Late- and post-orogenic Neoproterozoic intrusions of Jordan: implications for crustal growth in the northernmost segment of the East African Orogen

Approximately 90% of the upper crust exposed in southern Jordan is composed of intrusive rocks of latest Neoproterozoic age, grouped into two major subdivisions: the Aqaba (600–640 Ma) and the Araba (560–600 Ma) complexes. The Aqaba complex comprises several suites that range in composition from gabbro to high silica granite (45–80% SiO2) and follow a high-K calc-alkaline trend. This phase, which started with the emplacement of the Duheila Hornblendic Suite between 640 and 600 Ma, represents the main crust-forming stage in southwest Jordan. The Araba complex is a bimodal alkali-calcic to alkali igneous suite generated after development of a regional unconformity and deposition of the Saramuj Conglomerate. Mafic members of the Aqaba complex, the Duheila Hornblendic Suite, are enriched in LILE relative to the HFSE and are moderately enriched in REE [(La/Lu)n=5–11], traits typical of arc basalts. Geochemical modeling suggests derivation by 10–15% melting of amphibole-bearing spinel lherzolite, possibly above a subduction zone. The granitoids of the Aqaba complex are high in Ba, Sr and LREE, have low Y and have steep REE patterns [(La/Lu)n=20–25]. They have low initial 87Sr/86Sr (∼0.70305) and high εNd values (+2.3 to +5.0). These may have been generated by high degrees of partial melting (10–30%) of subducted oceanic crust, with or without a small proportion of ocean sediments. The mafic and intermediate end members of the Araba complex, the Araba Mafic to Intermediate Suite (48–65% SiO2) comprises quartz-diorites, quartz-monzodiorites, monzodiorites and monzogabbros. The mafic end member of this suite is enriched in LILE compared to the Duheila Hornblendic Suite, although its primitive compositions have similar REE patterns [(La/Lu)n=5–15]; and has been emplaced in a within-plate tectonic environment. Geochemical modeling supports formation of the Araba Mafic to Intermediate Suite by 10% partial melting of phlogopite-bearing spinel lherzolite. This hypothesis is supported by low initial 87Sr/86Sr (∼0.7035) and high εNd values (+2.4 to +4.2). The felsic end member of the Araba complex, the Humrat–Feinan Suite (68–80% SiO2), is characterized by moderate enrichments in LREE (La/Sm)n∼2, flat HREE patterns (Gd/Lu)n∼1, and strong negative Eu anomalies (Eu/Eu∗=∼0.07–0.53). These granites have geochemical features typical of A-type granite and can be derived by extreme fractional crystallization (∼80%) of the Araba Mafic to Intermediate Suite. The low initial 87Sr/86Sr (∼0.7048), high εNd values (+2.5 to +4.8), and spatial and temporal association with the Araba Mafic Suite support this model. Crustal evolution in the northernmost Arabian Nubian Shield occurred about the time of terminal collision between east and west Gondwanaland by the addition of magmas derived from mantle-derived melts. These melts changed markedly at about the time of collision, from a unimodal suite of convergent margin magmas characterized by deep (garnet-controlled) fractionation, to a bimodal suite of rift-related magmas characterized by shallow (feldspar-controlled) fractionation.