A HYBRID COMPOSITE DIKES SUITE FROM THE ARABIAN NUBIAN SHIELD, SOUTHWEST JORDAN: IMPLICATIONS FOR MAGMA MIXING AND WALL ROCK ASSIMILATION

Magma mixing and wall rock assimilation are widely accepted mechanisms for the diversification of igneous rocks. Evidence for these processes are often recognized in the field and supported by petrographic, geochemical and isotopic data.

The Arabian Nubian Shield (ANS) is an exemplary juvenile continental crust of Neoproterozoic age (1000-542 Ma). The post-collisional rift-related stage (~ 610 to 542 Ma) of its formation is characterized among others by the intrusion of several generations of simple and composite dikes.

This study documents a suite of hybrid composite dikes from the northernmost extremity of ANS in southwest Jordan. The petrogenesis of these dikes is discussed on the basis of field, petrographic, geochemical, and Rb/Sr isotopic data. These dikes give spectacular example for the interaction between basaltic magma and the granitic basement. This interaction ranges from brecciation and partial assimilation of the host alkali feldspar granite to almost complete assimilation of the granitic material. Field structures range from intrusive breccia (angular granitic fragments in a mafic groundmass) to the formation of hybrid composite dikes that are up to 15 meters in thickness.

The rims of these dikes are basaltic andesite in composition with alkali feldspar ovoids (up to 1 cm in diameter); while the central cores are of trachyandesitic to dacitic in composition and again with alkali feldspar ovoids and xenoliths from the dike rims.

A seven points Rb/Sr isochron from one of these composite dikes yields an age of 561±33 Ma and an initial ⁸⁷Sr/86Sr ratio of 0.70326 ± 0.0003 (2σ) and MSWD of 0.62.

Geochemical modeling using major, trace, rare earth elements and isotopes suggest the generation of the hybrid composite dike suite through the assimilation of 30% to 60% granitic crustal material by a basaltic magma, while the latter was undergoing fractional crystallization at different levels in the continental crust.