Tectono-Magmatic Evolution of the Antarctic Peninsula.

Wednesday 28th November 2012   

Dr. Conor Ryan (Tullow Oil) will present a lecture:

Tectono-Magmatic Evolution of the Antarctic Peninsula.

VENUE:                Geological Survey of Ireland                                     Beggars Bush, Haddington Road, Dublin 4

TIME:                    7.00pm MAP:                       http://www.gsi.ie/Contact+Us/Visiting+GSI.htm  

ABSTRACT

In northwestern Graham Land, rocks of the Antarctic Peninsula batholith and volcanic rocks of the Antarctic Peninsula Volcanic Group intrude and unconformably overlie low-grade metasedimentary ‘basement’ of the Trinity Peninsula Group (?Carboniferous to Triassic). The Mesozoic to Cenozoic subduction-related plutonic rocks range from gabbro to granite, with granodioritic compositions predominating. Mafic to intermediate hypabyssal intrusions cutting the plutonic, volcanic and ‘basement’ rocks are common. These igneous rocks form  a ‘normal’ calc-alkaline suite typical of active continental margins. Fractional crystallisation of a mafic parental magma seems to be the dominant process responsible for the range of igneous lithologies observed in the study area. New zircon U-Pb geochronological studies have unequivocally constrained the timing of plutonism at two locations along the Danco Coast to Late Cretaceous (Turonian) times. Isotopic systematics of the Danco Coast plutonic rocks reveal relatively low, mantle-like values. The Danco Coast granitoids are derived from subduction modified lithospheric mantle, or mafic underplate, which was added to the base of the crust from such a source. Thermobarometric data indicate that these granitoids were emplaced at shallow crustal depths, probably during a phase of extensional tectonics, which allowed these mantle or mafic lower crustal melts to avoid extensive interaction with middle to upper crustal material by passive emplacement.   New ⁴⁰Ar/³⁹Ar geochronological studies suggest that hypabyssal intrusions from the Danco Coast are likely to be early Palaeocene in age, whereas dykes from the Palmer Archipelago were emplaced in Oligocene to Miocene times. The ages of the Palmer Archipelago dykes extends the record of subduction-related magmatism into the Neogene in this part of the peninsula. Sr-Nd isotopic systematics of these dykes indicate generation of melts in an asthenospheric mantle source, and confirm a trend of progressively shallower depths of melting with time from lithospheric mantle garnet-bearing sources to asthenospheric spinel bearing mantle sources. Evidence is presented that suggest that the melts ponded initially at the base of the crust, before being emplaced at shallow crustal depths. Late Cretaceous to Palaeocene Danco Coast dykes were probably derived from both lithospheric and asthenospheric mantle sources. Late Cenozoic tapping of asthenospheric sources by trench proximal dykes may have been facilitated by rollback of the subducting slab with time, allowing asthenospheric mantle to migrate into the mantle wedge via enhanced corner flow and upwelling.  

CONOR RYAN

Graduated from Trinity’s Geology Department in 2002 after which he spent time working in Engineering Geology before undertaking his PhD at the University of Brighton and Natural History Museum, London.   His doctoral studies focussed on the tectonic evolution of the Antarctic Peninsula and he made two trips to the region. Upon completing his PhD he spent some time working in the Philippines on a gold exploration project before returning to Ireland in 2008 to take up a post with Tullow Oil.   A Chartered Fellow of the Geological Society of London and an accredited member of the Institute of Engineers of Ireland.