Journal of Conference Abstracts

Hybridisation Between Coexisting Diorite and Granophyre Magmas at Elizabeth Castle, Jersey, Channel Islands

R. A. Shortland (R.A.Shortland@derby.ac.uk)

Division of Earth Sciences, University of Derby, Derby DE22 1GB.

The Elizabeth Castle complex is situated on the western edge of the South East Granite Complex on the island of Jersey. Undisturbed parts of the complex consist of layered diorites intercalated with Fort Regent Granophyre which forms a series of sub-horizontal sheets sub-parallel to the layering. The sheets are interconnected vertically. Cross-cutting relationships indicate more than one episode of granophyre intrusion. Contact relationships indicate that the granophyre and the diorites were originally present as coexisting magmas. Much of the sheeted complex was later disrupted by further incursions of granophyre, producing a host of predominantly sub-angular diorite enclaves within granophyre. A minor lithology occurs intermittently along contacts between the granophyre and diorite. Field and petrographic evidence, along with major and trace element geochemistry, suggest that this is a hybrid resulting from interactions between the diorite and granophyre magmas. The nature of these interactions is currently under investigation.

The Origin of Greenstone Belts ­ Evidence from the Upper Greenstone Volcanic Sequence, Belingwe Belt, Zimbabwe

K. Silva¹ (ksilva@gl.rhbnc.ac.uk), M. Cheadle² (mjc44@liv.ac.uk), E. Nisbet¹ (nisbet@gl.rhbnc.ac.uk) & C. Brake^3,4 (cbrake@rednet.co.uk)

¹ Geology Dept., Queens' Building, Royal Holloway College, Egham Hill, Egham, Surrey TW20 OEX.

² Jane Herdman Labs., Department of Earth Sciences, University of Liverpool, Brownlow Street, Liverpool L69 3BX.

³ Department of Geology and Geophysics, University of Edinburgh, Grant Institute, West Mains Road, Edinburgh EH9 3JW.

^{4 c}/o K. Silva, at address 1 above for correspondence.

The volcanic component of the Upper Greenstone Sequence, Belingwe Greenstone Belt, Zimbabwe is Archaean (2.7 Ga) and consists of ~1km of komatiitic and komatiitic basalt flows of the Reliance Formation, overlain by ~6km of tholeiitic basalts of the Zeederberg Formation. Little sediment is incorporated within the sequence, implying rapid eruption with preservation by subsequent downwarping of the substrate. Sampling of the entire volcanic sequence has allowed detailed inspection of geochemical variations with stratigraphic height. These data suggest that >90% of the lavas were produced from a single source. Major and trace element data show that this sequence could evolve from a parental magma containing ~25 wt.% MgO by the fractionation of olivine, plagioclase, augite and minor chromite. A thin (0.5 km), chemically discrete unit constitutes a minor proportion of the Zeederberg Formation.

The volume of magma preserved in the synclinal Greenstone Belt is estimated at ~3000 km³. The Upper Greenstone sequence can be correlated over >2 x 10⁵ km² (Wilson et al., 1978) suggesting that >0.5 x 10⁶ km³ of magma may have been erupted. Comparable volumes of magma are found in younger flood basalts, such as the Deccan Traps, India. Together with the ultramafic nature of the parental magma, this suggests that these lavas were derived from a mantle plume. Thermodynamic modelling shows that komatiitic melts can be readily produced within the axes of Archaean mantle plumes, where, with a mantle potential temperature of 1900šC, up to 35 vol.% melting occurs between 350 and 100km depth. Less mafic basalts can only be produced indirectly, where plume-derived melts become trapped and fractionate before eruption. Therefore, whilst the Reliance Formation lavas may have been tapped directly from the plume, the Zeederberg basalts may have been allowed to pond, possibly in lower crustal magma chambers, before eruption.

Wilson, J.F., Bickle, M.J., Martin, A., Nisbet, E.G. & Orpen, J.L., Nature, 271, 23 -27 (1978).