Isotopic U-Pb ages of zircon from the granitoids of the central Sierra Nevada, California

Abstract

Sixty-two samples from well-established comagmatic granitoid sequences and certain unassigned formations and plutons of the central part of the Sierra Nevada batholith between latitudes 37° and 38° N. have been dated by the isotopic U-Pb method on zircon. The U-Pb ages indicate the following age distribution of the granitoids: (1) The axial part of the batholith is occupied by Cretaceous granitoid sequences that are progressively younger eastward over a 37-m.y. interval extending from about 125 m.y. to about 88 m.y. ago. (2) A single, but extensive, Triassic sequence with an optimum average age of about 210 m.y. is present in the east side of the batholith. (3) Plutons and granitoid sequences of Jurassic age, most of them with U-Pb ages between 186 and 155 m.y., occur in both margins and locally in the interior of the batholith. The distribution of Jurassic ages suggests that prior to the emplacement of the Cretaceous granitoids, Jurassic granitoids were widely distributed across the central Sierra Nevada but were not emplaced in a west-to-east succession as were the Cretaceous granitoids. Few of our ages fall between 155 and 125 m.y. However, a U-Pb age of 144 m.y. has been reported on the Sage Hen Flat pluton in the White Mountains, and U-Pb ages between 134 and 128 m.y. have been reported on remnants of older granitoids farther south in the Sierra Nevada, which are associated with roof pendants and septa. Also, numerous K-Ar ages on hornblende in the range of 152 to 131 m.y. have been reported on samples collected farther north along the west side of the batholith. The distribution of U-Pb ages is consistent with the interpretation that in the central Sierra Nevada, a belt of Cretaceous granitoids trending about N. 20° W. crosses a belt of Jurassic granitoids trending about N. 40° W. However, the U-Pb ages provide little support for the existence of five cyclic intrusive epochs for California and western Nevada. Comparison of the U-Pb ages on zircon with the K-Ar ages on biotite and hornblende shows generally good agreement for the younger granitoids but decreasing agreement for increasingly older granitoids. Most of the K-Ar ages on biotite and many on hornblende from older granitoids appear to have been reduced as a result of reheating by younger plutons. The dispersion of K-Ar ages reflects the complex structural and thermal history of the batholith.

Guadalupe pluton–Mariposa Formation age relationships in the southern Sierran Foothills: Onset of Mesozoic subduction in northern California?

“…The Sierran and Great Valley belts formed along the western margin of the North American plate, whereas the Franciscan was laid down on the approaching Farallon oceanic lithosphere. In northern California, the production of juvenile and reworked sialic crust was most voluminous in mid and Late Cretaceous time [Stern et al, 1981;Chen and Moore, 1982], but erosional debris from the dying calcalkaline arc was supplied to the fore arc and trench into Paleogene time [McLaughlin et al, 1982].…”

“…Dumitru et al (Early Cretaceous (circa 123 Ma) transition from nonaccretion behavior to strongly accretionary behavior within the Franciscan subduction complex, submitted to Tectonics, 2009) have suggested that the convergent margin was transformed from a nonaccretionary to an accretionary margin at $123 Ma. In any case, subsequent evolution of the continental margin involved the construction of three subparallel lithotectonic belts: (1) a massive Sierran batholith and its comagmatic but in-part earlier volcanic carapace [Stern et al, 1981;Bateman, 1992;Dunne et al, 1998;Irwin, 2003]; (2) a thick turbiditic section accumulating in the fore-arc basin as the Great Valley Group [Ingersoll, 1978[Ingersoll, , 1979[Ingersoll, , 1983Linn et al, 1992;DeGraaff-Surpless et al, 2002;Surpless et al, 2006;Wright and Wyld, 2007]; and (3) the rapidly deposited graywackes and intercalated dark shales of the Franciscan Complex in the offshore trench [Bailey et al, 1964[Bailey et al, , 1970Hamilton, 1969 et al, 2009]. The Sierran and Great Valley belts formed along the western margin of the North American plate, whereas the Franciscan was laid down on the approaching Farallon oceanic lithosphere.…”

“…[19] A Middle Jurassic episode of calcalkaline volcanismplutonism in the western Klamath Mountains and the Sierra Nevada and White-Inyo ranges, as well as fragmentary relics of the Red Ant blueschists in the northern Sierran Foothills [Stern et al, 1981;Wright and Fahan, 1988;Hacker and Goodge, 1990;Dunne et al, 1998;Ernst et al, 2002;Dickinson, 2008] suggest an important subduction component of transpression at $170-150 Ma; nevertheless, dominant ophiolite + chert-argillite rock units were stranded along the edge of the continent due to largely conservative plate motions during the mid-PaleozoicMiddle Jurassic assembly of northern California [Saleeby, 1981[Saleeby, , 1982[Saleeby, , 1983Silberling et al, 1987;Irwin, 2003]. The oceanic terrane collage was capped by terrigenous sediments and volcanics derived from an incipient arc; the latter must have started well before $160 Ma, more likely $170 Ma [Wright and Fahan, 1988;Dunne et al, 1998].…”

“…If the resultant calcalkaline arc and apron of derived volcanogenic sediments initially formed as a continuous curvilinear belt, this time interval evidently attended a progressive $200 km outboard migration of the Klamath terrane amalgam relative to the Sierra Nevada . Initiation of volcanism-plutonism and the clastic detritus shed oceanward from the growing arc include Middle and Late Jurassic granitoids [Stern et al, 1981;Bateman, 1992;Dunne et al, 1998;Dickinson, 2008] and their outboard erosional products such as the western Klamath Galice Formation [Harper et al, 1994;Miller and Saleeby, 1995;Gray, 2006] and the western Sierran Foothills Mariposa Formation [Sharp, 1988;Edelman and Sharp, 1989], both of OxfordianKimmeridgian age. Dating the onset of construction of this Upper Jurassic sequence in terms of Mariposa sedimentation, based on macrofossils and microfossils reported by Imlay [1961], Clark [1964], and Graymer and Jones [1994], may well define the change from earlier, chiefly strike-slip plate motion to later transpression involving a major component of lithospheric convergence along the Californian sector of the North American margin.…”

Late Cretaceous U/Pb zircon ages and Precambrian crustal inheritance in Ryoke granitoids, Kinki and Yanai districts, Japan.

“…1, 2, and 3). U/Pb zircon geochronology is the best tech nique for obtaining high-precision dates for gra nitic rocks to establish their crystallization ages (Stern et al, 1981).…”