Arm Of Sulawesi Research Articles

Hydrothermal Alteration Mineralogy Associated with Gold Mineralization in Buladu Area, Gorontalo, Northern Sulawesi, Indonesia

Geologically, the Buladu area is part of the North Sulawesi magmatic arc, located in the morphology of the mountains along the coastline of North Gorontalo district, Indonesia. Multiple subductions of North Sulawesi in the northern arm of Sulawesi and east Sangihe subduction in the east has triggered an active magmatism and volcanism. Formation and occurrences deposits of precious- and base metals have been found in the North Sulawesi Volcanic Arc. This paper describes a recent study on hydrothermal alteration mineralogy associated with one of the prospects in the region, namely Buladu gold prospect that for years has been operated by local artisanal miners. The study is focused on hand specimen and microscopic observation as well as XRD analysis. Lithologically, the study area is arranged by sandstone are members of Dolokapa volcano-sedimentary formation, above unconformity andesitic-basaltic volcanic breccia are members of the Pliocene Wobudu Formation and is intruded by granodiorite rocks. The petrographic study reveals that host rock of the mineralization is flow-volcanic breccia which shows porphyritic and flows structures. Resulting assemblages of secondary minerals are mineral associations characteristic of the magmatic-hydrothermal system the following imperative alteration zones could be recognized an outer zone of chloritic alteration and the inner zone of argillic alteration. The alteration showed characteristics of pervasive to selective alteration, where clay and quartz totally replaced groundmass, whereas chlorite and clay minerals were selectively altered plagioclase phenocrysts. Clay mineral species identified by XRD include chlorite and illite. These mineralogical features indicate that the hydrothermal alteration is an argillic type, which is spatially distal to the mineralization. Mafic mineral in andesite lava partly altered into mineral assemblage chlorite-epidote-quartz-carbonate show propylitic type, which is the outermost zone of mineralization.

LA-ICP-MS zircon U–Pb and muscovite K–Ar ages of basement rocks from the south arm of Sulawesi, Indonesia

The zircon U–Pb and muscovite K–Ar age from the Bantimala, Barru and Biru basement complexes in the South Arm of Sulawesi, Indonesia provide new information regarding the timing of magmatism, metamorphism and sedimentation in this region and have implications for the origin and evolution of the study area. The study area is at the juncture between the southeast margin of Sundaland and Bird's Head-Australia. The age of both the zircon U–Pb of detrital materials in the Bantimala Complex and the muscovite K–Ar of amphibolite in the Biru Complex fall in the Late Early Cretaceous (between 109 and 115Ma), which is a similar age range to previous data for both the sedimentary and metamorphic rocks. The youngest detrital zircon in the schist samples from the Barru Complex fall into the Triassic in age (between 243 and 247Ma). These age data indicate that the protolith of all three basement complexes were involved in the subduction system and metamorphosed in the late Early Cretaceous, but there are several differences in their deposition environment under and out of the influence of the late Early Cretaceous magmatism in the Bantimala and Barru Complexes, respectively. Felsic igneous activities are confirmed in the Late Cretaceous and the Eocene by the zircon U–Pb age of igneous rocks intruding or included as detrital fragments in three basement complexes. These dates are similar to those reported from the Meratus Complex of South Kalimantan. The detrital zircon age distributions of the basement rocks in the South Arm of Sulawesi display predominant Mesozoic (Cretaceous and Triassic) and Paleozoic populations with a small population of Proterozoic ages supporting the hypothesis that the West Sulawesi block originated from the region of the circum Bird's Head-Australian, namely the Inner Banda block. The absence of Jurassic zircon age population in the South Arm of Sulawesi suggests the division of the South Arm of Sulawesi from the Inner Banda block in early stage of rifting. Western Sulawesi is composed of several blocks separated from Inner Banda block with different histories, which is supported by the varieties of zircon population distribution in the basement rocks in the Western Sulawesi and also difference of general orientations of structural features between the Bantimala and Barru Complexes.

