Geochronology of subduction-related metamorphic rocks from the Central Indonesian Accretionary Collision Complex
Final Report Abstract
This study aimed at establishing a solid geochronologic foundation for in-depth understanding of subduction- and collision-related metamorphic processes in the Central Indonesian Accretionary Collision Complex. The results can be summarized as follows: a) Multipoint Rb–Sr mineral isochrons for samples from the Bantimala Complex display a narrow range of white mica ages (130–120 Ma), considerably smaller and more precise than the previous K–Ar age spectrum (137–113 Ma). Given that peak metamorphic temperatures >600 °C had been attained during eclogite-facies metamorphism, the new multipoint isochron ages are interpreted to postdate peak-HP conditions. b) Rb–Sr geochronology of HP/LT rocks and epidote amphibolites from the Luk Ulo Complex yielded a tight age cluster at ca. 119–117 Ma interpreted to reflect the timing of HP/LT metamorphism. Low peak metamorphic temperatures (ca. 400 °C) suggest that the new ages most likely constrain the time of mica growth or recrystallization during the HP stage. Neither petrological analysis nor geochronological dating has unambiguously confirmed different P–T–t paths for Luk Ulo eclogites with and without tourmaline that were postulated in an earlier study. Matching ages of high-pressure and lower pressure rocks are attributed to inheritance/incomplete resetting in the latter. c) The high precision of the Rb–Sr data of the Bantimala and Luk Ulo HP/LT rocks revealed previously unrecognized regional age differences that correspond to distinctive P–T conditions. These differences can be reconciled with a model suggesting formation in the same subduction complex. d) In the Meratus Complex only few HP/LT rocks escaped retrogression. A partially overprinted but still glaucophane-rich sample yielded a well-defined Rb–Sr isochron age (phengite, epidote) of 112.9 ± 0.7 Ma. More strongly overprinted samples yielded Rb–Sr dates between 115 to 110 Ma (± 3 to 6 Ma). The existence of Jurassic HP/LT rocks in South Kalimantan as described in the literature could not be verified. e) All Rb–Sr ages from the Bantimala, Luk Ulo and Meratus complexes are in the range of previous K–Ar data for similar rocks, indicating that existing data are not compromised by contamination with extraneous Ar (inherited or excess Ar). f) U–Pb zircon dating of HP/LT blocks provided first constraints for previously unknown protolith ages, indicating a Late Triassic–Early Jurassic time of formation (205–185 Ma) for the metabasic rock suite. Zircons that are older than the protolith age are interpreted to represent detrital grains from subducted sediments that had been picked up from a contaminated mantle source during melt formation. Metamorphic zircon rims in a strongly overprinted eclogite yielded U–Pb dates of ca. 133–110 Ma. g) In contrast to interpretations suggesting a relationship of high-grade metamorphic rocks (granulite and amphibolite) from the Meratus Complex to collision-related crustal thickening, it is more probable that these rocks either represent exotic continental slices from NW Australia or the metamorphic sole of the presumably Jurassic Meratus ophiolite.
Publications
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(2019). Age constraints on high-pressure/low-temperature metamorphism and sedimentation in the Luk Ulo Complex (Java, Indonesia). Lithos 324-325, 747–762
Hoffmann J, Bröcker M, Setiawan NI, Klemd R, Berndt J, Maulana A, Baier H
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(2019). Geochronology and Zr-in-rutile thermometry of high-pressure/low-temperature metamorphic rocks from the Bantimala Complex, SW Sulawesi, Indonesia. Lithos 324-325, 340–355
Böhnke M, Bröcker M, Maulana A, Klemd R, Berndt J, Baier H
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(2019). The distinctive tectonic and metamorphic history of the Barru Block, South Sulawesi, Indonesia: Petrological, geochemical and geochronological evidence. Journal of Asian Earth Sciences 172, 170–189
Maulana A, Christy AG, Ellis DJ, Bröcker M
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(2021). Rb–Sr geochronology of metamorphic rocks from the Central Indonesian Accretionary Collision Complex: Additional age constraints for the Meratus and Luk Ulo complexes (South Kalimantan and Central Java). Lithos 388-389
Alfing J, Bröcker M, Setiawan NI
