Geology of the Mutis Complex, Miomaffo, West Timor

Abstract

Mesozoic forearc assemblages are widespread across Indonesia, and these are also exposed in the overthrust terranes on Timor. The oldest rocks in the allochthonous Mutis Complex, West Timor are Mesozoic basaltic volcaniclastic rocks and melange containing blocks of normal mid-ocean ridge basalt (N-MORB) basalt (194 ± 5 Ma), amphibolite (metamorphism at 184 ± 6 Ma), garnet and actinolite-bearing schists, arkosic sandstone and volcanogenic sedimentary rocks. Based on the large component of Jurassic material, these rocks were probably derived from the Woyla terrane. The eastern part of the Miomaffo massif is complexly deformed with a dominant foliation overprinted by several fold and fault events including late normal cataclastic zones. This sequence is intruded by calc-alkaline andesitic dykes (possibly in the Eocene). The Central Sector of the massif is a block of high-strain greenschist facies rocks where the primary texture can no longer be recognised, but the bulk composition is the same as the volcaniclastic rocks in the east. Further west is an amphibolite facies metamorphic province. Amphibolite from the Western Sector have the same N-MORB composition as the basalt in the east, but the gneissic rocks have a higher proportion of arc-sourced protoliths. The peak metamorphic conditions were 650 °C and 0.9 GPa. This metamorphic event occurred at 37 Ma, reflecting subduction on the southeast margin of Sundaland, whereas the low-grade Mesozoic metamorphism of the Eastern Sector of the massif occurred in the accretionary prism along the Woyla Arc. The Mutis Complex at Miomaffo demonstrates the complex geological history of Mesozoic rocks in eastern Indonesia.

KEY POINTS

1. The Miomaffo massif demonstrates the strong oceanic character of the Banda terrane.

2. The anomalous Eocene metamorphic age of rocks ‘unconformably overlain’ by Cretaceous sediments is resolved.

Keywords:

• Timor
• Banda terrane
• metamorphic age
• MORB
• Cretaceous melange
• Mutis Complex

Acknowledgements

The authors gratefully acknowledge the logistic support provided for the fieldwork by the Indonesian geological survey, Bandung, and especially by Said Aziz. We would like to thank Alex Grady for suggesting the project. The paper was substantially improved by the constructive review of Myra Keep.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials.

Additional information

Funding

This project was partly funded by the ARC Research Hub for Transforming the Mining Value Chain [project number IH130200004].