Dynamics and age of formation of the Seram-Ambon ophiolites (Central Indonesia)

Abstract

Abstract. -The Seram-Ambon ophiolitic series comprise peridotites, websterites, gabbros and lavas. Petro-geochemical data show that the peridotites are weakly depleted rocks, except for the rare Cpx-free harzburgites. They underwent a sub-solidus metamorphic re-equilibration in the plagioclase field. The associated websterites and gabbros display various chemical features, allowing to define 3 types of websterites and 2 groups of gabbros. They have mostly BAB characteristics (presence of negative anomalies in Nb, Zr, Ti and Y), except the group 2 gabbros which have N-MORB features and the type 3 websterites which bear adakitic affinities. Lavas also display a variety of compositions, including high-Mg IAT and Mg-rich BABB with sub-alkaline affinities. Both IAT and BABB display high Th/Nb ratios which support an origin close to a continental crust environment.Our 20 to 15 Ma 40 K/ 40 Ar ages calculated for the BABB and 15-9 Ma for the IAT show that the basin and arc formed in a very short span of time, before their obduction 9-7 Ma ago [Linthout et al., 1997]. Considering the paleogeographic situation in the Miocene [Haile, 1979 ;Haile, 1981] and our data, we propose that the Seram-Ambon ophiolites formed during the early Miocene in a small, short-lived (10 Ma), transtensive basin bordered on its east by an active margin and on its western part by a passive continental margin over which it was later obducted towards the SW direction. Dynamique et âge de formation des ophiolites de Seram-Ambon (Indonésie centrale)Mots-clés. -Asie du Sud-Est, ophiolite, tholéiites riches en MgO, basaltes arrière-arc, rifting transtensif.Résumé. -L'île de Seram-Ambon est située au nord de la mer de Banda entre les îles de Sulawesi et de Nouvelle Guinée (fig. 1A). Sa position actuelle montre que son évolution géotectonique est et a été fortement liée, depuis le milieu du Cé-nozoïque, aux mouvements respectifs de la plaque Australienne qui remonte vers le NNW à raison de 7 cm.an -1 et de la plaque Philippine qui se déplace vers l'ouest à une vitesse d'environ 10 cm.an -1 [Minster et Jordan, 1978 ;De Mets et al., 1990].Plusieurs fragments d'ophiolites y sont présents. Ils sont pour l'essentiel localisés dans la partie ouest de l'île de Seram ( fig. 1B), sur les petites îles de Kelang situées à l'ouest et sur l'île d'Ambon présente au sud-est [Tjokrosapoetro et Budhitrisna, 1982] (fig. 1C). Ces trois secteurs réunis présentent tous les termes classiques d'une séquence ophiolitique, avec des péridotites, pyroxénites, gabbros et basaltes, mais aucun d'entre eux ne présente la totalité des termes d'une série ophiolitique complète. Les péridotites sont essentiellement des lherzolites à plagioclase, des harzburgites à spinelle riches en Cpx (3-3,5 %) pouvant contenir jusqu'à 2,2 % de plagioclase, et plus rarement des harzburgites sans Cpx qui contiennent quelques rubannements d'orthopyroxénites. Ces péridotites sont localement recoupées par des filonnets de chromitites, par des filons ou poches de webstérites de compositions chimiques variée...

Extreme extension across Seram and Ambon, eastern Indonesia: evidence for Banda slab rollback

“…This is by far the largest granitoid body observed on Seram or Ambon, and so it is strange that it was omitted from the latest GRDC map (Tjokrosapoetro et al, 1993a) and was not acknowledged by some subsequent studies that used this map (e.g. Monnier et al, 2003). The granite is almost identical to that previously described from Seram, and is also characterised by abundant schlieren of granulite residuum.…”

“…5). It has been a major focus of several previous studies (Schroeder van der Kolk, 1902;de Jong, 1923;Valk, 1945;Milsom, 1979;Milsom et al, 2001;Helmers et al, 1989;Linthout et al, 1989Linthout et al, , 1996Linthout and Helmers, 1994;Monnier et al, 2003;Martini et al, 2004) owing to the close association of well-exposed ultramafic rocks, large Mio-Pliocene granitoid bodies, and the Tehoru Formation and Taunusa Complex "metamorphic basement". The peninsula was first mapped in detail by Rutten and Hotz (1919).…”

“…The nature of the contact between these metamorphic units and the ultramafic complex to their south has previously been described as a SSW-dipping thrust above which the ultramafic complex was thought to have been obducted (e.g. Linthout et al, 1989Linthout et al, , 1996Linthout and Helmers, 1994;Monnier et al, 2003). In this scenario, the ultramafic complex was therefore considered as the base of an ophiolite with the schists and gniesses thought to represent its metamorphic sole, and the cordierite granite produced through sub-ophiolite anatexis before being emplaced into the over-riding peridotite.…”

Along‐ridge petrological segmentation of the mantle in the Oman ophiolite

“…Normalization values are from Sun and McDonough [1989]. patible elements, such as in Ti or Na [Allan and Dick, 1996;Arai et al, 1997;Dick and Natland, 1996;Girardeau and Francheteau, 1993;Hekinian et al, 1993;Kelemen, 1990;Monnier et al, 2003]. In the studied peridotites, no significant enrichment in Ti has been observed either in spinels (TiO 2 maximum value: 0.17% versus 0.4% to 0.5% at Hess Deep) or in the other phases, Cpx for instance (Table 4; see other mineral compositions in Tables 2, 3, and 5).…”

“…The theoretical evolution after partial melting [Arai, 1994a] shows a trend beginning from the Fertile MORB Mantle (FMM). Domains represent mineral compositions of fore-arc harzburgites [Ishii et al, 1992], abyssal peridotites [Dick and Bullen, 1984], and back-arc Cpx-free harzburgites [Monnier et al, 1995[Monnier et al, , 1999a[Monnier et al, , 1999b[Monnier et al, , 2000[Monnier et al, , 2003. magnesium (average MgO R % 47.1%, meaning 2.1% loss) and iron (average FeO* R % 8.5%, meaning 0.4% loss) losses and relative Si, Al, Ca and Cr enrichment (respectively +1.1%, +0.5%, +0.6% and +0.3%).…”