Aspects of the petrochemistry of the Phalaborwa Complex, northeastern Transvaal, South Africa
Date
1982
Authors
Eriksson, Susan Camenisch
Journal Title
Journal ISSN
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Abstract
The Phalaborwa Complex, northeastern Transvaal, South
Africa, consists of the main body of clinopyroxenites and
subordinate phoscorite, carbonatite and syenite which is
surrounded by numerous pipe-like bodies of syenitic
compositions and rare clinopyroxenites.
Clinopyroxenites of the main complex are characterized
by cumulus textures formed by separation and accumulation of
coprecipitating clinopyroxene, apatite and phlogopite.
Potassium feldspar is an intercumulus phase in feldspathic
pyroxenite. "Inch-scale" layering of clinopyroxene, apatite
and phlogopite formed as in situ cumulus layering near the
outer contact of the complex early in the cooling of the
magma. Breccias of monomineralic assemblages such as
glimmerite and massive pyroxenite reflect breaking up of
early formed rocks by magmatic currents.
Clinopyroxenes from clinopyroxenites are characterized
by Fe/(Fe+Mg) = 0.07-0.29, low T i 02 (0.00-0.25%), A 120 3
(0.00-1.63%), N a 20 (0.00-1.06%), and Cr, and high Wo
component and Sr. Micas from pyroxenites have Fe/(Fe+Mg) =
0.12-0.28, low T i 02 (0.17-1.73%) and have reverse and normal
pleochroism and increasing A1 with increasing Fe/(Fe+Mg).
Mineral compositions among feldspathic, massive and
micaceous pyroxenites overlap; no zonation of the complex
from outer contact inward is discernible with respect to the
Fe and Mg content. However, phlogopites in "inch-scale"
layering have low Fe/(Fe+Mg) of 0.12 and have reverse
pleochroism due to F e 3+ entry into the Al-deficient
tetrahedral site. Micas from phoscorite and carbonatites
have reverse pleochroism, Fe/(Fe+Mg) = 0.05-0.58, low T i 02
(0.00-0.84%) and decreasing A1 with increasing Fe/(Fe+Mg).
Olivines range from F o79 to F o91 and have very low Ni
content (<0.06% N i O ) . Olivines interpreted as xenocrysts
have Fo 84 to F o 8 7 . One of the olivine xenocrysts has an NiO
content of 0.29%.
Minerals from carbonatites have initial 87S r / 86Sr
ratios of 0.70393-0.70623 and 0.71022 and minerals from
clinopyroxenites have values of 0.71152-0.71242. Smallscale
inhomogeneities exist within samples. Postcrystallization
processes can account for variations within
samples, but cannot account for variations within a rock
type or for raising initial 87S r / 86Sr ratios of pyroxene
from 0.7039 to 0.7115. Magmas forming pyroxenites and some
carbonatites were generated in a high Rb/Sr mantle from
isotopically distinct sources. Hence, liquid immiscibility
and differentiation are not viable mechanisms for relating
these rocks to one another. Mixing of magmas and
assimilation of crust may account for isotopic variations
within a rock type.
U-Pb dating of uranothorianite and baddeleyite from
phoscorite and carbonatite yields an age of 2047+11/-8 m.y.
Rb-Sr dating of phlogopites gives widely disparate apparent
ages. Nine phlogopites yield an isochron of 2012 + 19 m.y.
One sample of phlogopite gave ages of 1661-2360 m.y. in nine
different determinations. Older micas may be present in the
Phalaborwa complex. Based on the two methods, a best age of
the Phalaborwa complex is 2030 + 18 m.y.
Two of the syenite pipes, Kgopoeloe and Spitskop, show
different levels of emplacement. Kgopoeloe is highly
brecciated from a fluid derived from the syenite. Spitskop
contains minor breccia and represents multiple injection of
syenite. At Spitskop, inward crystallization of the second
syenite forms a ring syenite and central syenite; both
syenites have cumulus enrichment of minerals.
Feldspathic pyroxenite of the Guide Copper Mine is
cogenetic with the pyroxenites of the main complex and
contains clinopyroxenes with oscillatory zoning.
Fluctuations in f02 may be attributed to formation of an
immiscible sulphide liquid.
Multiple intrusion of the Phalaborwa Complex is
proposed. Initial injection of potassic, probably
ultrabasic, liquid formed the pyroxenites. A second
intrusion of low-silica, carbonate-rich magma formed
phoscorite and banded carbonatite. A third intrusion of
carbonatite liquid formed the transgressive carbonatite.
Magmatic, cumulus processes dominated the formation of the
main complex. The syenites of Kgopoeloe and Spitskop are
not cogenetic with rocks of the main complex.
Description
A Thesis Submitted to the Faculty of Science,
University of the Witwatersrand , Johannesburg
for the Degree of Doctor of Philosophy
Keywords
Petrology--South Africa--Phalaborwa Complex, Geology--South Africa--Phalaborwa Complex