科研之友
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摘要
Abundant volcanism in the Central Kamchatka Depression (CKD) adjacent to the Kamchatka-Aleutian Arc junction occurs where the Pacific slab edge is subducting beneath Kamchatka. Here we summarize published data on CKD rocks and demonstrate a systematic south-to-north change of their compositions from moderately fractionated basalt-andesite tholeiitic series to highly fractionated basalt-rhyolite calc-alkaline series including high-magnesian andesites near the slab edge. Localized slab melting at the slab edge cannot explain these regional geochemical variations. Instead, we propose that the thermal state of the mantle wedge can be the key factor governing the composition of CKD magmas. We integrate the results from petrology and numeric modeling to demonstrate the northward decrease of the mantle wedge temperatures beneath CKD volcanoes, which correlates with decreasing slab dip, length of mantle columns, and magma flux. We envision two petrogenetic models, which relate the composition of erupted magmas to the subduction parameters beneath the CKD. The first model suggests that mantle temperature governs melt-peridotite equilibria and favors generation of andesitic primary melts in cold mantle regions above the shallowly subducting Pacific slab edge. Alternatively, mantle temperature may control magmatic productivity along the CKD, which decreases sharply toward the slab edge and thus allows more extensive magma fractionation deeper in the crust and mixing of highly evolved and mantle-derived magmas to generate Si-rich "primitive" magmas. These results point to a possible casual link between deep mantle and shallow crustal magmatic processes. Similar effects of mantle temperature on the composition and productivity of arc magmatism are expected elsewhere, particularly in volcanic regions associated with significant slab dip variation along the arc.
- 出版日期2008-7
