The MgO contents under study range from 8-16%. I guess one is high degree of partial melting of the mantle peridotites, and another is olivine cumulates. Can anyone recommond some related references which argue for these two aspects. Or are there other reasons for the high magnesian contents in basalts? Thank you.
You might have boninite basalts which are typically high in Mg (~15% or so) and are interpreted to form in fore-arc subductive environments, usually early on during subduction. You should also have elevated Si contents if you indeed have a boninite. Because of the higher Si content these often plot / are identified as andesites rather than basalts. The following papers might be useful in answering your questions: (1) http://petrology.oxfordjournals.org/content/35/3/577.abstract (2) https://pubs.er.usgs.gov/publication/70024714
Thank you for your answer, but they are indeed basalts instead of andesites. So I think boninite might be not a good candidate. Thank you again.
Please try these related references:
D. A. Gust, M. R. Perfit
Phase relations of a high-Mg basalt from the Aleutian Island Arc: Implications for primary island arc basalts and high-Al basalts.
Contributions to Mineralogy and Petrology
September 1987, Volume 97, Issue 1, pp 7-18.
S.I. Shkol'nik, V.G. Belichenko, L.Z. Reznitskii
High-magnesium picrite–basalt associations of the Tunka terrane (Baikal–Hövsgöl region) as an indicator of the back-arc basin spreading.
Russian Geology and Geophysics
Volume 54, Issue 2, February 2013, Pages 153–165.
Best regards,
G. Grundmann
Olivine accumulation is the most common explanation (Ni-Mg correlation), but you should also consider alteration processes which may add Mg. Detailed petrography is obviously important to distinguish between these possibilities.
I fully agree with the comment made by Dr. Kristoffer Szilas. Without knowing the mineralogy and without detailed petrographic studies any interpretation(s) made based on chemistry of the rocks/minerals is not advisable. For eg. WRA data of an olivine pyroxenite and a true komatiite when plotted in relevant diagram would occupy the field of komatiite - though one is plutonic and the other is volcanic. Dr .K.N. RAO
8 to 18 Wt MgO % with SiO2 bellow 50% is composition of picrite. These volcanics are common in volcanic trapps due to hot mantle plume and deep mantle batch melting. Example of Siberian, Deccan Caribbean...trapps; Literature about picritic volcanism is very large. Contact me if you need of more informations.
Mg-olivine fractionation would NOT enrich the residual magma in MfgO, but in FeOt. The explanation, if the rock is not a boninite, ha to be in the degree of partial melting of the mantle below. Such high MgO rocks are indeed picrites, and are frequent in the Caribbean Large Igneus Province (CLIP), which is exposed as tectonic blocks in the islands of Aruba and Curazao. as Christian Picard says. Also basaltic komatiites are high MgO rocks formed in the CLIP and elsewhere. Excellent basaltic komatiites belonging to the CLIP are exposed in Gorgona Island, in the west coast of Colombia (Echevarria et. al.) and I have found them in a small tectonic block emplaced in the Venezuela Coastal Range.
I can help to get some references which lights into the extreme possibility of asteroid impacts to generate high MgO basalts: Good luck
Green D. H. 1972. Archaean greenstone belts may include terrestrial equivalents of lunar maria? Earth and Planetary Science Letters 15, 263-270.
Jones, A. P., Price, D. G., DeCarli, P. S., Price, N. and Clegg, R. 2003. Impact decompression melting: A possible trigger for impact induced volcanism and mantle hotspots? In: Koeberl, C. and Martinez Ruiz, F. (Eds.) Impact markers in the stratigraphic record. Spinger, Berlin, pp. 91-120.
Can you provide a photomicrograph of the rock (having unusually high Mg content) and the list of minerals present? Just academic curiosity. Thanks. KNRAO
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