Geology of the Mutis Complex, Miomaffo, West Timor

Abstract

Mesozoic forearc assemblages are widespread across Indonesia, and these are also exposed in the overthrust terranes on Timor. The oldest rocks in the allochthonous Mutis Complex, West Timor are Mesozoic basaltic volcaniclastic rocks and melange containing blocks of normal mid-ocean ridge basalt (N-MORB) basalt (194 ± 5 Ma), amphibolite (metamorphism at 184 ± 6 Ma), garnet and actinolite-bearing schists, arkosic sandstone and volcanogenic sedimentary rocks. Based on the large component of Jurassic material, these rocks were probably derived from the Woyla terrane. The eastern part of the Miomaffo massif is complexly deformed with a dominant foliation overprinted by several fold and fault events including late normal cataclastic zones. This sequence is intruded by calc-alkaline andesitic dykes (possibly in the Eocene). The Central Sector of the massif is a block of high-strain greenschist facies rocks where the primary texture can no longer be recognised, but the bulk composition is the same as the volcaniclastic rocks in the east. Further west is an amphibolite facies metamorphic province. Amphibolite from the Western Sector have the same N-MORB composition as the basalt in the east, but the gneissic rocks have a higher proportion of arc-sourced protoliths. The peak metamorphic conditions were 650 °C and 0.9 GPa. This metamorphic event occurred at 37 Ma, reflecting subduction on the southeast margin of Sundaland, whereas the low-grade Mesozoic metamorphism of the Eastern Sector of the massif occurred in the accretionary prism along the Woyla Arc. The Mutis Complex at Miomaffo demonstrates the complex geological history of Mesozoic rocks in eastern Indonesia.

KEY POINTS

1. The Miomaffo massif demonstrates the strong oceanic character of the Banda terrane.

2. The anomalous Eocene metamorphic age of rocks ‘unconformably overlain’ by Cretaceous sediments is resolved.

Keywords:

• Timor
• Banda terrane
• metamorphic age
• MORB
• Cretaceous melange
• Mutis Complex

Introduction

The island of Timor was formed by the collision of the northwest margin of the Australian continent with the Banda Arc (Figure 1). Most modern reconstructions suggest that a thin passive margin sequence entered the trench at ca 5 Ma (Spakman & Hall, 2010). The onshore geology is consistent with this collision age (Berry et al., 2016; Duran et al., 2020; Keep & Haig, 2010). During the collision, an allochthonous terrane was thrust over the Australian continental margin and is now widely distributed across Timor (e.g. Audley-Charles, 1968; Earle, 1981; Haig et al., 2019; Harris, 2006; Sopaheluwakan et al., 1989). There has been extensive discussion of which units on Timor are part of the allochthonous terrane, but the list of probable allochthonous lithologies has largely converged in recent papers (Haig et al., 2019; Harris, 2011). These are known as the ‘lithotectonic units of the Banda terrane in Timor’ (Harris, 2006) or the ‘Overthrust Terrane Association’ (Haig et al., 2019). The allochthonous units are referred to here as the Banda terrane and include:

Figure 1. Location of Timor at the collision margin of Australia with Indonesia. Australian crust in orange. Sundaland and associated island arcs in green.

• Eocene to earliest Miocene siliciclastic, volcaniclastic and volcanic rocks; with isolated shelf carbonate buildups (includes Metan Formation in West Timor).

• Early Cretaceous to early Miocene deep-water carbonate.

• Late Mesozoic to Eocene outer shelf to bathyal siliciclastics (Palelo Group) including turbidites and debris-slide conglomerates (Haulesi Formation) and the carbonate chert association of the Noni Formation.

• Metamorphic rocks of the Lolotoi Complex (East Timor) and Mutis Complex (West Timor) referred to here as the Mutis Complex.

The present study concentrates on the rocks of the Miomaffo area (Figure 2) that are largely composed of the Mutis Complex and Palelo Group (Sopaheluwakan et al., 1989; van West, 1941). Early workers interpreted the Mutis Complex as basement to the Cretaceous Palelo Group (Haile et al., 1979; van West, 1941), and this concept has confused the interpretation. Standley and Harris (2009) recognised the observation that the Cretaceous Palelo Group unconformably overlies the Mutis Complex is not consistent with the Eocene metamorphic age reported for medium-grade metamorphic rocks of the Mutis Complex at Mosu, Boi, Mollo, Mutis, Usu, Lacluta and Bebe Susa massifs (Harris, 2006; Standley & Harris, 2009). The main aim here is to explain this discrepancy using the Miomaffo massif as an example. The stratigraphic relationships of Banda terrane at Miomaffo are shown in Figure 3.

Figure 2. Distribution of Banda terrane elements shown in pink (red for Miomaffo) based on Harris (2006).

Figure 3. Stratigraphy of the Banda terrane. Modified from Harris (2006).

One component of the eastern Sundaland basement not commonly reported in the Banda terrane is Mesozoic melange, despite this material being widespread across Sundaland (Wakita, 2000). Melanges are recognised in central Java, Sulawesi and Laut (South Kalimantan) islands. The melange from these locations typically has blocks of sandstone, siliceous shale, chert, limestone, basalt, rhyolite, schist and amphibolite in a foliated matrix. Metamorphic blocks have K/Ar mica ages in the range 180–110 Ma. Chert blocks contain radiolaria ranging in age from Middle Jurassic to Early Cretaceous (175–100 Ma). No rocks older than Jurassic were reported from these complexes by Wakita (2000), but Late Triassic to Early Jurassic protolith ages (205–185 Ma) were reported in the Bantimala Complex, South Sulawesi by Böhnke et al. (2019) and in the Woyla terrane, Sumatra (Barber, 2000). Given that the other Mesozoic lithologies of eastern Sundaland are widely reported in the Banda terrane, the lack of a distinctive melange element is surprising, but it may, in part, be due to difficulty in recognising melange due later deformation events and potential confusion with the Mio-Pliocene Bobonaro melange unit, widespread in Timor, despite the Bobonaro melange having a scaley clay matrix and containing clasts of autochthonous rocks (Barber et al., 1986; Harris et al., 1998).

