Peridotites and wherlites with original characteristics in the transition zone of the semall ophiolite (OMAN). Implications concerning the partial melting dynamics of the mantle at oceanic ridges
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The origin of basal lherzolites in the mantle section of harzburgite‐dominated ophiolites is enigmatic. The basal part of the mantle section is well exposed in the Muslim Bagh Ophiolite, Pakistan, which is one of the harzburgite‐dominated ophiolites of the Tethys Ophiolite Belt. In this contribution, we describe the basal lherzolite of the Muslim Bagh Ophiolite, Pakistan, and discuss its origin based on the trace‐element characteristics of its clinopyroxenes. The basal lherzolite exhibits porphyroclastic to mylonitic textures. Primitive mantle‐normalized trace‐element patterns of the porphyroclastic clinopyroxenes were characterized by low ratios of light rare‐earth elements (LREEs) to heavy rare‐earth elements (HREEs), low abundances of HREEs, and positive Sr anomalies. These geochemical characteristics are not consistent with a lherzolite and its clinopyroxenes, which is formed by a residue after a low degree of partial melting and melt extraction from peridotites in a mid‐ocean ridge setting. Instead, the compositions of the clinopyroxenes are consistent with open‐system melting induced by the infiltration of slab‐derived fluids into residual peridotites that had been depleted in REEs. The compositions of chromian spinels in the chromitites of the basal peridotite sequence are also consistent with their formation in an arc setting. We conclude that the basal lherzolites of the Muslim Bagh Ophiolite represent a residue after a relatively high degree of partial melting, and that the clinopyroxenes were added as metasomatic crystallization from slab‐derived arc‐related melts to this residual depleted peridotite in a subduction setting.
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Synthetic seismograms for the Samail ophiolite, computed using the full reflectivity technique, are used to test the hypothesis that this ophiolite complex can serve as a prototype for young oceanic crust. If so, then this ophiolite complex can be compared with the older (45 m.y.) Bay of Islands (BOI) ophiolite to develop an aging model for oceanic crust. Synthetics for this young emplacement age (5–8 m.y.) ophiolite are compared with ocean bottom hydrophone data from 0.5‐ and 4.5‐m.y.‐age crust obtained during the Rivera Ocean Seismic Experiment. Second‐arrival phases from reflected refractions from one ophiolite velocity‐depth model and data from 4.5‐m.y.‐age crust are in excellent agreement; this agreement suggests that the Samail ophiolite is a good working model for young oceanic crust. Earlier work has shown that the BOI ophiolite is a good prototype for mature oceanic crust. The BOI ophiolite data also suggest that alteration of the lower oceanic crust and upper mantle is limited. We use the petrologic and geochemical properties of the Samail and BOI ophiolites to develop a two‐stage model for crustal aging that directly limits alteration of the lower crust and upper mantle materials.
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Every few years the people who work on ophiolites and the people who work on oceanic crust decide that it is time to get together and compare notes. The ophiolite people are primarily but not exclusively traditional “land” geologists who combine geologic mapping with structural, petrologic, and geochemical studies; the ocean crust people are primarily (but not exclusively) marine geologists and geophysicists who drill and dive on the real thing. There is some overlap of people who do both, but since most do either one or the other, these get-togethers are a critical reality check for everyone.
GSA Special Paper 349 “Ophiolites and Oceanic Crust: New Insights from Field Studies and the Ocean Drilling Program,” presents results from a Penrose Conference on this topic convened by the editors in 1999 in northern California. In Penrose tradition, the meeting combined keynote speakers, short oral presentations, and poster sessions with extended discussion sessions, moderated and summarized, on the …
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Obduction
Ultramafic rock
Pillow lava
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Diorite
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Complete developmental ophiolitic suites have a sight of typical four layered textures, which shows similarities to nowadays oceanic crust. They are formed in an extend environment by the mid oceanic ridge. Because of tectogenesis make for ophiolitic suites dismember and tectonic emplacement in to the Continental suture zone. Along the faults zones ophiolitic mass occurred in the form of tectonic shect and tectonic melange. Ophiolite give main evidence for the oceanic crust formation and break. Ophiolite belt or paleo suture line has a direct mark, which defined paleo oceanic basin in the orogenic belt and defined block of continet block
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