藏南林子宗火山岩的年代學、地球化學和地體構造意義

Abstract

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 [