POLYPHASED DEFORMATION AND METAMORPHISM OF THE CRETACEOUS TO EOCENE SUCCESSIONS IN CENTRAL PALAWAN, PHILIPPINES

David, S. D., Jr.*, Suzuki, S.**, Yumul, G. P., Jr.***, Roffler, R.**** and Takemura, S.*****
* Mines and Geosciences Bureau, Quezon City, Philippines
** Department of Earth Sciences, Faculty of Sciences, Okayama University, Okayama Japan
*** National Institute of Geological Sciences, University of the Philippines, Quezon City, Philippines
**** Department of Geology, The University of Newcastle, NSW, Australia
***** Geosciences Institute, Hyogo University of Teacher Education, Hyogo, Japan

        The deformation and metamorphism of the Cretaceous to Eocene succession in Central Palawan was studied where the general lithology is defined by mudstones and sandstones. Modal and chemical analyses of sandstones from the sequence revealed that the clasts of sandstones are derived from Palawan continental block, which was rifted from the Asian mainland. The pelitic rocks in the study area gradationally change from slaty in the west to slightly schistose to the east.

        Structural analysis of the cleavage fabric revealed three stages of folding: F1, F2 and F3 phases with associated axial plane cleavages: slaty cleavage S1, crenulation cleavage S2 and crenulation cleavage S3 from the first stage to the third stage of folding. The folding also suggested that the sequence belongs to an overturned limb of large scale inclined fold with the youngest to the west.

        Metamorphic grade and condition based on the crytallinity (illite crytallinity) and b0 cell parameters of K-white micas during the first stage of deformation have been determined by X-ray diffraction analysis. It shows that the overall ‘illite crystallinity (IC)’ values decrease in grade from the E to the W, while the b0 cell parameters indicate moderate-high pressure conditions.

        From this study, the following tectonic history is proposed during the deformation of the Cretaceous to Eocene succession in Central Palawan:

• F1 folds and S1 schistosity developed under moderately high pressure-low temperature conditions which could have formed during the subduction of the Cretaceous to Eocene succession prior to the Middle Miocene.

• F2 folds and S2 formed as consequence of the collision of the ESE migrating Palawan Block with the Philippine Arc.

• F3 and associated cleavage S3 were formed during the continuos SE migrating of the Palawan Block.