Eastern Ophiolitic Belt Research Articles

Chapter 6 The mafic–ultramafic (ophiolitic) rocks of Myanmar

Mafic and ultramafic rocks are widely distributed in Myanmar and form major tectonic belts and suture zones representing remnants of the Mesotethyan Ocean. They occur as deformed and disrupted ophiolitic sequences and in forearc slivers as part of an accretionary wedge, intimately associated with siliciclastic and limestone units. They commonly host chromite–nickel–manganese, copper (gold) and jadeite mineralization. In this chapter, the nature, distribution, geological setting, petrological, mineralogical and ore characteristics of the mafic and ultramafic rocks in Myanmar are reviewed and described, and their tectonic setting and mineralization potential are discussed. Myanmar is characterized by north–south-trending major tectonic domains. From west to east, these include: the Rakhine coastal strip, an ensimatic foredeep; the Indo-Myanmar Ranges, an outer arc or forearc; the Western Inner-Myanmar Tertiary Basin, an inter-arc basin; the Central Volcanic Belt (Central Volcanic Line), an inner magmatic-volcanic arc; the Eastern Inner-Myanmar Tertiary Basin, a back-arc basin; and the Shan-Tenasserim massif, the ensialic Sino-Myanmar Ranges (Bender 1983; Khin Zaw 1989, 1990, 2017; Khin Zaw et al. 2015). Among these tectonic domains, ophiolitic rock associations occur in three belts trending nearly north–south parallel with each other (from west to east): the Western Ophiolitic Belt (WOB); the Central Ophiolitic Belt (COB); and the Eastern Ophiolitic Belt (EOB) (Fig. 6.1). The narrow WOB forms the longest belt, extending from the north to the southern part of Myanmar covering the eastern hills of the Naga, Chin and Rakhine ranges. According to Hutchison (1975), there are two ophiolite belts in Myanmar: the Naga Hill Line and the Mandalay Line. In this chapter the Naga Hill Line is termed the WOB. The Mandalay Line is the combination of the COB and the EOB. The southern continuation of the Mandalay Line is uncertain, due to a lack of comprehensive geological information. These …

Chapter 26 Copper deposits of Myanmar

This chapter presents a brief account of the copper deposits and occurrences in Myanmar. Myanmar has more than 70 copper occurrences, including world-class high-sulphidation Cu ± Au deposits west of Monywa on the west bank of the Chindwin River (Fig. 26.1, Table 26.1). In 1985, Myanmar commenced copper production with the development of the Sabetaung copper deposit at Monywa in central Myanmar. Copper matte was produced as a by-product from the Bawdwin Mine, a volcanic-hosted massive sulphide deposit, at the time the underground ore was being exploited. Copper is associated with epigenetic vein-type and epithermal gold ± silver deposits. Copper-bearing vein and skarn deposits are also present in Myanmar (e.g. Baladokhta, Mandalay region and Shweminbon, Kalaw township, southern Shan State). Myanmar has the potential for discovery of Cyprus-type copper ± gold deposits (e.g. Leymyetna) and porphyry Cu–Mo–Au deposits, such as Shangalon in the Kawlin–Wutho area in Sagaing State, northern Myanmar. Copper is associated with intrusive-related chromite–nickel–platinum deposits in ultramafic–mafic rocks (e.g. Tagaung Taung area, Thabeikkyin township). Copper is also associated with tin–tungsten mineralization in the Tanintharyi Region, southern Myanmar. Fig. 26.1. Locations of copper deposits and occurrences in Myanmar and their geological resource data. View this table: Table 26.1. Characteristics of selected copper deposits and occurrences in Myanmar Copper deposits and occurrences are distributed across five major geotectonic–metallogenic terranes of Myanmar. These five belts are as follows: 1. the Indo-Myanmar Ranges (IMR) or western ultramafic (ophiolitic) belt (Hla Htay et al. 2017; Kyi Khin et al. 2017); 2. the Central Magmatic Volcanic Arc (Myint Soe et al. 2017); 3. the Tagaung–Myitkyina Belt or Eastern Ophiolitic Belt (EOB) of Hla Htay et al. (2017), 4. the Mogok Metamorphic Belt (MMB), also known as the Mogok–Mandalay–Mergui Belt (MMMB) of Searle et al. (2007, 2017); 5. the Sino-Myanmar Ranges or Eastern Shan Highlands. The ore characteristics of the …

