Geological Setting Of Study Area
The Indonesia region is a part of crustal plate evolution produced by the subduction of the Indo-Australian oceanic plate along the southern margin of the Eurasian plate (also known as Sundaland). According to the global tectonic pattern, it was located in Island Arc and formed during the Mesozoic period (Batara and Xu, 2022), which was divided into three segments such as the Sumatra Arc comprising the Sumatra Island, the Sunda Arc from Java to Flores Island, and the Banda Arc for the island east of Flores (Batara and Xu, 2022; Soesilo et al. 2015). In addition, Java has an area of 127,000 km2 and is about 1000 km long, as well as central Java is transitioning between a continental basement in West Java and a predominantly intermediate basement in East Java (Isyqi and Ansori, 2021). On the other hand, several researchers conducted their study on Java Island (Fig. 1a) and divided it into three groups such as the Meratus trend (southwest-northeast), the Sunda trend (north-south), and the Java trend (west-east) based on gravity and seismic data (Satyana et al. 2021).
The structures on Java Island can be related to north-south compression due to the Indian Oceanic Crust Subduction (Batara and Xu, 2022). The structure formation mechanism is through wrench deformation; the four structural trends of Java Island are conformable to wrench tectonism (Satyana, 2014). A review of the volcanic arc history shows that Karangsambung would have been in a melange setting in Cretaceous times. Pre-tertiary and initial Tertiary rocks tectonically mixed up in the essential mass of claystone are interpreted as "melangue" (Ansori et al. 2019). These rocks are the oldest rock units in this area, recognized as the Luk Ulo Complex (Karangsambung area). As part of Luk Ulo Melange Complex, which also includes the collection of rock samples (Fig. 1b) associated with the stratigraphy of the study area (Fig. 1c), the Karangsambung area has both pre-Tertiary and post-Tertiary rocks, as shown in Fig. 1d.
The study area around Luk Ulo comprises many different types of rocks with complicated geological structures. The mélange complex of Luk Ulo is the oldest rock unit. The tectonic and stratigraphic unit consists of variable rock fragments enclosed in a groundmass of scaly clay and sheared black shale (Isyqi et al. 2019). The rock complex ranges from late Cretaceous to Paleocene, and the rock fragments are grouped into allochthonous and autochthonous. The allochthonous fragments consist of blue-green schist, cherts, red limestones, serpentinite, amphibolite, gabbro, peridotite, dacite, basalt, and pillow lavas, whereas the autochthonous fragments consist primarily of turbidite greywacke formations. These allochthonous and autochthonous rock fragments are considered a mélange complex comprised of a tectonic mixture of oceanic rocks (Jaya et al. 2017).
The Luk Ulo Melange Complex Formation is overlain by the Karangsambung Formation, which consists of polymict conglomerates and scaly clays that show grey to blackish colors. Based on the structure of the groundmass, like a scaly appearance on the clay matrix and formed fragments, the formation is interpreted as olisthostrome sediment deposits (Isyqi et al. 2019; Range, 2022; Suhendra et al. 2017). Further, Totogan formation has similar characteristics to the Karangsambung formation. The Waturanda formation, which is harmonic with the Totogan formation, consists of repetition bedding breccia and sandstone graywacke. Breccias generally consist of andesitic volcanic rocks, with the nature and size varying from gravel to boulders with a mass of rough sandstone (Isyqi et al. 2016; Miyazaki et al. 1998). The Penosongan formation is aligned with the Waturanda formation, consisting of intercalation sandstone–claystone, calcarenite, and tuff. It is dominated by white claystone tuff and marl coexisting with bedding sandstone, indicating the turbidite process (Batara and Xu 2022; Şahin et al. 2014). The bottom of this formation is characterized by bedding claystone and sandstone with succession towards carbonates which show a high constituent of carbonate mineral.
