ERUPTION AND EMPLACEMENT OF A BASALTIC WELDED IGNIMBRITE DURING CALDERA FORMATION ON GRAN-CANARIA

The 14.1 Ma old composite ignimbrite cooling unit P1 (45 km(3)) on Gra n Canaria comprises a lower mixed rhyolite-trachyte tuff, a central rh yolite-basalt mixed tuff, and a slightly rhyolite-contaminated basalti c tuff at the top. The basaltic tuff is compositionally zoned with (a) an upward change in basalt composition to higher MgO content (4.3-5.2 wt.%), (b) variably admixed rhyolite or trachyte (commonly <5 wt.%), and (c) an upward increasing abundance of basaltic and plutonic lithic fragments and cognate cumulate fragments. The basaltic tuff is divide d into three structural units: (I) the welded basaltic ignimbrite, whi ch forms the thickest part (c. 95 vol.%) and is the main subject of th e present paper; (II) poorly consolidated massive, bomb- and block-ric h beds interpreted as phreatomagmatic pyroclastic flow deposits; and ( III) various facies of reworked basaltic tuff. Tuff unit I is a basalt ic ignimbrite rather than a lava now because of the absence of top and bottom breccias, radial sheet-like distribution around the central Te jeda caldera, thickening in valleys but also covering higher ground, a nd local erosion of the underlying P1 ash. A gradual transition from d ense rock in the interior to ash at the top of the basaltic ignimbrite reflects a decrease in welding; the shape of the welding profile is t ypical for emplacement temperatures well above the minimum welding tem perature. A similar transition occurs at the base where the ignimbrite was emplaced on cold ground in distal sections. In proximal sections the base is dense where it was emplaced on hot felsic P1 tuff. The int ensity of welding, especially at the base, and the presence of spheric al particles and of mantled and composite particles formed by accretio n and coalescence in a viscous state imply that the now was a suspensi on of hot magma droplets. The flow most likely had to be density strat ified and highly turbulent to prevent massive coalescence and collapse . Model calculations suggest eruption through low pyroclastic fountain s (<1000 m high) with limited cooling during eruption and turbulent no w from an initial temperature of 1160 degrees C. The large volume of 2 6 km(3) of erupted basalt compared with only 16 km(3) of the evolved P 1 magmas, and the extremely high discharge rates inferred from model c alculations are unusual for a basaltic eruption. It is suggested that the basaltic magma was erupted and emplaced in a fashion commonly only attributed to felsic magmas because it utilized the felsic P1 magma c hamber and its ring-fissure conduits. Evolution of the entire P1 erupt ion was controlled by withdrawal dynamics involving magmas differing i n viscosity by more than four orders of magnitude. The basaltic erupti on phase was initially driven by buoyancy of the basaltic magma at cha mber depth and continued degassing of felsic magma, but most of the la rge volume of basalt magma was driven out of the reservoir by subsiden ce of a c. 10 km diameter roof block, which followed a decrease in mag ma chamber pressure during low viscosity basaltic outflow.