Global sea-level changes and substantial vertical displacement along the Monte Grande Fault (MGF) in the lower Rio Ica Valley of south-central Peru influenced the accumulation of bioclast-bearing and diatom-bearing Miocene siliciclastic sediments in an area of the East Pisco forearc basin (EPB) colloquially known as Laberinto. Two depositional hiatuses in the Laberinto area (similar to 17-14 Ma, similar to 12.5-10 Ma) manifest as sediment-filled erosional depressions a few kilometers in breadth. Erosion of the older depression was preceded by an similar to 18-Ma massive debris flow, possibly triggered by motion on the MGF causing lower Miocene lithoclastic olistoliths of up to two hundred meters length to spill off the footwall block. Sediment shed from the same footwall block may have formed previously recognized early Miocene deltas. From 14 to 13 Ma, the older depression filled with sediments herein assigned to the provisionally named Laberinto, Pampa, and Naranja members of the Pisco Formation, the latter member being characterized by marine delta foreset beds. The three members are at least partly correlative with the Pisco-0 sequence of the Pisco Formation. The younger depression was overrun at 10 Ma by debris flows of lithoclastic and granitic cobbles and boulders, then filled with diatomaceous silty sand with 5-m-sized lithoclastic olistoliths. The two lithologies constitute the provisionally named Mature Formation. Radiometric and newly revised biochronological data from throughout the EPB coupled with new diatom data from the Laberinto area have provided new insights into the correlation of sequences within the Chilcatay and Pisco formations and the interaction of local and basin-wide tectonism and global eustatic sea-level events across the basin.

