北蘇拉威西新生代岩漿活動的年代與地球化學特徵

Abstract

蘇拉威西位在歐亞板塊、菲律賓海板塊與澳洲板塊之間，由於板塊間的相對運動造就了複雜的地體構造，本研究透過鋯石鈾鉛定年、鋯石鉿同位素以及全岩主量及微量元素分析，來探索該地區的岩漿與構造的演化。分析結果顯示北蘇拉威西的岩漿活動分為主要的三期、五群，依序為：(1) 始新世至漸新世的拉斑質岩、(2a) 漸新世至中中新世的基性鈣鹼性岩、(2b) 漸新世至中中新世的酸性鈣鹼性岩(30-15 Ma)、(3a) 中中新世至上新世的中鉀鈣鹼性岩(10-4 Ma)以及 (3b) 晚中新世的高鉀鈣鹼性岩(7-5 Ma)，其中的第二期與第三期分別是依據岩性與鉿同位素的差異細分。 始新世至漸新世的拉斑質岩主要為玄武岩至玄武質安山岩，具有近似於N型中洋脊玄武岩(N-MORB)的稀土元素特徵((La/Yb)CN =0.5-0.8, CN：對C1 球粒隕石值標準化)，在蛛網圖當中呈現輕微的大離子親石元素富集與高場力鍵結元素的虧損，漸新世至中中新世屬於雙峰式的鈣鹼性岩，其中的基性岩(SiO2＝50-55 wt.%)主要由玄武岩構成，酸性岩(SiO2＝65-88 wt.%)則以I型花崗岩為主，基性岩於稀土元素圖解中呈現輕稀土元素較重稀土元素些微富集的分佈((La/Yb)CN =1.6-2.2)，在蛛網圖當中亦呈現中至強的大離子親石元素富集和高場力鍵結元素虧損的特徵，至於第二期的酸性鈣鹼性岩與第三期的鈣鹼性中酸性岩(閃長岩、安山岩與英安岩)，同樣大多都具有高的大離子親石元素富集以及強的高場力鍵結元素的虧損。 在漸新世至中中新世鈣鹼性岩中的岩漿鋯石具有近於虧損地函的εHf(t)值(+14.4-+17.1)和低鈾含量(128-374 ppm)，中中新世至上新世的中鉀鈣鹼性岩的岩漿鋯石同樣也具有近於虧損地函的εHf(t)值(+14.6-+15.6)和低鈾含量(125-234 ppm)，然而晚中新世的高鉀鈣鹼性岩卻具有明顯偏低的εHf(t)值(-2.5-+1.7)與高的鋯石鈾濃度(1058-1365 ppm)，並伴隨大量的繼承鋯石。 三期火成岩都具有隱沒相關的地化特徵，指示它們皆與隱沒作用存在直接或間接的關係。本研究推斷始新世至漸新世的岩漿活動是生成於洋弧相關的環境；漸新世至中中新世的岩石由其雙峰式岩性分佈的特徵指示了伸張環境下生成的可能，其中的基性岩可能是上湧的地函岩漿演化產物，而酸性岩則可能屬於下部地殼重熔的產物；晚中新世的高鉀岩石有別於第二期，具有顯著偏低的εHf(t)值指示了較老地殼物質的參與，依據其地化特徵上的高度不均質性、缺乏基性岩等，排除了單純由隱沒作用生成的推論。

Sulawesi is located at the junction of three major plates, namely, Eurasia, Australia and Philippine Sea, which underwent complex interactions in the Cenozoic.This study reports for the zircon U-Pb age and Hf isotope data of magmatic rocks from Northern Sulawesi. These, together with whole-rock geochemical analyses, are used to explore the magmatic and tectonic evolution in the region. The results indicate that there are 3 main stages of magmatism, with stage two and stage three can be further divided into two sub-groups based on different geochemical characteristics, respectively: (1) Eocene to Oligocene tholeiitic (TH) rocks, (2a) Oligocene to Miocene basic calc-alkaline (BCA) rocks, (2b) 30-15 Ma felsic calc-alkaline (FCA) rocks, (3a) 10-4 Ma median-K calc-alkaline (MKCA) rocks, and (3b) 7-5 Ma high-K calc-alkaline (HKCA) rocks. The TH group consists mainly of basalts that display N-MORB-like REE pattern [(La/Yb)CN =0.5-0.8]. They also show small LILE enrichment and slight HFSE depletion in a spidergram. The BCA group composed of basalts shows slight LREE enrichment [(La/Yb)CN =1.6-2.2]. They have strong LILE enrichment and strong HFSE depletion. All felsic rocks including FCA group, MKCA group and HKCA group exhibit strong LILE enrichment and strong HFSE depletion in a spider diagram. More specifically, magmatic zircons from I-type granitoids of the CA group are characterized by DM-like εHf(t) values (+14.4-+17.1) and low U concentrations (128-374 ppm). Those from the MKCA group, composed of rock types including diorite, andesite and dacite, also have small U (125-234 ppm) and DM-like εHf(t) values (+14.6-+15.6). Yet, those from dacites of the HKCA group have markedly higher U (1058-1365 ppm) and lower εHf(t) values (-2.5-+1.7), together with abundant inherited zircon grains. All three stages of magmatic rocks show subduction-related geochemical signatures. The TH group shows geochemical features consistent with an intra-oceanic arc setting. The 30-15 Ma calc-alkaline group composed of bimodal rocks indicate that they may form in an extensional setting. The last stageis heterogeneous. The MKCA rocks' DM-like εHf(t) values limit terrestrial or “old continental crust” contribution, whereas the HKCA rocks' lower εHf(t) values suggest significant contribution. Their heterogeneity and lack of basic rocks may indicate that they are not magmatism of a subduction zone.