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Title: Coarse calcite grains in hydrocarbon-rich tuffaceous mudstone in a Permian volcanic lake—An indicator for subaqueous volcanic deposition and related hydrocarbon generation
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Sedimentary Geology
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National Science Foundation
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  1. Abstract. Numerous studies have revealed genetic similarities between Tethyanophiolites and oceanic “proto-arc” sequences formed above nascent subductionzones. The Semail ophiolite (Oman–U.A.E.) in particular can be viewed as ananalogue for this proto-arc crust. Though proto-arc magmatism and themechanisms of subduction initiation are of great interest, insight isdifficult to gain from drilling and limited surface outcrops in marinesettings. In contrast, the 3–5 km thick upper-crustal succession of theSemail ophiolite, which is exposed in an oblique cross section, presents anopportunity to assess the architecture and volumes of different volcanicrocks that form during the proto-arc stage. To determine the distribution ofthe volcanic rocks and to aid exploration for the volcanogenic massivesulfide (VMS) deposits that they host, we have remapped the volcanic unitsof the Semail ophiolite by integrating new field observations, geochemicalanalyses, and geophysical interpretations with pre-existing geological maps.By linking the major-element compositions of the volcanic units to rockmagnetic properties, we were able to use aeromagnetic data to infer theextension of each outcropping unit below sedimentary cover, resulting ina new map showing 2100 km2 of upper-crustal bedrock. Whereas earlier maps distinguished two main volcanostratigraphic units, wehave distinguished four, recording the progression from early spreading-axisbasalts (Geotimes), through axial to off-axial depleted basalts (Lasail), topost-axial tholeiites (Tholeiitic Alley), andmore »finally boninites (BoniniticAlley). Geotimes (“Phase 1”) axial dykes and lavas make up ∼55 vol % of the Semail upper crust, whereas post-axial (“Phase 2”) lavasconstitute the remaining ∼45 vol % and ubiquitously coverthe underlying axial crust. Highly depleted boninitic members of the Lasailunit locally occur within and directly atop the axial sequence, marking anearlier onset of boninitic magmatism than previously known for theophiolite. The vast majority of the Semail boninites, however, belong to theBoninitic Alley unit and occur as discontinuous accumulations up to 2 kmthick at the top of the ophiolite sequence and constitute ∼15 vol % of the upper crust. The new map provides a basis for targetedexploration of the gold-bearing VMS deposits hosted by these boninites. Thethickest boninite accumulations occur in the Fizh block, where magma ascentoccurred along crustal-scale faults that are connected to shear zones in theunderlying mantle rocks, which in turn are associated with economicchromitite deposits. Locating major boninite feeder zones may thus be anindirect means to explore for chromitites in the underlying mantle.« less