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Textural and compositional variations in titanite constrain the roles of magma mixing and hydrothermal alteration in two plutons in central Utah: the Jurassic Notch Peak and the Oligocene Little Cottonwood stocks. In the Notch Peak intrusion, magmatic titanite grains usually have oscillatory zones combined with BSE-bright sector zones, in some cases surrounding simple unzoned cores. These grains are frequently overprinted by hydrothermal titanite with low concentrations of high field strength elements (HFSE). Magmatic titanite has an average δ18O of 6.0‰ and post-magmatic titanite is 6.2‰, as analyzed by SIMS. Average Zr-in-titanite temperatures are also similar, with 718 °C for magmatic and 711 °C for hydrothermal titanite. These observations indicate simple magmatic growth, followed by hydrothermal alteration by magmatic fluids. Titanite in aplite dikes and sills has lower concentrations of all trace elements except F. Many titanite grains in the aplites have late overgrowths of high-Fe titanite. This high-Fe titanite has δ18O of 6‰ and an average Zr-in-titanite temperature of 718 °C and likely precipitated from a last flush of exsolved magmatic water enriched in Cl and Fe. Titanite in the Little Cottonwood stock typically has distinct patchy cores with rounded and embayed ilmenite inclusions. Mafic enclaves have abundant titanite that is similar in texture and δ18O (5.1‰) to titanite in the host (δ18O = 4.9‰), but it has a slightly higher average Zr-in-titanite temperature (731 vs. 717 °C). The patchy cores in the enclaves have the highest average Zr-in-titanite temperature (759 °C) and distinctive REE patterns. The textural and compositional data indicate that a hotter, more reduced, ilmenite-bearing mafic magma mixed into an oxidized felsic magma, destabilizing existing ilmenite and allowing crystallization of titanite. In the granodiorite and in the enclaves, hydrothermal growth of titanite is evidenced by distinct narrow rims as well as anhedral titanite that grew between sheets of chloritized biotite. Secondary hydrothermal titanite typically has lower concentrations of most HFSE, but is relatively enriched in F, Mg, Mo, and U, and it has higher Nb/Ta and lower Th/U ratios. Post-magmatic titanite also has strikingly different REE patterns than magmatic titanite, including the absence of pronounced Eu anomalies and lower REE abundances. These chemical features are controlled by element solubilities in aqueous fluids. In most cases, hydrothermal titanite has δ18O values similar to magmatic titanite, indicating alteration and recrystallization from exsolved magmatic fluids. The involvement of meteoric water with low δ18O is evident locally; individual spots have δ18O as low as 1.7‰ in the Little Cottonwood stock. Titanite compositions and textures provide important insights into the origins of granitic rocks and can be used to distinguish separate batches of magma, gauge the evolution of magmatic rocks, assess mixing processes, and infer compositions of mixing components. Because titanite also forms hydrothermally, it retains hints about the composition, temperature, and oxygen fugacity of the hydrothermal fluids and reveals details about titanite-forming reactions. However, the Al-in-titanite geobarometer does not yield realistic pressures of crystallization and the use of titanite as a geochronometer is compromised by the development of U-rich hydrothermal titanite.

Polymetallic veins (Pb-Zn-Cu-Ag-Au) at the world-class Bingham Canyon, Utah, porphyry Cu-Au-Mo deposit have long been recognized, but poorly understood. They are laterally zoned outward from the center of the porphyry deposit transitioning from Fe-Cu to Pb-Zn-Cu-Ag-Au mineralization. Physical and chemical characterization of these polymetallic veins provide insight into the origin, timing, and controls of ore deposition. These sheared, sulfide-rich, NE/SW- trending veins are dominated by pyrite and multiple generations of quartz, with lesser amounts of other sulfide and gangue minerals. Gold (0.27–4.61 ppm) provides the most value to the ore, though the veins contain substantial Cu and Ag as well. Host rocks include Eocene monzonite and Paleozoic limestone and quartzite—all of which can contain economic ore lodes. Associated alteration is predominantly sericitic and argillic, with mineralization in wall rocks restricted to 1.5 m from the vein margins. Mineral assemblages vary with distance from the center of the main porphyry Cu-Au-Mo deposit and the modal abundances are dependent on the host rock. The appearance of both galena and sphalerite (and tennantite to an extent) occur along a boundary that creates a halo around the center of the associated porphyry deposit. This is accompanied by a shift in metal ratios and an increased concentration of chalcophile trace elements in sulfides from the polymetallic veins as determined by electron microprobe analyses (EMPA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Significant hosts of Ag include galena and tennantite, and Cu is hosted primarily in chalcopyrite, tennantite, and sphalerite. The main host of Au could not be determined, but Au could be focused along fractures or hosted in inclusions found in pyrite. The δ34S values of vein pyrite have a narrow range (2.3–3.4‰) suggestive of a magmatic source, whereas δ18O of quartz is more variable (11.5–14.0‰). These values are similar to several other polymetallic vein deposits associated with porphyry Cu deposits. This can be explained by fractionation of magmatic fluids at lower temperatures (350°–250°C) and/or mixing with exchanged 18O-rich meteoric water. Ore grades (Cu, Ag, Au) improve with distance from the center of the porphyry deposit; however, this is accompanied by higher concentrations of deleterious elements (e.g., Pb, As, Bi) for downstream processing. These polymetallic veins were created sequentially throughout the formation of the deposit. Initial joints in the sedimentary rocks probably formed as a result of emplacement of a barren equigranular monzonite intrusion, with continued dilation and propagation in all host rocks with each subsequent intrusion. The northeast orientation of the joints was controlled by the regional stress field, which is more apparent distal to the center of the Bingham deposit. Vein mineralization appears to postdate all intrusions and the porphyry Cu-Au mineralization; however, it may be related to the late fluids responsible for Mo mineralization in the main porphyry orebody that followed intrusion of the quartz latite porphyry. Quartz-sericite-pyrite mineralization associated with the veins precedes galena-sphalerite-pyrite mineralization. This was followed by late precipitation of chalcopyrite and tennantite and late normal faulting.

