40 Ar/ 39 Ar and Fission Track constraints on Miocene Tectonics in the Buruanga Peninsula, Panay Island, Central Philippines Monika Walia 1, T. F. Yang 1, T. K. Liu 1, C. H. Lo 1, L. S. Teng 1, W. M. Yuan 2, Ling Chung 1, G. P. Yumul 3, C. B. Dimalanta 4 1 National Taiwan University, Taipei, Taiwan 2 China University of Geosciences, Beijing, , China 3Department of Science and Technology, Bicutan, Taguig, Metro Manila 4 National Institute of Geological Sciences, University of Philippines, Diliman, Philippines

~7 cm/yr ~9 cm/yr  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

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}

[from Dimalanta, 2006]

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

Field Observations Quartz-rich sandstone, siliceous mudstone, shale

Field Observations mudstone- wackestone Massive to bedded Beds dipping NW

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

Field Observations Ultramafic rocks Dunites and harzburgites Highly serpentinized Highly fractured opx serpentine 1 mm

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

PN 1: Quartz Diorite

Early Miocene Ma Ma (Bellon & Rangin, 1991) qtz hbd bt plg 1 mm

PN 5: Intrusive Rock

PN 7B: Sandstone

PN 7B: Diorite

Hornblende Ar

No.Sample Mineral dated Nρsρs NsNs ρ i NiNi U (ppm) (± 2σ) Central age (Ma ± 1σ) P (χ 2 ) % 1.PN 1Zircon ±2.5<1 2.PN 1Apatite ±1.7<1 3.PN 5Zircon ± PN 5Apatite ± PN 7BZircon ±1.9<1 6.PN 7BApatite ±2.1<1 7.PN 8Zircon ±4.2<1 FISSION TRACK DATA

RESULTS 16.1 Ma 10.1 Ma13.6 Ma 50.9 Ma

RESULTS 9.3 Ma7.1 Ma 16.9 Ma

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

Ar-Ar Hornblende FT Apatite FT Zircon Ar-Ar Biotite Ar-Ar Hornblende

FT Zircon Ar-Ar Hornblende FT Zircon

 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 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 CONCLUSIONS

Thank you for the attention ~~