Geochronology, Geochemistry and Petrology of the Precambrian Sandia Granite, Albuquerque, New Mexico.

The Precambrian Sandia granite of north-central New Mexico belongs to the 1.2 - 1.6 Ga. crustal province of the western USA. The granite shows an intrusive contact with the metamorphic country rocks. The Rb-Sr whole rock isochron age of the country rocks is 1.61 (+OR-) 0.06 Ga, ((\u2787)Sr/(\u2786)Sr)(,o) = 0.705 (+OR-) 0.001. The culmination of the intrusion of the Sandia granite took place at 1.44 (+OR-) 0.04 Ga. ((\u2787)Sr/(\u2786)Sr)(,o) = 0.7054 (+OR-) 0.0005. Rb-Sr ages on biotite - whole rock pairs and an (\u2740)Ar/(\u2739)Ar dating of a biotite from the granite indicate final cooling to 300(DEGREES)-350(DEGREES)C at 1.33 Ga. This suggests slow cooling of the granite at rates which averged 4(DEGREES)C/Ma for about 100 Ma after its emplacement; during this period the Rb-Sr isotope system perhaps remained partially open. The Sandia granite shows compositional variation from granodiorite to quartz monzonite in both the northern and southern blocks. The field geology, petrology and geochemistry of the two blocks suggest that they form a single pluton. Both the Sandia granite and the mesocratic, two pyroxene granulite xenoliths therein give an (delta)(\u2718)0 value of +8.0 (+OR-) 0.5% indicating (meta)igneous source rocks for each of them. These values tend to rule out Condie\u27s (1978) favored hypothesis for magma generation of 30-50% partial melting of siliceous gradulites. Rather, they favor an alternative hypothesis, equally satisfactory from the geochemical viewpoint, involving 5-10% melt of a gabbroic (Condie, 1978) or two-pyroxene granulite parent rock in the lower crust. The Sandia granite, and by implication, the other high-Ca granitic rocks of the western USA (Condie and Budding, 1979) thus do not seem to represent addition of new mantle-derived materials to the middle-late Proterozoic crust of this section of the continent