ALBERT

Beschreibung: The link between mantle plumes and continental breakup remain a topic of debate. Here, a new 40Ar/39Ar age of 135.9 ± 1.2 Ma (2σ) and previous ages from the Bunbury Basalt – lava flows that are part of the Greater Kerguelen large igneous province (LIP) – reveal that 〉80% of magmatism in the southern Perth Basin was concomitant with the continental breakup of eastern Gondwana at ca. 137–136 Ma. New and existing isotope geochemical data show that only lithospheric and depleted asthenospheric sources were melted to form the Bunbury Basalt and most other early, ca. 147–124 Ma magmatic products part of the Greater Kerguelen LIP. All lines of evidence strongly point towards passive continental breakup of eastern Gondwana, including the restriction of 147–124 Ma magmatism to continental rifts, the lack of excess oceanic magmatism in this period and the 〉1000 km distance between the Kerguelen plume underneath Greater Indian lithosphere and the breakup nexus. It is not possible to reconcile the influence of a thermochemical plume with the observed geochemical, spatial and geochronological information. Instead, we posit that eastern Gondwana breakup occurred because it was proximal to the former suture zone associated with the ca. 550–500 Ma Kuunga Orogeny between Indo–Australia and Australo–Antarctica. Enrichment of the mantle with volatiles associated with subduction during the Kuunga Orogeny permitted partial melting when the continental crust was sufficiently attenuated in the Early Cretaceous. Repeated and protracted rifting of Greater India from Australo–Antarctic since the mid-Paleozoic eventually led to the rupture of the continental lithosphere and to mafic magmatism at ca. 137–136 Ma, approximately along the position of the former suture zone, without the influence of a mantle plume.

Beschreibung: Large Igneous Provinces mark important and consequential events that span almost the entirety of the Earth's history. These punctuated geologically significant events that include earth processes from the core to the atmosphere have been linked to climate changes, mass extinctions, and even large-scale tectonic shifts. To begin to understand the role LIPs play in these important moments across Earth's lifespan, a thorough understanding of how they form in the first place is imperative. The ∼180 Ma Karoo Continental Flood Basalt (CFB) Province has been heavily studied with one of the largest geochemical and geochronological databases available for a CFB; providing an excellent avenue to study in detail the petrogenesis of these large collections of magma. However, despite this large dataset, the origin of the Karoo CFBs is still debated, with many different and conflicting models proposed. Disparities in these models can often be traced down to the degree of influence a plume and/or the sub-continental lithospheric mantle (SCLM) had during the genesis of this magmatic province. Nine sills intruded into the Western Cape Province of the Karoo Basin were investigated using 40Ar/39Ar geochronology, whole rock geochemistry, and Sr, Nd, Pb isotopes. Five of these sills are peculiar as they contain hydrated minerals (biotite and hornblende), an unusual feature for Karoo basaltic rocks. Biotite and/or hornblende separates yielded statistically indistinguishable 40Ar/39Ar ages as plagioclase separates; indicating that the biotite and hornblende found within these samples are primary features requiring that water was present in the magma during the crystallization of those sills. Major and trace element geochemistry place these rocks into the low-Ti suite of the Karoo CFBs. Trace elements indicate the involvement of fluids over sediment within the source region for these Karoo magmas. All samples from the Western Cape Province display enriched Sr–Nd–Pb initial values with large variations in elemental concentrations for a restricted range of isotopic ratios. Assimilation models indicate these enriched isotopic contents and observed trends are not the result of crustal contamination and are representative of the composition of the source. The trace element and isotopic geochemistry as well as the primary hydrated minerals indicate that water present in the system at the time of formation came directly from one of the mantle sources, supporting the hypothesis that the SCLM was metasomatized by fluids plausibly as a result of the Proterozoic Kibaran Orogeny and/or the Permian Cape Orogeny. This first direct evidence of water within the mantle sources of tholeiitic CFBs, until now only theorized and modeled, potentially changes the fundamentals in how we approach our considerations of CFB generation as a whole.