The composite Sunrise Butte pluton, in the central part of the Blue Mountains Province, northeastern Oregon, preserves a record of subduction-related magmatism, arc-arc collision, crustal thickening, and deep-crustal anatexis. The earliest phase of the pluton (Desolation Creek unit) was generated in a subduction zone environment, as the oceanic lithosphere between the Wallowa and Olds Ferry island arcs was consumed. Zircons from this unit yielded a 206 Pb/ 238 U age of 160.2 ± 2.1 Ma. A magmatic lull ensued during arc-arc collision, after which partial melting at the base of the thickened Wallowa arc crust produced siliceous magma that was emplaced into metasedimentary rocks and serpentinite of the overthrust forearc complex. This magma crystallized to form the bulk of the Sunrise Butte composite pluton (the Sunrise Butte unit; 145.8 ± 2.2 Ma). The heat necessary for crustal anatexis was supplied by coeval mantle-derived magma (the Onion Gulch unit; 147.9 ± 1.8 Ma). The lull in magmatic activity between 160 and 148 Ma encompasses the timing of arc-arc collision (159–154 Ma), and it is similar to those lulls observed in adjacent areas of the Blue Mountains Province related to the same shortening event. Previous researchers have proposed a tectonic link between the Blue Mountains Province and the Klamath Mountains and northern Sierra Nevada Provinces farther to the south; however, timing of Late Jurassic deformation in the Blue Mountains Province predates the timing of the so-called Nevadan orogeny in the Klamath Mountains. In both the Blue Mountains Province and Klamath Mountains, the onset of deep-crustal partial melting initiated at ca. 148 Ma, suggesting a possible geodynamic link. One possibility is that the Late Jurassic shortening event recorded in the Blue Mountains Province may be a northerly extension of the Nevadan orogeny. Differences in the timing of these events in the Blue Mountains Province and the Klamath–Sierra Nevada Provinces suggest that shortening and deformation were diachronous, progressing from north to south. We envision that Late Jurassic deformation may have collapsed a Gulf of California–style oceanic extensional basin that extended from the Klamath Mountains (e.g., Josephine ophiolite) to the central Blue Mountains Province, and possibly as far north as the North Cascades (i.e., the coeval Ingalls ophiolite).

Nine U-Pb zircon ages were determined on plutonic rocks sampled from surface outcrops and rock chips of drill core from boreholes within the greater Los Angeles Basin region. In addition, lead-strontium-neodymium (Pb-Sr-Nd) whole-rock isotopic data were obtained for eight of these samples. These results help to characterize the crystalline basement rocks hidden in the subsurface and provide information that bears on the tectonic history of the myriad of fault systems that have dissected the Los Angeles region over the past 15 m.y. Seven of the nine samples have U-Pb ages ranging from 115 to 103 Ma and whole-rock Pb-Sr-Nd isotopic characteristics that indicate the crystalline basement underneath the greater Los Angeles Basin region is mostly part of the Peninsular Ranges batholith. Furthermore, these data are interpreted as evidence for (1) the juxtaposition of mid-Cretaceous, northern Peninsular Ranges batholith plutonic rocks against Late Cretaceous plutonic rocks of the Transverse Ranges in the San Fernando Valley, probably along the Verdugo fault; (2) the juxtaposition of older northwestern Peninsular Ranges batholith rocks against younger northeastern Peninsular Ranges batholith rocks in the northern Puente Hills, implying transposition of northeastern Peninsular Ranges batholith rocks to the west along unrecognized faults beneath the Chino Basin; and (3) juxtaposition of northern Peninsular Ranges batholith plutonic rocks against Late Cretaceous plutonic rocks of the Transverse Ranges along the San Jose fault in the northern San Jose Hills at Ganesha Park. These mainly left-lateral strike-slip faults of the eastern part of the greater Los Angeles Basin region could be the result of block rotation within the adjacent orthogonal, right-lateral, Elsinore-Whittier fault zone to the west and the subparallel San Jacinto fault zone to the east. The San Andreas fault system is the larger, subparallel, driving force further to the east.