Several gabbro, gabbroic diorite, and diorite intrusions were emplaced in the Neotethyan Urumieh-Dokhtar magmatic arc in the Kahak area of the Zagros Orogen. The plutonic rocks intruded Cenozoic volcanic and pyroclastic succession. U-Pb dating via LA-ICP-MS of zircon yields early to middle Eocene ages of ~53 to 38 Ma for the diorite and gabbroic diorite in Veshnaveh and Naragh areas and Miocene ages of 23 to 20 Ma for gabbroic rocks in the Kerogan and Fordou areas. The Eocene magmatic phase occurred quite possibly as three continuous pulses. The Eocene and Miocene intrusions have negative Nb-Ta anomalies like those of subduction-related magmas. The Pb isotopic compositions show the involvement of subducted sediment in the source of the studied samples. The Eocene intrusions are tholeiitic and enriched in incompatible-trace-elements (especially the HFSE) compared to the Miocene intrusions which are calc-alkaline and have lower concentrations of most incompatible trace elements. Sr-Nd isotopic compositions of the Eocene rocks lie close to the mantle array and near Bulk Silicate Earth, but the Miocene suite is shifted to higher (87Sr/86Sr)i. The higher (87Sr/86Sr)i, along with larger Ba, K, Pb, and Sr anomalies, in the Miocene suite imply more slab fluid input and/or crustal contamination. Apparently, the Eocene mafic magmas formed in slightly metasomatized sub-continental lithospheric mantle (SCLM), whereas the Miocene suite was derived from highly metasomatized, deeper levels of the SCLM. We suggest that slab roll back occurred between the Eocene and Miocene and transferred the location of mantle partial melting to greater depths.

Metallic sublimates coated by sulfides and chlorides line the vesicle walls of mafic volcanic lava and bombs from Kīlauea, Vesuvius, Etna, and Stromboli. The metallic sublimates were morphologically and compositionally similar among the volcanoes. The highest concentrations of S and Cl occurred on the surface of the sublimates, while internally they had less than 1 wt % S and Cl in most cases, leading us to classify them as alloys. The major components of the alloys were Cu, Sn, Co, and Ag based on electron microprobe analyses and environmental scanning electron microscope element maps. Alloy element maps showed a covariance of Cu-Sn, while Co and Ag concentrations varied independently. Laser ablation-inductively coupled plasma-mass spectrometry analysis of matrix glass and melt inclusions in bombs from Stromboli showed appreciable amounts of Cu, Co, and Sn. We propose a model for the origin of the metallic grains, which involves syneruptive and posteruptive magma degassing and subsequent cooling of the basalt vesicles. During syneruptive vapor phase exsolution, volatile metals (Cu, Co, and Sn) partition into the vapor along with their ligands, S and Cl. The apparent oxygen fugacity (fO2) in these vapor bubbles is low because of the relative enrichment of the exsolved gas phase in H2 relative to H2O in silicate melts, due to the much higher diffusivity of the former in silicate melts. The high fH2 and low fO2 induces the precipitation of metal alloys from the vapor phase. Subsequently, the reducing environment in the vesicle dissipates as the cooling vapor oxidizes and as H2 diffuses away. Then, metal-rich sulfides (and chlorides) condense onto the outer surfaces of the metal alloy grains either due to a decrease in temperature or an increase in fO2. These alloys provide important insights into the partitioning of metals into a magmatic volatile phase at low pressure and high temperature.

