Felsic igneous rocks within the 3.3‐ to 3.5‐Ga Barberton Greenstone Belt: High crustal level equivalents of the surrounding Tonalite‐Trondhjemite Terrain, emplaced during thrusting

Tectonics - Tập 6 Số 5 - Trang 529-549 - 1987
Maarten J. de Wit, Richard Armstrong, R. J. Hart, A. H. Wilson

Tóm tắt

Felsic igneous rocks within the circa 3.5‐Ga ophiolitic Onverwacht Group of the Barberton greenstone belt have traditionally been mapped as recurring volcanic units within a continuous stratigraphic succession. These felsic units have been interpreted as part of several mafic to felsic volcanic cycles making up this succession. Our field and geochemical data show that the felsic igneous rocks are not closely related to the mafic‐ultramafic rocks of the Onverwacht Group. Rather, the data indicate that these felsic units are high‐level equivalents of the widespread gneissic to isotropic trondhjemite‐tonalite plutons which surround and intrude the greenstone belt. The felsic igneous rocks are predominantly shallow‐level intrusions and subsurface felsic domes associated with minor volcanics and volcanoclastics and postdate the Onverwacht Group by 40–50 Ma. The felsic intrusions were preferentially emplaced along thrust zones, cutting the ophiolite basement and its overlying sedimentary cover. Structural and chemical data imply that the hydrated ophiolitic rocks of the Barberton greenstone belt are allochthonous and were thrust over similar simatic rocks from which the felsic igneous rocks were syntectonically derived by partial melting. Such structural imbrication of simatic crust can be best incorporated in a model advocating Archean intraoceanic obduction associated with shallow‐angle subduction, similar to that documented for the neotectonics of the Western Aleutians.

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Tectonics

Tập 6 Số 5

529-549

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