Mark D. Barton

Evaporitic-source model for igneous-related Fe oxide–(REE-Cu-Au-U) mineralization

We propose that many igneous-related Fe oxide-rich (REE-Cu-Au-U-bearing) deposits form by hydrothermal processes involving evaporitic ligand sources, either coeval salars or older evaporites. These deposits are abundant in both Phanerozoic and Proterozoic extensional continental and continent-margin settings. They commonly form in global arid zones, but they also occur where magmatism is superimposed upon older evaporites. Magmatic compositions exert only second-order control, mainly on alteration mineralogy and on element abundances. Hot S-poor brines generated by interaction with evaporitic materials are consistent with geologic settings and help rationalize the distinctive element enrichments (siderophile, lithophile) and hydrothermal alteration (sodic, locally alkaline) found in these systems. This model contrasts with immiscible oxide melt and magmatic-hydrothermal origins commonly proposed for these deposits, although all three mechanisms can occur. 31 refs., 3 figs., 1 tab.

Magmatism and the development of low-pressure metamorphic belts: Implications from the western United States and thermal modeling

Two-dimensional numerical modeling and geological and geophysical constraints from ancient and modern magmatic arcs demonstrate that magmatic heat advection is sufficient to produce low-pressure metamorphic belts in many areas, and that it is apparently necessary in some areas. In the western United States and other areas, regionally extensive low-pressure facies-series metamorphism (LPM) occurs where intrusions form >∼50% of the upper crust. Numerical models indicate that coalescence of thermal aureoles from multiple felsic intrusions can produce regionally extensive LPM where the abundance of intrusions exceeds ∼50%. This effect does not depend strongly on the rate of emplacement; LPM results even with complete cooling between intrusions. Models with geologically reasonable emplacement rates show that in an active magmatic arc, temperatures are near metamorphic maxima for only a small fraction of the time. Arc magmatism cannot sustain widespread thermal gradients of the magnitude indicated by the final distribution of LPM, a result consistent with heat-flow data in active arcs. Low-pressure metamorphic belts can thus develop through numerous local, short-lived metamorphic events while most of the crust remains considerably cooler. Metamorphic maxima largely depend on the biggest nearby intrusion; emplacement rates and other heat sources affect mainly the magnitude, not the distribution of metamorphism. A simulation based on the distribution and U-Pb geochronometry of the composite Sierra Nevada batholith compares favorably with the observed distribution of metamorphic grades and temperature histories; coeval medium-pressure facies-series metamorphism to the east of the batholith requires a different mechanism. Predicted surface heat flow is qualitatively similar in magnitude and distribution to that measured in modern arcs. Sequential intrusions can produce brief ( 10 km) uplift, and evidence for moderate over-all paleothermal gradients require high-level advection of heat. This model of relatively cool crust punctuated by local, short-lived thermal events differs significantly from the image of a broad, uniform thermal maximum that might be drawn from the spatial distribution of LPM; it has implications for crustal deformation igneous petrogenesis, and the interpretation of high metamorphic gradients in Archean terranes.

Metasomatism during subduction: products and possible paths in the Catalina Schist, California

