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

Yaron Be'eri-Shlevin, Yaron Katzir, John W. Valley

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90 Scopus citations

Abstract

Crustal recycling patterns during the evolution of the Neoproterozoic Arabian-Nubian Shield (ANS) were defined using the oxygen isotope ratio of zircon [δ18O(Zrn)]. Evidence for early (~ 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 δ18O(Zrn) values of several island arc complex (IAC) orthogneisses (6.9 to 8.2‰) and also from the average δ18O(Zrn) value of 6.4‰ determined for detrital zircons (~ 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 ~ 800 Ma show mantle-like δ18O(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 δ18O(Zrn) values of 5.0 to 7.9‰ indicating continued recycling of the felsic crust. The main sample set of this study comprises rocks from the mostly granitic, post-collisional calc-alkaline (CA2: ~ 635-590 Ma) and alkaline (AL: ~ 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 δ18O(Zrn) values of 5.7 and 5.8‰ pointing to the dominance of mantle-like δ18O sources in their formation. Nonetheless, grouping the two suites together reveals geographical zoning in δ18O(Zrn) where a large southeastern region of δ18O(Zrn) = 4.5 to 5.9‰ is separated from a northwestern belt with δ18O(Zrn) = 6 to 8‰ by a '6‰ line'. It is thus suggested that all CA2 and AL magmas of the northernmost ANS were derived from mantle-like δ18O 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 ~ 15-35%, high-δ18O 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 ~ 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 (~ 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 δ18O 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-δ18O pre-635 Ma crust. An important implication from our results is that petrogenesis of some high-δ18O 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.

Original languageEnglish
Pages (from-to)169-184
Number of pages16
JournalLithos
Volume107
Issue number3-4
DOIs
StatePublished - 1 Feb 2009

Keywords

  • A-type granites
  • Arabian-Nubian Shield
  • Magma sources
  • Oxygen isotopes
  • Zircon

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

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