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The Franciscan subduction complex of California is considered a type example of a subduction‐accretion system, yet the age of subduction initiation and relationship to the tectonic history of western North America remain controversial. Estimates for the timing of Franciscan subduction initiation are largely based either indirectly on regional tectonic arguments or from the ages of high‐grade blocks within mélange. Many of the high‐grade blocks record counterclockwise pressure‐temperature paths with early amphibolite overprinted by later eclogite and blueschist; however, their origin and significance with respect to subduction initiation have been debated. In contrast, some high‐grade blocks show evidence for clockwise pressure‐temperature paths and an early eclogite assemblage overprinted by later amphibolite. Zircon U‐Pb ages from inclusions in garnet and Lu‐Hf estimates of initial garnet growth ages from these samples record early eclogite metamorphism at ~176 Ma. Matrix zircon U‐Pb ages and Lu‐Hf estimates of final garnet growth ages record a barroisite‐amphibolite assemblage overprint of eclogite at ~160 Ma. Combined with petrologic data and existing geochronology, the data suggest that (1) Franciscan subduction was underway by no later than 180 Ma, (2) continuous subduction metamorphism occurred for at least 100 Ma, and (3) Franciscan subduction initiation predated the formation of the overlying Coast Range Ophiolite, supporting models that form the ophiolite above an east dipping Franciscan subduction zone. Subduction zones are places where dense ocean crust descends (or subducts) beneath more buoyant plates of continental or oceanic crust. Ancient subduction zones exposed at the Earth's surface provide important information on past plate movements and processes now occurring within active subduction zones. Determining the age that subduction began in ancient subduction zones is difficult because the earliest formed rocks are rarely preserved in the rock record. Our study focuses on the Franciscan Complex of California, which is considered a type example of an ancient subduction zone. The timing of Franciscan subduction initiation, however, has long been debated, resulting in conflicting models for the tectonic history of western North America. This study is unique because we dated the mineral zircon preserved as inclusions within garnet formed during the early stages of subduction. We also analyzed zircon outside of garnet formed during a younger event within the subduction history. The zircon inclusions within garnet yield an age of ~176 Ma, whereas the zircon outside the garnet had a younger age of ~160 Ma. These ages require that Franciscan subduction began by 180 Ma, significantly older than commonly believed and constrain models for the tectonic evolution of the western North American margin. Franciscan subduction began by about 180 Ma and continued as a single, long‐lived, east dipping subduction zone for approximately 100 MaThe metamorphic history of high‐grade blocks reflects several processes that operated at different times and places in the subduction zoneFranciscan subduction is older than the Coast Range Ophiolite, which likely then formed above an east dipping Franciscan subduction zone

Iceland's oldest silicic rocks provide unique insight into the island's early crustal evolution. We present new zircon U‐Pb ages bolstered with zircon trace element and isotopic compositions, and whole rock Nd, Hf, and Pb isotope compositions, from three silicic magmatic centers—Hrafnsfjörður, Árnes, and Kaldalón—to understand the petrogenesis of large silicic volcanic centers in the northern Westfjords, Iceland. Our data confirm Hrafnsfjörður as the oldest known central volcano in Iceland (∼14 Ma) and establish an older age for Árnes (∼13 Ma) than previously estimated. We also report the first U‐Pb zircon dates from Kaldalón (∼13.5 Ma). Zircon oxygen isotope compositions range from δ18O∼+2 to +4‰ and indicate involvement of a low‐18O component in their source magmas. Hrafnsfjörður zircon Hf (mean sample εHf∼ +15.3–16.0) and whole rock Hf and Nd (εHf= +14.5 to +15; εNd= +7.9 to +8.1) isotopic compositions are more radiogenic than those from Árnes (zircon sample εHf∼ +11.8–13; whole rock εHf= +12.8 to +15.1; εNd= +7.3 to +7.7), but Hrafnsfjörður whole rock Pb isotope compositions (208/204Pb = 37.95–37.96; 206/204Pb = 18.33–18.35) are less radiogenic than those from Árnes (208/204Pb = 38.34–38.48; 206/204Pb = 18.64–18.78). Kaldalón has zircon Hf isotope compositions of εHf∼+14.8 and 15.5 (sample means). These age and isotopic differences suggest that interaction of rift and plume, and thus the geodynamic evolution of the Westfjords, is complex. Isotopic compositions of Hrafnsfjörður and Árnes support involvement of an enriched mantle (EM)‐like mantle component associated with a pulsing plume that resulted in variable spreading rates and magma fluxes and highlight the heterogeneity of the Icelandic mantle. Iceland's oldest silicic rocks provide unique insight into the island's early crustal evolution. We present data from the trace mineral zircon, as well as whole rock Nd, Hf, and Pb isotope compositions, from three silicic magmatic centers—Hrafnsfjörður, Árnes, and Kaldalón—to understand the mechanisms of magma formation in the northern Westfjords, Iceland. Our data confirm Hrafnsfjörður as the oldest known central volcano in Iceland (∼14 Ma) and establish an older age for Árnes (∼13 Ma) than previously estimated. We also report the first dates from Kaldalón (∼13.5 Ma). Zircon oxygen isotope compositions suggest that silicic magmas in this area form from incorporation of at least a minor amount of pre‐existing crust material. Isotopic compositions from Hrafnsfjörður and Árnes differ significantly, despite those volcanoes being only ∼30 apart. The age and isotopic differences suggest that interaction of the Icelandic rift system and the Iceland mantle plume, and thus the geodynamic evolution of the Westfjords, is complex and doesn't follow a simple rifting scenario. Isotopic compositions of Hrafnsfjörður and Árnes rocks support involvement of an enriched mantle (EM)‐like mantle component associated with a pulsing plume that resulted in variable spreading rates and magma fluxes and highlight the heterogeneity of the Icelandic mantle. Zircon and whole rock data provide new insight on petrogenesis of some of the oldest silicic rocks in Iceland produced by a variety of—and in many cases, a combination of—processesThese data highlight nuances that suggest the geodynamic evolution of the Westfjords may be more complicated than previously proposedIsotopic compositions support involvement of an enriched‐mantle‐like component and a pulsing plume and highlight mantle heterogeneity Zircon and whole rock data provide new insight on petrogenesis of some of the oldest silicic rocks in Iceland produced by a variety of—and in many cases, a combination of—processes These data highlight nuances that suggest the geodynamic evolution of the Westfjords may be more complicated than previously proposed Isotopic compositions support involvement of an enriched‐mantle‐like component and a pulsing plume and highlight mantle heterogeneity

