Page navigation

When my app first starts up I have it display a login page. In the login button if they are able to login I want to then remove the login page and navigate to a tabbed page. In this tabbed page I'll have a settings page that would allow me to get back to the login page if needed. Right now I have the following but it doesn't work. The HomePage is shown but the back arrow to the login page shows up and I don't want that.

    public class LoginPage: ContentPage
    {
       public LoginPage() { // create controls here }

       public btnLogin_Clicked(object sender, EventArgs e){
          Navigation.PopAsync(); // remove this page (doesn't work)
          Navigation.PushAsync(new HomePage());
       }
    }

    public class App : Application
    {
       public App()
       {
          MainPage = new NavigationPage(new LoginPage());
       }
    }