Petrology and petrogenesis of a tertiary bimodal dolerite‐peralkaline/subalkaline trachyte/rhyolite dyke association from Lundy, Bristol Channel, UK

Abstract

The island of Lundy forms the southernmost igneous complex of the British Tertiary Volcanic Province (BTVP) and consists of granite ( = 90%)) emplaced into deformed Devonian sedimentary rocks (Pilton Shale) and associated with a swarm of dykes of doleriteibasalt, minor trachyte and rhyolite composition. The dolerites are of varied olivine basalt composition and are associated with peralkaline trachyte and subalkalineiperalkaline rhyolite with alkali feldspar and quartz f alkali amphibole f pyroxene mineralogy. The dyke swarm is therefore an anorogenic bimodal doleritei basalt--trachyte/rhyolitc BTVP association. Although the dyke association is bimodal in major element terms between doleriteibasalt and minor trachyteirhyolite, the mineralogy and trace element geochemistry indicate that the dykes may be regarded as a cogenetic dolerite-peralkaline trachyteirhyolite association with minor subalkaline rhyolites.Sr and Nd isotope data indicate derivation of these magmas from a similar BTVP mantle source (with or without minor contamination by Pilton Shale, or possibly Lundy granite).The petrogenesis of the Lundy dyke association is therefore interpreted in terms of extensive fractional crystallization of basaltic magma in a magma chamber of complex geometry below the (exposed) Lundy granite. Fractional crystallization of a representative dolerite magma (olivine f clinopyroxene k plagioclase) yields trachyte magma from which the crystallization of alkali feldspar (anorthoclase) & plagioclase (oligoclase) + Fe-Ti oxide + apatite results in peralkaline rhyolite. Rarer subalkaline rhyolites result from fractionation from a similar dolerite source which did not achieve a peralkaline composition so allowing the crystallization and fractionation of zircon. The basalt-(minor trachyte)/rhyolite bimodality reflects rapid crystallization of basalt magma to trachyte (and rhyolite) over a relatively small temperature interval (mass fraction of melt, F = t 0 . 1 5 ) . The rapid high level emplacement of basalt, trachyte and rhyolite dyke magmas is likely to have been associated with the development of a substantial composite bimodal basalt--(minor trachyte)/rhyolite volcano above the BTVP Lundy granite in the Bristol Channel.

Sr, Nd and O isotopic characters of quartz syenite in the Weiya magmatic complex from eastern Tianshan in NW China: Melting of the thickened juvenile lower crust

“…Several genetic models have been proposed to interpret the formation of syenites, which mainly include: (1) partial melting of crustal rocks in a closed system at pressures typical of the base of over-thickened crust (Huang and Wyllie, 1986;Deng et al, 1998;Tchameni et al, 2001), (2) partial melting of mantle peridotites with subsequent differentiation (Bailey, 1987;Whalen et al, 1996;Conceicão et al, 2000;Litvinovsky et al, 2002;Wang et al, 2005;Yang et al, 2005), (3) fractional crystallization of alkali basalt magma (Parker, 1983;Middlemost, 1985;Brown and Becker, 1986;Thorpe and Tindle, 1992) or fractionation caused by silicate liquid immiscibility (Rajesh, 2003), and (4) magma mixing processes, particularly mixing of basic and silicic melts with subsequent differentiation of the hybrid liquid (Barker et , 1975;Sheppard, 1995;Zhao et al, 1995;Wickham et al, 1995;Vernikovsky et al, 2003). In doing so, it is important to have an integrated study of major elements, trace elements and ragiogenic isotopes in continental igneous rocks in order to decipher the nature of their sources (e.g., Zhang, S.-B.…”

Field, petrographic and geochemical characteristics of Sullya alkaline complex in the Cauvery Shear Zone (CSZ), southern India: Implications for petrogenesis

“…The second model invokes partial melting of a metasomatized mantle (Sut-cliAe et al 1990; Lynch et al 1993). The third model involves the residual melts formed by differentiation of mantle derived alkali basalts (Parker 1983;Thorpe and Tindle 1992). The fourth model involves partial melting of the crustal rocks resulting from an inCux of volatiles (Lubala et al 1994) or in a closed system at pressures typical of the base of over-thickened crust (Huang and Wyllie 1981) with addition of alkalis.…”

The 186 Ma Dashibalbar alkaline granitoid pluton in the north-Gobi Rift of central Mongolia: Evidence for melting of Neoproterozoic basement above a plume

“…The genesis of syenites is still controversial. Commonly proposed models for the origin of syenites are (1) partial melting of metasomatized mantle (for example Lynch and others, 1993;Platt, 1996), (2) differentiation of alkali/transitional basaltic magmas (Thorpe and Tindle, 1992;Civetta and others, 1998;Ronga and others, 2010), (3) melting of underplated basalts (Sisson and others, 2005;Bonin, 2007;Jahn and others, 2009) and (4) magma mixing, typically mantle-derived basic magma with silicic crustal melts (for example Marks and Markl, 2001). Litvinovsky and others (1999) and Jahn and others (2009) argued that mixing of mantle-derived mafic magma with silicic crustal melts, followed by subsequent differentiation (for example Zhao and others, 1995) is not a viable mechanism as it requires an unusual proportion of end-members.…”