Cretaceous Sierra Nevada Batholith Research Articles

Andradite skarn garnet records of exceptionally low δ18O values within an Early Cretaceous hydrothermal system, Sierra Nevada, CA

Skarn garnets in the Mineral King roof pendant of the south–central Sierra Nevada within Sequoia National Park, California, USA reveal variable fluid chemistry with a significant component of meteoric water during metasomatism in the Early Cretaceous Sierra Nevada Batholith. We focus on andradite garnet associated with Pb–Zn mineralization in the White Chief Mine. Laser fluorination oxygen isotope analyses of δ18O of garnet (δ18O(Grt)) from sites along the skarn show a large range of values (− 8.8 to + 4.6‰ VSMOW). Ion microprobe (SIMS) analyses elucidate that individual andradite crystals are strongly zoned in δ18O(Grt) (up to 7‰ of variation). Total rare-earth element concentrations (∑REE) across individual garnets show progressive depletion of skarn-forming fluids in these elements during garnet growth. These findings support a skarn model of earliest red high-δ18O grandite garnet consistent with a magmatic-dominated equilibrium fluid (δ18O(H2O) as high as ≈ + 8‰). Later, green andradite crystallized in equilibrium with a low-δ18O fluid indicating a significant influx of meteoric fluid (δ18O(H2O) ≈ − 6 to − 5‰), following a hiatus in garnet growth, associated with late-stage Pb–Zn mineralization. Latest orange overprint rims have higher δ18O values (δ18O(H2O) ≈ 0–2‰), and depleted total REEs, suggesting influx of high-δ18O, trace-element depleted fluid derived from regional metamorphism of the carbonate host. Remarkably, low δ18O(Grt) values in the White Chief canyon skarn require a significant proportion of meteoric fluid available during > 400 °C andradite-forming metasomatism. Fluid flow was channelized at the pluton–wallrock contact, evidenced by the narrow extent of skarn.

The rise and fall of continental arcs: Interplays between magmatism, uplift, weathering, and climate

Continental arcs, such as the modern Andes or the Cretaceous Sierra Nevada batholith, are some of the highest topographic features on Earth. Continental arc volcanoes may produce more CO2 than most other types of volcanoes due to the interaction of magmas with sedimentary carbonates stored in the continental upper plate. As such, global flare-ups in continental arc magmatism may amplify atmospheric CO2 concentrations, leading to climatic warming. However, the high elevations of continental arcs may also enhance orographic precipitation and change global atmospheric circulation patterns, possibly increasing the efficiency of chemical weathering and drawdown of atmospheric CO2, which may subdue the climatic warming response to volcanic activity. To better evaluate the climatic response, we develop models that integrate magmatic crustal thickening, topographic uplift, isostasy and erosion. The topographic response is used to predict how soil formation rates, soil residence times, and chemical weathering rates vary during and after a magmatic episode. Although magmatism leads to crustal thickening, which requires topographic uplift, highest elevations peak ∼10 My after magmatism ends. Relatively high elevations, which enhance erosion and chemical weathering of the continental arc, persist for tens of million years after magmatism ends, depending on erosion kinetics. It has recently been suggested that the Cretaceous–Paleogene greenhouse (high atmospheric CO2 and warm climate) coincided with a global chain of continental arcs, whereas mid- to late Cenozoic icehouse conditions (low atmospheric CO2 and cold climate) coincided with a lull in continental arc activity after 50 Ma. Application of our models to the Sierra Nevada (California, USA) continental arc, which represents a segment of this global Cretaceous–Paleogene continental arc, reproduces the observed topographic and erosional response. Our models require that the newly formed continental arc crust remained high and continued to erode and weather well after (>50 My) the end of magmatism. Thus, in the aftermath of a global continental arc flare-up, both the total volcanic inputs of CO2 decline and the average weatherability of continents increases, the latter due to the increased proportion of widespread remnant topography available for weathering and erosion. This combination leads to a decrease in the long-term baseline of carbon in the ocean/atmosphere system, leading to cooling. Mid-Cenozoic cooling is often attributed solely to increased weathering rates associated with India–Eurasian collision and the Himalayan orogeny. However, the total area of now-extinct Cretaceous–Paleogene continental arcs is 1.3–2 times larger than that of the Himalayan range front and the Tibetan plateau combined, suggesting that weathering of these remnant volcanic arcs may also play a role in drawing down CO2 through silicate weathering and subsequent carbonate burial. In summary, if global continental arc flare-ups lead to greenhouse conditions, long-lived icehouse conditions should follow in the aftermath due to decreased CO2 inputs and an increase in regional weathering efficiency of remnant arc topography.

