Wehrlitization of lithospheric mantle beneath Fife, Scotland.
Résumé
Permo-Carboniferous mafic alkaline volcanism in Scotland sampled deep lithosphere beneath Lower Paleozoic basement. In the eastern part of Midland Valley terrane (central Scotland, Fife peninsula) volcanic and volcanoclastic rocks carry (among others) ultramafic xenoliths. The peridotite xenoliths give an insight into structure and composition of lithospheric mantle at the time of volcanic activity.We studied spinel lherzolites and wehrlites, occurring as usually <10 cm in diameter (up to 25 cm) xenoliths. The lherzolites have either protogranular or porphyroclastic texture, while wehrlites are equigranular. Clinopyroxene in porphyroclastic lherzolites and wehrlites is texturally secondary growing at the expense of orthopyroxene.Chemical composition of minerals is related to texture of the rock. Olivine forming protogranular lherzolites has Fo=88.35-88.80 and is Ca-poor (<900 ppm) which together with #Cr in spinel varying from 0.08 to 0.21 plot those rocks within the Olivine-Spinel Mantle Array (OSMA, Arai, 1994). Orthopyroxene in this group is chemically homogenous within a sample (Mg#=0.89-0.90, Al=0.15-0.19 a.p.f.u.); clinopyroxene in some samples is heterogeneous, but its composition varies in a narrow range (Mg#=0.89-0.92 and Al=0.22-0.34 a.p.fu.). The REE pattern of clinopyroxene from protogranular lherzolites varies from LREE-depleted to LREE-enriched one; it is always enriched in Th and U and depleted in Nb and Ta.Composition of minerals forming porphyroclastic lherzolites and wehrlites is strongly heterogeneous and varies in the same ranges. Olivine and spinel of some rare porphyroclastic lherzolites plot within OSMA (OlFo=88.50-88.80, Spl#Cr=0.12-0.20), but in majority of samples olivine composition grades toward lower forsterite contents (Fo=78.33-89.78) while Cr# is higher in spinel (Cr#=0.72-0.53) which locate them outside OSMA. Olivine has Ca content up to 2000 ppm. Orthopyroxene and clinopyroxene are chemically heterogeneous in terms of Mg# (0.82-0.89 and 0.81-0.93, respectively) and Al content (0.06-0.17 and 0.10-0.33 a.p.f.u., respectively). Clinopyroxene in porphyroclastic lherzolites and wehrlites has REE-enriched pattern and is enriched in Th and U and depleted in Nb and Ta.The clinopyroxene-orthopyroxene equilibrium temperatures for the protogranular lherzolites are usually ~980°C, only single sample gave temperature ~900°C (Brey and Köhler, 1991, JoP). Pyroxenes in porphyroclastic lherzolites and wehrlites are not equilibrated, but the elevated Ca content in olivine suggests that those rocks were affected by heating.The protogranular lherzolites from Fife are restites after relatively low (1-7%) degrees of partial melting and were further affected by cryptic metasomatism. Bonadiman et al. (2008, Geol. Soc.) suggested that the enrichment in Th and U may result from reaction with subduction-related melt(s), possibly related in central Scotland with Caledonian closure of Iapetus ocean. Composition of clinopyroxene from the most orthopyroxene-rich porphyroclastic lherzolites resembles that of clinopyroxene from protogranular rocks, but as the modal composition grades toward wehrlites, the clinopyroxene becomes more variable in composition but also LREE-enriched. We suggest, that the chemical and textural paths from protogranular to equigranular samples record different stages of wehrlitization process triggered by infiltration of LREE-enriched mafic melt(s).This study was possible thanks to project NCN UMO-2016/23/B/ST10/01905 from the Polish National Centre for Science and Polish-Austrian project WTZ PL 08/2018 .