Within the duration of the U.S. Geological Survey (USGS)–based Southern California Areal Mapping Project (SCAMP), many samples from the northern Peninsular Ranges batholith were studied for their whole-rock radioisotopic systematics (rubidium-strontium [Rb-Sr], uranium-thorium-lead [U-Th-Pb], and samarium-neodymium [Sm-Nd]), as well as oxygen (O), a stable isotope. The results of three main studies are presented separately, but here we combine them (>400 analyses) to produce a very complete Pb-Sr-Nd-O isotopic profile of an arc-continent collisional zone—perhaps the most complete in the world. In addition, because many of these samples have U-Pb zircon as well as argon mineral age determinations, we have good control of the timing for Pb-Sr-Nd-O isotopic variations. The ages and isotopic variations help to delineate at least four zones across the batholith from west to east—an older western zone (126–108 Ma), a transitional zone (111–93 Ma), an eastern zone (94–91 Ma), and a much younger allochthonous thrust sheet (ca. 84 Ma), which is the upper plate of the Eastern Peninsular Ranges mylonite zone. Average initial 87 Sr/ 86 Sr (Sr i ), initial 206 Pb/ 204 Pb ( 206 Pb i ), initial 208 Pb/ 204 Pb (average 208 Pb i ), initial epsilon Nd (average ε Ndi ), and δ 18 O signatures range from 0.704, 18.787, 38.445, +3.1, and 4.0‰–9.0‰, respectively, in the westernmost zone, to 0.7071, 19.199, 38.777, −5, and 9‰–12‰, respectively, in the easternmost zone. The older western zone is therefore the more chemically and isotopically juvenile, characterized mostly by values that are slightly displaced from a mantle array at ca. 115 Ma, and similar to some modern island-arc signatures. In contrast, the isotopic signatures in the eastern zones indicate significant amounts of crustal involvement in the magmatic plumbing of those plutons. These isotopic signatures confirm previously published results that interpreted the Peninsular Ranges batholith as a progressively contaminated magmatic arc. The Peninsular Ranges batholith magmatic arc was initially an oceanic arc built on Panthalassan lithosphere that eventually evolved into a continental margin magmatic arc collision zone, eventually overriding North American cratonic lithosphere. Our Pb-Sr-Nd data further suggest that the western arc rocks represent a nearshore or inboard oceanic arc, as they exhibit isotopic signatures that are more enriched than typical mid-ocean-ridge basalt (MORB). Isotopic signatures from the central zone are transitional and indicate that enriched crustal magma sources were becoming involved in the northern Peninsular Ranges batholith magmatic plumbing. As the oceanic arc–continental margin collision progressed, a mixture of oceanic mantle and continental magmatic sources transpired. Magmatic production in the northern Peninsular Ranges batholith moved eastward and continued to tap enriched crustal magmatic sources. Similar modeling has been previously proposed for two other western margin magmatic arcs, the Sierra Nevada batholith of central California and the Idaho batholith. Calculated initial Nd signatures at ca. 100 Ma for Permian–Jurassic and Proterozoic basement rocks from the nearby San Gabriel Mountains and possible source areas along the southwestern Laurentian margin of southern California, southwestern Arizona, and northern Sonora strongly suggest their involvement with deep crustal magma mixing beneath the eastern zones of the Peninsular Ranges batholith, as well as farther east in continental lithospheric zones. Last, several samples from the allochthonous, easternmost upper-plate zone, which are considerably younger (ca. 84 Ma) than any of the rocks from the northern Peninsular Ranges batholith proper, have even more enriched average Sr i , 206 Pb i , 208 Pb i , and ε Ndi signatures of 0.7079, 19.344, 38.881, and −6.6, respectively, indicative of the most-evolved magma sources in the northern Peninsular Ranges batholith and similar to radioisotopic values for rocks from the nearby Transverse Ranges, suggesting a genetic connection between the two.

We studied the formation of low-initial-Sr felsic plutons by using data from the Early Cretaceous western Peninsular Ranges batholith near Escondido, California. The systematically sampled Escondido plutons have a uniformly low initial 87 Sr/ 86 Sr isotope ratio of Sr i < 0.704, but a wide range of SiO 2 compositions, from 46 to 78 wt%, which fall in three distinct groups: 20% gabbros, 35% tonalites, and 45% granodiorites. These low-Sr i plutons are unique in having undergone one cycle of mantle melting to give basalt composition rocks, and a second cycle of arc basalt melting to give a range of SiO 2 plutons, but no third cycle of melting and contamination by old continental crust to yield high-Sr i rocks. After doing two-cycle partial melting and fractional crystallization calculations, it was recognized that mixing of gabbro and granodiorite magmas was necessary to yield the tonalites. The linear data pattern on Harker diagrams is interpreted as resulting from mixing of mafic magma from partial melting of the mantle and felsic magma from partial melting of the lower crust to form intermediate magma. These plutons provide a simplified two-cycle Phanerozoic example of the petrogenetic process for forming continental crust.

