Very low- to low-grade polimetamorphism in Jurassic-Cretaceous volcanic rocks to the south of Cholila, Chubut, Argentinian Patagonia

Abstract

In this paper we study the polymetamorphic processes of very low-grade metamorphism (VLGM) in subgreenschists facies to low-grade metamorphism (LGM) affecting the calcalkaline volcanic sequence of Middle or Upper Jurassic/Lower Cretaceous age in the North Patagonian Andes (Lat 42°30´–43° S), south of the Cholila locality, Chubut Province, Argentinian Patagonia. The sequence, deposited in marginal basins, is intruded by granitic rocks of Early Cretaceous age. The protolith is basaltic and andesitic in composition; the secondary minerals were deposited replacing the matrix, phenocrysts, infilling veins and amygdales.

Three metamorphic processes are recognized. The first regional process that affected the sequence occurred in a non- deformational environment of extensional type, with moderate to steep thermal gradient, and high ƒO₂. In this first stage, greenschists-facies temperatures of >300 °C at 2 kb are indicated by the observed assemblage of epidote–chlorite–actinolite–albite–quartz–magnetite–titanite +grandite (without pumpellyite nor prehnite), which formed at high XCO₂/XH₂O. Temperature and pressure then decreased to subgreenschists facies of VLGM in pumpellyite–actinolite facies with assemblages of pumpellyite– actinolite–epidote (without prehnite) and in prehnite–pumpellyite facies (with epidote–chlorite–albite– quartz–pumpellyite ± prehnite). In the northern area, the assemblage epidote, albite, chlorite, adularia, laumontite and scarce pumpellyite and prehnite indicates the zeolite facies.

Nearer to Cholila locality, there is evidence of a second stage of VLGM, probably related to the inversion of previous faults and to the uplift of the area in Upper Cretaceous times; the increased temperatures in this stage were generated by fluids related to magmatism. The presence of wairakite and yugawaralite together with laumontite is in accordance with a geothermal-type VLGM in the high- temperature zeolite facies. The equilibrium assemblage wairakite–yugawaralite–laumontite indicates P–T conditions of <220 °C and <0.5 kb. In the first two processes, the neutral to slightly alkaline hydrothermal fluids, the high PH₂O/P_total ratio, and the high ƒO₂ conditions in the stability field of hematite, favored the formation of calcsilicates and calcic zeolites. Later, new fractures affected the rocks and facilitated the circulation of new, more alkaline fluids; in some areas, fluids with acidic pH promoted the formation of illite and sericite, but at neutral pH, barrerite, offretite and smectite were deposited in amygdales at temperatures below 180 °C.