Geology Reference
In-Depth Information
Picroilmenite coexists with olivine, clinopyroxene and phlogopite at 1.0 GPa under
QFM buffer condition below 1,100
C. This phase shows lower TiO
2
and FeO
content at 1.5 GPa where it coexists with clinopyroxene and phlogopite. They
suggested that the phlogopite-bearing minette magma might have originated from a
metasomatized garnet-bearing peridotite source at deeper level in the mantle
(P
>
2.0 GPa) but can also be in equilibrium with a phlogopite-bearing wherlite
(
°
2.0 GPa under oxidizing as well as reducing mantle
condition. Because of their rapid crystallization rate and high liquidus temperatures,
a series of potassic daughter melts like K-rich latite and felsic minettes may originate
by segregation of ma
±
orthopyroxene) source at 1.7
-
c minette magmas during their ascent to the cooler continental
crustal environment. They thought that preservation of olivine in equilibrium with
phlogopite phenocrysts in primitive minettes suggest that assimilation was not
involved in their genesis. Their study supports the model that the phlogopite minette
magmas are brought to near surface condition at temperatures between 1,000 and
1,200
°
C and they got chilled rapidly during their emplacement.
13.2.11 Experimental Studies on a Phlogopite-Pyroxenite
Nodule from South-West Uganda
Melting experiments on a mantle-derived nodule consisting of clinopyroxene,
phlogopite and minor titanomagnetite, sphene and apatite, was conducted by Lloyd
et al. (1985) at 2.0 and 3.0 GPa between 1,175 and 1,300
C (Fig.
13.20
). The
nodule composition was selected on the basis of modal and chemical analyses of 84
mantle-derived nodules with metasomatic textures from the Katwe-Kikorango and
Bunyaruguru volcanic
°
fields of south west Uganda. At 3.0 GPa, 1,225 and
1,250
C, representing 20 to 30 % partial melting, the composition of glasses
compare favourably to those of the average composition of 26 highly potassic ma
°
c
lavas from the same region. Glasses produced by suf
ciently low degrees of partial
melting at 2.0 GPa could not be analysed. Glass compositions obtained by 20
30 %
-
melting at 3.0 GPa have high K
2
O (3.07
5.05 wt%), low SiO
2
(35.0
39.2 wt%),
-
-
high K/(K + Na) (0.54
0.71), (K + Na)/Al (0.99
1.08) and Mg/(Mg + Fe
t
)of
-
-
0.59
0.62. These results support the suggestion of Lloyd and Bailey (1975) that the
nodules represent the source material for the high K-rich lavas of south-west
Uganda. If this conclusion is correct it implies that anomalies in the mantle source
rock of phlogopite clinopyroxenite composition could produce the Uganda lavas by
relatively higher degrees of partial melting than normally considered. Highly
alkaline ma
-
c magmas could have been derived also from a pyrolitic mantle source.
Higher degree of melting is considered likely from different source regions, rich in
alkalis rather than radioactive elements. Steeper geotherms and increased
fluxing of
sub-rift mantle by degassing would also reduce higher degrees of partial melting.
Search WWH ::
Custom Search