In-site mineral geochemistry and whole-rock Fe isotopes of the quartz-magnetite-pyroxene rocks in the Wuyang area, North China Craton: Constraints on the genesis of the pyroxene-rich BIF

Abstract

Quartz-magnetite-pyroxene (QMP) rocks with high Fe contents are of greatly economic and petrological importance and generally occur within the oldest and largest exposures of supracrustal rocks preserved on Earth. However, the chemical composition and genesis of QMP rocks remain unclear. To constrain the protolith and genesis, we present new whole-rock geochemistry and Fe isotopic data, mineral EPMA and LA-ICP-MS analysis and X-ray mapping for the QMP rocks in the Wuyang area, North China Craton. The Wuyang QMP rocks have undergone high-temperature metamorphism and have special mineral assemblages with coexisting magnetite, quartz, clinopyroxene and variable orthopyroxene. Pyroxenes of the Wuyang QMP rocks show very distinct major, trace and rare earth element (REE) compositions from those of the nearby pyroxene-marbles and pyroxenite intrusions. Such difference excludes a contact metamorphic origin for pyroxenes of the QMP rocks. X-ray elemental mapping suggests that Fe-poor carbonate minerals (dolomite and calcite) and quartz inclusions are well preserved in these pyroxene grains and no reaction phenomenon can be observed between them, indicating that these pyroxenes are not the product of metamorphic decarbonation reactions of pre-existing sedimentary silica and Fe-rich carbonate phase. Shale-normalized REE + Y patterns of the whole-rock samples and pyroxenes display geochemical features consistent with classic banded iron formations (BIFs), which show light REE depletion, positive LaSN, EuSN and YSN anomalies. This indicates that the geochemical features are compositionally inherited from their original pure chemical sediments. The Wuyang QMP rocks are therefore considered as pyroxene-rich BIF. Neotocite and hisingerite inclusions in pyroxene grains from the QMP rocks could be metamorphic products of some early Fe- and Mn-rich silicates before pyroxene. Moreover, the Wuyang QMP rocks are enriched in light iron isotopes similar to other Fe- and Ca-rich minerals in Archean BIFs. Therefore, we inferred that pyroxene from the QMP rocks could result from dehydration of initial sedimentary hydrous Ca-, Mg- and Mn-containing Fe(II)-silicate gel during diagenesis and re-crystallization in later metamorphism, providing a new interpretation for the origin of pyroxene-rich BIF.