Megacrysts and salic xenoliths in Scottish alkali basalts: derivatives of deep crustal intrusions and small-melt fractions from the upper mantle

Abstract

ABSTR ACTCa-poor and typically Na-rich feldspar megacrysts are common associates of spinel lherzolitic and pyroxenitic xenoliths in Scottish alkalic basalts. Associated megacrysts and composite megacrysts and salic xenoliths include apatite, magnetite, zircon, biotite, Fe-rich pyroxene(s) and corundum. The salic xenoliths and related megacrysts are referred to collectively as the 'anorthoclasite suite': the majority of the samples are inferred to derive from the disaggregation of coarse-grained, typically Na-rich, syenitic protoliths at depth. Rare occurrences of euhedral anorthoclase megacrysts, together with zircon dating, imply that the suite crystallized at, or very shortly before, their entrainment by the basaltic host magmas. Some evidence suggests that the anorthoclasite suite protoliths lie within ultramafic (pyroxenitic) domains in the deep crust. The latter are inferred to be pegmatites, crystallized from carbonated trachytic magmas with widely variable Ca, Na, K, Ba and trace-element contents, and to have ranged from metaluminous to peraluminous. Crystal zonation and resorption textures within the salic xenoliths imply that the crystallization of the parent magmas was complex. Confirmation of this comes from cathodoluminescence studies of the feldspars showing that early ('primary') anorthoclases and potassian albites exhibit partial replacement by a more potassic feldspar. A third generation of potassic feldspar (enriched in an assortment of trace elements and deduced to have crystallized from a carbonated high-K melt) forms transecting zoned veins in which carbonate fills the axial zone.Whereas most of the anorthoclasite suite materials are inferred to have grown from metaluminous magmas, the occurrence of magmatic corundum in salic xenoliths indicates crystallization from magmas that were peraluminous. The corundum-bearing samples also contain Nb-rich oxide minerals and their associated feldspars have the highest rare-earth element (REE) contents. Accordingly, the peraluminous trachyte magmas are deduced to have been specifically enriched in high field-strength trace elements. It is proposed that formation of the anorthoclasite suite protoliths is a phenomenon closely related to that of salic glass 'pockets', well known from spinel lherzolite xenoliths around the world. Not only are there compositional affinities, but both sets of phenomena appear to have closely pre-empted the ascent of alkali basalt (host) magmas. We propose that the two sets of phenomena are linked and that the anorthoclasite suite derived from coarse-grained sheets, generated by the aggregation of salic melt fractions rising from the shallow mantle and heralding the onset of basaltic magmatism.

Sapphires related to alkali basalts from the Cerová Highlands, Western Carpathians (southern Slovakia): composition and origin

“…Similar models for corundum origin from upper mantle to lower crustal, fractionated syenitic/anorthoclasitic magma are also proposed for other sapphire occurrences (e.g. Coenraads et al 1995;Sutherland et al 1998Sutherland et al , 2009Upton et al 1999Upton et al , 2009Graham et al 2008).…”

Trace-element abundances in the shallow lithospheric mantle of the North Atlantic Craton margin: Implications for melting and metasomatism beneath Northern Scotland

“…They propose that chlorite is formed by relatively low-temperature alteration of phlogopite by a late H 2 O and CO 2 -rich fluid exsolved from kimberlites. Upton et al (2009) describe chloritic rims around ocelli from a range of Scottish monchiquite/nephelinitic dykes, and ascribe these to separation of a carbonate-rich melt/fluid. A similar effect seems a very plausible process within Loch Roag dyke, and the occurrence of LREE-rich bastnäsite would be consistent with a carbonatitic melt/fluid.…”

40Ar/39Ar dating of alkali feldspar megacrysts from selected young volcanoes of the Newer Volcanic Province, Victoria

“…They concluded that the majority of anorthoclase megacrysts (compositions in the range Ab 80-90 Or 20-10 ) are not phenocrysts, but crystallized from small-volume trachytic melts in the deep crust before being disaggregated and entrained by their host basaltic magmas. Based on the common association of these megacryst suites with alkaline basalt provinces, the presence of euhedral megacryst crystal forms and (limited) K-Ar and U-Pb data that suggest a close timing relationship between megacryst formation and host basalt magmatism, Upton et al (2009) suggested that the trachytic and basaltic magmas were broadly contemporaneous. However, some uncertainty remains about the exact location and nature of the host magma for the anorthoclase megacrysts.…”

“…This model was supported by Chapman and Powell (1976), who concluded that anorthoclase megacrysts (and associated cumulate nodules) formed from high-level differentiated magmas and were subsequently transported to the surface by later more basic magmas. Upton et al (2009) investigated megacrysts and xenoliths, including an anorthoclasite suite, from alkali basalts in Scotland. They concluded that the majority of anorthoclase megacrysts (compositions in the range Ab 80-90 Or 20-10 ) are not phenocrysts, but crystallized from small-volume trachytic melts in the deep crust before being disaggregated and entrained by their host basaltic magmas.…”

“…The primary aim of the present study was to determine new age data for younger NVP scoria cone volcanoes by using the 40 Upton et al (2009) suggests that anorthoclase megacrysts associated with many alkali basalt provinces are not phenocrysts, but are formed from small-volume trachytic melts in the deep crust immediately prior to their entrainment by basaltic magmas. Therefore, a further aim of the present study was to test whether alkali feldspar megacrysts entrained in the NVP volcanoes can be precisely and accurately dated using the 40 Ar/ 39 Ar method.…”