ABSTRACT This field trip will visit the southern Black Mountains volcanic center (SBMVC) and its associated products in northwestern Arizona. Post-volcanic extension and erosion of this semi-arid area afford the opportunity to examine a Miocene-aged (ca. 19–17 Ma) volcanic terrain in detail, using an approach that integrates fieldwork, petrography and geochemistry, and remotely sensed data. This approach was recently used in a three-year National Science Foundation Research Experience for Undergraduate (NSF REU) program, from which some results will be highlighted. This integrated approach can provide insight into the amounts and types of information that can be gleaned from various types of remotely sensed data of a volcanic terrain. Over three days, we will work our way through the volcanic section associated with the SBMVC as exposed near Oatman and Kingman, Arizona, to observe: (1) felsic to mafic shallow intrusions; (2) deposits from felsic to intermediate explosive and effusive eruptions; (3) deposits from sedimentary processes; and (4) the results of subsequent extensional faulting. The petrologic, physical volcanic, and morphologic characteristics of observed outcrops will be discussed at each field stop, and will be used as points of discussion while in the field, for observations of presumably volcanic terrains on other planetary bodies. An appended road log serves as a geologic guide to this volcanic center and associated rocks.

ABSTRACT Ion microprobe U-Pb zircon rim ages from 39 samples from across the accreted terranes of the central Blue Ridge, eastward across the Inner Piedmont, delimit the timing and spatial extent of superposed metamorphism in the southern Appalachian orogen. Metamorphic zircon rims are 10–40 µm wide, mostly unzoned, and dark gray to black or bright white in cathodoluminescence, and truncate and/or embay interior oscillatory zoning. Black unzoned and rounded or ovoid-shaped metamorphic zircon morphologies also occur. Th/U values range from 0.01 to 1.4, with the majority of ratios less than 0.1. Results of 206 Pb/ 238 U ages, ±2% discordant, range from 481 to 305 Ma. Clustering within these data reveals that the Blue Ridge and Inner Piedmont terranes were affected by three tectonothermal events: (1) 462–448 Ma (Taconic); (2) 395–340 Ma (Acadian and Neoacadian); and (3) 335–322 Ma, related to the early phase of the Alleghanian orogeny. By combining zircon rim ages with metamorphic isograds and other published isotopic ages, we identify the thermal architecture of the southern Appalachian orogen: juxtaposed and superposed metamorphic domains have younger ages to the east related to the marginward addition of terranes, and these domains can serve as a proxy to delimit terrane accretion. Most 462–448 Ma ages occur in the western and central Blue Ridge and define a continuous progression from greenschist to granulite facies that identifies the intact Taconic core. The extent of 462–448 Ma metamorphism indicates that the central Blue Ridge and Tugaloo terranes were accreted to the western Blue Ridge during the Taconic orogeny. Zircon rim ages in the Inner Piedmont span almost 100 m.y., with peaks at 395–385, 376–340, and 335–322 Ma, and delimit the Acadian-Neoacadian and Alleghanian metamorphic core. The timing and distribution of metamorphism in the Inner Piedmont are consistent with the Devonian to Mississippian oblique collision of the Carolina superterrane, followed by an early phase of Alleghanian metamorphism at 335–322 Ma (temperature >500 °C). The eastern Blue Ridge contains evidence of three possible tectonothermal events: ~460 Ma, 376–340 Ma, and ~335 Ma. All of the crystalline terranes of the Blue Ridge–Piedmont megathrust sheet were affected by Alleghanian metamorphism and deformation.

