Drowned in granite - retrieving the tectono-metamorphic history of the Janub metamorphic complex, the northernmost part of the Arabian-Nubian Shield

Abstract

The Janub Metamorphic Complex (JMC) in southern Jordan provides new correlative data constraining the transition from compressional to extensional tectonics in the northern Arabian-Nubian Shield. This constraint comes from the identification of extensional mylonitic shear zones affecting both the JMC and some intruding granitoids. The JMC comprises metamorphosed andesitic-dacitic-rhyolitic flows and pyroclastics, meta-volcanogenic sediments, and hornfels. Volcanism ceased by 618 ± 5 Ma. The volcanosedimentary sequence formed in an intra-arc basin in a mature island arc and was then buried and regionally metamorphosed at lower greenschist facies conditions in the waning accretionary phase between 618 and 615 Ma. This is based on the age of crosscutting plutons of the Rumman Suite, which triggered local high-T/low-P metamorphism at ∼ 615 Ma. Rumman granitoids and JMC rocks were deformed by narrow gently-dipping mylonitic shear zones, interpreted as extensional, that ceased operating by 605 Ma, the age of crosscutting undeformed dikes and plutons of the Yutum Suite. At 596 Ma, a late granite intruded the JMC and thermally metamorphosed some of the meta-sediments. This granite was deformed in a Najd-related brittle shear zone at ∼ 590 Ma as recorded by 40Ar/39Ar geochronology. The JMC can be correlated with the higher-grade Abu-Barqa Metamorphic Complex (ABMC), despite tectonics relations being obscured by younger intrusive rocks. The main metamorphic phase in the ABMC occurred at a depth of 18 km at ∼ 620–615 Ma, followed by gradual uplift and high-T metamorphism at 615–610 Ma and exhumation to the surface at ∼605 Ma. Extensional shearing in the JMC and exhumation of the ABMC probably developed in a core-complex like setting triggered by orogenic collapse. Ductile shearing related to collapse ceased before 605 Ma. The 590 Ma brittle shear zone is the first dated Najd-related structure from the basement of Jordan. The central setting of our data allows correlation of metamorphic complexes in Sinai and Saudi Arabia and is consistent with the final assembly of Gondwana, initiation of orogenic collapse, and transition to extensional tectonics occurring at ∼ 620–610 Ma in the northern Arabian-Nubian Shield.