A SECRET OF HAWAIIAN MELILITIC ROCKS
Atlantic Branch of P.P. Shirshov Institute of Oceanology RAS, Kaliningrad, Russia;
Melilitites are related to kimberlites [Gudfinnson, Presnall, 2005] and have some remarkable geochemical features, which share out them from basalts and evidence about their origin on the great depth, at low degree of partial melting of the enriched mantle substrate.
It was shown in Atlantic Ocean region, that melilitic rocks have origin on the continental plates, but not on the oceanic floor [Chernysheva, Kharin, 2007]. In the region of Pacific Ocean we know three points of melilitic rocks occurrence: 1) dykes and eruptive breccia of alnoites on the island Malaita, Solomon archipelago, near Australia (age 34 Ma), [Neal, Davidson, 1989]; 2) lava flows of melilite-olivine-nephelinites in Hamada region, Japan (age 5,7-6,1 Ma), [Tatsumi et al, 1999]; and 3) olivine-melilite-nephelinitic lavas on some islands of Hawaiian archipelago (age 0,5-3,5 Ma), [Frey, Clague, 1982; a.o.]. The first two occurrences are disposed on the rim of continental plates, which have deep roots bearing diamonds. But Hawaii have the only occurrence of melilitites without visible connection with continental plate.
The origination of Pacific Ocean is not clear up to now. Some researchers suppose, that drowned oceanic floor of Pacific can contents many relic blocks of delaminated continental plate, which have been transforming gradually to the “oceanic” crust. The ratios Ce/Pb and Nb/La in the rock are useful as signs of “continental” component in basalts, according to [Hofmann, 1997]. We have taken the data on basalt compositions from Mendocino and Murrei fracture zones, north-east from Hawaii [Melankholina et al., 2003], and the data of Site 801 [Proc.ODP, v.129], west of Central Pacific basin. Tholeiites from transforms have values of ratio Ce/Pb from 5 to 14,2 and in drill hole 801 from 2 to 5,4. The ratios Nb/La have values 0,7-1,1 and 0,9-1,4 subsequently. Normal oceanic basalts usually have Ce/Pb > 20-25; Nb/La about 2 and more. Therefore, some basalts of Pacific indeed had been contaminated by continental crust material. The same picture displays in many points of the south-west Pacific and in the Indian Ocean. So, our idea about possible existing of continental crust relics in Hawaiian region is worth attention.
Hawaiian melilitites are known in the youngest (“rejuvenated”) series of volcanics on the five western islands of archipelago, aside of the main “plume trunk” [Garcia et al., 2005]. Similar to some other melilitic rock occurrences (Greenland, Maimecha-Kotui province in Siberia, and others), there were two separate sources of magmas on Hawaii “working” simultaneously: shallow one providing tholeiites and mildly alkaline basalts, and very deep one providing melilitites. The main trigger of melilititic magmatism is usually a very strong rifting and breaking of the thick continental plates, sometimes up to ocean formation (Greenland). Mantle plumes are thought to be a conduktor of this orchestra. Many features of Hawaiian tectonic are hiden under water, and we don’t know yet, whether they are similar to others melilitic rock occurrences.
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