Petrological study of mantle xenoliths in alkali basalts, S. Korea

Category Petrology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Lee, Han-Yeang
Holding Date 29 September 2008

Mantle xenoliths in alkali basalt at Boun, Pyungtaeg, Gosung area, Jeju and Baegryung Island in South Korea are spinel lherzolites composed of olivine, orthopyroxene, clinopyroxene, and spinel. Minerals show homogeneous compositions. Olivine compositons have Fo89.0 to Fo90.2, low CaO (0.03 to 0.12 wt%), and NiO of 0.34 to 0.40 wt%; the orthopyroxene is enstatite with En89.0 to En90.0 and Al2O3 of 4 to 5 wt%; the clinopyroxene is diopside with En47.2 to En49.1 and Al2O3 of 7.42 to 7.64 wt% from Boun and 4.70 to 4.91 wt% from Baegryung. Spinel chemistry shows a distinct negative trend, with increaeing Al corresponding with decreasing Cr, and Mg#(100Mg/Mg+Fe) and Cr#(100Cr/Cr+Al) of 75.1 to 81.9 and 8.5 to 12.6, respectively.
The equilibrium temperatures of these xenoliths, taken as the average obtained from reasonable geothermometers, lie between 970 and 10200 C, and equilibrium pressures derived from couple of methods fall within the range of 12 to 19kbar (i.e., 42 to 63km). These temperatures and pressures are reinforced by considerations of the Al-isopleths in orthopyroxene to check its reliability. Magma pathway is also delineated from P-T estimates.
Mineral compositions of spinel-lherzolites in South Korea and eastern China are primitive; paleo-geotherms of both are quite similar, but degrees of depletion of the upper mantle could vary locally. This is demonstrated by eastern China, which has various depleted xenoliths caused by different degrees of partial melting. Geothermal gradient constructed from this study shows continental geotherm by Pollack and Chapman(1977) and oceanic geotherm by Mercier(1980).
REE geochemical study indicates that some samples have been attributed to cryptic metasomatism and lithospheric mantle beneath S. Korea is partially heterogeneous