印尼西爪哇新生代火山岩之年代學與地球化學特徵研究

Geochronological and geochemical characteristics of Cenozoic volcanic rocks from West Java, Indonesia

梁瑋琪(Leung Wai-Ki)

指導教授 ： 賴昱銘

國立臺灣師範大學/理學院/地球科學系/碩士(2022年)

https://doi.org/10.6345/NTNU202201025

摘要

西爪哇位在爪哇島西部，為印度-澳洲板塊隱沒至歐亞板塊下所產生的新生代巽他-班達隱沒帶之一部分，其基盤岩石為巽他古陸大陸基盤，由板塊間的相互運動隨著時空演化，展現了不同的地體構造與演化階段，使得岩漿活動沿著平行與垂直島弧方向，在年代或地球化學上都有系統性的變化。過去研究工作都以巽他島弧東段整體討論，但針對局部地區相關研究顯少，特別是自中新世以來持續有岩漿活動的西爪哇西部。因此，本研究在西爪哇西部5個火山岩體（Danau火山群、Gede火山群、Bayah Dome地區的北部、南部，以及Ciemas地區）採集了58個火山岩樣本，藉由鋯石鈾鉛定年結合全岩地球化學數據，以了解此區域火山岩漿的地球化學特徵、來源與成因機制，以及其地體構造與演化階段。
西爪哇西部新生代以來岩漿活動隨時間區分為三期，依序為中新世中期（17.0-11.4百萬年）、中新世晚期（9.8-5.4百萬年）及更新世（2.4-0.7百萬年），當中捕獲了白堊紀（145-140百萬年）與漸新世（26.0-22.1百萬年）的繼承鋯石，隨空間分布，中新世兩期出露在Bayah Dome地區，而更新世為西爪哇最年輕的火山岩體，並且遍布於整個西爪哇西部。在全岩地球化學上，三期岩漿皆為鈣鹼性序列的岩漿活動，由基性的玄武岩至中酸性的石英安山岩所構成（SiO2=50-69 wt.%），礦物組成有斜長石、角閃石、輝石類斑晶，部分酸性樣本含有少量的石英，並且都具有輕微的輕稀土元素相對富集（LREE/HREE=1.56-2.77）、微銪負異常（δEu=0.73-0.97），以及高場力鍵結元素虧損的現象，呈現典型的島弧岩漿地球化學特徵，代表在西爪哇西部三期岩漿的地球化學特徵大致相似。在全岩鍶釹同位素的比值上，中新世中期岩石具有εNd(0)值為+2.04至+3.78及87Sr/86Sr為0.7042至0.7057，中新世晚期岩石具有εNd(0)值為-0.80至+2.82及87Sr/86Sr為0.7043至0.7059，而更新世岩石具有εNd(0)值為-2.01至+3.61及87Sr/86Sr為0.7044至0.7069。
本研究的實驗結果，發現印度-澳洲板塊在西爪哇西部呈現多階段的隱沒作用，並可將島弧火山的空間分布與噴發時間相聯結，由Bayah Dome南部往北的Bayah Dome北部與Danau火山群，以垂直島弧方向有遠離海溝年代從17至2.4百萬年逐漸年輕的現象，並且推論三期的岩漿皆為地函楔和隱沒沉積物發生不同程度的部分熔融後形成的岩漿，混和後經結晶分異作用而噴發，繼而重建西爪哇西部新生代的地體演育過程與模擬圖。隨著巽他-班達島弧由東至西的地體構造演化，西爪哇西部源區早已在17百萬年前受到混染（εNd(0)值為+1.6至+3.8），並形成了中新世中期至更新世（17-2.4百萬年）岩漿，後來因中-東爪哇有斷裂的板塊隱沒至島弧下方過程中，以及隨著巽他海峽開始擴張，使得中-東爪哇與東蘇門答臘的岩漿活動停止，反映由東蘇門答臘至中-東爪哇（含西爪哇）的隱沒板塊相對較難隱沒，且其隱沒角度逐漸平緩，西爪哇西部岩漿活動以垂直島弧方向，由南往北的年代有逐漸年輕的現象。直到在2.4百萬年後，因中-東爪哇的斷裂板塊已隱沒至島弧下方，與從地球物理中發現東蘇門答臘-西爪哇隱沒板塊存有一個間隙，反映西爪哇西部曾發生隱沒板塊回捲，且角度再次變徒峭，從而有大量來自軟流圈的熱流上湧至源區，且在隱沒板塊上方有更多的沉積物熔體熔融，形成不同岩漿來源的特徵（εNd(0)值為+0.81至-2.01），以及更新世火山活動遍及整個西爪哇西部（由南部往北部）。