Physical and compositional measurements are made at the ~ 7 km-long (~ 2200 years B.P.) Kings Bowl basaltic fissure system and surrounding lava field in order to further understand the interaction of fissure-fed lavas with phreatic explosive events. These assessments are intended to elucidate the cause and potential for hazards associated with phreatic phases that occur during basaltic fissure eruptions. In the present paper we focus on a general understanding of the geological history of the site. We utilize geospatial analysis of lava surfaces, lithologic and geochemical signatures of lava flows and explosively ejected blocks, and surveys via ground observation and remote sensing. Lithologic and geochemical signatures readily distinguish between Kings Bowl and underlying pre-Kings Bowl lava flows, both of which comprise phreatic ejecta from the Kings Bowl fissure. These basalt types, as well as neighboring lava flows from the contemporaneous Wapi lava field and the older Inferno Chasm vent and outflow channel, fall compositionally within the framework of eastern Snake River Plain olivine tholeiites. Total volume of lava in the Kings Bowl field is estimated to be ~ 0.0125 km3, compared to a previous estimate of 0.005 km3. The main (central) lava lake lost a total of ~ 0.0018 km3 of magma by either drain-back into the fissure system or breakout flows from breached levees. Phreatic explosions along the Kings Bowl fissure system occurred after magma supply was cut off, leading to fissure evacuation, and were triggered by magma withdrawal. The fissure system produced multiple phreatic explosions and the main pit is accompanied by others that occur as subordinate pits and linear blast corridors along the fissure. The drop in magma supply and the concomitant influx of groundwater were necessary processes that led to the formation of Kings Bowl and other pits along the fissure. A conceptual model is presented that has relevance to the broader range of low-volume, monogenetic basaltic fissure eruptions on Earth, the Moon and other planetary bodies.

Rapakivi and alkali feldspar phenocrysts from the Devonian Deer Isle Granite Complex, Maine were investigated to see if magma mixing or isothermal decompression was responsible for their formation. Pb-isotope and trace-element analyses of alkali feldspar ovoids and their plagioclase inclusions were obtained by LA-ICP-MS to determine the characteristics of the initial magma; these were compared with the plagioclase mantles. Rapakivi grains show a decrease in Pb-isotopic values and an increase in Ba, La, and Ce concentrations from the ovoids and inclusions to the innermost plagioclase mantles adjacent to the ovoids. These variations, along with CL imagery of quartz phenocrysts, indicate an open system with compositional changes in the magma chamber occurring during rapakivi feldspar growth. Repeated episodes of magma mixing/replenishment by Ba and LREE enriched magmas with lower Pb-isotopic ratios (along with hybridized variations with the host magma) created localized dissolution of alkali feldspar and quartz. Alkali feldspar phenocrysts proximal to these zones of thermal perturbation were first resorbed and then mantled by plagioclase that records the more primitive signature of the more mafic melt. Pb-isotopic values and Ba, La, and Ce concentrations trend back to the initial magma composition at the mantle rims as the effects of the mixing event dissipated. For grains that were somewhat more distal to these zones of mixing, resorption of the alkali feldspar crystals did not occur, yet the fresh supply of Ba resulted in Ba-rich alkali feldspar mantles over Ba-poor alkali feldspar cores. Other alkali feldspar crystals were too far from the site of mixing; hence, they lack any evidence of open system crystallization. As crystallization of the chamber continued along solidification fronts, batches of cooler crystal-rich magmas settled en masse to the floor. Disaggregation of these batches during settling, and subsequent accumulation on the reservoir floor, brought grains with disparate crystallization histories together. Filter pressing of the cumulate pile flushed highly evolved melts/fluids out from interstitial pores to the high silica, upper portions of the chamber. The crystallization, transportation, and juxtaposition of disparate types of feldspar phenocrysts suggest that a relatively large and active magma chamber, periodically recharged by localized batches of melt, must have existed. In this case, it is clear that rapakivi mantled feldspars are the result of magma mixing; such interpretations may apply to many other occurrences as well.

