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Early Stages of Gondwana Break-Up |
1 Department of Mining Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK
2 Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
3 Department of Earth Sciences, The University, Leeds LS9 9JT
4 British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK
On the basis of geochemical signatures, Mesozoic magmatism in Antarctica is divided into the Ferrar Magmatic Province and the Dronning Maud Land Province. The tholeiitic magmatism of the Ferrar Magmatic Province is distinguished by such features as low Ti/Y (< 200) and Zr/Y (< 5.0) ratios, negative
In Coats Land, the Mesozoic tholeiitic magmatism is represented by doleritic sills and minor dykes which intrude Permo-Triassic sedimentary rocks. The dolerites can be subdivided into two series based on their TiO2 contents. Series 1 dolerites (TiO2 < 1.5%) can be further subdivided into three groups, which give Ar/Ar ages of 171±6 Ma (Group 1) and 193±7 Ma (Groups 2 and 3). It is only Group 2 magmas which have trace element and isotopic signatures akin to the Ferrar Magmatic Province. Group 1 dolerites have geochemical signatures which are transitional between the Ferrar Magmatic Province and Dronning Maud Land magma types.
The Ferrar Magmatic Province signature in Coats Land is confined to the early magmatic episode (193±7 Ma) and this appears to mark the initiation of rift related magmatism in this region. It is argued that extension was limited and that most of the melt was derived from the continental mantle lithosphere. In contrast, the younger rocks (176±5 Ma) have relatively lower initial 87Sr/86Sr and higher trace element ratios relative to the Ferrar Magmatic Province, and this appears to be associated with the later stages of rifting and relatively enhanced crustal extension which allowed for the encorporation of a small asthenosphere component.
Nd values (< –3) and relatively enriched initial 87Sr/86Sr ratios (> 0.709). All of these geochemical features indicate a major contribution from the continental mantle lithosphere in the generation of these magmas. In contrast, the Dronning Maud Land magmatism has elevated trace element ratios and Nd values (Ti/Y 250–600; Zr/Y 3.0–9.0; Nd –2 to +3) and lower initial 87Sr/86Sr ratios (< 0.707) relative to the Ferrar Magmatic Province. The trace element and isotopic correlations suggest that these magmas were derived by the mixing of an OIB like asthenospheric component with a continental lithosphere component. The transition between these two geochemical provinces is located in Coats Land.
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