Multi‐stage Process of the Bulqiza Chromitites, Eastern Ophiolitic Belt, Albania

Multi‐stage Process of the Bulqiza Chromitites, Eastern Ophiolitic Belt, Albania

Petrology of Cr‐rich Podiform Chromitites of Bulqiza, Eastern Ophiolitic Belt, Albania

Petrology of Cr‐rich Podiform Chromitites of Bulqiza, Eastern Ophiolitic Belt, Albania

Petrology of Cr‐rich Ophiolitic Chromitites of Bulqiza, Eastern Ophiolitic Belt, Albania

Petrology of Cr‐rich Ophiolitic Chromitites of Bulqiza, Eastern Ophiolitic Belt, Albania

Petrology and geochemistry of high Cr# podiform chromitites of Bulqiza, Eastern Mirdita Ophiolite (EMO), Albania

The ultramafic massif of Bulqiza, which belongs to the eastern ophiolitic belt of Albania, is a major source of metallurgical chromitite ore. The massif consists of a thick (>4km) sequence, composed from the base upward of tectonized harzburgite with minor dunite, a transitional zone of dunite, and a magmatic sequence of wehrlite, pyroxenite, troctolite and gabbro. Only sparse, refractory chromitites occur within the basal clinopyroxene-bearing harzburgites, whereas the upper and middle parts of the peridotite sequence contain abundant metallurgical chromitites. The transition zone dunites contain a few thin layers of metallurgical chromitite and sparse bodies are also present in the cumulate section. The Bulqiza Ophiolite shows major changes in thickness, like the 41–50wt.% MgO composition similar with forearc peridotite as a result of its complex evolution in a suprasubduction zone (SSZ) environment. The peridotites show abundant evidence of mantle melt extraction at various scales as the orthopyroxene composition change from core to rim, and mineral compositions suggest formation in a forearc, as Fo values of olivine are in 91.1–93.0 harzburgite and 91.5–91.9 in dunite and 94.6–95.9 in massive chromitite. The composition of the melts passing through the peridotites changed gradually from tholeiite to boninite due to melt–rock reaction, leading to more High Cr# chromitites in the upper part of the body. Most of the massive and disseminated chromitites have high Cr# numbers (70–80), although there are systematic changes in olivine and magnesiochromite compositions from harzburgites, to dunite envelopes to massive chromitites, reflecting melt–rock reaction. Compositional zoning of orthopyroxene porphyroblasts in the harzburgite, incongruent melting of orthopyroxene and the presence of small, interstitial grains of spinel, olivine and pyroxene likewise attest to modification by migrating melts. All of the available evidence suggests that the Bulqiza Ophiolite formed in a suprasubduction zone mantle wedge.

The structure of the ophiolitic beltin Albania inferred from geomagnetic anomalies

The ground magnetic measurements in Albania were used for the compilation of the Total Magnetic Field (TMF) anomaly map of Albania. The magnetic data were processed and interpreted in order to study the structure of the ophiolitic belts of Albania. The ophiolites of Albania are placed at the Mirdita zone and are divided into two parallel alignments which are called the eastern and western ophiolitic belts. They are associated with strong potential field anomalies and their characteristics are considered crucial for a better understanding of the tectonic settings of Albania. The ground TMF data used in this study were acquired over various campaigns (1990-1994) and cover most of Albania's territory. The data were compiled to a map after reduction to the epoch 1990.4. The strongest magnetic anomalies in Albania appear along the known ophiolitic belts which trend NE-SW to the north and NW-SE to the south. Several processing steps were applied to the unified and gridded data in order to obtain information on the distribution of the magnetic sources. The magnetic sources were subsequently modeled using a 2.5D inversion technique. The magnetic properties of the ophiolites determined from laboratory measurements on rock samples, while their lateral extent was calculated from the processing of the magnetic data and used as constraints to the inversion procedure. The bottom of the ophiolitic belts is considered to be predominated by harzburgites. They exhibit lower magnetization than other rocks of the complex, i.e. gabbros, basalt. In fact gabbros are associated with the observed high frequency magnetic anomalies. The modeling results indicate that the thickness of the ophiolites is reduced from east to west. The eastern ophiolitic belt has a maximum thickness of approximately 12 km at its northern section (Kukesi and Lura massifs). The magnetic sources appear with relatively small thickness at the western ophiolitic belt. Boundaries of the eastern ophiolites are characterized by vertical contacts.