The morphologies of craters on planetary surfaces reveal clues about the geologic mechanisms by which they originate and subsequently evolve, as well as the materials and physical variables inherent to the environment in which they formed. We carried out a quantitative multivariate analysis of shape descriptors derived from the outlines of craters formed by volcanic processes on Mars, Io, and Earth and by impact cratering on the Moon using elliptic Fourier analysis (EFA) and the Zahn-Roskies (Z-R) shape function. Canonical variate analysis (CVA) was used to construct a statistical model of differences between the crater groups to classify craters produced by various volcanic and impact processes. The classification model from canonical variate analysis of EFA shape descriptors yielded a 90% rate of success for the assignment of group membership among 406 examined craters. It correctly classified 138 of 154 (90%) ionian paterae,154 of 155 (99%) lunar impact craters, 31 of 35 (89%) terrestrial basaltic shield calderas, 32 of 38 (84%) terrestrial ash-flow calderas, and 12 of 24 (50%) martian basaltic shield calderas. The classification model from canonical variate analysis of Z-R shape function descriptors classified 84% of the total population of the examined craters correctly. The analysis correctly classified 96% of ionian paterae, 100% lunar impact craters, 51% terrestrial basaltic shield calderas, and 63% martian calderas, but only 16% of the terrestrial ash-flow calderas were correctly classified. Canonical variate analysis of EFA and Z-R results shows that the shapes of ash-flow calderas and paterae on Io differ the least of all groups included in this study, and basaltic shield calderas and martian calderas analyzed together also have few differences. The Z-R model successfully classifies more ionian patera and impact craters than the EFA classification model but performs poorly at classifying the other crater groups. This result shows that the descriptors convey different shape information. The Z-R model is robust in its ability to classify end-member differences in complexity while the EFA model is robust in its ability to reliably classify among more groups. These differences and similarities in shape confirm previously understood commonalities related to the origin and evolution of various types of craters. In general, basalt shield calderas on Earth and Mars are morphologically similar and are thought to have similar origins; this study confirms that the 2-D shapes of their craters are quantitatively correlated. Similarities have been noted between terrestrial ash-flow calderas and paterae on Io, principally in their large sizes, shallow magma chambers and complex evolution; this study confirms their shapes are also similar. Impact craters and ionian paterae are most dissimilar, as are their evolutions. This study demonstrates rigorous landform shape analysis can greatly increase our understanding of the diversity in craters and the processes involved in their formation.

We present new high-precision CA-ID-TIMS and in situ U–Pb ages together with Hf and O isotopic analyses (analyses performed all on the same grains) from four tuffs from the 15−10 Ma Bruneau–Jarbidge center of the Snake River Plain and from three rhyolitic units from the Kimberly borehole in the neighboring 10−6 Ma Twin Falls volcanic center. We find significant intrasample diversity in zircon ages (ranges of up to 3 Myr) and in δ18O (ranges of up to 6‰) and eHf (ranges of up to 24 e units) values. Zircon rims are also more homogeneous than the associated cores, and we show that zircon rim growth occurs faster than the resolution of in situ dating techniques. CA-ID-TIMS dating of a subset of zircon grains from the Twin Falls samples reveals complex crystallization histories spanning 104–106 years prior to some eruptions, suggesting that magma genesis was characterized by the cyclic remelting of buried volcanic rocks and intrusions associated with previous magmatic episodes. Age-dependent trends in zircon isotopic compositions show that rhyolite production in the Yellowstone hotspot track is driven by the mixing of mantle-derived melts (normal δ18O and eHf) and a combination of Precambrian basement rock (normal δ18O and eHf down to − 60) and shallow Mesozoic and Cenozoic age rocks, some of which are hydrothermally altered (to low δ18O values) by earlier stages of Snake River Plain magmatism. These crustal melts hybridize with juvenile basalts and rhyolites to produce the erupted rhyolites. We also observe that the Precambrian basement rock is only an important component in the erupted magmas in the first eruption at each caldera center, suggesting that the accumulation of new intrusions quickly builds an upper crustal intrusive body which is isolated from the Precambrian basement and evolves towards more isotopically juvenile and lower-δ18O compositions over time.

Mafic complexes in the central Sierra Nevada batholith record valuable geochemical information regarding the role mafic magmas play in arc magmatism and the generation of continental crust. In the intrusive suite of Yosemite Valley, major and trace element compositions of the hornblende-bearing gabbroic rocks from the Rockslides mafic complex and of the mafic dikes in the North America Wall are compositionally similar to high-alumina basalt. Of these rocks, two samples have higher Ni and Cr abundances as well as higher eNd values than previously recognized for the intrusive suite. Plagioclase crystals in rocks from the North America Wall and the Rockslides have prominent calcic cores and sharply defined sodic rims, a texture commonly associated with mixing of mafic and felsic magmas. In situ analyses of 87Sr/86Sr in plagioclase show no significant isotopic difference from the cores to the rims of these grains. We propose that the high 87Sr/86Sr (~0.7067) and low eNd (~−3.4) of bulk rocks, the homogeneity of 87Sr/86Sr in plagioclase, and the high δ18O values of bulk rocks (6.6–7.3 ‰) and zircon (Lackey et al. in J Petrol 49:1397–1426, 2008) demonstrate that continental crust was assimilated into the sublithospheric mantle-derived basaltic precursors of the mafic rocks in Yosemite Valley. Contamination (20–40 %) likely occurred in the lower crust as the magma differentiated to high-alumina basalt prior to plagioclase (and zircon) crystallization. As a consequence, the isotopic signatures recorded by whole rocks, plagioclase, and zircon do not represent the composition of the underlying lithospheric mantle. We conclude that the mafic and associated felsic members of the intrusive suite of Yosemite Valley represent 60–80 % new additions to the crust and include significant quantities of recycled ancient crust.

