Magma mingling/mixing vs. magmatic fractionation: Geneses of the Shirakawa Mo-mineralized granitoids, Central Japan

Abstract

Leucocratic biotite granites are main components in the Hatogaya pluton and the Hirase stock in the Shirakawa region of central Japan. Molybdenite-quartz vein mineralizations are widespread in and around the Hatogaya pluton and the Hirase stock, in which the largest is vein swarm of the Hirase mine. Mafic enclaves occur abundantly with graniric to granodioritic matrix in the northern part of the Hatogaya pluton, while they are rare in the Hirase granitic stock. The enclaves with generally round shape have mostly diabasic to fine plutonic textures under the microscope, and show interfingering and lobate contacts with the felsic matrix. The enclaves are quartz monzodiorite in composition containing SiO2 mostly around 60 %. They have felsic blebs, thus are considered a mingled magma of basaltic compositions originated in depth and a felsic magma generated from the Hida metamorphic-plutonic complexes or their basement. The mingled magma further mixed with and reacted with the felsic magma with SiO2 70 %, and then formed granodioritegranite of the high Na group (Na2O higher than 4.25 %). Thus, compositional variation of the northern part of the Hatogaya pluton was caused by the magma mingling. The mingling happened to be deeper level produced homogeneous granodiorite of the Mihoro pluton. Biotite granite of the low Na group (less than 4.25 %) could have originated in a granitic magma generated also from the Hida metamorphic-plutonic complexes or their basement. Most of the granites, occurring in the southern part of the Hatogaya pluton and Hirase stock, show high Rb/Sr ratio, strong Eu negative anomalies and flat REE patterns, and are thus considered as fractionated products of the SiO2 70 % original magma. The strong concentration of molybdenum in the Hirase stock can be explained by high degree of magmatic fractionation which produced MoS 2-rich residual melts, suitable fractures developed at the latest Cretaceous time, and preservation of the mineralized fractures at the present level of erosion.