Hind Ghanem | USGS Science

USGS2025

Geochemistry and spatial distribution of Neoproterozoic dike swarms from the northwestern tip of the Arabian-Nubian Shield: Implications for crustal extension

Ediacaran dike swarms in the Arabian-Nubian Shield (ANS) provide key insights into post-collisional tectonics and magmatism. This study presents new chemical data, two 40 Ar/ 39 Ar ages, and spatial distribution analysis for dike swarms in the northernmost ANS exposures in SW Jordan. Hornblende from a lamprophyre sill intruding the Saramuj Conglomerate yielded a plateau age of ∼592 ± 3 Ma, interpreted as the crystallization age of the sill. Micro-cores of sericite-rich domains in altered plagioclase phenocrysts from a dolerite dike intruding ∼586 Ma alkali granite produced ages of ∼580–570 Ma, interpreted as hydrothermal alteration ages, constraining dike emplacement to 586–580 Ma. The geochemistry of investigated dikes varies from alkaline to subalkaline and crosscuts calc-alkaline magmatic rocks. This transition reflects a shift from compression to extension (∼610-590 Ma), lithospheric thinning, and changes in mantle sources. After 590 Ma, magmatism became exclusively alkaline, forming A-type granitoids and dolerites, marking a shift to extensional magmatism. Dikes predominantly follow NE-SW and E-W orientations, with rare N-S, NW-SE, and WNW-ESE trends, suggesting a major horizontal extension direction from NW-SE to N-S. Dike density ranges from 5 to 13 %, averaging ∼9 %, with crustal extension estimates of 9–23 %, averaging 16 %. No correlation was found between dike composition, orientation, or order of emplacement, indicating consistent stress orientation during the final stage (605-580 Ma) of ANS evolution.

Journal of African Earth Sciences↗

USGS2020

40Ar/39Ar and U-Pb SIMS zircon ages of Ediacaran dikes from the Arabian-Nubian Shield of south Jordan

A spectacular feature of the Arabian-Nubian Shield (ANS) is the abundance of well-exposed and extensive Neoproterozoic dike swarms of multiple generations. These dikes are generally categorized into metamorphosed and unmetamorphosed post-orogenic dike swarms. The unmetamorphosed dikes in the northern ANS can be grouped into an old and young generations. We dated three dikes from the old generation of the unmetamorphosed dikes: a composite dike with latite margins and rhyolite core (607 ± 6 Ma, U-Pb), a biotite rhyolite dike (600 ± 4 Ma, 40Ar/39Ar age of biotite) and an andesite dike (594 ± 3, 40Ar/39Ar age of amphibole). We propose that these dikes representing the old generation were emplaced at different episodes extending approximately between 607 and 590 Ma. Time and composition equivalent plutonic rocks are common in Jordan and the northern ANS. These dikes crosscut the late to post-collisional granitoids and display a subduction-related character as evidenced from the Nb-Ta anomaly, suggesting a transitional magmatic activity from the orogenic to extensional environment. This generation of dikes is absent in the alkali feldspar A-type granite dated at 586 ± 5 Ma in Jordan and equivalent rocks in the northern ANS, which are crosscut only by the dolerite dikes which has an approximate crystallization age of ~579 Ma (40Ar/39Ar whole rock total gas age). Their within-plate character is supported by the absence of the Nb-Ta anomaly and the high field strength elements tectonic discrimination plots. We propose that these dolerite dikes represent the last Neoproterozoic igneous activity in the northern ANS, i.e. the magmatic activity was terminated ~50 m.y. before the estimated age of the Ram Unconformity at ~530 Ma. This age is in agreement with a previously suggested model of mantle lithosphere delamination from below the northern ANS after a significant crust-mantle thickening caused by the East African Orogeny. The thickening triggered exceptionally rapid uplift, followed by erosional unroofing of the ANS rocks, some lateral extension, and post-orogenic magmatic activity. This was followed by thermal relaxation and subsidence and the gradual denudation, erosion, and peneplanation that gradually developed until the approximate age of the unconformity at ~530 Ma.

Precambrian Research↗