Gary H. Girty

Provenance and Depositional Setting of Paleozoic Chert and Argillite, Sierra Nevada, California

ABSTRACT The Shoo Fly Complex, the remnants of an early to middle Paleozoic subduction system, contains the Culbertson Lake allochthon. Included in the Culbertson Lake allochthon are the post-Cambrian and pre-Upper Devonian Quartz Hill, Toms Creek, and McMurray Lake cherts. Geochemical data derived from these three units plot on the Fe2O3/TiO2 versus Al2O3/(Al2O3 + Fe2O3) and Lan/Cen versus Al2O3/(Al2O3 + Fe2O3) discrimination diagrams within either the continental margin-island arc field, or that part of the pelagic field overlapping the continental margin-island arc field. These relationships a e consistent with the presence of argillaceous turbidites interstratified with radiolarite, and suggest a relatively distal continental-island arc setting where muddy turbidity currents episodically interrupted pelagic deposition. Specimens from the Quartz Hill and Toms Creek cherts with Al2O3/TiO2 values > 20 make up Group I samples, whereas specimens with Al2O3/TiO2 values < 10 are classified as Group II samples. The Al2O3/TiO2 ratios of Group II specimens suggest derivation from mafic rock. Mean Th/Sc and Th/U values, as well as LREE-enriched patterns, are consistent with this interpretation, and indicate a source area dominated by alkalic seamount or ocean island material. The Al2O3/TiO2 ratios of Group I samples indicate a source area with an average andesitic to rhyodacitic composition. Chondrite-normalized REE distribution patterns, as well as mean Th/Sc and Th/U value , support such an interpretation. Samples from the McMurray Lake chert have Al2O3/TiO2 values ranging between 19 and 28, and REE patterns characterized by LREE enrichment, negative Eu anomalies, and slightly fractionated HREE patterns. These features, along with mean Th/Sc and Th/U values, indicate a source area dominated by old upper continental crust. The location of the source area(s) supplying material to cherts and argillites of the Culbertson Lake allochthon is unknown. However, existing tectonic models commonly portray the source of old upper continental crustal material in the Shoo Fly Complex as being located somewhere along the western North American continental margin, whereas the source of island-arc debris is portrayed as being located in a fringing arc system. Our data suggest that a relatively high-standing seamount or ocean island probably resided on the subducting oceanic plate. Thus, our work supports the idea that Al, Fe, Ti, Th, Sc, and the REEs in chert and argillite deposited in proximal continental-island arc settings can be used to assess the characteristics of sources supplying particulate matter to ancient t ench systems.

Provenance Determined by Ree, Th, and Sc Analyses of Metasedimentary Rocks, Boyden Cave Roof Pendant, Central Sierra Nevada, California

ABSTRACT REE, Th, and Sc analyses of metasedimentary rocks collected from the western sequence of the Boyden Cave roof pendant, when compared with similar analyses of uppermost Precambrian to Cambrian miogeoclinal mudstones from the Nopah Range, support the idea that rocks from the former area are a part of a large tectonically displaced fragment of the Cordilleran miogeocline, as proposed by R.A. Schweickert and M.M. Lahren. For example, REE distribution diagrams for samples from the western sequence of the Boyden Cave pendant show parallel light-REE enrichment trends and negative Eu anomalies that range from 0.61 to 0.86, and average 0.69. Likewise, REE distribution patterns for samples from the uppermost Precambrian to Cambrian Cordilleran miogeoclinal section in the Nopah Range also show p rallel light-REE enrichment trends, and Eu anomalies that range from 0.65 to 0.75 and average 0.68. On a La-Th-Se diagram, samples from the Boyden Cave pendant cluster with specimens from the Nopah Range. Finally, sediments from both areas have broadly similar Th/Sc and Th/U ratios. These results suggest that samples from the western sequence of the Boyden Cave pendant are composed of detritus like that shed from differentiated western North American upper continental crust during the early development of the Cordilleran miogeocline. Such an interpretation is consistent with: (1) the quartz-rich to slightly feldspathic character of sandstones in the Boyden Cave pendant, which implies a continental to transitional continental-block provenance; (2) a Precambrian U-Pb age for detrital zirco extracted from a single sandstone bed; and (3) the complete absence of volcanic material in the western sequence of the Boyden Cave pendant. Our results support the regional correlations and tectonic model proposed by R.A. Schweickert and M.M. Lahren, and further suggest that although underutilized, REE, Th, and Sc analyses are useful in evaluating stratigraphic, sedimentologic, and tectonic problems in complexly metamorphosed and deformed metaclastic terranes.