The Willow Creek mining district was the third-largest lode gold district in Alaska in the 20th century, having produced 19 metric tons (t) of gold from vein deposits and another 2 t of gold from associated placer deposits. The district is located in the southern Talkeetna Mountains, north of the Castle Mountain strike-slip fault system. Most gold occurs in widespread mineralized quartz veins hosted by granitic rocks of the Willow Creek pluton. A geochronologic study of granitic rocks, alteration minerals, and adjacent metamorphic rocks was conducted in order to better understand the timing of magmatism, thermochronologic evolution, and age of gold mineralization in the district. New U-Pb zircon dates from the Talkeetna batholith, including the Willow Creek pluton, range from 72.1 to 71.3 Ma. The U-Pb dates are similar to reported U-Late Cretaceous Pb and K-Ar dates from elsewhere in the study area, and tightly constrain the age of plutonic rocks that host the Willow Creek lode gold deposits. Granitic rocks that intrude the Willow Creek pluton along its northwestern margin yield U-Pb dates of 75.7 and 70.8 Ma. The 40Ar/39Ar thermochronologic data for the Willow Creek pluton and other granitoids reveal rapid postemplacement cooling through hornblende, muscovite, and biotite closure temperatures. Thermochronologic data from K-feldspars document slower cooling rates through lower temperatures, which may be related to slow uplift and exhumation during the Paleocene-Eocene. The data are compatible with a simple tectonothermal history following emplacement of the plutonic rocks, with essentially no evidence of significant thermal disturbance of the rocks after about 64 Ma. Uranium-thorium-lead dating of hydrothermal phosphate minerals (monazite and xenotime) and 40Ar/39Ar dates from hydrothermal sericite from wall rocks and veins indicate that the main stage of gold mineralization occurred at about 67 to 65 Ma. Combining U-Th-Pb data from monazites from the Independence, Talkeetna, Gold Bullion, and Fern deposits gave an average of 65.4 +/- 2.3 Ma; hydrothermal xenotime from the Independence vein also yields a statistically indistinguishable age of 64.5 +/- 2.3 Ma. These ages overlap 40Ar/39Ar dates of 67 to 66 Ma obtained from hydrothermal sericite associated with formation of the gold-bearing veins, and the age of a muscovite from a weakly mineralized pegamatite that cuts the Willow Creek pluton. In contrast to previous studies that suggested that at least some mineralization was Eocene in age, we found no compelling evidence for significant gold mineralization within the district at this time. The 40Ar/39Ar and K-Ar dates from graphitic white micas from schist south of the Hatcher Pass fault zone yield ages of 61 to 50 Ma that are probably related to uplift and cooling of these Late Cretaceous metasedimentary rocks. The absence of evidence for postemplacement thermal events affecting the plutonic rocks of the Willow Creek district to the north indicates that the schists and granitoids do not share a similar tectonothermal history, and that they have been juxtaposed by subsequent faulting along the Hatcher Pass fault zone. The Willow Creek mining district was the third-largest lode gold district in Alaska in the 20th century, having produced 19 metric tons (t) of gold from vein deposits and another 2 t of gold from associated placer deposits. The district is located in the southern Talkeetna Mountains, north of the Castle Mountain strike-slip fault system. Most gold occurs in widespread mineralized quartz veins hosted by granitic rocks of the Willow Creek pluton. A geochronologic study of granitic rocks, alteration minerals, and adjacent metamorphic rocks was conducted in order to better understand the timing of magmatism, thermochronologic evolution, and age of gold mineralization in the district. New U-Pb zircon dates from the Talkeetna batholith, including the Willow Creek pluton, range from 72.1 to 71.3 Ma. The U-Pb dates are similar to reported U-Late Cretaceous Pb and K-Ar dates from elsewhere in the study area, and tightly constrain the age of plutonic rocks that host the Willow Creek lode gold deposits. Granitic rocks that intrude the Willow Creek pluton along its northwestern margin yield U-Pb dates of 75.7 and 70.8 Ma. The 40Ar/39Ar thermochronologic data for the Willow Creek pluton and other granitoids reveal rapid postemplacement cooling through hornblende, muscovite, and biotite closure temperatures. Thermochronologic data from K-feldspars document slower cooling rates through lower temperatures, which may be related to slow uplift and exhumation during the Paleocene-Eocene. The data are compatible with a simple tectonothermal history following emplacement of the plutonic rocks, with essentially no evidence of significant thermal disturbance of the rocks after about 64 Ma. Uranium-thorium-lead dating of hydrothermal phosphate minerals (monazite and xenotime) and 40Ar/39Ar dates from hydrothermal sericite from wall rocks and veins indicate that the main stage of gold mineralization occurred at about 67 to 65 Ma. Combining U-Th-Pb data from monazites from the Independence, Talkeetna, Gold Bullion, and Fern deposits gave an average of 65.4 +/- 2.3 Ma; hydrothermal xenotime from the Independence vein also yields a statistically indistinguishable age of 64.5 +/- 2.3 Ma. These ages overlap 40Ar/39Ar dates of 67 to 66 Ma obtained from hydrothermal sericite associated with formation of the gold-bearing veins, and the age of a muscovite from a weakly mineralized pegamatite that cuts the Willow Creek pluton. In contrast to previous studies that suggested that at least some mineralization was Eocene in age, we found no compelling evidence for significant gold mineralization within the district at this time. The 40Ar/39Ar and K-Ar dates from graphitic white micas from schist south of the Hatcher Pass fault zone yield ages of 61 to 50 Ma that are probably related to uplift and cooling of these Late Cretaceous metasedimentary rocks. The absence of evidence for postemplacement thermal events affecting the plutonic rocks of the Willow Creek district to the north indicates that the schists and granitoids do not share a similar tectonothermal history, and that they have been juxtaposed by subsequent faulting along the Hatcher Pass fault zone.

The purpose of this report is to present geochronologic data for unaltered volcanic rocks, hydrothermally altered volcanic rocks, and mineral deposits of the Miocene Bodie Hills and Pliocene to Pleistocene Aurora volcanic fields of east-central California and west-central Nevada. Most of the data presented here were derived from samples collected between 2000–13, but some of the geochronologic data, compiled from a variety of sources, pertain to samples collected during prior investigations. New data presented here (tables 1 and 2; Appendixes 1–3) were acquired in three U.S. Geological Survey (USGS) 40Ar/39Ar labs by three different geochronologists: Robert J. Fleck (Menlo Park, CA), Lawrence W. Snee (Denver, CO), and Michael A. Cosca (Denver, CO). Analytical methods and data derived from each of these labs are presented separately. The middle to late Miocene Bodie Hills volcanic field (BHVF) is a large (>700 km2), long-lived (~9 million years [m.y.]), episodic eruptive complex (John and others, 2012) in the southern segment of the ancestral Cascades arc (du Bray and others, written commun., 2015) north of Mono Lake and east of Bridgeport, California (fig. 1). The field is near the west edge of the Walker Lane and the northwest edge of the Mina deflection where structures related to these shear zones may have localized magmatism. The Walker Lane (fig. 1) is a broad, northwest-striking zone of right-lateral shear that accommodates right-lateral motion between the Pacific and North America plates; the Mina deflection constitutes a 60-km-long right step in the Walker Lane (Faulds and Henry, 2008; Oldow, 1992, 2003; Stewart, 1988). The Bodie Hills volcanic field includes at least 31 volcanic rock units erupted from 21 significant volcanic eruptive centers. Four trachyandesite stratovolcanoes developed along the margins of the volcanic field and numerous silicic trachyandesite to rhyolite flow dome complexes erupted more centrally. Volcanism in the Bodie Hills volcanic field peaked at two periods, ~15.0 to 12.6 million years before present (Ma) and ~9.9 to 8.0 Ma, which were dominated by emplacement of large stratovolcanoes and large silicic trachyandesite-dacite lava domes, respectively. A final period of small-volume silicic dome emplacement began in the western part of the volcanic field at ~6 Ma and culminated at ~5.5 Ma (John and others, 2012).