The origin of amphibolites from metamorphic soles beneath the ultramafic ophiolites in Evia and Lesvos (Greece) and their geotectonic implication

The dismembered Jurassic ophiolitic complexes of the two Greek islands, Evia and Lesvos show significant similarities concerning their general structural, petrographic and geochemical characteristics. However, their geotectonic position is quite different, the Evia ophiolites belonging to the “Western Ophiolite Belt” of the internal Hellenides, whereas the Lesvos ophiolites seem to be part of the “Eastern Ophiolite Belt”, representing a possible link to the Pontides. Both ophiolites consist of large ultramafic masses, which are emplaced onto continental crust. The overthrust zones comprise flysch and ophiolitic mélange as well as amphibolitic metamorphic soles beneath the ultramafics. The identification of the protolithic character of the amphibolites as well as their metamorphic and deformation history allows a clarification of their complex emplacement processes from their original extensional oceanic ridge environment within the Pindos and Vardar marginal basins into a compressional supra-subduction zone, island arc and continental environment. On the basis of relict magmatic minerals and textures as well as on major and trace element bulk chemical composition, the medium to coarse grained Evia amphibolites are interpreted as former gabbroic and cumulate rocks similar to equivalent amphibolites dredged from numerous fracture zones in the Atlantic oceanic crust. Horizontal and vertical displacements which are able to produce such foliated and mylonitized gabbroic rocks suggest the existence of major deep reaching transform faults, which may be considered as one of the most important locations for the breakup of oceanic crust and lithosphere and subsequent incipient subduction. The Evia amphibolites could have been formed by a two-stage process: first, as gabbros that formed part of the gabbroic oceanic crust and secondly representing as small gabbroic and cumulate bodies within the ultramafic mantle. Either of these were transformed into low-grade epidote amphibolites under oceanic metamorphic facies conditions and deformed by transform fault displacement mechanism. Subsequently, within the Pindos intra-oceanic basin, these transform faults also facilitated obduction of slices of oceanic mantle and crust onto an accretionary wedge. In this respect, the Evia ophiolite complex shows a very similar emplacement process to that of the Othris, Vourinos and Pindos ophiolites, which can be interpreted using a combination of an intra-oceanic transform fault thrust and an obduction model. In this respect, the “Evia transform fault–overthrust model” sheds new light onto the discussion of the age of formation of oceanic crust, as well as on the timing of subduction and obduction. The so called “sole amphibolites“, especially those of only low-grade epidote-amphibolite facies may have already been formed during mid-ocean floor spreading and transform fault movements and not later during the ophiolite emplacement processes. Therefore all of the ages of oceanic crust and amphibolites, both from sedimentary and paleontological evidence as well as from radiometric data indicate a specific time window between the generation of the older parts of oceanic crust around 170 Ma (Bajocian) and their emplacement processes at approximately 155 to 150 Ma. This leads to the conclusion that the Pindos marginal oceanic basin could have at least an age spread of 15 to 20 Ma. In contrast, the Lesvos amphibolites are fine to medium grained with granoblastic and nematoblastc textures with relics of clinopyroxene and plagioclase. Cataclastic fabrics are also present as well as retrograde mineral assemblages from greenschist to prehnite facies metamorphic overprinting. According to bulk chemical analyses, these amphibolites are identified as MORB-type oceanic basalts as well as SSZ (supra-subduction zone/IAT —island arc tholeiite) basalts and olivine gabbros. The Lesvos amphibolites are seen as basaltic and gabbroic slices, which were incorporated into a supra-subduction, gabbro bearing island arc environment, overridden by larger slices of depleted lherzolite and harzburgite mantle. Since the amphibolites show in many cases highly deformed fabrics and a rather high-grade metamorphism as well as MORB basaltic compositions, an intra-oceanic thrusting cannot be excluded. Their origin can rather be placed in the Vardar oceanic basin than in the Pindos basin.

