Synchronous alkaline and subalkaline magmatism during the late Neoproterozoic-early Paleozoic Ross orogeny, Antarctica: Insights into magmatic sources and processes within a continental arc

Extensive exposure of intrusive igneous rocks along the Ross orogen of Antarctica-an ancient accretionary orogen on the margin of East Gondwana-provides an exceptional opportunity to study continental arc magmatism. There is significant petrologic and geochemical variability in igneous rocks within a ~ 500-km-long segment of the arc in southern Victoria Land. The conspicuous occurrence of carbonatite and alkaline silicate rocks (nepheline syenite, A-type granite, and alkaline mafic rocks) adjacent to large complexes of subalkaline granitoids is not adequately explained by traditional models for continental arc magmatism. Extensive geochemical analysis (> 100 samples) and zircon U-Pb geochronology (n = 70) confirms that alkaline and carbonatitic magmatism was partially contemporaneous with the emplacement of large subduction-related igneous complexes in adjacent areas. Major pulses of subalkaline magmatism were compositionally distinct and occurred at different times along the arc. Large bodies of subalkaline orthogneiss and granite (sensu lato) were emplaced over similar time intervals (ca. 25 Myr) to the north (ca. 515-492 Ma) and south (ca. 550-525 Ma) of the alkaline magmatic province, although the initiation of these major pulses of magmatism was offset by ca. 35 Myr. Alkaline and carbonatitic magmatism spanned at least ca. 550-509 Ma, overlapping with voluminous subalkaline magmatism in adjacent areas. The most primitive rocks from each area have similarly enriched trace element compositions, indicating some common characteristics of the magma sources along the arc. The samples from the older subalkaline complex have invariably low Sr/Y ratios (< 40), consistent with relatively shallow magma generation and differentiation. The younger subalkaline complex and subalkaline rocks within the area of the alkaline province extend to higher Sr/Y ratios (up to ~ 300), indicative of generation and differentiation at deeper levels. The significant spatial and temporal diversity in magmatism can be explained by a tectono-magmatic model that invokes alternating phases of extension and contraction in the overriding plate. The low Sr/Y magmatism of the earlier subalkaline complex and the largely contemporaneous alkaline and carbonatitic magmatism occurred during a phase of extension in the overriding plate ca. 550-525 Ma. The differences in composition of this early phase of magmatism can be explained by variable lithospheric thickness along the margin, with the alkaline magmas derived from partial melts of thick metasomatized lithosphere. The younger subalkaline complex was emplaced during a phase of contraction in the overlying plate ca. 515-505 Ma. The latest phase of (post-orogenic) magmatism occurred during extension in the overriding plate ca. 505-492 Ma.