2013

Abstracts Travel
Program Organizing committee

A SECRET OF HAWAIIAN MELILITIC ROCKS

Chernysheva E.A.  

Atlantic Branch of P.P. Shirshov Institute of Oceanology RAS, Kaliningrad, Russia;

elcher@atlas.baltnet.ru

 

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 dont know yet, whether they are similar to others melilitic rock occurrences.  

 

References

Gudfinnson G.H., Presnall D.C. Continious gradations among primary carbonatitic, kimberlitic, melilititic, basaltic, picritic and komatiitic melts in equilibrium with garnet lerzolite at 3-8 GPa // J. Petrol. 2005. V. 46. P. 1645-1659.

Chernysheva E.A., Kharin G.S. Alkaline magmatism in the history of the Norwegian-Greenland Basin // Petrologia (russian). 2007. V.15. N 3. P. 317-323.

Neal C.R., Davidson J.P. Un unmetasomatized source for the Malaita alnoite (Solomon Islands): petrogenesis // Geochim. Cosmochim. Acta. 1989. V. 53. P.1975-1990.

Tatsumi Y., Arai R., Ishizaka K. The petrology of a melilite-olivine nephelinite from Hamada, SW Japan // J. Petrol. 1999. V 40 (1). P. 497-509.

Clague D.A., Frey F.A. Petrology and trace element geochemistry of the HonoluluVolcanics, Oahu: implications for the oceanic mantle below Hawaii // J. Petrol. 1982. V. 23. P. 447-504.

Hofmann A.W. Mantle geochemistry: the message from oceanic volcanism // Nature. 1997. V. 395. P. 219-229.

Melankholina E.N., Sushevskaya N.M., Belyatsky B.V. Development of tholeiitic magmatism in the northern Pacific region // Doklady RAS (russian). 2003. V.391. N 1. P. 75-80.

Garcia M.O., Swinnard L., Weis D., et al. Petrology, geochemistry and geochronology of Kauai lavas over 4.5 Myr: implications for the origin of rejuvenated volcanism and the evolution of the Hawaiian plume // J. Petrol. 2010. V. 51 (7). P. 1507-1540.