李寄嵎2006-07-252018-06-282006-07-252018-06-281998http://ntur.lib.ntu.edu.tw//handle/246246/11704大陸東南火山帶為環太平洋火山帶的 一個重要組成部分，廣泛分布著中生代火 山岩（或稱為燕山期火山岩）。就體積而 言，基性岩類只佔了約5 ％，其餘為中、酸 性火山岩；而雙模式岩漿組合則零星分布 於浙、閩、贛、湘等地區，時代涵蓋侏儸 紀至白堊紀。本文擬藉由對浙江地區雙模 式火山岩之研究，探討其在大陸東南地區 燕山晚期大地構造演化上之意義。 浙江地區中生代火山活動可分為早晚 二期：(1)早期火山岩（或稱下火山岩系， 時代為晚侏儸紀至早白堊紀）由安山岩－ 英安岩－流紋岩組成連續的高鉀鈣鹼性系 列；(2)晚期火山岩（或稱上火山岩系，時 代為晚白堊紀）則往往為不連續的玄武岩 及高鉀流紋岩組成的雙峰式系列或者單獨 以玄武岩或流紋岩產出。此雙模式火山岩 多以夾層形態發育於地塹、半地塹型的斷 陷盆地或等軸狀火山窪地內，主要出露於 鄞縣玄曇地、天台九里坪及文成山門等 地。其中天台九里坪剖面不但涵蓋早晚兩 期，其頂部更為新第三紀玄武岩不整合覆 蓋，是本區出露最為完整的剖面。經由目 前有限之40 Ar/39 Ar 定年結果顯示，上火山 岩系之噴發年代約為108Ma 以後。 全岩地球化學分析結果顯示，浙江地 區雙模式火山岩之基性端員為玄武岩－玄 武安山岩(SiO2=43.6 －55.8%)，且由中鉀向 鉀玄岩系列(shoshonite)過渡；酸性端員之 SiO2 含量均大於70%，Na2O+K2O=6 ∼ 9%，屬於高鉀流紋岩。一般而言，兩種端 員均具有親岩漿性元素富集與Nb,Ta 負異 常；但是酸性端員更呈現明顯之Ba, Sr, Eu, P 及Ti 虧損，反映出長石礦物大量結晶分 異的演化過程，此一現象與沿海地區之A 型花崗岩(福建魁岐岩體91Ma ；浙江青田 岩體108Ma)頗為相似。 綜合野外產狀、區域構造特徵及地球 化學資料，我們認為浙江地區雙模式火山 岩可能為張裂型大地構造環境下的岩漿活 動產物。配合對鄰近地區相關岩類的系統 性定年資料，顯示大陸東南地區燕山晚期 張裂活動開始的時間，應早於110Ma 。Among North China Block, Indochina Block and Pacific plate, the greatest magmatic belts of Western Pacific are distributed in Huanan. These belts are formed after Indochina orogeny and collisions of North China and South china Blocks. Except the voluminous intrusives, the Mesozoic volcanics(or Yanshanian volcanics) are also widespread in this area. Generally, the basic parts are only 5% of the total volume, the others are the mediate and acidic volcanics. The Jurassic and Cretaceous bimodal magmatic suites are scatteredly distributed in Zhejiang, Fujian, Jiangxi, Hunan and Guangdong provinces. The Mesozoic volcanism of Zhejiang Province can be divided into two episodes. (1)Early episode(lower volcanic sequence, or Late Jurasic-Early Cretaceous sequence) consist predominantly of high-K calc-alkaline series (andesite-dacite-rhyolite); (2)Late episode (upper volcanic sequence, or Late Cretaceous sequence) are exposed in small fault-bounded basins and consist of bimodal basalt-rhyolite series. According to 40 Ar/39 Ar ages, the Late episode (upper volcanic sequence) began at 108Ma. Late Cretaceous bimodal basalt-rhyolite series mainly crop out in the eastern Zhejiang. The mafic member consist of basalt/ basaltic andesite (SiO2@43.6-55.8%), and belong to the middle K to Shoshonitic suites. Similar to Cretaceous A-type granites from the nearby region, the felsic member shows peralkaline nature [SiO2> 70%, Na2O +K2O@6-9%]. Generally, both members show enrichment in the incompatible trace elements and depletion in Nb and Ta. But the felsic member is characterized by the significant depletion of Ba, Sr, Eu, P and Ti which reflect the process of feldspar fractionation. From a series of studies of the Late Yanshanian granitoids in Zhejiang-Fujian areas since three years ago, the geochemical results show that some rock units have the characteristics of extension-type granite. This seems to imply the tectonic style of SE China has been changed from compression to extension since late Cretaceous. Therefore the studies of Mesozoic bimodal volcanism in the Zhejiang province shows that the extensional activity of SE China may be earlier than 110 Ma.application/pdf83432 bytesapplication/pdfzh-TW國立臺灣大學地質科學系暨研究所雙模式火山岩40 Ar/39 Ar 定年張裂活動bimodal volcanism40 Ar/39 Ar agesextensional activityreporthttp://ntur.lib.ntu.edu.tw/bitstream/246246/11704/1/872116M002008.pdf