Metamorphism, isotopic ages and composition of lower crustal granulite xenoliths from the Cretaceous Salta Rift, Argentina

LUCASSEN F.; LEWERENZ S.; FRANZ G.; VIRAMONTE JOSÉ G.; MEZGER K.

CONTRIBUTIONS TO MINERALOGY AND PETROLOGY

SPRINGER

Lugar: Berlin; Año: 1998 p. 325 - 341

0010-7999

Crustal xenoliths from basanitic dikes and necks that intruded into continental sediments of theCretaceous Salta Rift at Quebrada de Las Conchas, Provincia Salta, Argentina were investigated to get in- formation about the age and the chemical composition of the lower crust. Most of the xenoliths have a granitoid composition with quartz-plagioclase-garnet-rutile ‹ K- feldspar as major minerals. The exceedingly rare ma®c xenoliths consist of plagioclase-clinopyroxene-garnet‹hornblende. All xenoliths show a well equilibrated granoblastic fabric and the minerals are compositionally unzoned. Thermobarometric calculations indicate equilibration of the ma®c xenoliths in the granulite facies at temperatures of ca. 900 °C and pressures of ca. 10 kbar. The Sm-Nd mineral isochron ages are 95.1 ‹ 10.4 Ma, 91.5 ‹ 13.0 Ma, 89.0 ‹ 4.2 Ma (granitoid xenoliths), and 110.7 ‹ 23.6 Ma (ma®c xenolith). These ages are in agreement with the age of basanitic volcanism (ca. 130± 100 and 80±75 Ma) and are interpreted as minimum ages of metamorphism. Lower crustal temperature at the time given by the isochrons was above the closure temperature of the Sm-Nd system (>600±700 °C). The Sm-Nd and Rb-Sr isotopic signatures (147Sm/144Nd ˆ 0.1225± 0.1608; 143Nd/144Ndt0 ˆ 0.512000±0.512324; 87Rb/86Sr 0.099±0.172; 87Sr/86Srt0 ˆ 0.708188±0.7143161) and common lead isotopic signatures (206Pb/204Pb ˆ 18.43± 18.48; 207Pb/204Pb ˆ 15.62±15.70; 208Pb/204Pb ˆ 38.22±38.97) of the granitoid xenoliths are indistinguishable from the isotopic composition of the Early Paleozoic metamorphic basement from NW Argentina, apart from the lower 208Pb/204Pb ratio of the basement. The Sm-Nd depleted mantle model ages of ca. 1.8 Ga from granitoid xenoliths and Early Paleozoic basement point to a similar Proterozoic protolith. Time constraints, the well equili brated granulite fabric, P-T conditions and lack of chemical zoning of minerals point to a high temperature in a crust of nearly normal thickness at ca. 90 Ma and to a prominent thermal anomaly in the lithosphere. The composition of the xenoliths is similar to the composition of the Early Paleozoic basement in the Andes of NW Argentina and northern Chile. A thick ma®c lower crustseems unlikely considering low abundance of ma®c