Morphological and chemical variations of chromian spinel in dunite-harzburgite complexes from the Sangun zone (SW Japan): implications for mantle/melt reaction and chromitite formation processes |
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| Authors: | I Matsumoto S Arai |
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| Institution: | (1) Faculty of Education, Shimane University, Japan, JP;(2) Department of Earth Sciences, Kanazawa University, Japan, JP |
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| Abstract: | Summary ?Many ultramafic complexes, some of which have chromitite bodies, are exposed in the Sangun zone in central Chugoku district,
Southwest Japan. Harzburgite is always dominant over dunite, but the dunite/harzburgite ratio varies from complex to complex.
Large chromitite bodies are exclusively found in relatively dunite-dominant complexes or portions.
The degree of roundness, DR#=area/(round-length)2] (normalized by a circle’s value: 1/4π), of chromian spinel is variable, depending on lithology of the peridotites. Chromian
spinel is mostly anhedral or even vermicular (less than 0.4 in DR#) in harzburgite, and is most frequently euhedral or rounded
(within the range of 0.7 to 0.9 in DR#) in dunite.
The morphology of spinel is correlated with chemistry: the DR# is positively correlated with Ti content and Fe3+#(=Fe3+/(Cr + Al + Fe3+)), but is not related to Cr#. When chromitite is present in dunite, the spinel is relatively anhedral (vermicular) and low
in Ti and Fe3+# in the dunite whereas it is relatively euhedral and high in Ti and Fe3+# in surrounding harzburgite. We define these spinels as “extraordinary” spinels, which are commonly found in Wakamatsu mine
area in the Tari-Misaka complex, which exploits the largest chromite body in Japan. The rocks with the “extraordinary” spinels
show transitional lithologies (a gradual boundary, one meter to several tens of meters in width) between dunite and harzburgite
with “ordinary” spinels.
The formation of dunite and chromitite is interpreted as a result of the reaction of harzburgite with a relatively Ti-rich
magma (back-arc basin or MORB-like magma) and related magma mixing, as discussed by Arai and Yurimoto (1994). The dike-like occurrence of the dunite and chromitite indicates that the reaction took place along melt conduits
(=fractures) less than 200 m in width. Podiform chromitites were formed only when the reaction zone was relatively wide (several
tens of meters in width), that is, only when the degree of interaction was relatively high. The magma modified by the reaction
percolated, possibly by porous flow from the reaction zone outward, and changed the texture and chemistry of chromian spinel,
on the scale of several tens of meters. This type of melt transport, or melt flow through fractures with a melt percolation
aureole, may be prevalent in the uppermost mantle.
Received February 8, 2000;/revised version accepted December 22, 2000 |
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