Abstract
Late Cretaceous crystal-rich rhyolites and related granites are widely distributed in the Inner Zone of southwest Japan. A great deal has been written about these crystal-rich rhyolites; however, little attention has been paid to their crystal-rich character.
We focus on the remarkable crystal-rich character of these rocks with the aim of determining the magmatic conditions (i.e., temperature and initial water content) under which the Late Cretaceous Kizaki Rhyolite, and the associated Aoki Granite, formed within the Nishina Mountains, in the northeast of the Northern Japan Alps. The Aoki Granite contains unaltered fayalite and pyroxenes that retain information regarding the nature of the magma from which they formed. The Kizaki Rhyolite consists of a rhyolitic welded tuff, and its essential constituent (fiamme) is characterized by a crystal-rich nature (50%—60% by vol.). The Aoki Granite is remarkable for its lithological variability, but may be classified into three units: (1) fayalite- and pyroxene-bearing porphyritic granite, (2) chilled margin granite porphyry, and (3) mafic magmatic enclave-bearing finegrained biotite granite. These units have similar chemical compositions, characterized by high FeO*/MgO and high contents of CaO, Na2O + K2O, Ba, Zr, La, Ce, and Nd.
We analyzed the granite porphyry of the Aoki Granite using a geothermometer and the MELTS program, and results indicate that it was generated from hot and dry magma (850—900 °C, initial water content = 0.4%—1.0% by wt.). This also suggests that similar crystalrich rhyolites, widely distributed through the Inner Zone of Southwest Japan, were generated from similarly hot and dry magmas, and that such magmatism was vigorous during the Late Cretaceous at the eastern margin of the Eurasian continent.
