Primary and secondary structures, and structural evolution of La Clemira hornfels from range of Ambargasta, Santiago del Estero

Abstract

The hornfelses of La Clemira were generated by the thermal effect of the Ambargasta granitoids on sedimentary rocks during the middle Cambrian, and are preserved as kilometre-size roof pendants in the granitoids. A vertical and alternating sequence of metapelites and metapsammites, conserving the original bedding (S₀), with top to the east, preserves relics of crossbedding, load structures and convolute folds. These mesoscopic primary structures are unique in metamorphic rocks of the Eastern Pampean Ranges. Other post-metamorphic, mesoscopic secondary structures, mainly folds, were also recognized. Microscopically the hornfelses show some deformation, picked out by cordierite porphyroblasts with pressure solutions seams and by weakly oriented biotite in the matrix. This structural evidence allows the establishment of three post-metamorphic deformational stages: (1) upper Cambrian shortening parallel to beds, producing upright symmetrical folds, box folds and fault-propagation folds; (2) Cambrian-Ordovician boundary elongation, developing monoclinal folds grading to normal faults on short limbs; and (3) late jointing in granitoids, hypabyssal bodies and hornfelses. By comparison with described structures, very similar to those encountered in fold and thrust belts, and granitoid emplacement conditions (P < 0.3 GPa), it is possible to deduce that the post-metamorphic deformation was produced in an upper structural level, above the schistosity front.