Assessing Differences in Composition between Low Metamorphic Grade Mudstones and High-Grade Schists Using Logratio Techniques

Based on stratigraphic and petrologic similarities, it has been proposed that rocks exposed in roof pendants of the central Sierra Nevada, California are part of a tectonically displaced fragment of the Cordilleran miogeocline. We have identified significant geochemical similarities between metamudstones making up this miogeoclinal fragment and rocks comprising its proposed parents in the Mojave Desert-southern Great Basin region by geochemical analysis of samples collected from Snow Lake and Boyden Cave roof pendants, and from the Precambrian to Cambrian section of the Cordilleran miogeocline exposed in the Nopah Range, southeastern California. In order to circumvent the constant-sum problem inherent in geochemical data, the data were transformed into a continuous-variable format using logratio techniques. When the differences in means of aluminum-normalized logratioed data derived from the Snow Lake-Boyden Cave data set and the Nopah Range samples were calculated on an element by element basis, ~86% of the differences were not statistically significant at the 95% confidence level. The large number of compositional similarities identified between Snow Lake, Boyden Cave, and Nopah Range samples are consistent with the idea that the metasedimentary rocks of the central Sierra Nevada batholith are composed of material shed from the western North American interior and that roof pendants in the central Sierra Nevada are part of a displaced miogeoclinal fragment. This work demonstrates that rigorous statistical analyses of geochemical data transformed into a continuous-variable format can be a useful tool in evaluating the plausibility of lithostratigraphic correlations between metamorphosed, complexly deformed, and displaced metasedimentary rocks.

Tectonic implications of detrital zircon data from Paleozoic and Triassic strata in western Nevada and Northern California

U-Pb analyses of detrital zircons from various allochthonous assemblages of Paleozoic and early Mesozoic age in western Nevada and northern California yield new constraints on the sediment dispersal patterns and tectonic evolution of western North America. During early Paleozoic time, a large submarine fan system formed in slope, rise, basinal, and perhaps trench settings near the continental margin, west of continental shelf deposits of the Cordilleran miogeocline. Our detrital zircon data suggest that most of the detritus in this fan system along the western U.S. segment of the margin was derived from the Peace River Arch region of northwestern Canada, and some detritus was shed from basement rocks of the southwestern United States or western Mexico. In most cases, the detritus in the allochthonous assemblages was recycled through platformal and/or miogeoclinal sedimentary units prior to accumulating in offshelf environments. Lower Paleozoic rocks of the Roberts Mountains allochthon, Shoo Fly Complex, and Yreka terrane are interpreted to have been parts of this fan complex that accumulated along the central U.S. segment of the continental margin, probably within 1000 km of the miogeocline. During the mid-Paleozoic Antler orogeny, parts of the lower Paleozoic fan complex were deformed and uplifted, and strata of the Roberts Mountains allochthon were tectonically emplaced onto the continental margin. This orogeny was apparently driven at least in part by convergence of the Sierra-Klamath arc with the continental margin, as has been proposed by many previous workers, because these arc terranes are overlain by Mississippian clastic strata derived from the Roberts Mountains allochthon. Our data are not sufficient, however, to determine the polarity of the arc, or whether the arc formed along the continental margin or was exotic to western North America. Detrital zircon data indicate that following the Antler orogeny, clastic sediments derived from the Roberts Mountains allochthon were deposited both on the continental margin to the east and within intra-arc and backarc basins to the west. The occurrence of this detritus in terranes of western Nevada and northern California indicates that they were proximal to each other and to the continental margin during late Paleozoic time. The presence of upper Paleozoic volcanic and plutonic rocks and arc-derived detrital zircons in strata of the northern Sierra, eastern Klamath, and Black Rock terranes records the existence of a west-facing magmatic arc near the continental margin during late Paleozoic time. Our data are not supportive of scenarios in which these arc terranes were located farther north or thousands of kilometers offshore of the Nevada continental margin during late Paleozoic time. Following a second phase of uplift, erosion, and allochthon emplacement during the Permian-Early Triassic Sonoma orogeny, Middle and Upper Triassic strata now preserved in west-central Nevada accumulated in a backarc basin. Our data indicate that the basinal assemblages contain detritus from arc terranes to the west as well as the craton to the east.

