Formation of HFSE-Rich Glimmerites by Silicate Liquid-Liquid Immiscibility, Suzhou Pluton, Eastern China

Understanding the formation of high field strength element (HFSE) mineralization remains a challenge. Processes ranging from melt generation to final crystallization have been invoked to play a key role in the enrichment of HFSEs in magmatic-hydrothermal systems, yet the importance of crystal-liquid fractionation, liquid immiscibility, and/or fluid unmixing remains debated. Here, we present results of a study of granites and HFSE-mineralized glimmerites from the granitic Suzhou pluton, eastern China. We provide whole-rock major and trace element compositions, a description of mineral assemblages, and a detailed description of zircon textures, zircon Raman spectra, zircon major and trace element and delta O-18 compositions, and U-Pb ages. The granites and glimmerites have been dated at similar to 126 to 124 Ma and are thus coeval. The granites range from biotite and zircon poor to biotite and zircon rich and are in contact with glimmerites. The glimmerites form lenses and layers close to and along the margin of the Suzhou granite, comprising up to similar to 37 vol % biotite and up to similar to 2 vol % zircon and other HFSE-rich accessory minerals. The biotite-poor granites contain a single type of zircon (type-A: single crystals, oscillatory zoned, fully crystallized structure, relatively poor in trace elements, and relatively high delta O-18), whereas the biotite-rich granites and the glimmerites contain two zircon types (type-A crystals: same features as in the biotite-poor granites; type-B crystals: clustered, unzoned, partially metamict structure, rich in trace elements, and relatively low d18O). Both granite types are Si, Al, and Na rich, whereas the glimmerites are Fe, Ti, Mn, Mg, Ca, P, F, and HFSE rich and, compositionally, fall off simple fractionation trends. We interpret the textural, mineralogical, and compositional relationships to indicate that the glimmer-ites are the products of Fe-, F-, and HFSE-rich immiscible melts that unmixed from an alkali-rich, moderately reducing (similar to QFM + 0.5 and similar to QFM + 1.0; QFM = quartz-fayalite-magnetite buffer) Suzhou magma system at low crustal pressure. In addition, we note that the zircon textures and compositions are important recorders of the processes and conditions that led to the HFSE mineralization.

Automated summary

This study investigates the granites and HFSE-mineralized glimmerites from the Suzhou pluton in eastern China. The results reveal that the mineralization is a product of immiscible melts rich in iron, fluorine, and HFSE that separated from the Suzhou magma system at low crustal pressure. The textures and compositions of zircon are important indicators of the HFSE mineralization processes and conditions.