Original paper

Pyrophanite and high Mn ilmenite discovered in the Cretaceous Tono pluton, NE Japan

Sasaki, Kazuhiro; Nakashima, Kazuo; Kanisawa, Satoshi

Abstract

Pyrophanite and high Mn ilmenite have been discovered in the Tono plutonic rocks of Kitakami Mountains, NE Japan. The rocks in the pluton are classified into four rock facies; Marginal, Main, Arakawa-type and Central facies. The former three facies which are composed of quartzdiorite and granodiorite are believed to represent a series of continuous differentiation and the last facies, composed of tonalite and granodiorite, is similar but slightly different from the above differentiation. Magnetite, ilmenite and hematite are present in these rock facies, although ilmenite is absent in the Arakawa-type facies. Manganese content of ilmenite increases from the Marginal facies to the Main facies as differentiation proceeds. Especially, in the Central facies, the highest MnO content of 35 wt.% (74 mol. % MnTiO3) was detected in a composite-type ilmenite which exsolved hematite. This is the maximum value that has ever been reported from calc–alkaline igneous rocks, except for pegmatite. The presence of euhedral discrete hematite, myrmekite, euhedral biotite and intergranular muscovite in the Central facies, and an equilibrium temperature obtained by a two-feldspar geothermometer, show that crystallization of the Tono granitic magma continued to a low-temperature subsolidus stage under volatile-rich highly oxidized conditions. The pyrophanite crystallized in the greatly differentiated facies which solidified from an intrinsically volatile-rich magma.

Keywords

tono plutonfe-ti oxidespyrophaniteilmenitemagnetitehematiteoxidationvolatileoxygen fugacity