The petrogenesis of A-type magmas from the Amram Massif, southern Israel

The (similar to550-530 Ma) alkaline magmatic suite of the Amram Massif, southern Israel, was emplaced during the transition from an orogenic to an intra plate tectonic setting in the northeastern Arabian-Nubian Shield (ANS). The suite ranges from 45.6 to 78.8 wt % SiO(2), and consists of rhyolites, alkali quartz syenites, quartz syenites, monzonites, and co-magmatic mafic to felsic alkaline dikes. These rocks define a continuous chemical evolutionary trend and reveal a correlation between decreasing stratigraphic age and increasing silica content. The felsic members of the suite display A-type characteristics and are genetically linked through fractionation to the more mafic ones. Moderately positive initial epsilonNd values (+2 +/- 0.5), low initial (37)Sr/(86)Sr values (07036 +/- 2), high MgO and Fe(2)O(3) concentrations (4.10-8.95 and 10.0-12.5 wt %, respectively) and relatively flat rare earth element patterns [(La/Yb)(n) = 6.4 +/- 0.9] in the Amram mafic dikes (45.6-49.5 wt % SiO(2)), suggest their derivation from the sub-continental lithospheric mantle, above the garnet stability zone. The MELTS program was used to quantitatively model the chemical evolution of the suite. Extensive anhydrous fractionation (>90%), of plagioclase, alkali feldspar, clinopyroxene, olivine, and minor Ti-magnetite and apatite from parental mafic magmas, represented by the Amram mafic dikes, produced the rlyolitic compositions as well as the intermediate members of the suite. This suggests the presence of a large unexposed body of cumulate rocks at depth, as well as fusion of a large source-region (equivalent to an similar to5 km layer) in the lithospheric mantle. Regarded as a representative example for similar A-type outcrops in this region, this petrogenetic model further suggests that Neoproterozoic-Early Cambrian A-type magmatism in the northeastern ANS represents a significant post-orogenic addition of mantle-derived material to the juvenile crust. This magmatic episode was of a similar magnitude to that of the Cenozoic, extension-related, alkaline volcanism of the Arabian plate.