THE STUDY OF GABBROS ASSOCIATED WITH THE CRETACEOUS GRANITIC ROCKS, KITAKAMI MOUNTAINS

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The Early Cretaceous granites in the Kitakami Mountains are divided into six zones VIa, namely I to VI, and the last of which is divided further into subzones VIa and VIb(Katada 1974, Kanisawa and Katada,1988).1) Two kinds of gabbroic rocks associated with granitic plutons are recognized in the district,i.e., (1) olivine-pyroxene-hornblende gabbro series characterized by the association of An-rich plagioclase, olivine and pargasitic hornblende, and (2)biotite-olivine-pyroxene gabbro series characterized by the assemblage of relatively An-poor plagioclase, biotite and olivine with or without K-feldspar.2) Rocks of the olivine-pyroxene-hornblende gabbro series are associated with the plutons of Zones 1, 11 and V, and usually composed of hornblende gabbro, pyroxene-hornblende gabbro, and small amount of olivine-pyroxene-hornblende gabbro. Whereas, rocks of the biotite -pyroxene-hornblende gabbro series are composed of biotite-olivine-pyroxene gabbro/monzo- gabbro, biotite-pyroxene-hornblende gabbro/monzogabbro and associated with the plutons of Zones Ill, IV and VIb.3) Trace element patterns for granitic rocks indicate that they are rich in LILE and depleted in HFSE and show negative Nb anomaly similar to those of rocks of the island arc and active continental margins, but those of gabbroic rocks are irregular due to differences in modal proportions of minerals(Kanisawa and Katada,1988; Kanisawa and Yoshida,1989).4) Tectonic discrimination : Major element chemistry of the Cretaceous plutonic rocks of the district show continental are granites(CAG). Trace element characteristics, such as Y-, Rb- and Nb-SiO2, and Nb-Y, Rb-(Y+Nb) relations indicate those of the volcanic arc granites(VAG).5) The isotopic data are necessary to clarify the petrogenesis of granitic and gabbroic rocks. Further, it is considered that some of gabbro bodies indicate the sub-volcanic structure, so the detailed study on suitable body is necessary at this point.