The Fine Gold Intrusive Suite: The roles of basement terranes and magma source development in the Early Cretaceous Sierra Nevada batholith

The Fine Gold Intrusive Suite is one of the largest (>2000 km 2 ) and oldest intrusive complexes in the Sierra Nevada batholith (California, USA), and therefore contains a wealth of information about nascent magmatic processes in a convergent margin arc. Because the suite intrudes both accreted oceanic and/or island-arc terranes and continental crust, it provides perspective on how convergent margin magmatism recycles existing crust versus reworking of fringing island arcs into continental crust. Such insight informs our understanding of how continental crust formation may have operated in the Phanerozoic as compared to earlier in Earth history. New zircon U-Pb geochronology shows that the largely tonalitic suite was emplaced over ∼19 m.y. (124–105 Ma), in three pulses that young from west to east. The most recent domain is nested within the previous ones, such that lobes of magma protruding from the main bodies of the Bass Lake Tonalite (the primary member of the Fine Gold Intrusive Suite) are older than interior areas. Zircon δ 18 O (6.1‰–8.0‰) and eHf (–4.7 + 6.4) show temporal trends indicating that early magmas were source mixtures of mantle with as much as 45% Paleozoic to Mesozoic oceanic and/or arc rocks, whereas later magmas contain greater inputs (to 50%) of Proterozoic North American crust. Older domains in the suite were likely generated from isolated sources, including initial high Sr/Y (to ∼90), high Na 2 O magmas consistent with garnet-bearing sources inferred to be relatively deep. Higher 87 Sr/ 86 Sr, lower eHf, and higher Rb/Sr values in younger plutons show a source that tapped greater proportions of North American crust and was presumably more organized and larger, given its more homogeneous isotopic and trace element traits. Our findings also show that expression of the 87 Sr/ 86 Sr = 0.706 isopleth in arc magmas may be delayed until magma sources are sufficiently vigorous to melt and incorporate aged continental crust. Therefore, Sr i values of older stitching plutons may better record the position of discrete terrane boundaries, whereas younger plutons will record the magmatically average position of terrane boundaries. Although the Fine Gold Intrusive Suite is comparable to the Late Cretaceous voluminous intrusive suite of eastern Sierran suites in terms of duration and age zoning of magmatism, the influence of preexisting basement compositions and differing degrees of organization of the magma sources with age is more pronounced. In addition, the findings show that recycling of fringing arc terranes into continental crust is relatively rapid and that estimates of the growth of Phanerozoic continental crust from such reprocessing should be revised upward.

Accretionary Mesozoic–Cenozoic expansion of the Cordilleran continental margin in California and adjacent Oregon

The Mesozoic–Cenozoic Cordilleran orogen of California includes multiple accretionary belts incorporated sequentially into the continental margin since Middle Triassic time. Accreted tectonic elements include subduction complexes assembled along the Cordilleran margin, intraoceanic island arcs attached to the continental margin by Jurassic arc-continent collision, and subduction complexes associated with the flanks of the exotic island arcs. Systematic analysis of areal relations and geochronological data displayed on subregional geologic maps and summary chronostratigraphic diagrams allows the punctuated but quasi-continuous pattern of tectonic accretion to be discerned. Stitching plutons and sedimentary overlap successions constrain the times that successive accretionary belts were juxtaposed and amalgamated into the edge of the continental block. In the Klamath Mountains and Sierra Nevada, a continental-margin magmatic arc of Triassic–Jurassic age includes volcanic and plutonic components built upon and intruded into deformed Paleozoic assemblages that were accreted to the Laurentian margin before Middle Triassic time. The native arc assemblage is separated from intraoceanic Triassic–Jurassic arc assemblages exposed farther west by a compound suture belt of melange and broken formation derived from the remnant ocean basin that separated the east-facing intraoceanic arc system from the Cordilleran margin. Polarity reversal after Middle to Late Jurassic arc-continent collision was followed by accretion of a disrupted ophiolitic belt forming mafic basement in the subsurface of the Great Valley forearc basin. Subsequent forearc sedimentation accompanied the assembly of multiple belts of melange and broken formation that form the Mesozoic–Cenozoic Franciscan subduction complex of the California Coast Ranges. Franciscan subduction was largely coeval with intrusion of the dominantly Cretaceous Sierra Nevada batholith into the roots of the Cordilleran magmatic arc, but Franciscan accretion was just a late phase of continuing tectonic expansion that spanned more than 200 m.y. along the California continental margin.

