Sr-Nd-Hf-O isotope constraints on crustal contamination and mantle source variation of three Fe-Ti-V oxide ore deposits in the Emeishan large igneous province

Abstract

Magma-carbonate interaction and recycled oceanic crust in the mantle source are thought to have played a critical role in the genesis of magmatic Fe-Ti-V oxide ore deposits in the Emeishan mantle plume-related large igneous province in southwestern China. To test these hypotheses, we have carried out a combined study of zircon Hf-O isotopes and whole-rock Sr-Nd isotopes on three representative magmatic Fe-Ti-V oxide ore deposits associated with different types of wall-rock in the province: the Hongge and Taihe deposits with granitoid wall-rocks in most places, and the Panzhihua deposit with marble wall-rocks. Diopside-garnet marble xenoliths are common in the host gabbros of the Taihe deposit. The zircon separates used in this study are all from the mineralized units of the host layered mafic–ultramafic intrusions. The selected zircon grains yield U-Pb isotopic ages of 258.2 ± 2.8 Ma for Hongge intrusion, 263.3 ± 2.2 Ma for Taihe intrusion, and 257.6 ± 2.1 Ma for Panzhihua intrusion. The Hf-O isotopes of the zircon separates are εHf(t) from 3.1 to 4.7 and δ18O from 5.77 to 6.45‰ for Hongge, εHf(t) from 8.3 to 9.8 and δ18O from 4.12 to 4.56‰ for Taihe, and εHf(t) from 4.6 to 6.3 and δ18O from 5.49 to 6.10‰ for Panzhihua. The zircon δ18O values of the Taihe deposit are significantly lower than the mantle zircon values (5.3 ± 0.3‰), whereas those of the Hongge and Panzhihua deposits are higher than the mantle values. The initial 87Sr/86Sr and εNd(t) of the whole rock samples from the deposits are similar, from 0.7047 to 0.7052 and from 2.3 to 3.1, respectively. The observed decoupling of zircon Hf-O isotopes (low δ18O and high εHf(t) values) in the Taihe deposit is most consistent with a high-temperature altered, low δ18O oceanic gabbroic component in the plume source for this deposit. All of the isotope data together show that the parental magmas for these deposits experienced different types of crustal contamination. Contamination with the Precambrian metamorphic country rocks is much more significant in the parental magma for the Hongge deposit than those for the Panzhihua and Taihe deposits. On the other hand, contamination with marble country rocks is more pronounced in the parental magmas for the Panzhihua and Taihe deposits than in the parental magma for the Hongge deposit. An important new finding from this study is that these deposits are not linked to the same component of the mantle plume nor a single type of crustal contamination.