From magma to mush to lava: crystal history of voluminous felsic lavas in the Gawler Range Volcanics, South Australia
Ferguson, MRM and Ehrig, K and Meffre, S and Feig, S, From magma to mush to lava: crystal history of voluminous felsic lavas in the Gawler Range Volcanics, South Australia, Lithos, 346-347 Article 105148. ISSN 0024-4937 (2019) [Refereed Article]
The Gawler Range Volcanics (GRV) of South Australia are important extrusive analogues to the Mesoproterozoic Hiltaba Suite intrusive rocks of the Gawler Craton, which both host and are implicated in the formation of iron oxide-copper-gold deposits in the Olympic Province. We study three voluminous lavas in the GRV that were emplaced in less than one Ma and contain abundant glomerocrysts of pyroxene, titanomagnetite, plagioclase, K-feldspar, fayalite, apatite, and zircon. Within individual glomerocrysts, pigeonite can exhibit compositional zonation and a range of Y and heavy rare earth element compositions. Compositions of some pigeonite cores from within the same glomerocrysts vary, and crystal zonation does not have a systematic orientation with respect to the crystal's position within its host cluster. This suggests some phenocrysts did not grow adjacent to the other crystals found in individual clusters in the solidified rock, but rather aggregated or settled with other crystals following a stage of growth as independent, melt-bound phenocrysts. The aggregation of phenocrysts is also consistent with the multi-phase nature and diversity of grain-sizes present in the GRV glomerocrysts, rather than accumulations of crystals formed through winnowing of low-density phases or other mechanical sorting processes. In this scenario, glomerocrysts were formed partly by flocculation of independently nucleated crystals, and intergrew after aggregation during magma flow and/or as magma crystallinity increased in the magma reservoir prior to the emplacement of the lavas and the eruption trigger events. The last-erupted lava contains the highest proportion of glomerocrysts comprising complex and anhedral intergrowths. This is consistent with a normally zoned reservoir where the crystal:melt ratio and crystal concentration is greatest at depth. Loosely packed frameworks and free crystals are interpreted to occur in the middle and upper sections of the reservoir, respectively, represented by the first erupted Upper GRV lava and quartz-phyric portions of the lavas. Plagioclase is the most abundant phenocryst throughout the lavas but is present in only a subset of glomerocrysts. The low abundance and absence of feldspar and quartz in glomerocrysts is likely due to mineral resorption and subsequent separation and loss of these components from other glomerocrysts components prior to and during the eruption of the studied lavas. Once solid-solid crystal contacts were dissolved, the separation of the cluster components was driven by the stress placed on them by melt flow. Mafic magmatic enclaves recognised throughout the Upper GRV lavas likely represent the replenishing mafic magmas that provided the thermal and volatile input that drove crystal resorption. Continuing mafic replenishment and injection of sufficient volumes of magma into the Upper GRV reservoir may have also driven the eruption of the lavas. Prolonged fractionation and interaction with mafic magmas could have potentially conveyed or enhanced the distinctive A-type signature of these rocks, negating the requirement of a specialised precursor composition.
Gawler Range Volcanics, crystal clusters, silicic magma, magma rejuvenation, iron oxide deposits, pigeonite