臺灣中央山脈新康山地區變質火成岩之地球化學

Geochemical Studies of Metavolcanic Rocks in Shinkangshan Area, Central Range, Taiwan

呂婉瑄(W. S. Lyu) ； 宋聖榮(S. R. Song) ； 羅偉(W. Lo)

《鑛冶：中國鑛冶工程學會會刊》 55卷1期 (2011/03) Pp. 57-68

https://doi.org/10.30069/MM.201103.0005

摘要

臺灣中央山脈變質岩區有許多零星分佈之火成岩體，其中尤以出露於新康山地區的火成岩體範圍最廣。此些火成岩體的成因與臺灣島的地體構造演化有密不可分的關係。本研究以此中央山脈新康山地區的變質火山岩為對象，進行地球化學分析，期望對探討中央山脈的岩漿活動與地體構造演化提供重要的資訊。臺灣中央山脈新康山地區位於中央山脈南段，此地區出露之岩性為板岩偶夾千枚岩、變質砂岩和變質火山岩，變質火山岩其原岩產狀以火山熔岩和凝灰岩為主，由野外觀察新康山地區之變質火山岩部份具枕狀構造，推測此地區原岩可能形成於深洋環境，並且與其周圍之板岩與變質砂岩之沉積原岩同時期生成。又根據岩象學分析，新康山地區之變質火山岩礦物群屬於綠色片岩變質相，僅經歷一次的變質作用。依照主要元素結果顯示，新康山地區變質火山岩二氧化矽含量在48.29～51.35 wt.％。另由全岩地球化學之微量元素和稀土元素資料指示，新康山地區之變質火山岩，其輕稀土元素較重稀土元素富集，（La/Sm）N為1.88～2.87，（La/Yb）N為3.41～5.89，稀土元素型態分佈近似於洋島玄武岩，高場力鍵結元素和輕稀土元素比重稀土元素富集，且大離子半徑元素虧損的現象，具有板塊內部玄武岩的地球化學特性；而在鄰近新康山且與其相似的向陽山變質火山岩，其稀土元素分佈型態為平坦型，（La/Sm）N為1.23～2.07，（La/Yb）N為1.42～2.68，銪元素呈現輕微地負異常，釷富集，且鈮、鉭和鈦虧損的現象，具有火山島弧玄武岩的地球化學特性；由此可作為探討中央山脈地體構造演變之參考。

關鍵字

中央山脈 ； 板岩地層 ； 變質火山岩 ； 地球化學

並列摘要

Metavolcanic rock exposures are widely scattered in the metamorphic areas of the Central Range, Taiwan. The major concentration of these metavolcanic rock exposures can be found in the Shinkangshan area. We believe the petrogenesis of these metavolcanic rocks were closely associated with the orogenic process of the Taiwan Island. This research is intended to provide basic geochemical analysis of these metavolcanic rocks that will be significant for the future study of magmatism and tectonic history of the Central Mountain Range.From the field investigation along the road from Shinkangshan to shihdung area, these metavolcanic rocks were composed of lavas and volcanic tuffs exhibiting pillow-like structures. It is believed that the metavolcanic rocks are contemporaneous with the meta-sedimentary wallrocks.A major element geochemistry analysis showed that the silica content of the metavolcanic rocks in the Shinkangshan area ranges from 48.29 to 51.35 wt. %. While the whole-rock trace elements and rare earth elements geochemistry analysis indicate that the metavolcanic rocks on the Shinkangshan area with the rare earth elements ratio (La/Sm) N=1.88~2.87 and (La/Yb)N=3.41~5.89 are similar to that of oceanic basalts. Based on the geochemical characteristics of enrichment high field strength elements (HFSE), and light rare earth elements (LRE) relation to heavy rare earth elements (HREE), as well as depletion in large ion lithophile elements (LILE) suggested that the metavolcanic rocks of the Shinkangshan area are intra-plate basalts. Results of this study will greatly assist interpreting the tectonic evolution of the Central Range of Taiwan.