Supracrustal input to magmas in the deep crust of Sierra Nevada batholith: Evidence from high- δ18O zircon

Oxygen isotope ratios of zircon (Zc) from intrusives exposed in the Tehachapi Mountains, southern California, reveal large inputs of high- δ 18O supracrustal contaminant into gabbroic and tonalitic magmas deep (> 30 km) in the Cretaceous Sierra Nevada batholith. High δ 18O(Zc) values (7.8 ± 0.7‰) predominate in the deep parts of the batholith, but lower values (6.1 ± 0.9‰) occur in shallower parts. This indicates a larger gradient in δ 18O with depth in the batholith than occurs from west to east across it. Oxygen, Sr, and Nd isotope data show that the supracrustal contaminant was likely young (Paleozoic or Mesozoic), hydrothermally altered upper oceanic crust or volcanic arc sediments. Such rocks were subducted or underthrust beneath the Sierran arc during accretion of oceanic terranes onto North America. This component yielded high- δ 18O magmas that were added to the base of the batholith. On average, gabbros in the southern Sierra contain at least 18% of the subducted supracrustal component. Some tonalite and granodiorite magmas were additionally contaminated by Kings Sequence metasedimentary rocks, as evidenced by δ 18O(Zc) and initial 87Sr/ 86Sr that trend toward values measured for the Kings Sequence. Besides high δ 18O values in the southern Sierra, xenoliths in the central Sierra also have elevated δ 18O, which confirms the widespread abundance of supracrustal material in the sub-arc lithospheric mantle. In contrast to δ 18O(Zc), whole rock δ 18O values of many samples have undergone post-magmatic alteration that obscures the magmatic contamination history of those rocks. Such alteration previously prevented confident determination of the mass of young, hydrothermally altered mantle rocks that contributed to Sierran granitoids.

Geologic and geophysical investigation of two fine-grained granites, Sierra Nevada Batholith, California: Evidence for structural controls on emplacement and volcanism

The Johnson Granite Porphyry and the Silver Pass Porphyry are two late-stage porphyritic intrusions in the Cretaceous Sierra Nevada Batholith, California. Both intrusions are characterized by fine-grained textures, miarolitic cavities, pegmatite veins, and abundant xenoliths. Microstructures in the Johnson Granite Porphyry record magmatic flow. Magnetic fabrics in the Johnson Granite Porphyry, determined by anisotropy of magnetic susceptibility, show that the poles to magnetic foliations define a NNE-SSW girdle and magnetic lineations plunge shallowly to moderately to the WNW and ESE. A gravity survey and three-dimensional inversion indicate that the Johnson Granite Porphyry is shallow and tabular, extending to 1.5 km depth beneath the southwestern margin. A second gravity anomaly was delineated southeast of the Johnson Granite Porphyry, suggesting the intrusion continues to depth. The Silver Pass Porphyry is deformed by the dextral Rosy Finch shear zone and records a gradient of high- to low-temperature, solid-state fabrics. Field and magnetic foliations in the Silver Pass Porphyry are subvertical and lineations are shallowly plunging. We propose that regional shear zones controlled the emplacement of these intrusions. The Johnson Granite Porphyry was emplaced in a three-dimensional tension gash associated with the termination of the Bench Canyon shear zone. The Rosy Finch shear zone facilitated the emplacement of the Silver Pass Porphyry, probably as a tension fracture associated with the shear zone. The field observations and magnetic fabrics suggest that the Johnson Granite Porphyry may record an extrusive event, suggesting that volcanism may be related to deformation along active shear zones.

Missing Idaho arc: Transpressional modification of the 87Sr/86Sr transition on the western edge of the Idaho batholith

The North American Cordillera is characterized by subduction-related Mesozoic batholiths with voluminous middle Cretaceous plutonism. In the Cordillera, the Idaho batholith is anomalous, containing only a narrow (<10 km) band of middle Cretaceous plutons on its extreme western margin. These plutons are spatially coincident with the Late Cretaceous, crustal-scale, dextral transpressional western Idaho shear zone. Finite strain analysis indicates that there is significant east-west shortening in the western Idaho shear zone. A reconstruction using isotopic variations as strain markers, and comparing them to the undeformed middle Cretaceous Sierra Nevada batholith, results in a middle Cretaceous magmatic arc in western Idaho similar in width (∼85– 100 km) to other sections of the Cordilleran magmatic arc. The tectonic implications include: (1) a full magmatic arc existed in Idaho in the middle Cretaceous prior to Late Cretaceous deformation; (2) the currently sharp 87Sr/86Sr transition in western Idaho represents structural modification of the batholith after emplacement (i.e., the steep isotopic gradient results from deformation); and (3) as a result of Late Cretaceous shortening in the western Idaho shear zone, the main part of the Idaho batholith (i.e., post–ca. 90 Ma) intruded farther eastward into thick North American lithosphere. Consequently, the main part of the Idaho batholith has a stronger continental signature than the other coastal batholiths.

