Cretaceous metamorphism and plutonism in the Santa Cruz Mountains, Salinian block, California, and correlation with the southernmost Sierra Nevada

Abstract

New data on the ages of plutonism and metamorphism in the northern Santa Cruz Mountains, part of the enigmatic Salinian block of California, clarify the area9s geologic history. Uranium/lead dates on zircon, monazite, sphene, and apatite and initial Pb and Sr isotopic ratios provide a fingerprint for comparison to the rest of the Salinian block and to other terranes. The U/Pb zircon dates of plutonic rocks in the Santa Cruz Mountains fall into three groups. Emplacement ages of the two younger, undeformed groups are 91 Ma and between 99 and 103 Ma. Individual zircon analyses range from concordant to significantly discordant but have simple crystallization histories, coupled with either inheritance of Precambrian zircon or Pb loss. The older group of plutons includes recrystallized granite to well-foliated orthogneisses that share the regional fabric. Analyses from the older group are slightly discordant, and multistage histories that include inheritance, Pb loss, and homogenization of Pb complicate interpretation of their age. Emplacement ages for the older group are Early Cretaceous—greater than about 103 Ma and probably less than 130 Ma. Foliated and unfoliated plutons from the older group appear to bracket the age of Salinian deformation and metamorphism between about 103 and 130 Ma. Lower-intercept ages for detrital zircon from schist and quartzite further support a Cretaceous metamorphic age. The detrital zircon population contains a component at least 1.6 Ga, probably close to 1.7 Ga. Uranium/lead cooling ages measured on sphene and apatite are 96 to 93 and 93 to 88 Ma, respectively. This cooling history appears to be a response to intrusion and rapid cooling of the youngest plutons. Common Pb and Sr initial isotopic ratios are radiogenic compared with ratios of oceanic rocks and the western parts of the Cordilleran batholiths, but they are the least radiogenic in the Salinian block and fit the local geographic gradient of Pb and Sr initial ratios. Linear variation of Pb-isotope ratios between radiogenic wall rock and the least radiogenic rocks and correlation of inherited zircon with elevated Pb-isotope ratios suggest that plutons assimilated wall rock. Isotopic data and geologic histories of basement rocks in the northern Santa Cruz Mountains fit the geographic patterns documented in the rest of the Salinian block. More intriguingly, the Santa Cruz Mountains plutons are strikingly similar to parts of the southern Sierra Nevada batholith. Ages of plutonism and metamorphism, U/Pb zircon discordance systematics, petrography, and initial Sr isotopes match closely between the Santa Cruz Mountains crystalline rocks and units in the southernmost Sierra Nevada batholith. These data reinforce the long-standing view that the Salinian block and Sierra Nevada batholiths were adjacent during their formation. Prior to right-slip of about 310 km on the San Andreas fault, the probable westward continuation of the low-angle Rand fault system into the southern Sierra Nevada and the Salinian block appears to have offset these two crystalline suites about 125 km.