Different Localities of Potassium-Rich Silica-Undersaturated Igneous Rocks and Their Silica-Rich Variants - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 148

There is a diatreme at Serra do Bueno near the town of Bambui to the south of
Alto Paranaiba Igneous Province (19 ° 43 ′ S, 46 ° 03 ′ W). It is located close to several

K-rich ultrama c pipes. The Serra do Bueno diatreme is well exposed and com-

prises two facies, (1) a crater facies dominated by lapilli tuffs, and (2) a magmatic
hypabyssal facies consisting of kamafugitic (katungite, mafurite and ugandite)
intrusions. The 40 Ar/ 39 Ar analyses of olivine phenocrysts from the intrusion yielded
an isochron of 90 4 Ma.

The study of the intrusive rocks of Serra doBueno (Gibson et al. 1994) andCorrego
Varjao show that many rocks contain leucite, which is readily altered to analcite. The
alteration of leucite possibly perturbed the isotopic system, and hence age determi-
nation of rocks of Corrego-Varjao yielded two ages, one determined from a melilite
and other from a kamufugitic rocks, and the ages were found to be 77.8

±

± 1.1 and

57.3 0.6 Ma, respectively. According to Gibson et al. (1995) the magmatism in the

entire Alto Paranaiba Igneous Province might have taken place 80

±

90 Ma ago.
There is now a general consensus among petrologists from Brazil that in terms of
their constituent mineral assemblages and mineral chemistry the extrusive and
intrusive rocks of Alto Paranaiba Igneous Province are similar to the kamafugitic
family described from East Africa by Holmes (1950). However some of the K-rich
ultrama

-

c intrusives from Tres Ranchos, Indaia I and Limeria I may be termed as
kimberlite.
The K-rich lavas of the Alto Paranaiba Igneous Province are usually porphyritic
and have a fine-grained groundmass. They are characterized by olivine, clinopy-

roxene, ilmenite, perovskite and phlogopite. The accessory phases included
K-richterite, magnetite, leucite, kalsilite and glass. On the basis of their phenocryst
content they may be described as olivine- and clinopyroxene-phyric rocks.
The olivine-phyric lava at Presidente Olegario is composed of phenocrystal
olivine (up to 2 mm) and phlogopite (<1 mm, set in a groundmass of olivine,
diopside, phlogopite, richterite, perovskite, magnetite and glass).
In Carmo do Paranaiba, the K-rich lavas contain a large quantity of phenocrystal
clinopyroxene, phlogopite, apatite, and leucite together with microphenocrystal
ilmenite and perovskite set in a

fine-grained matrix of clinopyroxene, analcitized
leucite, magnetite, phlogopite, melilite and kalsilite. The pyroxene and phlogopite
phenocrysts are 1 cm in length and display compositional zoning.
According to Gibson et al. (1995), the K-rich lavas and hypabyssal rocks display
a continuous variation in MgO and FeO content from 11 to 16 wt%, and the mg-
number varies between 39 and 87. The lavas in general have high MgO content
(<20 wt%), but quite often between 18 and 20 wt%. The K 2 O/Na 2 O ratio varies from
1 to 16, the rocks are therefore, potassic to ultrapotassic and ma c to ultrama c. The

high CaO (11 - 18 wt%), low SiO 2 (45 - 32 wt%) and low Al 2 O 3 (5 - 8 wt%) contents

resemble kamafugites from East Africa.
The Mata do Lenco intrusion is the most extreme member of the group (K 2 O/
Na 2 O ratio = 8.1), and may be described as perpotassic (K 2 O/Al 2 O 3 = 0.77) and
peralkaline (K 2 O+Na 2 O/Al 2 O 3 = 0.86). The SiO 2 content is low (39 wt%) and
MgO content (19 wt%) is high. Chemistry of kamafugitic rocks from Alto Par-
anaiba Igneous Province from Brazil is given in Table 4.14 .