SHRIMP Geochronology in the Arabian Shield (Midyan Terrane, Afif Terrane) and Nubian Shield (Central Eastern Desert Terrane) Part V: Data Acquisition 2009
Allen Kennedy, Wieslaw Kozdroj, Khalid Kadi, Malgorzata Kozdroj, and Peter R. Johnson
The report presents SHRIMP zircon dating done on 23 Precambrian rocks collected in 2009 from the Arabian shield in the Kingdom of Saudi Arabia and the Nubian shield in Egypt (20 and 3 samples respectively). The work is a continuation of, and focused on the same tectonic units as a geochronological investigation started in 2008. Five samples of “layered rocks” represent volcanic bodies embedded in volcanosedimentary successions. Two samples are of mafic dikes cutting sedimentary rocks. The remaining fifteen samples are classified as “intrusive rocks” and include igneous rocks collected from a variety of undeformed plutonic complexes as well as from orthogneiss, representing meta-igneous rocks affected by regional metamorphism or localized shear zones. In most cases, zircon populations, as observed in cathodoluminescence (CL) images and dated by ion-microprobe (SHRIMP), document the highly complex history of their parent rocks. This complexity is due to factors such as: (1) the presence of old sources supplying inherited grains, (2) polyphase magmatic events with changing thermal conditions, (3) complicated dynamo-metamorphic transformations, (4) the influence of high-U sources producing radiation damage and (5) Pb disturbances (Pb loss) during the Phanerozoic. As a result, only six samples represent rocks that were produced during a single magmatic event, contain no inherited components, and were almost certainly unaffected by Pb loss. Eleven other samples contain inherited Archean, Paleoproterozoic or Mesoproterozoic zircons. Ten samples have zircons that yield Neoproterozoic ages older than the supposed age of the main crystallization and intrusion. Nonetheless, apart from one contaminated sample and two others that produced no preferred ages, it is possible for most of those complex samples to define a dominant population of zircons that yield reasonable ages, which can be interpreted as the time of crystallization of the parent magmas. Robust ages obtained from the Midyan terrane in the northwestern part of the Arabian shield include: ~706 Ma for meta-rhyolite of the Ghawjah formation (island-arc assemblage); 617.7±4.4 Ma for an andesitic dike cutting sediments of the Thalbah group (post-amalgamation basin); 687.8±2.6 Ma and 693 - 662 Ma for orthogneiss; and 569±15 Ma for post-kinematic granites localized within shear zones. One sample of basaltic/andesitic lava from the Afif formation (Afif terrane; eastern part of the Arabian shield) was dated as 630-640 Ma. Of similar age, 642.1±1.6 Ma, are porphyritic dacites. Other massive dacite/andesite (~586 Ma), basalt/andesite (~600 Ma), and a trachyte plug (~589 Ma) probably represent younger subvolcanic intrusives emplaced in the Afif formation. They are comparable in age with a sample of metagabbro (~600 Ma). A well-defined suite of plutonic rocks, all younger than the Afif formation, is identified by ages of 639.6±2.6 Ma (coarse-grained gabbro), 637.8±4.8 Ma (foliated granodiorite), 636.4±5.9 Ma (granite-porphyry), 613.2±3.8 Ma (calc-alkalic syeno-diorite of the Al Humayy pluton), 610.1±3.7 Ma (microgranite dike cutting the Al Humayy pluton) and 593.8±1.8 Ma (alkalic granite). The Al Humayy pluton intrudes sedimentary rocks of the Murdama group, whereas the others do not, indicating that the Murdama group must have been deposited between 636.4±5.9 Ma and 613.2±3.8 Ma. Robust ages from the Eastern Desert terrane (Nubian shield) were obtained for only two samples: 665.7±9.4 Ma for diorite representing a “meta-gabbro-diorite association” and ~580 Ma for an andesitic dike intruding Hammamat clastics.
Kennedy, A., Kozdroj, W., Kadi, K., Kodzroj, M.Z., and Johnson, P.R., 2010, SHRIMP geochronology in the Arabian Shield (Midyan terrane, Afif terrane) and Nubian Shield (Central Eastern Desert terrane) Part V: Data acquisition 2009: Saudi Geological Survey Open-File Report SGS-OF-2010-11, 80 p., 113 figs. 24 tables.