Metamorphosed silicic volcanic and hypabyssal rocks of middle Cretaceous (110 to 100 Ma) age occur in two roof pendants in the Kings Canyon area of the central Sierra Nevada. The metavolcanic remnants are similar in age to or are only slightly older than the voluminous enclosing batholithic rocks. Thus, high to surface levels of the batholith are implied for this region. This is interesting considering that deep-level (∼25 km) batholithic rocks of the same age as the metavolcanic rocks occur at the southern end of the range. Apparent structural continuity between these two regions suggests that the southern half of the range offers an oblique section through young (˜100 Ma) sialic crust. The middle Cretaceous ages of the two volcanic sequences are indicated by U/Pb zircon and Rb/Sr bulk-rock isochron data. The two isotopic systems agree very closely with one another. Some of the U/Pb systems within the Boyden Cave pendant are discordant due to the inheritance or entrainment of Proterozoic zircon. This is a common phenomenon in volcanic or plutonic rocks erupted or emplaced within the Kings sequence metamorphic framework, a belt of distinct pendants with abundant continent-derived sedimentary protoliths. In conjunction with other petrochemical parameters, lavas and magmas of this framework domain are shown to be contaminated with sedimentary admixtures. The contaminated domain of the batholith reflects the bounds of the Kings sequence framework, which along its eastern margin probably represents a major pre-batholith to early batholith tectonic break. The middle Cretaceous metavolcanic sequences were apparently built on two distinctly different early Mesozoic substrates separated by a major tectonic break. In the Boyden Cave pendant, the substrate may be represented by the shallow to deep-marine Kings sequence; to the east in the Oak Creek pendant, the substrate consists of a thick silicic ignimbrite sequence. In both areas the middle Cretaceous rocks and adjacent sequences share intense ductile deformation fabrics. Earlier views that considered these fabrics as an expression of Jurassic orogenic deformation are in error. Structural and age relations indicate that the fabrics developed between 105 and 100 Ma and during the medial phases of Cretaceous composite batholith growth.

Chemical and isotopic characteristics of plutons in the western United States reflect compositions and protoliths of subjacent source materials. A discontinuously exposed shear zone that extends along the length of the Sierra Nevada in California marks a boundary between two areas manifested geologically by wall-rock and roof-pendant lithologies of different ages, depositional environments, and structural histories. In addition, plutons on either side of the boundary have different chemical and isotopic compositions, which indicate that their source regions are of two fundamentally different lithosphere types. The western lithosphere type is called Panthalassan, whereas the eastern type is called North American. Isotopic investigations of plutons have defined an initial 87 Sr/ 86 Sr (Sr i ) = 0.706 line in each lithosphere type. However, δ 18 O more than +9 per mil in plutons with Sr i greater than 0.706 in the Panthalassan lithosphere indicates a significantly greater sedimentary component in the source materials for these plutons than for those plutons with similar Sr i but δ 18 O less than +9 per mil intruded into North American lithosphere. In contrast to the North American lithosphere, there is no evidence that a Proterozoic crystalline sialic basement exists where plutons have Sr i greater than 0.706 in the Panthalassan lithosphere. Instead, the plutons with Sr i greater than 0.706 intruded into Panthalassan lithosphere probably acquired that characteristic by assimilation of sediments derived from a Proterozoic sialic crust. Plutons with Sr i less than 0.706 have chemical and Nd isotopic characteristics that indicate time-integrated depletion in large ion lithophile elements in their source regions in the Panthalassan lithosphere relative to their sources in the North American lithosphere. The tectonic contact between the two lithosphere types may be the extension of the Sonora-Mojave megashear into northern California.

Potassium-argon ages of minerals are reported for 29 specimens of flows and tuffs from volcanic rocks that crop out in Nye and Esmeralda Counties, Nevada. Formation or group Epoch Range of K/Ar ages (m.y.) Range of K concentration (wt. percent) Biotite Alkali feldspar Thirsty Canyon Tuff Pliocene 6.2 to 7.5 absent 5.14 to 5.49 Piapi Canyon Group and Indian Trail Formation Pliocene t o Miocene 10.5 to 16 6.19 to 6.76 4.86 to 5.97 Older volcanic rocks Miocene 18 to 26 6.76 to 7.21 8.80 Precision of the age determinations on biotite and sanidine from these rocks is estimated by three sets of data (1) Duplicate analysis for argon and potassium; σ = 0.9 to 3.6 percent. (2) Age determinations of mineral pairs from seven specimens; σ = 1.2 to 3.0 percent. (3) Age determinations from 5, 4, and 2 widely separated localities in three welded tuffs; σ = 2.3 to 6.5 percent, (σ = population standard deviation at 95 percent confidence level.) For purposes of stratigraphie correlation, the precision estimates indicate that in order to be able to distinguish between two rocks at the 95 percent confidence level, their ages must differ by 3.4 to 8.5 percent if each rock is dated by a mineral pair. In contrast to the diversity reported for chemical compositions of the volcanic rocks dated, the potassium concentrations in biotite and sanidine from pyroclastic units in each of the three sequences appear to be remarkably consistent. A check of the published K/Ar ages in Nevada and Utah suggests that this characteristic of minerals in pyroclastic rocks is widespread.