Subduction of rocks into the mantle results in high-pressure metamorphism and the formation of eclogites from basaltic precursor rocks. In general, many kilometers of oceanic lithosphere are ultimately consumed prior to the subsequent continental slab subduction and collision. The exposure of the eclogites derived from oceanic subduction and continental subduction at the surface of Earth today record provide different P-T-t records of the subduction process. The Huwan shear zone in the Hong’an orogenic belt, marking a former ocean-continent transition zone, has been the focus of many studies on subduction-related high-pressure metamorphism. In this study, Lu-Hf garnet, U-Pb zircon, and Ar- Ar mica ages are combined with geochemical data to understand the origin of two coexisting eclogite bodies exposed along the Xuehe River in the Huwan Shear zone. In total, the results indicate that the two eclogites have different protoliths but experienced a similar metamorphic history. This observation requires new tectonic model for the coupled subduction of oceanic and continental crust in subduction zones. Combined geochemistry and zircon U-Pb geochronology suggest distinct oceanic and continental affinities for the eclogite protoliths. The Lu-Hf dates of 261.5 ± 2.4 Ma of the continental-type eclogite and 262.7 ± 1.7 Ma of the oceanic-type eclogite reflect garnet growth and are interpreted to closely approximate the age of eclogite-facies metamorphism. Therefore, both the geochemically oceanic- and continental-type eclogites underwent the same episode of Permian eclogite-facies metamorphism. The Permian Lu-Hf ages of ca. 262 Ma and the obtained Triassic Ar-Ar ages (~240 Ma) of the oceanic-type and continental-type eclogites imply coupled subduction and exhumation of oceanic and continental crustal materials in the Hong’an orogenic belt during the Permian and the Triassic. Though limited, the geochemical and geochronological results of this study, together with the discrepant Carboniferous dates for the nearby eclogites of previous studies, apparently suggest that the Huwan shear zone was not always a single coherent unit but instead comprises different tectonic slices that were metamorphosed at different times before final assembly. Some slices of the oceanic and continental crust underwent two subduction cycles during the Carboniferous and the Permian, whereas some eclogites registered only a single subduction-exhumation loop during the convergence between the South China Block and the North China Block in the Huwan shear zone. The consistent ages of the oceanic- and continental-type eclogites disfavor the traditional mélange model that high-pressure rocks are dismembered fragments that have been assembled and intercalated with rocks devoid of any high-pressure history at shallow crustal levels, forming a tectonic mélange.

Zircon inclusions, extracted from garnet from an ultrahigh-pressure eclogite in the eastern part of the Dabie orogen, were investigated with high spatial resolution U-Pb geochronology. The unzoned zircon rims have garnet and omphacite inclusions and show weak cathodoluminescence, low-Th/U ratios (0.02-0.18), flat high rare-earth elements patterns, and an absence of Eu anomalies, which suggest metamorphic growth in the presence of garnet and in the absence of feldspar. Dispersed concordia U-Pb ages, ranging from 205.1 ± 3.3 to 255.3 ± 6.1 Ma, defined a weighted mean 206Pb/238U age of 223 ± 8 Ma (mean square of weighted deviation, MSWD = 38), which is significantly younger than the Lu-Hf garnet-whole rock age of 242.2 ± 2.6 Ma. The U-Pb age probability distribution indicates two age populations at ca. 241 and ca. 216 Ma. The zircon rims, with younger ages, are distinguished from those with older ages by the occurrence of garnet and omphacite inclusions. Well-preserved prograde major- and trace-element zoning in garnet and the significant contribution of older garnet cores in bulk isotopic digestion suggest that the Lu-Hf age mostly reflects an early phase of garnet growth likely at amphibolite facies conditions. The new Lu-Hf age combined with the U-Pb ages of the zircon inclusions in garnet suggest that garnet growth was sustained over a time interval of ~25 m.y.

