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

Crustal xenoliths from basanitic dikes and necks that intruded into contine ntal sediments of the Cretaceous Salta Rift at Quebrada de Las Conchas, Pro vincia Salta, Argentina were investigated to get information 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-feldsp ar as major minerals. The exceedingly rare mafic xenoliths consist of plagi oclase-clinopyroxene-garnet +/- hornblende. All xenoliths show a well equil ibrated granoblastic fabric and the minerals are compositionally unzoned. T hermobarometric calculations indicate equilibration of the mafic xenoliths in the granulite facies at temperatures of ca. 900 degrees 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 ( mafic xenolith). These ages are in agreement with the age of basanitic volc anism (ca. 130-100 and 80-75 Ma) and are interpreted as minimum ages of met amorphism. Lower crustal temperature at the time given by the isochrons was above the closure temperature of the Sm-Nd system (>600-700 degrees C). Th e Sm-Nd and Rb-Sr isotopic signatures (Sm-147/Nd-144 = 0.1225-0.1608; Nd-14 3/Nd-144(t0) = 0.512000-0.512324; Rb-87/Sr-86 = 0.099-0.172; Sr-87/Sr-86(t0 ) = 0.708188-0.7143161) and common lead isotopic signatures (Pb-206/Pb-204 = 18.43-18.48; Pb-207/Pb-204 = 15.62-15.70; Pb-208/Pb-204 = 38.22-38.97) of the granitoid xenoliths are indistinguishable from the isotopic compositio n of the Early Paleozoic metamorphic basement from NW Argentina, apart from the lower Pb-208/Pb-204 ratio of the basement. The Sm-Nd depleted mantle m odel ages of ca. 1.8 Ga from granitoid xenoliths and Early Paleozoic baseme nt point to a similar Proterozoic protolith. Time constraints, the well equ ilibrated granulite fabric, P-T conditions and lack of chemical zoning of m inerals point to a high temperature in a crust of nearly normal thickness a t ca. 90 Ma and to a prominent thermal anomaly in the lithosphere. The comp osition of the xenoliths is similar to the composition of the Early Paleozo ic basement in the Andes of NW Argentina and northern Chile. A thick mafic lower crust seems unlikely considering low abundance of mafic xenoliths and the predominance of granitoid xenoliths.