Sr-Nd Isotopes and Geochemical Studies of Basement Rocks and Granitoids in the South Qilian Area, NW China

中國大陸西北地區南祁連基盤岩及花崗岩類之鍶－釹同位素及地球化學研究

董國安(Kuo-An Tung) ； 曾建元(Chien-Yuan Tseng)

《Collection and Research》 32期 (2019/12) Pp. 19-41

https://doi.org/10.6693/CAR.201912_(32).0003

摘要

The Qilian Orogeny Belt is situated between the Sino-Korean Plate and the Qaidam Plate. Its Precambrian strata are mainly composed of thick volcanic rocks and sedimentary rocks. During the Caledonian orogeny, fold faults were uplifted and subjected to medium-pressure facies. The highly metamorphic deformation was accompanied by intrusion with intermediate-felsic plutonic rocks. This study focused on basement rocks, granitoids, and mafic rocks and their main and trace elements; these rocks were collected from the middle part of the South Qilian area near Qinghai Lake at Gahai, Wulan, and Riyueshan. Sr-Nd isotopes were analyzed in the hope of allowing a preliminary discussion of the source, age, and thermal events of the basement rocks in the South Qilian area. Based on results of geochemical and isotope analyses, it was found that basement rocks at Gahai and Riyueshan were in the range of Nd isotope values (ε_(Nd) (1 Ga) -14.8 to -7.6), and their trace elements have the same characteristics. The trend obviously differed from base rocks in the Wulan area (ε_(Nd) (1 Ga) -8.4 to 2.2), and therefore, it is presumed that the Gahai and Riyueshan areas belong to the same plate, with an age of about 1.4 Ga, and the Wulan area is a different plate, aged about 1.7 Ga, and which was affected by three thermal events at about 1 billion, 500 million, and 280 million years ago, which respectively correspond to the Jinning, Caledonian, and Hercynian periods. In addition, results of the geochemical analyses, field observations, and lithofacies descriptions of granites indicate that they can be roughly classified into I-type and S-type granites, and granites collected in the Gahai and Riyueshan areas belong to Caledonian S-type granite, while granite in the Wulan area is an I-type granite with an age of 1.6 Ga, roughly the same age as the basement rocks.

關鍵字

Sr-Nd isotope ； geochemistry ； basement rocks ； granitoid ； Qilian Block

並列摘要

祁連造山帶位於中朝板塊及柴達木板塊之間，其前寒武紀之地層主要是由巨厚的火山岩和沈積岩所組成，加里東期造山運動時褶皺斷層隆起，遭受中壓相系的高度變質變形作用，並伴隨著中－酸性深成岩的侵入。本研究針對南祁連地區中段、青海湖附近之朵海、烏蘭及日月山等三個地點所採集之基盤岩、花崗岩類及基性岩類，分析其主要、微量元素及鍶、釹同位素，期望從中對南祁連地區基盤岩之來源、年代及所經歷之熱事件作初步探討。綜合基盤岩地化及同位素分析結果，得知朵海及日月山兩地之基盤岩在釹同位素（ε_(Nd) （1Ga） -14.8 and -7.6）範圍內，而且在微量元素特徵上具有相同趨勢，且與烏蘭地區之基盤岩明顯不同（ε_(Nd) （1 Ga） -8.4 and 2.2），故推測朵海及日月山地區屬同一板塊，年代約為14億年，而烏蘭地區則為另一板塊，年代約為17億年，且在約為10億年、5億年及2億8千萬年前，遭受三個熱事件作用，並可對比為晉寧期、加里東期及海西期。另從花崗岩類之地化分析結果、野外觀測、岩相描述可大致分為I-type和S-type兩種花崗岩，且朵海及日月山地區所採之花崗岩類皆屬於加里東期之S-type花崗岩，而烏蘭地區之花崗岩則為I-type之花崗岩，其年代為16億年，大致與基盤岩年代相同。

並列關鍵字

鍶－釹同位素 ； 地球化學 ； 基盤岩 ； 花崗岩類 ； 祁連地塊