The age and composition of the deep crust exposed in the Mariana forearc south of Guam, implications for the scale of Middle Eocene volcanism and climate change
Abstract
The sequence of lithologies exposed in the Mariana forearc southeast of Guam is similar to that of many ophiolites and includes widespread basaltic pillow lavas (termed forearc basalts or FAB; Reagan et al., 2010, G-cubed) that are thought to result from decompression melting associated with subduction initiation (SI). Ishizuka et al. (2011, EPSL) showed that the forearc lithologies east of the Bonin Islands were essentially identical to those of the Mariana forearc, and that the basaltic to gabbroic sections had ages of 51-52 Ma. Here, we report geochemistry and geochronology for deep crust lithologies collected during one Shinkai 6500 dive (6K-1229) in the Mariana forearc south of Guam. Gabbros at this location have compositions relating them to FAB and Zircon U-Pb ages of 51.5+/-0.7 Ma, exactly synchronous with similar rocks from the Bonin forearc 1,600 km to the north. Further south in the western Pacific, the Tonga-Kermadec forearc has an ophiolite-like assemblage with compositions and ages similar to those of the equivalent rocks in the IBM system (Bloomer and Fisher, 1987, J. Geol.; Acland, 1996, PhD Thesis, Durham; Todd et al., 2012, EPSL; Michibayashi et al. this meeting). To the north, the record of arc magmatism stretches back to at least 46 Ma in the western Aleutians (Jicha et al., 2006, Geology). Thus, SI could have occurred nearly simultaneously along much of the western margin of the Pacific plate. If so, then the resulting volume of basalt erupted near western Pacific trenches between 52 and 49 Ma would have been globally significant, perhaps exceeding the volumes of the largest igneous provinces. Another global event at about 51 Ma was the Early Eocene Climatic Optimum (EECO). This age marked the time when atmospheric CO2 values and thus global atmospheric temperatures were likely at or near their Cenozoic maxima (Zachos et al., 2008, Nature). The rise in δ18O for seawater toward the EECO began at about 58 Ma and the decline after ~51 Ma continued until ~45Ma. We postulate that the global temperature rise toward the EECO was associated with a relatively long period of enhanced volcanism on a global scale that vented excessive CO2 including the North Atlantic Igneous Province, which had a flare-up of activity between 55 and 56 Ma (Storey et al., 2007, Chem. Geol.), and the Siletzia terrane of the Northwestern United States, which erupted about 2.6 x 10^6 km3 of basalt between about 56 and 50 Ma (Pyle et al., 2009, GSA abstract). Basalts erupted at 51-52 Ma the Western Pacific could have emitted enough CO2 to cause the temperature maximum at the EECO. The slow cool-down thereafter might have been related to continued CO2 emission during the waning basaltic to boninitic magmatism that followed SI in the Western Pacific.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMPP31A2010R
- Keywords:
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- 1031 GEOCHEMISTRY / Subduction zone processes;
- 1616 GLOBAL CHANGE / Climate variability;
- 8185 TECTONOPHYSICS / Volcanic arcs;
- 8408 VOLCANOLOGY / Volcano/climate interactions