The southern Appalachian crystalline core is composed of lithotectonic assemblages that are largely sedimentary in origin. Sixteen paragneiss samples from the Blue Ridge and Inner Piedmont of North Carolina and Georgia, and one sample of Middle Ordovician rocks from the Sevier-Blountian clastic wedge in the Tennessee Valley and Ridge were sampled for sensitive high-resolution ion microprobe (SHRIMP) U-Pb detrital zircon geochronology, whole-rock geochemistry, and zircon trace-element analyses. Detrital zircon ages range from Archean (~2.7 Ga) to Middle Paleozoic (~430 Ma), with a notable abundance of Mesoproterozoic zircons (1.3–0.9 Ga). Many samples also contain moderate populations of slightly older Mesoproterozoic zircons (1.5–1.3 Ga). Minor populations of Paleoproterozoic (2.3–1.5 Ga) and Neoproterozoic (754–717 and 629–614 Ma) ages occur in several samples; however, Paleozoic detrital zircons (478–435 Ma) are restricted to samples from the Cat Square terrane. Depositional periods of the metasedimentary terranes are bracketed by detrital zircon, metamorphic, and magmatic ages, and include: (1) Mesoproterozoic, (2) Neoproterozoic to early Paleozoic, and (3) middle Paleozoic. A xenolith from the ~1.15 Ga Wiley Gneiss suggests a post–~1.2 Ga period of sedimentation prior to the ~1.15 Ga Grenvillian magmatic event. Detrital zircon populations of Neoproterozoic to Middle Ordovician suggest a mixed Laurentian provenance with Amazonian and peri-Gondwanan sources deposited in divergent and convergent plate settings. Blue Ridge and Inner Piedmont detrital zircon ages, whole-rock geochemistry, lithologic assemblages, and field relationships are compatible with deposition of immature clastic material in a rift and passive-margin setting from the Neoproterozoic to early Paleozoic. Occurrence of 1.3–0.9 Ga, 1.5–1.3 Ga, and 754–717 Ma detrital zircon ages indicate a dominantly Laurentian provenance for the Cartoogechaye, Cowrock, Dahlonega gold belt, Smith River allochthon, and Tugaloo terranes. Minor Paleoproterozoic populations in these terranes suggest input from distal terranes of the Laurentian midcontinent or the Amazonian craton. Transition to a convergent plate margin in the Middle Ordovician resulted in collision of central Blue Ridge and Tugaloo terranes and recycling of material from these terranes into the Mineral Bluff Formation and Sevier Shale. Ordovician and 629–614 Ma detrital zircons from the Cat Square terrane document the first occurrence of peri-Gondwanan material, which was deposited in a convergent setting between the Laurentian margin and the accreting Carolina superterrane during the Late Silurian to Devonian.

Abstract Extreme extension along the Colorado River has exposed the shallow to mid-crustal Spirit Mountain batholith and the roots of the roughly coeval Secret Pass Canyon volcanic center. Examination of the Spirit Mountain batholith reveals evidence for multiple replenishment and rejuvenation over a two million year period (ca. 17.5−15.3 Ma), with extensive coarse cumulate granites and leucogranite (high-silica rhyolites) sheets, mafic-felsic mingling and mixing, and a major dike swarm. The roots of the possibly related Secret Pass Canyon volcanic center comprise a large, very shallow, composite laccolith and smaller dikes, sills, and a volcanic neck. The volcanic sequence was emplaced within about a one million year period (ca. 18.5–17.3 Ma) and includes volcanogenic sediments, ignimbrites, domes, and block-and-ash flow deposits. An appended road log serves as a geologic guide to this magmatic region.

The mid-Miocene Aztec Wash pluton is divisible into a relatively homogeneous portion entirely comprising granites (the G zone, or GZ), and an extremely heterogeneous zone (HZ) that includes the products of the mingling, mixing and fractional crystallisation of mafic and felsic magmas. Though far less variable than the HZ, the GZ nonetheless records a dynamic history characterised by cyclic deposition of the solidifying products of the felsic portion of a recharging, open-system magma chamber. Tilting has exposed a 5-km section through the GZ and adjacent portions of the HZ. A porphyry is interpreted as a remnant of a chilled roof zone that marks the first stage of felsic GZ intrusion. Subsequent recharging by felsic and mafic magma, reflected by repeated cycles of crystal accumulation and melt segregation in the GZ and emplacement of mafic flows in the HZ, rejuvenated and maintained the chamber. Kilometre-scale lobes of mafic HZ material were deposited as prograding tongues into the GZ during periods of increased mafic input. Thus, they are lateral equivalents of the cumulate GZ granites with which they interfinger. Conglomerate-like units comprising rounded, matrix-supported intermediate clasts in cumulate granite are located immediately above the lobes. These ‘conglomerates’ appear to represent debris flows shed from sloping upper surfaces of the lobes. Thus, the GZ can be viewed as comprising distal facies, remote from the site of mafic recharging in the HZ, and the HZ as comprising proximal facies. Elemental chemistry suggests that the GZ cumulate granites represent a second-stage accumulation from an already evolved melt, and that coarse, more mafic, feldspar+biotite+accessory mineral ± hornblende rocks trapped between mafic sheets in the HZ are the initial cumulates. Fractionated melt accumulated roofward and laterally, and was the direct parent of the ‘evolved’ GZ cumulates. The most highly fractionated, fluid-rich melts accumulated at the roof.

