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A revised map of volcanic units in the Oman ophiolite: Insights into the architecture of an oceanic proto-arc volcanic sequence

A revised map of volcanic units in the Oman ophiolite: Insights into the architecture of an oceanic proto-arc volcanic sequence
A revised map of volcanic units in the Oman ophiolite: Insights into the architecture of an oceanic proto-arc volcanic sequence

Numerous studies have revealed genetic similarities between Tethyan ophiolites and oceanic “proto-arc” sequences formed above nascent subduction zones. The Semail ophiolite (Oman–U.A.E.) in particular can be viewed as an analogue for this proto-arc crust. Though proto-arc magmatism and the mechanisms of subduction initiation are of great interest, insight is difficult to gain from drilling and limited surface outcrops in marine settings. In contrast, the 3–5 km thick upper-crustal succession of the Semail ophiolite, which is exposed in an oblique cross section, presents an opportunity to assess the architecture and volumes of different volcanic rocks that form during the proto-arc stage. To determine the distribution of the volcanic rocks and to aid exploration for the volcanogenic massive sulfide (VMS) deposits that they host, we have remapped the volcanic units of the Semail ophiolite by integrating new field observations, geochemical analyses, and geophysical interpretations with pre-existing geological maps. By linking the major-element compositions of the volcanic units to rock magnetic properties, we were able to use aeromagnetic data to infer the extension of each outcropping unit below sedimentary cover, resulting in a new map showing 2100 km2 of upper-crustal bedrock.

Whereas earlier maps distinguished two main volcanostratigraphic units, we have distinguished four, recording the progression from early spreading-axis basalts (Geotimes), through axial to off-axial depleted basalts (Lasail), to post-axial tholeiites (Tholeiitic Alley), and finally boninites (Boninitic Alley). Geotimes (“Phase 1”) axial dykes and lavas make up ∼55 vol % of the Semail upper crust, whereas post-axial (“Phase 2”) lavas constitute the remaining ∼45 vol % and ubiquitously cover the underlying axial crust. Highly depleted boninitic members of the Lasail unit locally occur within and directly atop the axial sequence, marking an earlier onset of boninitic magmatism than previously known for the ophiolite. The vast majority of the Semail boninites, however, belong to the Boninitic Alley unit and occur as discontinuous accumulations up to 2 km thick at the top of the ophiolite sequence and constitute ∼15 vol % of the upper crust. The new map provides a basis for targeted exploration of the gold-bearing VMS deposits hosted by these boninites. The thickest boninite accumulations occur in the Fizh block, where magma ascent occurred along crustal-scale faults that are connected to shear zones in the underlying mantle rocks, which in turn are associated with economic chromitite deposits. Locating major boninite feeder zones may thus be an indirect means to explore for chromitites in the underlying mantle.

1869-9510
1181-1217
Belgrano, Thomas M.
6135b1b8-ca0f-41a6-a94a-6b6c3513dee3
Diamond, Larryn W.
2f4d31e5-b7df-41bb-9be1-975c823fc883
Vogt, Yves
449cffca-621d-4dca-8ade-167c3cc97c8c
Biedermann, Andrea R.
152dafb6-121a-4ab4-9c55-4bf81976753d
Gilgen, Samuel A.
634e2d69-8d77-4a8f-8e10-e01695f7aa7d
Al-Tobi, Khalid
00d422de-b5e8-4ea0-9b27-c49abe7ab2d7
Belgrano, Thomas M.
6135b1b8-ca0f-41a6-a94a-6b6c3513dee3
Diamond, Larryn W.
2f4d31e5-b7df-41bb-9be1-975c823fc883
Vogt, Yves
449cffca-621d-4dca-8ade-167c3cc97c8c
Biedermann, Andrea R.
152dafb6-121a-4ab4-9c55-4bf81976753d
Gilgen, Samuel A.
634e2d69-8d77-4a8f-8e10-e01695f7aa7d
Al-Tobi, Khalid
00d422de-b5e8-4ea0-9b27-c49abe7ab2d7

Belgrano, Thomas M., Diamond, Larryn W., Vogt, Yves, Biedermann, Andrea R., Gilgen, Samuel A. and Al-Tobi, Khalid (2019) A revised map of volcanic units in the Oman ophiolite: Insights into the architecture of an oceanic proto-arc volcanic sequence. Solid Earth, 10 (4), 1181-1217. (doi:10.5194/se-10-1181-2019).

Record type: Article

Abstract

Numerous studies have revealed genetic similarities between Tethyan ophiolites and oceanic “proto-arc” sequences formed above nascent subduction zones. The Semail ophiolite (Oman–U.A.E.) in particular can be viewed as an analogue for this proto-arc crust. Though proto-arc magmatism and the mechanisms of subduction initiation are of great interest, insight is difficult to gain from drilling and limited surface outcrops in marine settings. In contrast, the 3–5 km thick upper-crustal succession of the Semail ophiolite, which is exposed in an oblique cross section, presents an opportunity to assess the architecture and volumes of different volcanic rocks that form during the proto-arc stage. To determine the distribution of the volcanic rocks and to aid exploration for the volcanogenic massive sulfide (VMS) deposits that they host, we have remapped the volcanic units of the Semail ophiolite by integrating new field observations, geochemical analyses, and geophysical interpretations with pre-existing geological maps. By linking the major-element compositions of the volcanic units to rock magnetic properties, we were able to use aeromagnetic data to infer the extension of each outcropping unit below sedimentary cover, resulting in a new map showing 2100 km2 of upper-crustal bedrock.

Whereas earlier maps distinguished two main volcanostratigraphic units, we have distinguished four, recording the progression from early spreading-axis basalts (Geotimes), through axial to off-axial depleted basalts (Lasail), to post-axial tholeiites (Tholeiitic Alley), and finally boninites (Boninitic Alley). Geotimes (“Phase 1”) axial dykes and lavas make up ∼55 vol % of the Semail upper crust, whereas post-axial (“Phase 2”) lavas constitute the remaining ∼45 vol % and ubiquitously cover the underlying axial crust. Highly depleted boninitic members of the Lasail unit locally occur within and directly atop the axial sequence, marking an earlier onset of boninitic magmatism than previously known for the ophiolite. The vast majority of the Semail boninites, however, belong to the Boninitic Alley unit and occur as discontinuous accumulations up to 2 km thick at the top of the ophiolite sequence and constitute ∼15 vol % of the upper crust. The new map provides a basis for targeted exploration of the gold-bearing VMS deposits hosted by these boninites. The thickest boninite accumulations occur in the Fizh block, where magma ascent occurred along crustal-scale faults that are connected to shear zones in the underlying mantle rocks, which in turn are associated with economic chromitite deposits. Locating major boninite feeder zones may thus be an indirect means to explore for chromitites in the underlying mantle.

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Published date: 29 July 2019

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Local EPrints ID: 455460
URI: http://eprints.soton.ac.uk/id/eprint/455460
ISSN: 1869-9510
PURE UUID: 8b399d3a-aa85-45ed-a027-5a6bec2894bc

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Date deposited: 22 Mar 2022 17:39
Last modified: 12 Aug 2022 18:37

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Contributors

Author: Larryn W. Diamond
Author: Yves Vogt
Author: Andrea R. Biedermann
Author: Samuel A. Gilgen
Author: Khalid Al-Tobi

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