This study presents new data from the Miomaffo area, West Timor that provides additional insight into the nature of the Banda terrane. Evidence is presented for a Mesozoic melange origin for part of this unit. The structural relationship between medium-grade Eocene metamorphic rocks and a nearby sub-greenschist melange unit, which is closely associated with Late Cretaceous Palelo Group sedimentary rocks, is relevant to the problem outlined by Standley and Harris (2009).

Several conventional analytical techniques are used to support this research. To keep the report clear and concise, we have provided the description of these techniques along with the detailed results in the Supplemental papers. Reference is made to the relevant Supplemental papers where the results are discussed in the text.

The structure in the Mutis Complex is complex. At the level of this study, we cannot solve the complete deformation history of the Mutis Complex. Therefore, we have concentrated on the dominant foliation in the rock (Sd). Previous geological studies have assumed that the metamorphic rocks in the Mutis Complex all have the same deformation history, and we aim to evaluate this assumption. To keep the discussion clear, the structures are labelled by the sector in which they were found: SdE from the Eastern Sector, SdC from the Central Sector and SdW from the Western Sector. A major feature of the Mutis Complex geology is that most outcrops also have a lineation on the dominant foliation (Ld), and these lineations are labelled as LdE, LdC and LdW to indicate where they were measured.

Regional geology

The Banda terrane at Miomaffo was mapped by van West (1941) and remapped by Rosidi et al. (1979). The first modern petrology on any of the Mutis Complex rocks was from the Boi massif (Brown & Earle, 1983; Earle, 1981). Sopaheluwakan et al. (1989) reported reconnaissance level metamorphic petrology of the Miomaffo massif. The Miomaffo massif (Figures 4 and 5) includes the Mutis Complex and Palelo Group. The Mutis Complex is a complexly faulted and mylonitic group of Mesozoic ‘pelitic’, mafic and ultramafic rocks (Sopaheluwakan et al., 1989; van West, 1941) that have been variably metamorphosed. The Palelo Group was interpreted to be unconformable on the Mutis Complex (Earle, 1981; Haile et al., 1979; van West, 1941). It includes radiolarites and siliceous argillites (Noni Formation), and tuffaceous clastics and volcanic rocks (Haulesi Formation) of Cretaceous to Eocene age (Haig & Bandini, 2013; Haile et al., 1979; van West, 1941). The Haulesi Formation has been correlated with the Lasipu Formation on Sumba based on age and lithology (Haig et al., 2019). The Noni Formation has been compared to radiolarian cherts in South Sulawesi (Haile et al., 1979). Munasri and Harsolumakso (2023) have shown that the outcrop in the Miomaffo massif is Late Cretaceous in age. Andesitic dykes intrude the Mutis Complex and the associated Palelo Group and these dykes also intrude the Metan Formation (Eocene). Harris (2006) argued that the dykes can be no older than 35 Ma. These dykes were not found in the Western and Central sectors of the Miomaffo massif.

Figure 4. Geology of the Miomaffo massif along transects mapped in this study. Representative structural data shown for each structural domain. Sd, dominant lineation; Ld, lineation on Sd.

Figure 5. Geology interpretation of the Miomaffo massif based on this study, van West (1941), Rosidi et al. (1979) and Sopaheluwakan et al. (1989). The Mutis Complex is split into three sectors as discussed in the text. Schematic cross-section for the line A–B, shown as inset. Position of Bobonaro Melange on section is from Harris (2006).

Results

The present work involved detailed observations along the excellent river exposures that cut the Miomaffo massif (Figure 4). The Mutis Complex in the Miomaffo massif has very distinctive features based on the metamorphic and structural history that define three elements. The Eastern Sector (Figure 5) is a volcanic-derived sequence of clastic rocks and melange with widespread preservation of the primary rock texture. The Central Sector is a greenschist facies high-strain sequence with a very similar bulk rock composition to the Eastern Sector but dominated by the metamorphic texture. To the west is a medium-grade metamorphic sequence (Western Sector). Sopaheluwakan et al. (1989) reported the central and western sections as one continuous zoned metamorphic complex, but no transitional zone was observed in this study. The boundary was faulted, and no gradient in metamorphic texture and grade was observed. Based on these observations, the results of this work are discussed separately for each of these sectors.

