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

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