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Despite the widespread occurrence and economic significance of gold placer deposits, modern provenance studies of placer sediments remain largely qualitative. This study applies detrital zircon (DZ) geochronology to determine the source of zircon in placer deposits. We then evaluate the provenance of the zircon to assess whether the gold might have been derived from the same sources, thereby providing a case study of the use of DZ geochronology applied to placers. We present a new set of DZ U-Pb ages (n = 1058) and Lu-Hf (n = 61) isotopic data from four placer deposit samples collected from the Pioneer District of western Montana (USA). Each of the four samples yielded similar age spectra, with a range of U-Pb ages between 3000 and 25 Ma. We interpret that ≥250 Ma zircons were recycled from the Mesoproterozoic Belt Supergroup, Paleozoic–Mesozoic sedimentary rocks, and the Upper Cretaceous–Paleocene Beaverhead Group. Our 237 DZ U-Pb ages ≤250 Ma reveal two prominent age-probability peaks centered at ca. 69 Ma and ca. 26 Ma, which we interpret to record first-cycle derivation from the Royal stock and nearby Dillon Volcanics, respectively. We evaluate these data using an inverse Monte Carlo DZ unmixing model that calculates relative contributions from plausible source units, determining a 12% contribution from the Royal stock and a 43% contribution from the Beaverhead Group. A current absence of the Beaverhead Group in the hypothesized source region suggests complete erosion of the unit into the placer-bearing basin. Detrital zircon geochronology, Hf isotopic data, and the unmixing modeling results offer the first zircon-based support for previous interpretations that the Late Cretaceous Royal stock precipitated gold along its contact with overlying Proterozoic–Mesozoic sedimentary strata. Subsequent exhumation and erosion of the lode source led to gold deposition in the Anaconda metamorphic core complex supradetachment basin during the late Oligocene–late Miocene. The worldwide occurrence of gold placer deposits with unknown source areas provides abundant opportunity to apply these techniques elsewhere.

[Abstract Trench basins preserved along the southern margin of the Lhasa Terrane, Tibet, are sedimentologic records of convergent margin processes preceding Cenozoic India–Asia collision. We present new sedimentologic, petrographic and geochronologic data from the Rongmawa Formation and surrounding strata near Denggar, Tibet, to determine depositional environment, provenance and age. Depositional ages range from ca. 92 to 87 Ma and lithofacies are consistent with deposition by low‐ and high‐density turbidity currents and suspension settling of pelagic detritus in a deep‐marine, trench basin setting. Sandstone modal analyses and U–Pb geochronology indicate that trench basin detritus in this region was derived from the Lhasa Terrane. We interpret that the Cretaceous subduction trench received detritus from an axial sediment dispersal system that transported sediment from headwaters in the central‐southern Lhasa terrane near Lhasa City directly to the trench and then flowed westwards parallel to the trench. The preservation of trench basin strata deposited during Late Cretaceous time compared with the lack of trench deposits prior to ca. 92 Ma and after ca. 80 Ma suggests the margin experienced a period of significant accretion during this interval. In addition, deposition of trench basin strata occurred during Late Cretaceous adakitic magmatism and high‐temperature metamorphism, which are hypothesized to be explained by subduction of an oceanic ridge or subduction zone retreat and related upper plate extension along the southern margin of the Lhasa terrane. Subduction of an oceanic ridge may provide a mechanism to potentially erode forearc basin strata and promote increased sediment delivery directly to the trench., Provenance of Upper Cretaceous trench basin strata in southern Tibet are consistent with derivation from a margin‐parallel sediment dispersal system sourced near Lhasa City. Accumulation of trench strata along the southern margin of Asia during Late Cretaceous time suggests a period of accretion and increased sediment delivery directly to the trench possibly driven by a change in plate convergence and subduction of an oceanic ridge, respectively. ]

Fulong Cai et al, 2021, Configuration and Timing of Collision Between Arabia and Eurasia in the Zagros Collision Zone, Fars, Southern Iran, Tectonics, 40, Citation number, 10.1029/2021TC006762. To view the published open abstract, go to https://doi.org/10.1029/2021TC006762. Deposited by shareyourpaper.org and openaccessbutton.org. We've taken reasonable steps to ensure this content doesn't violate copyright. However, if you think it does you can request a takedown by emailing help@openaccessbutton.org.

Late Cretaceous trench basin strata were deposited in the subduction zone that consumed Neo-Tethyan oceanic lithosphere along the southern margin of the proto–Tibetan Plateau. We conducted detrital zircon (DZ) U-Pb geochronology on six trench basin samples (n = 1716) collected near Dênggar, Tibet (∼500 km west of Lhasa), to assess the provenance of these rocks and reconstruct Late Cretaceous sediment transport pathways. They contained DZ ages that point to a unique source around Lhasa city, north of the Late Cretaceous Gangdese magmatic arc. The modern Lhasa River catchment contains the requisite sources, and its main trunk transects the Gangdese magmatic arc, joining with the Yarlung River at a barbed junction at the India-Asia suture. We infer that the Lhasa River is an ancient feature that transported sediment to the subduction zone in Late Cretaceous time and persisted during India-Asia collision.

