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Title: | 西藏南部岡底斯岩體:年代學、地球化學及岩石成因 The Gangdese Batholith, Southern Tibet: Ages, Geochemical Characteristics and Petrogenesis |
Authors: | Da-Ren Wen 溫大任 |
Advisor: | 鍾孫霖(Sun-Lin Chung) |
Keyword: | 岡底斯岩體,西藏,年代學,地球化學, The Gangdese Batholith,Tibet Geochronology Geochemistry, |
Publication Year : | 2007 |
Degree: | 博士 |
Abstract: | 廣布於西藏南部拉薩陸塊的岡底斯岩體,過去以來一直被視為安地斯型活動大陸邊緣的重要組成,經由新特提斯洋板塊北向隱沒於亞洲大陸而形成。然而,活動期間跨越了亞洲與印度碰撞事件的岡底斯複式岩體,其年代、組成以及地殼演化始終缺乏詳細的調查。對此陸緣火山弧的瞭解程度,對於進一步探究新特提斯洋隱沒的動力學機制,特別是造山活動對岩漿演化的影響效應,至關重要。本研究結合25件鋯石離子探針年代數據、83筆全岩地球化學,以及65筆鍶–釹同位素分析資料,首次較系統性地報導岡底斯岩體之年代學和地球化學,並探討西藏南部的造山演化及地殼增生。年代學分析結果表明,岩漿活動可分為晚白堊紀(ca. 103-80 Ma)與早第三紀(ca. 65-46 Ma)兩時期,並確認約當八千萬年至六千八百萬年前間,有一岩漿休止期。在岩性組成上,岡底斯岩體包含一系列由輝長岩分異至花崗岩類的深成岩組合,成份的歧異性也相對表現在二氧化矽含量上,從43%分布至79%都有。此一中鉀至高鉀質鈣鹼系列的岩體,具有典型的島弧岩漿地球化學特徵,包括大離子半徑親石元素(銫、銣、鈾、釷、鉀及鋇等)之富集以及高場力元素(鈮、鉭及鈦)之虧損。基性至中性岩體的分異演化主要受控於結晶分異作用,而酸性岩類的成因則歸諸基性輝長岩質下部地殼的部份融熔,或混染部分地殼物質所致。此深成岩體之釹同位素初始值( The Gangdese Batholith, which emplaced extensively in the Lhasa terrane of southern Tibet, is widely regarded as the major constituent of an Andean-type convergent margin resulting from northward subduction of the Neo-Tethyan oceanic lithosphere under Asia. It is the largest Trans-Himalayan plutonic complex. However, the duration, composition and evolution of the Gangdese arc magmatism that spanned the accretion to collision of Asia with India, remain poorly investigated. In fact, knowledge of this continental arc is fundamental for constraining the geodynamics of the northward Neo-Tethyan subduction, particularly its consequential effect on the magmatic evolution. This thesis reports new results of a systematic study, including SHRIMP zircon dating of 25 samples, 83 geochemical and 65 Sr–Nd isotopic analyses. The data are used to delineate the source characteristics and draw implications for the orogenic evolution and crustal growth in southern Tibet. The geochronologic results suggest two distinct stages of plutonism in the Late Cretaceous (ca. 103-80 Ma) and early Paleogene (ca. 65-46 Ma). The results also confirm, if not refine, the notion that a magmatic gap or quiescent period existed between ca. 80 and 68 Ma. With diverse lithologies from gabbros to granites, the plutonic complex shows a large range of SiO2 contents from 43 to 79 wt.%, and typical geochemical characteristics in the medium-K to high-K calc-alkaline series arc magmas, such as enrichment in LILE (e.g., Cs, Rb, U, Th, K, and Ba) and depletion in some HSFE (Nb, Ta, and Ti). The differentiation from mafic to intermediate intrusives is most likely achieved by fractional crystallization, but the felsic granitoids are produced by partial melting of pre-exsiting gabbroic lower crust, or with some crustal contamination. The intrusives are predominantly characterized by depleted mantle sources with |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/27832 |
Fulltext Rights: | 有償授權 |
Appears in Collections: | 地質科學系 |
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