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Geochemistry of peridotite microxenolith enclosed in kimberlite from
Mozambique.
Yaser Maher Abdel Aziz
Faculty of Science, Assiut University, Egypt
Kimberlite
samples which have emplacement age near 138Ma (lower Cretaceous) occur
commonly as intrusive dykes in Permian to Jurassic Karoo sedimentary
rocks of Maniamba graben in Mozambique.These kimberlite enclose
peridotitic microxenolithes composed of coarse garnet peridotite (Iherzolite)
consisting of coexisting olivine, clinopyroxene, orthopyroxene and
garnet in a coarse protogranular equilibrium textures.
The xenoliths are characteristically enstatiite rich
with mg number between 90 and 92 which are comparable to compositions of
other cratonic peridotite in Southern Africa. Mineral chemistry
indicates that the garnet compositions is pyrope garnet with high MgO
(up to 21wt%), moderate Cr (2.7- 4.9 wt%) and low calcium (4-5.9 wt%)
contents.
Geothermobarometric caculations indicated that these
xenoliths are low temperature garnet peridotite. The estimated
tempaeature and pressure of equilibrium range from 900 - 1065 OC
and 14 - 24 kbar respectively. These are equivalent to depth range from
49-83 km characteristic to shallow uppermost upper mantle. These
conditions of equilibrium is comparable to coarse low temperature
cratonic garnet peridotites (1100 – 1200 OC) from South
Africa and worldwide cratons elsewhere. The high mg number of the
studied xenoliths are consistant with an origin as residues after the
extraction of ultramafic melts formed by melting processes that occurred
at greater depths than those at which magma separates beneath oceanic
spreading centers. |