Metasomatic processes of Paleozoic lithosheric mantle of Scotland Terranes

Mantle melting events occurring intermittently from the
late Carbonifeous to mid-Permian in northern and northwestern
Scotlandand relate on lithospheric extension,
generated small volumes of silica-deficient basic magmas,
which in some case, contain peridotitic xenoliths The present
work concerns mantle peridotites from several localities of
the Northern Highlands Terrane (Rinibar,Orkney- and Streap
Com’laidh) of Midland Valley Terrane (Rudduns Point). A
comparison with rare peridotite xenoliths from Hebridean
Terrane, Grampian Highland Terrane and Southern Uplands
Terrane which also experienced variable metasomatic
enrichment is also put forward, with the aim of studying the
evolutionary history of Scottish lithospheric mantle and
highlighting the metasomatic agents which affected this
portion of mantle during Paleozoic. Attention was mainly
focussed on major and trace element contents of minerals,
namely clinopyroxenes. In the Northern Highland Terrane,
clinopyroxenes from Streap Com’laidhare less LREE-enriched
and present less pronounced Ti and Zr anomalies (La/YbN
2.8-4.5; Zr* 0.42-0.57; Ti* 0.57-0.62) than clinopyroxenes
from Rinibar. Clinopyroxenes from Rinibar have been
variably enriched in LREE, with La/YbN ratios ranging from
2.7 to 32.2. Remarkable Ti and Zr negative anomalies,
although to variable extent, are noticed (Zr* 0.10 - 0.86; Ti*
0.07 - 0.28). Phlogopite is present, and can account for the
pronounced Ti negative anomaly in some clinopyroxenes.Two
different enrichment styles can be identified in the Orkney’s
lithospheric mantle: one presenting LREE-enrichment and
remarkable Zr (and Hf) and Ti negative anomalies, the other
with LREE-depletion (compared to MREE) and less
pronounced Ti (and Zr) negative anomalies, associated with
the presence of high-TiO₂ phlogopite. The two kind of
clinopyroxenes compare well with clinopyroxenes related to
carbonatite and kimberlite metasomatism respectively. A
progressive enrichment processes may also be envisaged
between clinopyroxenes in xenoliths from Midland Valley
Terrane, and those from Hebridian Craton.