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

北上山早白垩世花岗岩划分为6个VIa带，即I ~ VI带，其中VIa带又进一步划分为VIa和VIb亚带（Katada 1974, Kanisawa and Katada,1988）。(1)该区识别出两种与花岗质岩体伴生的辉长岩：(1)以富安斜长石、橄榄石和寄生角闪石组合为特征的橄榄石-辉石-角闪石系列；(2)以相对贫安斜长石、黑云母和橄榄石组合为特征的黑云母-橄榄石-辉长岩系列，其中钾长石或不含钾长石。2)橄榄石-辉石-角闪石辉长岩系列岩石与1、11、V带的岩体伴生，通常由角闪石辉长岩、辉石-角闪石辉长岩和少量橄榄石-辉石-角闪石辉长岩组成。黑云母-辉石-角闪石辉长岩系列岩石由黑云母-橄榄石-辉长岩/二长辉长岩、黑云母-辉石-角闪石辉长岩/二长辉长岩组成，并与ii、IV、VIb带的岩体伴生。3)花岗质岩石微量元素模式表明其富LILE，贫HFSE，表现出与岛弧和活动大陆边缘岩石相似的负Nb异常，而辉长岩由于矿物模态比例的差异而表现出不规则（Kanisawa and Katada,1988; Kanisawa and Yoshida,1989）。(4)构造判别：本区白垩系深成岩主要元素化学特征为花岗岩（CAG）。微量元素特征（Y -、Rb-、Nb- sio2）及Nb-Y、Rb-(Y+Nb)关系为火山弧花岗岩（VAG）特征。5)同位素资料对厘清花岗质和辉长岩的成因具有重要意义。进一步认为，部分辉长岩体为次火山构造，有必要对合适的辉长岩体进行详细的研究。

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.