Eastern Sector

Van West (1941) mapped the eastern section of the Miomaffo massif as predominantly ‘low metamorphic greenschists derived from basic igneous rocks’. Sopaheluwakan et al. (1989) mapped the eastern section as ‘amphibolitic greenschist’ and ‘metatuff’. However, these descriptions do not adequately reveal the complexity of the zone. The Eastern Sector is complexly faulted at the outcrop scale, with fault blocks of relatively low ductile strain, such that much of the original texture is preserved, varying from a few metres to hundreds of metres in size. The blocks are bounded by a range of structures including mylonite and cataclasite zones largely parallel to bedding (Figure 6a), cataclasite zones that postdate the andesite dykes (Figure 7c) and discrete faults (Figure 7d). This low-grade part of the complex is well exposed along the larger creeks. In a few places, a bedded structure is visible (Figure 7c), but the most common lithology is a melange made up of a wide range of blocks (e.g. Figures 7d and 8a) in a foliated matrix (Figures 6a and 7a, b). The blocks vary from millimetre scale to many metres. In some areas, the outcrops consist of a single lithology, and examples include metabasalt, siliceous argillite, arkosic sandstone and volcaniclastic rocks. Whether these larger domains are fault bounded or blocks in the melange is uncertain. Other lithologies seen only as smaller blocks are amphibolite, feldspathic schist, and green and black phyllite. The highest metamorphic grade of the blocks found was amphibolite facies with examples of amphibolite (Figure 9a) and garnet biotite quartz schist (Figure 9b). More common are actinolite-bearing metasandstones that are high in plagioclase. Late prehnite veins reflect the typical metamorphic grade of the melange matrix.

Figure 6. Photographs of the Eastern Sector of Miomaffo massif. (a) Melange with basaltic matrix and various lighter blocks of veins and volcaniclastic rocks. Shallowly dipping dominant foliation and early mylonite zone cut by late cataclastic fault zone to the left of hammer. (b) Texture of a section of late cataclasite. Hammer handle is 40 cm long.

Figure 7. Photographs of the Eastern Sector of the Miomaffo massif. (a) Eocene dyke (brown weathering) cuts across foliated melange. (b) Foliated melange. Open folds in dominant cleavage (SdE). (c) Eocene dyke (pale green) cuts across foliated and bedded mafic volcaniclastic rocks. Late cataclasite fault zone truncates left end of dyke. Hammer handle is 40 cm long. (d) Folded layers of volcaniclastic sandstone in dark foliated matrix. Dominant foliation shown as dashed line. Truncated by late cataclastic fault on the right-hand side.

Figure 8. Photographs of the Eastern Sector of Miomaffo Complex. (a) Isoclinal fold in feldspathic block in melange. (b) Close up of late cataclastic fault zone shown in Figure 6a. Hammer handle is 40 cm long.

Figure 9. Photomicrographs of Eastern and Central Sector rocks: (a) amphibolite from block in Eastern Sector melange, green hornblende (h) and clear plagioclase. (b) Garnet (g) porphyroblast in quartz biotite garnet schist outcropping as a block in the Eastern Sector melange. (c) Arkosic sandstone from Eastern Sector. Brown altered plagioclase with minor quartz and chlorite (bright green). Note basalt sand grain in the centre of image. (d) Andesitic dyke dominated by plagioclase (p) laths with minor quartz (q) and magnetite (m) in a chlorite (chl) calcite (c) altered matrix. (e) Fold in grey phyllite (Central Sector) with quartz–albite in light bands and chlorite muscovite graphite in dark bands. (f) Folded layering in green phyllite (Central Sector) with quartz–albite in light bands and chlorite (chl) epidote (ep) magnetite (m) in green layers cut by quartz (q) vein.

The Eastern Sector is deformed along discrete zones on higher strain with a wide range of fold styles. There are at least two generations of folding (Figure 7b, d) in the dominant foliation in the Eastern Sector (SdE: dominant foliation, Eastern Sector) but the structure is too complex to unravel within the scope of this project. The dominant foliation (Figures 6a, 7b, d and 8a) is subhorizontal in the north and moderately southeast dipping in the south (Figure 10). In the north, an amphibole lineation on this foliation trends NE–SW. In other outcrops, tight fold axes in the foliation have a similar trend. Many of the rocks in the south have L/S fabrics with the lineation trending southeast. All these lineations have been grouped as LdE (dominant lineation, Eastern Sector) on Figure 10. There are high-strain zones parallel to SdE (Figure 6a), and in two locations sheath fold geometries were observed along these zones.

Figure 10. Equal area stereographic projections of structural data. Dominant foliation and lineation on foliation surface shown for the Eastern Sector (SdE, LdE), Central Sector (SdC, LdC) and Western Sector (SdW, LdW). Bedding shown for all Palelo Group rocks. The late brittle and cataclastic faults in the Mutis Complex are also shown.

The underlying texture of the Eastern Sector rocks is typical of a mafic volcaniclastic rock (Figure 9c) or volcanic litharenite. However, in most samples, the fine texture is not preserved, and to aid description of the primary rock type, all samples were analysed using a portable X-ray fluorescence method (pXRF). pXRF can rapidly and inexpensively provide chemical concentrations of a range of geologically significant elements (with an atomic number of 12–92, Mg to U), commonly with instrument detection limits below 100 ppm (Ryan et al., 2017). In this study, pXRF analysis was used to assess the chemical composition of 200 samples collected including 45 from the Eastern Sector of the Mutis Complex. The analytical procedure is given in the Supplemental data 1. Where individual sample had two visually distinct compositional domains, both domains were analysed.

