鍾孫霖臺灣大學：地質科學研究所李皓揚Lee, Hao-YangHao-YangLee2007-11-262018-06-282007-11-262018-06-282007http://ntur.lib.ntu.edu.tw//handle/246246/54825在藏南拉薩陸塊廣泛出露的林子宗火山岩，一般認為形成於印度-亞洲碰撞之前，是新特提斯洋板塊向北隱沒所造成的安地斯山型島弧岩漿產物。本研究在拉薩陸塊中系統性採集了火山岩從事鋯石鈾-鉛和全岩氬-氬定年，希望進一步了解林子宗火山岩的時空分佈，再進行地球化學分析工作，藉以探討林子宗火山岩的岩石成因。定年結果顯示，一般習稱的林子宗火山岩其實是兩期火山活動的產物，即：（1）白堊紀活動期，介於112~71 Ma，普遍分佈在拉薩陸塊上；（2）早第三紀活動期，介於63~43 Ma，僅分佈在拉薩陸塊南部。根據原始定義，後者才是“真正的”林子宗火山岩，並可由老至年輕分為典中組、年波組及帕那組等三個火山地層單元。整體而言，從白堊紀到早第三紀，藏南火山活動展現南向遷移而且規模加劇的特徵，並在50 Ma左右形成廣泛的岩漿“大噴發”（magmatic “flare-up”）。 地球化學資料顯示，林子宗火山岩的成份從玄武岩變化到流紋岩（SiO2=45~80 wt.%），性質以鈣鹼質為主，都具有大離子半徑元素（LILE）富集和高場力鍵結元素（HFSE）虧損的典型島弧岩漿特徵。在大噴發時期產生的火山岩（即：帕那組）最不均質，組成從低鉀拉斑質、鈣鹼質變化到鉀玄質，並且呈現不同程度的輕稀土元素富集，我們據此可將本期林子宗火山岩區分為鈣鹼質系列（calc-alkaline suite）、低鉀系列（low-K suite）、高稀土系列（high-REE suite）和鉀玄系列（shoshonitic suite）四類。 就大部分的鈣鹼質林子宗火山岩（63~50 Ma）而言，它們的鍶釹同位素比值 [The Linzizong volcanic successions that crop out in the Lhasa terrane, southern Tibet have been widely regarded as the products of northward subduction of the Neo-Tethyan oceanic slab that resulted in an Andean-type convergent margin along South Asia before its collision with India. In this study, 2 zircon U-Pb ages and 26 whole-rock Ar-Ar ages along with geochemical analysis of 129 volcanic rocks collected from the Lhasa terrane were carried out to better constrain the temporal-spatial distribution and petrogenesis of the Linzizong volcanism. The age results delineate two discrete episodes of volcanic activity. These are (1) a widespread Cretaceous episode (ca. 112~71 Ma), and (2) an intense, but geographically confined, Paleogene episode (ca. 63~43 Ma) that occurred only in the southern part of the Lhasa terrane. The second episode is here re-defined as the “real” Linzizong volcanic successions that comprise, from bottom to top, Dianzhong, Nianbo and Pana Formations. The age results furthermore indicate a southward migration and intensification of volcanism, which culminated at ca. 50 Ma (“flare-up” period). The Linzizong volcanic successions consist dominantly of calc-alkaline rocks that range from basalt to rhyolite compositions (SiO2=45~80 wt.%) and show typical arc-lava geochemical features marked with LILE enrichment and HFSE depletion in the spidergram. Those formed during the flare-up period, i.e., the Pana Formation, however, show significant geochemical variations from low-K tholeiitic through calc-alkaline to shoshonitic compositions coupled with increasing enrichment in LREEs. Therefore, the volcanic successions are divided into four suites, namely, calc-alkaline suite, low-K suite, high-REE suite and shoshonitic suite. Most Linzizong calc-alkaline rocks (ca. 63 to 50 Ma) show Sr and Nd isotope compositions [誌 謝 I 摘 要 II ABSTRACT IV LIST OF FIGURES X LIST OF TABLES XIII LIST OF PLATES XIII CHAPTER 1．INTRODUCTION 1 CHAPTER 2．GEOLOGICAL BACKGROUND AND SAMPLING 5 CHAPTER 3．ANALYTICAL METHODS 12 3.1. Ar-Ar dating 12 3.2. Zircon U-Pb dating 12 3.3. Major- and trace-element measurements 13 3.4. Sr-Nd isotope analysis 14 3.5. Zircon Hf isotope analysis 15 CHAPTER 4．ANALYTICAL RESULTS 18 4.1. Geochronological results 18 4.1.1. Whole-rock Ar-Ar ages 18 4.1.2. Zircon U-Pb ages 23 4.2. Major- and trace-element results 27 4.2.1. Major-element compositions 27 4.2.1.1. Southern Province (Linzizong volcanic successions) 27 4.2.1.2. Northern Province and Sangri Group 40 4.2.2. Trace-element compositions 41 4.2.2.1. Southern Province (Linzizong volcanic successions) 41 4.2.2.2. Northern Province and Sangri Group 52 4.3. Whole-rock Sr-Nd and zircon Hf isotope results 58 4.3.1. Whole-rock Sr-Nd isotope compositions 58 4.3.1.1. Southern Province (Linzizong volcanic successions) 58 4.3.1.2. Northern Province and Sangri Group 63 4.3.2. Zircon Hf isotope data 63 CHAPTER 5．DISCUSSION 65 5.1. Temporal-spatial distribution of the Linzizong volcanic successions 65 5.2. Geochemical changes in magmas through time 70 5.3. Source components: mantle wedge, asthenosphere and crust 73 5.4. Zircon Hf isotopic constraints 76 5.5. Magma generation of the Linziong volcanic successions 79 5.5.1. Dianzhong and Nianbo Formations (ca. 63 to 50 Ma) 79 5.5.2. Pana Formation (ca. 50 to 43 Ma) 80 5.6. Geodynamic significance inferred from age and geochemical constraints 85 5.6.1. ca. 63 to 50 Ma 85 5.6.2. ca. 50 to 43 Ma 88 CHAPTER 6．CONCLUSIONS 94 REFERENCES 96 PLATES 109 APPENDIX 111 Table DR-1 111 Refereed Paper 1．Miocene Jiali faulting and its implications for Tibetan tectonic evolution EPSL (2003) 120 Refereed Paper 2．藏南林周盆地林子宗火山岩的時代、成因及其地質意義：鋯石U-Pb年齡和Hf同位素證據 岩石學報（2007） 138 Refereed Paper 3．Age and Geochemical Characteristics of the Linzizong Volcanism for Neotethyan Slab Breakoff in Southern Tibet (Submitted to Geology) 14620424318 bytesapplication/pdfen-US林子宗火山岩西藏年代地球化學新特提斯洋隱沒板塊脫離Linzizong volcanic successionsTibetagegeochemistryNeo-Tethyan subductionslab breakoffthesishttp://ntur.lib.ntu.edu.tw/bitstream/246246/54825/1/ntu-96-F88224102-1.pdf