Garnet pyroxenite xenoliths and pyropic megacrysts in Scottish alkali basalts

Abstract

Synopsis Pyropic garnet occurs within garnet pyroxenite xenoliths and as discrete crystals in Carboniferous–Permian alkali basaltic tuffs and hypabyssal intrusions. Eight localities are now known from the Midland Valley together with another in the NW Highlands. The xenoliths are inferred to have been derived from minor pyroxenitic intercalations in spinel lherzolite within the lithospheric mantle. Although pyrope crystals at Elie Ness are regarded as high-pressure phenocrysts cognate with the basanitic host, it is indeterminable whether discrete garnets at the other localities are phenocrysts or xenocrystal fragments derived from garnet pyroxenites. All, however, whether phenocrystic, xenocrystic or integral components of pyroxenitic xenoliths, are regarded as cognate in the sense of being products of high-pressure crystallization of late Palaeozoic basanite magmas. Kaersutite, phlogopite, hercynitic spinel, ilmenite and pseudomorphs after orthopyroxene occur as minor components in some of the garnet pyroxenite xenoliths. Mineral compositional data (electron- and ion-microprobe) are presented, with comparisons to xenolith-megacryst suites elsewhere. The garnets have strong similarities with each other and with megacryst populations from alkali basalts outside the U.K. It is concluded that the pyroxenites originally crystallized from basaltic magmas as high-pressure cumulates from depths of >50 km and represent some of the deepest samples from the Scottish lithosphere.

Pyroxenites and megacrysts from Vitim picrite-basalts (Russia): Polybaric fractionation of rising melts in the mantle?

“…The typical BMG clinopyroxenes give two arrays of cooling from 1300 to 1200°C around 2.2 and 2.5 GPa. An inclined PT path continues the trend derived from this cloud, forming two additional (Nimis and Taylor, 2000) for the clinopyroxenes, the solid circles are for data used for calibration in this work (Nimis and Taylor, 2000): (Taylor, 1998;Walter, 1998;Brey and Kohler, 1990; and in addition from Brey et al, 2008), the open circles are for the extended data set (Foley et al, 2009;Walter, 1998 (Ashchepkov and Andre, 2002;Litasov et al, 2000) and worldwide pyroxenites (Frey, 1980;Pearson and Nowell, 2004;Upton et al, 2003;Downes, 2007;Zheng et al, 2009) using polymineral thermobarometry (Brey and Kohler, 1990) (see Appendix A equation set 1) and monomineral methods (Nimis and Taylor, 2000) Nimis and Taylor (2000) thermometer and Ashchepkov (2002) barometer (symbols are as in Fig. 3).…”

“…2-20). They differ from GL in having lower modal pyroxene abundances (Ashchepkov et al, 1994;Ionov et al, 2005a,b), but in general they are more fertile than SL from most localities in Europe (Witt-Eickschen and Kramm, 1998;Witt-Eickschen et al, 2003;Upton et al, 1999Upton et al, , 2003Downes, 2001) and North America (Wilshire and Shervais, 1975).…”

“…Garnet peridotites and pyroxenite xenoliths and pyrope megacrysts are not frequent in alkali basalt (Beeson and Jackson, 1970;Chapman, 1976;Kepezhinskas, 1979;Nixon and Boyd, 1979;Delaney et al, 1979;Kovalenko et al, 1987;Mukhopadyay and Manton, 1994;Ashchepkov et al, 1996;Stern et al, 1999;Upton et al, 2003;Bjerg et al, 2005). The picrite-basalt tuffs from Vitim (Russia) and closely located volcanic centers (Ashchepkov et al, 2003) contain a nearly complete set of megacrysts, peridotites and pyroxenites as described in the literature.…”

The mantle of Scotland viewed through the Glen Gollaidh aillikite

“…A thinning of the NAC margin south and east is also borne out in the dominance of various mantle mineral classes. Anomalous samples occur as garnets reported in Elie Ness, Fife and in Dunaskin Glen and the Heads of Ayr (Upton et al 2003) with inferred pressures up to 20 kbar. However, more typically, there is also a progression from dominant, deep-sourced Mg chromite (Clare Island; Upton et al 2001), to Mg-Al chromite (Loch Roag, Upton et al 2011; with minor Mg chromite, Glen Gollaidh), to Mg-Al chromite with Mg-Cr-Al spinel (Rinibar) to shallow-sourced Mg-Cr-Al spinel (off-craton at Streap Com'laidh and Fidra).…”

Geochemistry and Petrology of Lavas in the Upper Onverwacht Suite, Barberton Mountain Land, South Africa

“…However, crystal/melt partition coefficients may vary considerably as a function of temperature, pressure and composition (e.g., Pearce and Norry, 1979). Yb is strongly concentrated in pyrope (e.g., Upton et al, 2003), while its distribution coefficient may be less than 1 in highpressure majorité (e.g., Ohtani et al, 1989;Corgne and Wood, 2004). Thus, if majorité was involved in the generation of the komatiitic melts, Yb would not necessarily display a negative anomaly.…”