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Tourmaline nodules: products of devolatilization within the final evolutionary stage of granitic melt?

By
Dražen Balen
Dražen Balen
University of Zagreb, Faculty of Science, Horvatovac 95, Zagreb, Croatia
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Igor Broska
Igor Broska
Slovak Academy of Sciences, Geological Institute, Dúbravská cesta 9, Bratislava, Slovakia
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Published:
January 01, 2011

Abstract

The origin of tourmaline nodules, and of their peculiar textures found in peripheral parts of the Moslavačka Gora (Croatia) Cretaceous peraluminous granite are connected with the separation of a late-stage boron-rich volatile fluid phase that exsolved from the crystallizing magma. Based on field, mineralogical and textural observations, tourmaline nodules were formed during the final stage of granite evolution when undersaturated granite magma intruded to shallow crustal horizons, become saturated and exsolved a fluid phase from residual melt as buoyant bubbles, or pockets. Calculated P–T conditions at emplacement level are c. 720 °C, 70–270 MPa, and water content in the melt up to 4.2 wt%.

Two distinct occurrence types of tourmalines have been distinguished: disseminated and nodular tourmalines. Disseminated tourmaline, crystallized during magmatic stage, is typical schorl while nodular tourmaline composition is shifted toward dravite. The increase of dravite in nodular tourmaline is attributed to mixing of the fluid phase from the residual melt with fluid from the wall rocks.

The pressure decrease and related cooling at shallow crustal levels can be considered as a major factor controlling fluid behaviour, formation of a volatile phase, and the crystallization path in the Moslavačka Gora granite body.

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Geological Society, London, Special Publications

Granite-Related Ore Deposits

A. N. Sial
A. N. Sial
Federal University of Pernambuco, Recife, Brazil
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J. S. Bettencourt
J. S. Bettencourt
Universidade de Sao Paulo, Brazil
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C. P. De Campos
C. P. De Campos
Ludwig-Maximilians-Universität, Munich, Germany
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V. P. Ferreira
V. P. Ferreira
Federal University of Pernambuco, Recife, Brazil
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Geological Society of London
Volume
350
ISBN electronic:
9781862395985
Publication date:
January 01, 2011

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