Extent of magma mixing as determined from pumice fragments, Tenerife, Canary Islands: An electron microprobe and Nomarski interference study of feldspar and host glass.

Feldspar and glass from two ignimbrite deposits from pre-calderan rocks of the Diego Hernandez Formation, Tenerife, Canary Islands are studied in detail using electron microprobe (EMP) and Nomarski Differential Interference Contrast (NDIC) to better understand magma mixing events. Banded pumice fragments contain contrasting magma compositions from basaltic trachyandesite (mafic) to phonolite (felsic) recognized by their black to green color, respectively. Both physical and chemical magma mixing are identified in banded pumice based on EMP and NDIC. Phonolitic magma is homogenous with a distinctive feldspar population ($\approx$Or$\sb{40}$), whereas the magic member ranges from basaltic trachyandesite to trachyandesite, with two feldspar populations (An$\sb{40}$ and An$\sb{70}$). A chemical mixing event is proposed between an alkali basalt (with feldspar An$\sb{70}$) and a trachyandesite (feldspar An$\sb{40}$) to form a hybrid melt of trachyandesite. Physical mixing between the hybrid and phonolite melts occurs just prior to or during the eruptive event forming banded pumice fragments. Both ignimbrites studied show evidence of injection of mafic magma with the presence of calcic plagioclase ($\approx$An$\sb{70}$) and other mafic minerals, but only one is associated with caldera collapse of Diego Hernandez. Incomplete reaction textures and wider range in glass chemistry indicate incomplete mixing between pre-existing trachyandesite and injected alkali basalt for the collapse event suggesting a more violent injection and subsequent eruption of this ignimbrite leading to collapse of the magma chamber. Distal deposits of collapse event are more widespread as compared to the ignimbrite not associated with collapse which also indicates the larger scale related to caldera collapse