Cretaceous metamorphism and plutonism in the Santa Cruz Mountains,Salinian Block, California, and correlation with the southernmostSierra Nevada

Abstract

New data on the ages of plutonism and metamorphism in the northernSanta Cruz Mountains, part of the enigmatic Salinian block ofCalifornia, clarify the area's geologic history. Uranium/lead dates onzircon, monazite, sphene, and apatite and initial Pb and Sr isotopicratios provide a ""fingerprint"" for comparison to the rest of theSalinian block and to other terranes.The U/Pb zircon dates of plutonicrocks in the Santa Cruz Mountains fall into three groups. Emplacementages of the two younger, undeformed groups are 91 Ma and between 99and 103 Ma. Individual zircon analyses range from concordant tosignificantly discordant but have simple crystallization histories,coupled with either inheritance of Precambrian zircon or Pb loss. Theolder group of plutons includes recrystallized granite towell-foliated orthogneisses that share the regional fabric. Analysesfrom the older group are slightly discordant, and multistage historiesthat include inheritance, Pb loss, and homogenization of Pb complicateinterpretation of their age. Emplacement ages for the older group areEarly Cretaceous-greater than about 103 Ma and probably less than 130Ma.Foliated and unfoliated plutons from the older group appear tobracket the age of Salinian deformation and metamorphism between about103 and 130 Ma. Lower-intercept ages for detrital zircon from schistand quartzite further support a Cretaceous metamorphic age. Thedetrital zircon population contains a component at least 1.6 Ga,probably close to 1.7 Ga. Uranium/lead cooling ages measured on spheneand apatite are 96 to 93 and 93 to 88 Ma, respectively. This coolinghistory appears to be a response to intrusion and rapid cooling of theyoungest plutons.Common Pb and Sr initial isotopic ratios areradiogenic compared with ratios of oceanic rocks and the western partsof the Cordilleran batholiths, but they are the least radiogenic inthe Salinian block and fit the local geographic gradient of Pb and Srinitial ratios. Linear variation of Pb-isotope ratios betweenradiogenic wall rock and the least radiogenic rocks and correlation ofinherited zircon with elevated Pb-isotope ratios suggest that plutonsassimilated wall rock.Isotopic data and geologic histories of basementrocks in the northern Santa Cruz Mountains fit the geographic patternsdocumented in the rest of the Salinian block. More intriguingly, theSanta Cruz Mountains plutons are strikingly similar to parts of thesouthern Sierra Nevada batholith. Ages of plutonism and metamorphism,U/Pb zircon discordance systematics, petrography, and initial Srisotopes match closely between the Santa Cruz Mountains crystallinerocks and units in the southernmost Sierra Nevada batholith. Thesedata reinforce the long-standing view that the Salinian block andSierra Nevada batholiths were adjacent during their formation. Priorto right-slip of about 310 km on the San Andreas fault, the probablewestward continuation of the low-angle Rand fault system into thesouthern Sierra Nevada and the Salinian block appears to have offsetthese two crystalline suites about 125 km.

11 samples from this reference