TRIASSIC MORB MAGMATISM IN THE SOUTHERN MIRDITA ZONE (ALBANIA)

In southern Albania the Mirdita Ophiolitic nappe is characterized by subophiolitic complexes in which remnants of volcanic ophiolite sequences of Triassic age have been identified, either as rare blocks of variable dimension in the Rubik Complex, or as a thin tectonic unit (the Porava Unit), sited immediately under the main ophiolitic masses of the Eastern Ophiolite Belt. In this paper the results of petrological investigations on basalts and biostratigraphical studies on associated radiolarian cherts included in these subophiolitic complexes units are presented. Biostratigraphical investigations indicate that cherts have ages ranging from Middle to Late Triassic. The associated basalts are represented by both high- Ti mid-ocean ridge basalts (MORB) and alkaline ocean island basalts (OIB). MORB rocks mainly consist of basalts and ferrobasalts with a mild enrichment in low field strength elements and flat rare earth element patterns and, subordinately, by basalts strongly depleted in incompatible elements and light rare earth elements. The chemistry of slightly enriched MORB is consistent with a generation in a mid-ocean ridge setting, from somewhat enriched sub-oceanic mantle source(s), whereas depleted MORB generated from a primitive MOR-type mantle source. The OIB rocks imply a generation in a within-plate oceanic setting from a mantle source enriched by plume chemical components. Basalts and associated cherts from southern Albania subophiolitic melanges represent remnants of a Triassic oceanic lithosphere, which testify for the existence, from northern Albania to southern Greece, of a Middle to Late Triassic oceanic basin located between the Adria and Eurasia plates. The occurrence in the Rubik Complex and Porava Unit of MOR basalts generated from differently enriched sources, as well as of alkaline OIBs, suggests that the early stage of oceanic spreading was variably associated with a plume activity.

The geological setting of volcanogenic sulphide orebodies in Albanian ophiolites

Cu–Fe sulphides with minor Zn, Au and Ag occur as massive, disseminated and vein mineralizations in volcanosedimentary rocks, volcanics and occasionally associated intrusions of a sheeted dyke complex in ophiolites in Albania. The ophiolites form two sub-parallel units, the Western Ophiolite and Eastern Ophiolite Belts, stretching across the country from north-west to south-east, and have mid-ocean ridge and island arc (boninite supra-subduction type) affinities, respectively. The ores have been extensively prospected and were mined until the 1990s.The Western Ophiolite Belt comprises a volcanosedimentary series, pillow basalts, ultramafic rocks, gabbros and plagiogranites. The most significant orebodies are in volcanosedimentary rocks at Rubik, Palaj-Karme and Porave. The orebodies with massive chalcopyrite–pyrite ores vary in shape occurring between hematised radiolarian chert and tholeiitic basalt pillow lavas. Mapping the shape of the main orebody at Rubik shows a series of mounds comparable to the collapse of 'black smoker' chimneys. Massive and disseminated sulphides are hosted by MORB affinity basalts at Kachinar, Derven and Rehova. A geophysical induced polarisation survey has indicated further resources at depth at West Kachinar.The Eastern Ophiolite Belt consists of a basalt–andesite–rhyolite series cut by a sheeted dyke complex and underlain by quartz diorite–plagiogranite, gabbro, gabbronorite and harzburgite. It includes a 25-km long belt of a significant longitudinal continuity of mineralization between Chafe Mal and Reps. Massive sulphide ores underlain by disseminated veins and stockworks are hosted by a Middle–Upper Jurassic lower basalt–andesite succession at Reps, Spach, Maja e Made, Tuch, Lak Rosch, Paluce and Fushe Arres. Alteration includes chloritization, epidotization, silicification, argillisation, zeolitization and carbonatisation. The Upper Jurassic, upper dacite volcanoclastic, andesite–dacite, boninite succession hosts massive pyrite–chalcopyrite–sphalerite ore underlain by few or no disseminated veins or stockworks at Gurth Spach, Munelle and Chafe Bar. At Perlat, massive ores are in an andesite pillow lava. Induced polarisation anomalies indicate further orebodies at depth in this highly thrusted area. Sulphide veins and other quartz–lode sulphides occur in some gabbros. An unusual orebody at Gjegjan occurs in a tectonic mélange of volcanics, greenschists and amphibolites.Similarities between Albania and other ophiolitehosted volcanogenic deposits at Cyprus and Oman include a clustering of orebodies along the general strike of the ophiolite, the occurrence of massive ores at successive horizons, and the presences of vein, stockwork and disseminated mineralization surrounding and beneath the massive sulphide zones; but, in Albania, ochres and umbers are not found, the age is older and the age-span greater. The Albanian orebodies range in size from 0·3–10·0 Mt. Those in the Western Ophiolite Belt are comparable in size to modern seafloor deposits at ocean ridges. Eastern Ophiolite Belt orebodies are similar in size to others interpreted as having formed in a suprasubduction environment. The Eastern Ophiolite Belt offers the greatest potential for locating future orebodies, possibly beneath thrust planes.

