Grant EAR0309799 from the National Science Foundation, Petrology and Geochemistry, to Robert J. Stern, 2003
Testing Models of Crustal Growth in the Neoproterozoic Arabian-Nubian Shield
Robert J. Stern
Center for Lithospheric Studies, University of Texas at Dallas, Richardson, TX 75083, USA.
Project Summary
The proposed research project is designed to test the hypothesis that an important component of the growth of continental crust results from the addition of mantle plume material, namely by the accretion of oceanic plateaux or Large Igneous Provinces (LIP). The Arabian-Nubian Shield (ANS) is an unparalleled natural laboratory for studying how continental crust forms by plate tectonic processes. It represents the bulk of the juvenile continental crust that formed during the Neoproterozoic (variously 1000 or 900 Ma to about 544 Ma; Knoll, 2000), and has the thickness (ca. 40 km) and velocity structure of typical post-Archean crust. The ANS was generated by accretion of about a dozen Neoproterozoic tectonostratigraphic terranes, the juvenile nature of which is demonstrated by the abundance of ophiolites and initial radiogenic Sr, Nd, and Pb isotopic compositions. The ANS is superbly exposed and, due to co-operation with the Saudi Geological Survey, highly accessible.
The proposed project is a two-year study of mildly metamorphosed volcanic and plutonic units located within two field areas along the southern flank of the Neoproterozoic Bi'r Umq-Nakasib suture zone in western Saudi Arabia. This is the oldest suture zone in the ANS and contains one of the most complete Penrose ophiolites in the entire shield. The main non-ophiolitic lithostratographic units in the study areas belong to the Samran and Mahd Groups, both of which have previously been interpreted as arc-related sequences. Preliminary studies of the Samran Group in 2001 have revealed geochemical characteristics of some volcanic units similar to basalts of the Ontong Java plateau, a submarine LIP in the western Pacific Ocean. Units in Sudan that correlate with those in the proposed study areas have already spawned a controversy regarding their tectonic setting (e.g. Abdelsalam and Stern, 1993b; Schandelmeier et al., 1994).
The objectives of the proposed study are to constrain the nature and timing of crustal growth by studying the lithostratigraphy, petrology, geochemistry, geochronology, and isotopic composition of volcanic rocks of the Samran and Mahd Groups in order to test whether or not these volcanic sequences represent accreted crust generated by a mantle plume. Two field seasons are planned (2002-2003 and 2003-2004), during which time a comprehensive suite of samples will be collected from the major volcanic units and associated magmatic suites. These samples will be analyzed for major and trace element compositions by ICP-OES and ICP-MS, respectively, with special emphasis on immobile trace elements (HFSE and REE). U-Pb zircon geochronology will be performed by SHRIMP-RG and TIMS, and isotopic compositions of Sr, Nd, Hf, and Pb will be determined by TIMS. Detailed structural and lithostratigraphic information collected during several traverses along and across regional trends will form the basis for interpretation of the analytical results.
Intellectual Merit of the Proposed Activity: The hypothesis that juvenile crustal growth may in large part be accomplished by additions from mantle plumes challenges the dominant arc accretion model for continental growth, both globally and for the ANS in particular. This hypothesis has been suggested for the evolution of the ANS (Stein and Goldstein, 1996), but has not been tested. The proposed research promises to contribute fundamentally to scientific understanding of the mechanisms and timing of continental crust formation in the Neoproterozoic, with significant implications for modern processes.
Broader Impacts of the Proposed Activity: This project has been developed by and for a doctoral student and so is a focused confluence of research and education. Understanding Neoproterozoic crustal growth has important implications for understanding global changes during this time in the atmosphere, hydrosphere, and biosphere, including global-scale glaciations and the explosive radiation of metazoan life. Results of this study will also contribute to the MARGINS "Rupturing Continental Lithosphere"experiment, as the opening of the Red Sea (a MARGINS focus site) is largely controlled by reactivation of Precambrian structures such as the Bi'r Umq-Nakasib suture zone. The interaction of U.S. investigators through this study with scientists from the Saudi Geological Survey (SGS), formed in 1999, promises to have a major impact on how the organization develops. The SGS encourages the participation of U.S. scientists and students interested in geologic problems in Saudi Arabia, and the success of this proposal should lead to more involvement of U.S. investigators. The co-operation of NSF-supported research teams and SGS scientists promises to advance overall understanding of Earth systems and processes.