Low-grade metamorphism of Cambro-Ordovician successions in the Famatina belt, Southern-Central Andes: Burial-inversion history linked to the evolution of the proto-Andean Gondwana margin

the metamorphic p-t conditions of low-grade units from the famatina belt, central andes of argentina, were estimated through petrography, x-ray diffraction, and electron microscopy. for the middle-upper cambrian negro peinado formation a tectono-metamorphic event associated with intense intrafoliar folding, with estimated temperatures between 290 and 400°c (kicis: 0.16-0.27a°29, biotite blastesis and compositional homogeneity in dioctahedral micas) and intermediate pressure conditions (white mica b parameter: 9.010á-9.035á), was recognized. the achavil formation (middle-upper cambrian) presents a main metamorphic event associated with temperatures between 200 and 290°c (kicis: 0.26-0.41a°29) and intermediate- to low-pressure conditions (white mica b parameter values: 8.972á-9.017á). some illitic substitution in dioctahedral micas also indicates lower metamorphic grade than the negro peinado formation. for upper cambrian to middle ordovician sequences a burial metamorphic pattern, with a progressive decrease in metamorphic grade from volcancito formation to cerro morado group (ca. 490-465 ma; kicis: 0.31-0.69a°29) and absence of tendency changes linked to strati-graphic discontinuities was proposed. mica and chlorite are the main phyllosilicates in the oldest units, while ilt/ sme (r3) mixed-layer is almost the only one in the youngest. white mica b parameter indicates intermediate- to low-pressure conditions for all these sequences. this burial metamorphic pattern presents a marked break as the youngest ordovician unit (la aguadita formation, after ca. 452 ma) records higher metamorphic conditions (ikcis: 0.28-0.19a°29) than units from the ordovician arc, with estimated temperatures between 270 and 330°c and intermediate-pressure conditions. our results indicate that basin contraction and inversion processes related to the ordovician ocloyic orogeny involved at least two well-discriminated and not superposed metamorphic episodes in this region.