Evolution of the Sudbury igneous complex southern metamorphic aureole and controls on anatexis

Abstract

The South Range metamorphic aureole around the 1.85 Ga Sudbury Igneous Complex (SIC) is spatiotemporally connected to the world-class ore deposits of the Sudbury mining camp. Defining the physiochemical expression and understanding the evolution of the metamorphic aureole are therefore of economic interest to mineral exploration efforts. The importance of studying the SIC metamorphic aureole is highlighted by new insights into low-P/high-T (LP-HT) metamorphism of basalts including, LP-HT mineral assemblages, partial melting, melt mobilization, phase equilibria modelling of metabasalts at relatively LP-HT conditions, element mobility during metamorphic processes, and retrograde zircon formation with concurrent mobilization and fractionation of Zr-Hf. The South Range metamorphic aureole is best preserved in Paleoproterozoic Elsie Mountain Formation (EMF) metabasalts that form a large proportion of the immediate footwall to the SIC along its southern margin, which also includes the Murray and Creighton granites. Mapping of the metamorphic aureole in the EMF metabasalts defines 3 metamorphic zones: 1) an up to ca. 500 m wide pyroxene-hornfels zone (PHZ) extending from the SIC contact and characterized by a peak metamorphic mineral assemblage of plagioclase-clinopyroxene-orthopyroxene-magnetite-ilmenite estimated to reflect peak temperatures of ≥925 °C; 2) a pyroxene-granofels zone (PGZ) extending from the PHZ and up to 750 m from the SIC contact characterized by a similar twopyroxene assemblage, but typically with abundant retrograde high-Ti hornblende; 3) a hornblende-hornfels zone (HHZ) extending from the PGZ and to at least 1000 m from the SIC contact characterized by a hornblende-plagioclase-quartz-ilmenite ± biotite ± magnetite assemblage indicating temperatures of up to 680 °C. Field evidence for partial melting and melt mobilization in the EMF metabasalts consist of mainly macroscopic leucocratic patches that locally coalesce. Microtextural evidence for partial melting includes optical continuous quartz domains containing plagioclase and pyroxenes locally with euhedral crystal faces, and relatively low-Ca plagioclase and quartz frameworks around mainly relatively high-Ca plagioclase representing nucleation from a melt onto existing crystals. Phase equilibria modelling using bulk rock compositions indicate that partial melting resulted in 10-20% melt generation in the PHZ, and probably even higher degrees of melting is recorded locally. Compared to the granites where partial melts have been traced as dikes for hundreds of meters injecting back into the SIC, no backinjections were documented to emanate from the EMF metabasalts. This indicates that while a high-T metamorphic aureole developed in the metabasalts, the granites were continuously experiencing high degrees of partial melting preventing the development of a metamorphic aureole even some time after solidification of the SIC. Thus, the width of the high-T contact aureole is wider in the EMF metabasalts than in the granites. This is also true in a comparison to the contact aureole documented in the North Range Archean gneisses. Furthermore, the estimated peak contact metamorphic temperatures in the EMF metabasalts are in better agreement with previous thermal models that required substantial thermomechanical erosion (800 m) of North Range footwall rocks to match the width of the observed contact aureole. Thus, the process of thermomechanical erosion might have been less significant in the EMF basalts and perhaps other mafic lithologies. Trace element geochemistry of the EMF metabasalts successfully permits a subdivision of the PHZ into Hornfels A and B zones. The Hornfels A zone defines the metabasalts are in better agreement with previous thermal models that required substantial thermomechanical erosion (800 m) of North Range footwall rocks to match the width of the observed contact aureole. Thus, the process of thermomechanical erosion might have been less significant in the EMF basalts and perhaps other mafic lithologies. Trace element geochemistry of the EMF metabasalts successfully permits a subdivision of the PHZ into Hornfels A and B zones. The Hornfels A zone defines the inner most ca. 250 m, and is characterized by metabasalts that generally show relative depletion in LILE, REE and HFSE. Thus, trace element systematics in the EMF metabasalts of the Hornfels A zone accentuates the potential for metamorphic processes including devolatilization reactions and partial melting to severely mobilizing not only relatively easy mobilized elements, e.g., LILE, but also the relatively immobile HFSE. Thus, the trace element systematics has the potential to identify high-T parts of the metamorphic aureole where the micro- and macroscopic petrographic evidence has subsequently experienced obliteration by tectonometamorphic events. The defining geochemical characteristic of Hornfels A samples is a pronounced negative Zr-Hf anomaly (Zr/Zr* < 0.67) that is associated with sub-chondritic Zr/Hf values. Furthermore, zircon with uncharacteristic textures forming poikilitic, branching, and interstitial networks are observed exclusively in Hornfels A samples, and yield an U-Pb age of 1850 ± 24 Ma. The zircon textures, age, relation to high-T mineral assemblage, and chemistry suggests crystallization from trapped melt films during retrograde cooling. In combination with the whole rock trace element geochemistry these observations provides strong circumstantial evidence that Zr-Hf was mobilized in silicate melts, and that a 250 m zone from the SIC contact experienced melt segregation. Important to mineral exploration efforts is the observation that the width of the contact aureole in the EMF basalts appear to correlate with the thickness of the SIC that is thought to have a primary control on the location of contact deposits. Also, the width of the high-T contact aureole might provide a limiting factor for the extent to which low-S Cu-PGE rich mineralization can penetrate into the footwall.