The Eastern Sector rocks (Figure 11) have low SiO₂ (average 46 wt%) and high Fe₂O_3T (12 wt%), MgO (9 wt%), CaO (7.6 wt%) and K₂O (0.4 wt%). In comparison, average pelitic compositions are 64 wt% SiO₂, 7.6 wt% Fe₂O_3T, 2.4 wt% MgO, 0.65 wt% CaO and 4 wt% K₂O (Forshaw & Pattison, 2023). On the sedimentary classification diagram of Herron (1988), the samples plot in the field of Fe shales (Figure 12). The compositions are like average mid-ocean ridge basalt (MORB) compositions (Gale et al., 2013). Since most of the metasedimentary rocks were identified as volcaniclastic in texture, we plotted the compositions on volcanic classification diagrams (Figure 11). Based on the immobile trace-element diagram of Pearce (1996) the Eastern Sector samples mostly plot in the tholeiitic basalt field with a 20% overlapping into the andesite field. Using the Nb–Zr–Y diagram of Meschede (1986), most samples plot in the normal MORB (N-MORB) field. We conclude that the Eastern Sector melange and volcaniclastic rocks are derived from N-MORB basalt. Some of the samples show a compositional trend indicating other sedimentary components (Figure 13). Eight samples are enriched in SiO₂ but depleted in Zr showing a trend towards a chert composition indicating addition of biogenic silica (Figure 13). Two samples trend towards higher silica with enrichment of Zr typical of sorting process found in epiclastic sedimentary rocks. Three samples include a carbonate component (marl) high in Mg as well as Ca and show a trend towards low SiO₂ and Zr due to dilution. Two-thirds of the samples are basaltic volcaniclastic rocks.

Figure 11. Composition of metasedimentary rocks measured by pXRF. Six major elements are plotted against SiO₂. Most of the sedimentary rocks are of volcanoclastic origin and are shown on igneous geochemical diagrams (Meschede, 1986; Pearce, 1996) to identify possible source rocks. WPAB, within plate alkali basalt; WPT, within plate tholeiite; E-MORB, enriched mid-ocean ridge basalt; N-MORB, normal mid-ocean ridge basalt; VAB, volcanic arc basalt.

Figure 12. Composition of metasedimentary rocks of the Mutis Complex using the classification proposed by Herron (1988).

Figure 13. Zr plotted against SiO₂ for Eastern and Central sectors sedimentary rocks showing compositional trends towards siliceous argillite, limestone and sandstone (by sorting). See text for interpretation. Analyses are by portable XRF.

The metabasalt blocks in the Eastern Sector have a tholeiitic composition (Supplemental data 2). The amphibolite blocks in the melange have the same composition as the metabasalt. These rock types are plotted as ‘amphibolite blocks’ on the trace-element classification diagram (Figure 14a) of Pearce (1996), which shows they fall in the tholeiitic basalt field as expected from their major element composition. Using the tectonic setting classification diagram of Pearce (2014), both amphibolite and metabasalt blocks from the Eastern Sector have compositions typical of N-MORB compositions (Figure 14b).

Figure 14. Composition of basaltic rocks at Miomaffo. (a) Volcanic rock classification after Pearce (1996); and (b) tectonic setting of basalt after Pearce (2014). ‘Amphibolite blocks’ includes both greenschist and amphibolite facies blocks in the Eastern Sector. ‘Metabasalt’ is from the Central Sector. ‘Eocene amphibolite’ is from the Western Sector. E-MORB, enriched mid-ocean ridge basalt; N-MORB, normal mid-ocean ridge basalt; OIB, ocean island basalt.

One amphibolite block in the Eastern Sector melange has an Ar/Ar isochron age of 184 ± 6 Ma (Supplemental data 3). One of the blocks of garnet-bearing schists contains detrital zircons indicating a maximum depositional age of 201 ± 4 Ma (Supplemental data 4). The Eastern Sector Mutis Complex is nominally unconformably overlain by Palelo Group (Harris, 2006) but all the contacts inspected in this study were faults or lacked critical outcrops. A Late Cretaceous chert exposed along the Noil Toko (River Toku) as described by Munasri and Harsolumakso (2023) is interpreted here as a block in the melange (see discussion below) and provides the maximum age for the melange.

The dykes that intrude the Miomaffo massif (Figure 7a, c) are 1–3 m wide and show little petrographic variation, being fine-grained and equigranular (Figure 9d). They mainly consist of plagioclase, augite, titanomagnetite and ilmenite. Phenocrysts are rare. All the samples studied have been metamorphosed to an albite–prehnite–quartz–chlorite assemblage. The Eocene dykes in the Eastern Sector have 54–60 wt% SiO₂ (Supplemental data 2). Using the volcanic classification diagram of Pearce (1996), they plot on the boundary between basalt and andesite (Figure 14a). They are low in Nb and fall in the arc-related field using the tectonic setting diagram of Pearce (2014) indicating they have a typical calc-alkaline affinity (Figure 14b; Supplemental data 2). The dykes also intrude the Haulesi Formation (Figure 4) indicating they are less than 80 Ma. Similar dykes intrude Eocene stratigraphy in other Banda terrane klippe implying that at least some of the dykes are less than 35 Ma (Harris, 2006).

There is a distinctive generation of late faults that cut all rock types in the Miomaffo massif, including the Palelo Group and the andesitic dykes (Figures 6a, b, 7c, d and 8b). The late faults typically form narrow cataclastic zones (Figures 6a and 8b). Where observed, they have normal offsets. These faults have a wide range on orientations (Figure 10) including steep faults striking NS and NE–SW, and shallowly south-dipping faults.

Central Sector: low grade, high strain

Most rocks in the Central Sector are fine-grained and white, green or dark grey (graphitic) with a foliated texture (phyllite) (Figure 15). A few rocks have relict sandstone textures. Metabasic rocks are very common in the southern outcrop along the Noil Niti (River Niti) and less common in the northern exposure along the Noil Noni (Figure 4). Cherts and marls are rare. The Central Sector rocks have a typical greenschist facies mineral assemblage. The most common green rocks (Figure 9f) have the assemblage epidote, actinolite, albite, chlorite with minor titanite and carbonate. Stilpnomelane was detected in a few samples. The grey rocks (Figure 9e) are quartz–albite–muscovite ± biotite. Many rocks are clay altered, but it is not clear how much of this is due to weathering, and whether any clay alteration is hydrothermal in origin. We did not find any quartz–muscovite-rich rocks (i.e. pelites).