Sedimentary and metasedimentary rocks within the southern Appalachian Blue Ridge and Inner Piedmont contain a valuable record of Late Proterozoic Laurentian margin evolution following the breakup of Rodinia. Paleogeographic reconstructions and increasing amounts of geochronologic and isotopic data limit the derivation of these paragneisses to the Laurentian and/or west Gondwanan craton(s). Southern Appalachian crystalline core paragneiss samples have ε Nd values between –8.5 and –2.0 at the time of deposition and contain abundant 1.1–1.25 Ga zircon cores with Grenville 1.0–1.1 Ga metamorphic rims. Less abundant detrital zircons are pre-Grenvillian: Middle Proterozoic 1.25–1.6 Ga, Early Proterozoic 1.6–2.1 Ga, and Late Archean 2.7–2.9 Ga. Blue Ridge Grenvillian basement has almost identical ε Nd values and displays the same dominant magmatic core and metamorphic rim zircon ages. Based on our data, nonconformable basement-cover relationships, and crustal ages in eastern North America, we contend that the extensive sedimentary packages in the southern Appalachian Blue Ridge and western Inner Piedmont are derived from Laurentia. ε Nd values from Carolina terrane volcanic, plutonic, and volcaniclastic rocks are isotopically less evolved than southern Appalachian paragneisses and Blue Ridge Grenvillian basement, easily separating this composite terrane from the mostly Laurentian terranes to the west. Neoproterozoic and Ordovician, as well as Grenvillian and pre-Grenvillian, zircons in eastern Inner Piedmont paragneisses indicate that these samples were deposited much later and could have been derived entirely from a Panafrican source or possibly a mixture of Panafrican and recycled Laurentian margin assemblages.

A number of Grenvillian basement massifs occur in the southern Appalachian Blue Ridge. The largest are contained in the Blue Ridge anticlinorium, which extends northward from its widest point in western North Carolina to Maryland. The Tallulah Falls dome, Toxaway dome, and Trimont Ridge area contain small internal basement massifs in the eastern and central Blue Ridge of the Carolinas and northeastern Georgia. All are associated with Paleozoic antiformal culminations, but each contains different basement units and contrasting Paleozoic structure. The Tallulah Falls dome is a broad foliation antiform wherein basement rocks (coarse augen 1158 ± 19 Ma Wiley Gneiss [ion microprobe, 207 Pb/ 206 Pb], medium-grained 1156 ± 23 Ma [ 207 Pb/ 206 Pb] and 1126 ± 23 Ma [ 207 Pb/ 206 Pb] Sutton Creek Gneiss, and medium-grained to megacrystic 1129 ± 23 Ma Wolf Creek Gneiss [sensitive high resolution ion microprobe, SHRIMP, 207 Pb/ 206 Pb]) form a ring and spiral pattern on the west, south, and southeast sides of the dome. Basement rocks are preserved in the hinges of isoclinal anticlines whose axial surfaces dip off the flanks of the dome. The Wiley Gneiss was intruded by Sutton Creek Gneiss. The Toxaway dome consists predominantly of coarse, banded 1151 ± 17 Ma and coarse augen 1149 ± 32 Ma (SHRIMP 206 Pb/ 238 U) Toxaway Gneiss folded into a northwest-vergent, gently southwest- and northeast-plunging antiform that contains a boomerang structure of Tallulah Falls Formation metasedimentary rocks in the core near the southwest end. The coarse augen gneiss phase constitutes a larger proportion of the Toxaway Gneiss toward the northeast. Field evidence indicates that the augen phase intruded the banded Toxaway lithology; U/Pb isotopic ages of these lithologies, however, are statistically indistinguishable. The Trimont Ridge massif occurs in an east-west–trending antiform west of Franklin, North Carolina, and consists of felsic gneiss that yielded a 1103 ± 69 Ma SHRIMP 207 Pb/ 206 Pb age. An ε Nd -depleted mantle model age of 1.5–1.6 Ga permits derivation of all of these basement rocks (including most from the western Blue Ridge) from eastern granite-rhyolite province crust, except the Mars Hill terrane rocks, which yield 1.8–2.2-Ga model ages. The small Grenvillian internal massifs were probably rifted from Laurentia during the Neoproterozoic, and became islands in the Iapetus ocean that were later swept onto the eastern margin of Laurentia during Ordovician subduction and arc accretion. These massifs were additionally penetratively deformed and metamorphosed during the Taconian and Neoacadian orogenies.

