Abstract
The Schwaner Mountains in southwestern Borneo form a large igneous province with a complex magmatic history and poorly known tectonic history. Previously it was known that Cretaceous granitoids intruded metamorphic rocks of the Pinoh Metamorphic Group assumed to be of Paleozoic age. Jurassic granitoids had been reported from the southern Schwaner Mountains. Most ages were based on K-Ar dating. We present new geochemistry, zircon U-Pb and40Ar/39Ar age data from igneous and metamorphic rocks from the Schwaner Mountains to investigate their tectono-magmatic histories. We subdivide the Schwaner Mountains into three different zones which record rifting, subduction-related and post-collisional magmatism. The Northwest Schwaner Zone (NWSZ) is part of the West Borneo Block which in the Triassic was within the Sundaland margin. It records Triassic to Jurassic magmatism during early Paleo-Pacific subduction. In contrast, the North Schwaner Zone (NSZ) and South Schwaner Zone (SSZ) are part of the SW Borneo (Banda) Block that separated from NW Australia in the Jurassic. Jurassic granitoids in the SSZ are within-plate (A-type) granites interpreted to have formed during rifting. The SW Borneo (Banda) Block collided with eastern Sundaland at c. 135 Ma. Following this, large I-type granitoid plutons and arc volcanics formed in the NWSZ and NSZ between c. 90 and 132 Ma, associated with Cretaceous Paleo-Pacific subduction. The largest intrusion is the c. 110 to 120 Ma Sepauk Tonalite. After collision of the East Java-West Sulawesi (Argo) Block, subduction ceased and post-collisional magmatism produced the c. 78 to 85 Ma Sukadana Granite and the A-type 72 Ma Sangiyang Granite in the SSZ. Rocks of the Pinoh Metamorphic Group mainly exposed in the NSZ, previously assumed to represent Paleozoic basement, contain abundant Early Cretaceous (110 to 135 Ma) zircons. They are interpreted as volcaniclastic sediments that formed contemporaneously with subduction-related volcanic rocks of the NSZ subsequently metamorphosed during intrusion of Cretaceous granitoids. There are no igneous rocks older than Cretaceous in the NSZ and older than Jurassic in the SSZ and there is no evidence for a continuation of a Triassic volcanic arc crossing Borneo from Sundaland to the east.
Highlights
SE Asia is known to have been formed from continental fragments of Australian Gondwana origin from the Late Palaeozoic onwards to form the Sundaland continent
For tonalite sample LD10-100 (Figure 5B) 25 ages by SHRIMP gave a weighted mean age of 114.2 ± 0.7 Mineral Age (Ma) (MSWD 1.1). 15 concordant U-Pb zircon ages were obtained by LA-ICP-MS for tonalite LD10-038 (Figure 5C) and a coherent cluster of 12 were used to calculate a weighted mean age of 114.4 ± 1.1 Ma (MSWD 1.3)
Tonalite LD10-103 (Figure 5E) from the easternmost NSZSSZ boundary yielded 50 concordant U-Pb zircon ages determined by LA-ICP-MS and 45 were used to calculate a weighted mean age of 88.1 ± 0.6 Ma (MSWD 2.8)
Summary
SE Asia is known to have been formed from continental fragments of Australian Gondwana origin from the Late Palaeozoic onwards to form the Sundaland continent. New mapping, sampling and dating (e.g., Setiawan et al, 2013; Davies et al, 2014; Breitfeld et al, 2017; Hennig et al, 2017) and new reconstructions (Hall et al, 2009; Metcalfe, 2009) have cast doubt on previously accepted ideas. Despite their name, the Schwaner Mountains form a relatively low area (Figure 2A), with elevations mainly between 100 and 500 m, there are a few high peaks of 1000 to 2000 m. The evidence shows there was igneous activity within the Sundaland and Australian margins before the collision of SW Borneo with Sundaland, followed by subduction-related magmatism in the Schwaner Mountains, and further magmatism that occurred after subduction ceased
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