Miocene to recent extension in NW Sulawesi, Indonesia

The Malino Metamorphic Complex (MMC) in the western part of the North Arm of Sulawesi (Indonesia) has previously been suggested to be a metamorphic complex exhumed in the Early – Middle Miocene. This idea was based on limited K–Ar and 40Ar/39Ar age data, but no structural data were presented to provide evidence for the mechanism of exhumation. Here we present new field observations, micro-structural analyses and a revised stratigraphy of NW Sulawesi based on new age data, to provide better constraints on the timing and mechanism of exhumation. The data presented here suggest that the MMC is a metamorphic core complex which underwent lithospheric extension during the Early – Middle Miocene. Although the MMC experienced significant extension, there is no evidence that it was exhumed during this time. There is no contact between the MMC and the Pliocene Ongka Volcanics, contradicting a previously inferred unconformable contact. Pliocene undeformed granitoids intruding the MMC indicate the complex was still at depth during their emplacement. Furthermore, Pliocene and Pleistocene cover sequences do not contain metamorphic detritus. A second phase of extensional uplift was accommodated by brittle faulting from the Late Miocene-Pliocene onwards, during which the MMC was exhumed. This extension is widespread, as indicated by synchronous exhumation of the adjacent Palu Metamorphic Complex in West Sulawesi, and rapid subsidence offshore in Gorontalo Bay. It is linked to northward slab rollback of the southward-subducting Celebes Sea since the Pliocene. GPS data show rapid northward motion of the North Arm of Sulawesi with respect to the Celebes Sea, indicating that this process is ongoing at present day.

Hydrothermal alteration and timing of gold mineralisation in the Rumbia Complex, Southeast Arm of Sulawesi, Indonesia

The Rumbia Mountains, which in this study named Rumbia schist Complex is an east-west oriented, composed by a high-pressure/low-temperature, and a medium-pressure/low-temperature metamorphic rocks. Identified as mica schist, glauchopane schist, and green schist. Rumbia complex known as the location of gold deposits prospects discovered by local communities since 2007. The results of research showed that the metamorphic rocks are as hosts. There are two phase of gold mineralization that occurs in this area, namely: 1) Associated with tectonic deformation and metamorphic rocks exhumation, and 2) gold-related hydrothermal deposits. Radiometric age dating used 40Ar/39Ar geochronology, indicate that the first of gold mineralisation in the Rumbia Complex occurred ∼23 million years ago, and the second gold mineralisation were subsequently overprinting at 7 million years ago.

Preliminary Result of The Influence of Earthquake Stress Change and The Implication for Soputan Volcano and Lokon Volcano

On February 6, 2016, an eruption occurred on the Northern Sulawesi arm, particularly Soputan volcano. One day earlier, Lokon volcano located close to Soputan volcano was decreased its status from standby to alert level by the Center for Volcanology and Geological Hazard Mitigation (CVGHM). The different reactions of two volcanoes proposed the question why the increment activity just happened in Soputan volcano. This uniqueness made us to suggest that static stress of earthquake may control the magmatic systems. We investigate here the earthquake-volcanism interaction through static stress changes by using Coulomb failure stress associated with an earthquake occurred in the Northern Molluca Sea on 25 November 2015. We slice the same dip for the each region in vertical cross sections. Therefore, the Coulomb failure stress pattern can be investigated beneath the study area. Our results suggest that Coulomb failure stress was increased by 0.3 × 10-3 to 0.4 × 10-3 bar below the Soputan’s region. Lokon’s region, the stress was reduced by -0.1 × 10-3 to -0.4 × 10-3 bar. The positive change may perturb magma overpressure leading to eruption and promoted volcanic earthquakes. The situation was very different that Lokon volcano ran into reduction activity and volcanic earthquakes were discourage due to stress shadow. We show that the difference volcanic response were likely controlled by static stress of the earthquake.

Bats (Chiroptera) recorded in the lowland of Southeast Sulawesi, Indonesia with notes on taxonomic status and significant range extensions

Abstract This paper reports on a bat survey conducted in November 2011 in Mangolo Nature Park and Rawa Aopa Watumohai National Park, both lowland forests located in Southeast Sulawesi. We recorded 22 species of bats that represents nearly 1/3 of the total bat species known to occur on Sulawesi. Three of these are endemic to Sulawesi and adjacent islands, whereas one species, Myotis cf. ridleyi was identified as a new distributional record for this island and with further investigation could prove to be an undescribed species. Our record of Chironax melanocephalus tumulus provided a range extension to the southeastern arm of Sulawesi. Two specimens of Hipposideros boeadii were topotypes and represent the first collections after the description of the type specimen. Collections of Rhinolophus arcuatus from this survey were only the second record of this species from island and represent a range extension. Specimens of Megaderma spasma celebensis were the first records of this species from Southeast Sulawesi. Species are discussed individually with external, cranial and dental measurements summarized. Based on this survey, the number of bat species now documented from the lowlands of Southeast Sulawesi represents the highest diversity yet recorded from a site on Sulawesi. This region is therefore a high priority for conservation and a hotspot for bat research in Indonesia, especially Sulawesi.