This map updates the geology of Baranof Island based on fieldwork, petrographic analyses, paleontologic ages, and isotopic ages. These new data provide constraints on depositional and metamorphic ages of lithostratigraphic rock units and the timing of structures that separate them. Kinematic analyses and thermobarometric calculations provide insights on the regional tectonic processes that affected the rocks on Baranof Island. The rocks on Baranof Island are components of a Paleozoic to Early Tertiary oceanic volcanic arc complex, including sedimentary and volcanic rocks that were deposited on and adjacent to the arc complex, deformed, and accreted. The arc complex consists of greenschist to amphibolite facies Paleozoic metavolcanic and metasedimentary rocks overlain by lower-grade Triassic metasedimentary and metavolcanic rocks and intruded by Jurassic calc-alkaline plutons. The Paleozoic rocks correlate well in age and lithology with rocks of the Sicker and Buttle Lake Groups of the Wrangellia terrane on Vancouver Island and differ from rocks of the Skolai Group that constitute basement to type-Wrangellia in the Wrangell Mountains. The Jurassic intrusive rocks are correlative with plutons that intrude the Wrangellia terrane on Vancouver Island but are lacking in the Wrangell Mountains. The rocks accreted beneath the arc complex are referred to as the Baranof Accretionary Complex in this report and are correlated with the Chugach Accretionary Complex of southern and southeastern Alaska and with the Pacific Rim Complex on Vancouver Island. Stratigraphic correlations between upper- and lower-plate rocks on Baranof Island and western Chichagof Island with rocks on Haida Gwaii and Vancouver Island, in addition to correlative ages of intrusive rocks and restorations of the Fairweather-Queen Charlotte, Chatham Strait, and Peril Strait Faults that define the Baranof-Chichagof block, suggest Baranof Island was near Vancouver Island at the time of initiation of arc magmatism in the Early Jurassic. Early Eocene plutons that intruded the accretionary complex outboard of the arc on Baranof Island are attributed to anatectic melting of trench sediments resulting from subduction of a spreading center. Oligocene intrusive rocks on Baranof Island correlate in age and composition with intrusive rocks in the Kano Plutonic Suite on Haida Gwaii, and similar magmatic sources are inferred.

