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We report the discovery of MAGAZ3NE J095924+022537, aspectroscopically-confirmed protocluster at $z = 3.3665^{+0.0009}_{-0.0012}$around a spectroscopically-confirmed $UVJ$-quiescent ultra-massive galaxy (UMG;$M_{\star}=2.34^{+0.23}_{-0.34}\times10^{11} {\rm M}_\odot$) in the COSMOSUltraVISTA field. We present a total of 38 protocluster members (14spectroscopic and 24 photometric), including the UMG. Notably, and in markedcontrast to protoclusters previously reported at this epoch which have beenfound to contain predominantly star-forming members, we measure an elevatedfraction of quiescent galaxies relative to the coeval field($73.3^{+26.7}_{-16.9}\%$ versus $11.6^{+7.1}_{-4.9}\%$ for galaxies withstellar mass $M_{\star} \geq 10^{11} {\rm M}_\odot$). This high quenchedfraction provides a striking and important counterexample to the seemingubiquitousness of star-forming galaxies in protoclusters at $z>2$ and suggests,rather, that protoclusters exist in a diversity of evolutionary states in theearly Universe. We discuss the possibility that we might be observing either"early mass quenching" or non-classical "environmental quenching." We alsopresent the discovery of MAGAZ3NE J100028+023349, a secondspectroscopically-confirmed protocluster, at a very similar redshift of $z =3.3801^{+0.0213}_{-0.0281}$. We present a total of 20 protocluster members, 12of which are photometric and 8 spectroscopic including a post-starburst UMG($M_{\star}=2.95^{+0.21}_{-0.20}\times10^{11} {\rm M}_\odot$). ProtoclustersMAGAZ3NE J0959 and MAGAZ3NE J1000 are separated by 18 arcminutes on the sky (35comoving Mpc), in good agreement with predictions from simulations for the sizeof "Coma"-type cluster progenitors at this epoch. It is highly likely that thetwo UMGs are the progenitors of Brightest Cluster Galaxies (BCGs) seen inmassive virialized clusters at lower redshift.

Spectroscopic Confirmation of a Protocluster at $z=3.37$ with a High Fraction of Quiescent Galaxies