Mg- and Cr-rich staurolite and Cr-rich kyanite in high-pressure ultrabasic rocks (Cabo Ortegal, northwestern Spain)

Metamorphosed ultrabasic rocks, likely derived from cumulate protoliths, occur intercalated within eclogites in the Cabo Ortegal complex (northwest Spain). Mg- and Cr-rich staurolite coexists with Cr-rich kyanite in these rocks. The staurolite has X_Mg > 0.74 and contains >6 wt% Cr₂O₃, and kyanite contains >4 wt% Cr₂O₃. Associated minerals are tschermakitic- to magrresio-hornblende, garnet, and zoisite. Mg- and Cr-rich staurolite and Cr-rich kyanite were formed through reactions involving partial destruction of Cr-rich spinel. Green pleochroism in staurolite and blue pleochroism in kyanite presumably result from Cr substitution for Al in the octahedral sites of these minerals. This substitution accounts for Cr enrichment in both minerals. Staurolite has greater affinity for Cr relative to Al than kyanite. In both there is, however, a significant difference between observed and ideal Al = Cr substitution. Mg enrichment in staurolite is controlled by cation substitution involving ^[4]Fe sites. X_Mg values of staurolite, although showing considerable variation, may be as high as those of associated garnets but are consistently lower than those of associated amphibole (X_Mg > 0.9). Although the Cr contents of kyanite and staurolite could result from the Cr-rich bulk composition, the occlurences of small, colorless, Cr-poor kyanite included in matrix minerals and of blue, Cr-rich, matrix kyanite suggest increasing Cr substitution in kyanite with increasing pressure and temperature.