Geology topics

Ronald W. Kistler

Publications and source records attributed to Ronald W. Kistler.

7 recordsLinked to original sources

USGS2014

Age and isotopic systematics of Cretaceous borehole and surface samples from the greater Los Angeles Basin region: Implications for the types of crust that might underlie Los Angeles and their distribution along late Cenozoic fault systems

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.

Book chapter↗

USGS2014

Pb-Sr-Nd-O isotopic characterization of Mesozoic rocks throughout the northern end of the Peninsular Ranges batholith: Isotopic evidence for the magmatic evolution of oceanic arc–continental margin accretion during the Late Cretaceous of southern California

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.

Book chapter↗

USGS1995

The Grayback Pluton: Magmatism in a Jurassic back-arc environment, Klamath Mountains, Oregon

The Jurassic Grayback pluton was emplaced in a back-arc setting behind a contemporaneous oceanic arc. Th\alphae main stage of the pluton consists of an early, reversely zoned tonalite to gabbro that was intruded by synplutonic noritic and gabbroic magmas. Late-stage activity was characterized by intrusion of tonalitic and granitic dikes, many of which contain mafic enclaves and hybrid zones. Most mafic rocks in the pluton are calc-alkaline, with characteristic magnesian clinopyroxene, calcic cores in plagioclase, and elemental abundances similar to H 2 O-rich arc basalts. However, some mafic rocks contain relatively Fe-rich clinopyroxene, lack calcic cores in plagioclase, and are compositionally similar to evolved high-alumina tholeiite. Compositional variation in the main stage can be modeled in part by fractional crystallization and crusted assimilation during which parental calc-alkaline basalt evolved to granitic compositions. Cumulates related to this process are represented by modally variable melagabbro and pyroxenite. Mixing of basaltic and tonalitic magmas accounts for the compositions of most main-stage intermediate rocks, but mixing of basaltic and granitic magmas was uncommon until late in the pluton's history. Oxygen, Sr and Nd isotopic data indicate that virtually all main-stage magmas in the pluton contain a crustal component. Isotopic and trace element data further suggest that late-stage tonalitic dikes represent melts derived from older, metavolcanic arc crust Deep crustal contamination of main-stage rocks took place below the level of emplacement, probably in a magma-rich zone where basalts ponded and mixed with crustal melts. The Grayback pluton illustrates the diversity of Jurassic back-arc magmatism in the Klamath province and demonstrates that ancient magmatism with arc-like features need not be situated in an arc setting.

Oregon↗

USGS1992

Petrology of the Caribou Mountain Pluton, Klamath Mountains, California

The Caribou Mountain pluton is a small trondhjemitic body that intruded semipelitic schist of the Stuart Fork terrane in late Middle Jurassic to Early Cretaceous time. Its emplacement followed the intrusion of an adjoining body of hornblende quartz diorite called the Middle Fork pluton and the mode of its emplacement was as an asymmetric ballooning diapir (Davis, 1963), as shown by concentric foliation, radial late-stage dikes, foliated enclaves, and folded blocks of schlieren-banded tonalite. Coarse-grained hornblende-bearing trondhjemite is the dominant rock type in the Caribou Mountain pluton, and it is called the ‘main trondhjemite’. It was followed by medium-grained ‘late trondhjemite’ and by late-stage trondhjemitic and granodioritic dikes. All the trondhjemitic rock types are characterized by low alkali contents, high light rare earth elements, low initial 87 Sr/ 86 Sr, and low δ 18 O. However, the late trondhjemite has higher Na 2 O and a higher initial 87 Sr/ 86 Sr value than the main trondhjemite, and the two units cannot be related by fractional crystallization. The late granodioritic dikes are richer in Ba, Rb, Y, and Sc than the late trondhjemite and probably reflect assimilation of Stuart Fork metasedimentary rocks by late-stage trondhjemitic magma. Mafic enclaves in the main trondhjemite contain xenocrysts of quartz and plagioclase derived from the host by magma mixing. The enclaves have K 2 O, Ba, and Rb contents similar to, or higher than those of the host rocks. Their rare earth element (REE) patterns display strong middle REE enrichment caused by accumulation of hornblende, probably as the result of filter pressing. The main trondhjemite cannot be derived from Middle Fork magma because the initial 87 Sr/ 86 Sr of the Middle Fork pluton is lower than that of the trondhjemite. The absence of parental mafic magmas of appropriate composition suggests that the Caribou Mountain trondhjemitic magmas formed by partial melting of an amphibolitic source rock compositionally similar to low-K tholeiite.