Magmatic sulfides in 97 samples of volcanic and intrusive rocks from the Tertiary Bingham (Cu-Au-Mo) and Tintic (Ag-Pb-Zn-Cu-Au) districts, Utah, were examined to help better understand the fate of magmatic sulfides during intrusion and eruption. Our findings show that shallowly emplaced dikes and sills have erratic but locally high concentrations of sulfides. Volcanic rocks and large porphyry intrusions from these districts typically have at least two orders of magnitude fewer sulfides than the dikes. Sulfide concentrations vary dramatically across these dikes and sills; for example, in one sill in Castro Gulch, Bingham district, sulfide abundance increases from 9 ppm by volume in the center to more than 2,000 ppm near the margin. Chalcophile metals show corresponding changes in abundance. For example, the whole-rock copper content of the sill ranges from 23 ppm in the center to 35 ppm along the margins. The textures of sulfide grains (interpreted to reflect recrystallization, resorption, and degassing) even in the most sulfide-rich samples, commonly have been modified, suggesting that no sample preserves all of its original magmatic sulfide content. Immiscible liquids of monosulfide solid solution crystallized as pyrrhotite, pyrrhotite and chalcopyrite, or pyrite and chalcopyrite with declining temperature and pressure. These locally recrystallized to pyrite and chalcopyrite or to pyrite and an Fe oxide as they are oxidized. The alteration and preservation textures change from subspherical sulfide blebs near the margins of dikes and sills, to partially altered sulfides farther in, to complete absence of sulfides in the vast majority of intrusions (except where small sulfides are completely enclosed by phenocrysts). Sulfide concentrations appear to vary according to cooling rate and inferred pressure at the time of quenching or crystallization of the matrix. Most of the sulfides along the quenched margins of these dikes and sills are in the matrix. Slower cooling coupled with removal of magmatic volatiles, including sulfurous gases (e.g., H2S, SO2), allows the resorption or oxidation of magmatic sulfides to occur during final crystallization of a magma. Together, these processes remove greater than 90 percent of the original endowment of magmatic sulfides. This probably explains the low-magmatic sulfide abundances of slowly cooled, large porphyritic intrusions, and most importantly, allows metals and sulfur to participate in the formation of porphyry deposits. The relative abundances of base metals lost from the center of the sill are similar to the relative abundances of the metals in the Bingham deposit (production and reserves), suggesting that these processes also may have operated at a larger scale.

Unusual monotonous intermediate ignimbrites consist of phenocryst-rich dacite that occurs as very large volume (>1000 km 3 ) deposits that lack systematic compositional zonation, comagmatic rhyolite precursors, and underlying plinian beds. They are distinct from countless, usually smaller volume, zoned rhyolite–dacite–andesite deposits that are conventionally believed to have erupted from magma chambers in which thermal and compositional gradients were established because of sidewall crystallization and associated convective fractionation. Despite their great volume, or because of it, monotonous intermediates have received little attention. Documentation of the stratigraphy, composition, and geologic setting of the Lund Tuff – one of four monotonous intermediate tuffs in the middle-Tertiary Great Basin ignimbrite province – provides insight into its unusual origin and, by implication, the origin of other similar monotonous intermediates. The Lund Tuff is a single cooling unit with normal magnetic polarity whose volume likely exceeded 3000 km 3 . It was emplaced 29.02±0.04 Ma in and around the coeval White Rock caldera which has an unextended north–south diameter of about 50 km. The tuff is monotonous in that its phenocryst assemblage is virtually uniform throughout the deposit: plagioclase>quartz≈hornblende>biotite>Fe–Ti oxides≈sanidine>titanite, zircon, and apatite. However, ratios of phenocrysts vary by as much as an order of magnitude in a manner consistent with progressive crystallization in the pre-eruption chamber. A significant range in whole-rock chemical composition (e.g., 63–71 wt% SiO 2 ) is poorly correlated with phenocryst abundance. These compositional attributes cannot have been caused wholly by winnowing of glass from phenocrysts during eruption, as has been suggested for the monotonous intermediate Fish Canyon Tuff. Pumice fragments are also crystal-rich, and chemically and mineralogically indistinguishable from bulk tuff. We postulate that convective mixing in a sill-like magma chamber precluded development of a zoned chamber with a rhyolitic top or of a zoned pyroclastic deposit. Chemical variations in the Lund Tuff are consistent with equilibrium crystallization of a parental dacitic magma followed by eruptive mixing of compositionally diverse crystals and high-silica rhyolite vitroclasts during evacuation and emplacement. This model contrasts with the more systematic withdrawal from a bottle-shaped chamber in which sidewall crystallization creates a marked vertical compositional gradient and a substantial volume of capping-evolved rhyolite magma. Eruption at exceptionally high discharge rates precluded development of an underlying plinian deposit. The generation of the monotonous intermediate Lund magma and others like it in the middle Tertiary of the western USA reflects an unusually high flux of mantle-derived mafic magma into unusually thick and warm crust above a subducting slab of oceanic lithosphere.

