Qualitative thermobarometry of inverted metamorphism in the Pelona and Rand Schists, southern California, using calciferous amphibole in mafic schist

Abstract The Rand and Pelona Schists consist of eugeoclinal rock types overlain by continental basement along the Vincent‐Chocolate Mountains (VCM) faults. Both schists display inverted metamorphic zonation, defined in part by a systematic variation in composition of calcic to sodic‐calcic amphibole in mafic schist structurally upward. The compositional progressions include increase of total A1, A1IV and Ti, but decrease in the ratios of Na/(Na + Ca) to A1/(A1 + Si), and NaM4 to (A1VI+ Fe3++ Ti). These variations imply that structurally high rocks belong to a lower‐pressure metamorphic fades series than those at depth. This result is consistent with previous views that the inverted metamorphic zonations represent intact structural sequences. Amphibole composition is dependent not only on structural position (i.e. P‐T), but also upon bulk‐rock composition. The important controls are whole‐rock Mg/(Mg + Fe2++ Mn) and Fe3+/Fe2+. The greatest impact of these factors, however, is on the absolute values of Na and Al, rather than their ratio. Thus, interpretation of facies series is not seriously hindered by compositional variability. Sodic amphibole in epidote blueschists from the Rand Schist is extensively replaced by sodic‐calcic amphibole. Sodic‐calcic amphibole in the Rand Schist and Pelona Schist is, itself, rimmed by actinolitic amphibole. Similar blueschist to greenschist transitions in other metamorphic terranes are typically attributed to exhumation. In the Rand and Pelona Schists, the sequence probably formed during burial.