The thermochronology for several suites of Mesozoic metamorphic and plutonic rocks collected throughout the northern Peninsular Ranges batholith (PRB) was studied as part of a collaborative isotopic study to further our understanding of the magmatic and tectonic history of southern California. These sample suites include: a traverse through the plutonic rocks across the northern PRB (N = 29), a traverse across a central structural and metamorphic transition zone of mainly metasedimentary rocks at Searl ridge (N = 20), plutonic samples from several drill cores (N = 7) and surface samples (N = 2) from the Los Angeles Basin, a traverse across the Eastern Peninsular Ranges mylonite zone (N = 6), and a suite of plutonic samples collected across the northern PRB (N = 13) from which only biotite Ar-40/Ar-39 ages were obtained. These geochronologic data help to characterize five major petrologic, geochemical, and isotopic zonations of the PRB (western zone, WZ; western transition zone, WTZ; eastern transition zone, ETZ; eastern zone, EZ; and upper-plate zone, UPZ).Apparent cooling rates were calculated using U-Pb zircon (zr) and titanite (sphene) ages; Ar-40/Ar-39 ages from hornblende (hbl), biotite (bi), and K-feldspar (Kf); and apatite fission-track (AFT) ages from the same samples. The apparent cooling rates across the northern PRB vary from relatively rapid in the west (zr-hbl similar to 210 degrees C/m.y.; zr-bio similar to 160 degrees C/m.y.; zr-Kf similar to 80 degrees C/m.y.) to less rapid in the central (zr-hb similar to 280 degrees C/m.y.; zr-bio similar to 90 degrees C/m.y.; zr-Kf similar to 60 degrees C/m.y.) and eastern (zr-hbl similar to 185 degrees C/m.y.; zr-bio similar to 180 degrees C/m.y.; zr-Kf similar to 60 degrees C/m. y.) zones. An exception in the eastern zone, the massive San Jacinto pluton, appears to have cooled very rapidly (zr-bio similar to 385 degrees C/m.y.). Apparent cooling rates for the UPZ samples are consistently slower in comparison (similar to 25-45 C/m.y.), regardless of which geochronometers are used.Notable characteristics of the various ages from different dating methods include: (1) Zircon ages indicate a progressive younging of magmatic activity from west to east between ca. 125 and 90 Ma. (2) Various geochronometers were apparently affected by emplacement of the voluminous (ETZ and EZ) La Posta-type plutons emplaced between 99 and 91 Ma. Those minerals affected include K-feldspar in the western zone rocks, biotite and K-feldspar in the WTZ rocks, and white mica and K-feldspar in rocks from Searl ridge. (3) The AFT ages record the time the rocks cooled through the AFT closure temperature (similar to 100 degrees C in these rocks), likely due to exhumation. Throughout most of the northern traverse, the apatite data indicate the rocks cooled relatively quickly through the apatite partial annealing zone (PAZ; from similar to 110 degrees C to 60 degrees C) and remained at temperatures less than 60 degrees C as continued exhumation cooled them to present-day surface temperatures. The ages indicate that the western "arc" terrane of the WZ was being uplifted and cooled at ca. 91 Ma, during or shortly after intrusion of the 99-91 Ma La Posta-type plutons to the east. Uplift and cooling occurred later, between ca. 70 Ma and ca. 55 Ma, in the central WTZ, ETZ, and EZ rocks, possibly as upwarping in response to events in the UPZ. The UPZ experienced differential exhumation at ca. 50-35 Ma: Cooling on the western edge was taking place at about the same time or shortly after cooling in the younger samples in the ETZ and EZ, whereas on the east side of the UPZ, the rocks cooled later (ca. 35 Ma) and spent a prolonged time in the apatite PAZ compared to most northern traverse samples.Apparent cooling rates from Los Angeles Basin drill core samples of plutonic rocks show that four are similar to the WTZ thermal histories, and two are similar to the WTZ histories, indicating that the eastern part of the Los Angeles Basin area is underlain by mainly western zone PRB rocks.Thermal histories revealed by samples from Searl ridge indicate that the WTZ magmatism intruded the metasedimentary rocks prior to their deformation and metamorphism at ca. 97 Ma. Both low-grade schists and metasandstones of the western side of the ridge and high-grade gneisses of the eastern side of the ridge have thermal histories consistent with eastern zone rocks-suggesting a temporal/thermal relationship between the western transition zone and the eastern zones.Limited ages from six samples across the Eastern Peninsular Ranges mylonite zone (EPRMZ) indicate that this zone underwent cooling after emplacement of the youngest UPZ rocks at 85 Ma, suggesting that thrusting along the EPRMZ was either coeval with emplacement of the UPZ plutonic rocks or occurred shortly afterwards (similar to 10-15 m.y.). Alternatively, the EPRMZ thrusting may have occurred at temperatures under similar to 180 degrees C at yet a later date.The geochronology presented here differs slightly from previous studies for similar rocks exposed across the middle and southern portions of the PRB, in that our data define a relatively smooth progression of magmatism from west to east, and the transition from western, oceanic-arc plutonism to eastern, continental arc plutonism is interpreted to have occurred at ca. 99-97 Ma and not at ca. 105 Ma.

