Petrogenesis of a ~900 Ma mafic sill from Xuzhou, North China: Implications for the genesis of Fe-Ti-rich rocks

Abstract

The Niutishan sill, one of the ~900 Ma mafic sills in Xuzhou (Jiangsu province, North China), has a thickness of ~30 m. We have identified chemical parameters that define three differentiated interlayers from thirty-eight samples in a ~22 m profile. The sill reveals a succession consisting of an upper border zone of fine-grained dolerite (UZ), a middle zone of quartz-bearing gabbro (MZ), a coarse-grained apatite-enriched gabbro (LZc), a layer of Fe-Ti oxide-enriched gabbro (LZb) and a deeper medium-grained massive gabbro (LZa) at the bottom of the lower zone, capped by an uppermost chilled margin (CM) of plagioclase-clinopyroxene-porphyritic dolerite. Its significantly differentiated interlayers offer an opportunity to investigate the petrogenesis of high-Fe-Ti gabbro. The chilled margin composition suggests a highly evolved ferro-basaltic parental magma (whole-rock Mg#: 39, with ~2.9 wt% TiO2 and ~14.2 wt% FeOt), which likely represents a residual magma derived from a deep-seated magma chamber after significant removal of olivine. The internal compositional variation was controlled by fractional crystallisation with successive presence of plagioclase and Fe-poor clinopyroxene, followed by Fe-Ti oxides plus Fe-rich clinopyroxene, and finally apatite and albite-oligoclase. This sequence is consistent with the result of thermodynamic modelling of MELTS at a low H2O content of ~0.2 wt% and in a temperature range from 1200 to 780 °C and at a pressure of 70 MPa. The saturation of clinopyroxene before Fe-Ti oxides in liquids is interpreted to imply low-fO2 and water-unsaturated conditions, which led the residual magma to an Fe-Ti- enriched trend. After the saturation and mechanical settling of clinopyroxene primocrysts from the parental magma, large amounts of Fe-Ti oxides crystallised from the evolved Fe-Ti-rich melt. Further crystallisation and solidification of the crystal mush led to the formation of the major Fe-Ti oxide-rich layer and expelled interstitial melt upward to mix with the more evolved residual melts, leading to the saturation of apatite. Meanwhile, the residual melts gradually tended to Si enrichment, which matches the observed compositions (enriched incompatible elements) of naturally occurring quartz- gabbro in the MZ. As a result, viscosity would increase, trapping highly evolved melts and impeding liquid separation, as exhibited by the silicic mesostasis of graphic intergrowths of quartz and alkali feldspar, and occasionally calcite. This sill is also characterized by considerable amounts of titanite at the bottoms of the UZ and LZ, which was likely formed during the late evolutionary stages as products of reaction between the primary Ti-rich minerals and a late igneous fluid/melt at temperature near 584 °C. The F-rich and Cl-poor apatite (XF > 0.95) and Ti-rich magnetite (7.2–9.7 wt% Ti) from Fe-Ti (P)-rich layers indicate a magmatic origin from high-temperature Fe-Ti-rich melts. It is thus concluded that the significantly differentiated Niutishan sill was formed by the combined processes of fractionation, accumulation and compaction under water-unsaturated and low-fO2 conditions, which most likely occurred in an intra-continental rather than a subduction-related environment. Further synthetic analysis suggests conclusion that this magmatism may record a break-up event.