Petrology of Igneous Rocks at Middle Valley, Juan de Fuca Ridge (original) (raw)
1994, Proceedings of the Ocean Drilling Program
Leg 139 of the Ocean Drilling Program drilled igneous rock at four sites in Middle Valley: Site 855 from the eastern boundary fault of the valley; Site 856 from late mafic sills that crosscut a major sulfide deposit; Site 857 from a series of highly altered sills of variable chemistry; and Site 858 from a topographic high beneath an active site of hydrothermal discharge. The oldest rocks are from Site 855. These are N-MORBs containing refractory megacrysts of olivine, calcic plagioclase and Cr-rich clinopyroxene. The rocks have [La/Ce] n =0.76-0.82, and [Ce/Yb] n = 0.76-0.89. Igneous rocks from Site 856 are relatively fresh (but more altered than those from Site 855) young sills that postdate the main phase of axial magmatism and associated hydrothermal alteration. These are some of the most primitive compositions from the northern Juan de Fuca Ridge and contain refractory megacrysts of olivine and chromian spinel, with plagioclase of intermediate compositions (slightly more sodic than those for Site 855). The rock compositions are strongly depleted in LREE, with [La/Sm] n = 0.28-0.29 and [Ce/Yb] n = 0.37-0.39. Samples from the chilled contacts with sediments have spurious trace element chemistry due to alteration. Atopographic high drilled at Site 858 is composed of N-MORB and T-MORB lavas that must have erupted above the sediments, suggesting a relatively old age, near that of the basement. They are aphyric to slightly phyric with sodic plagioclase and magnesiochromite. Pseudomorphs of small mafic phenocrysts are inferred to be replacements of olivine. These rocks are relatively homogeneous with [Ce/Yb] n > 1.3, suggesting a lower percentage of partial melting compared to the other sites. They have [La/Ce] n = 0.84-0.94 and a variably developed negative Eu anomaly. The sill complex drilled at Site 857 is extensively altered. Trace elements (especially REE) suggest that they are derived from heterogeneous mantle source regions, with some mixing. One source region is similar to that of Site 855 with [La/Ce] n <0.82 and [La/Yb] n <l. The second source is similar to that of Site 858 with [La/Ce] n >0.82 and [La/Yb] n >l. Although the age relationships among the sills are difficult to constrain, the former compositions tend to be the most altered, and likely of greater age. Thus the tectonic scenario is that a robust magma chamber formed the Site 855 basalts and fed the older sills at Site 857. Magmas constructed the Site 858 topographic high and basalts from a similar mantle source region fed some of the sills at Site 857. Sills of intermediate composition suggest mixing between these magmas, either in the source region or in crustal magma chambers. The Site 856 sills postdate the ridge jump to West Valley after the Middle Valley rift failed.
Sign up for access to the world's latest research.
checkGet notified about relevant papers
checkSave papers to use in your research
checkJoin the discussion with peers
checkTrack your impact
Loading Preview
Sorry, preview is currently unavailable. You can download the paper by clicking the button above.