Petrology, geochemistry and tectonic significance of serpentinized ultramafic rocks from the South Arm of Sulawesi, Indonesia

Serpentinized ultramafic rocks occur in two separate basement complexes in the South Arm of Sulawesi, the Bantimala and Barru Blocks. We present petrographic, mineral chemical and geochemical data for these rocks, and interpret them in terms of petrogenesis and tectonic setting. The rocks of both blocks show strong serpentinization of original anhydrous silicates. The Bantimala ultramafics consist mainly of peridotite (harzburgite and dunite) and clinopyroxenite, with lenses of podiform chromitite. Metamorphism is evidenced by the occurrence of amphibolite-facies tremolite schist. In contrast, the Barru ultramafics consist of harzburgite peridotite and podiform chromitite, which also show an amphibolite-facies overprint that in this case may be related to intrusion by a large dacite/granodiorite body. Whole-rock trace element analyses and spinel compositions show that the Barru harzburgite is depleted relative to primitive mantle, and has had some melt extracted. In contrast, the Bantimala dunite, harzburgite and clinopyroxenite are cumulates. Both are derived from a supra-subduction zone environment, and were obducted during the closure of small back-arc basins. If there has been no rotation of the blocks, then the Bantimala ultramafics were emplaced from an ENE direction, while the Barru ultramafics were emplaced from the WNW. The ultramafic suites from these two blocks are juxtaposed with metamorphic assemblages, which were later intruded by younger volcanics, particularly in the Barru Block.

The age of undeformed dacite intrusions within the Kolaka Fault zone, SE Sulawesi, Indonesia

We present petrologic, geochemical and U–Pb sensitive high resolution ion microprobe (SHRIMP) data from previously undocumented dacite intrusions from the SE Arm of Sulawesi. The dacites occur in a strand of a major fault (the Kolaka Fault) that crosses the SE Arm of Sulawesi and northern Bone Bay. U–Pb SHRIMP dating shows the “Kolaka Dacite” yields zircon grains and overgrowths that range between ca. 4 and 7Ma, indicating active magmatism in SE Sulawesi at this time. The youngest age population (4.4±0.2Ma) from this range is interpreted to be the maximum crystallization age for the dacite. The Kolaka Dacite is undeformed, and so potentially intruded during or after movement within a strand of the Kolaka Fault. The dacites may have otherwise been emplaced passively along existing foliation planes in the country rock schist. Additional U–Pb data were collected from inherited zircons, yielding ages between 8Ma and 1854Ma. We consider that these inherited zircons are xenocrysts, derived from either (1) a partially melted protolith and/or (2) xenocrysts assimilated during ascent of the magma. In either case, the inherited zircons record the age of the basement rocks beneath this part of SE Sulawesi. These inherited zircon cores show that the SE arm of Sulawesi is underlain by Proterozoic or younger material, validating earlier ideas that the crust here was derived from Gondwana.

Morphostructure Control Towards the Development of Mahawu Volcanic Complex, North Sulawesi

DOI: 10.17014/ijog.v7i1.134The studied area, situated in northeastern part of North Sulawesi Arm, is dominantly occupied by the Mahawu, Linau, Tompusu, and Kasurutan volcanic rocks. Using remote sensing data, such as landsat image, black and white panchromatic aerial photograph, and IFSAR image, morphology-origin unit and morphology lineament can be interpreted. Four morphology-origin units, those are Mahawu Volcano Complex, Intra-montane Plain structure, Linau Volcano Complex, and Lacustrine Plain are recognized. Furthermore, morphological lineament pattern was statistically processed to find out the general stress direction in the area to determine the probability of the structural morphology occurrence in the Mahawu Volcano Complex. The result shows that generally the development pattern of volcanic cones are irregular, except the Mahawu Volcano Complex showing a linear pattern. This lineament pattern is interpreted as a NW - SE fault pattern controlling the rise of magma. At least, two tectonic and two eruption periods occurred regularly at different time from the Quaternary age till the present.