Recent detrital zircon U-Pb geochronology reveals an increasing proportion of Grenville-age (ca. 0.95–1.3 Ga) and ca. 300–480 Ma grains in late Paleozoic strata of the SW United States. These grain populations are interpreted to have been sourced from the Appalachian orogen, though the precise timing, transport mechanisms, and pathway(s) of sediment dispersal remain unclear. We combine 35,796 published detrital zircon U-Pb ages from Ordovician to Pennsylvanian strata of southern Canada, northern Mexico, and the U.S. with new data (1,628 ages) from Kansas, Missouri, Montana, and South Dakota. These data are integrated with sedimentary structural data and paleogeographic reconstructions to reveal temporal and spatial patterns of the sediment routing system at continent scale. In Ordovician time, North America was partitioned into western, central, and eastern domains in which strata were derived primarily from the Peace River Arch, the Superior Craton, and the Appalachians, respectively. Silurian–Devonian time saw limited integration of these domains, corresponding with the delivery of Appalachian-derived detritus to the Midcontinent via prograding deltas and westward-flowing rivers. Appalachian detritus flowed westward in Mississippian time, accumulating in the Appalachian foreland and continuing westward through Mississippi, Arkansas, Missouri, Oklahoma, Kansas, Colorado, Arizona, and California along the continental shelf. Given that North America was at equatorial latitudes and was inundated by the Kaskaskia sea at this time, westward dispersal likely occurred by trade wind–driven longshore drift, waves, tides, and marine currents, with the possible added contribution of hurricanes. Modern analogs for the southern margin of North America during Mississippian time (e.g., the Great Barrier Reef and the east coast of South America) indicate that long-distance (>1000 km) shelf-parallel sediment transport is readily accomplished through fair-weather processes and extreme events. Finally, Appalachian-derived detritus became widespread throughout North America following regression of the Kaskaskia sea in Pennsylvanian time, likely via fluvial, deltaic, and aeolian processes.

North-trending rifts throughout south-central Tibet provide an opportunity to study the dynamics of synconvergent extension in contractional orogenic belts. In this study, we present new data from the Dajiamang Tso rift, including quantitative crustal thickness estimates calculated from trace/rare earth element zircon data, U-Pb geochronology, and zircon-He thermochronology. These data constrain the timing and rates of exhumation in the Dajiamang Tso rift and provide a basis for evaluating dynamic models of synconvergent extension. Our results also provide a semi-continuous record of Mid-Cretaceous to Miocene evolution of the Himalayan-Tibetan orogenic belt along the India-Asia suture zone. We report igneous zircon U-Pb ages of ~103 Ma and 70–42 Ma for samples collected from the Xigaze forearc basin and Gangdese Batholith/Linzizong Formation, respectively. Zircon-He cooling ages of forearc rocks in the hanging wall of the Great Counter thrust are ~28 Ma, while Gangdese arc samples in the footwalls of the Dajiamang Tso rift are 16–8 Ma. These data reveal the approximate timing of the switch from contraction to extension along the India-Asia suture zone (minimum 16 Ma). Crustal-thickness trends from zircon geochemistry reveal possible crustal thinning (to ~40 km) immediately prior to India-Eurasia collision onset (58 Ma). Following initial collision, crustal thickness increases to 50 km by 40 Ma with continued thickening until the early Miocene supported by regional data from the Tibetan Magmatism Database. Current crustal thickness estimates based on geophysical observations show no evidence for crustal thinning following the onset of E–W extension (~16 Ma), suggesting that modern crustal thickness is likely facilitated by an underthrusting Indian lithosphere balanced by upper plate extension.

The Yarlung Suture Zone in Southern Tibet marks the boundary between India and Asia–formerly separated by an ocean basin–and is a critical record of the tectonic processes that created the Tibetan Plateau. The Yarlung Suture Zone is also a frontier research area, as difficulty of access has limited research activity, providing ample opportunities for new discoveries. This paper documents field research conducted by the authors along the Yarlung suture zone in eastern Xigaze (Shigatse, Rikaze) County, ∼250 km west of the city of Lhasa, in July 2017. The goal of this research was to map the Suture Zone structure in detail, and more specifically to understand the branching relationships between two major fault systems—the Great Counter Thrust and Gangdese Thrust. A summary of early geological exploration is included to provide context for this research.

