印尼東側達伊島漸新世島弧岩漿活動的年代與地球化學特徵

Abstract

在班達隱沒系統中韋伯海盆普遍被認為是個從3 Ma開始張裂形成的弧前盆地，而達伊島即位在韋伯海盆南緣；達伊島主要由第四紀的石灰岩與輝長岩類組成，後者在沒有定年資料的情況下被部分文獻認為是韋伯海盆延伸的產物。本文將首次報導達伊島上火成岩的鋯石鈾鉛定年(N =19)、全岩鍶-釹同位素(N = 13)以及鋯石鉿同位素分析(N = 15)的結果，還有全岩的主量以及微量元素的資訊，其中包括輝長岩(N = 38)、安山岩(N = 4)、花崗閃長岩(N = 4)、流紋岩(N = 1)以及花崗岩(N = 3)。根據達伊島火成岩礦物組成、全岩主量及微量元素的結果可將其分成兩類：晶堆岩以及非晶堆岩，前者係指早期結晶並堆積在岩漿部底部的材料，並且具有高Mg# (Mg# > 70)和/或斜長石富集(Eu/Eu* > 1.5)的特徵，後者則可細分為矽質岩石(N = 14)以及鈣鹼岩石(N = 1)，這兩種岩石在蛛網圖中皆出現Ta、Nb、Ti虧損的情形，指示了岩石成因與隱沒作用相關；此外，分析的結果還表明鈣鹼岩石的輕稀土元素富集且全岩鍶-釹同位素顯示有其有受到陸源物質影響的跡象，至於矽質岩石則相反地呈現平坦的稀土元素或輕稀土虧損的情形，而全岩鍶-釹同位素與鋯石鉿同位素的結果皆指出矽質岩石的岩漿源區具有虧損地函的性質；另外，鋯石鈾鉛定年的結果指出這些火成岩具有兩種年代特性：(1)以岩漿鋯石為主以及(2)以繼承鋯石為主，其中以岩漿鋯石為主的岩樣(N = 15)年代介於32 ~ 25 Ma之間，並且集中在ca. 30與26 Ma，此類岩石涵蓋了晶堆岩、鈣鹼岩石以及基性及酸性矽質岩石，至於以繼承鋯石為主的樣本則僅有晶堆岩及中性的矽質岩石(N = 4)；此外，本文亦有針對基性岩(N = 2)進行礦物相的觀察以及礦物成分分析，結果表明這些基性岩中出現高鎂普通角閃石和陽起石等角閃石族礦物富集的情形，且在背向散射電子影像中亦發現高鎂普通角閃石受蝕變影響而漸變成陽起石的證據，另一方面根據相圖所提供角閃石與輝長岩共存的壓力條件可推斷這些角閃石是在中、下部地殼中由含水的熔體結晶而來。根據鋯石鈾鉛定年的結果可知達伊島火成岩之岩石成因與韋伯海盆的張裂並無關係，而全岩化學分析的亦顯示達伊島之矽質岩石符合島弧矽質系列岩漿的特徵，即這些火成岩皆屬漸新世島弧岩漿活動的產物。

Dai Island is situated at the southern extension of the Weber Deep, a forearc basin that started rifting in the Banda subduction system in 3 Ma. The island is composed mainly of Quaternary limestones and gabbroic rocks, the latter were widely related to the Weber deep rifting. Here this study reports the first set of zircon U-Pb ages (N = 19) and Hf isotope ratio (N = 15), whole-rock Sr-Nd isotope signatures of igneous rocks from Dai (N = 13), and together with whole-rock major and trace element data, including gabbro (N = 38), andesite (N = 4), granodiorite (N = 4), rhyolite (N = 1) and granite (N = 3). Igneous rocks in Dai can be divided into 2 groups by mineral assemblages, major and trace element data: cumulates and non-cumulates. Cumulates are considered as the products of early-stage crystallization and deposited in the bottom of the magma chamber, furthermore, cumulates show low contents of SiO2, REE concentration, and high-Mg (Mg# > 70) and/or Eu (+) anomaly (plagioclase-bearing and Eu/Eu* > 1.5). The major element data indicate that non-cumulates are either tholeiitic or calc-alkaline composition, with the former named “Tholeiitic rocks” (TH; N = 14) and the latter “Calc-alkaline rock” (CA; N = 1). The tholeiitic and calc-alkaline rocks are relative depletion of Ta-Nb-Ti in the spidergram, indicating a subduction-related petrogenesis. Furthermore, CA is LREE-enriched, coupled with (87Sr/86Sr)i = 0.7068 and ℇNd(t) = −10, which indicates that it may be affected by continental materials. Additionally, zircon Hf isotope data, whole-rock trace element data, and Sr-Nd isotope data of tholeiitic rocks show flat-REE or LREE-depleted, and the features of the depleted mantle source. Our zircon U-Pb ages also suggest that the rock samples mainly contained magmatic zircons, including cumulates, calc-alkaline rock and mafic and felsic tholeiitic rocks, are 32 ~ 25 Ma, and that inherited zircons commonly appear in cumulates and intermediate tholeiitic rocks (N = 4). Moreover, this study also reports the result of thin section observation and mineral composition analysis in mafic rocks (N = 2), the results show that there are lots of amphibole group minerals, including magnetism hornblende and actinolite, enriched in mafic rocks, and that actinolites were formed by the alteration of magnetism hornblende. In accordance with the phase diagram of gabbro and amphibole, these amphiboles were crystallized from water-saturated magma in the middle and lower crust. According to zircon U-Pb ages, whole-rock Sr-Nd isotope, major and trace element data and zircon Hf isotope results, we can learn that the igneous rocks in Dai island were not the products of Weber Deep rifting, but mostly an island arc tholeiitic rocks, that is, this study considers that most of the igneous rocks in Dai are the relict of a Oligocene island arc.