He tells us that they went out immediately and fled from the tomb. That sounds like fear. Then he tells us that trembling and astonishment fell upon them. That sounds a lot like fear, too. But the Greek word translated here as astonishment is almost the exact equivalent of our word, ecstasy.
Ecstasy is quite unlike fear.
It's not at all difficult to imagine that the women were ecstatic, that they were beside themselves with emotion. They were ecstatic, but they were silent about it. They said nothing to anyone about what they had seen, and here the element of fear returns. The closing words of Mark's entire Gospel tell us simply that the women were afraid. Those words bring Mark's Gospel to its conclusion.