Deep Sea Sediment Gravity Flow Deposits in Gulf of Tomini, Sulawesi

Micro plate collision against the Eastern Arm of Sulawesi since Pliocene has resulted in a major supply of terigenous sediments into Late Miocene rift-basins in Gulf of Tomini. Studies on offshore multi-channel seismic reflection data complemented by published on-land geological data indicate a series of tectonic events that influenced the depositional system in the Gulf of Tomini, During the Late Neogene, alternating pulses of terigenous sediments were deposited in the basins in the form of deep-sea slump-turbidite-pelagic sediments. A sediment gravity flow deposit system at the slope and the base of the basins changed gradually into a deep-sea pelagic fill system toward the center of the basins. Three tectono-stratigraphy sequences (A, B, and C) separated by unconformities indicating the Late Neogene history and the development of the basins were identified. These tectonic processes imply that the earlier sediments in the Gulf of Tomini are accomplished by a differential subsidence, which allows a thickening of basin infill. The Pliocene-Quaternary basin fill marks the onset of a predominant gravity flow depositional system.

Tectonic re-interpretation of the Banggai-Sula–Molucca Sea margin, Indonesia

Abstract High resolution multibeam bathymetric and seismic data from the area north of the Banggai-Sula Islands, Indonesia, provide a new insight into the geological history of the boundary between the East Sulawesi ophiolite, the Banggai-Sula microcontinent and the Molucca Sea collision zone. Major continuous faults such as the Sula Thrust and the North Sula–Sorong Fault, previously interpreted to bound and pass through the area are not seen. The south-verging Batui Thrust previously interpreted offshore to the east of Poh Head cannot be identified. In the areas where the thrust was interpreted there is a north-vergent thrust and fold zone overlain by almost undeformed sediments. Gently dipping strata of the Banggai-Sula microcontinent margin can be traced northwards beneath younger rocks. In the east, rocks of the Molucca Sea collision complex are deformed by multigenerational folds, thrusts and strike-slip faults. There is a series of small thrusts between the leading edge of the collision complex and the foot of the slope. In the west a zone of transpression close to the East Arm of Sulawesi is the termination of the dextral strike-slip Balantak Fault extending east from Poh Head.

Petrologic, isotopic, and radiometric age constraints on the origin and tectonic history of the Malino Metamorphic Complex, NW Sulawesi, Indonesia

The Malino Metamorphic Complex (MMC) is located at the western end of the north arm of Sulawesi. It consists of mica schists and gneisses (derived from proximal turbidite and granitoid protoliths), with intercalations of greenschist, amphibolite, marble, and quartzite, forming an E-W elongated dome-like structure bounded on all sides by faults. The age of the MMC is constrained between Devonian and Early Carboniferous. This Paleozoic age, the presence of Archean and Proterozoic inherited zircons, and the isotopic signature of the mica schists and gneisses indicate that the terrane was derived from the New Guinea-Australian margin of Gondwana. Similarities with basement rocks in the Bird’s Head suggests a common origin. Greenschists forming a discontinuous selvage (metamorphic carapace) around the complex were derived from adjacent autochthonous Paleogene formations. The rocks of the MMC show a Barrovian-type progression from greenschist through epidote-amphibolite to amphibolite facies. P–T estimations suggest a depth of burial of up to 27–30 km. K/Ar and 40Ar/39Ar cooling ages of 23–11 Ma, and a 7 Ma age for unconformably overlying volcanic rocks, indicate that the complex was exhumed during the Miocene. Two tectonic scenarios are considered: 1. the continental fragment docked with Sulawesi during the Mesozoic and was exhumed as a metamorphic core complex during the Miocene; 2. it was subducted beneath the north arm during the late Oligocene and then rapidly returned back to the surface.

Stratigraphy and tectonic setting of the Cretaceous and Paleogene volcanic-sedimentary successions in northwest Sulawesi, Indonesia: implications for the Cenozoic evolution of Western and Northern Sulawesi

