I've noticed that with the more recent Gmail updates for Android, when you click on one of your emails, a new Activity opens (I'm assuming it's not a fragment because of the back arrow).
The new activity's back arrow does not just appear as they do by default, however. The hamburger menu from the main interface spins into an arrow in an animation (see the video here: http://material-design.storage.googleapis.com/publish/material_v_3/material_ext_publish/0B3T7oTWa3HiFbFRfT196SWRyS2s/animation_delightfuldetails_wellcrafted.webm)
(Note: The newly opened email MAY be a fragment, because the toolbar doesn't change & the default new activity animation doesn't play. I'm not sure which one it is.)
Clarification: I do know how to get the hamburger menu to turn into an arrow when it's pressed/when the navigation drawer is opened. In fact, I purposefully disabled said animation because it goes against specs. (see this article: http://www.androidpolice.com/2014/10/30/google-turns-design-inconsistency-ten-latest-round-navigation-drawers/). I want to know, however, if the same animation is possible when creating a new fragment/activity, whichever one Gmail probably uses.