Journal ArticleDOI
Geochemistry and Petrogenesis of Amphibolites, Kolar Schist Belt, South India: Evidence for Komatiitic Magma Derived by Low Percentages of Melting of the Mantle
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TLDR
The Kolar Schist belt of the Dharwar Craton of South India is an Archean greenstone belt dominated by metavolcanal amphibolites as discussed by the authors.Abstract:
The Kolar Schist Belt of the Dharwar Craton of South India is an Archean greenstone belt dominated by
metavolcanic rocks. The mafic metavolcanic rocks occur as komatiitic and tholeiitic amphibolites. The komatiitic
amphibolites occur along the margins of the N-S trending, synformal belt. They are much less abundant than the
tholeiitic amphibolites and have 14 to 21-3 wt. per cent MgO. The komatiitic amphibolites from the west/central part
of the belt have two distinctive REE patterns: (1) those enriched in the middle to light REE but depleted in Ce
relative to Nd; and (2) those with patterns that are convex up, i.e. depleted in both light and heavy REE, although
more depleted in the light REE. Associated tholeiites have light REE depleted to flat REE patterns. Komatiitic and
tholeiitic amphibolites from the eastern part of the belt have enriched light REE patterns. The tholeiitic
amphibolites from the Kolar Schist Belt are similar to the TH I and TH II types of Archean tholeiites of Condie
(1981). The komatiitic amphibolites are similar to komatiites and komatiitic basalts of Barberton Mountainland, but
have higher FeO and TiO 2 abundances and lower Yb/Gd ratios. The petrogenetic interpretations for these
rocks are based primarily on a modification of the MgO-FeO diagram of Hanson & Langmuir (1978), and modelling of
Zr, Ni and REE. All of the rocks have undergone some fractionation. While the modelling does not give accurate
temperatures, pressures, compositions and extents of melting of the mantle sources for the various amphibolites, it
does present an approach which can be used for estimating these parameters. For example, the komatiitic amphibolites
appear to be derived from melts generated by 10 to 25 per cent melting of the mantle over a range of depths and
temperatures greater than 80 km and 1575°C. The variation in the P-T conditions of magma generation is possibly
due to adiabatic melting in mantle diapirs with a range of FeO/MgO ratios. If the tholeiitic amphibolites are
derived from similar mantle sources (it is not clear that they are), their parent melts may have been generated by
similar extents of melting, but at depths of less than 80 km. The komatiitic amphibolites from the west central part
of the belt were generated from light REE depleted mantle, whereas those from the eastern part of the belt appear to
have been generated from light REE enriched mantle. The sources for the komatiitic amphibolites in both areas were
significantly enriched in FeO relative to pyrolite. Thus, a light REE depleted and a light REE enriched source
appear to have provided mafic volcanics with similar major element chemistry to this belt during its
evolution.read more
Citations
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Precambrian Continental Crust of India and Its Evolution
B. P. Radhakrishna,S. M. Naqvi +1 more
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An empirical thermal history of the Earth's upper mantle
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Journal ArticleDOI
Archean crustal evolution in China: The Taishan complex, and evidence for juvenile crustal addition from long-term depleted mantle
Bor-ming Jahn,Bernard Auvray,Q.H. Shen,Dunyi Liu,Zong-qing Zhang,Y. J. Dong,X. J. Ye,Q. Z. Zhang,J. Cornichet,J. Mace +9 more
TL;DR: The main framework for crustal evolution of the late Archean Taishan Complex in the Shandong Province of China is constructed using data from field investigation, petrographic examination, geochemical analysis and isotopic age determination as discussed by the authors.
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