From Island Arc to Craton: Timescales of Crustal Formation Along the Neoproterozoic B’ir Umq Suture Zone, KSA
Ulysses S. Hargrove, Robert J. Sterna, William R. Griffina, Peter R. Johnson, Mohamed G. Abdelsalam
The Neoproterozoic Hijaz and Jeddah arc terranes that flank the Bi’r Umq suture zone (BUSZ) in western Saudi Arabia record some of the earliest magmatic and deformational events in the juvenile part of the Arabian-Nubian Shield (ANS). Detailed U-Pb ion microprobe (SHRIMP-RG) analyses were conducted on zircon from 31 plutonic and volcanic samples, the ages of which reveal a complex and protracted (820-520 Ma) tectono-magmatic history that is comparable to that of correlative units in eastern Sudan. The results define four magmatic episodes (M1-M4) that record the transition of the ANS from discrete arcs to composite terranes to craton. The earliest igneous activity occurred simultaneously on both sides of the suture at ca. 825-800 Ma during M1, and possibly began as early as ca. 854 Ma. M1 was followed by a 15 Ma hiatus in magmatism that was accompanied by deformation, denudation, and exposure of the M1 units. Rifting of the arc terrane and emplacement of the Tharwah ophiolite, the age of which is tentatively reinterpreted to be 777 ± 17 Ma, may have occurred during this hiatus. Igneous activity resumed during the M2 magmatic episode (785-745 Ma), which represents the main crust-forming event, presumably above a subduction zone. M2 magmatism was contemporaneous with suture-related deformation, as recorded by ca. 785 and 750 Ma metatonalite plutons that exhibit structural evidence of syntectonic emplacement. Following the end of deformation and a nearly 50 Ma hiatus in magmatism along the BUSZ, magmatism was briefly reactivated during the M3 (700-633 Ma) episode and was associated with local extension. The final magmatic episode, M4 (598-566 Ma), as defined by previous studies, involved emplacement of numerous granitoids that are similar to postorogenic A-type igneous rocks present throughout the northern ANS and Saharan Africa. The origin of some of these has previously been attributed to anatexis of the lower continental crust as a result of orogenic crustal thickening, and they represent the cratonization of the ANS. The results also provide new insights into the role of older continental material in construction of the ANS. Although the core of the shield where our samples originate is considered juvenile, with no crust older than ca. 870 Ma, some volcanic rocks and posttectonic granites from the BUSZ contain zircons that yield abundant early Neoproterozoic to Archean Pb-Pb ages; one of these (2840 Ma) is the oldest yet reported from the juvenile core of the Arabian Shield. Paleoproterozoic-Archean inherited zircon may be derived from basement of that age exposed in the southeastern ANS, but sources for abundant Mesoproterozoic inherited zircon do not occur anywhere in the shield. They could have been assimilated from terrigenous sediments that were shed from nearby passive margins, transported by glaciers, or subducted and then resurrected by partial melts of the subducted slab. Alternatively, they were assimilated from cryptic pre-Neoproterozoic crust that underlies the “juvenile” core of the ANS. Some zircon morphologies suggest a sedimentary (detrital) origin, whereas others are more consistent with in situ extraction of juvenile grains from older igneous basement. Inheritance in the Tharwah ophiolite supports the presence of older basement, as it is best explained by rifting of continental crust. The abundance of inherited zircon along the BUSZ greatly increases the known extent of the “contaminated” shield and suggests that previously unrecognized pre-Neoproterozoic crust played a significant, although cryptic, role in the evolution of the juvenile core of the ANS.
Hargrove, U.S., Stern, R.J., Griffin, W.R., Johnson, P.R., and Abdelsalam, M.G., 2006, From island arc to craton: timescales of crustal formation along the Neoproterozoic B’ir Umq suture zone, KSA: Saudi Geological Survey Technical Report SGS-TR-2006-6, 69 p., 4 tables, 32 figs.