Petrology and geochemistry of mafic dykes from the Muslim Bagh Ophiolite (Pakistan): implications for petrogenesis and emplacement
Mohammad Ishaq KakarKhalid MahmoodMohammad ArifMehrab KhanAndrew C. KerrMohibullah MohibullahAimal Khan Kasi
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Abstract:
Two different types of mafic dykes are found in the Muslim Bagh Ophiolite: 1) a sheeted dyke complex and 2) a mafic dyke swarm. Relative to the host plutonic section, the sheeted dykes are poorly developed, implying that they formed in an oceanic setting with a low and intermittent supply of magma, probably because of cyclic accumulation of crystals at the base of the magma chamber. Both the sheeted dykes and the dyke swarms have been metamorphosed to greenschist/amphibolite facies conditions. With the exception of the upper level gabbros and sheeted dykes, the dyke swarms crosscut almost the whole ophiolite suite as well as the metamorphic sole rocks, but are truncated structurally at the contact with the underlying mélange and sediments. Hence, the injection of the dyke swarms postdates the formation of both the main Muslim Bagh Ophiolite and the metamorphic sole rocks, but predates the accretion of the mélange and the final emplacement of the ophiolite onto the Indian plate margin. Both the sheeted dykes and dyke swarms are tholeiitic and have a geochemical signature of either island arc tholeiites (IAT) or are transitional between mid-oceanic ridge basalts and IAT. Oceanic rocks with such characteristics, especially their enrichment in large-ion lithophile elements, are generally thought to have formed by processes involving a subduction zone component in the source region by fluids released from the subducting slab. The Muslim Bagh Ophiolite sheeted dykes originated in the late Cretaceous, in a supra-subduction zone tectonic setting related to the subduction of a narrow branch of the Neo-Tethys Ocean, followed by a subduction rollback due to splitting of the nascent arc in the Tethys Ocean. This intra-oceanic subduction led to the formation of a metamorphic sole, followed by the off-axis intrusion of mafic dykes into the ophiolite through a slab window. The Muslim Bagh Ophiolite was accreted to the Bagh Complex and finally obducted onto the Indian Platform.Keywords:
Petrogenesis
Pillow lava
Greenschist
Island arc
The Nidar Ophiolite is located between the North Himalayan nappes and the Indus Suture Zone in NW Himalaya in eastern Ladakh (India). Based mainly on geochemical argument, this ophiolite is classically interpreted as a relic of an intra-oceanic arc (Maheo et al. 2000; Maheo et al. 2004), which developed at around 140 Ma, prior to the collision between the Indian and Eurasian plates (Ahmad et al. 2008). From top to bottom, this ophiolite is composed of various sedimentary rocks (radiolarites, polygenic conglomerates and carbonates), volcanic rocks (pillow lavas, basaltic to andesitic in composition), gabbros (Fe- and layered gabbros, pegmatites and minor troctolites), serpentinites, dunites, pyroxenites and peridotites (mainly harzburgites). The Nidar Ophiolite underwent an anchizonal metamorphism with preservation of primaries structures (layering) and volcanic textures (pillow lavas). This study is mainly focused on new field observations across the ophiolite and the surrounding units. A new detailed geologic map of the ophiolite between the Nidar village and Kyun Tso area is presented. The upper part of the ophiolitic complex is an alternation of volcanic and sedimentary rocks (500- 1000 m thick) and the lower part consists of large outcrops of gabbros (3000m thick). These mafic rocks are separated from the serpentinized ultramafic rocks by a 200m thick ophiolitic breccia and continental Indus Molasse slices. The Nidar Ophiolite is made up of the classical rock type succession (ultramafites, gabbros, pillow basalts, radiolarites), but the internal structure is far more complex than previously suggested. New field data (geologic and structural maps, lithologic sections, etc.) coupled with new geochemical analysis will help to constrain the geodynamic context and deformation history. Ahmad, T., T. Tanaka, H.K. Sachan, Y. Asahara, R. Islam, et P.P. Khanna. 2008. « Geochemical and isotopic constraints on the age and origin of the Nidar Ophiolitic Complex, Ladakh, India: Implications for the Neo-Tethyan subduction along the Indus suture zone ». Tectonophysics 451 (1–4): 206‑ 24. Maheo, Gweltaz, Herve Bertrand, Stephane Guillot, Georges Mascle, Arnaud Pecher, Christian Picard, et Julia De Sigoyer. 2000. « Temoins d’un arc immature tethysien dans les ophiolites du Sud Ladakh (NW Himalaya, Inde) ». Comptes Rendus de l’Academie des Sciences - Series IIA - Earth and Planetary Science 330 (4): 289‑ 95. Maheo, Gweltaz, Herve Bertrand, Stephane Guillot, Igor M. Villa, Francine Keller, et Paul Capiez. 2004. « The South Ladakh ophiolites (NW Himalaya, India): an intra-oceanic tholeiitic arc origin with implication for the closure of the Neo-Tethys ». Chemical Geology 203 (3–4): 273‑ 303.
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AbstractAbstractCu–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.Keywords: SULPHIDE OREBODIESALBANIAN OPHIOLITES
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The claim by Furnes et al . (Reports, 23 March 2007, p. 1704) that Greenland metavolcanic rocks require Paleoarchean sea-floor spreading is incompatible with their own data. The purported sheeted dikes have the composition of pyroxenitic komatiite and could not have fed the adjacent ferroandesitic pillow lavas. Neither type has ophiolitic analogs, and both are likely ensialic.
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Back-arc basin
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The Lanong ophiolite,located in Jiangcuo,Bange,Tibet,is a key element within the Baila-Lanong ophiolite belt.Characterized by well preserved pillow lavas,the Lanong ophiolite consists of blocks of peridotites,gabbros,diabases and pillow lavas.Trace elements analysis indicates that the pillow lavas are enriched in Sr,Ba and Th,delpleted in Nb,Ta and Ti,clearly displaying compositional characteristics of the island-arc volcanic rocks.On the Chondrite-normalized trace element diagram,all lava samples show flat patterns,with (La/Yb)N and (Ce/Yb)N ratios similar to those of N-MORB.In conclusion,the Lanong ophiolite is of SSZ affinity,and should be the relics of a back arc basin.Combining the terranes' distribution with the discrimination results of geochemical diagrams,a simple tectonic evolution model is suggested:the Neotethys oceanic crust to the north began to southwardly subduct in middle-late Jurrasic,then a spreading sub-centre was formed on the obducting slab south of the subdction zone,which developed into a back arc basin subsequently.As the subdction going on,the back arc basin closed and only portions of the back arc crust were preserved by accretion.
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The Horokanai ophiolite is a segment of metamorphosed oceanic crust and upper mantle, tectonically replaced into the Kamuikotan Zone of Hokkaido, Japan. Metamorphic grade, ranging from the zeolite faciles (Zone A) through the greenschist facile (Zone B) and the greenschist-amphibolite transitional facile (Zone C), to the amphibolitic and granulizes facile (Zone D) increases progressively downwards with zone boundaries subparallel to the ophiolite pseudostraitigraphy. The granulite facile rocks include both metagabbros and their underlying ultranafic rocks. Coexisting minerals from several tens of samples covering all the minerals zones were analysed by means of an electronprode microanalyser; the results are presented, along with brief consideration of their compositional variation with metamorphic grade. The faciles series of metamorphism of the Horokanai ophiolite corresponds to the low-pressure type with a temperature range of 100-750 °C, which is broadly comparable to that inferred for ocean-floor metamorphism. The major difference is the presence of the granulite facile rocks in the Horokanai ophiolite and its absence in ocean-floor metamorphism.
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