New field and laboratory data from NW Sulawesi, Indonesia, integrated with existing information, provide a tectonostratigraphic framework for the Cretaceous and Paleogene of this region. The study area straddles two distinct provinces, referred to as ‘Western’ and ‘Northern Sulawesi’. Western Sulawesi forms the (rifted) continental margin of eastern Sundaland. It consists of a metamorphic basement, partly of Australian origin, overlain by Late Cretaceous turbidites that were deposited in a fore-arc setting (Latimojong Formation). These are in turn covered by volcanic-sedimentary successions that were deposited during the early Middle Eocene to earliest Miocene. They represent a transgressive cycle from syn-rift siliciclastics, through nummulitic limestone and associated shelf sediments, to deeper marine mudstones and turbidites (Budungbudung and Tinombo Formations). During the same period Northern Sulawesi developed as an oceanic island arc, characterized by bimodal volcanism (Papayato Volcanics), founded on (back arc?) oceanic crust. The Cretaceous and Paleogene volcanic and sedimentary suites show lithological and geochemical characteristics that reflect the contrasting tectonic setting of the two provinces. The tectonic relationship between the two domains is not clear. They probably formed a more or less continuous belt throughout the Cenozoic, but were definitely connected not later than the early Miocene. Strong deformation in the Paleogene formations in the northern part of NW Sulawesi and their unconformable relationship with the overlying formations may be the result of the collision of the north arm of Sulawesi with a continental fragment, of Australian derivation, during the early Miocene, or it may be related to the formation of a metamorphic core complex in a mid-Miocene extensional tectonic setting. A second major tectonic event, which commenced in the Pliocene and is still ongoing, affected the entire region.

Evolution of an accretionary complex along the north arm of the Island of Sulawesi, Indonesia

Abstract Seismic reflections across the accretionary prism of the North Sulawesi provide excellent images of the various structural domains landward of the frontal thrust. The structural domain in the accretionary prism area of the North Sulawesi Trench can be divided into four zones: (i) trench area; (ii) Zone A; (iii) Zone B; and (iv) Zone C. Zone A is an active imbrication zone where a decollement is well imaged. Zone B is dominated by out‐of‐sequence thrusts and small slope basins. Zone C is structurally high in the forearc basin, overlain by a thick sedimentary sequence. The subducted and accreted sedimentary packages are separated by the decollement. Topography of the oceanic basement is rough, both in the basin and beneath the wedge. The accretionary prism along the North Sulawesi Trench grew because of the collision between eastern Sulawesi and the Bangai–Sula microcontinent along the Sorong Fault in the middle Miocene. This collision produced a large rotation of the north arm of Sulawesi Island. Rotation and northward movement of the north arm of Sulawesi may have resulted in southward subduction and development of the accretionary wedge along North Sulawesi. Lateral variations are wider in the western areas relative to the eastern areas. This is due to greater convergence rates in the western area: 5 km/My for the west and 1.5 km/My for the east. An accretionary prism model indicates that the initiation of growth of the accretionary prism in the North Sulawesi Trench occurred approximately 5 Ma. A comparison between the North Sulawesi accretionary prism and the Nankai accretionary prism of Japan reveals similar internal structures, suggesting similar mechanical processes and structural evolution.

Present-day stress and deformation field within the Sulawesi Island area (Indonesia) : geodynamic implications

Present-day stress and deformation field within the Sulawesi Island area (Indonesia) : geodynamic implications

Stratigraphy, sedimentation and palaeogeographic significance of the Triassic Meluhu Formation, Southeast arm of Sulawesi, Eastern Indonesia

The Triassic Meluhu Formation, which is widely distributed in the Southeast Arm of Sulawesi, consists of sandstone, siltstone, and mudstone, with limestone intercalations in the upper part. The formation is underlain by a basement of metamorphic rocks and is unconformable overlain by Palaeogene carbonates of the Tampakura Formation. The Meluhu Formation is divided into three members from the base to the top: the Toronipa Member, dominated by sandstone; the Watutaluboto Member, consisting of mudstone, siltstone and shale; and the Tuetue Member, characterised by the presence of marl and/or limestone. The Toronipa Member is interpreted to have been deposited by a meandering river system, which extended east–west. The environment in the depositional basin later changed to a tide-dominated delta system in which the Watutaluboto Member was developed. Subsidence of this tidal delta led to a shallow marine environment in which the Tuetue Member was deposited. Petrographic studies suggest that the source area was topographically rugged and occupied by metamorphic rocks, overlain by sandstone and volcanic rocks. These studies also suggest that the basin in which the Meluhu Formation was deposited was located in a humid tropical region with a high rainfall. The results of palaeomagnetic study shows that the formation underwent a clockwise rotation through about 25 °, and that the basin was located at a palaeolatitude of 20 °S. The Meluhu Formation, similar to the other Triassic sediments in eastern Indonesia, was deposited in a rift graben. The sediments were deposited during the early stages of rifting, graben formation and subsidence within the NW Australian continental plate. Subsequently, a continental fragment, including the Meluhu graben, separated from Australia to become an allochthonous terrane before colliding with Sulawesi.

