Numerous subvolcanic intrusions crop out at this erosion level. A , B , C represent the approximate trace of the schematic cross section of Fig.  3 .

Based on extensive field observations (including several mapped transects), whole rock geochemistry, isotopic and geochronologic data, we have reconstructed the evolution of the FNVC (Fig.  3 ) from the beginning of volcanism at 9.7 Ma to the last subvolcanic intrusion emplaced at 6.1 Ma (Halter et al.

Although compositional evolution is towards more mafic compositions in individual stocks, the most silica-rich intrusions of each stock define a continuous trend of increasing SiO ₂ content between 8.5 and 6.1 Ma (Fig.  3 ).

after initiation of extrusive activity, the continuous compositional evolution of subvolcanic intrusions with time can be taken as evidence for the gradual build-up and growth of a subvolcanic magma chamber (Fig.  3 and Halter et al.