HR: 1340h
AN: V13B-1472    [Abstracts]
TI: What is the Significance of Adakitic Granitoids and Zircon Inheritance in Juvenile Arc Rocks of the Neoproterozoic Makkah Batholith, Saudi Arabia?
AU: * Hargrove, U S
EM: ushgeo@utdallas.edu
AF: University of Texas at Dallas, Department of Geosciences 2601 N. Floyd Rd., Mail Stop FO21, Richardson, TX 75083 United States
AU: Stern, R J
EM: rjstern@utdallas.edu
AF: University of Texas at Dallas, Department of Geosciences 2601 N. Floyd Rd., Mail Stop FO21, Richardson, TX 75083 United States
AU: Kimura, J
EM: jkimura@riko.shimane-u.ac.jp
AF: Shimane University, Department of Geoscience, Matsue City, 690-8504 Japan
AU: Johnson, P R
EM: johnson.pr@sgs.org.sa
AF: Saudi Geological Survey, P.O. Box 54141, Jiddah, 21514 Saudi Arabia
AB: Neoproterozoic plutonic rocks of the Makkah batholith along the northern margin of the Jiddah terrane in western Saudi Arabia record $>$50 Ma of arc magmatism. Results of the first detailed survey using U-Pb zircon geochronology by SHRIMP-RG and trace-element geochemistry by ICP-MS provide important new constraints on subduction-related magmatism in the Arabian-Nubian Shield (ANS), but have also illuminated new problems. Samples from several dozen composite plutons ranging in composition from gabbro to granodiorite or granite were analyzed. SHRIMP-RG data for the plutons reveal 3 magmatic pulses at ca. 800, 775, and 750 Ma. Phase I (ca. 800 Ma) produced plutons that yielded U-Pb zircon ages of 803+/-17 Ma (previously reported zircon age of 816+/-3 Ma) and 804+/-5 Ma. The 803-Ma granite is nonconformably overlain by a potentially glacial diamictite at the base of the arc volcanic sequence. Phase II (ca. 775 Ma) plutons yielded ages of 781+/-8 Ma (previously reported zircon age of 760+/-10 Ma), 782+/-7 Ma, 776+/-6 Ma, and 770+/-5 Ma (previously reported zircon age of 769+/-5 Ma). Phase III (ca. 750 Ma) plutons yielded ages of 755+/-5 Ma, 747+/-5 Ma, 750+/-5 Ma, and 747+/-9 Ma. SHRIMP-RG was used to overcome a long-recognized problem of inheritance in the region. Inherited zircons of Mesoproterozoic age occur in one arc suite and in post-tectonic, A-type alkalic granites, and zircons as old as Archean occur in the arc volcanic rocks. The source of the inheritance is enigmatic. Remnants of Archean-Paleoproterozoic continental crust are exposed in the Afif terrane far to the SE. Could arc plutons of the Makkah batholith have interacted with previously unrecognized Pre-Neoproterozoic continental crust beneath the Jiddah terrane? Alternatively, did shallow arc magmas interact with buried glacial deposits and inherit far-traveled detrital zircons? The plutons exhibit geochemical signatures expected for juvenile oceanic arc rocks. They are I-type granitoids of the low- to medium-K series, show strong subduction signatures, and plot as volcanic arc granites on discrimination diagrams. Nearly half the samples show characteristics of adakites (high Sr/Y $<$130, low Y, steep REE patterns). Adakitic rocks previously recognized in the ANS were attributed to the subduction and melting of young, hot oceanic crust. However, the production of adakites from slab-melting should wane with time. That adakitic rocks occur in all 3 magmatic pulses ($>$50 Ma) suggests they were produced by a more sustained mechanism, such as melting of thickened eclogitic or garnet-granulitic lower crust. Anatexis in the lower crust is an appealing explanation for the adakitic chemistries, and for the source of the inherited zircons, but is not consistent with the arc characteristics and juvenile isotopic signatures of the Jiddah terrane. Could subduction of young oceanic crust be sustained long enough to produce adakitic magmas for $>$50 Ma (e.g. by oblique subduction of ridge segments)? If so, could the subduction of ridge segments at ca. 25 Ma intervals have produced the 3 magmatic pulses? Also, could large volumes of juvenile arc magmas melt enough older crust to inherit abundant zircons yet retain their original arc characteristics and isotopic signatures? We are using this opportunity to invite constructive dialogue to help resolve these questions.
DE: 9619 Precambrian
DE: 1020 Composition of the crust
DE: 1035 Geochronology
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting