MONAZITE AS A MONITOR OF SUPRASOLIDUS CORDIERITE GROWTH AND BREAKDOWN: AN EXAMPLE FROM THE NANGA PARBAT MASSIF

Chris Yakymchuk has made significant contributions to understanding accessory mineral behaviour in high-temperature (T) metamorphic rocks. His work has enabled metamorphic petrologists to effectively apply accessory mineral petrochronology to migmatites, leading to an enhanced understanding of crustal melting. Many previous studies have targeted garnet (gt) bearing rocks, as the strong partitioning of HREE+Y into gt allows the trace element (TE) chemistry of dateable accessory minerals to monitor the growth or consumption of peritectic gt. As such, gt-absent rocks from high-T metamorphic terranes have often been overlooked for petrochronology.

Here, we present a petrochronological study on a gt-absent cordierite (crd) migmatite from the Nanga Parbat Massif (NPM). Phase equilibria modelling, Ti in biotite thermometry, and preserved mineral textures constrain a P-T path characterized by initial peritectic crd growth due to biotite breakdown melting at 720 °C, 0.4 GPa, followed by simultaneous decompression and cooling to the solidus, resulting in partial crd breakdown and melt crystallization. Despite the absence of gt in the rock and any evidence for the earlier stability of gt, monazite (mnz) U-Th-Pb and TE data show that nearly all of the mnz chemical variance is accounted for by changes in HREE+Y. Mnz initially grew at ~10 Ma with low HREE+Y, followed by a systematic increase in HREE+Y until ~2 Ma. REE patterns of early mnz show strong HREE depletion, with an atypical inflection of REE slope between Tm and Lu. Existing mineral/melt TE partitioning data suggest crd may fractionate HREE in the absence of gt, and that Tm and Yb are less compatible in crd than Er and Lu. HREE fractionation by crd may thus explain the atypical REE slope between Tm and Lu in mnz, and we interpret systematic temporal changes of HREE in mnz to track: 1) peritectic crd growth at 10 Ma and 2) progressive crd breakdown and concomitant melt crystallization during cooling and crossing of the solidus at 2 Ma, consistent with systematic trends in U, Th/U, and Eu/Eu*.

These data show that mnz can be used to date gt-absent reactions involving crd. When compared to P-T-t paths of other samples from the NPM, this rock records a distinctly older and lower-P melting history, suggesting disparate P-T-t paths and differential mass transport within the core of the NPM.