Extensive volcanic and intrusive igneous activity, partly localized along regional structural zones, characterized the southern Toquima Range, Nevada, in the late Eocene, Oligocene, and Miocene. The general chronology of igneous activity has been defined previously. This major episode of Tertiary magmatism began with emplacement of a variety of intrusive rocks, followed by formation of nine major calderas and associated with voluminous extrusive and additional intrusive activity. Emplacement of volcanic eruptive and collapse megabreccias accompanied formation of some calderas. Penecontemporaneous volcanism in central Nevada resulted in deposition of distally derived outflow facies ash-flow tuff units that are interleaved in the Toquima Range with proximally derived ash-flow tuffs. Eruption of the Northumberland Tuff in the north part of the southern Toquima Range and collapse of the Northumberland caldera occurred about 32.3 million years ago. The poorly defined Corcoran Canyon caldera farther to the southeast formed following eruption of the tuff of Corcoran Canyon about 27.2 million years ago. The Big Ten Peak caldera in the south part of the southern Toquima Range Tertiary volcanic complex formed about 27 million years ago during eruption of the tuff of Big Ten Peak and associated air-fall tuffs. The inferred Ryecroft Canyon caldera formed in the south end of the Monitor Valley adjacent to the southern Toquima Range and just north of the Big Ten Peak caldera in response to eruption of the tuff of Ryecroft Canyon about 27 million years ago, and the Moores Creek caldera just south of the Northumberland caldera developed at about the same time. Eruption of the tuff of Mount Jefferson about 26.8 million years ago was accompanied by collapse of the Mount Jefferson caldera in the central part of the southern Toquima Range. An inferred caldera, mostly buried beneath alluvium of Big Smoky Valley southwest of the Mount Jefferson caldera, formed about 26.5 million years ago with eruption of the tuff of Round Mountain. The Manhattan caldera south of the Mount Jefferson caldera and northwest of the Big Ten Peak caldera formed in association with eruption of a series of tuffs, principally the Round Rock Formation, mostly ash-flow tuff, about 24.4 million years ago. Extensive 40Ar/39Ar dating of about 60 samples that represent many of the Tertiary extrusive and intrusive rocks in the southern Toquima Range provides precise ages that refine the chronology of previously dated units. New geochronologic data indicate that the petrogenetically related Corcoran Canyon, Ryecroft Canyon, and Mount Jefferson calderas formed during a period of about 560,000 years. Electron microprobe analyses of phenocrysts from 20 samples of six dated units underscore inferred petrogenetic relations among some of these units. In particular, compositions of augite, hornblende, and biotite in tuffs erupted from the Corcoran Canyon, Ryecroft Canyon, and Mount Jefferson calderas are similar, which suggests that magmas represented by these tuffs have similar petrogenetic histories. The unique occurrence of hypersthene in Isom-type tuff confirms its derivation from a source beyond the southern Toquima Range.

The Jílové deposit in the central part of the Bohemian Massif represents a vein to stockwork type of orogenic-type gold deposit. It is hosted by Neoproterozoic rocks of the Jílové Belt and by various magmatic dikes related to the ~355 to ~335Ma Central Bohemian Plutonic Complex. The deposit is situated along the terrane boundary of the Teplá Barrandian and Moldanubian units.

In northwestern Arizona, the high-standing, relatively unextended Colorado Plateau abruptly gives way across a system of major west-dipping normal faults to a highly extended part of the Basin and Range province known as the northern Colorado River extensional corridor. The transition from unextended to highly extended upper crust is unusually sharp within this region, contrasting with a broad transition zone elsewhere. The southern White Hills lie near the eastern margin of the extensional corridor in northwestern Arizona and contain a large east-tilted half graben that chronicles Miocene extension and constrains the timing of structural demarcation between the Colorado Plateau and Basin and Range province during Neogene time. This growth-fault basin is bounded on the east by the west-dipping Cyclopic and Cerbat Mountains fault zones. Greater tilts in the hanging walls suggest that these faults have listric geometries. The stratigraphy in the half graben consists of Miocene volcanic rocks intercalated with an eastward-thickening wedge of synextensional fanglomerates. Tilts in the Miocene units decrease up section from similar to 75 degrees to 5 degrees. Recent Ar-40/Ar-39 dating (11 new dates) of variably tilted volcanic rocks in the growth-fault basin and regional relations constrain the timing of east-west extension between ca. 16.6 and <9 Ma, with peak extension from ca. 16.6 to 15.2 Ma. Capping 8.7 Ma basalts are tilted 5 degrees-10 degrees and record the waning stages of extension. Thus, the sharp boundary between the Colorado Plateau and Basin and Range began developing by ca. 16.5 Ma and has changed little since ca. 9 Ma. Major extension and basin development significantly lowered base level within the extensional corridor and induced headward erosion into the western margin of the Colorado Plateau, which ultimately facilitated development of the western Grand Canyon. Abundant clasts of 1.7 Ga megacrystic granite in the eastward-thickening fanglomerates within the growth-fault basin suggest a partial provenance from the Garnet Mountain area along or near the western margin of the Colorado Plateau beginning as early as ca. 16 Ma and continuing to ca. 9 Ma.

