Holocene glaciation of the central Sierra Nevada,California

Sediment cores from two bedrock-dammed lakes in North Fork Big Pine Creek, Sierra Nevada, California, preserve the most detailed and complete record of Holocene glaciation yet recovered in the region. The lakes are fed by outwash from the Palisade Glacier, the largest (～1.3 km²) and presumably longest-lived glacier in the range, and capture essentially all of the rock flour it produces. Distinct late-Holocene (Matthes) and late-Pleistocene (Recess Peak) moraines lie between the modern glacier and the lakes. The lakes have therefore received continuous sedimentation from the basin since the retreat of the Tioga glacier (Last Glacial Maximum) and capture rock flour related to all post-LGM advances. A total of eight long cores (up to 5.5 m sediment depth) and one short surface sediment short core preserve a coherent record of fluctuating rock flour flux to the lakes through the Holocene. Age constraints on rock flour spikes in First and Second lakes based on 31 14C-dated macrofossils indicate Holocene glaciation began ～3200 cal yr B P, followed by a possible glacier maximum at ～2800 cal yr B P and four distinct glacier maxima at ～2200, ～1600, ～700 and ～250-170 cal yr. B.P., the most recent maximum being the largest.Reconstruction of the equilibrium-line altitudes (ELA) associated with each distinct advance recorded in the moraines (Recess Peak, Matthes, and modern) indicates ELA depressions (relative to modern) of ～250 m and 90 m for Recess Peak and Matthes advances, respectively. These differences represent decreases in summer temperatures of 1.7–2.8 °C (Recess Peak) and 0.2–2° (Matthes), and increases in winter precipitation of 22-34 cm snow water equivalent (s.w.e.) (Recess Peak) and 3-26 cm s.w.e. (Matthes) compared to modern conditions. Although small, these changes are significant and similar to those noted in the Cascade Range to the north, and represent a significant departure from historical climate trends in the region.     

Late Pleistocene Glaciations in the Northwestern Sierra Nevada, California

Pleistocene fluvial landforms and riparian ecosystems in central California responded to climate changes in the Sierra Nevada, yet the glacial history of the western Sierra remains largely unknown. Three glacial stages in the northwestern Sierra Nevada are documented by field mapping and cosmogenic radionuclide surface-exposure (CRSE) ages. Two CRSE ages of erratic boulders on an isolated till above Bear Valley provide a limiting minimum age of 76,400±3800 ¹⁰Be yr. Another boulder age provides a limiting minimum age of 48,800±3200 ¹⁰Be yr for a broad-crested moraine ridge within Bear Valley. Three CRSE ages producing an average age of 18,600±1180 yr were drawn from two boulders near a sharp-crested bouldery lateral moraine that represents an extensive Tioga glaciation in Bear Valley. Nine CRSE ages from striated bedrock along a steep valley transect average 14,100±1500 yr and suggest rapid late-glacial ice retreat from lower Fordyce Canyon with no subsequent extensive glaciations. These ages are generally consistent with glacial and pluvial records in east-central California and Nevada.     

Holocene environmental change resets lichen surface dates on Recess Peak glacial deposits in the Sierra Nevada,California

Development of an accurate chronology for glacial deposits in the Sierra Nevada has long been problematic given the lack of suitable organic material for radiocarbon dating. Lichenometry initially appeared promising as ages showed an increase from cirque headwalls to down-canyon moraines. However, while Recess Peak lichen age estimates range from 2 to 3 ka, recent work shows these deposits to be at least 10 ka older. Here, we present evidence for a late Holocene reset of Recess Peak lichen ages by significant post-depositional climate change. Following late-Pleistocene deposition of Recess Peak moraines, warming through the mid-Holocene allowed forests to advance into shallow basins eliminating local inverted tree lines. This produced a partial canopy where shading killed the original post-Pleistocene crustose lichen colonies. Late-Holocene cooling resulted in forest retreat from these basins as alpine tree line fell. Lichens then recolonized the re-exposed Recess Peak deposits. We conclude that while Recess Peak lichen ages are accurate to within the dating uncertainty of the technique, existing lichen ages actually date the timing of post-mid-Holocene cooling and recolonization, and not the original emplacement of these deposits. Thus, applications of Lichenometry should consider post-depositional environmental change when interpreting the meaning of these dates.     

