Petrogenesis of late-stage, high-K bimodal magmas within a continental arc: an example from the Ross Orogen, Antarctica

A bimodal suite of lamprophyric and felsic porphyry dikes, forming part of the Neoproterozoic-Ordovician Ross orogeny, occurs throughout the Dry Valleys of Southern Victoria Land, Antarctica. Lamprophyres are mafic dikes enriched in incompatible elements and hydrous minerals and, in orogenic settings, are frequently associated with post-subduction extension, marking the end of subduction-related magmatism. Although the geologic units comprising the Ross orogen have been extensively mapped and studied, only sparse geochemical and geochronological data for these dikes exist; the Ross orogen provides an opportunity to gain insight into orogenic processes, particularly those underlying the end of subduction. A primary goal of this project was to use field relationships and U-Pb zircon geochronology to clarify cross-cutting relationships between lamprophyres, porphyry dikes, and other intrusive units in order to document the timing of the youngest magmatism and end of the Ross orogeny in Southern Victoria Land. Another was to use whole rock major and trace-element geochemistry to determine the petrogenetic relationship between lamprophyre and porphyry dikes and finally improve our understanding of the geochemistry of lamprophyre dikes. Geochronological data from 19 samples in tandem with field observations indicate that lamprophyres are cross-cut by, and older than, porphyry dikes, and that emplacement of these dikes occurred in quick succession following the cessation of voluminous calc-alkaline magmatism. Dates are consistent with similar dikes from Northern Victoria Land, suggesting similar emplacement ages for dikes along much of the Ross orogen margin. Whole-rock geochemical data from 213 samples indicate that the most primitive lamprophyres are enriched in Mg, Cr, and Ni as well as incompatible elements such as LILE and LREE, indicating that they drive from an enriched mantle source. The presence of negative HFSE anomalies and Pb enrichment suggests a subcontinental lithosphere source enriched by previous addition of upper continental crust-derived sediments, with the LILE enrichment and hydrous character being consistent with metasomatism of the source region. The results are similar to other “orogenic” lamprophyres that are sourced from metasomatized mantle contaminated by continental detritus. Porphyry dikes form co-linear arrays with lamprophyres in major and trace element trends, but are separated from the majority of lamprophyres by a gap of approximately 10 wt. % SiO2, indicating a bimodal suite of magmas with limited, local mixing. Lamprophyres and porphyry dikes have similar trace element distributions, and this, in tandem with cross-cutting relationships, is consistent with the porphyry dikes representing a melt from a lamprophyre-like source. The rarity of porphyry dikes described at a reconnaissance level south of the Dry Valleys suggests that this may have been a localized event, while the absence of porphyry dikes coexisting with lamprophyres in otherwise similar tectonic settings elsewhere in the world suggests that the occurrence of bimodal suites may relate to specific occurrences of underplating rather than crustal mixing