Formation and composition of the lower continental crust: Evidence from Scottish xenolith suites
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abstract
Crustal xenoliths occur in a variety of Carboniferous and Tertiary alkali basaltic rocks across Scotland and provide indications of the nature of the deep crust beneath the major lithotectonic zones from the Archean Lewisian of the far NW to the Paleozoic terranes in the SE. Sr, Nd, and Pb isotopic and representative chemical compositions are presented for 35 mafic xenoliths from the different age and tectonic zones of Scotland and for 21 felsic, gneissose xenoliths, from the Midland Valley and Southern Uplands in the south. The coexistence of mafic granulite and spinel lherzolite xenoliths at several of these localities, together with seismic velocity data, provide evidence that the mafic xenoliths, and possibly some of the felsic xenoliths, may be derived from 20–30 km depth. The absence of garnet from the mafic assemblages suggests that in most areas the crust was probably not much more than 30 km thick in the Permo‐Carboniferous. The extremely variable Al2O3 (up to 27 wt %) and MgO contents at any one location, together with positive Eu and Sr anomalies in chondrite‐ and mantle‐normalized diagrams, provide evidence that many of the mafic xenoliths represent cumulates from basaltic magmas. The Loch Roag mafic xenoliths, erupted through the depleted high‐grade metamorphic Lewisian Complex, are especially rich in K (average 1.3 % K2O) and Rb (average 54 ppm) with K/Rb extending as low as 100. The K/U ratios for the mafic xenoliths are high, ranging to over 100,000 at Loch Roag and in the SW Midland Valley. The relative concentrations of U, K, Rb, Pb, Nd, Sm, Eu and Sr in most of the mafic Xenoliths, while displaying regional characteristics, display intra‐site variations that are consistent with crystal‐liquid fractionation. There is little evidence of U or Rb depletion via fluid or melt extraction during metamorphism. The concentrations of U in the mafic xenoliths display as much intrasite uniformity as the rare earth elements and correlate with Zr concentrations. U/Zr ratios are similar to those found in ocean island basalts. The mafic xenoliths display a marked decrease in ϵNd and 206Pb/ 204Pb with increasing age of the upper crust from SE to NW, as also observed in the ∼400 Ma Caledonian granites. Sm‐Nd crustal residence ages for the Loch Roag suite are particularly variable ranging from 1.6 to 3.6 Ga. However, these isotopic compositions and model ages appear to be, in part, the product of crustal assimilation by the igneous precursors to the mafic xenoliths. Model ages, mineral ages and scattered whole rock isochrons indicate that many of the precursors to these Xenoliths were in fact formed and metamorphosed during the late Proterozoic and Paleozoic and provide evidence of widespread deep crustal magma reservoirs, underplating, crustal assimilation and arc accretion at these times. Nd and Sr isotopic data for coexisting pyroxenite xenoliths at Loch Roag indicate that they may represent contaminated ultramafic cumulates that were cogenetic with the precursors to the mafic xenoliths. The Pb isotopic compositions of the Midland Valley mafic and felsic xenoliths are similar and relatively uniform despite large variations in Nd and Sr isotopic compositions. This is explicable if the crust in this region was relatively uniform and similar in composition to the mantle and/or if crustal Pb dominated in arc environments. The average major element composition of the Scottish mafic xenolith suite is similar to that reported from other regions such as the Sierra Nevada. This is taken as evidence that the processes of crystal fractionation and formation of mafic and ultramafic cumulates during underplating are relatively consistent between different regions. Average U/Pb ratios of mafic and felsic xenoliths are effectively identical (0.068 and 0.064, respectively). The average Sm/Nd ratio (0.20) for the entire suite is lower and average K/U ratio (27,000) higher than estimates of average lower crust based on Queensland mafic xenoliths. These compositions as well as those of basement rocks, sedimentary supracrustal rocks and granites provide evidence of systematic chemical differences between the Scottish and Australian lithosphere. The compositional features of the Paleozoic Scottish crust such as low Sm/Nd and high K/U may have been sustained in part by recycling of Archean and early Proterozoic lithosphere through sedimentary processes, subduction and arc volcanism. The average Sm‐Nd crustal residence age and Pb isotopic composition of the mafic xenoliths at Loch Roag is very similar to that of the host Lewisian. Conservative mass balance models imply an average ratio of Lewisian‐derived Nd to parent magma Nd in the xenoliths of about 2. Such large degrees of contamination indicate that although underplating may be significant process in reconstituting Archean lithosphere, it does not necessarily result in substantial changes to the average age of the crust.
