Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Saha, A. Articles by Wortman, G. L. Search for Related Content GeoRef GeoRef Citation

Geochemical evidence for a subducted infant arc in Franciscan high-grade-metamorphic tectonic blocks

Aniki Saha^,1, Asish R. Basu^,1, John Wakabayashi^,2 and Gregory L. Wortman^#,3

¹ Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA
² 1329 Sheridan Lane, Hayward, California 94544, USA
³ Department of Earth and Environmental Sciences, California State University, Fresno, California 93740, USA

Some of the evidence of tectonic and chemical processes that occur in subduction zones is preserved in blueschists and eclogites, as validated by the geochemical data presented from high-grade-metamorphic tectonic blocks from the Franciscan Complex in central California. Combining major elements, compatible and incompatible trace element concentrations and ratios, and Sr-Nd-Pb isotopic data, we conclude that the protoliths of these high-grade Franciscan blocks were arc lavas with no continental crust–derived components. Although the protoliths underwent high-grade metamorphism at the inception of subduction, the tectonic affinity of the protoliths can be distinguished by our geochemical study. Our geochemical data are significant in that (1) the protoliths of these high-grade blocks have a different mode of origin than the protoliths of the lower-grade Franciscan basalts that were incorporated later in the subduction complex, as the latter generally are considered to be oceanic-island basalts (OIB) and derived from mid-oceanic-ridge basalt (MORB); and (2) the protoliths of the high-grade blocks were formed over a pre-Franciscan subduction zone.

Based on the geochemical evidence for an arc origin and the available geochronological data, we suggest that protoliths of the high-grade tectonic blocks of the Franciscan Complex and the Coast Range Ophiolite formed in the same infant-arc setting. Initiation of Franciscan subduction in this arc crust led to the formation of the high-grade blocks beneath the young Coast Range Ophiolite. Continued subduction resulted in subduction of older oceanic crust of MORB and OIB origin, some of which was scraped off to form lower-grade metabasites within the Franciscan Complex.

Key Words: Franciscan • arc • subduction • geochemistry • high-grade metamorphism

This article has been cited by other articles:

				U. Horodyskyj, C.-T. A. Lee, and P. Luffi Geochemical evidence for exhumation of eclogite via serpentinite channels in ocean-continent subduction zones Geosphere, October 1, 2009; 5(5): 426 - 438. [Abstract] [Full Text] [PDF]

				W.G. Ernst, C. A. Snow, and H. H. Scherer Contrasting early and late Mesozoic petrotectonic evolution of northern California Geological Society of America Bulletin, January 1, 2008; 120(1-2): 179 - 194. [Abstract] [Full Text] [PDF]

				T. Tsujimori, J. G. Liou, and R. G. Coleman Finding of high-grade tectonic blocks from the New Idria serpentinite body, Diablo Range, California: Petrologic constraints on the tectonic evolution of an active serpentinite diapir Geological Society of America Special Papers, January 1, 2007; 419(0): 67 - 80. [Abstract] [Full Text] [PDF]