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THE IGWISI HILLS EXTRUSIVE KIMBERLITES

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<NASA-TM-X-74222) THE IGHISI HILLS
EXTBUSIVE KIMEEBLIIES (NASA) 20-p
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THE IGWISI HILLS EXTRUSIVE "KIMBERLITES"
By ARCH M. REID,* C.H. DONALDSON,! J.B. DAWSON,^ R.W. BROWN,§
and W.I. RiDLEYt<">
ABSTRACT
The Igwisi Hills are a group of volcanic hills in Tanzania where a unique series of eruptions has produced
volcanic rocks that have many of the features of kimberlites. The rocks are "igneous conglomerates" that are
characterized by the presence of abundant ellipsoids of olivine in a fine grained matrix. The matrix contains major
amounts of carbonate, serpentine, and complexly zoned Mg-Al-Cr spinels, minor perovskite, apatite and limonite.
Large magnesian olivines are highly rounded and some have partially recrystallized to more Fe-rich and Ni-poor
subhedral to euhedral grains. Several olivine ellipsoids are partly or wholly rimmed by a black coating consisting
of perovskite, and Mg-Al spinel. Mineral inclusions within the olivines include chrome pyrope, similar in composition to garnets in kimberlites, low Al enstatite, low Al magnesian chrome diopside, Mg-Al chromite, and a
highly magnesian phlogopite (Mg/Mg + Fe ~ .94). All of these are apparently primary phases in equilibrium with
olivine. The total assemblage is similar in many respects to that found in garnet peridotite xenoliths such as those
from the Lashaine volcano in northern Tanzania.
The Igwisi irruptives apparently contain material derived from phlogopite-bearing, garnet peridotites with a
primary mineral assemblage (assuming that all these phases coexist at depth) indicative of equilibrium at upper
mantle temperatures and pressures. This primary assemblage was disrupted and brought rapidly to the surface
in a gas-charged, carbonate-rich fluid. Rapid upward transport, extrusion, and rapid cooling have preserved
inclusions of upper mantle ultramafics in a carbonate-rich matrix and have tended to prevent reaction between
inclusions and matrix that might otherwise have yielded a more typical kimberlite.
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INTRODUCTION
The Igwisi Hills are a small group of volcanic hills northwest of Urambo in central
Tanzania (4°51' S., 3P55' E.) described and mapped by SAMPSON (1953). In contrast to the
recent volcanics erupted along rift valleys encircling the Tanzania shield, the Igwisi tuffcones lie within the shield (DAWSON, 1971a). The hills consist of three craters with diameters
of 400,250 and 200 yards and rims that stand 50-150 feet above the surrounding plain. The
form of the craters suggests a young geological age. From a breach in the largest crater a
small flow half a mile in' length and breadth has occurred (SAMPSON, 1953). The rocks from
Igwisi were divided by SAMPSON into tuffs, that are homogeneous, and lavas that are highly
variable in grain size and vesicularity. The lavas contain a great abundance of rounded
olivine grains that are most common towards the base of the flows.
BASSETT (1954).described the Igwisi lavas as a two-component mixture of olivine, derived
from duriite, and an aluminous cement, derived perhaps from limestone plus bauxitic or
* NASA Johnson Space Center, Houston, TX 77058, U.S.A.
t Lunar Science Institute, 3303 NASA Road 1, Houston, TX 77058, U.S.A.
$ Department of Geology, University of St. Andrews, Scotland.
§.J-pckJieed_Electronics Corp7l68!l El Camino Real, Houston, TX 77058, U.S.A.
t^PresenTaddress: Lamont-Doherty Geological Observatory;.Palisades, N.Y. 10964, U.S.A.
199
200
A. M. REID, C. H. DONALDSON, J. B. DAWSON, R. W. BROWN AND W. I. RIDLEY
limonitic deposits. The Igwisi crater then acted as "a natural aluminous cement concrete
mixer". The possible relationships between the Igwisi rocks and kimberlites were explored
by FOZZARD (1956), who summarizes earlier evidence on this subject. FOZZARD looked for
and did not find any significant variability in olivine in the Igwisi samples and thus found
no evidence of separate origins for the various olivine types from large xenocrysts to small
grains. FOZZARD stressed the similarities to kimberlites and provided data on the high Nb
content of both Igwisi samples and kimberlites.