Detrital zircon geochronology of the Shoo Fly Complex, northern Sierra terrane, northeastern California

U-Pb analyses have been conducted on 92 individual detrital zircon grains from 4 of the main thrust sheets of the Shoo Fly Complex. Samples from the Culbertson Lake allochthon, Duncan Peak allochthon, and Lang sequence yield mainly 1.80-2.10, 2.20-2.45, and 2.55-2.70 Ga ages, which suggests that sediments in these units originated in a cratonal region containing Paleoproterozoic and Archean igneous rocks. These ages match those of basement provinces from the northwestern Canadian shield, suggesting a provenance link with northwestern North America during early Paleozoic time. The Sierra City melange, however, has significantly different zircon ages of 551-635 and 1170-1319 Ma, with only a subordinate population of >1.8 Ga grains. These grains apparently were derived originally from an outboard Neoproterozoic-Cambrian(?) volcanic arc and from 1.0-1.7 Ga basement rocks of southwestern North America. The occurrence of all three sets of ages in a sandstone that accumulated outboard of the Lang, Culbertson Lake, and Duncan Peak thrust sheets indicates that most rocks of the Shoo Fly Complex formed inboard of a volcanic arc located in proximity to the southern portion of the Cordilleran margin.

Early Cretaceous intra‐arc ductile strain in Triassic‐Jurassic and Cretaceous continental margin arc rocks, Peninsular Ranges, California

The Cuyamaca-Laguna Mountain shear zone (CLMSZ) lies along the axis of the Peninsular Ranges batholith, separating it into an eastern and western plutonic zones. The shear zone involves Triassic-Jurassic and Early Cretaceous plutonic units which intruded the Triassic Julian Schist and transects the eastern edge of a cryptic lithospheric boundary, separating oceanic crust on the west from continental crust on the east. The Julian Schist crops out on either side of the cryptic lithospheric boundary and is interpreted to represent an overlap sequence. This structural/stratigraphic relationship indicates that the contrasting lithospheric types must have been juxtaposed prior to approximately the Triassic time, and as a result, the CLMSZ probably developed in an intra-arc setting. At least two periods of deformation produced the polygenetic CLMSZ. Structures that formed during D1 include S1 and L1. In Triassic-Jurassic and Early Cretaceous orthogneisses, S1, a pervasive NW striking and NE dipping mylonitic gneissosity, obliterates nearly all traces of an older magmatic fabric. L1 plunges steeply to the NE, lies within the plane of S1, and is locally a well-developed stretching lineation. D1 structures can be traced from the ∼115 Ma Oriflamme Canyon protomylonite into the adjacent Julian Schist and are represented by a well developed S-C mylonitic structures indicative of NE–SW contraction. D1 structures in the Oriflamme Canyon protomylonite and in the ∼118 Ma Pine Valley granodiorite developed while these plutons were incompletely solidified. Hence D1 probably occurred between ∼118 and ∼115 Ma and had culminated in the 105 My emplacement of the Las Bancas tonalite. Normal convergence, ∼125 to 115 Ma, between the North American and Farallon plates is coincident with D1 and the syntectonic emplacement of the Pine Valley granodiorite and the Oriflamme Canyon protomylonite. This relationship suggests that the mechanically weak, thermally and melt-softened cryptic lithospheric interface between oceanic and continental lithosphere may have yielded during the normal convergence event, resulting in the concentration of strain into the CLMSZ during arc magmatism. Such a conclusion underscores the possibility of using intra-arc structures to deduce convergence patterns, as elegantly argued in several recent papers. A >12-km long normal sense shear zone transects D1 structures and formed during D2. Mesoscopic structure associated with the normal sense shear zone includes S2, L2, and C2. D2 structures are the record of NE–SW extension between ∼105- and ∼94 Ma. They may be related to the vertical loading of the CLMSZ by the hanging wall block of the westward verging Santa Rosa and Borrego Springs mylonite belts or they may represent an early, local response to magmatically and structurally overthickened, gravitationally unstable crust. In the latter interpretation, D2 structures are the harbingers of Tertiary-aged, gravity-driven collapse of the SW Cordilleran margin.

Provenance of Paleozoic mudstones in a contact metamorphic aureole determined by rare earth element, Th, and Sc analyses, Sierra Nevada, California

Rare earth element (REE), Th, and Sc analyses of mudstones from the contact metamorphic aureole of the Middle Jurassic Emigrant Gap composite pluton support the idea that the lower Paleozoic Lang, Black Oak Spring, and Zion Hill sequences are composed of detritus derived from a continental landmass. On REE distribution diagrams, our samples exhibited (1) no systematic change as a result of varying metamorphic grade, (2) light-REE enrichment trends, and (3) Eu anomalies that vary from 0.46 to 0.93 and average 0.66. On a La-Th-Sc ternary diagram, our data cluster within the fields of passive-margin sediment and post-Archean Australian shale (PAAS). These new data—in conjunction with the quartz-rich character of sandstones, the absence of volcanic material, and Precambrian detrital zircon ages—suggest that clastic material in the Lang, Black Oak spring, and Zion Hill sequences was derived from a continental landmass that may have been North America or, alternatively, some continental fragment oceanward of early Paleozoic North America.