The Mars Hill terrane (MHT), a lithologically diverse belt exposed between Roan Mountain, North Carolina–Tennessee, and Asheville, North Carolina, is distinct in age, metamorphic history, and protoliths from the structurally overlying Eastern Blue Ridge and underlying Western Blue Ridge. MHT lithologies include diverse granitic gneisses, abundant mafic and sparse ultramafic bodies, and mildly to strongly aluminous paragneisses. These lithologies experienced metamorphism in the granulite facies and are intimately interspersed on cm to km scale, reflecting both intrusive and tectonic juxtaposition. Previous analyses of zircons by high-resolution ion microprobe verified the presence of Paleoproterozoic orthogneiss (1.8 Ga). New data document a major magmatic event at 1.20 Ga. Inherited and detrital zircons ranging in age from 1.3 to 1.9 Ga (plus a single 2.7 Ga core), ubiquitous Sm-Nd depleted mantle model ages ca. 2.0 Ga, and strongly negative ε Nd during Mesoproterozoic time all attest to the pre-Grenville heritage of this crust that was suggested by previous whole-rock Pb and Rb-Sr isotope studies. A single garnet amphibolite yielded a magmatic age of 0.73 Ga, equivalent to the Bakersville dike swarm, which cuts both the MHT and the adjacent Western Blue Ridge. Zircons from this sample display 0.47 Ga metamorphic rims. Zircons from all other samples have well-developed ca. 1.0 Ga metamorphic rims that date granulite-facies metamorphism. Silica contents of analyzed samples range from 45 to 76 wt%, reflecting the extreme diversity observed in the field and the highly variable protoliths. The MHT contrasts strikingly with basement of the adjacent Eastern and Western Blue Ridge, which comprise relatively homogeneous, 1.1 to 1.2 Ga granitic rocks with initial ε Nd values near 0. It appears to have more in common with distant Paleoproterozoic crustal terranes in the Great Lakes region, the southwestern United States, and South America.

The inland edge of the Jurassic magmatic belt passes through the eastern Mojave Desert, where it was emplaced in ancient continental crust. Three intrusive units exposed there—the Ship and Clipper Mountains plutons and a dike swarm in the Old Woman and Piute Mountains and Kilbeck Hills—are broadly similar to each other and to other intrusions of Jurassic age, but they differ from one another in detail and all show very clear evidence for interaction with the ancient crust. All three intrusive units are primarily metaluminous and range from mafic to moderately felsic in composition. The Ship Mountains pluton and dikes included both mafic and felsic magmas that mingled locally. The Clipper Mountains pluton comprises a compositional continuum from hornblende gabbro through granodiorite, at least partly a result of crystal accumulation processes. The ca. 160-Ma Clipper Mountains pluton was emplaced syntectonically with thrusting at a depth of approximately 15 km. The ca. 145-Ma dike swarm intruded at approximately 12 km, and the Ship Mountains pluton at <5 km. The Ship Mountains pluton, which is not well dated, initially overlay the dike swarm prior to Late Cretaceous and Tertiary extension and may have a similar age. The intrusions are all enriched in incompatible elements and have isotopic compositions that are more evolved than any plausible mantle source (high 87 Sr/ 86 Sr, low ε Nd , high 207 Pb/ 204 Pb and 208 Pb/ 204 Pb compared with 206 Pb/ 204 Pb). Ship Mountains and most dike samples are less evolved in Nd and Sr than the Mojave crust, but the Clipper Mountains Nd-Sr array is coincident with the less evolved portion of the field of ancient Mojave crust. Extremely strong U-Pb inheritance in Clipper zircons and moderate inheritance in dike zircons verifies the crustal component. We interpret Ship and dike rocks to be hybrids of ancient enriched mantle-derived mafic magmas and the ancient crust; the Clipper Mountains pluton could represent a restite-rich magma entirely derived from the Mojave crust, although a modest mantle contribution is likely.

