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Effect of compositional variations on log responses of igneous and metamorphic rocks. II: acid and intermediate rocks

By
R. Pechnig
R. Pechnig
Angewandte Geophysik, RWTH Aachen University of Technology, Lochnerstrasse 4-20, 52056 Aachen, Germanyr.pechnig@geophysik.rwth-aachen.de
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H. Delius
H. Delius
Department of Geology, University of Leicester, Leicester, LE1 7RH, UK
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A. Bartetzko
A. Bartetzko
Angewandte Geophysik, RWTH Aachen University of Technology, Lochnerstrasse 4-20, 52056 Aachen, Germanyr.pechnig@geophysik.rwth-aachen.de
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Published:
January 01, 2005

Abstract

An extensive data-set of petrophysical down-hole measurements exists for boreholes drilled into continental crystalline crust. We selected boreholes covering a range of different types of plutonic rocks and gneisses in amphibolite or high-grade metamorphic rocks. According to Serra's concept of electrofacies, a specific set of log responses should characterize one rock type. Here, we concentrate on the detection of compositional variations between rock types. Bulk composition of the protoliths influences the mineralogical composition of the metamorphic rock, and we demonstrate how this impacts on the down-hole measurements. Integration of logging data with geochemical core data and mineralogical descriptions allows the calibration of the log responses to rock types. The relationship of the log responses with core data shows a remarkably good correlation, and diagnostic trends are detected. From the logs, potassium and neutron porosity are particularly helpful in distinguishing different types of gneisses and igneous rocks with respect to their protoliths. The proportions of amphibole/pyroxene, mica + K-feldspar and feldspar + quartz in the rocks seem to control the direction of correlation in a cross-plot, i.e. positive or negative, depending on increasing or decreasing mineral proportions. This is true for all boreholes, and a generalized classification scheme could be developed for these crystalline rocks.

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Contents

Geological Society, London, Special Publications

Petrophysical Properties of Crystalline Rocks

P. K. Harvey
P. K. Harvey
University of Leicester, UK
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T. S. Brewer
T. S. Brewer
University of Leicester, UK
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P. A. Pezard
P. A. Pezard
Université de Montpellier II, France
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V. A. Petrov
V. A. Petrov
IGEM, Russian Academy of Sciences, Russia
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Geological Society of London
Volume
240
ISBN electronic:
9781862394889
Publication date:
January 01, 2005

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