Microtextural characterisation of the Lower Zone in the western limb of the Bushveld Complex, South Africa: evidence for extensive melt migration within a sill complex

Abstract

by plagioclase, clinopyroxene and other late stage phases, shows variable ratios of these minerals ranging from ca. 21:15:64 to 75:17:8. In comparison to modelled expected ratios, most of the analysed rocks have higher amounts of early crystallising interstitial phases (e.g. plagioclase, clinopyroxene), relative to late crystallising phases (e.g. quartz, alkali feldspar). Therefore, interstitial melt must have migrated at different stages of fractionation during cumulate solidification, as a consequence of either compaction or displacement by convecting interstitial liquids. Two samples, however, show the opposite: late phases are relatively more abundant than early ones, which is consistent with a convection-driven replacement of primitive interstitial liquid by more evolved liquid. These results have important implications for the interpretation of the Lower Zone and, by extension, for layered intrusions in general: (1) interstitial sulphide mineralisation may be introduced into a cumulate through infiltrating melts, i.e. the liquid components of a sulphur-saturated crystal mush are not withheld from further migration, upon interaction with a cumulate pile; (2) most importantly, late stage minerals, such as zircon, rarely crystallise from trapped liquid that was initially in equilibrium with the cumulate. Therefore, dating of interstitial zircon from cumulates is unlikely to record the actual timing of emplacement, but merely the crystallisation of a later episode of residual melt that migrated through the cumulate.

The formation of three-grain junctions during solidification. Part I: observations

“…Indeed, one of the highest values of calculated interstitial liquid is found in sample 949′11″, which contains only 4.8 vol.% interstitial minerals, and is clearly strongly adcumulate in nature. This suggests that calculations of liquid content based only on the bulk rock P 2 O 5 are unreliable, due perhaps to the migration of interstitial liquids during solidification (Karykowski and Maier 2017), or to the crystallisation of apatite in channels through which late-stage liquids flowed (Meurer and Meurer 2006).…”

The formation of three-grain junctions during solidification. Part I: observations

“…Indeed, one of the highest values of calculated interstitial liquid is found in sample 949′11″, which contains only 4.8 vol.% interstitial minerals, and is clearly strongly adcumulate in nature. This suggests that calculations of liquid content based only on the bulk rock P 2 O 5 are unreliable, due perhaps to the migration of interstitial liquids during solidification (Karykowski and Maier 2017), or to the crystallisation of apatite in channels through which late-stage liquids flowed (Meurer and Meurer 2006).…”

Formation of the Flatreef deposit, northern Bushveld, by hydrodynamic and hydromagmatic processes

“…They reported mostly < 1% phlogopite and hornblende in the LZ and lower portion of the CZ, albeit with many samples having several per cent mica. Similarly, Karykowski and Maier (2017) showed that the LZ can locally be highly enriched in phlogopite (up to 6 modal %). Maier and Eales (1997) observed that within the UCZ, olivine-bearing cumulates tend to be relatively mica rich, and Maier (1995) reported abundant small (< 0.5 mm) phlogopite from an olivine-bearing UCZ troctolite.…”

Braided peridotite sills and metasomatism in the Rum Layered Suite, Scotland

“…The formation of layered intrusions via sill emplacement is likely more widespread than suggested in the literature and not limited to thin peridotite layers (e.g. Karykowski and Maier 2017). The protracted emplacement of smallvolume sills into a developing crystal mush will inevitably have complicated the cooling history of the complex as a whole.…”

“…The varying members within so-called cyclic sequences could very well be intercalated sills and the pre-existing cumulate, irrespective of scale (e.g. Renner and Palacz 1987;Karykowski and Maier 2017). More complex sequences of cumulate might represent the pre-existing cumulate, the intruding sill, and an intermediary (hybrid) lithology formed from the reaction between the two components, which would appear to link the layers by magma fractionation (e.g.…”

The formation of three-grain junctions during solidification. Part I: observations

“…Indeed, one of the highest values of calculated interstitial liquid is found in sample 949′11″, which contains only 4.8 vol.% interstitial minerals, and is clearly strongly adcumulate in nature. This suggests that calculations of liquid content based only on the bulk rock P 2 O 5 are unreliable, due perhaps to the migration of interstitial liquids during solidification (Karykowski and Maier 2017), or to the crystallisation of apatite in channels through which late-stage liquids flowed (Meurer and Meurer 2006).…”

Formation of the Flatreef deposit, northern Bushveld, by hydrodynamic and hydromagmatic processes

“…They reported mostly < 1% phlogopite and hornblende in the LZ and lower portion of the CZ, albeit with many samples having several per cent mica. Similarly, Karykowski and Maier (2017) showed that the LZ can locally be highly enriched in phlogopite (up to 6 modal %). Maier and Eales (1997) observed that within the UCZ, olivine-bearing cumulates tend to be relatively mica rich, and Maier (1995) reported abundant small (< 0.5 mm) phlogopite from an olivine-bearing UCZ troctolite.…”

Braided peridotite sills and metasomatism in the Rum Layered Suite, Scotland

“…The formation of layered intrusions via sill emplacement is likely more widespread than suggested in the literature and not limited to thin peridotite layers (e.g. Karykowski and Maier 2017). The protracted emplacement of smallvolume sills into a developing crystal mush will inevitably have complicated the cooling history of the complex as a whole.…”

“…The varying members within so-called cyclic sequences could very well be intercalated sills and the pre-existing cumulate, irrespective of scale (e.g. Renner and Palacz 1987;Karykowski and Maier 2017). More complex sequences of cumulate might represent the pre-existing cumulate, the intruding sill, and an intermediary (hybrid) lithology formed from the reaction between the two components, which would appear to link the layers by magma fractionation (e.g.…”

The formation of three-grain junctions during solidification. Part I: observations

“…Indeed, one of the highest values of calculated interstitial liquid is found in sample 949′11″, which contains only 4.8 vol.% interstitial minerals, and is clearly strongly adcumulate in nature. This suggests that calculations of liquid content based only on the bulk rock P 2 O 5 are unreliable, due perhaps to the migration of interstitial liquids during solidification (Karykowski and Maier 2017), or to the crystallisation of apatite in channels through which late-stage liquids flowed (Meurer and Meurer 2006).…”

Formation of the Flatreef deposit, northern Bushveld, by hydrodynamic and hydromagmatic processes

“…They reported mostly < 1% phlogopite and hornblende in the LZ and lower portion of the CZ, albeit with many samples having several per cent mica. Similarly, Karykowski and Maier (2017) showed that the LZ can locally be highly enriched in phlogopite (up to 6 modal %). Maier and Eales (1997) observed that within the UCZ, olivine-bearing cumulates tend to be relatively mica rich, and Maier (1995) reported abundant small (< 0.5 mm) phlogopite from an olivine-bearing UCZ troctolite.…”

Braided peridotite sills and metasomatism in the Rum Layered Suite, Scotland

“…The formation of layered intrusions via sill emplacement is likely more widespread than suggested in the literature and not limited to thin peridotite layers (e.g. Karykowski and Maier 2017). The protracted emplacement of smallvolume sills into a developing crystal mush will inevitably have complicated the cooling history of the complex as a whole.…”

“…The varying members within so-called cyclic sequences could very well be intercalated sills and the pre-existing cumulate, irrespective of scale (e.g. Renner and Palacz 1987;Karykowski and Maier 2017). More complex sequences of cumulate might represent the pre-existing cumulate, the intruding sill, and an intermediary (hybrid) lithology formed from the reaction between the two components, which would appear to link the layers by magma fractionation (e.g.…”