Field Characteristic And Petrography Investigation
The granite, metapsammites, and metagranite rocks were found as boulders along the Luk Ulo River. The sampling points started from the junction between Long River and Luk Ulo River to the upstream Luk Ulo River heading northeast (Fig. 1d). A total of 59 samples of granite, metapsammite, and metagranite rocks were taken from 8 stations (Fig. 2). Out of these, 22 samples were chosen for geochemical analysis.
These rocks were shown to be about 1 m in diameter, rounded in shape, and transported away from their source. No outcrop was found after tracking along the Luk Ulo River as far as 10 km from the junction between those two rivers, and all rock samples are classified as floating samples. They are easily distinguished from other rocks due to their coarse texture and bright colour. Other rocks like volcanic (basalt), gabbro, quartzite, and serpentinite are also found. It is quite challenging to find high-pressure metamorphic rocks along this river, as they mainly crop out in the Muncar River. Most of the samples are metamorphic with granitic composition (metagranitoids). Several rock samples were considered fresh granitoids as they were not metamorphosed when observed as hand specimens (no foliation) and in their thin sections (having a graphic texture). Few samples showed good schistosity, and directly classified them as schists, while one sample was classified as gneiss when it showed excellent gneissic foliation. The mineral chemistry of garnet from granitoids can be considered metamorphic garnet, as it is characterized by zoning concerning Fe-Mg-Mn-Ca elements. These rock samples can be classified as group 2 based on texture and mineral compositions. The first group is hornblende-bearing granite, metapsammites, and metagranite rocks, which indicate magmatic activity. The second group is garnet-bearing granite, metapsammites, and metagranite, which commonly have foliation that indicates metamorphic origin.
Samples Collection And Analytical Technique
The rock samples have been collected to determine the major, trace, and rare earth elements in the study area and analyzed at the departments of Kyushu University, Japan, and Balai Penyelidikan and Pengembangan Teknologi Kebencanaan Geologi, Yogyakarta, Indonesia. In total, 22 samples of metapsammite and metagranite rock were analyzed at the laboratory of Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG) for X-Ray Fluorescence (XRF) analysis with the model (Rigaku ZSX Primus II, Japan) for evaluating entire rock chemistry data. Further, the same samples were analyzed at Kyushu University, Japan, using inductively coupled plasma mass spectrometry (ICP-MS) with an Agilent 7500cx quadrupole model for trace and rare-earth element analysis. Six samples were analyzed at earth and environmental division laboratories of Kyushu University for Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) analysis to obtain Uranium-Lead (U-Pb) Zircon dating and a summary of geochronology analysis. La-ICP-MS is a technique that often identifies rare earth elements (REEs) and other trace elements. The results from all studies were then interpreted to understand tectonic evolution and magmatism activity related to metapsammite and metagranite rocks from Luk Ulo Complex, Karangsambung, Kebumen, Central Javas Island, Indonesia.
Accurate identification is achieved using these approaches, with a margin of error that is greater than 5% for REEs and 10% for the remaining trace elements (Hosseini et al. 2014). The zircon grains of each sample were separated from 80 mesh and whole rock powder by panning, magnetic separations, and hand-picking techniques. The zircon was mounted in epoxy resin and polished to an exposed mid-grain section. Internal structures of each analyzed zircon grain were observed using a scanning electron microscope equipped with electron dispersive spectrometer (SEM-EDS). Cathodoluminescence images are acquired for samples to be analyzed because they provide a powerful tool for placing laser pits in homogenous portions of crystals. They can also help determine the origin of zircon grains. Although U-Pb zircon dating was performed using an Agilent 7500cx quadrupole ICPMS with a New Wave Research UP-213 Nd-YAG UV (21 nm) laser ablation (LA) system installed at the Kyushu University with the analytical conditions are follow. The weighted mean U/Pb ages are used, and uncertainties are given for the 95% confidence level. The raw data acquisition and calibration of the isotopic ratios of zircon were processed offline using GLITTER software. Data reduction and processing were conducted using the computer program ISOPLOT version 3.75, provided by K.R. Ludwig at the Berkeley Geochemical Centre, University of California. Analytical errors shown in this study are 95% confidence levels (2σ error) and analyzed data for zircon grains.