Fission-track dating of volcanics in the northern part of the Taiwan-Luzon Arc: eruption ages and evidence for crustal contamination

Abstract

Igneous rocks from the Coastal Range and offshore islets of eastern Taiwan, Camiguin island and northern Luzon of the Philippines, which collectively constitute the northern part of the Taiwan-Luzon Arc, have been dated by the fission-track technique. The pooled ages range widely from 49.9 Ma to 0.02 Ma. Some of the samples produce bimodal or even multiple age distributions in composite probability density plots, indicating the presence of inherited “ages” in the dated zircons. A χ 2 test shows that most of the χ 2 ages are in good agreement with the youngest peak date. Except for one intrusive basement sample and five volcanic samples affected by hydrothermal alteration, most of the χ 2 ages can be interpreted as the ages of volcanic eruptions. The inherited ages of the zircon populations present in many samples record thermal activities which occurred prior to the eruption of their host magmas. This suggests that upper level crustal contamination has taken place, thereby introducing some xenocrystic zircons into the magmas erupted. The fission-track ages are always younger than or occasionally equal to the ages obtained by other methods for the samples from the same localities. Of six samples dated by the ArAr stepwise heating method four samples have plateau age close to the FTD ages. Some samples do not contain inherited, partially annealed FTD ages and the corresponding KAr ages are consistent with the FTD ages. A detailed study of Rb Sr isotopic systematics of phenocryst minerals by Lan C. Y., Shen J. J. S. and Lee T. (1986). A RbSr isotopic study of andesites from Lutao, Lanhsu, and Hsiao-Lanhsu, eruption ages and isotopic heterogeneity. Bull. Inst. Earth Sci. 6, 211–226 showed isotopic disequilibrium suggesting retention of unequilibrated older volcanic products. Many of the KAr dates, which are commonly older than the eruption ages determined by the FTD technique, may reflect inherited argon. Based on results from a combination of dating methods, volcanism in the northern part of the Taiwan-Luzon Arc can be deduced to have been active since 26 Ma. The igneous activity in the Taiwan Segment (northern part of North Luzon Arc as exposed in the Coastal Range) ceased at 2.2 Ma, because of the arc-continent collision. Further southward, away from the present location of the collision zone, the volcanoes are still active.