大屯火山群年代的不確定性

Abstract

過去已有許多研究嘗試利用各種定年方法與定年材料求得大屯火山群最近一次噴發的年代。然而，各研究結果互不相容，最後一次噴發年齡仍未有定論。大多數研究於20世紀末期進行，與近期相關研究則相隔逾二十年。隨著時間推移，定年設備與技術持續進步，且對不同定年方法之機制及所涉元素與礦物特性之理解更為深入，因而有機會重新評估前人定年結果，並與近期研究成果比較，以判定何種方法最適合用於大屯火山群最近噴發年代的測定。 本研究於磺嘴山、七星山、紗帽山、烘爐山及竹子山等岩流露頭及岩芯樣本上，分別進行鈾釷定年與氬氬定年分析，並將結果與前人之鉀氬、氬氬、鈾鉛鋯石、鈾釷鐳及核飛跡等定年結果進行比較。 重複樣本在鈾釷與氬氬定年分析中呈現顯著差異，推測主因為樣本組成複雜。鈾釷分析中識別出兩組具有顯著不同同位素組成與年齡之角閃石。各別井孔中的岩芯樣本基質的氬氬結果年齡相近，暗示岩流可能連續快速噴發；而氬氬角閃石年齡則較其基質年代為早。與先前資料比較，氬氬基質與鈾釷鐳定年所測年代通常為研究區中最年輕，且與同類研究結果相當吻合；核飛跡年代則始終屬於最老群，而鉀氬及氬氬全岩年代分佈廣泛。大屯火山群岩體年齡過於年輕，不適合鈾鉛鋯石分析，日後或需考慮採用鈾釷鋯石定年。整體而言，由於岩石組成複雜可能導致測得年代偏舊，實際噴發年齡很可能較目前測得結果年輕。

Multiple studies in the past have attempted to obtain the most recent eruption age of the Tatun Volcanic Group (TVG) using various dating methods and dating materials available. Yet results among studies are incomparable, and the age of the last eruption remains inconclusive. Most studies were conducted at the end of the 20th century, with a two-decade gap before those done in the 21st century. With the passing of time, better equipments and techniques have been developed, and with a better understanding of the mechanics of different dating methods and the characteristics of the elements and minerals involved, it is now possible to re-evaluate previous dating results and compare them with more recent results to determine which method is most suitable for dating recent eruption ages in TVG. In this study, U-Th and Ar-Ar dating analyses were conducted on groundmass and mineral separates from Huangzuei, Cising, Shamao, Honlu, and Chutze lava flow outcrops and boreholes, then compared with past K-Ar, Ar-Ar, U-Pb zircon, U-Th-Ra, and fission track results. Duplicate samples produced divergent ages for both U-Th and Ar-Ar analyses. This is attributed to the samples being multiphase. Two amphibole populations of vastly different isotopic compositions and ages were identified in the U-Th analysis. Within each borehole, the Ar-Ar groundmass results show similar ages, suggesting that the lava flows erupted in quick succession. Ar-Ar hornblende ages predate their groundmass counterparts. But compared to previous data, Ar-Ar and U-Th-Ra results typically rank among the youngest ages in an area and compare well with similar studies. Fission track ages are consistently among the oldest, while K-Ar and Ar-Ar whole rock ages span across a broad range. The TVG rocks were too young for U-Pb zircon analysis, and U-Th zircon analysis may be needed. All bulk analyses may appear older due to multiphase, therefore, eruption age is likely younger than measured results.