摘要
青藏高原南部拉萨地体晚三叠世-早侏罗世时期岩浆活动的地球动力学背景仍是尚待解决的一个重要科学问题。本文报道了南部拉萨地体西部打加错地区新发现的安山玢岩的锆石U-Pb年代学、Hf同位素和全岩地球化学数据。打加错安山玢岩主要由蚀变细粒斜长石、辉石和角闪石组成。2件样品的LA-ICP-MS锆石U-Pb定年结果分别为204±4Ma和203±2Ma,表明打加错地区在晚三叠世发生了安山质岩浆活动。结合近期报道的打加错地区辉石角闪二长闪长岩的年龄(207.3±3.6Ma)记录,表明南部拉萨地体西部打加错地区发生了晚三叠世岩浆活动。2件安山玢岩的锆石εHf(t)值多为正值(分别为-2.2^+8.8和-2.0^+10.7),可比于南部拉萨地体中东部其它地区同期岩浆记录的Hf同位素成分(如:工布江达南部203Ma正长花岗岩的锆石εHf(t)为-4.9^+2.7;日喀则东部约205Ma二长花岗岩的锆石εHf(t)为+11.9^+15.8),这表明南部拉萨地体晚三叠世岩浆活动从东工布江达向西延伸到打加错一带,东西延伸约800km。打加错地区2件安山玢岩样品(SiO2=56.1%~59.0%)以较低Mg#(45.8~48.7)和Al2O3含量(16.4%~17.0%)为特征,属于偏铝质钾玄质-高钾钙碱性火山岩。本文获得的新数据,结合最近报道的早侏罗世桑日群岩浆活动记录以及打加错地区上三叠统地层研究新进展,本文更赞成打加错地区和南部拉萨地体其它地区的晚三叠世岩浆活动形成于与班公湖-怒江洋壳南向俯冲有关的弧后环境。
The geodynamic setting of the Late Triassic-Early Jurassic magmatic activities in the Lhasa Terrane remains hotly debated. This paper reports zircon LA-ICP-MS U-Pb dating,Hf isotopic,and whole-rock geochemical data of the andesite porphyrites from Dajiacuo in the western segment of the southern Lhasa subterrane. The Dajiacuo andesite porphyrites consist mainly of hornblende,pyroxene,and altered fine-grained plagioclase. Two andesitic porphyrite samples have been dated at 204 ± 4Ma and 203 ±2Ma,respectively,indicating the present of the Late Triassic andesitic magmatism in this region. In combination with the presence of pyroxene-bearing amphibole monzodiorite of 207. 3 ± 3. 6Ma recently reported from Dajiacuo,it is suggesting that the Late Triassic magmatism was active in the western segment of the southern Lhasa subterrane. These two andesitic porphyrite samples mainly show positive zircon εHf（ t） values（-2.2 ～ ＋8.8 and-2.0 ～ ＋10.7,respectively）,comparable to those of the coeval magmatic records reported from the middle and eastern segments of the southern Lhasa subterrane（ e. g.,ca. 203 Ma syenogranite to the south of Gongbogyamda display zircon εHf（ t） of-4.9 ～ ＋2.7,and ca.205 Ma monzogranite to the east of Xigaze exhibit zircon εHf（ t） of＋ 11. 9 ～ ＋ 15. 8）. Such similarity indicates that the Late Triassic magmatic activity extended from Gongbogyamda in the east to Dajiacuo in the west,covering over 800 km along the length of the southern Lhasa subterrane. Two andesitic porphyrite samples（ SiO2= 56. 1% ～ 59. 0%） are characterized by low Mg^#（ 45.8 ～48.7） and Al2O3（ 16.4% ～17.0%） contents and are shoshonitic or highK calc-alkaline. These new data,together with the data of volcanic rocks recently reported from the Lower Jurassic Sangri Group and the new identification of the Upper Triassic strata around the Dajiacuo,enable us to favor that the Late Triassic magmatic activity documented from Dajiacuo and other segments of the southern Lhasa subterrane
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2014年第10期3100-3112,共13页
Acta Petrologica Sinica
基金
中国科学院青藏高原专项(B类)(XDB03010301)
国家973项目(2011CB403102)
国家自然科学杰出青年基金项目(41225006)联合资助
