Estimation of errors in measurement of illite crystallinity: The limits and problems of application to accretionary complexes.

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  • イライト結晶度の測定誤差の評価　付加コンプレックスに適用する際の限界と問題点
  • イライト ケッショウド ノ ソクテイ ゴサ ノ ヒョウカ フカ コンプレックス ニ テキヨウ スル サイ ノ ゲンカイ ト モンダイテン

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Abstract

The errors of the illite crystallinity (IC) and its factors were examined on samples from argillaceous rocks of the Urayama Unit of the Jurassic Chichibu accretionary complex in the Kanto Mountains, central Japan.All sedimented slides for X-ray diffractometer (XRD) were controlled in the thickness of clay film on slides and treated by the ethylene glycol solvation. Combined errors of XRD machine and intra-sample were estimated to be about 1.5% (coefficient of variation) together. The IC values of black shales have the intra-outcrop errors of 4.6%, and also the intra-area (2×2.5 km2) variations within 9%. The intra-area variations decrease up to approximately 6% except for samples near major faults bounding tectonostratigraphic units. The IC values of melange-type rock show the intra-outcrop error of 11%, which are much larger than those of black shale. Quantitative analyses of lithological effects for the IC values indicate that the IC values tend to decrease by increase of detrital muscovite and sandstone fragments, and vary with distribution of grain size (<2μm). These data show that both effects of faults and lithological characters of melange-type rock cause to increase the IC values errors.The above mentioned results suggest that in case of the IC analysis for regional thermal maturity of the accretionary complexes, we should avoid sampling rocks near faults and we need to estimate the intra-outcrop errors of the IC values for melange-type rock and black shale.

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