California↗

USGS1990

Chapter 15: Two different lithosphere types in the Sierra Nevada, California

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.

California↗

USGS1990

Geochemistry and intrusive history of the Ashland pluton, Klamath Mountains, California and Oregon

The Ashland pluton is a calc-alkaline plutonic complex that intruded the western Paleozoic and Triassic belt of the Klamath Mountains in late Middle Jurassic time. The pluton comprises a series of compositionally distinct magma pulses. The oldest rocks are hornblende gabbro and two-pyroxene quartz gabbro with initial 87 Sr/ 86 Sr = 0˙7044, δ 18 O = 8˙7%, and REE patterns with chondrite normalized La/Lu = 7. These units were followed by a suite of tonalitic rocks (La N /Lu N = 7) and then by a suite of K 2 O- and P 2 O 5 rocks of quartz monzodioritic affinity (La N /Lu N = 13–21; La N /Sm N = 2˙4–3˙) The quartz monzodioritic rocks were then intruded by biotite granodiorite and granite with lower REE abundances but more fractionated LREE(La N /Lu N = 13–19; La N /Sm N = 4˙3–6 and they, in turn, were host to dikes and bosses of hornblende diorite. The latest intrusive activity consisted of aplitic and granitic dikes. Combined phase equilibria and mineral composition data, indicate emplacement conditions of approximately P total = 2˙3kb, P H2O between 1˙5 and 2˙2 kb, and f O2 between the nickel-nickel oxide and hematite-magnetite buffers. Successive pulses of magma display increasing SiO 2 together with increasing δ 18 O and decreasing initial 87 Sr/ 86 Sr. The isotopic data are consistent with either (1) combined fractional crystallization of andesitic magma and concurrent assimilation of crustal material characterized by low Sr 1 and high (δ 18 O or, more probably, (2) a series of partial melting events in which sources were successively less radiogenic but richer in 18 O Each intrusive stage displays evidence for some degree of crystal accumulation and/or fractional crystallization but neither process adequately accounts for their compositional differences. Consequently, each stage appears to represent a distinct partial melting or assimilation event. The P 2 O 5 -rich nature of the quartz monzodiorite suite suggests accumulation of apatite. However, the suite contains abundant mafic microgranitoid enclaves and most apatite in the suite is acicular. These observations suggest that magma mixing affected the compositional variation of the quartz monzodiorite suite. Mass balance calculations are consistent with a simple mixing process in which P 2 O 5 -rich alkalic basalt magma (represented by the mafic microgranitoid enclaves) was combined with a crystal-poor felsic magma (represented by the tonalite suite), yielding a quartz monzodioritic magma that then underwent differentiation by crystal fractionation and accumulation.

California, Oregon↗

USGS1974

Nature of the angular unconformity between the Paleozoic metasedimentary rocks and the mesozoic metavolcanic rocks in the eastern Sierra Nevada, California

Two major wall-rock sequences, the Paleozoic metasedimentary rocks and the Mesozoic metavolcanic rocks, in the eastern Sierra Nevada, California, are separated by an angular unconformity rather than by a fault as has been proposed by other investigators. The unconformity is parallel to formation contacts in the younger metavolcanic rocks and crosscuts formation contacts in the older metasedimentary rocks. Locally, basal conglomerate with pebbles derived from the older metasedimentary rocks occurs just above the unconformity. The older metasedimentary rocks consist mostly of former marl, chert, and mudstone; they contain three generations of superposed structures. The younger metavolcanic rocks consist mostly of former andesite flows, tuff, and graywacke; these contain only the younger two generations of superposed structures. The sinuous trace of the unconformity is primarily the result of superposed deformations of regional scale which occur in most roof pendants of the eastern Sierra Nevada.

California↗