U.S. National Science Foundation Continental Dynamics Program (EAR-1008527); U.S. National Science Foundation Instrumentation and Facilities program (EAR-1338583) to the Arizona LaserChron Center; China National Science Foundation (41490610); Geological Society of America (student research grant).

NSF Continental Dynamics grant [EAR-1008527]; NSF Instrumentation and Facilities program [EAR-1338583]; Chinese Academy of Sciences [XDB0301401]; National Natural Science Foundation of China [41490610]

The provenance of Sibumasu terrane sedimentary rocks and their tectonic relationships with surrounding terranes exposed in Southeast Asia record separation and accretion of Gondwana-derived terranes during Late Paleozoic and Mesozoic time. This paper reports sandstone petrographic and U–Pb detrital zircon geochronologic data from Ordovician to Lower Jurassic strata within the Sibumasu terrane in Shan State, Myanmar. The Ordovician strata are composed of limestone and siltstone. The Lower Silurian Linwe and Upper Silurian Namhism Formations are comprised of limestone, silty sandstone, conglomerate and sandstone, respectively. Sandstones from both Ordovician and Silurian strata are dominated by 567–470 Ma and 982–917 Ma detrital zircons that are interpreted to be sourced from the eastern Gondwana supercontinent. The Carboniferous unit is composed of metasedimentary rocks (phyllite, slate, quartzite, and meta-marl). Sandstones from Carboniferous units show a strong 1165–1070 Ma detrital zircon age peak that is not present in the Ordovician and Silurian strata. These zircon grains were most likely derived from both the Albany–Fraser Province in Southwest Australia and Maud Province in Antarctic. The Upper Triassic to Lower Jurassic, shallow marine Loi-an Group consists of thin- to medium-bedded sandstone and mudstone that unconformably overly the Permian to Middle Triassic Plateau Limestone Group. Sandstones from the Loi-an Group contain abundant Permian to Triassic detrital zircons that are interpreted to have been derived from the Sukhothai Arc of the western Indochina terrane. Formation of this arc is attributed to eastward (present coordinates) subduction of Paleo-Tethyan oceanic lithosphere beneath Indochina. Therefore, we propose that the Sibumasu terrane was juxtaposed against northwestern Australia as part of the Gondwana supercontinent during Paleozoic to Early Permian time. During the Late Triassic and Early Jurassic, Sibumasu strata record an abrupt influx of Permian and Triassic zircon grains, signifying a change in provenance from Gondwana to the Sukhothai Arc. These data are consistent with a tectonic model involving rifting of the Sibumasu terrane from Gondwana and subsequent docking with the Indochina/Simao terranes during Mesozoic time.

The long-term (>1 Ga) thermal histories of cratons are enigmatic, with geologic data providing only limited snapshots of their evolution. We use zircon (U–Th)/He (zircon He) thermochronology and age-composition correlations to understand the Proterozoic–Phanerozoic thermal history of Archean Wyoming province rocks exposed in the northern Laramide ranges of western North America. Zircon He ages from the Wind River Range (54 dates) and Bighorn Mountains (32 dates) show negative correlations with effective uranium (eU), a proxy for radiation damage. Zircon dates from the Bighorns are between 960 Ma (low-eU) and 20 Ma (high-eU) whereas samples from the Wind Rivers are between 582 Ma (low-eU) and 33 Ma (high-eU). We applied forward modeling using the zircon radiation damage and annealing model ZrDAAM to understand this highly variable dataset. A long-term t – T path that is consistent with the available geologic constraints successfully reproduced age-eU correlations. The best fit to the Wind Rivers data involves two phases of rapid cooling at 1800–1600 Ma and 900–700 Ma followed by slower cooling until 525 Ma. During the Phanerozoic, these samples were heated to maximum temperatures between 160 and 125 °C prior to Laramide cooling to 50 °C between 60 and 40 Ma. Data from the Bighorn Mountains were successfully reproduced with a similar thermal history involving cooler Phanerozoic temperatures of ∼115 °C and earlier Laramide cooling between 85 and 60 Ma. Our results indicate that age-eU correlations in zircon He datasets can be applied to extract long-term thermal histories that extend beyond the most recent cooling event. In addition, our results constrain the timing, magnitude and rates of cooling experienced by Archean Wyoming Province rocks between recognized deformation events, including the >1 Ga period represented by the regionally-extensive Great Unconformity.

