Abstract 8
The Jabal Aja Ring Complex (ARC) is a Late Ediacaran composite pluton of post-collisional A- 9
type granites in the northeastern part of the Arabian Shield. It is an elliptical body with 10
discontinuous ring-shaped outcrops due to later faulting. Field relationships enable the 11
recognition of two main phases of magmatic activity in the ARC. The early phase represents the 12
core of the intrusion (monzogranite, syenogranite, granophyre, and alkali feldspar granite) 13
surrounded by the last phase (alkali volcanics and alkaline/peralkaline granites). The contacts 14
between two phases are gradational and/or sharp, indicating their emplacement within a very 15
short time period before the complete crystallization of the earlier phase. The ARC is alkaline to 16
peralkaline rare-metal granites with the common geochemical characteristics of post-collisional 17
intraplate A-type rocks. The pronounced negative Eu anomalies (Eu/Eu* = 0.14–0.25) reflect 18
extreme magmatic fractionation and perhaps the effects of late fluid–rock interaction. All A-type 19
rocks of the ARC retain a positive Nb-Ta anomaly that increases from the early phase to the 20
most evolved alkaline/peralkaline granites. They define geochemical signatures reflecting their 21
derivation from same magma sources that evolved through fractional crystallization with crustal 22
contamination. The geochemical characteristics of the A-type granites of ARC reflect re-melting 23
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of previously formed arc material. The mafic microgranular enclaves (MMEs) represent 24
metamorphic rocks derived from/in the deeper part of the crust, and represent the source rocks 25
from which the granitic magma was generated by partial melting. The early phase of magmatism 26
of the ARC represents melts derived from partial melting of lower/middle crustal sources during 27
the post-collisional stage that fractionated to give the various granitic rocks. The absence of 28
mafic xenoliths in the late stage of the ARC indicates that the contribution of crustal sources to 29
the magma generation is absent in the late stage.