The Genesis of Zoned Skarns in the Sierra Nevada, California

Zoned skarns occur at plutonic-metamorphic contacts, in veinscutting marble, and at contacts between marble and interlayeredamphibolite and biotite-rich rocks. For P = 2 kb, fluid inclusionsand P-T-X_CO2 stability relations of calc-silicate assemblagessuggest T< 650 °C and a H₂O-rich fluid (X_CO2 < 0.1).Small-scale, Ca-rich endoskarns are common near exoskarns. Massbalance calculations suggest that: (a) the formation of exoskarnrequires the influx of solute in an aqueous solution from uncontaminatedmagma in addition to material derived from the endoskarn, (b)some ‘limestone assimilation’ is required to formendoskarns, and (c) skarn formation was essentially a constant-volumeprocess. Applying chromatographic theory, compositional profilesof garnet and pyroxene across zoned skarns suggest that infiltrationmetasomatism was an important process, although diffusion metasomatismappears to have produced local compositional gradients at theinfiltration ‘fronts’. Fluid flow calculations showthat thick exoskarns could readily form by intergranular infiltration of aqueous solutions. Reciprocal diffusional exchangeis suggested as a dominant mechanism in the formation of zonedskarns formed at contacts between interlayered metamorphic lithologies.     

Studies of batholithic feldspars: Sierra Nevada,California

Co-existing plagioclase and alkali feldspars of the Sierra Nevada granites and plagioclases of the mafic inclusions have been analysed using an ARL EMX electron microprobe analyser. Each Sierran rock type contains co-existing feldspar pairs within specific compositional ranges. Core plagioclase compositions of the mafic inclusions are only slightly higher or lower in anorthite than the host rock plagioclases and cluster between An30 and An40. The chemical inhomogeneity of the Sierran potash feldspars and this effect on the Barth k value prohibits the use of the feldspars as geothermometers for these particular rocks. Results of the electron microprobe, x-ray, and petrographic study and the experimental hydrothermal investigation of the granites suggest but do not prove that both the plagioclase composition and the mafic inclusion mineralogy can be explained in terms of a model which considers the inclusions to be the refractory residue left over from the partial melting of crustal material.Submitted to the Faculty of the Department of the Geophysical Sciences, The University of Chicago, in partial fulfillment for the degree of Doctor of Philosophy.     

Paleofire reconstruction for high-elevation forests in the Sierra Nevada, California, with implications for wildfire synchrony and climate variability in the late Holocene

Here, we present two high-resolution records of macroscopic charcoal from high-elevation lake sites in the Sierra Nevada, California, and evaluate the synchroneity of fire response for east- and west-side subalpine forests during the past 9200 yr. Charcoal influx was low between 11,200 and 8000 cal yr BP when vegetation consisted of sparse Pinus-dominated forest and montane chaparral shrubs. High charcoal influx after ∼ 8000 cal yr BP marks the arrival of Tsuga mertensiana and Abies magnifica, and a higher-than-present treeline that persisted into the mid-Holocene. Coeval decreases in fire episode frequency coincide with neoglacial advances and lower treeline in the Sierra Nevada after 3800 cal yr BP. Independent fire response occurs between 9200 and 5000 cal yr BP, and significant synchrony at 100- to 1000-yr timescales emerges between 5000 cal yr BP and the present, especially during the last 2500 yr. Indistinguishable fire-return interval distributions and synchronous fires show that climatic control of fire became increasingly important during the late Holocene. Fires after 1200 cal yr BP are often synchronous and corroborate with inferred droughts. Holocene fire activity in the high Sierra Nevada is driven by changes in climate linked to insolation and appears to be sensitive to the dynamics of the El Niño-Southern Oscillation.     

n-Alkane evidence for the onset of wetter conditions in the Sierra Nevada,California (USA) at the mid-late Holocene transition, ~ 3.0 ka

n-Alkane biomarker distributions in sediments from Swamp Lake (SL), in the central Sierra Nevada of California (USA), provide evidence for an increase in mean lake level ~ 3000 yr ago, in conjunction with widespread climatic change inferred from marine and continental records in the eastern North Pacific region. Length distributions of n-alkane chains in modern plants growing at SL were determined and compared to sedimentary distributions in a core spanning the last 13 ka. As a group, submerged and floating aquatic plants contained high proportions of short chain lengths (< nC₂₅) compared to emergent, riparian and upland terrestrial species, for which chain lengths > nC₂₇ were dominant. Changes in the sedimentary n-alkane distribution over time were driven by variable inputs from plant sources in response to changing lake level, sedimentation and plant community composition. A shift toward shorter chain lengths (nC_21, nC₂₃) occurred between 3.1 and 2.9 ka and is best explained by an increase in the abundance of aquatic plants and the availability of shallow-water habitat in response to rising lake level. The late Holocene expansion of SL following a dry mid-Holocene is consistent with previous evidence for increased effective moisture and the onset of wetter conditions in the Sierra Nevada between 4.0 and 3.0 ka.     