The Bingham porphyry Cu–Au–Mo deposit, Utah, may only be world-class because of substantial contributions of sulfur and metals from mafic alkaline magma to an otherwise unremarkable calc-alkaline system. Volcanic mafic alkaline rocks in the district are enriched in Cr, Ni, and Ba as well as Cu, Au, platinum group elements (PGE), and S. The bulk of the volcanic section that is co-magmatic with ore-related porphyries is dacitic to trachytic in composition, but has inherited the geochemical signature of high Cr, Ni, and Ba from magma mixing with the mafic alkaline rocks. The volcanic section that most closely correlates in time with ore-related porphyries is very heterogeneous containing clasts of scoriaceous latite, latitic, and minette, and flows of melanephelinite, shoshonite, and olivine latite in addition to volumetrically dominant dacite/trachyte. Bingham ore-related porphyries show ample evidence of prior mixing with mafic alkaline magmas. Intrusive porphyries that have not been previously well-studied have several chemical and mineralogical indications of magma mixing. These "mixed" lithologies include the hybrid quartz monzonite porphyry, biotite porphyry, and minette dikes. Even some of the more silicic latite and monzonite porphyries retain high Cr and Ba contents indicative of mixing and contain trace amounts of sapphire (

This paper reports the existence of abundant magmatic sulfides in INTRODUCTION latite lava vitrophyres and vent-facies dikes from the Bingham and The eruption of sulfur-rich magmas from El Chichon Tintic districts in central Utah. These dikes and flows appear to (Luhr et al., 1984) and Mt Pinatubo (Westrich & Gerlach, be comagmatic with the ore-related monzonite intrusions. In addition, 1992) in recent years has attracted attention to the limited analytical data suggest that the magmatic sulfides contain most of solubility of sulfur in intermediate magmas and the factors the Cu and Ag present in the latites. Magmatic sulfides are preserved potentially responsible for the formation of magmatic only in the least oxidized and least degassed lavas and vitrophyres. sulfides and anhydrite. However, the occurrence of magPetrography of comagmatic dikes and intrusions suggests that matic sulfides in intermediate magmas is not universal; resorption and oxidation (S-degassing) of the globules may make many volcanic suites appear to be sulfide unsaturated. the metal content available to a magmatic ore-fluid. Minette and Inasmuch as magmatic sulfides may sequester chalcophile shoshonite lavas occur in subordinate amounts in the latitic volcanic metals, what would their presence signify in magmatic fields associated with latite–monzonite intrusions and hydrothermal systems that formed porphyry Cu and mesothermal vein ore deposits of the Tintic (Ag–Pb–Zn–Cu–Au) and Bingham deposits? Do they sequester a significant portion of the (porphyry Cu–Au–Mo) districts, Utah. The ages, chemical and magmatic Cu, Pb, Zn, Ag, and Au, thereby restricting modal compositions, and field relations of the mafic alkaline lavas metal availability of subsequent ore-forming fluids? suggest that such magmas mixed and fractionated at shallow crustal We have examined the volcanic rocks from the Tintic levels to form the ore-related latites and monzonites. The unaltered and Bingham mining districts (Fig. 1), Utah, and found mafic alkaline lavas in these districts are some of the most Cu-rich that almost all of the volcanic units contain magmatic lavas in the Great Basin province. sulfides. Some quenched dikes and intrusion margins also have preserved magmatic sulfides comparable with those in the volcanic rocks. Several lines of evidence indicate that the volcanic and intrusive rocks are comagmatic (Keith et al., 1989, 1991; Waite, 1996). The Tintic district and East Tintic sub-district in Utah have recorded over

