IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P30

Date/Time: July 24 Poster

The initiation of crustal recycling processes in island arcs: insights from the Soufriere
Volcanic Centre, Saint Lucia
Jan Lindsay1 , Axel Schmitt2 , Robert Trumbull3 , Daniel Stockli4 , Philip Shane1 , Tracy Howe1
1

School of Environment, The University of Auckland, Auckland, New Zealand, 2 Department of Earthand Space Sciences, University of California Los Angeles, USA, 3 GFZ German Research Centre forGeosciences, Telegrafenberg, 14473 Potsdam, Germany, 4 Department of Geological Sciences, Jackson
School of Geosciences, University of Texas at Austin, USA
E-mail: j.lindsay@auckland.ac.nz

Extensive intermediate pyroclastic flow deposits, lava flows, domes and block-and-ash-flow deposits from theSoufriere Volcanic Complex (SVC), Saint Lucia, collectively represent a conspicuous shift in composition anderuption style for this island arc segment. Compositionally, these dominantly silicic (61.6 to 67.7 wt. percent SiO2)
SVC deposits are distinct from older more mafic deposits in Saint Lucia. They contain accessory zircons, and manyof the most recent rocks have phenocrystic quartz. SVC deposits occur in and around the Qualibou Depression,a c. 10 km diameter wide sector collapse structure of poorly constrained age. Here, we re-investigate the onsetof silicic volcanism in Saint Lucia, and the relationship between individual SVC pyroclastic flows and the sector
collapse through (U-Th)/He, U-Th and U-Pb zircon chronostratigraphy, aided by mineralogical and geochemicalcorrelation.

The oldest SVC units comprise pyroclastic deposits at Micoud (640+/-8 ka), Bellevue (264+/-8 ka), AnseJohn (104+/-4 ka) and La Pointe (59.8+/-2.1 ka), and at Anse Noir and Piaye (undated). These were all previouslygrouped with or associated with the so-called Choiseul tuff, whereas our chronology demonstrates that the units
represent individual periods of activity spanning a range of ages. Choiseul pumice at the type locality has yielded a(U-Th)/He zircon age of 515+/-61 ka, overlapping with the age for Morne Tabac (532+/-42 ka), a dome truncated by
the depression escarpment. Younger SVC units comprise Morne Bonin (273+/-15 ka), Gros Piton and Petit Piton(71+/-3 ka and 109+/-4 ka, resp.), Belfond (13.6+/-0.4 ka) and Terre Blanche (15.3+/-0.4 ka). The younger units areclearly domes within the Qualibou Depression whereas the nature of Morne Bonin (dome or mega-block) remains
poorly constrained. Belfond and Terre Blanche have whole rock geochemistry and mineral assemblages similar to
the Belfond pyroclastic flow deposit, the most recent widespread pyroclastic flow with nearly-concordant (U-Th)/Heand 14C ages of 20 ka. The Belfond and Terre Blanche domes possibly represent late-erupted degassed portionsof the magma that produced the Belfond pyroclastic flow.

The geochemical characteristics and similar zircon age distributions of the silicic lava domes and pyroclastic
flows of the SVC suggest that these share a common magma source beneath the Qualibou depression. The
onset of activity from this proto-Qualibou centre occured much earlier than previously thought (by c. 500 k.a.), andmarks a change from largely quartz- and zircon-free andesitic magmatism to more evolved magma compositions.
We interpret crystal recycling in SVC magmas, and the presence of co-genetic plutonic enclaves, to indicate theprogressive build-up of a mid-crustal batholith, and thus a major change in the crustal structure of this island arc
segment.

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