Geochemistry

Much of the Indonesian and Philippine archipelagos is constructed from Cenozoic igneous rocks, including volcanic arcs, plutonic complexes, and widespread basaltic volcanism. Understanding the origin, composition, and evolution of these rocks is essential for reconstructing the tectonic development of Southeast Asia.

Magmatic rocks record information about the mantle sources from which they were derived, the melting conditions that generated them, and the crustal rocks through which the magmas passed during ascent and storage. As a result, geochemical analysis provides a powerful tool for investigating tectonic processes such as subduction, arc magmatism, crustal growth, rifting, and continental collision.

The SE Asia Research Group conducts geochemical studies across the region with the aim of understanding the origin of magmatism, the nature of the underlying crust and mantle, and the relationship between volcanic activity and regional tectonic evolution. With the support of consortium members, this programme includes the collection and analysis of igneous rocks from key locations across Southeast Asia and the integration of geochemical datasets with tectonic reconstructions, geochronology, and regional geology.

Analytical approaches commonly include major and trace element geochemistry, radiogenic isotope systems (such as Sr, Nd, Pb and Hf), and stable isotope analyses, which together allow detailed reconstruction of magma genesis and evolution.

Magmatism as a Record of Tectonic Processes

Igneous rocks provide a direct record of tectonic processes occurring within the Earth. Different tectonic environments—such as subduction zones, continental rifts, or post-collisional settings—produce magmas with distinctive geochemical signatures.

By analysing the chemistry of volcanic and plutonic rocks it is possible to determine:

the nature of the mantle source region
the role of fluids and melts released from subducting slabs
the contribution of continental crust during magma evolution
the processes of fractional crystallisation and magma differentiation
the tectonic setting in which the magmas formed

This information is particularly valuable in Southeast Asia, where many crustal blocks are poorly exposed or buried beneath younger sediments or volcanic rocks.

Crustal Structure and Hidden Basement

In many parts of Southeast Asia the nature of the underlying basement is uncertain or completely concealed beneath younger volcanic and sedimentary sequences. Geochemistry offers an indirect method for investigating these hidden crustal components.

Acidic volcanic rocks are especially useful in this respect because their magmas often spend long periods stored within the crust before eruption. During this time they may assimilate surrounding crustal rocks, incorporating geochemical signatures that reveal the composition of the basement through which they passed.

By combining geochemical and isotopic analyses, it is therefore possible to identify ancient continental fragments, arc crust, or other crustal blocks that may not be exposed at the surface.

Selected Regional Advances:

Geochemistry of Acid Volcanoes: Sunda Arc and Sulawesi

This project examined the geochemistry of volcanic rocks from the Sunda Arc and Sulawesi in order to investigate the nature and origin of basement rocks beneath these regions.

Samples were collected from volcanic centres across Sumatra, Java, Bali, Lombok, and Sulawesi, and analysed using trace element geochemistry together with radiogenic isotopes including Sr, Nd, Pb and Hf, as well as stable oxygen isotopes.

Magmatic rocks act as probes of the geological history of the regions through which they pass. The chemistry of these magmas reflects the integrated effects of mantle melting, interaction with subducted sediments and fluids, and contamination from crustal rocks during magma storage and ascent.

Acidic volcanic rocks are particularly valuable because their magmas typically have long residence times within the crust. This increases the likelihood that they will interact with crustal material and therefore acquire geochemical signatures that reveal the nature of the underlying basement.

The project built on earlier work that identified previously unrecognised crustal components beneath Java. By extending this approach across the Sunda Arc and Sulawesi, the research aimed to determine whether ancient continental fragments occur more widely within the arc basement, and to understand the relationships between these crustal blocks and regional tectonic evolution.

Geochemical and Sr–Nd–Hf–O Isotopic Constraints on Sunda Arc Volcanism

This study focused on volcanic centres within the Sunda island arc, which formed as a result of northward subduction of the Indo-Australian Plate beneath the Eurasian Plate.

Detailed geochemical and isotopic analyses were carried out on volcanic rocks from several centres in Java, including the Salak and Gede volcanic complexes in West Java and the Ijen volcanic complex in East Java. These analyses aimed to determine the relative contributions of different source components involved in magma generation.

Variation in the composition of Sunda Arc lavas reflects several interacting factors, including differences in the thickness and composition of the overriding plate, the amount and type of sediment on the subducting plate, and the processes operating within the crust beneath individual volcanic centres.

The research demonstrated that fractional crystallisation plays a major role in controlling the major and trace element compositions of Sunda Arc magmas, although the importance of individual differentiation processes varies between volcanic centres. By understanding these processes at specific volcanoes it becomes possible to investigate broader geochemical variations along the arc and their tectonic significance.

Cenozoic Magmatism of Borneo

This project investigated the geochemistry and timing of Cenozoic igneous activity across Borneo, with the aim of understanding the tectonic processes that shaped the island during this period.

Igneous activity in Borneo ranges from basaltic to rhyolitic compositions and spans a long interval from the earliest Cenozoic to the Quaternary. Previous studies suggested that several phases of magmatism occurred, but their tectonic significance remained uncertain.

The research identified at least five major phases of igneous activity in Kalimantan and surrounding regions:

Paleocene
Eocene
late Oligocene to early Miocene
late Miocene to Pliocene
Plio-Pleistocene

These phases correlate broadly with igneous activity recognised in Sabah and Sarawak. In Sabah, volcanic rocks occur in several regions including the Dent and Semporna peninsulas, the Sandakan area, and the Kinabalu region. Earlier geological surveys divided these rocks into Oligo-Miocene andesites and dacites and younger Pliocene–Quaternary volcanic series, but geochemical data for many units remained limited.

This project sought to characterise the geochemistry of these rocks and provide a geochronological framework to better understand the tectonic evolution of Borneo. The work addressed several key objectives:

determining magma differentiation histories and crustal contamination processes
identifying the nature of mantle sources and the influence of subduction-related components
investigating spatial and temporal changes in volcanism across the island
linking magmatic activity to regional tectonic events, including subduction of the proto–South China Sea and later tectonic reorganisation

The results contribute to understanding the accretionary history of Borneo, the evolution of its volcanic arcs, and the development of the surrounding sedimentary basins. They also provide insight into the composition of crustal blocks within the region and the processes responsible for crustal growth in Southeast Asia.

Regional Significance

Geochemical studies provide a crucial perspective on the geological evolution of Southeast Asia. By analysing the chemistry and isotopic composition of igneous rocks, SEARG research helps to reconstruct the tectonic processes responsible for arc formation, crustal growth, and regional magmatism.

When integrated with plate tectonic reconstructions, geochronology, and field geology, geochemical data allow the magmatic history of the region to be linked directly to the broader tectonic evolution of Southeast Asia and the southwest Pacific.

These studies therefore play a central role in understanding how volcanic arcs formed, how crustal blocks interacted and collided, and how magmatism reflects the dynamic plate tectonic environment that characterises this part of the world.