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

~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”

{Queano et al., 2007}

[from Dimalanta, 2006]

The rock units are The rock units are limestone, chert, limestone, chert, siliceous mudstone and quartz-rich siliceous mudstone and quartz-rich sandstonesandstone

In the east of the peninsula, In the east of the peninsula, pyroclastic pyroclastic flow flow deposits and clastic rocks deposits and clastic rocks (polymictic (polymictic conglomerates, arkosic conglomerates, arkosic and lithic sandstones and lithic sandstones and shale)and shale)

Serpentinized peridotites Serpentinized peridotites (harzburgite (harzburgite and dunite) in the northeastern part of and dunite) in the northeastern part of the study area the study area

Field ObservationsField Observations

Quartz-rich Quartz-rich sandstone,sandstone, siliceous siliceous mudstone,mudstone, shaleshale

Field ObservationsField Observations

mudstone-mudstone-wackestonewackestone

Massive to beddedMassive to bedded

Beds dipping NWBeds dipping NW

Field ObservationsField Observations

Plio-PleistocenePlio-Pleistocene (Cruz & Lingat, 1996)(Cruz & Lingat, 1996)

LimestoneLimestone

Field ObservationsField Observations

Ultramafic rocksUltramafic rocks

Dunites Dunites and and harzburgitesharzburgites

Highly Highly serpentinizedserpentinized

Highly fracturedHighly fractured

opx

serpentine

1 mm

Late Middle MioceneLate Middle Miocene

Units:Units:

1. 1. Quartz-poor Quartz-poor sandstone, sandstone, mudstone, shalemudstone, shale

2. 2. ConglomeratesConglomerates with harzburgite and with harzburgite and gabbro clastsgabbro clasts

Field ObservationsField Observations

PN 1: Quartz Diorite

PN 1: Quartz Diorite

Early MioceneEarly Miocene20.820.8++1 Ma1 Ma

19.519.5++1 Ma 1 Ma

(Bellon & Rangin, 1991)(Bellon & Rangin, 1991)

qtz

hbdbt

bt

plg 1 mm

PN 5: Intrusive Rock

PN 7B: Sandstone

PN 7B: Diorite

Hornblende Ar

No. SampleMineral

datedN ρs Ns ρ i Ni

U(ppm)(± 2σ)

Central age

(Ma ± 1σ)P (χ2) %

1. PN 1 Zircon 51 2.52 2625 2.18 2274 436.8 11.8±2.5 <1

2. PN 1 Apatite 45 0.10 174 0.7 1176 17.6 12.6±1.7 <1

3. PN 5 Zircon 16 3.14 365 0.64 74 127.5 50.9±10.3 91.4

4. PN 5 Apatite 20 0.17 91 0.87 470 21.7 16.2±2.4 20.3

5. PN 7B Zircon 31 0.99 1143 1.13 1301 - 8.6±1.9 <1

6. PN 7B Apatite 31 0.05 120 0.33 838 - 13.2±2.1 <1

7. PN 8 Zircon 22 1.91 451 1.21 286 - 16.3±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

40Ar/39Ar RESULTSPN-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

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 partTectonic 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

CONCLUSIONS

Thank you for the attention ~~