The Igwisi volcanics are extrusive rocks that show several remarkable similarities to
kimberlites and also some differences. MITCHELL (1970), summarizing the earlier evidence,
concludes that the Igwisi rocks are not extrusive kimberlites. MITCHELL'S conclusions are
based on the high Al content of the Igwisi samples (BASSETT, 1954) and on the absence of
several of the characteristic minerals of kimberlite such as pyrope, chrome diopside and
magnesian ilmenite. These conclusions were in part negated by DAWSON (1971b) who
discovered both chrome diopside and garnet in the Igwisi rocks. The work reported here
is based on a suite of Igwisi samples, collected by DAWSON, that show phases characteristic
of kimberlites.
Extrusive equivalents of kimberlites are apparently very rare. The evidence for their
occurrence is reviewed by DAWSON (1971b) who discusses two possible candidates—the
Igwisi Hills extrusives and the meimechites of northwestern Siberia (MooR and SHEINMANN,
1946; BUTAKOVA and EGOROV, 1962).
PETROGRAPHY
The Igwisi volcanic rocks are gray-brown olivine porphyries with large green olivines
set in a fine-grained carbonate-rich matrix. The rocks with the higher olivine contents
resemble conglomerates as the larger olivine grains are all rounded and substantially larger
than the grain size of the matrix. Some samples have abundant white or cream-colored
carbonate amygdules, indicative of a gas-charged magma. Non-cognate xenoliths of hornfelsed material occur in the tuffaceous specimens. In general the tuffs are lighter colored
than the lavas, are less coherent and contain a greater variety of inclusions. Xenoliths of
well-foliated biotite gneiss are abundant in the lowest lavas of the inner wall of the northeast
crater.
The texture of the Igwisi volcanics is dominated by the presence of highly symmetrical
oblate spheroids of olivine, up to 2.2 x 1 cm. Some of the olivine spheroids show a subparallel en-echelon configuration. Many of the olivine spheroids are faintly brown single
crystals. Polycrystalline aggregates are also common but in most cases these clearly have
recrystallized from single crystals. The grains in the polycrystalline aggregates are smaller
(.14 cm or less), generally euhedral to subhedral, strain-free and colorless. They have fewer
fractures than the primary olivines and tend to be concentrated around the margins of the
spheroids. Granular mosaics of unoriented olivines are common within the spheroids or
adjacent to the peripheries, especially at regions of maximum curvature (Figs. 1, 4). The
peripheral relationship indicates that recrystallization followed rounding.
Most olivine spheroids that are single crystals are optically homogeneous but a few
do show narrow (0.05 cm) rims that appear as normal zones. Less than 10% of the spheroids
are mantled by an opaque rim (Fig. 1) that is thickest at the highly curved ends and thinnest
or totally absent along the sides of the spheroids.
THE IGWISI HILLS EXTRUSIVE
KIMBERLITES
FIG. IA
FIG. IB
FIG. 1. Reflected light photomicrograph oflgwisi sample t134A3. Large rounded grains are olivines.
The largest spheroid, shown in detail in Fig. Ib, is 7.75 /an long and consists of olivine surrounded
by a rim of spinel and perovskite.
201
216
A. M. REID, C. H. DONALDSON, J. B. DAWSON, R. W. BROWN AND W. I. RIDLEY
higher temperatures are required. Magmatic corrosion would accord with the observation
that nearly all the spheroids are, or apparently were, olivine single crystals, and also with
the presence of spinel-perovskite rims. However, there seems to be no evidence from other
localities that magmatic corrosion of olivines can generate spheroidal shapes. It should also
be noted that the other primary phases occur surrounded by olivine single crystals. This
texture is difficult to reconcile with normal igneous crystallization.
Alternatively, the shapes of the olivines, resembling conglomerate pebbles, may well be
erosional. Similar spheroidal olivines occur in kimberlites and are occasionally rimmed by
perovskite (DAWSON and HAWTHORNE, 1973). The mechanism of erosion is not at all clear
but may well be related to upward transport. McGETcmN (1968) has suggested that material
in kimberlite pipes may be transported upwards at extremely high speeds. The shapes of the
Igwisi olivine spheroids may be related to abrasion under these unique conditions.
