Petrography and geochronology of the Kuki granite, Kitakami Mountains, northeastern Japan: Shallow crustal intrusion and emplacement processes of granitic magma

Abstract

Understanding the crustal evolutionary dynamics of island arc-trench systems requires a thorough evaluation of magma chamber processes, and especially of shallow crustal granitic magma intrusion and emplacement processes. To address this, we studied the petrography and geochronology of the Kuki granite, Kitakami Mountains, northeast Japan, as an example of a non-adakitic granite at the magmatic flare-up stage. Analysis of paired crystallization temperatures and pressures of hornblende grains reveals the temperature-pressure (i.e., depth) history of the granitic magma. The pressure and temperature history of the Kuki granite indicates no change in the magma depth with cooling from 800 to 730 °C, and geobarometric calculations indicate the emplacement of magma at a depth of approximately 9-10 km. Simultaneous determination of the zircon U-Pb age, Ti concentration, and Th/U yields 1) the time-temperature history of granitic magma before its solidification, and 2) the correlation between temperature and Th/U in the magma. The magma chamber cooled from ∼ 900 to 700 °C at approximately 125 Ma. The change in Th/U with temperature indicates the progression of fractional crystallization in the cooling magma chamber, and greater fractional crystallization in the magma chamber at temperatures above 800 °C than below 800 °C.