The Baikal Rift is a zone of active lithospheric extension, located in the Sayan-Baikal fold belt between the Siberian Craton and the Aldan Shield. Rifting was initiated in this region ~35 Mya. The ~20 to 3 Ma Vitim Volcanic Field (VVF) lies on the eastern shoulder of the rift and consists of ~5000 km<sup>3</sup> of alkali basalts and basanites, with minor nephelinites and tholeiitic basalts. 140 drill core samples have been used to study temporal and spatial variations in the volcanic pile, which is up to 700 m thick in places. The VVF lavas have been analysed by XRF and ICP-MS for their whole-rock chemistry. Variations in major element abundances (e.g. MgO, which varies from 14.6-3.3 wt.%) reflect fractional crystallisation of olivine, clinopyroxene and to a lesser extent, plagioclase. Prior to eruption, the nephelinitic magmas appear to have crystallised in chambers in the upper crust (at 1 kbar) whereas the tholeiitic basalts appear to have undergone fractionation at the base of the crust (9 kbar). At a given MgO content, Na<sub>2</sub>O, P<sub>2</sub>O<sub>5</sub> and K<sub>2</sub>O and incompatible trace elements decrease systematically from nephelinites through to tholeiitic basalts, suggesting that partial melting is also responsible for variations in whole-rock chemistry. <sup>87</sup>Sr/<sup>86</sup>Sr and <sup>143</sup>Nd/<sup>144</sup>Nd vary from 0.70391 to 0.70487 and 0.51272 to 0.51287 respectively, and indicate that the Vitim magmas have not assimilated significant quantifies of continental crust. Fractionation-corrected compositions may therefore be used to study mantle processes operating beneath the VVF. Nephelinites, basanites, alkali basalts and tholeiites display a wide range in [La/Yb]<sub>n</sub> ratios (5-33) which is interpreted as evidence of variation in the degree of partial melting. Trace element ratios, e.g. Ba/Sr and La/K, suggest that phlogopite and possibly amphibole are present as residual mantle phases during generation of the nephelinites and basanites but not the tholeiites.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:599321 |
| Date | January 2002 |
| Creators | Garner, J. S. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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