For its Stradivari Anniversary Concert on Saturday, the Library of Congress drew a set of notably mellow and silver-toned, Antonio Stradivari-crafted instruments from its collection and placed them in the eloquent hands of the Toronto-based Cecilia String Quartet. The ensemble opened its recital with a lean-toned reading of Mozart’s D-Minor Quartet K. 421, spun of gossamer textures and the merest hint of vibrato, and closed with Mendelssohn’s E-Minor Quartet Op. 44, No. 2 in a performance bursting with emotion.

Between those standard-repertoire bookends came a cannily programmed pair of contemporary works. Sofia Gubaidulina’s 1971 String Quartet No. 1 starts with the quartet members sharing the same note but quickly devolves into combative interplay that shrieks, murmurs, whines and sputters until the four players are isolated musically (and physically, having gradually retreated to every corner of the stage), essentially babbling to themselves. Written during a dark Soviet period, the work (particularly in the Cecilia’s razor-sharp performance) felt prescient and pointedly in touch with our own country’s zeitgeist.

The Cecilia String Quartet. (Lisa-Marie Mazzucco)

The U.S. premiere of Canadian composer Kati Agócs’s 2015 “Tantric Variations” sounded, in a way, like the mirror-image of Gubaidulina’s quartet. Here the musicians begin the work with toneless scraping that gradually coalesces into musical form. Troubled, searching conversations take shape among the four instruments. At times, they are jagged and confrontational, at others blossoming into the kind of unashamedly soaring, tonal melodies that would not be out of place in the Mozart or Mendelssohn quartets on the program. They conclude in euphonious concord. It was as if the very string-quartet structure that Gubaidulina had dismantled in her piece were being reconstituted here and given a sense of hope by Agócs.

The Cecilia, which commissioned “Tantric Variations,” played it gorgeously. The concert marked the 60th anniversary of the gift of the first of five Stradivari to the Library of Congress.