Geochronology and geochemical evolution of the wadi turabah felsic plutonic ring complex, Central Arabian Shield

Abstract EN

Rb-Sr whole rock isochrons and major element oxide data for the Wadi Turabah felsic plutonic ring complex are discussed .

Felsic rocks of the Wadi Turabah ring structure indicate a complex history of evolution and consist of gneissic granodiorite-granite suite (GGGS), massive granodiorite- granite suite (MGGS) , and a ring dike granite.

The GGGS and north trending metavolcanic and metasedimentary layered sequences (classified as Baish and Bahah) have suffered polyphase deformation and metamorphism in the upper and lower grades ofthc greenschist and amphibolite facies .

resp ectively.

The ring dike granite is the last phase in the magmatic evolution of the ring complex .

Rb-Sr whole rock data for six samples of the GGGS defines an apparent age of 535± II Ma (20') with an initial ll7Srf86Sr ratio ofO .7037±0.00016 (20') and a MSWD (mean square of weighted deviat es) value of 6.6.

The age determinedon five samples ofMGGS is 416±9 Ma (20') with an init ial ll7Srf86Sr ratio of O.704±0.0002 (20') and a MSWD value of 9.01.

The ring dike granite yields a four point isochron age of 117±0.4 Ma with an initial B7Srf86Sr ratio ofO .7089±O .OOOI5 (2a) and a MSWD valu e of O.Ol.

Major clement oxide and Rb-Sr data indicate that the felsic plutonic rock s within the ring structure arc highly differentiated and deriv ed from fractional crystallization of magma generated from the lower crust or upper mantle .

The chemical data also indicate that the ring dike granite is fractionated from magma of ternary minimum composition in the system Si02NaAISiO~- KAISiO, under water pressure of I Kb.