Oligo-Miocene A-type granites and granophyres from Yemen: isotopic and trace-element constraints bearing on their genesis
Tommasini, Simone; Poli, Giampiero; Manetti, Piero; Conticelli, Sandro
European Journal of Mineralogy Volume 6 Number 4 (1994), p. 571 - 590
published: Jul 27, 1994
manuscript accepted: Mar 28, 1994
manuscript received: Sep 10, 1993
ArtNo. ESP147050604005, Price: 29.00 €
Abstract Oligo-Miocene A-type granites and granophyres from Yemen were emplaced during the early stages of lithospheric break-up and thinning prior to the onset of rifting and sea-floor spreading of the Red Sea. New trace element data together with Sr isotope are reported in the present paper. The Yemen granites and granophyres have high alkali (7-11 wt.%), high field strength element, and Zn contents along with low Sr and Ba contents. They are characterised by high FeOtot/MgO, Ga/Al values. The initial Sr isotope values range from 0.704 to 0.721. Y/Nb, Yb/Ta, and Ce/Nb values are similar to those of Yemen Trap Series basalts and clustered to the field of OIB. These trace element ratios argue against the origin of the Yemen granites and granophyres by crustal anatexis processes, and provide evidence for a genetic link with coeval basalts of the Yemen Trap Series. Furthermore, two groups of high silica magmas can be recognised, which are characterised by Nb/Y values of ~ 1 (Low Nb/Y Granites) and ~ 2 (High Nb/Y Granite). The overall geochemical characteristics of the Yemen granites and granophyres, along with their Sr isotope composition, support an origin for the Low Nb/Y and High Nb/Y Granites by a two-stage Assimilation and Fractional Crystallisation process (AFC) commencing from two types of the Yemen Trap Series basalts with slightly different within-plate signature. Each of the two AFC stages could be occurred in distinct magma reservoirs located in the lower (Step I) and middle-upper crust (Step II).