Assessing Changes in Elemental Mass as a Result of Chemical Weathering of Granodiorite in a Mediterranean (Hot Summer) Climate

ABSTRACT In ascending order, relatively unweathered granodiorite, 1.97 m of friable granodiorite, and 0.91 m of very coarse loamy sand make up a well developed weathering profile that developed on a Cretaceous granodiorite lying within a Mediterranean (hot summer) climatic belt in the Peninsular Ranges of southern California, U.S.A. Textural, modal, and chemical data indicate that in the friable granodiorite and in the very coarse loamy sand, plagioclase was leached at a rate faster than was K-feldspar while quartz remained little changed. In addition, molecular Ca and Na were probably leached at a faster rate from plagioclase and titanite than K was leached from K-feldspar and biotite. Mass-balance calculations show that 3.4% (± 2.0%) of the original bulk mass of the friable granodiorite was removed during its development. This loss of bulk mass was accommodated through the removal of 4%, 9%, 18%, 11%, 17%, 29%, and 14% of the masses of Si, Ca, K, Na, P, Ba, and Sr respectively. In addition, the masses of Fe and Y were increased by 12% and 52% respectively. Of the 3.4% loss of original bulk mass, 99% is due to the loss of Si, Ca, K, and Na mass. The severity of chemical alteration is greatest in samples analyzed from very coarse loamy sand. This conclusion is supported not only by textural relationships, but also by mass balance relationships that show that 11.2% (± 2.1%) of the original bulk mass of the very coarse loamy sand was removed during its development. This loss in mass was accommodated through the removal of 12%, 12%, 42%, 14%, 41%, 80%, 21%, 14%, 10%, and 40% of the masses of Si, Mn, Ca, K, Na, P, Ba, Zn, Rb, and Sr, respectively. In contrast, during development of the very coarse loamy sand the masses of Ti and Fe were increased by 17% and 22%, respectively. Of the 11.2% loss of original bulk mass, 99% is attributable to the removal of Si, Ca, K, and Na mass. Data summarized above, and in work by others, imply that plutoniclastic sediment derived from sources lying within a wide range of climatic conditions, do not reflect directly the bedrock from which they were derived, but instead will mirror the compositions of the weathering profile.

Petrology and provenance of pre–Late Devonian sandstones, Shoo Fly Complex, northern Sierra Nevada, California

Between Quartz Hill and Fall Creek Mountain, California, the Shoo Fly Complex includes the Poison Canyon and Red Hill formations. Field, biostratigraphic, and U-Pb zircon data indicate that both formations are most likely post-Cambrian and pre-Middle Devonian in age. Medium-grained and pebbly sandstone in both formations was deposited in submarine-fan environments by subaqueous sediment gravity flows laden with continentally derived detritus. The source of terrigenous detritus in the Poison Canyon and Red Hill formations included rocks that contained zircons as old as ∼2.09 b.y. The depositional basin in which the Poison Canyon and Red Hill formations accumulated was floored by oceanic lithosphere and was close enough to some continental landmass to have received sand-sized detritus derived from it. The continental landmass may have been located in western North America, but there is nothing in the data presented here that precludes the possibility that it was located on some foreign continent.

A Matlab 5 Program for Calculating the Statistics of Mass Change

Evaluating the statistics of elemental mass transfer associated with metasomatism is a complex and time-intensive process. Hence, we developed a Matlab 5 program, herein referred to as SACD, that has proven to be fast, efficient, and thorough at statistically analyzing elemental and rock-mass changes. The essence of SACD is that it calculates all pertinent statistics that stem from the null hypothesis, H0, that there is no change in elemental mass brought on by metasomatism. The results output by SACD were checked against published data from the Wepawaug Schist, Connecticut that was metamorphosed under Barrovian conditions, probably during the Acadian orogeny. Elemental and total rock-mass changes computed by SACD are nearly identical to those in the published literature but show that, contrary to the published literature, acceptance of H0 carries with it an unacceptable risk. We urge anyone teaching the principles of elemental mass transfer to students to make use of this tool. It will free students from...