The relative timing and magnitude of middle Tertiary extension and volcanism in the Great Basin (northern Basin and Range province) of the western United States remain controversial. To constrain the timing, we present 31 stratigraphic sections from the central part of the province, together with data from other studies in the Great Basin. Especially significant in this record of regional paleogeographic and associated tectonic conditions are thick sections of many well-dated ash flow sheets emplaced during the period of the most voluminous, or peak, volcanic activity about 31–20 Ma. From these data we make the following conclusions: (1) Extension prior to the period of peak volcanism was apparently localized. (2) Extension during peak volcanism (the ignimbrite flareup) was minor and in places possibly related to magmatic processes in the shallow crust, rather than to regional tectonic processes. Angular unconformities and interbedded epiclastic deposits within sequences of volcanic rocks from 31 to about 22–20 Ma that would manifest synvolcanic faulting, tilting, and erosion are limited. (3) In the Great Basin as a whole, major extension and peak volcanism correlate poorly in space as well as time. (4) Essentially dip-slip faults cutting the entire conformable volcanic sequence are common in the Great Basin and indicate a widespread episode of extension after peak volcanism. Southward sweeping Tertiary volcanism in the Great Basin reflects migration of the mantle magma supply that powered crustal magma systems. We suspect this migration was related to progressive southward foundering and steepening of dip of a subducting oceanic plate (after an earliest Tertiary near-horizontal configuration beneath the continental lithosphere) and consequent backflow of asthenospheric mantle into the widening wedge between the plates. In the northern Great Basin, where the sweep was rapid, we postulate that relatively small volumes of mantle-derived magma were inserted as dikes into the lower, locally extending crust which was unusually warm because of Mesozoic compressional thickening; crustal magma systems so powered were repeatedly tapped to feed modest volume eruptions of chiefly intermediate composition lava and minor silicic ash flow tuff. As the sweep stagnated in the central southern Great Basin, copious volumes of mafic magma were inserted into the crust, apparently mostly as extensive horizontal sheets, or sills, in a nonextending, uplifting crust in a state of nearly isotropic horizontal stress. These sills and the high mantle power input optimized crustal magma generation, creating huge volumes of silicic magma that vented as large volume ash flows, chiefly about 31–20 Ma. After about 22–20 Ma, the volcanic-capped plateau collapsed in a widespread network of north striking extensional faults as plate boundary compressive forces were overcome by spreading forces within the uplift. Eruption of lava again became the dominant mode of volcanism.

Inclusions of quenched mafic magma occur in a 21 Ma rhyolite lava and precursory non-welded tuff that form the Spor Mountain Formation in west-central Utah. The mafic inclusions are not lithic inclusions; no comparable volcanic unit was present at the surface when the Spor Mountain Formation erupted. Mineral and bulk compositions preclude liquid immiscibility. The mafic inclusions show clear morphologic and textural effects of magma mingling shortly before eruption of the rhyolite. Globular inclusions from both units are vesicular, phenocryst-poor, plagioclase-sanidine-clinopyroxene-orthopyroxene-magnetite-ilmenite latites and trachytes with quench temperatures of about 1000°C. Although overlapping in SiO2, TiO2, Zr, and Hf concentrations, inclusions from the underlying tuff lack negative Eu anomalies and are enriched in P2O5, K2O, Al2O3, Sr, Pb, Cr, and Ni whereas those hosted in the overlying lava have small negative Eu anomalies and two-fold enrichments in Fe2O3, MnO, HREE, Y, Ta, Th, Rb, and Cs. The most reasonable explanation for the differences between the two sets of inclusions lies in selective chemical exchange between the rhyolite lava and the mafic inclusions after eruption. Limited mechanical mixing occurred after the inclusions solidified and became chemically modified. The textures of the inclusions in the lavas and the elements selectively mobilized in the inclusions imply that vapor-phase transport occurred in this low-pressure volcanic environment. If such substantial variations in inclusion compositions can arise during what must have been a short period of time before chemical reactions were halted by rapid cooling, it seems unlikely that the compositions of mafic inclusions formed by magma mingling in slowly cooled granites preserve their original compositions, mineralogies, or information about their ultimate sources. Using the compositions of such chemically modified inclusions as end-members for mixing calculations may lead to erroneous results regarding the significance of magma mixing in plutonic rocks.

Topaz rhyolites are fluorine-rich alkaline silicic lavas and shallow intrusives that are characterized by the presence of topaz (Al 2 SiO 4 F 2 ) in gas cavities, commonly associated with Mn-Fe garnet, bixbyite, pseudobrookite, specularite, quartz, and other minerals. In the western United States, Cenozoic (0.5-50 m.y. old for dated examples) topaz rhyolites occur on both sides of the Colorado Plateau (in Colorado, New Mexico, Arizona, Utah, and Nevada) and in Idaho and Montana. They also occur in a single linear belt in Mexico. Their enrichment in lithophile (fluorophile) elements (Li, Rb, Cs, U, Th, Nb, Ta, Sn, W, Be, etc.) leads to the term rare metal rhyolites. Similar F-rich rocks from Mongolia and the Soviet Union have been called ongonites. Topaz rhyolites appear to represent a special class of the bimodal or high silica rhyolites of the western United States.Their extensional tectonic setting and geochemical characteristics suggest that topaz rhyolites are the extrusive equivalents of anorogenic or residual (A- or R-type) granites. Their petrogenesis presumably involves partial melting of Precambrian continental crust (they appear to be restricted to areas of such crust) in the presence of a high heat flow (which tends to enrich F in solids at the expense of H 2 O). Mafic magmas may provide the heat for melting. Further differentiation may depend on (1) zone refining during ascent, (2) extreme fractional crystallization, (3) dehydration due to early pyroclastic volcanism, and (4) apical enrichment of near-surface magma chambers due to liquid state processes.Practical interest in topaz rhyolites results, in part, from the spatial and genetic association with volcanogenic deposits of Be, U, Sn, and F. Spor Mountain, Utah, provides the best example of this type of mineralization. Topaz rhyolite lavas from productive districts tend to be relatively phenocryst rich, poor in miarolitic cavities, and granophyrically crystallized. Vitrophyres, where present, are exceptionally rich in F and fluorophile elements, and rare earth patterns are exceptionally fiat (low La/Yb ratios) and have pronounced negative Eu anomalies. Near-surface country rocks penetrated by the volcanic vents were reactive (carbonate rocks) and water tables were presumably high.Most topaz rhyolite lavas are not associated wth volcanogenic mineralization. These nonproductive lavas are generally phenocryst poor, distinctly flow banded, lithophysa rich, and spherulitically-crystallized. Vitrophyres are less enriched in F and fluorophile elements, country rocks are nonreactive, and water tables could have been low.Topaz rhyolites of both the productive and nonproductive type may be valuable indicators of subsurface mineralization. Their distribution coincides very closely with that of (1) fluorite and silver-base metal districts, (2) the central and eastern tungsten belts of Kerr (1946), and (3) topaz-rich porphyry Mo-W deposits of the Climax and Henderson type. Topaz rhyolite volcanic vent areas might then reflect the existence of large F-rich magma chambers below with (1) subvolcanic breccia, porphyry, and greisen vein deposits of Mo, W, Sn, and other elements, (2) base and precious metal veins and fluorite-rich replacements, and (3) more deeply seated rare metal pegmatites.

