龜山島高鎂安山岩之岩漿成因

Abstract

菲律賓海板塊以每年大約七公分的速度向西北方隱沒至歐亞大陸板塊下方，形成了琉球島弧及沖繩海槽弧後盆地，根據Chung et al. (2000) 的研究，沖繩海槽最南部分 (Southernmost Part of Okinawa Trough, SPOT) 不是一般認定的單純的弧後盆地，因此有島弧火山噴發所造成的龜山島座落其中。過去傳統上將龜山島火山岩歸類為典型的島弧鈣鹼系列安山岩 (SiO2 ≈ 60 wt.%) ，但本研究顯示若干岩樣的鎂含量較高 (MgO ≥ 5 wt.%) ，可視為高鎂安山岩，本研究中擬利用詳細的地球化學資料結合地球物理證據，探討此種岩石的成因。在微量元素組成上，龜山島高鎂安山岩具有富集的大離子岩石圈 (Cs,Rb,Ba) 、釷、鈾及鉛等元素，以及虧損的高場力鍵結元素 (Nb,Ta,Ti) ，與世界其他地區的島弧岩漿顯示相似的特性；此外，它們的主量和微量元素成份和Rudnick and Fountain (1995) 所提出的平均大陸地殼的組成近乎相同。在同位素組成上，龜山島高鎂安山岩具有低的釹同位素 (εNd ≈ –4.3 〜 –5.0)、高的鍶同位素 (87Sr/86Sr ≈ 0.706) 及鉛同位素比值 (206Pb/204Pb ≈ 18.76, 207Pb/204Pb ≈ 15.69, 208Pb/204Pb ≈ 39.06)，似乎反映了明顯的大陸地殼訊號。然而，歸納以上所有地球化學資料，本研究發現龜山島高鎂安山岩無法利用傳統上認為的基性岩漿的結晶分異加上地殼混染 (AFC) 模式解釋，而更可能是由隱沒的菲律賓海洋板塊加上沉積物熔融所形成。地球化學模擬計算結果顯示的岩漿成因過程為：隱沒的沉積物及海板塊分別發生部份熔融，這兩種岩漿以大約1：1的比率混合後，再與地幔楔以9：1的比率發生岩漿─地幔楔交互作用，即可造成龜山島高鎂安山岩。這樣的岩漿成因模式需要一個特殊的熱源，可能與SPOT的地體環境有關；而地震層析成像研究報導，在本地區隱沒帶的四十公里深處有一個明顯的低速區，與本研究所推測的部份熔融作用吻合。

Kueishantao is an emerged volcanic islet located at the western end of the Southernmost Part of Okinawa Trough (SPOT). The Okinawa Trough, extending from SW Kyushu, Japan to NE Taiwan, is widely regarded as a backarc basin that is built behind the Ryukyu arc-trench system owing to subduction of the Philippine Sea plate underneath the Eurasian plate. The SPOT, however, is not a simple backarc basin but an embryonic rift zone in which early arc volcanism occurs as a result of the Ryukyu subduction. The Kueishantao is one of such volcanoes thus formed in the SPOT and consists mainly of andesitic lava flows dated to be ~7000 yr old. In this study, we report whole rock major and trace element, and Sr-Nd-Pb isotope compositions of the Kueishantao andesites. The results indicate that some of the samples have unexpectedly high magnesium, with MgO ≥ 5 wt.% and Mg# > 0.5, relative to their silica contents (SiO2 ≈ 60 wt.%), which allow them to be coined as high-Mg andesites (HMAs). In the incompatible element variation diagram, these Kueishantao HMAs exhibit enrichments in the large ion lithophile elements and Th, U and Pb, and depletions in the high field strength elements, features typical of arc lavas from the Ryukyu subduction zone as well as convergent margins worldwide. More interestingly, their overall geochemical compositions are very similar to those of the mean continental crust proposed by Rudnick and Fountain (1995). The Kueishantao HMAs have uniform isotope compositions, with low εNd (–4.3 to –5.0), high Sr (87Sr/86Sr ≈ 0.706) and Pb (18.76, 15.69 and 39.06 of 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb, respectively) ratios. Such “continental” isotopic signatures have led previous workers to argue significant crustal contamination during magma ascent as a major petrogenetic process, but our evaluation shows that this simple binary mixing model fails to explain their geochemical and Pb isotope systematics. We propose, instead, that the Kueishantao HMAs result from partial melting of the altered Philippime sea crust and overlying subducting sediments, followed by a melt-mantle wedge interaction. This interpretation is in consistency with seismic tomographic data beneath the SPOT area characterized by a complex collision/extension/subduction tectonic context off NE Taiwan.