HIIT cardio will result in significant fat loss - far more significant than 60-70% max heart rate. Lower intensity exercise utilises fat as a primary energy fuel source during exercise, but the net adipose tissue (fat tissue) breakdown is far greater with HIIT cardio over a 24 hour snapshot.

Meanwhile with regards to the breakdown of muscle mass, HIIT cardio can actually produce an anabolic effect with regards to muscular hypertrophy (building muscle). Of course, significant amounts of HIIT cardio (or any cardio for that matter) will result in significant muscle atrophy (breakdown). But this simply is achieved through a caloric defecit - and could be just as effectively achieved through a diet deficient in calories.

It's very easy to get confused on this topic, particularly with the infamous "fat burning zone"...gah that is something I have a real beef about.

For more reading on the subject, check out these articles:

The Fat Burning Zone

(Begin with the one above, it's an article I had published in several fitness/bodybuilding magazines and provides a more general overview on the topic. The articles below analyse the subject in greater detail.)

Myths Under the Microscope - Fat Burning Zone

Myths Under the Microscope - Fasted Cardio

Myths Under the Microscope - Final Discussion