Trace element compositions of apatite from the middle zone of the Panzhihua layered intrusion, SW China: Insights into the differentiation of a P- and Si-rich melt

Abstract

Abstract The Panzhihua layered intrusion in the ~ 260 Ma Emeishan large igneous province is composed of melagabbro and Fe–Ti oxide ore bodies in the lower zone (LZ) and the lower part of the middle zone (MZa), and Fe–Ti oxide-poor leucogabbro in the upper part of the middle zone (MZb) and upper zone (UZ). Cumulus apatite grains occur in the ~ 500- to 600-m-thick MZb, which makes up 25–30% of the ~ 2-km-thick intrusion. Apatite grains from the MZb show two compositional reversals in the composition of Sr, which divide the MZb into three sub-units from the base upwards, MZb1, MZb2 and MZb3. There is 1–3 vol.% apatite in the MZb1 and MZb2 and 2–5 vol.% apatite in the MZb3. Both apatite and plagioclase have an overall trend of decreasing Sr in each sub-unit. Most apatite grains from the MZb1 and MZb2 have negative Eu anomalies (Eu/Eu* = 0.70–0.98) on chondrite-normalized REE plots and some at the top of the MZb2 have positive Eu anomalies (Eu/Eu* = 1.09–1.18), whereas all grains from the MZb3 have positive Eu anomalies (Eu/Eu* = 1.11–1.25). We consider that the Panzhihua intrusion formed due to immiscibility of ferrobasaltic magmas in a large convection cell at high temperatures. The immiscible Fe-rich melt tended to move towards the base of the chamber, whereas the Si-rich melt moved upwards due to density differences. Crystallization of Fe–Ti oxides from the Fe-rich melt at high temperatures may result in the enrichment of P in the residual magmas. The upward moving residual P-rich magmas may have mixed with Si-rich melt to form a P- and Si-rich melt in the upper part of the chamber, from which the MZb formed. Double-diffusive convection circulated in the P- and Si-rich melt to form stratified magma layers. Magma mixing between the stratified magma layers resulted in the compositional reversals of apatite along the boundaries. Negative Eu anomaly of apatite in the MZb1 and MZb2 is attributed to prior crystallization of plagioclase, whereas replenishment of a syenitic magma to the MZb3 may result in the positive Eu anomaly of apatite in the MZb3. The immiscibility model can explain why the apatite-rich MZb is above the major Fe–Ti oxide layers in the Panzhihua intrusion.