Uranium-Pb dating of zircons in pre- and syn-ore dykes, as well as gold-related monazite and xenotime, in combination with Ar analysis of metamorphic and hydrothermal mica, shows that the majority of gold mineralisation at Kalgoorlie, including ductile-brittle Fimiston-, brittle-ductile Oroya- and brittle Mount Charlotte-style gold, was deposited within a short period between about 2645Ma and 2640Ma. This giant ore system formed after 2690 to 2680Ma basic magmatism, after deposition of volcanosedimentary Black Flag Beds and coeval intrusion of felsic porphyry dikes at c.2670Ma and post-peak regional metamorphism pre-c.2650Ma. Gold mineralisation could have overlapped the end of homblende-phyric dike emplacement between 2655Ma and 2645Ma, but was broadly coeval with lamprophyre dike intrusion at 2650Ma to 2640Ma. Therefore, gold was deposited in both ductile and brittle structures at broadly the same time, under similar P-T conditions. Also, despite a late overprinting low strain, thermal event at 2610 to 2600Ma, the geometry of the Gold Field and its contained lode systems has remained essentially the same since the time of gold mineralisation.

We report geochronological and geochemical data for the calc-alkalic Lowland Creek volcanic field (LCVF) in west-central Montana. Ar-40/Ar-39 age determinations show that the LCVF was active from 52.9 to 48.6 Ma, with tuff-forming eruptions at 52.9 +/- 0.14 and 51.8 +/- 0.14 Ma. These dates span the age range of vigorous Eocene igneous activity in the Kamloops-Absaroka-Challis belt. The LCVF evolved upward from basal rhyolites (SiO2 > 71 wt%) to dacites and andesites (SiO2 < 62 wt%). Compositional change parallels a transition from early explosive volcanism to late effusive activity. Four geochemical components can be detected in the rocks. A component with Pb-206/Pb-204 < 16.5 and epsilon(Nd) near -15 is predominant in anhydrous, two-pyroxene dacites; hydrous rhyolites, rhyodacites, and dacites with epsilon(Nd) below -10 are dominated by a second component; hydrous rocks with Pb-206/Pb-204 > 18.3 and epsilon(Nd) > -9 contain a third component; and an andesite with low epsilon(Nd) content and epsilon(Nd) near -9 probably contains a fourth component. The first three components probably derive from the lower and middle crust, whereas the fourth is probably from the lithospheric mantle.

There has been a long-standing controversy regarding the timing and number of gold mineralization events at Kalgoorlie. Uranium-Pb dating of zircons' and hydrothermal monazite and xenotime, as well as Ar-40/Ar-39 analysis of metasomatic fuchsite and white mica. are used to date pre- to synore dikes. alteration, and orebodies in order to resolve this issue. The majority of gold mineralization at Kalgoorlie, including ductile-brittle Fimiston-, brittle-ductile Oroya- and brittle Mount Charlotte-style gold, are different expressions of a complex mineralizing system that was active at broadly the same time at ca. 2.64 Ga. Gold mineralization was thus deposited in both ductile and brittle structures at approximately the same crustal level at broadly the same time, limier similar-PT conditions. This giant ore system formed after ea. 2.69 Ga basic magmatism, intrusion of the Golden Mile Dolerite sill at 2680 +/- 9 Ma, and intrusion or calc-alkaline feldspar-quartz porphyry dikes at 2670 5 Ma. Gold mineralization was broadly coeval with lamprophyre dike intrusion at 2642 6 Ma and overlapped the waning stages of hornblende and albite-bearing porphyry dike emplacement at 2650 6 Ma and regional metamorphism. Subsequent brittle deformation in the Kalgoorlie gold held way, accompanied by hydrothermal activity that may have led to some late gold mineralization or remobilization in extensional quartz vein arrays in the Golden Mile between about 2.61 and 2.60 Ca. This late hydrothermal activity and associated brittle deformation marks the last event to significantly affect the rocks it Kalgoorlie and may be related to uplift lift and final cooling of the terrane. Despite this late event, the geometry of the Kalgoorlie gold field and its contained lode systems has remained essentially the same since the time of gold mineralization.

