Origin Of The Proterozoic Massif-type Nelsonite

Models for the nelsonite formation are currently highly contentious, with liquidimmiscibility and fractional crystallisation as frequently proposed formationmechanisms. The nelsonites in the Damiao massif anorthosite complex in the NorthChina Craton and experimental evidence are revisited for the existence of silica-freeCaO-FeO-Fe2O3-TiO2-P2O5immiscible nelsonitic liquids. Our results of differentialscanning calorimetry (DSC) demonstrate that nelsonite with the composition ofone-third apatite and two-thirds Fe-Ti oxides by weight completely melts well above1450oC, which is in good agreement with numerous experimental studies of theCaO-P2O5-FexO system in connection to metallurgy. Thus, the composition cannot bemolten at temperatures relevant for crystallisation of the Damiao magma. A review ofexperimental studies of liquid immiscibility and analyses of natural immiscibleglasses show that all the liquids on the Fe-and P-rich side of the miscibility gap haveat least20wt.%of aluminosilicate components. Main results of this study come fromthe analyses of apatite-hosted melt inclusions in Damiao nelsonite. The inclusionsrange from~3to200μm in diameter. They are ubiquitous and meet all themorphological criteria of primary melt inclusions crystallised into assemblages ofdaughter minerals. Almost all of them contain vermiculite and chlorite, and somecontain biotite, amphibole, phlogopite and Fe-Ti oxide. Out of dozens analysedinclusions, only three have high contents of SiO2(62.1-73.8wt.%) and low contentsof FeO (0.25-2.35wt.%). Bulk compositions of other inclusions show largevariations in SiO2(20.79-50.16wt.%) and FeOt (13.44-32.78wt.%). With a fewexceptions, the inclusions are very low in CaO (0.04-1.51wt.%) and high in Al2O3(10-21.17wt.%). Despite the high Fe content, the compositions significantly differfrom those of the typical immiscible Fe-rich melts. It appears that the cumulus apatitecrystallised from Fe-rich, hydrated silicate melt. We propose that the inclusions atDamiao record a trend of intercumulus melt evolution, which might have beenthrough liquid-liquid immiscibility and most likely have been strongly affected byseparation of a hydrothermal fluid phase and the losses of alkali and Ca silicatecomponents from the melt into the fluid.