婆羅洲沙巴東南部中新世以來火山岩之年代學與地球化學特徵

Abstract

東南亞地區的婆羅洲位於三大板塊，歐亞板塊（Eurasian Plate）、菲律賓海板塊（Philippine Plate）以及澳洲板塊（Australian Plate）聚合邊緣的交匯處，被一系列的邊緣海盆所包圍著。沙巴位於婆羅洲東北部，其基盤岩層為中生代的蛇綠岩套，並覆蓋著新生代沉積岩體，中新世以後的火山岩主要出露在沙巴東南部，晚中新世（7~8 Ma）的花崗岩侵入岩體出露在京那巴魯山（Mt. Kinabalu）。本研究分析沙巴火山岩鋯石鈾鉛定年以及鉿同位素結合全岩地球化學數據，能更完整的了解此區域火山岩的成岩作用以及其構造解釋。 鋯石鈾鉛定年結果可將沙巴東南部區域的火山岩分為兩期，分別為中期中新世（12~11 Ma）到晚期中新世（9~7 Ma）及更新世（小於0.23 Ma），更新世火山岩為整個婆羅洲最年輕的火山岩體。在全岩地球化學結果可將此區域火山岩分為兩類，中新世兩期的火山岩主要為鈣鹼性序列，從玄武質到流紋質都有包含（SiO2＝45~71 wt.%），微量元素呈現出高場強元素（high field strength elements，HFSEs）虧損，大離子親石元素（large ion lithosphile elements, LILEs）富集，推斷為隱沒相關的大陸島弧地球化學特徵。更新世階段則都為洋島型玄武質安山岩，岩性都較為基性（SiO2 = 54 wt.%），其高場強元素虧損的現象並不明顯，指示洋島型玄武質安山岩呈現了板內的地球化學特徵。在同位素方面，中新世的鈣鹼性火山岩全岩ƐNd(t)值爲+1.0至+5.0和鋯石ƐHf(t)值爲+17至+3，洋島型玄武質安山岩全岩ƐNd(t)值爲-3.2至+0.8和鋯石ƐHf(t)爲+5至-2，顯示中新世鈣鹼性火山岩同位素值接近虧損地函端元，在分析結果中可將洋島型玄武質安山岩分為兩類型，他們可能來自於同個岩漿源區，但在岩漿上升時經歷的過程不同，造成兩類型岩石在微量元素及同位素上有些許差異。 從實驗結果可得知，沙巴東南部區域的火山岩在年代及地球化學特徵上可以區分為兩期，第一期岩漿活動從中新世中期開始，一直持續到中新世晚期停止，這時期是隱沒作用相關的大陸島弧鈣鹼性岩漿活動，且並非位在鄰近位置的蘇祿海洋島弧隱沒系統所造成的岩漿活動。同位素結果可見此期受到地殼物質混染的程度隨著年代遞減而增加；第二期更新世岩漿活動轉變為洋島型，此期岩石呈現了板內的地球化學特徵，岩石源區為融熔程度較低的上部地函的，且可能在岩漿上升過程受到地殼物質混染。

Borneo is situated in South East Asia characterized by the occurrence of a suite of marginal basins that are located at the junction of three converging major plates (Eurasia, Philippine Sea and Australia). Sabah that exists in northeastern Borneo has a basement of the Mesozoic ophiolitic complex, overlain by Cenozoic sedimentary formations associated with post-Miocene volcanic rocks in Southeast Sabah and late Miocene (~7-8 Ma) granitic intrusions in Mt Kinabalu. This study reports new zircon U-Pb ages and Hf isotopes together with whole-rock geochemical data of the Sabah volcanic rocks, which remain poorly studied, to better understand their petrogenesis and tectonic significance. Our age data allow us to divide the volcanic rocks into three stages, a predominant Middle Miocene (12~11 Ma) stage, Late Miocene (9~7 Ma) stage and a subordinate Pleistocene (<0.3 Ma) stage. The Pleistocene volcanic rocks are the youngest volcanic products identified so far in entire Borneo. Both of the Middle Miocene stage and the Late Miocene consist calc-alkaline rocks, which vary from basaltic to rhyolitic composition and show subduction-related geochemical features such as depletion in high field strength elements (HFSEs, e.g., Ta, Nb and Ti) and enrichment in large ion lithophile elements (LILEs). The Pleistocene stage consists exclusively of OIB-type basaltic rocks that show no or insignificant HFSE depletion. Zircons from an evolved sample (SiO2=54 wt.%). The Miocene calc-alkaline rocks display generally high despite various whole rock ƐNd(t) values from +1.0 to +5.0 and magmatic zircons ƐHf(t) values from +17 to +3. The Pleistocene OIB-type basaltic andesites display significantly lower ƐNd(t) values from -3.2 to +0.8 and ƐHf(t) values from +5 to -2. The Pleistocene OIB-type basaltic andesites can be divided two type, these two types may come from the same magma chamber and have different processes during magma ascent. As the results, the Miocene calc-alkalic volcanism in Southeast Sabah is interpreted as the westernmost part of the Sulu arc system that initiated in the Middle Miocene and ended in the Late Miocene. The Pleistocene OIB-type basaltic andesites, which show intraplate geochemical characteristics, may have resulted from small degree melting of the convecting upper mantle.