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| Titel |
Zircon mineral trace element chemistry as a function of metamorphic grade along a traverse of lower Archean crust, Tamil Nadu, south India |
| VerfasserIn |
Daniel Harlov, Daniel Dunkley, Edward Hansen, Tomokazu Hokada |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250036795
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| Zusammenfassung |
| Trends in whole-rock and mineral chemistry are seen along a 95 km traverse of lower
Archean granitic orthogneissic crust, in the Eastern Dharwar Craton, Tamil Nadu, south India
(Hansen and Harlov 2007 J Petrol 48, 1641). Going from north (amphibolite facies) to south
(granulite-facies) along the traverse, chemical trends include whole-rock depletion of Rb, Cs,
Th, and U; enrichment in Ti and F with depletion in Fe and Mn in biotite and amphibole;
increases in Al with decreases in Mn in orthopyroxene; and enrichment of fluorapatite in F
coupled with depletion in Cl. In the northern most portion of the traverse the principal
REE-bearing minerals are allanite and titanite. South of a clinopyroxene isograd, separating
the granulite- and amphibolite-facies zones, monazite grains independent of fluorapatite
are the major REE- and Th-bearing phase. Further south independent monazite is
rare but Th-free monazite inclusions are common in fluorapatite. During prograde
metamorphism, independent monazite was replaced by REE-rich fluorapatite in
which the monazite inclusions later formed. The loss of independent monazite
was accompanied by a loss of whole-rock Th and possibly a small depletion in
LREE.
Zircon grains along the traverse preserve domains of magmatic zoning with ages between
ca. 2.70Ga and 2.55Ga, recording the emplacement of granitic protoliths. Magmatic
zircon was modified during metamorphism in two distinct ways: (i) zircon along
cracks, growth zones and margins is replaced by U-enriched zircon, commonly with
abundant silicate inclusions; and (ii) grain margins are dissolved and overgrown
by faceted rims of U and Th depleted zircon. Type (i) replacement textures are
variably found in samples along the whole traverse, whereas type (ii) overgrowths
appear near the clinopyroxene isograd and increase in proportion to the original
protolith zircon southwards, such that most samples in the southern half of the
traverse contain only minor remnants of protolith zircon relative to overgrowth zircon.
Thorium and U contents of magmatic zircon do not have simple relationships to
whole-rock Th-U-Zr contents, or to either type of metamorphic zircon. Instead, high U –
low Th compositions of type (i) zircon may reflect equilibration with Th-bearing
phases at amphibolite to lower granulite-facies conditions, whereas type (ii) low
U – low Th overgrowths reflect whole-rock depletion in Th and U (but not Zr).
Ages for type (ii) overgrowths cluster around 2.5Ga, similar to those obtained for
identical zircon from the southern margin of the Eastern Dharwar Craton (Clark et al.
2009, Gondwana Res. 16, 27), whereas ages from type (i) zircon scatter towards
protolith ages, consistent with partial resetting through recrystallisation of magmatic
zircon.
Whole-rock U-Th-Zr compositions are decoupled from magmatic zircon, and coupled
with type (ii) overgrowths, demonstrating that chemical changes along the traverse were
produced during metamorphism, rather than reflecting differences in the protoliths. Most
mineralogical features along the traverse can be accounted for by progressive dehydration and
oxidation reactions. Trace element depletion is best explained by the action of externally
derived low-H2O activity brine migrating from a source at greater depth, possibly preceded or
accompanied by partial melting. |
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