Roberts Mountains allochthon and the western margin of the Cordilleran miogeocline in the Northern Ritter Range pendant, eastern Sierra Nevada, California

The truncated southwestern edge of the Roberts Mountains allochthon is exposed in the Northern Ritter Range pendant in the eastern Sierra Nevada, structurally overlying parautochthonous rocks of the Cordilleran miogeocline. The Northern Ritter Range pendant exposes units that have the same stratigraphic affinities and structural relationships as rocks of the Antler orogenic belt in Nevada. Paleozoic metasedimentary rocks exposed in the pendant consist of two major units: (1) a structurally complex, disrupted chert and argillite unit interpreted to be correlative with the Roberts Mountains allochthon; and (2) a stratigraphically coherent siliceous and calcareous unit, the Rush Creek sequence, that is interpreted to be part of a transitional outer shelf and slope assemblage of the lower Paleozoic Cordilleran miogeocline. In the Northern Ritter Range pendant, the Roberts Mountains allochthon structurally overlies the Rush Creek sequence along a north-striking, steeply dipping fault zone that may be a preserved remnant of the Roberts Mountains thrust, which in north-central Nevada emplaced the allochthon over outer shelf and slope strata of the Cordilleran miogeocline during the Late Devonian–Early Mississippian Antler orogeny. These stratigraphic and structural belts are truncated on the southwest side of the pendant by the Gem Lake shear zone, a northwest-trending dextral strike-slip fault associated with the Cretaceous Sierra Nevada batholith. The Northern Ritter Range pendant thus defines both the southwestern limit of the Antler orogenic belt and the westernmost exposures of parautochthonous miogeoclinal rocks in the central Cordillera.

Thermobarometric constraints on the depth of exposure and conditions of plutonism and metamorphism at deep levels of the Sierra Nevada Batholith, Tehachapi Mountains, California

We present thermodynamic estimates of pressures, temperatures, and volatile activities in variably deformed, gabbroic to granitic, Cretaceous (115–100 Ma) batholithic and framework rocks of the Tehachapi Mountains, southernmost Sierra Nevada, California. Al contents of hornblende in granitoids imply igneous emplacement at ∼8 kbar in the southernmost Tehachapi Mountains, with lower pressures (3–7 kbar) to the north. Metamorphic pressures and temperatures for garnet‐bearing paragneisses and metaigneous rocks were estimated on the basis of garnet‐hornblende‐plagioclase‐quartz and garnet‐biotite‐plagioclase‐quartz thermobarometers. Disparate results for the metaigneous rocks from the latter system point to the difficulty of applying pelite‐based thermobarometers to rocks of contrasting composition and mineralogy. Preferred pressures cluster at 7.1–9.4 and 3.6–4.3 kbar. Incomplete knowledge of reaction histories, however, limits our interpretation of the lower pressures because they are minimum estimates. The ∼4‐kbar samples are all from a small area and, if our interpretation is correct, they imply a local, more shallow event superimposed on crust once residing at deeper structural levels. Garnet‐hornblende and garnet‐biotite temperatures are less coherent, likely owing to retrograde Fe‐Mg exchange, and range from 570° to 790°C, The majority of the rocks are igneous and affected by recrystallization and metamorphism during subsolidus cooling; they are not granulites. Country rock paragneisses are typically migmatized at “peak” metamorphic conditions near that of the wet granite solidus (&gt;690°C). Veinlike paragenesis of garnet in the metaigneous rocks suggests formation related to the presence of a fluid phase. Thermodynamic estimates of volatile activities in these garnet‐bearing assemblages suggest variable, mostly CO2‐rich fluid compositions, in the absence of any pervasive fluid flux. The igneous rocks of the Tehachapi Mountains were thus intruded at depths of ∼30 km, making them the deepest known exposed components of the Cretaceous Sierra Nevada batholith. Metamorphism occurred at these great depths and, perhaps, locally after ∼15 km of uplift before ∼87 Ma, implying an uplift rate of 1.2 mm/yr. (A minimum uplift rate is 0.6 mm/yr.) This original uplift and possible subsequent uplift events may have been related to underthrusting of a block of Rand Schist from what is now the southeast, with concomitant widespread ductile deformation. The deduced pressure‐temperature and uplift history is similar to those of high‐pressure/high‐temperature Cretaceous batholithic rocks in Salinia and the San Gabriel Mountains, but direct correlation is not wan‐anted. When compared with higher‐level intrusive rocks from analogous portions of the Sierra Nevada batholith to the north, the Tehachapi rocks reveal a deep batholith that is more heterogeneous and somewhat more mafic on average, but displaying a similar level of isotopic hybridization involving mantle and crustal sources. The batholith is quartz‐rich at these levels, suggestive of a weak, ductile middle crust susceptible to prolonged deformation and possible delamination.