Sillimanite-Cordierite Xenocrysts in S-type Tonalite, Hidaka Metamorphic Complex, Hokkaido, JapanRoberto Weinberg, Monash University, Australia Andy Tomkins, Kyle Rebryna, Monash University, Australia |
In the
Nikambetsu river in southern Hidaka
Metamorphic Complex, there is a series of outcrops of
sillimanite-cordierite tonalite with biotite rims. Two thin sections
were studied: one had retrogressed (sericitized), rounded grains of
poikilitic cordierite, in a
cordierite+orthopyroxene+garnet+muscovite+biotite tonalite,
representing a tonalite formed through melting at high grade, the other
one, from the Nikambetsu river had porphyroblasts of sillimanite in the
core, surrounded by cordierite (?) rimmed by biotite, forming an
elliptical grain like in the photographs below. The rock is thus:
(sericite+chlorite) cordierite+sillimanite+muscovite+biotite tonalite.
The cordierite around the sillimanite needs to be confirmed by probing,
and it is partly retrogressed to sericite and chlorite. Thus, many of
the sillimanite grains depicted below could represent complex grains
possibly representing the partial break down of sillimanite+biotite to
form cordierite. This page shows several outcrops with long
sillimanite rimmed by biotite and defining a lineation in the tonalite.
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a) Pon Nikambetsu and Sarurugawa rivers: cordierite and sillimanite in tonalite
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| Figure 1a. Block from Sarurugawa with sillimanite grain rimmed by biotite, identical features to the outcrops in Nikanbetsu a few kilometers away. Similar grains could be found in migmatites of the Nakanogawa Group sedimentary rocks in the surroundings (Figs 2b,c). However, in general the cordierite grains in tonalite outcrops around this river are generally rounded and lack the biotite rim, similar to that in Fig. 2f below. | Tonalite from Pon Nikambetsugawa with rounded blueish, poikilitic coridierite grains, in center of the photograph left of the coin. Notice also xenoliths from the source. |
b) Sarurugawa river: sillimanite in migmatite
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| Migmatite from Sarurugawa showing a number of rounded biotite-rimmed sillimanite and possibly cordierite grains (e.g. left of the coin). | Migmatite from Sarurugawa with euhedtral biotite-rimmed sillimanite grains (e.g. large grain below the coin, and euhedral cross-section in the middle of the photograph). |
c) Nikambetsu river: sillimanite (possibly with cordierite) xenocrysts in tonalite
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| Figure 2a. Tonalite with a xenolith and two large, long sillimanite grains surrounded by a rim of biotite. | Figure 2b. Section across the length of two cordierite grains surrounded by biotite rim. Note the horizontal biotite aggregate close to the coin, indicating the remains of the rim of another sillimanite grain. |
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| Figure 2c. Typical outcrop of S-type tonalite of the southern Hidaka metamorphic complex with aligned sillimanite grains, parallel to the long dimension of sedimentary xenoliths. |
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| Figure 2d:. Tonalite with aligned sillimanite grains with rims ov biotite, or just the remainder rim of biotite (center bottom part). | Figure 2e.Same as d). |
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| Figure 2f. Cordierite grain (up and right from the coin) different from the more typical grains shown, in that it is rounded and rimmed by a fine layer of a light coloured mineral. This grain is found close to a xenolith in a heterogeneous part of the outcrop. More typical euhedral biotite-rimmed sillimanite grains are also found (down and left of the coin). |