Mantle xenolith‐bearing Maastrichtian to Tertiary alkaline magmatism in Oman

Mantle xenolith‐bearing alkali basalts, basanites and tephrites with 1.5–2 K2O and 4–5 wt% Na2O occur as small (<100 m) plugs and dikes in the Batain and Haushi‐Huqf areas, and WNW of Muscat. Their black color and the common presence of peridotite xenoliths allow a separation from older alkaline rocks and from the ophiolitic extrusive rocks. Two main dike directions, approximately E‐W and N‐S oriented, are observed. Intrusions seem to occur where these two fault systems intersect. The basalts crosscut the Upper Maastrichtian siliciclastic rocks of the Fayah Formation and the nappe stack of the Eastern Ophiolite Belt of the Batain area. They have not been observed intruding the Tertiary shallow marine carbonates although K‐Ar whole rock dating on these lavas yielded Late Eocene 37 ± 1 to 44 ± 1 Ma ages. The rocks are aphyric and fine‐grained with microgranular, and less common microtrachytic texture. They contain magmatic olivine (Fo80–82), nepheline (Ne82–86Ks14–18), clinopyroxene (Di46En25Wo28) with 2.5 wt% Al2O3, accessory phlogopite (XMg0.6, with 5.5 wt% TiO2), in a microcrystalline or glassy matrix. Plagioclase is only observed in few samples. Locally occurring mm to cm‐sized immiscible leucocratic melt droplets consist of potassium feldspar (An1–4Ab37–39Or60–70), plagioclase (An20–22Ab65Or13–15), nepheline (Ne85Ks15), phlogopite (XMg0.8, with ∼6–7 wt% TiO2), and titanomagnetite with ∼8 wt% TiO2. More than 95% of the xenoliths are <5cm‐sized weakly to clearly foliated spinel peridotites of mantle origin. Sedimentary xenoliths (hornfelses) are rare and lower crustal xenoliths were not found. The peridotite xenoliths consist of olivine (Fo90–92), enstatite (En89Fs09Wo02) with 3.1–3.3 wt% Al2O3, diopside (Di45En26Wo29) with 3–6 wt% Al2O3, and Cr‐spinel with ∼60 mol% spinel, 20% hercynite, and 15–16% magnesiochromite. The age, chemical composition, and structural position suggest a relation with local tectonic movements associated with plate reorganization in the Owens Basin region prior to the Red Sea opening.