Contact Metamorphosed Ultramafic Rocks in the Western Sierra Nevada Foothills, California

Within the western Sierra Nevada metamorphic belt, linear bodiesof alpine-type ultramafic rock, now composed largely of serpentineminerals, parallel the regional strike and commonly coincidewith major fault zones. Within this metamorphic belt, east ofSacramento, California, ultramafic rocks near a large maficintrusion, the Pine Hill Intrusive Complex, have been emplacedduring at least two separate episodes. Those ultramafic rocks,evidently unaffected by the Pine Hill Intrusive Complex andcomposed largely of serpentine minerals, were emplaced alonga major fault zone after emplacement of the Pine Hill IntrusiveComplex. Those ultramafic rocks, contact metamorphosed by thePine Hill Intrusive Complex, show a zonation of mineral assemblagesas the igneous contact is approached: olivine+antigorite+chlorite+tremolite+Fe-Cr spinel olivine+talc+chlorite+tremolite+Fe-Crspinel olivine+anthophyllite+chlorite+tremolite+Fe-Cr spinel olivine+orthopyroxene+aluminous spinel+hornblende+Fe-Cr spinel.Superimposed on these mineral assemblages are abundant secondaryminerals (serpentine minerals, talc, chlorite, magnetite) whichformed after contact metamorphism. Correlation of observed mineralassemblages with the experimental systems, MgO-SiO₂-H₂O andMgO-Al₂O₃-SiO₂-H₂O suggests an initial contact temperature of775±25 °C for the Pine Hill Intrusive Complex assumingPtotal P_fluid PH₂O. The pressure acting on the metamorphic rockduring emplacement of the intrusion is estimated to be a minimumof 1.5 kb.     

Isotopic variation in the Tuolumne Intrusive Suite,central Sierra Nevada,California

Granitoid rocks of the compositionally zoned Late Cretaceous Toulumne Intrusive Suite in the central Sierra Nevada, California, have initial⁸⁷Sr/⁸⁶Sr values (Sri) and¹⁴³Nd/¹⁴⁴Nd values (Ndi) that vary from 0.7057 to 0.7067 and from 0.51239 to 0.51211 respectively. The observed variation of both Sri and Ndi and of chemical composition in rocks of the suite cannot be due to crystal fractionation of magma solely under closed system conditons. The largest variation in chemistry, Ndi, and Sri is present in the outer-most equigranular units of the Tuolumne Intrusive Suite. Sri varies positively with SiO₂, Na₂O, K₂O, and Rb concentrations, and negatively with Ndi, Al₂O₃, Fe₂O₃, MgO, FeO, CaO, MnO, P₂O₅, TiO₂, and Sr concentrations. This covariation of Sri, Ndi and chemistry can be modeled by a process of simple mixing of basaltic and granitic magmas having weight percent SiO₂ of 48.0 and 73.3 respectively. Isotopic characteristic of the mafic magma are Sri=0.7047, Ndi=0.51269 and ¹⁸O=6.0, and of the felsic magma are Sri=0.7068, Ndi=0.51212 and ¹⁸O=8.9. The rocks sampled contain from 50 to 80% of the felsic component. An aplite in the outer equigranular unit of the Tuolumne Intrusive Suite apparently was derived by fractional crystallization of plagioclase and hornblende from magma with granudiorite composition that was a product of mixing of the magmas described above. Siliceous magmas derived from the lower crust, having a maximum of 15 percent mantle-derived mafic component, are represented by the inner prophyritic units of the Tuolumne Intrusive Suite.     

