Ferropicrite from the Lalibela area in the Ethiopian large igneous province

摘要

Ferropicrite (FeO* = 14.0 wt% and MgO = 13.9 wt%) and picritic ferrobasalt (FeO~* = 14.7 wt% and MgO = 10.8 wt%) lava flows are found near Lalibela in the Oligocene (～ 30 Ma) Ethiopian large igneous province (LIP) in association with ultratitaniferous transitional basalt and picrite of the second high-Ti (HT2) series. The dominant phenocryst in the studied samples is Mg-rich olivine (up to Fo_(88.9)) with high CaO contents (to 0.42 wt%) without any kink band structure, indicating that the olivines are crystallized from a magnesian melt. Spinel microphenocrysts and inclusions in olivine are characterized by extremely high Cr# (79-84), moderate Mg# (18-51), moderate Fe~(3+)# (11-39) and high TiO_2 (3.6-14.8 wt%). The clinopyroxene phenocrysts are Mg#=74-88, TiO_2 = 0.84-1.82 wt%, and Al_2O_3 = 1.2-3.2 wt% in the cores. The REE contents of clinopyrox-enes display enrichment in LREE (La_N/Yb_N = 1.2-1.9) and MREE (Eu_N/Yb_N = 3.3-4.3) relative to HREE. Relative depletion of HFSE (e.g., Nb and Zr) is also observed. In general, these trace element characteristics of clinopyroxene are similar to those of the HT2 basalts. Some of the clinopyroxene crystals show strong reverse zoning with abrupt increase in Cr, Ni and Mg# from core to rim, which may be resulted from reaction of melt with mantle peridotite during magma ascent. Bulk rock chemistry of the studied samples exhibits very low Al_2O_3/TiO_2 (1.8-2.2) and high Zr/Y (8.2-10.2) ratios. These may indicate the important role for garnet during melting and that the lavas were formed by a small degree of partial melting which apparently contradicts to the high Cr# of spinel. In view of low Cr/Al ratio of the bulk rock, the high Cr# of spinel suggests very high temperature of the magma (and the mantle plume). The origin of these ferropicrite and picritic ferrobasalt could be attributed to high pressure partial melting of peridotite-eclogite (pyroxenite) mixture that possibly incorporated recycled oceanic crust components.