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

Abstract

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 (ΣREE) content and low FeO/MgO ratio. However, quartz syenite from the Sibolga Julu, which exhibits I‐type characteristics, has high ΣREE 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 ΣREE 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 ΣREE 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.

Geochemical characteristics and Sr‐Nd‐Hf isotope compositions of Late Triassic post‐collisional A‐type granites in Sarudik, SW Sumatra, Indonesia

“…The occurrence of the Sarudik A2‐type granites, together with the graben‐horst structural style, implies a magmatic event triggered by post‐collisional extension during the late Triassic, which may explain the existence of closely coeval granitoids in Sijunjung (206 ±3 Ma, biotite K‐Ar) (Silitonga & Kastowo, 1975), Sulit Air (203 ±6 Ma, biotite K‐Ar) (McCourt & Cobbing, 1993), Sungei Isahan (197 ±2 Ma, muscovite K‐Ar) (Schwartz, 1987) and Sibolga (206–219 Ma, hornblende and biotite K‐Ar) (Aspden et al, 1982; Fontaine & Gafoer, 1989). The deduction also shows the flaw in the model proposed by Setiawan et al (2017), who suggested that the A‐type granitoids in Sibolga were formed in continental rift setting during the Triassic.…”

“…3 As suggested by Barber et al (2005) Cobbing, 1993), Sungei Isahan (197 ±2 Ma, muscovite K-Ar) (Schwartz, 1987) and Sibolga (206-219 Ma, hornblende and biotite K-Ar) (Aspden et al, 1982;Fontaine & Gafoer, 1989). The deduction also shows the flaw in the model proposed by Setiawan et al (2017), who suggested that the A-type granitoids in Sibolga were formed in continental rift setting during the Triassic.…”

“…The oldest rock is the Kuantan Formation of Permian, mainly composed of quartzites, quartz sandstones, phyllites and shales, which is intruded by the Sibolga Granitoid Complex and overlain by Cenozoic volcanics and sediments. More recently, Setiawan, Takahashi, and Imai (2017) identified I-and A-type granitoids in the Sibolga Granitoid Complex based on geochemical data and proposed a shift of tectonic settings in Sibolga and surroundings by linking I-type to early Permian subduction of Paleo-Tethys beneath the West Sumatra Block and A-type to extension induced by strike slip movement of the West Sumatra Block along the MSTZ relative to the East Sumatra Block during the early Permian to the early Triassic, respectively. However, this model is rather speculative for the lack of constraint from high-precision dating in his study.…”

Geochemistry of Granitoids in Central Sumatra: An Identification of Plate Extension during Triassic

“…Three Tethyan oceans were involved in the development of the Southeast Asia terranes, namely, the Devonian-Triassic Palaeo-Tethys, the Early Permian-Late Cretaceous Meso-Tethys, and the Late Triassic-Late Cretaceous Ceno-Tethys (Jiang et al, 2017;Hutchinson, 2014;Metcalfe, 2013). The subduction of the Indian Plate beneath Asia was considered to have initiated in the Eocene (Ali and Aitchison, 2005;Irzon et al, 2018;Setiawan et al, 2017).…”

“…Most of the Mesozoic intrusions in Sumatra are I-type, subduction-associated, and situated in the Barisan Mountains (Barber, 2000;McCourt et al, 1996;Zhang et al, 2018). However, several studies notified the presence of anorogenic A-type intrusions, namely Sibolga Granite (Setiawan et al, 2017;Zhang et al, 2020) and Sijunjung Granite (Irzon et al, 2018) in the central Sumatra region. Therefore, more complete petrography and geochemistry data on these plutons are required to explain the tectonic history of Sumatra.…”