Figure 15. Typical structure within the Central Sector rocks. (a) Intersection lineation (LdC) where the dominant foliation (SdC) is the axial plane to tight folds; (b) intrafolial boudinage layers in metabasic rock; (c) small horst structure in a boundary between pale green and grey phyllite; and (d) isolated isoclinal fold hinge within SdC.

The dominant structure in the Central Sector is a transposition layering and associate cleavage (SdC) (Figure 15b, d). The transposed layering is refolded by tight to open folds (Figures 9e, f and 15a, d) and cut by ductile shear zones and multiple generations of faults (Figure 15c) including the late cataclastic faults that cut the Eocene dykes in the Eastern Sector.

The dominant foliation in the Central Sector (SdC) dips shallowly north (average 20°/030°) and is folded around an axis plunging down dip (Figure 10). In a third of the sites, the dominant lineation is parallel to this fold axis and is at least in part an intersection lineation associated with tight folds (Figure 15a). The SE-plunging lineations on SdC (Figure 10) include stretching lineations and isoclinal fold hinges. In the southern metabasalt exposures, SdC dips shallowly NE (Figure 10). The dominant lineations in the metabasalt are amphibole and feldspar lineations plunging SE.

The pXRF composition of these samples shows the same range of bulk composition (average SiO₂ 46 wt%, Fe₂O_3T 12 wt%, MgO 10 wt%, CaO 8 wt%, K₂O 0.1 wt%) as the Eastern Sector with compositions typical of basaltic volcanoclastic rocks (Figure 11). No typical pelitic compositions were found.

Six samples from the large metabasalt exposure along the Noil Niti have basaltic composition (46–52 wt% SiO2) with an LREE depleted pattern (Supplemental data 2). The samples plot in the sub-alkaline basalt field (Figure 14a) of Pearce (1996). In most trace-element classification diagrams, they plot in the N-MORB field (e.g. Figure 14b). Zircon recovered from a pegmatitic patch in metabasalt from this area has an Early Jurassic U/Pb crystallisation age of 194 ± 5 Ma (Supplemental data 4).

Western Sector: medium grade

The Western Sector of the Miomaffo Complex is composed of two mappable units (Figure 4), amphibolite and grey gneiss (Figure 16a). Amphibolite, typically containing brown hornblende, plagioclase, epidote and magnetite, is a common composition both as mappable units (Figure 4) and interlayered with the grey gneiss that is the dominant composition in the other map unit. In the grey gneiss, garnet is a distinctive minor mineral visible in hand specimens (Figure 16b, c). Most outcrops are dominated by a gneissic layering including lenticular, boudinaged and isoclinally folded (transposed) quartz veins (Figure 16b), but at a few locations it is possible to recognise an earlier clastic texture with angular fragments of felsic igneous rocks visible (Figure 16c). In thin-section, the gneisses are plagioclase-rich with garnet–biotite–staurolite and muscovite as the dominant mineralogy (Figure 17).

Figure 16. Typical rocks within the Western Sector. (a) Grey felsic gneiss overlying amphibolite; (b) transposed quartz veins in higher strain gneiss; and (c) relict volcaniclastic texture in garnet-bearing gneiss. Legend: q, transposed quartz vein; g, garnet porphyroblast; clasts, felsic igneous rocks clasts preserved with deformed gneissic texture.

Figure 17. Photomicrograph of the sample 63738 showing evidence for pre-metamorphic deformation events. Note the foliation preserved with the relict clasts outlined in white and tight fold preserved within staurolite porphyroblasts. The position of SdW is shown in red. Internal foliation (Si) in staurolite outlines early fold. St, staurolite; Grt, garnet; Bi, biotite.

The dominant foliation in Western Sector (SdW) is a gneissic layering. In most samples, this is composed of feldspar-rich layers and darker bands with more biotite. The dominant lineation in the Western Sector (LdW) is an intersection lineation associate with isoclinal fold hinges. In the amphibolite, the foliation varies from a schistosity to a banded (gneissic) texture, and the dominant lineation is mostly due to elongate plagioclase. Despite the wide variety in appearance, SdW consistently dips 60°/025° (Figure 10), and the dominant lineation (LdW) trends 020°.

The mesoscopic structure of the Western Sector appears simpler than the Central and Western sectors. However, there is microstructural evidence of early (pre-SdW) deformation history preserved within the relict clasts and staurolite (e.g. Figure 17). It is possible all the older deformation events recognised in the Eastern Sector also affected the Western Sector rocks but were obscured during the later metamorphic event. The late cataclastic faults are common in the Western Sector.

The Western Sector amphibolite has the same bulk composition as the metabasalt blocks in the melange (Figure 9; Supplemental data 2). The three analysed samples plot in the MORB field based on trace-element data (Figure 14b). The garnet-bearing gneiss includes more compositions with K₂O > 1 wt% (Figure 11) but still not close to that typical of pelitic compositions. The SiO₂ varies up to 75 wt%. If the gneiss were originally a volcaniclastic rock, the immobile trace elements (Zr/Ti vs Nb/Y diagram of Pearce, 1996) would be consistent with an andesitic source (Figure 11). We interpret the preservation of igneous clasts in some outcrops, the high plagioclase content and the trace-element composition as evidence that the original sedimentary rocks in the Western Sector were in part derived from a volcanic arc.