Melting of lower continental crust in a young post-collision setting; a geochemical study of Plio-Quaternary acidic magmatism from central Sulawesi (Indonesia)

Abstract Acidic potassic calc-alkaline (CAK) magmas have been emplaced in the central part of the western arm of Sulawesi from 6.5 to 0.6 Ma, mostly as peraluminous dacites, rhyolites and granites. They overlay or crosscut a high-grade metamorphic basement including lower crustal garnet peridotites and granulites, the latter showing evidences for incipient melting during rapid uplift. Major and trace element data coupled with a Sr, Nd and Pb isotopic study of the CAK magmas and their lower crustal basement rocks demonstrate that they share a number of common features, including radiogenic Sr and Pb and unradiogenic Nd signatures, consistent with those of Australian granulites and Indian Ocean sediments. We propose that the CAK magmas derived from the anatexis of lower crustal rocks of Australian origin (the Banggai-Sula microcontinent) during the phase of uplift which followed their collision with the Sundaland margin (the western arm of Sulawesi) during the Middle Miocene, and possibly the breakoff of the subducted Molucca Sea slab.

The 1996 Earthquakes in Sulawesi, Indonesia

We study the rupture process of the 1 January ( Mw = 7.9), 16 July ( Mw = 6.6), and 22 ( Mw = 7) July 1996 earthquakes in Sulawesi, Indonesia. A teleseismic body waveform inversion of very broadband records shows that these events occurred beneath the accretionary prism. The main shock was due to a well determined fault with a strike of 53°N and a very shallow dip of 7°. From its source-time function duration (30 sec) and the aftershock distribution we estimate a rupture area of 90 × 60 km2 and an average slip of 1.80 m. The surface displacement computed with our best model fits well the displacement vector at the only available GPS station at Tomini. The tsunami generated by the mainshock had an approximate source radius of 45 km which roughly agrees with the rupture size estimated above. The earthquakes took place in a relay zone between the trench and the Palu-Koro transcurrent fault. They ruptured a shallow dipping thrust fault which corresponds to the subduction interface under the North Sulawesi arm. The slip vectors of all the events have a NNW orientation parallel to the direction of convergence between the North Sulawesi arm and the Celebes Sea. Among the peculiarities of the rupture process, we found that the July 22 aftershock was the only event to have a well defined precursor 1.8 sec before the main P -wave onset.

Continental block collision in the eastern arm of Sulawesi (Indonesia). Structure and geodynamic interpretation

Abstract Recent investigations in East Indonesia lead us to consider the eastern arm of Sulawesi as the result of a collision between two continental blocks: the Tokala block to the west and the Banggai–Sula block to the east. The Tokala block results from the Oligocene obduction of an ophiolitic Asiatic basin onto the passive margin of a Gondwanian block (Banda block), with collision with the Asiatic active margin (western arm of Sulawesi) near the end of the Oligocene or at the beginning of the Miocene. The Tokala Block was then collided by the Irian Jaya derived Banggai–Sula block in the Early to Middle Pliocene times or later.

Geochemical response to varying tectonic settings: an example from southern Sulawesi (Indonesia)

The South arm of Sulawesi was an active continental margin from approximately 60 to 10 Ma, when it collided with the microcontinental fragment of Buton. Pre-collisional samples analyzed for this study are characterized by a geochemical signature typical of arc volcanics: high LILE/HFSE ratios; 87Sr/ 86Sr slightly higher than MORB; 143Nd/ 144Nd ratios similar to MORB. Syn-collisional samples have more enriched isotopic signatures, and are relatively potassium rich. This is interpreted to reflect a larger contribution from subducted sediments, added to the mantle wedge as a silicic melt. Melting of subducted sediments is interpreted to result from a decrease in subduction rate and an increase of temperature in the slab. Magmatism that postdates the collisional event by 10 Ma is characterized by higher Nb/Y ratios than the pre- or syn-collisional samples, and Sr and Nd isotopic signatures intermediate between these two groups. This is likely to reflect melting of a subduction-modified mantle, with a significant contribution from the sub-continental lithospheric mantle. Comparison with post-collisional magmatism from other areas of the world suggests that trace element signatures are similar, but isotopic characteristics are variable. The latter are likely to reflect both the age of the sub-continental lithospheric mantle and the time lag between cessation of subduction and formation of the post-collisional magmas.