The Jurassic South Albanian ophiolites: MOR- vs. SSZ-type ophiolites

Within the western belt of the southern Albanian ophiolites, the Voskopoja ophiolite consists of three subunits: Voskopoja, Morava and Rehove. These are predominantly lherzolites with minor harzburgites and dunites in the mantle section. Above come ultramafic and mafic cumulates including wehrlites, troctolites and olivine gabbros. Gabbronorites are restricted to the Morava subunit. Isotropic clinopyroxene gabbros, extrusives and sediments are present only in Rehove and Voskopoja. The volcanic section is dominated by basaltic breccias, including megablocks with sheeted dykes, pillow lavas and isolated dykes. The basaltic breccias grade upwards into sandstones, in turn, interlayered with argillites and cherts of Jurassic age. The basalts are predominantly clinopyroxene–plagioclase basalts, either aphyric or plagioclase phyric. Geochemically, they are divided into four groups: (1) an intermediate Ti and Zr group with low Ni (hereafter called low-Ni group), (2) an intermediate Ti–Zr group with high Ni (hereafter called high-Ni group), (3) a high-Ti–Zr group and (4) a low-Ti–Zr group. The high-Ni content in group 2 is interpreted as originating from olivine and spinel xenocrysts. Apart from the high-Ni content, groups 1 and 2 are comparable with the volcanics of the “low- to high-Ti intermediate ophiolites.” By contrast, group 3 is more comparable to the high-Ti ophiolitic extrusives in the western ophiolite belt of northern Albania. Group 4 consists SSZ-type basalts and is widespread in the volcanics of the eastern ophiolite belt. Comparison of the ultramafic–mafic cumulates and the basaltic volcanics with those in the northern part of the western belt in Albania and the Pindos ophiolite indicates that there is a systematic variation in petrography and geochemistry from north to south in the western belt, with an increasingly distinct SSZ signature towards the south. Ultramafic and mafic cumulates, as well as basalts from the Shebenik massif in the eastern belt, are similar to those of Voskopoja, implying a genetic relationship.

Geochemical Characterization of Podiform Chromite Ores from the Ultramafic Massif of Bulqiza (Eastern Ophiolitic Belt, Albania) and Hints for Exploration

Four main podiform occurrences of chromite have been distinguished in the ultramafic Bulqiza massif, Albania; they are associated from the bottom to the top with: i) basal harzburgites (lower tectonite sequence), ii) dunitic lens-bearing harzbugites (middle-upper tectonite sequence), iii) layered chromitite-bearing dunites of transitional zone, and, iiii) the stratigraphically lowermost part of the magmatic section. However, only the occurrences hosted by rocks ii) and iii) are economically important. Chromite compositions similar to those of other podiform chromites are bimodal. Thus, chromites from occurrences ii) and iii) are richer in Cr than chromites from the other two occurrences. Accessory chromite disseminated throughout the massif is richer in Fe than orebody chromite, displaying similar range of the Cr/Cr+Al ratios, with accessory residue chromite matching the characteristic (Cr/Cr+Al >0.6) range of type-III Alpine peridotites. Both orebody and accessory chromites show compositional ranges correlating with host-rock compositions, i.e., the Cr-rich chromites are hosted by the most refractory ultramafic rocks. Therefore, the Cr/Cr+Al and Mg/Fe tot (Cr/Fe tot ) ratios of chromite are useful for exploration of high-grade, metallurgical chromite ore at Bulqiza and in other massifs of similar geodynamic setting.

West‐northwest directed obduction of the Batain Group on the eastern Oman continental margin at the Cretaceous‐Tertiary boundary

The Batain coast area in eastern Oman is dominated by allochthonous Permian to Late Maastrichtian sedimentary and volcanic rocks (Batain Group), unconformably overlain by neoautochthonous Tertiary sediments. The allochthonous rocks of the Batain coast were previously attributed to the Hawasina complex, the Permian to Coniacian/Santonian sedimentary infill of the neo‐Tethyan Hawasina basin off northern Oman. Previous structural interpretations suggested that the Batain Group, along with the Hawasina complex and the Semail ophiolite, was obducted in the Coniacian to Campanian from NE to SW onto the northern Oman continental margin. Results of our work in the Batain area differ from previous interpretations, with most significant differences concerning timing and direction of obduction. Our results show that WNW directed tectonic movements formed a fold‐and‐thrust belt and led to the obduction of allochthonous rocks onto the east Oman continental margin during latest Maastrichtian/earliest Paleocene times. This is coeval with emplacement of ophiolitic fragments along the eastern coast of Oman (eastern ophiolite belt) but is about 15–20 Myr later than emplacement of Hawasina complex and Semail ophiolite in northern Oman. Postemplacement structural evolution during the Tertiary involved intraplate extension, possibly reflecting the Red Sea/Gulf of Aden opening, and late Tertiary shortening related to convergence between Arabia and Eurasia. Late Tertiary contractional deformation resulted in refolding of the Batain nappes and in folding of the overlying Tertiary sediments. A palinspastic reconstruction of the Batain area indicates that the Permian to Upper Cretaceous sediments were formerly deposited in the Batain basin, a part of the proto‐Indian Ocean, along the present‐day eastern Oman margin. This leads us to propose that Permian breakup of Gondwanaland created both continental margins of Oman and led to the opening of two major basins: the neo‐Tethyan Hawasina basin in the north and the proto‐Indian Ocean Batain basin in the east, the latter separating Arabia from greater India.

