The Yingfengling clinopyroxene megacrysts are characterized by low Mg# (0.62 ∼ 0.47) and heavy rare earth element (HREE), and high Al2O3, TiO2, Na2O and middle rare earth element (MREE) contents, with significant composition variation. As Mg of clinopyroxenes decreases, Al2 O3 (11.58% to 6.97%), TiO2 (2.28% to 1.06%) and HREE decrease. These variation correlations are markedly different from those of clinopyroxene megacrysts from other localities worldwide. Most incompatible elements, such as Nb, Sr, Zr, Hf and LREE-MREE, increase with decreasing Mg#, similar with other clinopyroxene megacrysts all over the world. Low Mg# and Ni, Co and Cr contents indicate that the parental magma of these megacrysts is a strongly evolved residual melt (Mg# = 0.27∼0.35). Fractional crystallization modeling suggests that the parental magma of the Mg-richest megacryst (Lz-54) found in Yingfengling was derived probably from a primitive tholeiitic melt by about 70-80% fractional crystallization, and that other megacrysts were the products of this parental magma through 25 ∼ 80% fractional crystallization of clinopyroxene, garnet, plagioclase and ilmenite. T-P calculations indicate that the Yingfengling megacrysts were crystallized in the upper mantle of 56 ∼ 60 km deep (ca. 1.73GPa ∼ 1.83GPa) and high temperature of 1290°C ∼ 1130°C, which is in favor of garnet crystallization. By comparing with the clinopyroxene megacrysts from other localities in compositions and formation conditions, it is suggested that the distinct composition variation exhibited by the clinopyroxene megacrysts from different volcanic rocks was principally controlled by both primary composition of parental magma and crystallization condition. The megacryst parental magma intruding into the upper mantle caused the different extent of metasomatism to wall-rock peridotites. Relatively early fluid-poor magma resulted in the weak metasomatism of peridotite, in which only highly active elements were greatly modified, such as Sr and La, and most trace elements have little change. Late magma with abundant F-, P- rich fluid caused the strong metasomatism effect on the peridotites with remarkable change of most trace elements. It is suggested that the fluid amount of metasomatic melt is the key factor affecting the metasomatism of peridotites.
|Translated title of the contribution||Geochemistry of clinopyroxene megacrysts from Yingfengling basalt and the metasomatism effect of their parental magma on mantle peridotites|
|Number of pages||13|
|Journal||Acta Petrologica Sinica|
|Publication status||Published - 2003|
- Clinopyroxene megacrysts
- Leizhou Peninsula
- Mantle metasomatism