Contrib Mineral Petrol (1989) 101:57-68 Contributions to Mineralogy and Petrology 9 Springer-Verlag1989 Origin of Sr, Nd and Pb isotopic systematics in high-Sr basalts from central Arizona James H. Wittke 1 *, Douglas Smith 1 and Joseph L. Wooden 2 1 Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78713, USA 2 United States Geological Survey, Menlo Park, CA 94025, USA Abstract. Alkalic and tholeiitic basalts were erupted in the central Arizona Transition Zone during Miocene-Pliocene time before and after regional faulting. The alkalic lava types differ from the subalkaline lavas in Sr, Nd and Pb isotopic ratios and trace element ratios and, despite close temporal and spatial relationships, the two types appear to be from discrete mantle sources. Pre-faulting lava types include: potassic trachybasalts (87Nr/a6Sr=0.7052 to 0.7055, end=--9.2 to --10.7); alkali olivine basalts (878r/ 86Sr = 0.7049 to 0.7054, end = --2 to 0.2); basanite and haw- aiites (87Sr/S6Sr=0.7049 to 0.7053, end=--3.5 to --7.8); and quartz tholeiites (S7Sr/S6Sr=0.7047, eNa=--l.4 to -- 2.6). Post-faulting lavas have lower s 7Sr/S6Sr (< 0.7045) and end from --3.2 to 2.3. Pb isotopic data for both pre- and post-faulting lavas form coherent clusters by magma type with values higher than those associated with MORB but within the range of values found for crustal rocks and sulfide ores in Arizona and New Mexico. Pb isotopic sys- tematics appear to be dominated by crustal contamination. Effects of assimilation and fractional crystallization are in- adequate to produce the Sr isotopic variations unless very large amounts of assimilation occurred relative to fractiona- tion. It is impossible to produce the Nd isotopic variations unless ancient very unradiogenic material exists beneath the region. Moreover the assumption that the alkalic lavas are cogenetic requires high degrees of fractionation inconsistent with major- and trace-element data. Metasomatism of the subcontinental lithosphere above a subduction zone by a slab-derived fluid enriched in Sr, Ba, P and K could have produced the isotopic and elemental patterns. The degree of metasomatism apparently decreased upward, with the alkalic lavas sampling more modified regions of the mantle than the tholeiitic lavas. Such metasomatism may have been a regional event associated with crustal formation at about 1.6 Ga. Disruption and weakening of the subcontinental lithosphere in the Transition Zone of the Colorado Plateau by volcanism probably made deformation possible. Introduction The isotopic and trace element characteristics of continental alkalic basalts may be partly determined by assimilation * Present address: Department of Geology and Geophysics, The University of Wyoming, Laramie, WY 82071, USA Offprint requests to: J.H. Wittke of crust, and so it is commonly difficult to use continental basalts as probes of mantle sources and as indicators of source processes. In particular, the effects of source metaso- matism and crustal contamination may be difficult to distin- guish. Metasomatic introduction of incompatible elements (e.g., LREE, Sr, Rb, K, P) into the source regions prior to melting has been proposed and examined by numerous authors as summarized in Menzies and Hawkeworth (1987). Indeed, such mantle metasomatism may be a necessary pre- cursor to alkalic magmatism. In contrast, however, it is possible that the combined effects of fractional crystalliza- tion and assimilation of wallrock material may produce some important geochemical characteristics attributed to metasomatism in many continental basalts (e.g., Taylor 1980; DePaolo 1981 a). The Cenozoic basalts of the Hickey and Perkinsville Formations in the Transition Zone of the Colorado Plateau in central Arizona offer a particularly good opportunity to examine source processes and to distinguish them from the effects of crustal contamination. Alkaline lavas of di- verse compositions and subalkaline lavas were erupted in the Black Hills area during a period of only about 10 m.y. - Hickey lavas within only about 4 m.y. During this time, the margin of the Colorado Plateau was disrupted by nor- mal faulting, possibly accompanied by a change in the na- ture of the underlying mantle source regions, such as the replacement of enriched subcontinental lithosphere by as- thenosphere inferred to take place on the eastern margin of the Plateau by Perry et al. (1987). Some of the Black Hills lavas are unusually Sr-rich (up to 3200 ppm) and thus are less sensitive than most lavas to contamination by crus- tal Sr. The Black Hills volcanics display a wide range of basalt compositions within a small area allowing an evalua- tion of the temporal link between volcanism and extensional deformation to be made without the uncertainties inherent in a regional study. Major- and trace-element analyses com- bined with Sr, Nd and Pb isotopic data were used to evalu- ate the importance of contamination and metasomatism and the role of changing tectonic environment and crustal character on the genesis of these basalts. Geology Late Miocene and Pliocene basalts of the Hickey and Perkinsville formations crop out in central Arizona about 40 km southwest of Flagstaff near the town of Jerome (Fig. 1) in the Transition Zone of the Colorado Plateau. The Transition Zone separates the