ISOTOPIC COMPOSITION OF GRANITOIDS VERSUS VOLCANIC ROCKS IN NORTHERN NEVADA: MAGMA SOURCES, BASEMENT PROVINCES, AND THE JURASSIC FACTOR

Many studies of the radiogenic isotopic composition of granitoids in northern Nevada have shown a west-to-east increase in 87Sr/86Sr and decrease in 143Nd/144Nd, interpreted to reflect a change in basement from Phanerozoic (west) to Precambrian (east) marked by the 0.706 Sr line and parallel eNd = -7 line. With rare exception, little attention has been paid to correlations between isotopic composition and the age of granitoid plutons. The plutonic record includes Jurassic-Early Cretaceous (“JEC”, 170-100 Ma), Late Cretaceous (“LC”, 85-75 Ma), and Cenozoic (“CZ”, < 45 Ma, accompanied by volcanic activity). Wright and Wooden (1991) showed that JEC plutons are distinct isotopically from LC and CZ plutons, that eNd values in JEC plutons can be less negative than -7 on either side of the eNd = -7 line, and that an Archean basement terrane underlies northeastern Nevada. Here, we present major, trace element, and Sr-Nd isotopic data for JEC, LC, and CZ intrusive rocks in two areas: Majuba Hill and the East Range in NW Nevada (Phanerozoic basement) and the Pequop Mountains area of NE Nevada (Archean basement). Sampled plutons (with mafic inclusions) from the East Range include the JEC Lee Peak and Inskip Canyon, and CZ Granite Mountain plutons, which are mostly metaluminous. The East Range CZ samples have fairly uniform initial isotope ratios (Sr ~ 0.705; eNd +4 to -4), but the JEC samples are highly heterogeneous (Sr 0.705-0.710; eNd =5 to -12). Isotope ratios do not correlate with SiO2 content or peraluminosity. At Majuba Hill, JEC, LC and CX samples cover a comparatively small isotopic range (Sr 0.704 to 0.706; eNd +1 to +3). The Pequop Mountains study area is mostly underlain by Archean basement. Pequop JEC are metaluminous to peraluminous, and have the lowest initial Sr (0.703-0.706) and least negative eNd (-1 to -4). Pequop LC are muscovite-garnet bearing and have the highest Sr (> 0.713) and most negative eNd (-21). Pequop CZ are metaluminous and have intermediate initial ratios (Sr 0.708 – 0.713; eNd (-7.4 to -21.2). The LC and CZ samples from both study areas follow the geographic/isotopic patterns identified in previous studies in northern Nevada: low Sr and less negative eNd in the west, high Sr and low eNd in the east. Jurassic intrusions do not follow this pattern, and should not be used in drawing isotopic (tectonic) boundaries.