U.S. National Science Foundation [EAR-1008527]; China National Science Foundation [41490610]; Geological Society of America

The Xigaze forearc basin records the evolution of the southern Lhasa terrane convergent margin, largely affected by Neo-Tethyan subduction processes, prior to the Paleocene Tethyan Himalaya–Eurasia collision. New geologic mapping and U-Pb detrital-zircon geochronologic data from the Lazi region, 340 km southeast of Lhasa, show that forearc basin sedimentation began ca. 110 Ma conformably atop the Yarlung–Tsangpo ophiolitic melange. By this time, the arc–trench system along the southern margin of the Lhasa terrane (Eurasia) consisted of an accretionary complex, overlying ophiolitic melange, the Xigaze forearc basin, and the Gangdese magmatic arc. There is no geological evidence in the Lazi region for more than one subduction zone between the southern Lhasa terrane margin and India. Sedimentological facies analysis from Albian to Santonian clastic and carbonate sedimentary rocks preserved in the Xigaze forearc basin indicate deep-marine sedimentation characterized by hemipelagic carbonate and volcanogenic sediment-gravity-flow deposits. Sandstone modal petrographic and U-Pb detrital-zircon geochronologic data reveal Asian continental margin, Gangdese magmatic arc, and central to northern Lhasa terrane provenance. During this time, basin fill was deposited in cycles of high and low sediment flux characterized by alternating successions of clastic turbidite inner- to outer-fan deposits and hemipelagic limestone and marlstone sequences. Along-strike differences in the timing of initial forearc basin sedimentation are likely the result of intra-basin topography and/or diachronous development of ophiolitic forearc basement.

The Oligocene–Miocene Kailas Formation is exposed along strike for ∼1300 km within the southernmost Lhasa terrane. In this study, we documented the sedimentology, structure, and age of this unit exposed between 87°E and 90°E. Within this region, the Kailas Formation is composed of continental deposits dominated by conglomerate and sandstone, with lesser volumes of siltstone and paleosols. These rocks were deposited nonconformably on Gangdese Batholith and related volcanic rocks along their northern boundary, whereas to the south, the south-dipping Great Counter Thrust places them in contact with Xigaze forearc and melange units. We interpret the Kailas Formation to have been deposited in alluvial-fan and fluvial environments with sediment principally derived from the north. Based on sedimentology and structural relationships, we interpret these rocks to have formed in a north-south extensional setting. New zircon U-Pb ages from volcanic tuffs and flows show that Kailas Formation deposition is younger to the east: Deposition occurred between 26 Ma and 24 Ma in western Tibet (81°E), at 25–23 Ma north of Lazi (87.8°E), at 23–22 Ma near Dazhuka (89.8°E), and as late as 18 Ma southwest of Lhasa (92°E). Overall, basin development propagated eastward at a rate of ∼300 mm/yr. This pattern and rate of propagation are similar to that of the temporal-spatial distribution of adakitic and ultrapotassic magmatism within the Lhasa terrane to the north, which has been interpreted as a record of slab breakoff. Magmatism lags several million years behind Kailas basin development at most locations. We interpret the Kailas basin to have formed as the result of Indian slab shearing and breakoff, which began in western Tibet around 26 Ma and reached eastern Tibet by ca. 18 Ma.

Mesozoic strata along northern margin of the Lhasa terrane near Dingqing, Tibet provide a semi-continuous record of the Bangong-Nujiang Ocean closure and the subsequent Lhasa-Qiangtang collision. In this study, we present results of sandstone petrographic, detrital zircon U Pb, and Cr-spinel geochemical data to determine the provenance of the Mesozoic strata (from the Triassic Quehala Group and the Mid-Jurassic to Lower Cretaceous Xihu, Lagongtang, and Duoni Formations, which are young from ~220 Ma to ~100 Ma) in this region, thereby allowing for interpretation of their tectonic setting. The similar 1200–900 age cluster from the lower Xihu Formation to that of the Triassic Quehala Group and the distinct age peaks at ~200 Ma and ~146 Ma from the upper Xihu Formation suggest a Lhasa terrane provenance to the south. Distinctive age clusters of 300–210 Ma and ~1800 Ma and Cr-spinel composition analysis of the Lagongtang Formation indicate a provenance shift from the Lhasa terrane to the Qiangtang terrane and the Bangong-Nujiang suture zone to the north. The Lagongtang Formation was deposited in a bathyal-abyssal to shallow-slope environment from north to south, that we interpret as the foredeep depozone and distal foredeep of a peripheral foreland basin system that developed due to flexural subsidence related to the Lhasa-Qiangtang collision and terrane accretion. The age of the Lhasa-Qiangtang collision is constrained by a ~140 Ma tuffite U Pb age at the base of the foreland basin strata. An angular unconformity between the Xihu and Lagongtang Formation, which we interpret as the result of the Lhasa-Qiangtang collision. Our results indicate that the Lhasa-Qiangtang collision initiated around ~140 Ma in the Dingqing region, simultaneously with previous determinations 1200 km to the west near Gaize area. Therefore, we prefer quasi-simultaneous onset of collision along-strike to zippering collision models wherein the east collision age is older than the west.