On Santa Catalina Island, southern California, lawsonite-albite to amphibolite facies metasedimentary, metamafic, and metaultramafic rocks show veining and chemical alteration that reflect fluid flow and mass transfer at 15 to 45 km depths in an Early Cretaceous subduction zone. In many exposures, multiple generations of cross-cutting syn- and post-kinematic veins record fluid transport and metasomatism during various stages of prograde metamorphism and uplift. Mineralogy and whole-rock compositions demonstrate chemical redistribution, especially of Si, Al, and alkali elements (Na, K), but also of many trace elements, particularly B and LILE (Rb, Cs, Sr, and Ba). Evidence exists for mass transfer, at both local and larger scales, via mechanical mixing, diffusional, and fluid-mediated transfer processes. Highest-grade, amphibolite facies rocks contain feldspar + quartz ± mica ± amphibole leucosomes and pegmatites attributed to migmatization; the leucosomes and pegmatites reflect high-PT mass transfer in felsic silicate liquids. Veining and replacement in blueschist grade rocks comprise three contrasting types of assemblages: (1) silica-saturated (quartz-rich), (2) potassic (white-mica ± quartz-rich), and (3) sodic and silica-undersaturated (albite/Na-amphibole-rich, quartz-absent). Evidence for silicification and alkali exchange also occurs in greenschist and amphibolite facies units. In all units, the evidence for metasomatism (e.g., veins; stable isotope homogenization; rinds on blocks) is particularly abundant in melange zones, in which melange matrix compositions resulting from mechanical mixtures of mafic, ultramafic, and sedimentary rocks were shifted by metasomatic additions and subtractions during melange formation. Geochemical evidence (particularly stable isotope data) indicates that the blueschist, greenschist, and amphibolite units exchanged with fluids of similar compositions. The diverse metasomatic features in the Catalina Schist provide evidence regarding fluid sources and paths. Based on the stable isotope data, the H2O-rich, low-salinity (∼ 1 to 2 equivalent wt. % NaCl), C/1bO/1bH/1bS/1bN fluids are believed to have been derived from low-grade, largely sedimentary parts of the subduction zone (analogs for fluid sources are the low-grade units). Metasomatic changes could be driven by flow across boundaries between contrasting lithologies and by variations in pressure and temperature along the fluid flow paths. Simple predictions of mass changes along different P-T paths suggest that both mechanisms could be effective at producing the range of observed features, even though the required equilibrium constants are only poorly estimated at the relevant P-T conditions. Decreasing T and P favors fixing of K, Si, C, and H in rocks, whereas increasing T (± moderately decreasing P) should fix Na but leach most other components. The Si-rich, K ± Si-rich, and Na-rich/Si-poor assemblages are thus consistent with differing fluid P-T flow paths. Regular differences are expected in silica precipitation/dissolution, alkali exchange, and hydrogen-alkali exchange reactions, among others. Silica ± carbonate addition, consistent with the majority of veins observed, is likely the consequence of cooling ± decompression whereas sodic (± silica-undersaturated) assemblages would be expected for rarer, but geologically plausible up-T fluid flow paths. A composite fluid flow path, first up-grade, then down P and T. is indicated for the silica addition to the largely ultramafic amphibolite-facies melange. Although mass balance and physical constraints appear to preclude pervasive major element metasomatism on large scales, focussing of fluids would likely produce pervasive changes in significant volumes (e.g., up to km-scale melange zones). Vein mineralogy would record the paths even at small fluxes. Study of the Catalina Schist demonstrates the significance of metasomatism at all scales, but indicates that large-scale changes in vein mineralogy and bulk composition are in some cases attributable to fluid flow over large distances. Comparison with other areas and elementary theoretical considerations suggest that these processes may be widely developed and that their petrographic and geochemical effects potentially give insight into the dynamics of subduction zones.

Fluid flow and metasomatism in a subduction zone hydrothermal system: Catalina Schist terrane, California

On Santa Catalina Island, southern California, bluechist to amphibolite facies metasedimentary, metamafic, and meta-ultramafic rocks show veining and alteration that reflect fluid flow and mass transfer at 25-45 km depths in an Early Cretaceous subduction zone. Synkinematic and postkinematic veins record fluid transport and metasomatism during prograde metamorphism and uplift. Vein and host-rock mineralogy and whole-rock compositions demonstrate large-scale chemical redistribution, especially of Si and alkali elements. Veins and host rocks trend toward isotopic equilibration with aqueous fluids with {delta}{sup 18}O{sub SMOW}=+13{per thousand} {plus minus} 1{per thousand}. The likely source for these fluids is in lower temperature, sediment-rich parts of the subduction zone. Carbon isotope systematics support this conclusion and indicate the influence of an organic C source. Quartz solubility relations indicate the importance of fluid-flow paths in chemical redistribution during subduction. These results document large-scale fluid flow and the complexity of possible metasomatic and mechanical mixing processes at intermediate levels of subduction zones. The record of subduction-zone mass transfer in the Catalina Schist is compatible with the record inferred for greater depths from geochemical and petrologic studies of arc magmatism.

Tectonic and metasomatic mixing in a high-T, subduction-zone mélange: insights into the geochemical evolution of the slab-mantle interface