Oceanic-type high-pressure eclogite from the Huwan mélange in the western part of the Dabie orogen, north of the classic ultrahigh-pressure terrane, has been investigated with combined Lu-Hf and Sm-Nd geochronology. The eclogites are thought to have formed during the subduction of oceanic crust prior to the subduction of the Yangtze craton beneath the North China block. Garnet Lu-Hf and Sm-Nd garnet-whole rock dates yield similar ages of 257.4 ± 1.4 and 252.5 ± 2.7 Ma, the youngest ages reported thus far from this mélange. Despite the partially preserved prograde major- and traceelement zoning in garnets, the Lu-Hf and Sm-Nd ages mostly reflect the high-pressure eclogite-facies metamorphism instead of the early phase of garnet growth. The data define a new high-pressure age population for the Dabie orogen, which provides a more detailed insight into the subduction history of the Yangtze craton before continental collision. The new Lu-Hf and Sm-Nd dates for these oceanictype eclogites, combined with existing geochronological information, range from 315 to 253 Ma. This broad interval suggests that the overall subduction of oceanic crust occurred over a period of ~60 Ma and that different slices with distinct pressure-temperature histories underwent HP metamorphism at distinct times. A model of a continuous evolution from oceanic to continental subduction is favored, and the onset of the deeply continental subduction occurred probably not before ca. 257 Ma.

Ophiolite metamorphic soles preserve important records of ophiolite emplacement, but there have been few detailed investigations into their non‐mafic portions. We present new thermobarometric and petrochronologic data from a metasediment and mafic restite in the upper Wadi Tayin sole exposure in the Samail (Oman‐UAE) Ophiolite. Thermodynamic modeling suggests metasedimentary garnet nucleation at ~4 kb, ~550°C and final growth at 7.5 ± 1.2 kbar, 665 ± 32°C, occurring by 93.0 ± 0.5 Ma (Lu‐Hf isochron). Zircon U‐Pb dates of 106.9 ± 2.3 (detrital) and 98.7 ± 1.7 to 94.1 ± 1.6 Ma (metamorphic) bracket the initiation of metamorphism, and monazite U‐Pb dates from ~97–89 Ma suggest a lengthy period of growth or recrystallization. A mafic titanite U‐Pb age of 92.2 ± 1.8 Ma records the earliest possible juxtaposition of high‐ and lower‐grade sole rocks. These and other data suggest that (i) the Wadi Tayin sole preserves an inverted metamorphic, metasomatic, and age gradient,(ii) metasediment metamorphism occurred during, or soon after, crystallization of the overlying ophiolite (≤96.5 Ma); and (iii) sole metasediments define a thermal gradient continuous with hotter, higher‐Pamphibolites. Some of these data conflict with existing models for sole formation, and we propose several hypotheses to explain them. Cooling of the sole below Ar closure by ~92 Ma suggests that strain rapidly partitioned away from the sole, leading to large‐scale, thin‐skinned thrust emplacement of the ophiolite >100 km across the continental margin and the late, cool underthrusting of the continental margin. Prograde, amphibolite‐facies sole metamorphism at Wadi Tayin bracketed to ~99–93 MaSediment subduction in the metamorphic sole coincided with or postdated ophiolite crystallizationIntegration with existing data permits several testable models for metamorphic sole formation

As one of two well‐exposed intraoceanic arcs, the Talkeetna arc of Alaska affords an opportunity to understand processes deep within arcs. This study reports new Lu‐Hf and Sm‐Nd garnet ages, 40Ar/39Ar hornblende, mica and whole‐rock ages, and U‐Th/He zircon and apatite ages from the Chugach Mountains, Talkeetna Mountains, and Alaska Peninsula, which, in conjunction with existing geochronology, constrain the thermal history of the arc. Zircon U‐Pb ages establish the main period of arc magmatism as 202–181 Ma in the Chugach Mountains and 183–153 Ma in the eastern Talkeetna Mountains and Alaska Peninsula. Approximately 184 Ma Lu‐Hf and ∼182 Ma Sm‐Nd garnet ages indicate that 25–35 km deep sections of the arc remained above ∼700°C for as much as 15 Myr. The 40Ar/39Ar hornblende ages are chiefly 194–170 Ma in the Chugach Mountains and 175–150 Ma in the Talkeetna Mountains and Alaska Peninsula but differ from zircon U‐Pb ages in the same samples by as little as 0 Myr and as much as 33 Myr, documenting a spatially variable thermal history. Mica ages have a broader distribution, from ∼180 Ma to 130 Ma, suggesting local cooling and/or reheating. The oldest U‐Th/He zircon ages are ∼137 to 129 Ma, indicating no Cenozoic regional heating above ∼180°C. Although the signal is likely complicated by Cretaceous and Oligocene postarc magmatism, the aggregate thermochronology record indicates that the thermal history of the Talkeetna arc was spatially variable. One‐dimensional finite difference thermal models show that this kind of spatial variability is inherent to intraoceanic arcs with simple construction histories.