Like many granites, the Late Cretaceous intrusives of the eastern Mojave Desert, California, have heretofore provided useful but poorly focused images of their source regions. New studies of lower crustal xenoliths and inherited accessory minerals are sharpening these images. Xenoliths in Tertiary dykes in this region are the residues of an extensive partial melting event. Great diversity in their composition reflects initial heterogeneity (both igneous and sedimentary protoliths) and varying amounts of melt extraction (from <10% to >70%). Mineral assemblages and thermobarometry suggest that the melting event occurred at T≥750°C at a depth of about 40 km. Present-day Sr, Nd, and Pb isotopic ratios indicate a Mojave Proterozoic heritage, but unrealistic model ages demonstrate the late Phanerozoic adjustment of parent/daughter ratios. A link between these xenoliths and the Late Cretaceous granites, though not fully documented, is probable; in any case, they provide invaluable clues concerning a crustal melting event, recording information about nature of source material (heterogeneous, supracrustal-rich), conditions of melting (moderately deep, moderately high T, accompanied by partial dehydration), and melt extraction (highly variable, locally extensive). The Old Woman-Piute granites contain a large fraction of inherited zircon and monazite. A SHRIMP ion probe investigation shows that these zircons record a Proterozoic history similar to that which affected the Mojave region. Zonation patterns in zircons, and to a lesser extent monazites and xenotimes, document multiple phases of igneous, metamorphic, and sedimentary growth and degradation, commonly several in a single grain. Low Y in portions of the cores of inherited zircons and monazites and in monazites and outer portions of zircons from the xenoliths appear to indicate growth in equilibrium with abundant garnet.

Plutonism was widespread from mid-Mesozoic through Paleogene time in the Cordilleran Interior of the United States (CI, defined as the region underlain by broadly autochthonous ancient crust inland from the Sierra Nevada batholith). Intrusive activity here was broadly synchronous with intrusion of the coastal Sierra Nevada and Peninsular Ranges batholiths, but in detail, timing in the CI differed from that nearer the coast: Triassic plutons are absent in the CI, the Jurassic intrusive peak is less pronounced than in the Sierra Nevada, and the most intense plutonic activity occurred later than in the Sierra Nevada and Peninsular Ranges. Granitoid rocks of the CI, especially the younger ones, have clear isotopic signatures of ancient crustal source components and are commonly strongly peraluminous; both of these characteristics represent major contrasts with the coastal batholiths. The North American craton, which forms the basement of the CI and from which the plutons were to a considerable extent derived, underwent intense orogeny and high-grade metamorphism during the Early Proterozoic. For the next 1.5 b.y. the continent remained intact and free of orogenic modification. Compressional tectonics, manifested both by thin-skinned thrusting and ductile nappe formation, as well as plutonism, characterized Mesozoic reactivation. Emplacement of the dominant Cretaceous CI plutons was roughly synchronous with the major deformation. CI plutonism was distinctive in several respects: (1) although no continental collision and no apparent extensional tectonism were involved, it extended far inland (800 km at present; probably >400 km at the time of intrusion) in old, previously stable crust; (2) intermediate metaluminous (<65 percent SiO 2 ) as well as felsic peraluminous granitoids show clear isotopic evidence for major crustal input; (3) strongly peraluminous granitoids have moderate concentrations of large-ion lithophile elements and fairly high Sr contents, not the high LIL and low Sr that characterize such rocks in other belts in ancient crust. Although CI magmatism extended remarkably far inland and evidence for a major subcrustal component is in many cases absent, it almost certainly was directly related to a subduction-induced thermal regime. Crustal thickening by sediment accumulation or overthrusting was probably of little importance in inducing magmatism. The thermal regime necessary for extensive Cretaceous–early Paleogene anatexis was probably brought about by increased mantle heat flux, perhaps resulting from lithospheric erosion, and/or by emplacement of subcrustal magmas at deep levels.

The Late Cretaceous Old Woman–Piute Range batholith includes both metaluminous and strongly peraluminous granitoid series that intruded the reactivated craton of southeastern California shortly after the orogenic peak. Whole-rock Sr, Nd, and O, feldspar Pb, and zircon U-Pb isotopic compositions, in combination with major- and trace-element and petrographic data, indicate that although these series are not comagmatic, they both were generated primarily by anatexis of Proterozoic crust. Differences between the two rock types are functions of source compositions: peraluminous granitoids were apparently generated from an intermediate to felsic source, metaluminous granitoids from more mafic igneous material with a possible modest subcrustal contribution. No sedimentary input is required in production of the peraluminous granites, and in fact, chemically mature sedimentary material is ruled out as an important contributor— that is, these are not S-type granites. Lead-isotope data reveal that the crust that yielded both magma series had undergone an ancient high-grade uranium depletion event, but independent evidence indicates that at the time of anatexis this crust was by no means anhydrous.