|Tatun Volcano Group, Taiwan
Mt.Huangzuei is the prominent volcanic cone in stage six of the Tatun Volcano Group [TVG], a series of seven volcanic stages manifested in approximately thirty volcanic origins. All of the volcanoes are situated less than 20km from the Taipei metropolitan area which is populated by over 7 million people, or only slightly smaller than the New York Metropolitan area. A combination of high weathering rates and lack of a historical eruption create the common belief that these volcanoes are extinct.
The eruptions consist of calc-alkaline andesite attributed to arc volcanism of the nearby Philippine Sea plate being subducted into the Eurasian plate (Teng, 1996, Geology) or post-collisional collapse (Wang et al., 1999, Tectonophysics and 2004, Journal of Petrology). Previously reported K-Ar analyses by Juang and Chen (1989, Bulletin of Central Geological Survey, in Chinese), Tsao (1994, Bulletin of Central Geological Survey, in Chinese), and 40Ar/39Ar whole rock analyses by Lee (1996, masters thesis, National Taiwan University) from a number volcanic cones spanning the entirety of the TVG all have been used to suggest the onset of volcanism between 2.5-2.8 Ma and ending 0.3 Ma, with only sporadic younger volcanic events ceasing at 0.2 Ma.
However, recent volcanological and seismic observations show that the Tatun Volcano Group is behaving like a dormant volcano with an active magma chamber. Monitoring of high-frequency seismicity in the form of isolated events or spasmodic bursts driven by the circulation of hydrothermal fluids in a pattern similiar to those reported in other dormant Quaternary volcanoes (Konstantinou et al., 2007). Helium isotopic compositions, HCl and Sulfur monitoring in many hydrothermal events since August 2004 have also suggested new magma supply or opening of fractures to the Mt.Cising region of Tatun (Lee, Volcanology, 2008). Proxies for eruptions including radiocarbon dated charcoal and volcaniclasic pumice have been observed in paleolake sediments in the volcanic region, these 14C dates suggest an eruption had occurred .006-.023 Ma (Belousov, 2010; Chan and Lin, 2002)
40Ar/39Ar laser step-heating analyses of nine samples from Mt.Huangzuei volcano performed in the AGES Lab suggest far younger ages than previously reported. Hand samples and a core were all previously collected from four eruption stages on Huangzuei volcano. These four stages were documented based on field observation, petrography, petrology, major element analysis, REE, and Sr-Nd isotopic analysis (Lai et al., TAOS, 2010).
Samples were prepared and analyzed by step-heating 3-5 grains of groundmass hand-picked from the magnetic 250-150μm grain size fraction. All of the isochron ages obtained so far are significantly younger than any age previously reported for Mt.Huangzuei. Our current work consists of improving the gas cleanup in the extraction stage, and improving the initial 40/36 estimate of airs for a highly resolved mass discrimination to lower the error estimates.
Gary Mesko, Sidney Hemming, Su-chin Chang
Sheng-Rong Song (National Taiwan University)
Belousov, A., Belousovoa, M., Chen, C.H., and Zellmer, G., Deposits, caracter and timing of recent eruptions and gravitational collapses in Tatun Volcano Group, northern Taiwan: Hazard-related issues, Journal of Volcanology and Geothermal Research, v.191, 3-4, p.205-221. doi:10.1016/j.jvolgeores.2010.02.001.
Chen, W., Yang, C., Yang, Y., and Liu, J., 2003, Volcanic landform and sequences of the Tatun Volcanoes. Bulletin of the Central Geological Survey 16, 99-123.
Juang, W.S., and Chen, J. C., 1989, Geochronology and geochemistry of volcanic rocks in northern Taiwan, Bulletin of Central Geological Survey 5, 31-66. (in Chinese)
Lai, Y.M., Lin, Y.J., Song, S.R., Tsai, Y.W., Hsieh, Y.C., and Lo, W., 2010, Topography and Volcanology of the Huangtsuishan Volcano Subgroup, Northern Taiwan. Terrestrial, Atmospheric and Oceanic Sciences, v.21, p. 1-11, doi:10.3319/TAO.2010.02.22.04(TH)
Lee, S. F., 1996, Volcanic sequence study of the Tatun Volcano Group: The Chihsinshan Subgroup, Master thesis, National Taiwan University, Taipei. (in Chinese)
Teng, L.S., Extensional collapse of the northern Taiwan mountain belt, Geology 1996, 24, p. 949-952. doi: 10.1130/0091-7613(1996)024<0949:ECOTNT>2.3.CO;2.
Tsao, S. J., 1994, Potassium-argon age determination of volcanic rocks from the Tatun volcano group, Bulletin of Central Geological Survey 9, 137-154. (in Chinese)
Wang, K.L., Chung, S.L., Chen, C.H., Shinjo, R., Yang, T.F., and Chen, C.H., Post-collosional magmatism around northern Taiwan and its relation with opening of the Okinawa Trough, Tectonophysics, 308 (1999), p. 363-376.
Wang, K.L., Chung, S.L., O'Reilly, S.Y., Sun, S.S., Shinjo, R., and Chen, C.H.,
Geochemical Constraints for the Genesis of Post-collisional Magmatism and the Geodynamic Evolution of the Northern Taiwan Region, J. Petrology 2004 45: 975-1011; doi:10.1093/petrology/egh001.