Petrochemistry of Granitoids in Sibolga and its Surrounding Areas, North Sumatra, Indonesia

Petrochemical studies were conducted on granitoids in Sibolga and surrounding areas in North Sumatra, Indonesia. The granitoids presented the characteristics of A- and I-type ilmenite series. The syenogranites from Sarudik, the syenogranite, quartz syenite, and quartz alkali feldspar syenite from Sibuluhan Sihaporas, and the monzogranites from Sibolga Julu are A-type granitoids that have high SiO2, CaO, Al2O3, Na2O+K2O, and Rb content, a high FeO/MgO ratio, and low Ba, Sr, and Zr content. The alkali feldspar syenite from Sarudik, the quartz alkali feldspar syenite from Tukka, and the quartz syenite from Adian Koting show I-type characteristics. I-type granitoids have lower SiO2, Na2O+K2O, Rb, Yb, and total REE (sigma REE) content and low FeO/MgO ratio. However, quartz syenite from the Sibolga Julu, which exhibits I-type characteristics, has high sigma REE content but a low FeO/MgO ratio. The granitoids consisted mainly of K-feldspar megacrysts, quartz, plagioclase, and biotite with hornblende in some cases. Accessory minerals were zircon, apatite, allanite, and titanite. Cordierite and corundum occurred as xenocrysts and inclusions in K-feldspar in syenogranite (Sarudik) and quartz syenite (Adian Koting). SiO2 and sigma REE content and the Rb/Sr ratio are positively correlated to each other. The REE are enriched in quartz alkali feldspar syenites from Sibolga Julu and Sarudik, quartz syenite from Tarutung, and quartz alkali feldspar syenite, syenogranite, and quartz syenite from Sibuluhan Sihaporas. These highly-differentiated granitoids were formed within the plate settings. In contrast, the sigma REE content of hornblende-bearing granitoids that were formed in volcanic arc settings is low. The coexistence of cordierite and corundum with biotite and hornblende indicates that the metaluminous magma was contaminated by metasedimentary basement rocks.