Origin and evolution of lithospheric mantle beneath the juvenile Arabian-Nubian Shield: Evidence from the Natash peridotite xenoliths, Southern Eastern Desert, Egypt

Compared with the Archean cratonic mantle, much less is known about the genesis and composition of the subcontinental lithospheric mantle (SCLM) beneath the juvenile terranes. In this study, we present petrology and mineral chemistry analyses of peridotite xenoliths carried by Cretaceous volcanics from the Natash area in the western Arabian-Nubian Shield (ANS). Combined with mantle xenoliths from the eastern ANS, we aim to constrain the nature, origin, and evolution of SCLM beneath the juvenile terrane. The Natash peridotite xenoliths, including well-preserved clinopyroxene (Cpx) and spinel (Sp), are categorized into two groups. Group A peridotites display exceptionally high Mg# (molar100 x Mg/(Mg + Fe-total)) in Cpx (95.6-96.7) and Cr# (molar100 x Cr/(Cr + Al)) in Sp (45.5-51.7), signifying a high degree of melting and refractory nature. Cpx in Group A demonstrates low Ti/Eu (532-1785) and high (La/Yb)(N) (5.34-26.29) ratios, indicative of carbonatitic metasomatism. Comparatively, Group B peridotites exhibit relatively high Mg# in Cpx (93.8-95.5) and Cr# in Sp (15.9-26.2), indicating a low degree of melting and fertile nature. Cpx trace element patterns subdivide Group B into three sub-groups. Cpx in Group B1 are characterized by high Ti/Eu (3934-5622) but low (La/Yb)(N) (0.91-2.98), implying silicate melt metasomatism. Both Groups B2 and B3 Cpx show steep rare earth element (REE) patterns, negative anomalies in high field-strength elements, and high (La/Yb)(N) (3.62-7.57). However, Cpx in B3 exhibit higher Ti/Eu ratios and water-soluble element contents than those in B2 peridotites. In addition, the spoon-shaped REE patterns are prominently unique to Cpx in Group B3. Hence, it is inferred that Group B2 were influenced by carbonatitic melts, whereas the evolved small-volume CO2-H2O-rich fluids infiltrated into Group B3 samples. Furthermore, the high Sr contents (71-404 ppm) but low Sr-87/Sr-86 (0.7003-0.7034) ratios of Cpx in Natash peridotites suggest that the various metasomatic agents all originated from the asthenosphere. Collectively, the Natash peridotite xenoliths exhibit similar nature and metasomatism processes with those from Cenozoic volcanics in the eastern ANS, and they all show affinity to continental peridotites. The pMELTS simulation suggests that the refractory peridotites have a high melting pressure (typically >3 GPa), while the fertile samples register a low melting pressure. Consequently, we infer that the SCLM beneath the juvenile ANS was formed through the cooling of upwelling asthenosphere, incorporating the ancient mantle fragments, and subsequently underwent asthenosphere-derived carbonatitic and silicate melt/fluid metasomatisms.