Mixing between enriched lithospheric mantle and crustal components in a short-lived subduction-related magma system, Dry Valleys area, Antarctica: Insights from U-Pb geochronology, Hf isotopes, and whole-rock geochemistry

Abstract

Granitoids associated with the Neoproterozoic-early Paleozoic Ross orogeny are extensively exposed in the Dry Valleys region of southern Victoria Land, Antarctica, affording an exceptional opportunity to gain insight into the temporal and spatial scales of continental arc magmatism. Samples spanning 150 km along strike and 50 km across strike were selected for isotopic and geochemical analysis. Zircon U-Pb geochronology and the first Hf isotope data for Dry Valleys granitoids, coupled with whole-rock elemental data, reveal mixing between enriched lithospheric mantle and Precambrian crustal components and indicate that the principal phase of magmatism in the Dry Valleys area was restricted to a period of 23 m.y., from ca. 515 to 492 Ma. This relatively short period of magmatism contrasts with other segments of the Ross orogen, in which magmatism spanned greater than 100 m.y. Most calc-alkaline intrusions spanned 515-500 Ma, while postkinematic granitoids with alkali-calcic geochemical signatures spanned 505-492 Ma, indicating a transitional shift to an overall extensional tectonic regime. Zircon e Hf(i) values range between -0.3 and -7.2, with two-stage depleted-mantle model ages ranging from 1.5 to 1.9 Ga. Low e Hf(i) values in mafic samples are consistent with derivation from an enriched subcontinental lithospheric mantle source, while the large-volume granitic intrusions show evidence for increasing assimilation of old crust over time. A broadening of the e Hf(i) range to more negative values in the younger intrusions may reflect crustal thickening or underplating of fertile continental material into the source region of the arc.

Coupled garnet Lu–Hf and monazite U–Pb geochronology constrain early convergent margin dynamics in the Ross orogen, Antarctica

“…Figure shows a timeline summarizing the existing geochronology of magmatism and metamorphism associated with the Ross orogeny. Published U–Pb ages for igneous rocks in southern Victoria Land span c. 550–485 Ma (summarized in Hagen‐Peter et al ., ). Read () reported an age of 557 ± 5 Ma for an alkaline gabbroic pluton in the Walcott Glacier area, and zircon U–Pb data (Hagen‐Peter, ) confirm that magmatism commenced by 565 ± 7 Ma in this area.…”

“…The inception of widespread magmatism in the area c . 565–550 Ma (Read, ; Hagen‐Peter, ; Hagen‐Peter et al ., ) coincides with growth of early high‐Y monazite with pronounced negative Eu anomalies, and local magmatism may have been the cause of partial melting in OU82864.…”

“…Magmatism in northern Victoria Land (nVL) spanned c . 545–495 Ma (summarized in Hagen‐Peter et al ., ). Eclogite facies metamorphism occurred in this region at c .…”

“…The onset of magmatism was transient along the Ross orogen (Hagen‐Peter et al ., ), and different segments of the orogen experienced different metamorphic histories at different times (Goodge, ). However, a paucity of geochronology for metamorphic rocks in the Ross orogen makes it difficult to integrate the metamorphic and deformational history with the well‐established magmatic history of the arc.…”

“…The major pulse of arc magmatism, including the emplacement of the ~1000 km 2 Bonney Pluton, in the Dry Valleys area, occurred at c. 515–490 Ma (Fig. ; summarized in Hagen‐Peter et al ., ).…”