Named after the great Catalan cellist Pablo Casals, this group — violinists Abel Tomás Realp and Vera Martinez Mehner, violist Jonathan Brown, and cellist Arnau Tomás Realp — is both exacting and exciting. 2010’s excellent Metamorphosis presents a set by three Hungarian composers, Béla Bartók, György Ligeti, and György Kurtág. Bartok’s string quartets are major landmarks, and his Fourth String Quartet (1928) is both a wonder of structural symmetry and a rich catalogue of instrumental gestures. Cuarteto Casals masterfully brings to life Bartok’s expansive timbral palette, one created by the use of assertive pizzicato, vibrato-free passages, glissandi, and much more. Ligeti’s First String Quartet (1953-54) clearly builds on Bartok’s innovations while striking out in new directions. The piece’s subtitle, “Métamophosis Nocturnes,” suggests the shape-shifting, mysterious nature of this powerful work, which can veer from violent rhythms to quiescent lyricism. Kurtag’s 12 Microludes for String Quartet (1977-78), whose short sections each begin on a different note of the chromatic scale, conjures a rich sound world.