Different Localities of Potassium-Rich Silica-Undersaturated Igneous Rocks and Their Silica-Rich Variants - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 191

4.11 K-Rich Rocks from Paraguay

The Serra Geral formation of eastern Paraguay has been the site of alkaline mag-
matism since Mesozoic times (Comin-Chiaramonti et al. 1997). These igneous
rocks include:
(1) Late Permian-Early Triassic sodic intrusions and lavas,
(2) Early Cretaceous potassic igneous rocks and very rarely-occurring sodic lavas,
(3) Late Cretaceous-Oligocene sodic lavas.

Two distinct magmatic events are dominant in the Asuncion-Sapucai graben
(ASU) of eastern Paraguay: (a) widespread potassic magmatism and eruption of
tholeiites of Serra Geral formation (Early Cretaceous) and (b) Asuncion sodic
magmatism (Late Cretaceous-Oligocene). The potassic rocks form a compositional
continuum from moderately to strongly potassic rock series. Comin-Chiaramonti
et al. suggested the presence of two potassic suites i.e. (i) basanite to phonolite and
(ii) alkali basalt to trachyte and their intrusive analogues. The sodic rocks include
ankaratrites, nephelinites and phonolites. Two similar but distinct parental magmas
have been inferred for the potassic suites, both characterized by strongly fraction-
ated REE and negative Ta

Ti anomalies. Slightly positive Ta and Nb anom-
alies distinguish the sodic rocks from their potassic counterparts. The rocks of
Asuncion-Sapucai graben have been divided by them into three groups:

Nb

- -

(1) Potassic type (K 2 O/Na 2 O varying between 1 and 2,
(2) Highly potassic type (K 2 O/Na 2 O > 2) and
(3) Transitional type (K 2 O/Na 2 O < 1) (Table 4.13 ).
There are varieties of rock types such as theralites, essexites, gabbros and ijo-
lites. These rocks are holocrystalline with diopsidic pyroxene (Wo 44 - 51 Fs 8 - 17-
Hd 31 - 41 ), olivine (Fo 75 - 82 to Fo 44 - 66 ), mica (Ti-phlogopite to Ti-biotite), Ti-
magnetite, alkali feldspar, nepheline (Ne 64 - 80 Ks 20 - 36 ) amphibole.

Leucite crystals pseudomorphed by analcite and plagioclase are common in
potassic and transitional group of rocks.
Basanites, tephrites and phonotephrites typically show porphyritic textures with
phenocrysts of clinopyroxene (Wo 40 - 50 Fs 10 - 19 Hd 31 - 41 ), olivine (Fo 60 - 85 ), and leu-
cite pseudomorphed by sanidine + nepheline, in glassy groundmass containing
microlites of clinopyroxene

± leucite ±

ilmenite, Ti-phlogopite-bio-
tite, alkali feldspar (Or 15 - 88 ), nepheline ss (Ne 44 - 59 Ks 17 - 26 ) and analcite. Phenocrystal
plagioclase (up to An 74 ) is present in potassic and transitional rock type. Accessory
phases are amphibole (pargasite-kaersutite), apatite with or without zircon.
Phonolites are characterized by phenocrysts of leucite pseudomorphs, alkali
feldspar (Or 47 - 75 ), clinopyroxene (Wo 48 - 50 Fs 11 - 34 ), Fe-pargasite, nepheline

± olivine, Ti-magnetite ±

± bio-

tite magnetite or hematite.

Glassy groundmass contains microlites of alkali feldspar, nepheline, clinopyrox-
ene

± sphene ± melanite (Ti- andradite up to 68 wt%) ±

± Ti- andradite ± magnetite or hematite. Analyses of rocks from Paraguay are

given in Table 4.13 .