Late Pleistocene vegetation of Kings Canyon,Sierra Nevada,California

Seven packrat midden samples make possible a comparison between the modern and late Pleistocene vegetation in Kings Canyon on the western side of the southern Sierra Nevada. One modern sample contains macrofossils and pollen derived from the present-day oak-chaparral vegetation. Macrofossils from the six late Pleistocene samples record a mixed coniferous forest dominated by the xerophytic conifers Juniperus occidentalis, Pinus cf. ponderosa, and P. monophylla. The pollen spectra of these Pleistocene middens are dominated by Pinus sp., Taxodiaceae-Cupressaceae-Taxaceae (TCT), and Artemisia sp. Mesophytic conifers are represented by low macrofossil concentrations. Sequoiadendron giganteum is represented by a few pollen grains in the full glacial. Edaphic control and snow dispersal are the most likely causes of these mixed assemblages.The dominant macrofossils record a more xeric plant community than those that now occur on similar substrates at higher elevations or latitudes in the Sierra Nevada. These assemblages suggest that late Wisconsin climates were cold with mean annual precipitation not necessarily greater than modern values. This conclusion supports a model of low summer ablation allowing for the persistence of the glaciers at higher elevations during the late Wisconsin. The records in these middens also suggest that S. giganteum grew at lower elevations along the western side of the range and that P. monophylla was more widely distributed in cismontane California during the Pleistocene.     

Microgranitoid enclave swarms in granitic plutons, central Sierra Nevada, California

Microgranitoid enclaves are common in granitic plutons worldwide, occurring individually and in homogeneous or heterogeneous swarms. Three plutons in the central Sierra Nevada batholith contain swarms with mostly heterogeneous suites of enclaves in the intermediate composition range, and occur in a number of two-dimensional shapes, specifically as dikes, small rafts, lenses, pipe/vortices and large massive shapes. Swarms are characterized by various features, including the nature of their boundary with the host, their planar or non-planar character, internal geometry, density of enclave packing, presence or absence of schlieren and crystal aggregates, and axial ratios and degree of preferred alignment of enclaves. We propose that heterogeneous enclave swarms form by one, or some combination of, the following mechanisms: (1) velocity-gradient sorting parallel or normal to the flow, (2) gravitational sorting or (3) break-up of heterogeneous dikes. Common sites where enclave swarms form include pluton margins or internal viscosity walls, within fractures, and near the pluton roof.     

Geophysical evidence for Holocene lake-level change in southern California (Dry Lake)

Bird, B. W., Kirby, M. E., Howat, I. M. & Tulaczyk, S. 2009: Geophysical evidence for Holocene lake-level change in southern California (Dry Lake). Boreas , 10.1111/j.1502-3885.2009.00114.x. ISSN 0300-9483.
Ground penetrating radar (GPR) data are used in combination with previously published sediment cores to develop a Holocene history of basin sedimentation in a small, alpine lake in southern California (Dry Lake). The GPR data identify three depositional sequences spanning the past 9000 calendar years before present (cal. yr BP). Sequence I represents the first phase of an early Holocene highstand. A regression between <8320 and >8120 cal. yr BP separates Sequence I from Sequence II, perhaps associated with the 8200 cal. yr BP cold event. Sequence II represents the second phase of the early-to-mid Holocene highstand. Sequence IIIa represents a permanent shift to predominantly low lake stands beginning ∼5550 cal. yr BP. This mid-Holocene shift was accompanied by a dramatic decrease in sedimentation rate as well as a contraction of the basin's area of sedimentation. By ∼1860 cal. yr BP (Sequence IIIb), the lake was restricted to the modern, central basin. Taken together, the GPR and core data indicate a wet early Holocene followed by a long-term Holocene drying trend. The similarity in ages of the early Holocene highstand across the greater southern California region suggests a common external forcing – perhaps modulation of early Holocene storm activity by insolation. However, regional lake level records are less congruous following the initial early Holocene highstand, which may indicate a change in the spatial domain of climate forcing(s) throughout the Holocene in western North America.     

Biotites from Granitic Rocks of the Central Sierra Nevada Batholith, California

Biotites from plutonic recks of the central Sierra Nevada andInyo Mountains, California, have been examined and characterizedby powder X-ray diffraction and optical and chemical methods. Compositions of the biotites define a trend in the compositionaltriangle Fe+3 Fe+2Mg. When related to the experimentally studiedternary system KFe₃+3AlSisO₁₂H-₁-KFe₃+2 AlSi₃O₁₀(OH)₂-KMg₃AlSi₃O₁₀(OH)₂and coupled with the estimated positions of biotite solid solutionsfor different oxygen buffers, the trend suggests that oxygenfugacities in magmas during biotite crystallization were slightlyhigher than those defined by the Ni-NiO buffer. The compositionaldata also suggest that magmas were ‘buffered’ withrespect to oxygen by oxides existing within the magmas themselves. Correlation between the Fe/(Fe+Mg) ratio, an inferred temperatureindicator, and other elements is generally poor, which suggeststhat factors other than temperature at the time of crystallizationexerted an important influence on compositions.     

