First report of geo- and thermochronological results from the Cordillera Central, Luzon, Philippines

Nakajima, Toru   ; Sueoka, Shigeru   ; Nagata, Mitsuhiro   ; Kohn, B. P.*; Ramos, N. T.*; Tsutsumi, Hiroyuki*; Tagami, Takahiro*

Geo- and thermochronological methods were applied to dioritic rocks from the Cordillera Central, Luzon, Philippines, to understand their emplacement and exhumation history in the island arc mountains. Zircon U-Pb ages range from 32.54 0.70 to 6.11 0.15 (2SE) Ma, indicating that dioritic magmas intruded the middle and upper crust intermittently during Tertiary magmatism. Zircon fission-track (ZFT) pooled ages range from 35.63 2.17 to 6.91 0.36 (2SE) Ma, and are generally comparable with the U-Pb ages at each locality. These results suggest that the diorites cooled rapidly below 250-350 degree, (i.e. through the partial annealing zone of the ZFT system), following their intrusion. On the other hand, the zircon and apatite (U-Th-Sm)/He (ZHe and AHe) weighted mean ages, ranging from 11.71 0.36 to 8.82 0.26 and 9.21 0.52 to 0.98 0.088 (2SE) Ma respectively, indicate a decrease in cooling rates at a lower temperature range, especially through the partial retention zone of the AHe system. This observation suggests that the ZFT ages reflect initial cooling of the dioritic magma, whereas the AHe ages reflect the cooling history associated with regional exhumation following the initial cooling phase. Spatial distribution of the AHe ages suggest that rapid exhumation of the Cordillera Central during the Quaternary resulted from the block-like uplift of the entire mountain range.