By 1970, the two geologists had established that eruptions of alkali basalt, followed by extrusion of silicic peralkaline lavas, had occurred episodically at Edziza for the past 10 million years and that volcanism was accompanied by east-west extension and incipient rifting.
32.
Lherzolite is known from the lower ultramafic part of ophiolite complexes ( although harzburgite is more common in this setting ), from alpine-type peridotite massifs, from fracture zones adjacent to mid-oceanic ridges, and as xenoliths in kimberlite pipes and alkali basalts.
33.
The name " picrite " can also be applied to an olivine-rich alkali basalt : such picrite consists largely of phenocrysts of olivine and titanium-rich augite pyroxene with minor plagioclase set in a groundmass of augite and more sodic plagioclase and perhaps analcite and biotite.
34.
High-alumina basalts have aluminium contents of 17 19 wt % Al 2 O 3; boninites have magnesium contents of up to 15 percent MgO . Rare feldspathoid-rich mafic rocks, akin to alkali basalts, may have Na 2 O + K 2 O contents of 12 % or more.
35.
From west to east these zones are : the trachyte stratovolcano of the "'Sete Cidades Massif "'; a field of alkali-basalt cinder cones and lava flows with minor trachyte; the trachyte stratovolcano of the "'�gua de Pau Massif "'; a field of alkali-basalt cinder cones and lava flows with minor trachyte and tristanite; the trachyte stratovolcano of Furnas; and the Nordeste shield, which includes the Povoa��o caldera and consists of alkali basalt, tristanite and trachyte.
