南蘇拉威西新生代岩漿活動的時空變化與地體意義

Abstract

南蘇拉威西位處西太平洋造山帶南緣，是亞洲大陸與澳洲大陸碰撞的最前緣。此造山帶由西向東展現了現在進行式的隱沒與碰撞共存地體，因此，位處碰撞最前緣的南蘇拉威西是研究隱沒至碰撞地體轉換及其相應岩漿活動特徵的首選之地。 南蘇拉威西的新生代岩漿活動依時空變化與地球化學特徵可分為三期、五個系列，依序為：分布於研究區域中心的始新世 (53-36 Ma) 鈣鹼性系列；廣泛出露於整個南蘇拉威西的晚中新世 (11-5 Ma) 低鈮超鉀質、低鈮鉀質與高鈮超鉀質鹼性系列；以及侷限於蘇拉威西最南端的更新世 (3-1 Ma) 高鈮鈉質鹼性系列。 始新世鈣鹼性系列具有高場力鍵結元素虧損 ((Nb/La)n =0.17-0.40) 、輕稀土元素富集 ((La/Yb)n =3.26-7.79) 、以及近似於中洋脊玄武岩的初始同位素組成 ((87Sr/86Sr)i =0.7035-0.7040) 等特徵；晚中新世低鈮超鉀質 (K2O/Na2O >2) 系列的高場力鍵結元素虧損程度高 ((Nb/La)n =0.10-0.20) 、輕稀土富集程度高 ((La/Yb)n =8.16-19.1) 、初始同位素接近全矽酸鹽地球 ((87Sr/86Sr)i =0.7060) ，低鈮鉀質 (K2O/Na2O >1) 系列帶有最強的高場力鍵結元素虧損訊號 ((Nb/La)n =0.13-0.32) 、相對低的輕稀土富集程度 ((La/Yb)n =4.66-21.3) 、以及接近中洋脊玄武岩的初始同位素組成 ((87Sr/86Sr)i =0.7044-0.7060) ，高鈮超鉀質系列則具低的高場力元素虧損程度 ((Nb/La)n =0.64-0.78) 、極高的輕稀土富集程度 ((La/Yb)n =17.9-19.5) 、以及亦接近全矽酸鹽地球的初始同位素組成 ((87Sr/86Sr)i =0.7062)；此外，更新世的高鈮鈉質系列則以高場力鍵結元素虧損程度低 ((Nb/La)n =0.19-0.64) 、輕稀土富集程度高 ((La/Yb)n =8.40-26.6) ，以及位於前兩期岩漿活動間的初始同位素比值 ((87Sr/86Sr)i =0.7044-0.7052) 為特徵。 本研究根據分析所得的年代及地球化學特徵，推斷始新世的岩漿活動應生成於陸弧環境，其岩漿生成源區為地函楔，屬於隱沒帶岩漿活動；晚中新世與更新世的岩漿活動則是班達板片兩度回捲造成的岩石圈地函熔融產物，前者顯著的表現了不均質岩石圈地函的特徵，後者不能排除有軟流圈地函的參與，然此二期岩漿活動皆屬碰撞帶岩漿活動。

Southern Sulawesi located in the southern part of the Western Pacific Orogenic belt, is an ideal natural laboratory for studying the geological processes involved in the transition from subduction to collision related to the interaction between the Eurasian and Australian plates. This study reports zircon U-Pb age and geochemical data of Cenozoic igneous rocks collected from Southern Sulawesi. The age data indicate 3 episodes of magmatism: (1) Eocene (53-36 Ma), (2) late Miocene (11-5 Ma), and (3) Pleistocene (3-1 Ma). The first episode occurred in the central part of our studied area, the second one erupted extensively, and the youngest one can only be found at the southernmost part of Southern Sulawesi. The geochemical data, however, define 5 series of magmatic rocks: (1) Eocene calc-alkaline (CA), (2) late Miocene low niobium ultrapotassic (LNb UK), (3) low niobium potassic (LNb K), (4) high niobium ultrapotassic (HNb UK), and (5) Pleistocene high niobium sodic alkaline (HNb Na). The Eocene CA samples are characterized by low (Nb/La)n (0.17-0.40), medium (La/Yb)n (3.26-7.79), and N-MORB-like (87Sr/86Sr)i (0.7035-0.7040). The late Miocene LNb UK rocks are highlighted by low (Nb/La)n (0.10-0.20), high (La/Yb)n (8.16-19.1), and BSE-like (87Sr/86Sr)i (0.7061). The LNb K rocks also show low (Nb/La)n (0.13-0.32) and high (La/Yb)n (4.66-21.3), but with N-MORB-like (87Sr/86Sr)i (0.7044-0.7060). Yet, the HNb UK rocks are featured by high (Nb/La)n (0.64-0.78), high (La/Yb)n (17.9-19.5), and BSE-like (87Sr/86Sr)i (0.7062) instead. Besides, the Pleistocene HNb Na series with high (Nb/La)n (0.19-0.64) and high (La/Yb)n (8.40-26.6) have (87Sr/86Sr)i between former two episodes (0.7044-0.7052). Based on age and geochemical results, we attribute the magma sources of Eocene rocks to a mantle wedge, of late Miocene rocks to a heterogeneous lithospheric mantle, and of Pleistocene rocks to both lithospheric and asthenospheric mantle. The Eocene magmatism might occur in a subduction-zone setting, and the late Miocene and Pleistocene magmatic activities were probably produced by the rollback of Banda slab which caused the opening of Banda basin during the Neogene in a collision-zone setting.