Across the Salmon River suture in western Idaho, where allochthonous Permian to Cretaceous oceanic rocks are juxtaposed against Proterozoic North American rocks, a wide variety of plutonic rocks are exposed. Available data indicate much variation in composition, source, and structural state of these plutons. The plutonic rocks were long described as the western border zone of the Cretaceous Idaho batholith but limited pre-existing age data indicate more complicated origins. Because the affinity and age of the plutonic rocks cannot be reliably determined from field relations, TIMS U-Pb dating in conjunction with Sr, Nd, and Pb isotopic studies of selected plutons across the suture in western Idaho were undertaken. The data indicate three general groups of plutons including (1) those that intruded the island arc terranes during the Triassic and Jurassic, those that intruded near the western edge of oceanic rocks along the suture in the Early Cretaceous, and the plutons of the Idaho batholith that intruded Proterozoic North American rocks in the Late Cretaceous. Plutons that intruded Proterozoic North American rocks commonly include xenocrystic zircons and in several cases, ages could not be determined. The least radiogenic Sr and most radiogenic Nd are found among the Blue Mountains superterrane island arc samples. Suture-zone plutons have isotopic characteristics that span the range between Idaho batholith and island arc samples but mostly follow island arc signatures. Plutons of the Idaho batholith have the most radiogenic initial Pb and Sr ratios and the least radiogenic Nd of the samples analyzed.

Paleomagnetic data from Proterozoic mafic dikes in southwestern Montana provides evidence for two distinct episodes of subparallel dike emplacement at ca. 1450 and 780Ma. Published geochemical data from dikes in the southern Tobacco Root Mountains has identified three distinct compositional groups, termed groups A, B, and C. Geochronological data from the group A dikes yielded a Sm–Nd age of 1448±49Ma. Emplacement of these dikes is thought to reflect mafic magmatism associated with extension accompanying development of the adjacent Mesoproterozoic Belt Basin. Paleomagnetic results from these dikes and a group C dike yield antipodal magnetizations with a group-mean direction of D=225.0°, I=61.8° (k=27.9, α95=7.7°, N=14 independent means/24 sites). The average paleomagnetic pole (8.7°N, 216.1°E, A95=10.3°) is considered to be primary on the basis of positive baked contact tests and similarity to poles of ca. 1.45–1.4Ga from intrusions elsewhere in North America, but is discordant with respect to poles from age equivalent sedimentary rocks of the Meosoproterozoic Belt Supergroup. 40Ar/39Ar dates from geochemical group B dikes are consistent with published U–Pb dates that demonstrate dike emplacement at 780Ma as part of the regional Gunbarrel magmatic event. Hornblende concentrates from the group B dikes yield 40Ar/39Ar apparent ages of 778–772Ma, whereas biotite from a baked contact zone yielded a plateau date of 788Ma. Paleomagnetic results from the group B dikes yield a mean direction of D=301.5°, I=−17.1° (k=65.7, α95=4.0°, N=12 independent means/23 sites) with a paleomagnetic pole at 14.6°N, 127.0°E (A95=3.2°). The combination of geochronologic data, results of a baked contact test, and spatial agreement of the paleomagnetic poles with poles of similar age elsewhere in North America indicates that this is also a primary magnetization associated with dike emplacement. Paleomagnetic data from some of the Tobacco Root Mountains dikes provide evidence that they were partially to completely remagnetized during latest Cretaceous to early Tertiary time, perhaps due to thermal affects associated with emplacement of the Late Cretaceous Tobacco Root Batholith. The overall agreement of paleomagnetic poles from the Proterozoic dikes with those of age equivalent rocks elsewhere in North America and agreement of the secondary magnetization with expected directions for the latest Cretaceous/early Tertiary indicate that the rocks of the Tobacco Root Mountains have not experienced significant tilting or vertical axis rotation since the Mesoproterozoic. The new paleomagnetic poles from this study thus provide key data for refining Meso- and Neoproterozoic parts of the North American APW path.

