A TECTONIC CONTROL ON THE TIMING, CHEMISTRY AND SCALE OF VOLUMINOUS PULSED INTRUSIVE MAGMATISM: EVIDENCE FROM THE LASSITER COAST INTRUSIVE SUITE, ANTARCTICA

We present the evolution of a ~13,000 km² intrusive suite in Antarctica and show how its pulsed emplacement, chemistry and volume were controlled by syn-magmatic deformation by utilising field, structural, geochemical, geochronological and magnetic anisotropy data.

The Lassiter Coast Intrusive Suite (LCIS) of the Antarctic Peninsula provides a unique opportunity to observe the evolution of a pulsed syn-tectonic intrusive suite. The LCIS is one of the largest magmatic events in West Antarctica, and its timing and scale during the Mid-Cretaceous period of global deformation and extensive trans-Pacific magmatism renders it of both regional and global significance. Studying 26 of the suite’s composite intrusions (~1200 km² of magmatism), the suite is dominantly granodioritic, but ranges from pyroxenite to granite. Combining U-Pb ages with field and magnetic anisotropy (AMS) data shows two discrete pulses of largely batholith-style syn-orogenic magmatism associated with peaks in tectonic compression, the first pulse (118 Ma) distinctly more mafic than the much more voluminous and felsic second phase (113 Ma). They are followed by a period of reduced tectonic compression and ultimately post-compressional extension associated with waning and isolated post-orogenic felsic plutonism (110-95 Ma).

This data highlights the causal link between voluminous pulsed magmatism and crustal deformation, with magmatic flare-ups deriving from pulses in tectonic compression. This non-steady state may also allow the compositional evolution of magma required to develop the differentiated continental crust, with early magmatism aiding the assimilation potential and consequent felsic composition of those that follow through crustal heating and provision of fresh, warm mafic-intermediate material for subsequent partial melting.