西藏東緣鮮水河斷層形成時間與貢嘎山花崗岩體的形成時間及其剝蝕歷史

Emplacement and Denudation History of Gongga Shan Granite and timing of Xianshuiho fault in Eastern Tibet

曾嚴俊(Tseng,Yenchun)

指導教授 ： 李元希

國立中正大學/理學院/地球與環境科學系應用地球物理與環境科學研究所/碩士(2012年)

摘要

在西藏東緣, 沿著鲜水河斷層發育了長約100公里，寬約10公里的貢嘎山花崗岩體，並發育成為西藏東緣貢嘎山脈最高峰（7556m）。此花崗岩體在地質圖上顯示為西藏東緣唯一的新生代花崗岩體，周圍以三疊紀的板岩為主，過去研究認為此花崗岩體形成時間約13Ma與18Ma，導因以左移為主的鲜水河斷層活動所致。然而鮮水河斷層截切過此花崗岩體，顯示兩者形成時間關係仍有疑義，本研究利用鋯石U-Pb定年、Ar-Ar定年與核飛跡定年分析此花崗岩體形成時間與剝蝕歷史，並釐清印度與歐亞板塊碰撞何時影響此區域。
U-Pb定年結果顯示此花崗岩體並非全都是新生代花崗岩體，許多年代為中生代160-180Ma的花崗岩體。且新生代岩體有好幾群年代，分別約為10Ma、14Ma、16Ma、18Ma、35Ma與50Ma，顯示花崗岩漿的入侵並非一次完成，由於新生代早期花崗岩侵入年代開始於50Ma，而過去研究認為鮮水河斷層在晚中新世開始，此兩者年代差異甚大，因此花崗岩體成因應與鮮水河斷層活動無關，可能反應印度板塊與歐亞板塊碰撞過程中，變形集中於既有弱面並伴隨局部地殼融熔作用所致。
在花崗岩體內部顯示鋯石核飛跡年代分為7-9Ma與3-5Ma兩群。5Ma反應鮮水河斷層開始活動時間，7-9Ma反應岩體抬升時的冷卻年代。岩體外圍鋯石核飛跡年代約10-12Ma之間，顯示區域性抬升剝蝕發生於10-12Ma之前。

關鍵字

鮮水河斷層 ； 貢嘎山花崗岩

並列摘要

In eastern Tibet the Gongga granite was developed along the Xienshuiho fault which is near 100 km in length. In southern part of the Gongga granite is also associated with the highest peak 7556 m in eastern Tibet. Previous researches considered that this granite was developed in 13-18 Ma and resulted from the faulting heating of the left lateral Xienshuiho fault and also is considered the Xienshui fault started since 13-18 Ma to present.

In this study we used the zircon U-Pb dating to identify the emplacement timing of the granite and muscovite and biotite Ar-Ar dating and fission track dating to identify the exhumation history of the Gongga granite and developing timing of the Xienshuiho fault.

The zircon U-Pb ages show the granite is composed of Mesozoic and Cenozoic granite which is different from previous result that all the Gongga granite is Miocene granite. In addition we found that the emplacement timing of the granite 50Ma, 35Ma ,18Ma, 16Ma, 14Ma, and 10Ma which are also different from previous results. This emplacement mechanism of the Gongga granite probably results from the collision event between Eurasia plate and India plate since 50 Ma.

The Ar-Ar ages and fission track ages near 5 Ma along the Xienshuiho fault which indicates the developing timing of the Xienshuiho fault is near from 5 Ma to now rather than 13-18 Ma. The zircon fission track ages show near 10 Ma surrounding the Gongga granite which indicates the timing of the rapid exhumation started from 10 Ma age.

並列關鍵字

Xienshuiho fault ； Gongga granite

參考文獻

許晉瑋 (2010)西藏高原東緣鲜水河斷層帶熱定年研究。國立台灣大學地質學研究所碩士論文，111頁。

Cosmic ray exposure ages of tektites by the fission-track technique. J.

temperature,pressure, and ionization on the formation and stability of fission tracks inminerals and glasses. J. Geophys.Res 70, 1497-1502.

Fleischer, R., L., Price, P. B., and Walker, R. M. (1975). Nuclear tracks

of Asian Earth Sciences 22 (2004) 465–481