關鍵字

印尼 ； 西爪哇 ； 巽他島弧 ； 新生代岩漿活動 ； 鋯石鈾鉛定年法 ； 全岩地球化學

並列摘要

West Java is situated in the western part of Java and underlain by continental basement rocks of Sundaland. It is part of Sunda-Banda subduction zone and was built through multiple events of Cenozoic magmatism produced by Indian-Australian Plate subducted beneath Eurasian Plate. With temporal and spatial evolutions of interaction between the plates, magmatism displays along-arc and across-arc variations of geochronological and geochemical constraints. Previous studies have focused on discussing the systematic variations in the eastern part of the Sunda arc, however, relevant studies of temporal and spatial evolution in the western part of West Java, where the magmatism has been continuing since Miocene, remain poorly studied. Therefore, this study collected 58 volcanic rocks from 5 volcanic complexes (Danau Volcanic Complex, Gede Volcanic Complex, Bayah Dome (North), Bayah Dome (South), and Ciemas) in the western part of West Java, and presented new zircon U-Pb ages and whole-rock geochemical data, to better understand geochemical characteristics of multiple magmatic events, magma genesis and tectonic evolution of the subduction environment.
The zircon U-Pb results show 3 stages of magmatic events, from Mid-Miocene (17.0-11.4 Ma) to Late Miocene (9.8-5.4 Ma) and Pleistocene (2.4-0.7 Ma), including Cretaceous (145-140 Ma) and Oligocene (26.0-22.1 Ma) inherited zircons. We found that the Mid to Late Miocene volcanic rocks are exposed in the Bayah Dome (North & South), while the youngest volcanic rocks of Pleistocene spread throughout the whole study area. In the whole-rock geochemical results, volcanic rocks in the West Java are composed of calc-alkaline basalt to dacite (SiO2=50-69 wt.%) and mineral assemblages are plagioclase, hornblende, and pyroxene ± quartz. All calc-alkaline volcanic rocks display slightly enrichment in light rare earth elements (LREE/HREE=1.56-2.77) and slightly negative europium anomalies (δEu=0.73-0.97). In multi-element distribution patterns show depletion in high field strength elements and exhibiting a typical geochemical characteristic of island arc magma. For whole-rock Sr-Nd isotopes, the calc-alkaline samples from Mid-Miocene display εNd(0) values from +2.04 to +3.78 and 87Sr/86Sr values from 0.7042 to 0.7057, those from the Late Miocene display εNd(0) values from -0.80 to +2.82 and 87Sr/86Sr values from 0.7043 to 0.7059, and those from Pleistocene display εNd(0) values from -2.01 to +3.61 and 87Sr/86Sr values from 0.7044 to 0.7069.
This study suggested that the multi-stages subduction of the Indian-Australian Plate occurred in West Java and the spatial distribution of 5 volcanic complexes can be linked with magmatic events. We found that the magmatism became progressively younger from the south of Bayah Dome to the north of Bayah Dome and Danau Volcanic Complex. The mantle source is probably derived from partial melting of the mantle wedge and subducted sediments, further undergoing the processes of fractional crystallisation to erupt in the Cenozoic Era. Furthermore, this study establishes a new tectonic evolution model of the western part of West Java based on the above results and discussions. The calc-alkaline magmatism has already undergone source contamination (εNd(0) values = +1.6 to +3.8) before 17 Ma and further formed the Mid-Miocene to Pleistocene (17-2.4 Ma) arc magmatism. During the 17-2.4 Ma period, the tectonic evolution of the Sunda arc from east to west, including the process of slab breakoff and further subducting beneath the Central-East Java, and extension of Sunda Strait caused magmatism of East Sumatra and Central-East Java to cease, reflecting the subducted slab from East Sumatra to Central-East Java (including West Java) shifted from high to low-angle subduction, and causing continuous magmatism in the West Java to migrate from the south to the north in the study area. Until 2.4 Ma, the tearing slab segment of Central-East Java subducted underneath the greater depth, and a gap is found in the slab beneath the boundary of East Sumatra-West Java through geophysics, these tectonic evolutions might reflect subducting slab beneath the western part of West Java rollback with steep angle subduction, with extra heat flow ascending from the asthenosphere and melting more subducted sediments. This mechanism changed the magmatic composition with εNd(0) values from +0.81 to -2.01 and the Pleistocene volcanism distributed in the whole study area (from the south to north).

並列關鍵字

Indonesia ； West Java ； Sunda Arc ； Cenozoic magmatism ； Zircon U-Pb dating ； Whole-rock geochemical analysis

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