High-silica topaz-bearing rhyolites of Cenozoic age are widely distributed across the western USA and Mexico. They are characteristically enriched in fluorine (>0.2 wt.%) and incompatible lithophile elements (e.g. Li, Rb, Cs, U, Th, Be). In addition to topaz, the rhyolites contain garnet, bixbyite, pseudobrookite, hematite and fluorite in cavities or in their devitrified groundmasses. Magmatic phases include sanidine, quartz, oligoclase and Fe-rich biotite. Allanite, fluorite, zircon, apatite and magnetite occur in most; pyroxene, hornblende, ilmenite and titanite occur in some. The rhyolites crystallized over a wide temperature interval (850° to 600° C) at \(f_{0_2 } \) that ranges from QFM to NNO. The REE patterns of most topaz rhyolites are almost flat (La/YbN=1 to 3) and have deep Eu anomalies (Eu/Eu*=0.01 to 0.02). Both parameters decrease with differentiation. Titanite-bearing rhyolites have prominent middle REE depletions. Topaz rhyolites appear to have evolved from partial melts of a residual granulitic source in the Precambrian lower crust. According to the proposed model, the passage of hot mafic magmas through the crust produced partial melts as a result of the decomposition of F-rich biotite or amphibole. An extensional tectonic setting allowed these small batches of magma to rise without substantial mixing with contemporaneous mafic magmas.

A geochemically and mineralogically diverse group of granitoids is present within an area of 900 km2 in the southern Snake Range of eastern Nevada. The granitoids exposed range in age from Jurassic through Cretaceous to Oligocene and include two calcic intrusions, two different types of two-mica granites, and aplites. The younger intrusions appear to have been emplaced at progressively more shallow depths. All of these granitoid types are represented elsewhere in the eastern Great Basin, but the southern Snake Range is distinguished by the grouping of all these types within a relatively small area.

Analysis of fluorine and chlorine in 228 samples of granitoids' from the Basin and Range province of the western United States suggests that at least three types of granitoids can be distinguished: (1) fluorine-poor granitoids of the northwestern Great Basin (mean F = 0.041 wt %, maximum = 0.11 wt %) intrude a variety of allochthonous oceanic or island-arc terranes that were probably accreted to North America during the Paleozoic and Mesozoic eras; (2) fluorine~rich Precambrian granites generated during "anorogenic" magmatism of Proterozoic -age in the southwestern United States (mean F = 0.118 wt %, maximum = Q.35 wt %); and (3) a large group of granitoids with moderate fluorine contents (meanF = 0.053 wt %, maximum = 0.13 wt %). Thislast group consists of granitoids ofthe eastern Great Basin and the southern Basin and Range province that occur throughout the autochthonous continental terrane of the western United States. These differences'in fluorine concentration do not appear to be the result of regional differences in the degree of magma evolution. Instead, this variability is attributed to magma contamination by, or generation from, a comparatively fluorine-rich res­ ervoir in the continental crust underlying the southern and eastern portions of the -Basin and Range province and to the absence of this reservoir in the northwestern Great Basin. This interpretation is consistent with the geologically established boundaries ofthe"exotic" terranes and with the Sr and Nd isotope compositions of rocks from these regions. Chlorine contents (mean CI = 0.013 wt %, range = 0.005-0.050 wt %) show no regional differences and are uniformly low in these granitoid rocks.