The mineral assemblage in the grey gneiss is quartz–plagioclase–biotite–staurolite–ilmenite ± margarite ± paragonite (Supplemental data 5). Six samples were analysed by EPMA to determine the mineral compositions and calculate the average P–T method in Thermocalc (Holland & Powell, 2011). The P–T conditions calculated from using the garnet-rim compositions are all similar (Figure 18), and the average P–T condition across all samples is 650 °C and 0.9 GPa. However, the evidence suggests that the sample from the south (63703) may have a peak metamorphic temperature 50 °C lower than that of the other samples. Sample 63738 has experienced a higher temperature metamorphism than other samples. This sample comes from the far west in an area with limited outcrop. It is separated from the other northern samples by a fault. There is a consistent temperature across the north of the Western Sector until the metamorphic grade jumps at this fault contact. The variation in metamorphic grade does not match the smooth gradient from greenschist to amphibolite facies reported by Sopaheluwakan et al. (1989). The greenschist facies rocks of the Central Sector are separated from the amphibolite facies rocks by faults, and the variation in grade within each fault block is small.

Figure 18. Average P–T for amphibolite facies samples in the Mutis Complex. The sample in red (63580) is from a block in the Eastern Sector melange. All other samples are from the Western Sector. P–T conditions calculated using rim garnet composition and Thermocalc (Holland & Powell, 2011).

A garnet–staurolite gneiss from the Western Sector contains detrital zircon in the range 10–20 μm diameter (Supplemental data 4). However, cathodoluminescence (CL) imaging showed that many grains had extensive metamorphic rims around a core with complex growth zoning typical of magmatic zircons. The best-preserved grains have a crystallisation age of 201 ± 4 Ma. The other grains are more difficult to interpret. Half of the grains show Pb mobility patterns (discordant). Two grains are concordant with an age of 160 Ma, and two other grains are close to concordant at 85 Ma. Based on the CL image evidence for metamorphic rims on these zircons, we interpret the 180 Ma and 85 Ma as a mixture of inherited grains and metamorphic rims. The zircon closure temperature by diffusion (Oriolo et al., 2018) is higher than the metamorphic grade reached in these rocks. We therefore interpreted the 201 Ma age as detrital and a maximum age for deposition. The protolith age is no older than Jurassic. No old grains typical of continental source rocks were analysed.

Three samples of amphibolite were selected for K/Ar hornblende analysis. The details of mineral separation and analytical methods are provided in Supplemental data 3. The hornblende has a consistent K/Ar age near 40 Ma. Two of the hornblende separates were then analysed by ⁴⁰Ar/³⁹Ar methods. These analyses produced inverse concordia ages (Supplemental data 3) of 35.7 ± 1.1 Ma and 37.1 ± 0.4 Ma. These ages are very similar to metamorphic ages recorded from seven other Mutis Complex blocks ranging from 38 to 31 Ma (Harris, 2006). We consider the ⁴⁰Ar/³⁹Ar hornblende age as the definitive metamorphic age for the Western Sector, and it is these data that support our interpretation that the 85 Ma U/Pb zircon age from the gneisses is a mixed age and not the age of peak metamorphism.

Two of the grey gneiss samples studied in thin-section have porphyroblastic apatite. One of these had sufficient U content (5 ppm) to allow a U/Pb apatite age to be measured (Supplemental data 6). The age of metamorphism calculated using this method is 31 ± 23 Ma, consistent with the Ar/Ar age measured for the Western Sector.

Other Banda terrane rocks

The Palelo Group (Noni Formation, Haulesi Formation) is exposed on the margin of the Eastern Sector melange. No unconformity surface was found, and all the mapped contacts were faults, although the southeastern contact (Figure 4) is approximately parallel with the bedding in the Palelo Group and could be a fault along the unconformity surface. The Noni Formation in this southern exposure is deformed (Figure 19b), but the deformation is brittle in style. The outcrop of ‘Noni Formation’ reported by Haile et al. (1979) is further north along the Noil Noni (Figure 4). This is the outcrop reported as Late Cretaceous by Munasri and Harsolumakso (2023). This block is tightly folded and deformed (Figure 19a) in the style typical of blocks in the Eastern Sector melange sequence rather than the normal Noni Formation outcrops that directly underlie the Haulesi Formation, 2 km to the south (Figure 4). Haile et al. (1979) argued that the tightly folded block was originally unconformable on the melange, but the combination of high strain and limited outcrop prevented us from confirming this relationship. If the chert block described by Haile et al. (1979) were a block in the melange it would be the youngest block known from the melange.

Figure 19. (a) Tightly folded chert (possible Noni Formation) on the margin of the late fault zone near Desa Noiltoko (Figure 4) with deformation typical of the Eastern Sector melange. (b) Strongly faulted grey chert (normal Noni Formation) on the boundary between Mutis Complex melange and Haulesi Formation (Palelo Group).

The Haulesi Formation (Late Cretaceous to Eocene; Rosidi et al., 1979) is folded and faulted in the Miomaffo massif, but no examples were found with a strong foliation typical of the Mutis Complex. The Haulesi Formation is steeply dipping to the south (Figure 10).

