Nd-Sr-Hf-O isotope provinciality in the northernmost Arabian-Nubian Shield: implications for crustal evolution

International audienceMulti-isotope study including whole-rock Nd-Sr, single zircon Hf, and SIMS delta(18)O analyses of zircons sheds light on magma sources in the northernmost Arabian-Nubian Shield (ANS) during similar to 820-570 Ma. Reconnaissance initial Nd and Sr isotope data for the older rocks (similar to 820-740 Ma) reaffirms previous estimates that early crustal evolution in this part of the shield involved some crustal contamination by pre-ANS material. Prominent isotope provinciality is displayed by post-collisional calc-alkaline and alkaline igneous rocks of similar to 635-570 Ma across a NW-SE transect across basement of the Sinai Peninsula (Egypt) and southern Israel. Silicic rocks of the NW-region are characterized by lower epsilon Nd(T)-epsilon Hf(T) and higher Sr(i) and delta(18)O compared with rocks of the SE-region, and the transition between the regions is gradual. Within each region isotope ratios are independent of the extent of magma fractionation, and zircon cores and rims yield similar delta(18)O values. Comparison with southern segments of the ANS shows that the source for most similar to 635-570 Ma rocks can be modeled as the isotopically aged lower-intermediate crust in the ANS core (SE-region) and its northern, more contaminated ANS margins (NW-region). Nevertheless, Nd-Sr isotope enrichment of the lithospheric mantle is indicated by some basic magmas of the NW-region displaying the most enriched Nd-Sr isotope compositions. Comparison of Nd and Hf depleted mantle model ages for rocks of the SE-region may indicate that crustal formation events in the ANS geographical core took place at 1.1-1.2 Ga and were followed by crustal differentiation starting at similar to 0.9 Ga

Provenance of Neoproterozoic sediments in the Särv nappes (Middle Allochthon) of the Scandinavian Caledonides: LA-ICP-MS and SIMS U-Pb dating of detrital zircons

We present U-Pb age data for detrital zircons from dike-intruded Neoproterozoic sedimentary rocks of the Caledonian Middle Allochthon in central Sweden and Norway. Detrital zircons from 11 samples from the Särv, Sætra and upper Leksdal nappes (informally referred to as the Särv nappes) are clustered within ca. 0.9-1.75. Ga, but display a bimodal distribution with major ca. 1.45-1.75. Ga and ca. 0.9-1.2. Ga components. An apparent increase of younger (0.9-1.2. Ga) components to the northwest reflects varying source terranes. Detrital zircons from an additional sample from the lower part of the Leksdal Nappe, of uncertain affiliation to the Särv has a prominent 1.75-1.85. Ga component supporting previous suggestions that this part of the nappe belonged to a more proximal basin. Comparison of the Särv age probability patterns with data from basement windows and basement slices within the Middle Allochthon in central Sweden and Norway supports the derivation of the sediments from the attenuated Baltican continental crust on which they were presumably deposited. Similar comparisons suggest that derivation from the southern segment of the Fennoscandian Shield or from eastern segments of Laurentia is less likely, mostly because they include also older components.We infer that the ca. 200. km wide belt of attenuated Baltican continental crust included northern extensions of Mesoproterozoic to early Neoproterozoic terranes exposed in the southern part of the Fennoscandian Shield and the easternmost part of Laurentia, which at ca. 900. Ma were still adjacent. Pre-1.75. Ga terranes of the Fennoscandian Shield were probably isolated from the Särv distal basin(s) by intra-cratonic basins and uplifted margins associated with early development of this extended continental crust. The significantly older ages in the lower part of the Leksdal Nappe and its inferred more proximal position support this model. The proposed northern extension of Mesoproterozoic-early Neoproterozoic terranes can explain in a simpler way the occurrence of such detritus in many Caledonide-Appalachian allochthons exposed at the margins of the North Atlantic, with no need to infer large displacement along the axis of the Caledonide Orogen or to postulate selective transport of Grenville-age material from the south over large distances. One of our Särv samples located at the Norwegian coast revealed Caledonian reworking at ca. 395. Ma. This age agrees with ages of late-tectonic amphibolite-facies metamorphism and pegmatite intrusion recorded in this part of the Caledonides