The Lower Paleozoic Silgara Formation is exposed in the Pescadero-Aratoca area，southwestern Santander Massif，and consists of metapelites and interlayered metabasic rocks intruded by Triassic-Jurassic calc-alkaline granites．A sequence of metamorphic zones was developed during Caledonian low-P/T metamorphism．The regional metamorphic grade of the Silgara Formation decreases southwestward，from the sillimanite zone，through the staurolite and garnet zones，to the biotite-zone．Metamolphism has occunrred under conditions of high-temperature and low-pressure（Buchan type metamorphism），and reflects the high heat flow that exists in this part of the Santander Massif．
Garnet in the pelitic schists preserves chemical zoning，with decreasing X_<Mn>，and increasing X_<Fe> and X_<Mg> from core to rim，suggesting craystal growth during increasing temperature．X_<Ca> increases outwards and reaches a maximum at mantle，and is dependant on metamolphic pressure（maximum X_<Ca> indicates peak metamorphic pressure）．