The Mutis Complex at Miomaffo is separated into two blocks by a narrow zone near the village Desa Noiltoku (Figure 4), where a range of lithologies are exposed. These rocks are typical of the Gondwana lithologies that underlie the Banda terrane klippen (Harris, 2011). They include Miocene sedimentary rocks of the Noil Toko Formation and Triassic limestone (Haile et al., 1979; Munasri & Harsolumakso, 2023; Rosidi et al., 1979). The Miocene sedimentary rocks in this zone are deformed into tight upright folds with poles to bedding oriented along a great circle defining a fold axis plunging 13/201 (Figure 20a) and crosscut by at least two generations of faults. The most common small-scale faults are sinistral faults striking 160°. The dextral faults strike 060° indicating E–W shortening (Figure 20b). Some of the west-dipping N–S-striking faults have normal displacement. This high-strain zone postdates the emplacement of the Miomaffo massif. Using the structural data from this work and following the existing paradigm for the Banda terrane (Harris, 2006) a cross-section of the Miomaffo massif (Figure 5) shows these Miocene beds as a horst exposing the underlying autochthonous Australian craton sequences.

Figure 20. Structure of the Miocene Noil Toku Formation in the fault zone near Desa Noiltoko (Figure 4) plotted on equal area stereographic projection. (a) Poles to bedding showing great circle distribution defining a fold axis plunging 13°/201°; and (b) small faults with striations showing movement direction. Where the sense of movement was visible, these are shown as red (for sinistral) and blue (for dextral) dots and arrows.

Discussion

The Mutis Complex at Miomaffo includes outcrops of amphibolite and greenschist facies metamorphic rocks and the Palelo Group typical of the Banda terrane. The Eastern Sector is composed of mafic volcaniclastic rocks and melange with a similar whole-rock composition. We found no normal pelitic compositions or evidence for any continental provenance in the Mutis Complex. The composition is unusual. In reviewing the literature, the only similar compositions we found were mafic sands reported from Colorado and Oregon. Van de Kamp and Leake (1985) proposed these mafic sands as the type example of sediments derived exclusively from mafic rocks. In the Eastern Sector, the volcaniclastic rocks are closely associated with MORB basalts. We concluded that these rocks were originally deposited far from continental sources and isolated from arc volcanic sources. Given that the deformed rocks were accumulated in a forearc setting (Harris, 2006) before thrusting onto Timor, we conclude they are probably off-scraped oceanic sediments. One metamorphic block in this sequence has a maximum protolith age of 200 Ma. Another block was metamorphosed at 180 Ma. We conclude that Jurassic Ocean crust was being subducted, and at least part of the sequence was already accreted to an arc by 180 Ma. However, if the chert described by Munasri and Harsolumakso (2023) were a block in the melange, the accretion may have continued until the Late Cretaceous. Harris (2006) argued convincingly that in many places, the Cretaceous Noni Formation unconformably overlies metamorphic rocks of the Mutis Complex. These observations are consistent with the deposition of Noni Formation on an outer arc ridge that was still active so that locally parts of the Noni Formation are deformed while elsewhere chert deposition continues. Near the end of the Cretaceous, arc volcanics become a major source in deposition with the transition to the greywackes of the Haulesi Formation. We conclude that the Palelo Group unconformably overlies rocks of the Eastern Sector. Since the dominant foliation in the Eastern Sector (SdE) is not found in these rocks, it must be fully developed before the end of the chert deposition (Noni Formation) in the latest Cretaceous.

The geological history of the Central Sector is less constrained. The bulk rock composition is the same as the Eastern Sector, and the age of deformed metabasalt in this sequence is 200 Ma (Lower Jurassic). However, there were no locations where the Noni Formation is closely associated with this unit. There are also no known examples of Eocene andesitic dykes. The major deformation is younger than 200 Ma, but we have no lower bound to the age. Either the Central Sector was not close to the Eastern Sector in the Eocene, or the higher strain foliation (SdC) is younger than the dykes.

The rocks of the Western Sector were deposited after 200 Ma. The amphibolite has a MORB composition similar to that of metabasalt in the east, but the metasedimentary rocks have an additional source that is higher in K₂O and in SiO_2. We found no evidence for continental sources in the provenance and consider a contribution from an arc source is more likely based on the chemical composition. These rocks were metamorphosed at ca 37 Ma. The dominant foliation (SdW) is defined by metamorphic minerals and therefore formed after the deposition of the Palelo Group and probably after the intrusion of the andesitic dykes in the east. Given the difference in grade of metamorphism, the Western Sector was more than 20 km from the Eastern Sector at 37 Ma. The unconformable relationship of the Palelo Group with the Mutis Complex is likely for the Eastern Sector but impossible for the Western Sector. We conclude that the Mutis Complex was assembled in its present form after 37 Ma. The general form of this assemblage is a series of fault-bounded slices dipping to the east (Figure 5).

The Mutis Complex at Miomaffo is dominated by mafic compositions largely of oceanic affinity but includes some evidence of island arc sources in the Western Sector. The basalts formed in the Lower Jurassic. Park et al. (2014) described very similar rocks at Fohorem (Figure 2), 80 km to the east. The Fohorem exposures contain Middle Jurassic andesite. Park et al. (2014) concluded that the Lolotoi block at Fohorem came from an oceanic island arc in the Meso-Tethys that was later accreted to Sundaland. The reconstruction of Hall (2012) shows an oceanic arc (Wolya Arc) subducting Meso-Tethys oceanic crust (>160 Ma) before it was accreted to Sundaland to become the Woyla terrane (Figure 21). The Woyla terrane (Barber, 2000) includes Jurassic–Cretaceous volcanic arcs and imbricated oceanic rocks typically with prehnite–pumpellyite to greenschist facies metamorphism, but higher metamorphic grades are reported locally (Barber & Crow, 2005). Radiolarian cherts range from Triassic to mid-Cretaceous. Continental material is absent from the definitive Woyla terranes. The Woyla Arc accreted to Sundaland by 95 Ma (Advokaat et al., 2018; Hall, 2012). The resulting Woyla terrane is unlike other pre-Eocene rocks described from Sumatra (Barber et al., 2005). The lithologies found in the Mutis Complex at Miomaffo are more consistent with an origin in the Woyla terrane than the Cretaceous accretionary rocks of Sulawesi, which include pre-Mesozoic sources (Böhnke et al., 2019) and are assumed to have continental crust as basement (Hall & Sevastjanov, 2012).

