  This paper is devoted to the analysis of blow-up solutions for the fractional
nonlinear Schr\"odinger equation with combined power-type nonlinearities \[
i\partial_t u-(-\Delta)^su+\lambda_1|u|^{2p_1}u+\lambda_2|u|^{2p_2}u=0, \]
where $0<p_1<p_2<\frac{2s}{N-2s}$. Firstly, we obtain some sufficient
conditions about existence of blow-up solutions, and then derive some sharp
thresholds of blow-up and global existence by constructing some new estimates.
Moreover, we find the sharp threshold mass of blow-up and global existence in
the case $0<p_1<\frac{2s}{N}$ and $p_2=\frac{2s}{N}$. Finally, we investigate
the dynamical properties of blow-up solutions, including $L^2$-concentration,
blow-up rate and limiting profile.
