Very low grade metamorphism in the Precuyano Unit, Neuquén Basin, Argentina

RUBINSTEIN, N. BEVINS, R. ROBINSON, D. Y SRUOGA, P.

Kiel

Congreso; 20th Colloquium on Latin American Earth Sciences; 2007

Deutsche Forschungsgemeinschaft-Universidad de Kiel

Very low grade metamorphism in the Precuyano Unit, Neuquén Basin, Argentina The Neuquén Basin covers more than 160,000 km2 between 30°and 40°S in the western Andean Cordillera. It developed as an ensialic intra–arc and back–arc basin from early Jurassic times and contains a thick Mesozoic–Cenozoic sedimentary succession. The onset of an extensional regime related to the initial stages of the break–up of Gondwana can be identified as early as latest Triassic times. The widespread rift system comprised several elongate troughs with a sedimentary and volcanic infill of more than 2000 m. These subparallel hemi–grabens with alternating polarity were affected by tectonic inversion during the Andean Orogenic Cycle. Several periods of tectonic reactivation took place during Triassic–Jurassic extension, controlling the characteristics and distribution of the syn–rift facies. The initial infill of these depocenters consisted of bimodal volcanic, volcaniclastic and continental epiclastic rocks, grouped in the so–called Precuyano Unit, dated as developing between 219 and 182 Ma. This unit is characterized by lateral lithological variations, including tuffs, ignimbrites, rhyolites and epiclastic rocks, and has produced oil and gas since 1960. The study of chip samples from the Granito Negro x–1 oil producer borehole, fig.1, reveals a stratigraphic sequence that is remarkably different from the rest of the equivalent sections of the Nequén Basin, which is probably linked to the first stages of the rifting, Rubinstein et al., 2005. This is composed mainly of alkaline basaltic lavas, along with minor interbedded rhyolitic lavas, ignimbritic deposits and organic matter–bearing lacustrine shales. The lavas are vesicular, with porphyritic textures and intergranular to hyalopilitic growndmasses. Two different types of basalts are present, one dominated by plagioclase feldspar with minor clinopyroxene and scarce olivine, the other dominated by plagioclase feldspar and opaque minerals with minor clinopyroxene. They show variable alteration in the studied section, depending on their vesicularity and their primary glass content. The alteration paragenesis is dominated by chlorite along with iron oxides, carbonates, titanite, epidote, iddingsite–bowlingite, along with minor prehnite and pumpellyite, as well as very scarce albite, zeolites and quartz occurring as a replacement of intersticial areas, primary minerals, and infilling of vesicules. This paragenesis corresponds to very low–grade metamorphism in the prehnite– pumpellyite facies. Some 48 samples from the basaltic lavas were examined in this study for their secondary mineralogy by petrographic methods, from a total depth range of 980 m. Three samples were investigated further by electron microprobe techniques, from depths of 2626 m, 2788 m, and 3450 m. All show a consistent assemblage, including especially the metamorphic grade indicator minerals prehnite, pumpellyite and epidote; critically, actinolite is absent. This, combined with the evidence from the epidote chemographic projection, which shows broadly parallel tie–lines for chlorite–pumpellyite in the three samples with KD Mg–Fe ratios in the range 0.26–0.84,1 suggests that metamorphic conditions for the section were at sub–greenschist levels. From the studies of Frey et al. 1991, this would suggest temperatures in the range 175–275°C. Critically, these temperatures are well above those for the oil–gas window. In relation to the overall model, the sub– greenschist facies metamorphism agrees with an extensional tectonic environment, generating higher heatflow as a result of crustal thinning and raised thermal gradients, as proposed for the Welsh Basin by Bevins and Robinson1993. In this way we suggest that the metamorphism was contemporaneous with basin development and prior to oil migration.2 between 30°and 40°S in the western Andean Cordillera. It developed as an ensialic intra–arc and back–arc basin from early Jurassic times and contains a thick Mesozoic–Cenozoic sedimentary succession. The onset of an extensional regime related to the initial stages of the break–up of Gondwana can be identified as early as latest Triassic times. The widespread rift system comprised several elongate troughs with a sedimentary and volcanic infill of more than 2000 m. These subparallel hemi–grabens with alternating polarity were affected by tectonic inversion during the Andean Orogenic Cycle. Several periods of tectonic reactivation took place during Triassic–Jurassic extension, controlling the characteristics and distribution of the syn–rift facies. The initial infill of these depocenters consisted of bimodal volcanic, volcaniclastic and continental epiclastic rocks, grouped in the so–called Precuyano Unit, dated as developing between 219 and 182 Ma. This unit is characterized by lateral lithological variations, including tuffs, ignimbrites, rhyolites and epiclastic rocks, and has produced oil and gas since 1960. The study of chip samples from the Granito Negro x–1 oil producer borehole, fig.1, reveals a stratigraphic sequence that is remarkably different from the rest of the equivalent sections of the Nequén Basin, which is probably linked to the first stages of the rifting, Rubinstein et al., 2005. This is composed mainly of alkaline basaltic lavas, along with minor interbedded rhyolitic lavas, ignimbritic deposits and organic matter–bearing lacustrine shales. The lavas are vesicular, with porphyritic textures and intergranular to hyalopilitic growndmasses. Two different types of basalts are present, one dominated by plagioclase feldspar with minor clinopyroxene and scarce olivine, the other dominated by plagioclase feldspar and opaque minerals with minor clinopyroxene. They show variable alteration in the studied section, depending on their vesicularity and their primary glass content. The alteration paragenesis is dominated by chlorite along with iron oxides, carbonates, titanite, epidote, iddingsite–bowlingite, along with minor prehnite and pumpellyite, as well as very scarce albite, zeolites and quartz occurring as a replacement of intersticial areas, primary minerals, and infilling of vesicules. This paragenesis corresponds to very low–grade metamorphism in the prehnite– pumpellyite facies. Some 48 samples from the basaltic lavas were examined in this study for their secondary mineralogy by petrographic methods, from a total depth range of 980 m. Three samples were investigated further by electron microprobe techniques, from depths of 2626 m, 2788 m, and 3450 m. All show a consistent assemblage, including especially the metamorphic grade indicator minerals prehnite, pumpellyite and epidote; critically, actinolite is absent. This, combined with the evidence from the epidote chemographic projection, which shows broadly parallel tie–lines for chlorite–pumpellyite in the three samples with KD Mg–Fe ratios in the range 0.26–0.84,1 suggests that metamorphic conditions for the section were at sub–greenschist levels. From the studies of Frey et al. 1991, this would suggest temperatures in the range 175–275°C. Critically, these temperatures are well above those for the oil–gas window. In relation to the overall model, the sub– greenschist facies metamorphism agrees with an extensional tectonic environment, generating higher heatflow as a result of crustal thinning and raised thermal gradients, as proposed for the Welsh Basin by Bevins and Robinson1993. In this way we suggest that the metamorphism was contemporaneous with basin development and prior to oil migration.