Crustal evolution and recycling in a juvenile continent: Oxygen isotope ratio of zircon in the northern Arabian Nubian Shield

Crustal recycling patterns during the evolution of the Neoproterozoic Arabian-Nubian Shield (ANS) were defined using the oxygen isotope ratio of zircon [delta O-18(Zrn)]. Evidence for early (similar to 870-740 Ma) crustal recycling in the northernmost ANS (southern Israel and Sinai, Egypt) is given by laser fluorination analysis of bulk zircon separates, which yield higher than mantle 5180(Zrn) values of several island arc complex (IAC) orthogneisses (6.9 to 8.2%.) and also from the average delta O-18(Zrn) value of 6.4 parts per thousand determined for detrital zircons (similar to 870-780 Ma) from the Elat-schist; the latter representing the oldest known rock sources in the region. These results indicate prolonged availability of surface-derived rocks for burial or subduction, melting, and assimilation at the very early stages of island arc formation in the ANS. Other IAC intrusions of similar to 800 Ma show mantle-like delta O-18(Zrn) values, implying that not all magmas involved supracrustal contribution. Much younger (650-625 Ma) deformed syn-collisional calc-alkaline (CA1) intrusions are characterized by delta O-18(Zrn) values of 5.0 to 7.9 parts per thousand indicating continued recycling of the felsic crust. The main sample set of this studycomprises rocks from the mostly granitic, post-collisional calc-alkaline (CA2: similar to 635-590 Ma) and alkaline (AL: similar to 608-580 Ma) magmatic suites. Despite having distinct geochemical characteristics and petrogenetic paths and spans of magmatic activity, the two suites are indistinguishable by their average 6180(Zrn) values of 5.7 and 5.8 parts per thousand pointing to the dominance of mantle-like 6180 sources in their formation. Nonetheless, grouping the two suites together reveals geographical zoning in F)180(Zrn) where a large southeastern region of 6180(Zrn) =4.5 to 5.9 parts per thousand is separated from a northwestern belt with delta O-18(Zrn)= 6 to 8 parts per thousand by a '6 parts per thousand line'. It is thus suggested that all CA2 and AL magmas of the northernmost ANS were derived from mantle-like delta O-18 reservoirs in the mafic lower-crust and the lithospheric-mantle, respectively. However, while in the northwestern belt these magmas intruded a thick crustal section and assimilated similar to 15-35%, high-delta O-18 IAC+ CA1 material, magmas in the southeastern region intruded a thinner crust and little or no contamination occurred. The proposed NW-SE variance in crustal thickness during the late Neoproterozoic fits well with the geometry of the fan shaped rifting model proposed by Stern [Stern, R.J., 1985. The Najd Fault System, Saudi Arabia and Egypt: a late Precambrian rift related transform system. Tectonics 4,497-511.] for this region. Deep parts of the lithosphere were beginning to rift at similar to 630 Ma, allowing the asthenospheric mantle to rise and transfer heat to the lithosphere. This resulted in vast melting of the mafic lower-crust to produce the batholithic CA2 magmas. Later (similar to 610 Ma) percolation of lithospheric-mantle melts (possibly along deep seated lithospheric-scale faults) introduced AL magmas to shallow levels of the crust. Intrusion of CA2 and AL mantle-like 6180 parent magmas into the thinned southeastern crust did not involve assimilation of older crust whereas similar intrusion into the thicker northwestern crust resulted in mild assimilation of high-delta O-18 pre-635 Ma crust. An important implication from our results is that petrogenesis of some high-delta O-18 AL magmas of the northernmost ANS involved assimilation of supracrustal material. Felsic intrusions of the AL suite were previously described as A-type granites derived solely from mantle melts with no crustal components. Our results contribute to the A-type petrogenesis debate by showing that their formation can involve recycling of crustal material. (c) 2008 Elsevier B.V. All rights reserved.