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Sillimanite-Cordierite Xenocrysts in S-type Tonalite, Hidaka Metamorphic Complex, Hokkaido, Japan

Roberto Weinberg, Monash University, Australia

Andy Tomkins, Kyle Rebryna, Monash University, Australia

 

 

Copyright 2004-2011 by Roberto Weinberg. All rights reserved. Unlimited permission to copy or use is hereby granted for non-profit driven enterprise, subject to inclusion of this copyright notice and acknowledgment of the source URL: users.monash.edu.au/~weinberg.

 

I would very much appreciate an email stating how this material will be used: Roberto Weinberg, Monash University, Australia. Thanks, RW.

 

DISCLAIMER. The material on this website has not undergone the scrutiny of Monash University and does not conform to its corporate web design. It is entirely based on a free-spritied, curiosity-driven research effort by the author, and therefore in no way expresses the official position of the University.

 

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.

 

waterfall

CLICK ON PHOTOS FOR HIGH RESOLUTION VERSION

 

 


 

a) Pon Nikambetsu and Sarurugawa rivers: cordierite and sillimanite in tonalite

sillimanite rimmed by biotite in tonalite
cordierite tonalite
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

biotite-rimmed
sillimanite in migmatite
biotite-rimmed
sillimanite
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

biotite-rimmed
sillimanite in tonalite
biotite-rimmed
cordierite in tonalite
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.
S-type tonalite with sillimanite and xenoliths
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.

 

biotite-rimmed
sillimanite in tonalite
biotite-rimmed
sillimanite in tonalite
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).
cordierite in tonalite
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).