Crustal evolution of the Neoproterozoic Bi'r Umq suture zone, Kingdom of Saudi Arabia: Geochronological, isotopic, and geochemical constraints

Neoproterozoic igneous rocks along the Bi'r Umq suture zone (BUSZ) in western Saudi Arabia record some of the earliest tectonomagmatic events in the juvenile (mantle-derived) part of the Arabian-Nubian Shield (ANS). New geochemical, isotopic, and geochronological data provide new insight into crustal evolution in this part of the ANS. U-Pb zircon ages from 31 units reveal a complex and protracted tectonomagmatic history. Four magmatic episodes are defined: M1 (825-800 Ma) pretectonic; M2 (785-745 Ma) syntectonic; and M3 (700-633 Ma) and M4 (598-566 Ma) post-tectonic episodes. Deformation of M1 units and emplacement of the Tharwah ophiolite (777 +/- 17 Ma) during a 15 Ma hiatus in arc magmatism may be rift-related. Suture-related deformation during M2 is recorded by 785 and 750 Ma syntectonic plutons, similar to the timing along strike in Sudan. Most BUSZ igneous rocks are geochemically indistinguishable from those produced in modern convergent margin settings, but variations between magmatic episodes reflect different petrogeneses. Samples with high Sr/Y and strong LREE-enrichment, typical of adakites, are common and were likely produced by lower crustal anatexis and/or slab melting. BUSZ igneous rocks have highly positive initial &egr;Nd (+8.5 to +3.9) and Nd model ages (830-560 Ma) that approximate crystallization ages (813-573 Ma) and are comparable to exposed juvenile upper ANS crust and to xenoliths from the lower crust and lithospheric mantle. The data confirm that most of the 40-km-thick ANS crust is juvenile and was extracted from depleted mantle largely during the interval 830-740 Ma. However, Neoproterozoic samples containing zircon inherited from much older sources yield slightly lower initial &egr;Nd, suggesting that some juvenile magmas assimilated older continental material. Paleoproterozoic-Archean zircon may be inherited from basement exposed in the ANS periphery, but Mesoproterozoic sources are unknown in the shield. Zircon morphologies are consistent with detrital origins and in situ extraction from igneous basement, although inheritance in the ophiolite argues for the latter (i.e. during rifting of older arc crust). The abundant inherited zircon along the BUSZ greatly increases the known extent of the "contaminated" shield and suggests that the role of pre-Neoproterozoic crust was significant in the evolution of juvenile parts of the ANS.