaline province consists of an early volcanic suite, a series of 14 interfering circular and elliptical alkaline plutons, numerous dykes and a silicified carbonatitic complex. The alkaline rocks, excluding the dykes, are restricted to a narrow NE-trending belt that has been traced for nearly 80 km.The plutons can be divided into two groups defined by the silica-oversaturated or silicaundersaturated character of the rock units within each pluton. It has been possible to deduce from textural relationships and whole-rock and mineral chemistry that the two groups can be derived from a single pulaskitic magma type, in turn producible by minor fractionation of a mantle-derived trachytic parent. Sr isotopic data indicate that the trend from pulaskite through quartz syenite to granite was developed by progressive crustal contamination due to slow rates of emplacement. In contrast, the undersaturated trend towards foyaite shows no evidence of crustal influence, the plutons probably being emplaced at a greater rate. These plutons exhibit marked sub-vertical crystal layering parallel to the intrusion walls suggesting that crystal-fluid processes were responsible for the fractionation trend.A separate strongly undersaturated magma batch is represented in the Velasco province and occurs as a melasyenitic dyke-swarm. Some dykes are strongly enriched in rare-earth elements (REE), concentrated in the mineral britholite, and indicate interaction with a hydrous phase enriched in REE.

Significance of Mineral Chemistry of Syenites and Associated Rocks of Elagiri Complex, Southern Granulite Terrane of the Indian Shield

“…According to other workers (Wallace and Green, 1988;Sweeney, 1994;Harmer, 1999) low degree melting of a carbonated mantle peridotite can give rise to the generation of carbonatite, associated with alkaline rocks. Geochemically continental syenites show large diversity and those syenites are considered to have been derived in a variety of tectonic settings (Fitton, 1987;Fletcher and Beddoe-Stephens, 1987) from silica undersaturated alkali basaltic magma. It is likely that the bulk of continental syenites formed by fractional crystallization of mantle derived alkali basalts with or without interaction with the pre-existing continental crust (Miyazaki et al 2003 and references therein).…”

Petrogenesis of the Neoproterozoic Alkaline Ultrapotassic Suítes of Northeastern Brazil: Major- and Trace-Element Evidence from Pyroxene Chemistry and Numerical Modeling

“…Zhao et al (1995) reviewed the different models regarding the sources for syenitic magmas: (1) inter- Thompson et al (1982), to compare the diopside-bearing syenites of the Casa Nova suite with typical ultrapotassic rocks (A) and alkali basalts (B). action between alkali basalt liquid and pre-existing continental crust (Fitton, 1987), proposed for syen ites and trachytes of East Greenland (Nielsen, 1987), Bolivia (Fletcher and Beddoe-Stephens, 1987), southern Australia (Ewart, 1982), and the Massif Central, France (Downes, 1987); (2) partial melting of amphibolites with alkali basalt composi tion at the base of lower crust (Smith et al, 1988), proposed for the Mount Dromedary monzonite; and (3) partial melting of metasomatized mantle (Bailey, 1987).…”

Kûngnât, revisited. A review of five decades of research into an alkaline complex in South Greenland, with new trace-element and Nd isotopic data

“…ALKALINE complexes emplaced by cauldron subsidence are common in continental extensional zones. Examples involving syenites and peralkaline granites include the Zomba-Malosa complex, Chilwa alkaline province, Malawi (Woolley and Jones, 1987), the Oslo graben, Norway (Oftedahl, 1978), the Evisa-Calasima complex, Corsica (Bonin, 1988), plutons of the Velasco alkaline province, eastern Bolivia (Fletcher and Beddoe-Stephens, 1987), the White Mountain Batholith, New England, USA (Eby, 1987), plutons of the Adrar des Iforas, Mali (Liégeois and Black, 1987), several of the Niger-Nigerian ringcomplexes (Bowden et al, 1987) and Center 1, Coldwell Complex, Ontario (Mitchell and Platt, 1978;Platt and Mitchell, 1982). Whilst sharing a common emplacement mechanism, the complexes may also show differences in petrography, mineral compositional trends and geochemical evolution.…”