Wed 21st June @ noon, Rm 312.222
Abstract:
To develop a comprehensive understanding of geological processes a multidisciplinary approach much be utilized. To provide insights on the origin and duration models of LIPs generation, this research focus’ on three Continental Flood Basalts (CFB) located throughout the Southern Hemisphere; the ca. 511 Ma Kalkarindji CFB Province located in the northwest of Australia; the Karoo CFB Province located in southern Africa and parts of Antarctica; and the Tasmanian Dolerite portion of the Ferrar CFB Province outcropped in Antarctica and Tasmania.
An extensive geochemical database of the Kalkarindji igneous constituents display one of the most geochemically homogeneous CFBs currently known. Interpretations from the geochemical data preclude the involvement of the assimilation of crustal material during the emplacement of the Kalkarindji magmas. Instead the geochemistry indicate the enriched characteristics are native to the mantle source. Calculations utilizing the high 207Pb/204Pb and elevated 208Pb/204Pb for moderate 206Pb/204Pb conclude the enriched geochemical characteristics can be reproduced from an ancient enrichment to the sub-continental lithospheric mantle (SCLM) ca. 2.5 Ga into the source region of the Kalkarindji magmas. Using the geochemical data a model of mantle warming for ca. 700 Ma coupled with decompression melting of the anciently fertilized SCLM that was triggered during a large scale rotation of Gondwana ca. 500 Ma is presented for generation of the magmas of the Kalkarindji Province. An important constraint on this model is the ability to cause SCLM melting, which requires previous charging by fluids to provide a fertile and/or relatively hydrated SCLM.
The integration of 40Ar/39Ar geochronology with geochemical analyses of dolerite intrusions into the Western Cape province of the Karoo CFBs display the first direct evidence of a hydrated source for a CFB province. 40Ar/39Ar plagioclase geochronology indicate these Western Cape intrusions are some of the first magmatic expressions of the Karoo CFB province at ca 183.5 Ma. Precluding any evidence of a considerable amount of crustal contamination, the primary hydrated minerals (biotite and hornblende) are representative of a hydrated source. Furthermore, the geochemical and geographic location of the Western Cape intrusions with other low-Ti rocks of the Karoo LIP indicate subduction during the Kibaran Orogeny provided hydrated material to the base of the lithosphere preferentially proximal to the subduction zone. The direct evidence of H2O in the source region of tholeiitic CFBs provides evidence needed to suggest the SCLM could contribute notably in the generation of CFBs.
The advent of multi-collector noble gas mass spectrometers provides the ability to continue to push various geochronological techniques to new horizons. The first geologically meaningful 40Ar/39Ar plateau ages for terrestrial pyroxene has been accomplished on dolerites from the Kalkarindji and Ferrar CFBs. A thorough comparison of these 40Ar/39Ar pyroxene plateau ages with previously published results from a multitude of geochronological methods indicate these results represent a true geological event. Irradiation induced recoil resulting from extensive exsolution in some of the samples can be mitigated through a thorough petrographic investigation when selecting samples for 40Ar/39Ar geochronology. The ability to utilize the 40Ar/39Ar technique with the mineral pyroxene establishes unprecedented geochronological opportunities to the study of LIPs as well as metamorphic and thermochronologic applications.