Hornblendes from Granitic Rocks of the Central Sierra Nevada Batholith, California

Twenty samples of hornblendes from rocks of 14 plutonic unitsin the central Sierra Nevada and Inyo Mountains, California,have been studied in detail. Optical, density, single-crystaland powder X-ray diffraction, and major and minor element chemicaldata are reported. The compositions of the hornblendes show only limited correlationwith the chemistry of the rocks in which they occurred. Hornblendesfrom granitic rocks of the eastern Sierra Nevada and Inyo Mountainshave a wide range of tetrahedral aluminum content which is oftenas low as three-quarters of an atom per formula unit, whereashornblendes from younger granitic rocks elsewhere in the SierraNevada batholith contain more than one atom of tetrahedral aluminumper formula unit. Because an increase of aluminum in tetrahedralco-ordination is considered indicative of higher temperaturesof crystallization, the observed differences in the hornblendessuggest that older plutonic rocks of the batholith may havebeen metamorphosed regionally or may have been affected by widespreadhydrothermal action prior to consolidation of later graniticrocks.     

Holocene vegetation and climate change recorded in alpine bog sediments from the Borreguiles de la Virgen,Sierra Nevada,southern Spain

High-resolution pollen and magnetic susceptibility (MS) analyses have been carried out on a sediment core taken from a high-elevation alpine bog area located in Sierra Nevada, southern Spain. The earliest part of the record, from 8200 to about 7000 cal yr BP, is characterized by the highest abundance of arboreal pollen and Pediastrum, indicating the warmest and wettest conditions in the area at that time. The pollen record shows a progressive aridification since 7000 cal yr BP that occurred in two steps, first shown by a decrease in Pinus, replaced by Poaceae from 7000 to 4600 cal yr BP and then by Cyperaceae, Artemisia and Amaranthaceae from 4600 to 1200 cal yr BP. Pediastrum also decreased progressively and totally disappeared at ca. 3000 yr ago. The progressive aridification is punctuated by periodically enhanced drought at ca. 6500, 5200 and 4000 cal yr BP that coincide in timing and duration with well-known dry events in the Mediterranean and other areas. Since 1200 cal yr BP, several changes are observed in the vegetation that probably indicate the high-impact of humans in the Sierra Nevada, with pasturing leading to nutrient enrichment and eutrophication of the bog, Pinus reforestation and Olea cultivation at lower elevations.     

Fabric studies within the Cascade Lake shear zone, Sierra Nevada, California

The Cascade Lake shear zone occurs on the eastern margin of the Tuolumne Intrusive Suite, Sierra Nevada Batholith, California. Foliation in the zone is NNW trending and subvertical, and lineation is moderately south plunging. Deformation is syn-tectonic with emplacement of the Cathedral Peak granodiorite. A deformation gradient exists toward the NE margin of this pluton, with higher strains and lower temperatures of deformation found near the contact. We compare fabric data collected very densely in this shear zone using several techniques: field fabrics, 3D orientation of K-feldspar megacrysts, and AMS (anisotropy of magnetic susceptibility) analysis. In general, the results from the three different methods are in agreement. Deformation in this shear zone is part of a larger pattern of deformation within the Cathedral Peak granodiorite, as recorded by AMS analysis, and dextral shearing associated within the last stage of plutonism within the Sierra Nevada magmatic arc.     

Mesoscopic structural fabric of early Paleozoic rocks in the northern Sierra Nevada, California and implications for Late Jurassic plate kinematics

Analysis of the mesoscopic structure of the early Paleozoic Shoo Fly complex, northern Sierra Nevada, California, reveals three phases of deformation and folding. The first phase of folding is pre-Late Devonian and the second two are constrained by regional relations as due to the Late Jurassic Nevadan orogeny. Main phase Nevadan deformation produced penetrative slaty cleavage which is steep, NNW-trending and parallel to tectonostratigraphic terranes of the region. Cleavage is axial-planar to ubiquitous isoclinal similar folds. Fold axes define a NNW-trending girdle with a distinct, near-vertical maximum. Main phase Nevadan folds have nearly ideal class 2 orthogonal thickness geometry although some class 1C forms exist in more competent units. The overall geometry of main phase folds suggests formation by progressive deformation in a flattening regime with cleavage as the flattening plane and a steep extension axis defined by the fold axis maximum. A steep extension axis direction for main phase Nevadan deformation is supported by analysis of interference relations where folds of this generation deform pre-Late Devonian folds. Late Nevadan folds range from kink flexures to ideal class 2 similar folds with incipient axial-planar cleavage. The kinematic significance of late Nevadan folds cannot be evaluated because of their varying style and orientation throughout the northern Sierra Nevada.Penetrative ductile deformation and near-vertical extension during the Nevadan orogeny was synchronous with accretion of oceanic and/or island arc rocks against the western margin of the northern Sierra Nevada. The kinematic framework of deformation defined for Nevadan deformation is consistent with essentially orthogonal convergence of these exotic terranes with the Sierran margin and argues against a transform/transpressive regime.     