Abstract The Catalina Schist (California) contains an amphibolite-grade (0.8–1.1 GPa; 640–750 °C) melange unit consisting of mafic and ultramafic blocks in high-Mg, schistose melange matrix with varying modal proportions of talc, chlorite, anthophyllite, calcic-amphibole, enstatite, and minor phases including zircon, rutile, apatite, spinel, and Fe–Ni sulfides. This melange unit is interpreted as a kilometer-scale zone of tectonic and metasomatic mixing formed within a juvenile subduction zone, the study of which may yield insight into chemical mixing processes at greater depths in subduction zones. Relationships among the major and trace element compositions of the mafic and ultramafic blocks in the melange, the rinds developed at the margins of these blocks, and the surrounding melange matrix are compatible with the evolution of the melange matrix through a complex combination of infiltrative and diffusional metasomatism and a process resembling mechanical mixing. Simple, linear mixing models are compatible with the development of the melange matrix primarily through simple mixture of the ultramafic and mafic rocks, with Cr/Al ratios serving as indicators of the approximate proportions of the two lithologies. This conclusion regarding mafic–ultramafic mixing is consistent with the field observations and chemical trends indicating strong resemblance of large parts of the melange matrix with rinds developed at the margins of mafic and ultramafic blocks. The overall process involved development of metasomatic assemblages through complex fluid-mediated mixing of the blocks and matrix concurrent with deformation of these relatively weak rind materials, which are rich in layer silicates and amphibole. This deformation was sufficiently intense to transpose fabrics, progressively disaggregate more rigid, block-derived materials in weaker chorite- and talc-rich melange, and in some particularly weak lithologies (e.g., chlorite-, talc-, and amphibole-rich materials), intimately juxtapose adjacent lithologies at the (sub-)cm scale (approaching grain scale) sampled by the whole-rock geochemical analyses. Chemical systematics of various elements in the melange matrix can be delineated based on the Cr/Al-based mixing model. Simple mixing relationships exhibited by Al, Cr, Mg, Ni, Fe, and Zr provide a geochemical reference frame for considerations of mass and volume loss and gain within the melange matrix. The compositional patterns of many other elements are explained by either redistribution (local stripping or enrichment) at varying scales within the melange (Ca, Na, K, Ba, and Sr) or massive addition from external sources (Si and H2O), the latter probably in infiltrating H2O-rich fluids that produced the dramatic O and H isotopic shifts in the melange. Melange formation, resulting in the production of high-variance ultramafic assemblages with high volatile contents, may aid retention of volatiles (in this case, H2O) to greater depths in subduction zones than in original subducted mafic and sedimentary materials. The presence of such assemblages (i.e., containing minerals such as talc, chlorite, and Mg-rich amphiboles) would impact the rheology of the slab–mantle interface and perhaps contribute to the low-velocity seismic structure observed at/near the slab–mantle interface in some subduction zones. If operative along the slab–mantle interface, complex mixing processes such as these, involving the interplay between fluid-mediated metasomatism and deformation, also could impact slab incompatible trace element and isotopic signatures ultimately observed in arc magmas, producing “fluids” with geochemical signatures inherited from interactions with hybridized rock compositions.

Igniting flare-up events in Cordilleran arcs

High-fl ux pulses of magmatism that make up most of the exposed North American Cordilleran arcs are derived primarily from upper plate lithospheric source materials, and not the mantle wedge as most models would predict, based on a compilation of thousands of previously published Sr, Nd, and O isotopic data. Mass balance calculations show that no more than 50% of that mass can be mantle-derived. Flare-ups must have fundamentally developed simultaneously with crustal/lithospheric thickening, thus implying a connection. Subduction erosion from the trench side, and retroarc shortening from the foreland side are the main tectonic shortening processes that operate in conjunction with high fl ux magmatism during subduction, and therefore are likely triggers for fl are-up events in arc. These arcs represent the sites of crustal differentiation, and thus contribute to net continental growth, only if dense residual lower crust was returned to the convective mantle. Isotopic data shown here suggest that if convective removal of batholithic roots takes place, it must be a consequence and not a cause of episodic fl are-ups. The Altiplano-Puna Volcanic Complex in South America may be the most recent continental arc segment in flmode.

Metasomatism and partial melting in a subduction complex Catalina Schist, southern California

Partial melting accompanied high-pressure amphibolitization of eclogitic blocks and metasomatism of blocks and their peridotite matrix in a unit of the Catalina Schist, a subduction zone metamorphic terrane. Migmatitic blocks contain leucocratic zones, veins, and pods with albitic plagioclase ? quartz ? muscovite. Mineralogically similar dikes and veins cut the ultra-mafic matrix; a few veins can be traced back into migmatitic blocks. Uniform phase proportions and igneous textures indicate that the dikes and veins crystallized from silicate melts. Field relations, mineral and bulk compositions, and comparisons with experimental data suggest that the migmatites and dikes are partial melts of variably metasomatized amphibolites. Element-partitioning, phase-equilibrium, and fluid-inclusion data are compatible with partial melting of the amphibolites at P = ∼8–11 kbar and T = ∼640–750 °C, in the presence of a low-salinity aqueous fluid. Although the Catalina metamorphism occurred at much shallower levels than those inferred for subduction-related magmatism, the process envisioned is one of multistage, slab-derived metasomatic contamination of the mantle in the “hanging wall” of a subduction zone.