Figure 21. Early Cretaceous reconstruction of Tethys from Hall (2012) showing the oceanic Woyla Arc just before accretion to Sundaland to form the Woyla terrane. The Woyla Arc is subducting Meso-Tethys oceanic crust older than 160 Ma. Note the Australian crustal blocks just before they accrete to Sundaland to form the basement of southwest Kalimantan, east Java and west Sulawesi.

Hall (2012) reports that there was little or no convergence at the southern margin of Sundaland from 90 to 45 Ma, and there was little volcanism during this interval. In the Banda terrane, this time is also quiescent with the deposition of the Palelo Group (Harris, 2006). Most authors have supported the correlation of the Palelo Group and overlying volcanic rocks and limestones with the widespread continental margin facies on Sundaland (Haig et al., 2019; Harris, 2006).

Convergence restarted at 45 Ma (Hall, 2012). This is reflected in extensive Eocene calc-alkaline volcanism of the Metan Formation (West Timor; Harris, 2006) and Barique Group (East Timor; Haig et al., 2019). In the Mutis Complex, we see widespread medium-grade metamorphism at 40–33 Ma (Harris, 2006; this paper). Thus, the dominant foliation in the Western Sector (SdW) formed on the southern margin of Sundaland. In contrast, SdE formed in the forearc of the Cretaceous Woyla Arc before its accretion to Sundaland.

Standley and Harris (2009) noted the contradiction implicit in the Eocene age of metamorphism in rocks that are unconformably overlain by unmetamorphosed Upper Cretaceous sandstones. They concluded that the metamorphism was associated with extension and that all the basal contacts of the Palelo Group are extensional faults leading to a core complex model. However, this explanation does not fit the observations at Miomaffo. While the bedding in the Palelo Group parallels the contact (Figure 4), the foliation (SdE) in the melange sequence is not parallel to the contact. In addition, Eocene andesitic dykes are younger than SdE but are not found in the Western Sector. A much simpler interpretation is that the medium-grade high-strain blocks have been juxtaposed with the low-strain low-grade blocks in the forearc of the Sunda Arc after 35 Ma.

A suite of late faults in the Miomaffo massif have a distinctive cataclastic style. These faults have a wide range of orientations (Figure 10). The most common is steep and striking NNE parallel to the late fault zone that crosses the complex and affects unconsolidated rocks that are largely Miocene sediments (Haile et al., 1979; Rosidi et al., 1979). Where visible, these faults have normal offsets. It is possible that the late cataclasite zones result from extensional collapse after the Mutis Complex was thrust over Timor.

Summary

Mesozoic accretionary assemblages are widespread across Indonesia, and these can be seen in the overthrust terranes on Timor. The oldest rocks in the allochthonous Mutis Complex, West Timor are exposed in the Eastern Sector of the Miomaffo massif. They include Mesozoic melange containing blocks of Jurassic N-MORB basalt, amphibolite (metamorphism at 175 Ma), garnet and actinolite-bearing schists, arkosic sandstone and volcanogenic rocks. The sedimentary provenance is largely Mesozoic MORB with a small component of island-arc-volcanic provenance. Prehnite is common as a late metamorphic mineral in the matrix. The accretionary material is complexly deformed with a dominant foliation and widespread cataclasite zones. This sequence is intruded by calc-alkaline andesitic dykes (possibly in the Eocene). To the west of the melange is a block of high-strain greenschist facies rocks (isoclinal folding and transposition layering) where the block in matrix texture cannot be recognised, but the range of bulk compositions is the same. Further west across several faults is an amphibolite facies metamorphic block, containing amphibolite with the same N-MORB composition as the metabasalt in the east. The peak metamorphic conditions in the Western Sector were 600–720 °C, 0.6–1.1 GPa. The garnet gneiss in this area has a similar bulk composition to the melange further east but with a higher component of andesite in the source. The metamorphic assemblages overprint low-T isoclinal folds, preserved within metamorphic porphyroblasts. Peak metamorphism occurs at ca 37 Ma reflecting an event within the active margin of Sundaland. At ca 5 Ma, NW Australia collided with this margin, and the Banda terrane was thrust over Timor. A late generation of cataclastic faults zones cuts all the pre-existing lithologies. The complex is cut by a younger fault zone that affects the underlying unconsolidated sedimentary rocks and contains evidence of both strike slip and normal fault movement. The Miomaffo massif is an excellent example of the complex geological history of rocks along the boundary between Indonesia and Australia.

Acknowledgements

The authors gratefully acknowledge the logistic support provided for the fieldwork by the Indonesian geological survey, Bandung, and especially by Said Aziz. We would like to thank Alex Grady for suggesting the project. The paper was substantially improved by the constructive review of Myra Keep.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials.

Additional information

Funding

This project was partly funded by the ARC Research Hub for Transforming the Mining Value Chain [project number IH130200004].