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Isotopic mantle signatures and metamorphic overprinting

Research Group:
Andrea Möller, Thorsten Geisler (Münster),
Ernst Pernicka (Freiberg), Helmut Schleicher,
Wolfgang Todt (MPI Mainz), Shrinivas Viladkar (Bombay)

In Cooperation with:
Max Planck-Institut für Chemie Mainz (W. Todt)
St. Xavier's College Bombay (S. Viladkar)
HASYLAB/DESY Hamburg (K. Rickers, G. Falkenberg)
Geology Department University Chennai


GEOLOGY

The carbonatite complexes of Tamil Nadu are characterised by the rock association carbonatite-syenite-pyroxenite±dunite. They are intruded within the Precambrian gneisses of the Southern Ghats Terrain (SGT) and situated within or along a dominant Panafrican shear zone between the Madras Block and the Northern Block of the SGT. Their mode of appearance differs rather strongly according to their geographic position: in the North porphyric structures of some syenites of the Sevattur complex are indicative for a shallow intrusion level, in the South the foliation and fabric of the Pakkanadu rocks reveal formation at higher p-T conditions.

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Geological sketch maps of Tamil Nadu carbonatite occurrences
Landsat 7 image of Samalpatti and Sevathur area (detail of N-44-10-2000)
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For the interpretion of the isotopic data, the question of a possible metamorphic overprinting of the carbonatites is highly important. The existence of a metamorphic event is most evident for the Pakkanadu complex by folding textures and the growth of large monazite crystals. But also the Samalpatti carbonatites show the effect of tectonic stress, e.g. by dominant stress twinning of calcite and by shearing. These features may result from the location of the complexes within or nearby the dominant Panafrican shear zones of the area.
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Large monazite crystals in foliated carbonatite, Pakkanadu)
Xenolith of fenitized gneiss in carbonatite, Samalpatti
Stress twinning in calcite, Samalpatti

GEOCHEMISTRY
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PM normalized trace element geochemistry of Sevattur carbonatites and calcsilicate marbles
Comparison of PM normalized trace element geochemistry of Tamil Nadu carbonatites
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PM normalized trace element pattern for phlogopites and biotites from Tamil Nadu carbonatite - pyroxenite - syenite complexes




Zr/Nb vs. Rb/Nb for syenites from Tamil Nadu carbonatite complexes
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Trace element and REE partitioning in a natural apatite - carbonatite melt system
using synchrotron XRF microprobe analysis

Inclusions of calcite within large euhedral apatite crystals from the carbonatite complex of Sevattur represent inclusions of a primary carbonatitic melt (calcite I) from which the apatites have crystallized. The apatites themselves are embedded into a younger calcite-carbonatitic melt (calcite II).
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Using the synchrotron XRF microprobe at beamline L at HASYLAB/DESY (Hamburg), the concentrations of the trace elements Ba, Sr, Y, Zr, Th, La, Ce, Nd, Sm, Gd, Dy, and Er were determined both in melt inclusions as well as in host apatites and younger carbonatite matrix. Unexpectedly high REE concentrations were found not only in apatite but also in calcite, especially of the younger matrix phase, in agreement with the whole rock geochemistry. The data reveal an equilibrium distribution between melt inclusions and host apatite that allows the calculation of partition coefficients for elements of interest.

ISOTOPE GEOCHEMISTRY AND GEOCHRONOLOGY
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Our geochronological data on monazites from Pakkanadu yield an Pb/Pb age of 759±3 Ma (b) which is concordant with electron microprobe (EMS) dating (a) on the same minerals (750±11 Ma; Möller et al. 2000). In contrast, the Pb/Pb total rock isochron age of Sevattur carbonatites (Schleicher et al. 1997) is slightly higher (801±11 Ma). Whereas the mineral ages may refer to a Panafrican metamorphic event, the meaning of the Pb/Pb total rock age is still open.
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The Pb isotope geochemistry of the Tamil Nadu carbonatites prove a mixing relationship between an upper mantle reservoir and a second component, which may be either an enriched mantle like EM II or a crustal component (Schleicher et al. 1998). Whether this mixing signature is a characteristic of the upper mantle or was achieved during or after the intrusion into the crust cannot deduced from our Pb isotopic data.

The Nd and Sr isotopic systems, which in carbonatites are highly resistant against crustal contamination, show a signature with very low present day 143Nd/144Nd ratios between 0.5116 and 0.5122 but rather elevated 87Sr/86Sr ratios around 0.705. To interpret these data in terms of mantle signatures, an age correction has to be done which is especially sensitive for the Nd isotopic data. If an age of 800 Ma is assumed according to the Pb/Pb total rock dating of Sevattur (Schleicher et al. 1997) the Nd-Sr signature of the Tamil Nadu carbonatites reveal a composition of an ancient enriched mantle, even somewhat more enriched than modern EM I.

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