1 National Taiwan University, Taipei, Taiwan

1. 40Ar/39Ar and Fission Track constraints on Miocene Tectonics in the Buruanga Peninsula, Panay Island, Central Philippines Monika Walia1, T. F. Yang1, T. K. Liu1, C. H. Lo1, L. S. Teng1, W. M. Yuan2, Ling Chung1, G. P. Yumul3, C. B. Dimalanta4 1National Taiwan University, Taipei, Taiwan 2China University of Geosciences, Beijing, 100083, China 3Department of Science and Technology, Bicutan, Taguig, Metro Manila 4National Institute of Geological Sciences, University of Philippines, Diliman, Philippines

2. The Philippines is sandwiched between two oppositely dipping subduction zones • The Philippine island arc is built on oceanic crust which was modified by several episodes of magmatism ~7 cm/yr ~9 cm/yr

4. Age of collision “The Palawan Microcontinental block, which was rifted from Mainland Asia started to collide with the Philippine Mobile Belt during Early to Middle Miocene” {Yumul et al., 2003 and reference therein} “Paleomagnetic reconstructions show that the Palawan microcontinental Block-Philippine Mobile Belt collision occurred in the Late Miocene, somewhat later than is commonly envisaged” {Queaño et al., 2007}

5. [from Dimalanta, 2006]

6. The rock units are limestone, chert, siliceous mudstone and quartz-rich sandstone In the east of the peninsula, pyroclastic flow deposits and clastic rocks (polymicticconglomerates, arkosic and lithic sandstones and shale) Serpentinizedperidotites(harzburgite and dunite) in the northeastern part of the study area

7. Field Observations Quartz-rich sandstone, siliceous mudstone,shale

8. Field Observations mudstone-wackestone Massive to bedded Beds dipping NW

9. Field Observations Plio-Pleistocene (Cruz & Lingat, 1996) Limestone

10. Field Observations Ultramafic rocks Dunitesand harzburgites Highly serpentinized Highly fractured serpentine opx 1 mm

11. Field Observations Late Middle Miocene Units: 1. Quartz-poor sandstone, mudstone, shale 2. Conglomerates with harzburgite and gabbroclasts

12. PN 1: Quartz Diorite

13. Early Miocene 20.8+1 Ma 19.5+1 Ma (Bellon & Rangin, 1991) PN 1: Quartz Diorite qtz bt hbd bt plg 1 mm

14. PN 5: Intrusive Rock

15. PN 7B: Sandstone

16. PN 7B: Diorite

17. Hornblende Ar

18. FISSION TRACK DATA

19. RESULTS 13.6 Ma 10.1 Ma 50.9 Ma 16.1 Ma

20. RESULTS 16.9 Ma 7.1 Ma 9.3 Ma

21. 40Ar/39Ar RESULTS PN-8 (Hornblende) PN-1B (Hornblende) PN-1C (Biotite) 19.35±0.09 Ma PN-1C (Hornblende) 18.69±0.03 Ma PN-1B (Biotite) 84.07±0.37 Ma 18.49±0.03 Ma 30.6±0.19 Ma

22. Ar-Ar Hornblende Ar-Ar Hornblende Ar-Ar Biotite FT Zircon FT Apatite

23. Ar-Ar Hornblende Ar-Ar Hornblende FT Zircon FT Zircon

24. CONCLUSIONS • Ar-Ar hornblende ages suggest the time of intrusion of the • diorite bodies in the area – • ~84 Ma in the eastern part and ~30 Ma in the western part • Tectonic activity is recorded by the zircon FT and Ar-Ar system over a temperature range of ~250° C to ~500° C around 17-20 Ma • This may be inferred as the time of collision of the • microcontinent with the mobile belt • We interpret the last stage of cooling recorded by the fission • track system as representing the timing of later successive • events/soft collisions ranging from 9-13 Ma • In the western coast of the island, apatite ages show partial • resetting and zircon ages are unaffected by the tectonic • event being away from the suture zone

25. Thank you for the attention ~~