Late Cretaceous/early Tertiary convergence between the Indian and Arabian plates recorded in ophiolites and related sediments

Remnants of ocean floor forming the Eastern Ophiolite Belt in Oman and the Western Ophiolite Belt in Pakistan have a common plate-tectonic history culminating in emplacement at the Cretaceous/Tertiary boundary. Fragments of ocean floor in these two belts have ages between 150 and 65 Ma and recorded tectonic events in the early Indian Ocean at 150 Ma, 130-120 Ma, 110-100 Ma and 70-65 Ma. New radiometric and chronostratigraphic ages and paleomagnetic and sedimentary information are used to relocate these ophiolites in the frame of the evolving Indian Ocean, which was characterised by the stepwise breakup of Gondwana at 158 Ma (East and West Gondwana), 130 Ma (Southern Atlantic, breakup of East Gondwana), 95-84 Ma (Madagascar and India/Seychelles), 65 Ma (India and Seychelles) and, finally, at 40 Ma, rifting between Africa and Arabia. The 150-Ma-old oceanic rocks of Masirah Island in Oman originally formed the extension of the basins now preserved along the eastern edge of the Afro-Arabian plate. Masirah drifted together with India-Seychelles when a new ridge formed at approximately 130 Ma separating microplates (e.g., Kabul Block) from the northern edge of Greater India. Parts of this new ocean were later emplaced to form the western ophiolite belt in Pakistan. Consumption of oceanic crust and emplacement of ophiolites during the closure of the Neotethys occurred not only north of India (± Seychelles), but also along two or three subduction zones between the Afro-Arabian plate and India/Seychelles, documented by the formation of metamorphic soles beneath ophiolites. Relics of ocean floor formed approximately 65–70 Ma ago, now form the upper units in the Western Ophiolite belt of Pakistan where they overlie an accretionary prism of pillow lavas of dominantly (Aptian-) Albian age and sedimentary rocks. The breakup of India and the Seychelles at 65 Ma was associated with the eruption of flood basalts (Deccan and Seychelles) and enhanced counter-clockwise movement of India. This breakup possibly caused the Late Jurassic/Early Cretaceous fragments of ocean floor (e.g., Masirah) to be thrust onto the eastern edge of Oman. The ophiolites, which were emplaced onto the western edge of India, drifted as part of this plate farther north where collision with Eurasia and the accreted microplates occurred in the Eocene (∼ 55 Ma).

Tectonics of the ophiolite belt from Naga Hills and Andaman Islands, India

The ophiolitic rocks of Naga Hills-Andaman belt occur as rootless slices, gently dipping over the Paleogene flyschoid sediments, the presence of blue-schists in ophiolite melange indicates an involvement of the subduction process. Subduction was initiated prior to mid-Eocene as proved by the contemporaneous lower age limit of ophiolite-derived cover sediment as against the accreted ophiolites and olistostromal trench sediment. During the late Oligocene terminal collision between the Indian and Sino-Burmese blocks, basement slivers from the Sino-Burmese block, accreted ophiolites and trench sediments from the subduction zone were thrust westward as nappe and emplaced over the down-going Indian plate. The geometry of the ophiolites and the presence of a narrow negative gravity anomaly flanking their map extent, run counter to the conventional view that the Naga-Andaman belt marks the location of the suture. The root-zone of the ophiolite nappe representing the suture is marked by a partially-exposed eastern ophiolite belt of the same age and gravity-high zone, passing through central Burma-Sumatra-Java.