麻豆传媒

听

The Scourian granulite facies terrain comprises the most ancient portion of the Precambrian Lewisian complex of N.W. Scotland. It is composed of a cogenetic series of gneisses from ultramafic to felsic in composition--dominated by felsic gneisses of tonalitic chemistry, with lesser mafic and ultramafic varieties, and very minor granitic rocks. The terrain was affected by a granulite facies metamorphic event at approximately 2600 to 2900 m.y. at which time it became severely depleted in the incompatible elements U, Th and Rb, and possibly K. The physical conditions of granulite facies metamorphism have been estimated at 916 + 33^oC, 11 + 1 kb, at very low partial pressures of water. Isotopic data restrict any previous upper crustal history to less than 150 m.y.

New REE data for whole rock and mineral separates of the Scourian complex show that:

1. overall, the Scourian complex has a small, but distinct, positive Eu anomaly.
2. individual rock type REE patterns are unlike most REE patterns of upper crustal rocks of equivalent major element chemistry.
3. whole rock REE abundances largely reflect the REE abundances of the major rock-forming minerals.
4. mineral REE patterns, abundances and distribution coefficients are very similar to those from dacitic rocks.
5. the REE content of the Scourian mineral phases is not a strong function of rock type.
6. the concordant granitic, and most of the discordant pegmatitic, rocks have anomalous REE abundances (low absolute concentrations and large positive Eu anomalies), unlike upper crustal granitic rocks.

The preferred hypothesis of this study is that the Scourian complex is the residuum, left after the generation and removal of melts of granitic composition. Trace element partial-melting modelling suggests that approximately 20% partial melting of a LRE enriched upper crustal Scourian precursor (moderately high in Ba) could account for virtually all of the trace element features of the Scourian rocks.

Evidence from geophysics, geochemistry and general geology indicate that granulite facies terrains are samples of lower crust. It is suggested, although tentatively at this stage, that the above mode of formation is not unique to the Scourian complex but is the mechanism whereby the heat-producing elements are concentrated in the upper crust. The REE data suggest that it may be possible to balance the observed continental upper crustal negative Eu anomaly with a positive Eu anomaly in the lower crust, yielding an overall continental crust with normal or chondritic Eu abundances.

The majority of the granitic rocks of the Scourian complex are interpreted as having formed, at a late stage of Scourian metamorphism, by disequilibrium partial melting of the Scourian gneisses, or by equilibrium partial melting with sub-solidus re-equilibration with the host gneisses.

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Pages: 252
Supervisor: Gunter Muecke