Calibrating Late Quaternary terrestrial climate signals: radiometrically dated pollen evidence from the southern Sierra Nevada,USA

We constructed a radiometrically calibrated proxy record of Late Pleistocene and Holocene climate change exceeding 230,000 yr duration, using pollen profiles from two cores taken through age-equivalent dry lakes—one core having greater age control (via ²³⁰Th alpha mass-spectrometry) and the other having greater stratigraphic completeness. The better dated of these two serial pollen records (Searles Lake) served as a reference section for improving the effective radiometric age control in a nearby and more complete pollen record (Owens Lake) because they: (1) are situated ∼90 km apart in the same drainage system (on, and immediately leeward of, the eastern flank of the Sierra Nevada), and (2) preserved strikingly similar pollen profiles and concordant sequences of sedimentological changes. Pollen assemblages from both lakes are well preserved and diverse, and document serial changes in Late Pleistocene and Holocene plant zone distribution and composition in the westernmost Great Basin; they consist of taxa now inhabiting montane forest, woodland, steppe, and desert-scrub environments. The studied core intervals are interpreted here to be the terrestrial equivalent of marine δ¹⁸O stages 1 through 9; these pollen profiles now appear to be among the best radiometrically dated Late Pleistocene records of terrestrial climate change known.     

Metamorphism of Calcareous Rocks in Three Roof Pendants in the Sierra Nevada, California

Two roof pendants in the Hope Valley area, Alpine County, containabundant calc-silicate assemblages which can be related to univariantor invariant equilibria in the CaO-Al₃O₃-SiO₂-H₂O-CO₂ system.Such assemblages are considered to represent components of reactionsthat buffered the chemistry of the pore fluid. Through dataobtained from microprobe analysis it is concluded that solidsolution in plagioclase, garnet, and clinozoisite are importantvariables such that on a TXco₂ projection each sample had aunique path during metamorphism. Differences in the plagioclasecomposition of nearby samples with assemblages related by thereaction: grossularite_(s.s)+quartz = anorthite_(s.s.)+wollastonite, suggest unique equilibration temperatures for assemblages inlocal domains. In the Twin Lakes pendant in Fresno County, thereaction: clinohumite+calcite+CO₂= 4forsterite+dolomite+H₂O, is importantin magnesian marbles. Contrasting parageneses, which are relatedby this equilibrium, are considered to reflect variations influid composition. Constrasting assemblages in calc-silicaterocks, which are linked by the reactions: calcite+quartz= wollastonite+CO₂, tremolite+calcite= dolomite+diopside+CO₂+H₂O, exist down to the scale of a thin section. Variation in Ti contentof idocrase may be an important factor in assemblages linkedby reactions involving this phase. This study suggests that during contact metamorphism of calcareousrocks in the Sierra Nevada, H₂O and CO₂ behaved as ‘initialvalue components’ (Zen, 1963) whose activities were controlledby reactions withion local systems.     

Petrology of the Guadalupe Igneous Complex South-western Sierra Nevada Foothills California

The Guadalupe igneous complex is one of several late Jurassic,mesozonal plutons in the foothills of the Sierra Nevada. Severallines of evidence suggest that the different rock types, rangingfrom eucrite to leucogranophyre, originated mainly through fractionalcrystallization of a parent basaltic magma. Important differencesbetween this complex and other large differentiated basic intrusionsinclude the presence of steeply inclined but weak layering inthe gabbroic rocks and an abundance of hydrous mafic mineralsrelative to olivine and pyroxene, which are not representedby iron-rich end-members in the granitic differentiates. Chemically,the differentiation trend is marked by a large variation infelsic parameters and only a weak enrichment in iron. This trend,which is similar to that of calc-alkaline suites of orogenicregions, may have been influenced by abnormally high pressuresof water and oxygen for a crystallizing basaltic magma.