The geochemical studies on the plutonic rocks of the Geochang, the central part of the Yeongnam Massif, were carried out in order to constrain the petrogenesis and the paleotectonic environment. The objects of this study are Biotite granite, Hornblende granite, Haeinsa granite near Gajo-myeon and Gaya-myeon.
Based on the detailed petrographical studies, the Biotite granites are plotted in granite and granodiorite. Heainsa granite and hornblende granite are plotted in granite, quartz syenite, and monzonite after Streckeisen(1976).
The granitic rocks in the studied area belong to the calc-alkaline magma series. Especially, characteristics of trace elements and REE pattern suggest that these rocks are produced in the subduction zone related to calc-alkaline series. Also, the studied granitic rocks correspond to peraluminous and I-type. Chondrite-normalized REE patterns show that LREE are enriched much more than HREE, and have weak Eu(-) anomaly. It is similar to pattern of Jurassic granitoids in the South Korea.
High Resolution Secondary Ion Mass Spectrometer (SHRIMP-lle) zircon U-Pb age determinations show the ages of 89.61±0.89 Ma, 89.77 ± 0.42 Ma and 87.2 ± 1.3 Ma for the Biotite granite, and ages of about 188 Ma for Biotite granite near unyang-myeon. And ages of 89.61 Ma for Hornblende granite, near Gajo-myeon, which are very different from those near Namha-myeon. Lastly, the ages of 193.1 ± 1.3 Ma, 194.3 ± 4.0 Ma, 195.8 ± 3.1 Ma, 195.5 ± 1.9 Ma, 193.8 ± 1.3 Ma, 192.4 ± 1.4 Ma, 193.8 ± 2.7 Ma, 187.7 ± 3.3 Ma, and 188.2 ± 3.6 Ma for the Haeinsa granite.
Therefore, Biotite granite in this study turned out to be Late Cretaceous yet these results weren''t the same as those by Park et al.(2006). In that study, the ages of Biotite granite turned out to be Jurassic. Haeinsa granite can be important clues to understand the whole rocks during the Mesozoic igneous activities occurred on the Yeongnam massif. During the Mesozoic, there are two magmatic activities in this area. One is the Jurassic and the other is the Cretaceous.
Besides, Some samples of Haeinsa granite belonging to alkaline should be studied further.