Some part of the deformation apparent within the olivines, namely the deformation
that is restricted to the zones of maximum curvature, clearly occurred after the olivines
attained their spheroidal shape. In addition, much of the olivine recrystallization is obviously related to shape and took place after the olivines had become rounded. The chemical data
indicate that recrystallization was accompanied by some chemical migration (gain of Ca,
loss of Ni, increase in Fe/Mg). Recrystallization and deformation may in part be a response
to mechanical stress during transport. Recrystallization was rapid or rounding did not occur
in the later stages of rapid transport. Hopefully further study of the unique textures of the
Igwisi rocks will provide insight into the complex processes of disruption and/or corrosion,
transport and erosion involved in the formation of the Igwisi volcanics.
IGWISI HILLS AND KIMBERLITES
The Igwisi rocks resemble kimberlites and the question arises as to whether these are
indeed extrusive kimberlites. Their similarities to kimberlites include (1) the two-component
nature of the rocks with high P-T minerals in a low P-T matrix; (2) the presence of chrome
pyrope, Al enstatite, chrome diopside, chromite and olivine; (3) the highly oxidized volatilerich matrix with serpentine, calcite, magnetite, perovskite; (4) the bulk composition,
assuming the lower Al values we find are representative, and particularly the high Sr, Zr
and Mb contents; (5) the occurrence in a narrow isolated vent within a stable shield area.
The Igwisi rocks are in several respects similar to the intrusive kimberlite of the Benfontein
sill described by DAWSON and HAWTHORNE (1973). The minerals in both are very similar as
are several textural features including rimming of olivines and flow textures defined by
carbonate laths.
Differences from kimberlite include the lack of magnesian ilmenite, the scarcity of
matrix phlogopite and the overall low alkali content. The high carbonate content coupled
with high Sr and very low Rb more closely resembles carbonatite than kimberlite. It is
apparent that other data such as oxygen isotope ratios are necessary to a complete study.
To some extent the classification of the Igwisi rocks is a function of the definition of kimberlite used. The similarities to kimberlite are such that the Igwisi rocks should be included
within that group with the presumption that there are "kimberlites and kimberlites".
One objective of this study has been to follow up the discovery by DAWSON (1971b) of the
additional phases contained within the Igwisi olivines. If the high temperature assemblages
are representative of mineral proportions at depth, they are derived from extremely olivine-
THE IGWISI HILLS EXTRUSIVE "KIMBERLITES"
217
rich garnet peridotites. The minerals of the spheroids are remarkably similar to garnet
peridotite minerals in xenoliths in South African and Lesothan kimberlites and in the
Lashaine volcano in northern Tanzania. There is growing evidence for the presence within
the upper mantle over large regions of Africa of extensive volumes of garnet peridotite. The
major minerals of these peridotites have equilibrated at high temperatures and pressures
and show little compositional variability. Phlogopite with a high Mg/Fe ratio (high Ti at
Lashaine, low Ti at Igwisi) is a sporadic constituent.
Present also within the mantle are volatile-rich fluids such as those which contributed
to the Igwisi "kimberlitic" matrix. These fluids may be derived by small degrees of partial
melting of pristine mantle material or may be generated by metamorphic differentiation at
depth. Their existence is evident but the mechanism for their formation is obscure. Such
fluids may enrich magmatic liquids formed by partial melting at depth or may accumulate
in technically stable areas ultimately to be released and transported upward as part of a
volatile-rich kimberlite "magma".
ACKNOWLEDGEMENTS
We thank J. M. Rhodes for the trace element data and the water determination, E. K. Gibson for the carbon analysis, J. R. Smyth for the X-ray powder pattern and R. B. Merrill for
constructive criticism of the manuscript. C. H. Donaldson thanks the Lunar Science
Institute for a Visiting Graduate Fellowship and the Natural Environment Research Council of the United Kingdom for financial support. J. B. Dawson acknowledges the help of
the Carnegie Trust for the University of Scotland and the travel fund of the University of
St. Andrews for financing field work in East Africa in 1966 and also the Department of
Mineral Resources, Dodoma, Tanzania for help in visiting the Igwisi Hills. W. I. Ridley was
supported by the Lunar Science Institute, which is operated by the Universities Space
Research Association under contract no. NSR-09-051-001 with the National Aeronautics
and Space Administration. This paper is Lunar Science Institute Contribution no. 171.
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_".
BUTAKOVA, E. LT. 2nd EGOROV, L. S. (1962) Meimecha-Kotui complex of alkaline and ultrabasic rocks. In:
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