西藏高原東緣鮮水河斷層帶熱定年學研究

40Ar/39Ar Thermochronological Study of Xianshuihe Fault Zone, Eastern Tibet

許晉瑋(Jin-wei Hsu)

指導教授 ： 羅清華

國立臺灣大學/理學院/地質科學研究所/碩士(2010年)

https://doi.org/10.6342/NTU.2010.02808

摘要

鮮水河斷層位於西藏高原東緣，截切了散佈著早中生代或新生代花岡岩體之古生代至早中生代沉積岩層。本研究沿鮮水河斷層採集花岡岩、偉晶岩與片麻岩岩石樣本，挑選鋯石、鉀長石與雲母類礦物進行鈾鉛定年與氬-氬定年研究，並結合熱年代學概念，以期了解鮮水河斷層帶活動歷史，並推論此區域大地構造演化意義。
各礦物樣本以人工挑選方式進行礦物分離，並以雷射階段加溫與雷射單礦物燒熔方式進行氬-氬定年分析。結果顯示雲母類礦物年代分布於兩群： 5.5-4.2 Ma與 153-141 Ma。採自貢嘎山地區之鉀長石年代約為4.4-4.2 Ma，而松潘甘孜地塊中的鉀長石則顯示了較老的年代 (142-44 Ma)。花岡岩中的鋯石鈾鉛定年結果則呈現了213+/-4 Ma，211+/-3 Ma與9.2+/-1.7 Ma之年代。
經計算各礦物之封存溫度，結合定年結果可推論出當地冷卻歷史與冷卻速率。並考慮前人所報導之鈾鉛、銣鍶與核飛跡年代後，研究結果顯示在貢嘎山地區自9 Ma起之冷卻速率高達100C/Myr，此外亦發現另一快速冷卻事件發生於14-9 Ma。其餘採自鮮水河斷層以北的花岡岩則呈現兩階段快速冷卻過程：第一階段為150-140 Ma前以約10C/Myr之速率冷卻，之後直至30-20 Ma後才以10C/Myr第二階段冷卻。然而，出露於鮮水河斷層以南的花岡岩僅於約140 Ma之前顯示一期快速冷卻事件。此結果表示在松潘甘孜地塊在侏儸紀期間曾發生侵蝕剝露事件 (至約150-140 Ma止)，印度歐亞間碰撞亦於松潘甘孜地塊東部導致再次抬升。
本研究成果顯示鮮水河斷層沿線之花岡岩體形成於早中生代，且曾經歷一段至早第三紀為止的長時間緩慢冷卻歷史。此外，來自貢嘎山之花岡岩、偉晶岩、片麻岩的年輕的氬-氬年代指出：鮮水河斷層之剪切運動確實發生於中新世與上新世，此運動反映了自從第三紀起始的印度-歐亞間碰撞。

關鍵字

氬氬定年 ； 鮮水河斷層帶 ； 西藏高原

並列摘要

In the eastern margin of the Tibet plateau, the Xianshuihe fault zone transacts Paleozoic-early Mesozoic sedimentary sequences which were intruded by a number of the Early Mesozoic or Cenozoic granites. Granite, pegmatite and gneiss samples were collected along the Xianshuihe fault zone for the present U-Pb and 40Ar/39Ar geochronological study, aiming to reveal the movement history for the fault zone and to explore its implications to regional tectonics.
The results show mica 40Ar/39Ar ages clustering in two groups: 5.5-4.2 Ma and 153-141 Ma. The younger ages appear in micas obtained from gneiss, granite and pegmatite in Kongashan, whereas the older ages were derived from granites in the Songpan-Garze fold belt. Two K-feldspars from granites in Kongashan display ages of 4.4+/-0.2 and 4.2+/-0.3 Ma, while other K-feldspars from the granites collected from outcrops in the Songpan-Garze fold belt show much older ages as 142+/-3 Ma, 70.6+/-1.8 Ma and 44.1+/-1.8 Ma. However, these 40Ar/39Ar ages are still much younger than their respective zircon U-Pb ages (213+/-4 Ma, 211+/-3 Ma) except one with 9.2+/-1.7 Ma.
Considering their respective closure temperatures, these radiometric ages would indicate a cooling rate of 100C/Myr for the Kongashan area since ca. 9 Ma. The granites collected from the outcrops north of Xianshuihe fault zone in Songpan-Garze fold belt show two stages of rapid cooling, with a rate of 10C/Myr prior to ca. 150-140 Ma and 10C/Myr after 30-20 Ma, respectively. Other granite from the outcrop south of Xianshuihe fault only displays one rapid cooling stage prior ca. 140 Ma. This would suggest that an erosion and exhumation event in the Songpan-Garze fold belt may have occurred in Early Cretaceous (ca. 150-140 Ma). The collision between India and Eurasia caused further uplift in the eastern Songpan-Garze fold belt during Late Oligocene-Miocene.
The present thermochronological study suggests that the granitic rocks cropping out along the Xianshuihe fault zone may have emplaced in the Early Mesozoic and experienced a prolonged cooling history until the Late Tertiary. While, the young radiometric ages from granite, gneiss and pegmatite samples indicate that the shearing activity of the Xianshuihe fault zone most likely occurred in the Miocene and Pliocene, in response to the active India-Eurasia collision starting since Tertiary.

並列關鍵字

40Ar/39Ar dating ； Xianshuihe fault zone ； Tibet

參考文獻

Yao, H., Zhao, Z., Qiao, Y., Li, C., Wang, S., Wang, Y., Chen, Y., and Jiang, F., 2007, Magne-tostratigraphic dating of the Xigeda formation in Mianning, Sichuan and its significance: Quaternary Sciences, v. 27, p. 74-84.

Albarede, F., Feraud, G., Kaneoka, I., and Allegre, C., 1978, 39Ar-40Ar dating: The importance of K-feldspars on multi-mineral data of polyorogenic areas: Journal of Geology, v. 86, p. 581-598.

Allen, C., Luo, Z., Qian, H., Wen, X., Zhou, H., and Huang, W., 1991, Field study of a highly active fault zone: The Xianshuihe fault of southwestern China: Geological Society of America Bulletin, v. 103, p. 1178-1199.

Arne, D., Worley, B., Wilson, C., Chen, S., Foster, D., Luo, Z., Liu, S., and Dirks, P., 1997, Differential exhumation in response to episodic thrusting along the eastern margin of the Tibetan Plateau: Tectonophysics, v. 280, p. 239-256.

Berger, G., and York, D., 1981, Geothermometry from 40Ar/39Ar dating experiments: Geochimica et Cosmochimica Acta, v. 45, p. 795-811.