The papers in this volume describe petrologic, structural, and geochemical studies related to geographic areas adjacent to and including the Salmon River suture zone. We therefore start this volume by defining and giving a general description of that suture zone. The western margin of the North American continent was the setting for complex terrane accretion and large-scale terrane translation during Late Cretaceous and Eocene time. In western Idaho, the boundary that separates the Paleozoic-Mesozoic accreted oceanic, island-arc rocks on the west from Precambrian continental metamorphic and sedimentary rocks on the east is called the Salmon River suture zone (SRSZ). Readers will note that the term 'Salmon River suture zone' is used in the title of this volume and in the text of several of the papers and the term 'western Idaho suture zone' is used in several other papers in this volume. Both terms refer to the same geologic feature and reflect historical usage and custom; thus no attempt has been made by the editors to impose or demand a single term by the various authors of this volume. The suture zone is marked by strong lithologic and chemical differences. Rocks adjacent to the suture zone are characterized by high-grade metamorphism and much structural deformation. In addition, the zone was the locus of emplacement of plutons ranging in composition from tonalite to monzogranite during and after the final stages of accretion of the oceanic terrane to the North American continent. The contents of this paper consists of seven chapters.

New geochronological data from hydrothermal phosphate minerals indicate that the majority of gold mineralization at Kalgoorlie, including Fimiston-, Oroya- and some Charlotte-style lodes, are broadly the same age at 2.65-2.63 Ga. This giant ore system formed after c. 2.69 Ga basic magmatism, intrusion of the Golden Mile Dolerite sill at c. 2680-2675Ma, as well as deposition of volcanosedimentary Black Flag Beds and coeval felsic porphyry dyke intrusion at c. 2.67 Ga. Mineralization overlapped but outlasted hornblende-phyric and lamprophye dyke intrusion and peak regional metamorphism at c. 2.65-2.64 Ga. It was broadly coeval with both ductile and brittle deformation, which were probably related to a prolonged regional deformation event that was accompanied by dynamic mineral growth. Subsequent localised brittle faulting was accompanied by hydrothermal activity that led to some late Charlotte-style gold mineralization after 2.62 Ga.

The margin of northern Venezuela is a complex zone representing the orogenic events from basement formation to subsequent subduction and exhumation during transpressional collision. This boundary zone has six east-west-trending belts that each record a different segment of its development. This geologic complexity requires radiometric ages to unravel, and we herein provide 48 new ages including U-Pb ( 4), Rb-Sr ( 2), Ar-40/Ar-39 ( 24), zircon and apatite fission-track ( 17), and C-14 ( 1) ages to constrain the evolution of three of these belts. These three belts are the Cordillera de la Costa, Caucagua-El Tinaco, and Serran a del Interior belts.In the Cordillera de la Costa belt, U-Pb geochronologic data indicate portions of the basement igneous and metaigneous rocks formed in the Cambro-Ordovician (513-471 Ma). New Ar-40/Ar-39 data from Margarita Island indicate that some of the subduction complex was rapidly cooled and exhumed, whereas other portions indicate slower cooling. This contrasts with new Ar-40/Ar-39 data from the Puerto Cabello portion of the subduction complex that has Eocene to Oligocene (42-28 Ma) cooling ages. New fission-track data imply the entire Cordillera de la Costa belt from Puerto Cabello to La Guaira (similar to 150 km) was uplifted at the same time.In the Caucagua-El Tinaco belt, the oldest Ar-40/Ar-39 amphibole ages from the Tinaquillo ultramafi c complex are Jurassic ( 190 Ma). Additional amphibole Ar-40/Ar-39 cooling ages are older than previously recorded in either the Tinaco or Tinaquillo complex. One amphibole Ar-40/Ar-39 cooling age for the Tinaco complex is similar to previous U-Pb results.New apatite fission-track results from the Serran a del Interior foreland fold and thrust belt are synchronous with exhumation in the Cordillera de la Costa belt. In addition, several zircon fission-track ages in the Serran a del Interior belt are older than their fossil ages, indicating a Cretaceous minimum provenance age for Miocene beds.Significant new findings from these geochronologic studies include ( 1) several igneous and metaigneous bodies that may be correlated with orogenic events in the Appalachians occur within the subduction melange; ( 2) the Tinaquillo complex may record Jurassic rifting; ( 3) Cretaceous source rocks for the Serran a del Interior sedimentary strata; ( 4) exhumation of the subduction complex is segmented because two regions have significantly different cooling histories, with Margarita Island exhumed in the Cretaceous, whereas to the west, the Puerto Cabello region has widespread Paleogene cooling and exhumation ages; and ( 5) earthquake activity in 1812 caused uplift as recorded by exposure of Recent corals.