Target database memory exceed available shared memory
MEMORY_TARGET needs /dev/shm filesystem with required size. By default /dev/shm size is half of the system memory. So you have to change line in /etc/fstab from:

tmpfs                   /dev/shm                tmpfs   defaults          0 0


to something like:

tmpfs                   /dev/shm                tmpfs   defaults,size=49G 0 0


df -h /dev/shm will show you tmpfs size. To avoid reboot after editing /etc/fstab you can do:

umount tmpfs
mount -a


But with such amount of memory I would go with ASMM (SGA_TARGET) and huge pages. Especially if you will have many clients connected. Overhead to manage 48GB of 4kB pages will be quite big.