The Geochemistry of Feldspar-Free Volcanic Rocks.

Abstract

Highly silica-undersaturated magmas which form feldspar-free volcanic rocks have been erupted at some time on most continents and in all major ocean basins. In this study 488 oceanic and continental feldspar-free volcanic rocks have been analyzed for major and trace elements. The results are presented in a new compilation covering five continents and three oceans. This type of volcanism is associated with intraplate volcanic activity and occurs within extensional tectonic regimes in continental areas or as late-stage, post-erosional volcanism on oceanic islands. It is restricted almost entirely to the continental circum-cratonic Proterozoic lithosphere and to the margins of ocean basins where the Phanerozoic oceanic lithosphere is older. The magmas responsible for oceanic (OIN) and continental (CON) nephelinites are dominated by asthenosphere-derived melts. Nephelinitic rocks are enriched in elements which partition strongly into carbonate melts. Small silicate melt fractions may have been enriched by a carbonate precursor liquid derived by 0.1 % melting of the same source, the carbonate addition being ca. 10% for GIN and up to 20% for CON. K and Rb variation in OIN and CON indicate derivation from amphibole. (ca. 2%)Tgamet Iherzolite sources by carbonate-silicate liquid mixing. Amphibole is residual at small degrees of partial melting but consumed before larger degrees of melting appropriate to most OIB are reached. In a few somewhat K-enriched sources phlogopite is indicated. OIN are characterised by HIMU and EMI trace-element ratios but amphibole implies shWlow sources rather than the deep mantle location proposed for the HIMU component. OIN and CON are enriched in carbonate-phobic incompatible elements. This may be the result of remobilization of asthenosphere-derived melts from the base of the lithosphere. Continental lithosphere involvement is also indicated by the greater variability of CON in incompatible element compositions. Potassic rocks from the Western Rift (WR) and New South Wales (NSW) are extremely enriched in incompatible elements. This may be evidence of metasomatised lithospheric mantle involvement in magma genesis. Carbonate liquids are involved in most WR magmas but not in NSW magmas where H20 -rich fluids are indicated. Phlogopite (probably with amphibole) dominates both sources. Olivine-poor nephelinites (COP) are also enriched in incompatible elements but have similar normalised profiles to OIN and CON. COP may be derived by liquid immiscibility from highly carbonated asthenospheric magmas and are related to CON by two-stage silicate-carbonate liquid partitioning effects, crystal fractionation, and contamination by lithospheric mantle.