New major and trace element analyses of Ocean Drilling Program Leg 170 and 205 mafic igneous samples are presented along with Sr and Nd isotopic results. Samples were cored from a sill unit (Subunit 4A) and a lower igneous unit (Subunit 4B) whose lower boundary was not drilled. The samples are dominantly microcrystalline to medium-grained glomerocrystic plagioclase-clinopyroxene gabbro in a nearly holocrystalline groundmass. Samples are dominantly low-K subalkaline tholeiites with trace element systematics that correlate with unit stratigraphy. Using isotopic data, trace element ratios little affected by fractionation, and abundance data from the least fractionated samples, it is possible to model their mantle sources and subsequent igneous processing. Melting models suggest that small differences in degrees of partial melting can generate the two geochemical groups identified within the igneous complex. Crystal fractionation and accumulation have further modified the melt compositions, particularly within the lower stratigraphic unit. Mixing models indicate that these units are derived from a mantle source that is 50%–70% enriched compared to depleted mid-ocean-ridge source mantle. In the context of the complex regional tectonic and volcanic history of the Cocos plate, this igneous complex likely represents melts of depleted upper mantle that were previously enriched during transit near the Galápagos hotspot. A postenrichment change in the local–regional tectonic stress regime may have triggered decompression melting and emplacement of the igneous complex far from any active spreading center or plume. Preexisting plate fractures, ridge jumps, and abandoned spreading centers may have facilitated distal reach of plume-overprinted material.
1Dreyer, B., Chavagnac, V., Morris, J., and Font, L., 2006. Source and petrogenesis of the igneous complex cored during ODP Leg 205: implications for off-axis plume-ridge interaction on the Cocos plate. In Morris, J.D., Villinger, H.W., and Klaus, A. (Eds.), Proc. ODP, Sci. Results, 205 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/205_SR/212/212.htm>. [Cited YYYY-MM-DD]
2Department of Earth and Planetary Sciences, Washington University, One Brookings Drive, St. Louis MO 63130, USA. Correspondence author: dreyer@levee.wustl.edu
3University of Southampton, National Oceanography Centre, Southampton, Waterfront Campus, European Way, Southampton SO14 3ZH, UK.
4Department of Earth Science, University of Durham, Durham DH1 3LE, UK.
Initial receipt: 22 June 2005
Acceptance: 18 January 2006
Web publication: 5 September 2006
Ms 205SR-212