An Archaean sill complex and associated supracrustal
Archaean supracrustal rocks on Arveprinsen Ejland comprise mafi c and felsic volcanic rocks overlain by an epiclastic sedimentary sequence invaded by a mafi c to ultramafi c sill complex. The latter has a strike-length of 7500 m and a cumulative preserved thickness of 2000–2500 m and amounts to near...
| Main Authors: | , , , , , |
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| Format: | Text |
| Language: | English |
| Subjects: | |
| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.618.2268 http://www.geus.dk/publications/bull-gl/nr181/nr181_p087-102.pdf |
| Summary: | Archaean supracrustal rocks on Arveprinsen Ejland comprise mafi c and felsic volcanic rocks overlain by an epiclastic sedimentary sequence invaded by a mafi c to ultramafi c sill complex. The latter has a strike-length of 7500 m and a cumulative preserved thickness of 2000–2500 m and amounts to nearly 50 % of the exposed thickness of the supracrustal rocks. Chilled and locally peperitic contacts are developed between component sills and the inter-sill metasedimentary septa. The sub-alkalic sill complex and mafi c lavas and tuffs are high-magnesium tholeiites and basaltic komatiites whereas the felsic rocks are calc-alkaline rhyolites and dacites. Chondrite- and MORB-normalised spider diagrams affi rm the close similarity of the mafi c volcanic rocks and the sill complex; they are also consistent with a tholeiitic or komatiitic affi nity. Tectonomagmatic discrimination plots suggest an ensialic arc-related setting for the sill complex and the mafi c and felsic volcanic rocks. The sill complex was progressively emplaced, as an upward-younging sequence of component sills, beneath 2 to 2.5 km of seawater and substantially less than 0.5 km of wet sediment. Sills formed when the magmatic pressure exceeded the effective overburden pressure of the sediment plus the vertical tensile strength (T o) of the host materials. Intrusion was probably promoted by the drop in T o at the interface between contact-lithifi ed and poorly lithifi ed strata. The thickness of the sill complex was accommodated by dilational lifting plus the capacity of an intrusion to create space through expulsion of water from wet sediment. |
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