A chemical and isotopic study of the Laramide granitic belt of northwestern Mexico: Identification of the southern edge of the North American Precambrian basement

Along the Laramide belt of northwestern Mexico, granitic rocks of similar bulk composition show isotopic and trace element signatures that help to delineate the position of the southern edge of the North American Precambrian basement. In the northern part, the Laramide plutons (the ‘‘northern granites’’) intruded Proterozoic crystalline rocks and a thick Late Proterozoic through Paleozoic miogeoclinal cover of North American affinity. In the central part, the granitic bodies (the ‘‘central granites’’) were emplaced into a sequence of Paleozoic eugeoclinal rocks overlain by Late Triassic clastic units. The southern part of the belt (the ‘‘southern granites’’) intruded a less-known crust characterized by middle to late Mesozoic island-arc‐related volcanic and sedimentary rocks of the Guerrero terrane. Data from a suite of metaluminous to slightly peraluminous calc-alkalic granitic rocks along the belt display north-to-south geochemical and isotopic variations, which could correlate with the type of intruded basement. The northern and central granites are characterized by strongly fractionated, light rare earth element (REE)‐enriched patterns, which display generally pronounced negative europium anomalies, whereas the southern granites have lower total REE enrichments and much flatter chondrite-normalized slopes displaying almost no europium anomalies. Isotopic results also suggest regional variations, as shown by the following initial Sr and eNd ranges: 0.7070 to 0.7089 and24.2 to25.4, respectively, for the northern granites; 0.7060 to 0.7079 and23.4 to25.1 for the central granites; and 0.7026 to 0.7062 and20.9 to 14.2 for the southern granites. On the basis of their isotopic similarities, the Proterozoic mafic to intermediate lower crust revealed by xenoliths from young volcanic flows in southern Arizona and northern Mexico is interpreted as a reasonable parental source for the northern and central granites; however, mantle-derived melts are not excluded. The more primitive southern granites are interpreted to come from a source that lacked Proterozoic basement. Instead, they were probably derived by mixing of juvenile mantle melts with partial melts of the lower parts of the Guerrero terrane. In general, the north-to-south compositional variations of the Laramide granitic rocks of northwestern Mexico reflect the crustal structure underneath the batholiths. The Sr and Nd data indicate that the edge of the North American Precambrian basement extends approximately southeastward from the coastal batholith of central Sonora; then, about 200 km south of Hermosillo in southern Sonora, the edge bends eastward and continues to the east beneath the Sierra Madre Occidental volcanic province.

Iron Oxide(–Cu–Au–REE–P–Ag–U–Co) Systems

The Fe oxide(–Cu–Au–REE–P–U) family of Cu, Fe, and/or Au deposits (or IOCG) represents a geochemically coherent but geologically diverse group that formed globally from the Archean to the Holocene. IOCG systems exhibit intense, voluminous Na–Ca–K–Fe(–H) hydrothermal alteration related to flow of moderately to highly saline metal-rich, sulfur-poor brines. These fluids account for the characteristic sulfide-poor, oxide-rich mineralogy and the alkali-rich character of the alteration and for the varied contents of Cu, Au, and other metals. Associated igneous rocks range from mafic to felsic, subalkaline to alkaline. Metal enrichments vary with host-rock type and sulfur availability. Geologic settings are tectonically diverse but commonly have evidence for contemporaneous or older evaporitic environments. Magmatism drives most systems, yet clearly amagmatic examples occur. Geochemical and petrologic studies demonstrate igneous-dominated sources for some solutes and permissive evidence for a connection to magmatic fluids. In many cases, a central role for nonmagmatic saline fluids is evident. The geochemistry of the latter fluids rationalizes the key distinguishing features of the IOCG family. The diversity of the IOCG family parallels that seen in other major families of deposits; their distinctive attributes indicate that they comprise a separate class of (mainly) terrestrial hydrothermal systems.

In situ oxygen isotope analysis of monazite as a monitor of fluid infiltration during contact metamorphism: Birch Creek Pluton aureole, White Mountains, eastern California

Monazite from the hydrothermal aureole of the Cretaceous two-mica Birch Creek Plu- ton in the White Mountains of eastern California records the infiltration of magmatic fluids into the metasedimentary Early Cambrian Deep Spring Formation. Monazite in the Birch Creek Pluton displays concentric, euhedral magmatic zoning, 18 O 8.7 0.2‰, and Th-Pb magmatic ages of 78.0 0.7 Ma. The middle Deep Spring Formation 0.5 km east of the contact underwent moderate- to low-temperature alteration by F-rich mag- matic fluids; monazite displays patchy zoning but has similar 18 O values (8.7 0.4‰) and Th-Pb ages (78.3 1.6 Ma) to monazite in the Birch Creek Pluton. In contrast, monazite from the upper Deep Spring Formation 0.6 km west of the contact and outside the mapped hydrothermal zone shows concentric zoning, 18 O 5.2 0.3‰, and partially reset detrital ages from 583 to 1069 Ma. Deep Spring Formation monazite within the hydrothermal alteration zone dissolved and reprecipitated during magmatic fluid infiltra- tion, whereas monazite outside the zone was unaffected. In contrast, Deep Spring zircon within the hydrothermal alteration zone preserved its magmatic zoning and Cambrian- Precambrian U-Pb ages. Zircon can reliably date events prior to hydrothermal activity, whereas monazite, being more susceptible to alteration by fluids, is useful for mapping the extent and timing of fluid infiltration events.