Geochemistry of Cenozoic igneous rocks of Borneo

Kai Kim Chiang, completed PhD

SRTM90 DEM of Sabah

SRTM90 DEM of Sabah

Igneous activity is a direct response to chemical and tectonic processes, different tectonic settings being characterised by distinctive igneous rocks and frequently possess diagnostic geochemical signatures. Arc-like magmatism in Borneo is thought to be due to the subduction of the proto-South China Sea beneath NW Borneo. In SE Sabah, the igneous rocks are thought to be an extension of the subduction of the Sulu Sea in a SE direction along the Sulu trench. In the Plio-Pleistocene, there is an abrupt change in the geochemistry of the igneous rocks that suggests a continental rift setting.

 

There is a wide range of basaltic to rhyolitic igneous rocks in Kalimantan with ages from the earliest Cenozoic through to the Quaternary. From the temporal and spatial distribution of these rocks described by previous authors, at least five phases of igneous activity can be identified which may be linked to major regional tectonic events. These occur in the Palaeocene, the Eocene, the late Oligocene to early Miocene, the late Miocene to Pliocene and the Plio-Pleistocene. These can be correlated with similar phases of igneous activity in Sabah and Sarawak. In Sabah Cenozoic igneous rocks are found in the Dent and Semporna peninsulas in SE Sabah, near Sandakan, and the Kinabalu region. Intermediate intrusive rocks are known from Central Sabah but no geochemical data is available for these rocks and they are undated. The Borneo Geological Survey divided Cenozoic igneous rocks in SE Sabah into two groups according to their stratigraphic positions: Oligo-Miocene andesites and dacites and Pliocene-Quaternary series of basalts, andesites and dacites. Here, the Cenozoic igneous rocks are divided into three groups. Oligo-Miocene andesites and dacites, Late Miocene-Pliocene andesites and dacites, and Plio-Pleistocene basaltic lavas. The Cenozoic igneous rocks of Sarawak are found in three separate regions: West Sarawak, the Balingian valley and the Upper Ranjang valley of central Sarawak. Four phases of igneous activity can be identified which appear to coincide with Phases II – V of the igneous activity in Kalimantan and Sabah.

The objectives of this work were to characterise the geochemistry of Tertiary igneous rocks in Borneo. The data will be used to provide a geochronological framework with which to constrain the tectonic processes operating in the region during the tectonic evolution of the island during the Cenozoic. The character and timing of magmatic activity is also critical in establishing the accretionary history of Borneo and rates of crustal growth. Some particular objectives of this project were:

  • To determine the differentiation history of the rocks. Geochemical data can provide evidence for transport and crustal level processes that affect magma during emplacement. This includes fractional crystallisation and contamination of the magma with overlying by basement rocks. This is important in constraining the nature of the basement and the dynamics of the magma plumbing system, and for quantifying any modification of intrinsic magmatic characteristics derived from a melt’s source.
  • To determine the source and nature of the cntributing melts. Detailed trace element interpretation of the samples will allow the characterisation of the underlying mantle and its modification by subduction components, such as fluids or melts from the sediment cover and fluids from the subducting slab.
  • To study the temporal and spatial changes in volcanism in Borneo. This includes geochemical changes to the sub-arc mantle, and the composition of the overlying crust.
  • To compare the geochemical features of the Borneo rocks with igneous activity in the surrounding regions and link the type of magmatism to the Cenozoic regional tectonics. This includes the possible identification of the oceanic crust and its affinities, the presence of any continental fragments and their provenance and the timing and mechanism is of plate tectonics in the region and their consistency with current tectonic models for the region.