ABSTRACT Granitoid batholiths dominated by felsic to intermediate compositions are commonly associated with mafic plutons and enclaves; however, the genetic relationship between the apparently coeval but compositionally dissimilar magmas is unclear. Here, we reviewed the age and lithogeochemical and Nd-Sr isotopic compositions of some classic plutonic rocks emplaced in the Northern Highlands, Grampian and Connemara terranes of the Caledonide orogen of Scotland and Ireland. The Northern Highlands terrane consists mostly of Neoproterozoic metasedimentary rocks of the Moine Supergroup and is located north of the Great Glen fault. The Grampian terrane also consists of Neoproterozoic metasedimentary rocks (Dalradian Supergroup) and is located south of the Great Glen fault in both Scotland and Ireland. Amphibolite-facies metasedimentary rocks in the Connemara terrane are correlated with the Dalradian Supergroup, and the terrane is bounded by splays of the Highland Boundary and Southern Uplands faults. These three terranes were intruded by Silurian–Devonian mafic and felsic to intermediate plutonic rocks that display field evidence for mingling and mixing and have a similar range (between ca. 437 and 370 Ma) in emplacement ages. This range implies they were intruded during and after the late Caledonian Scandian orogenic event that resulted from the mid- to late Silurian collision of amalgamated Avalonia and Baltica with Laurentia and the final closure of the Iapetus Ocean. Our review supports the contention that the Great Glen fault represents a major compositional boundary in the Silurian lithosphere. Felsic to intermediate plutons that occur north of the Great Glen fault are more enriched in light rare earth elements and Ba-Sr-K compared to those to the south. Isotopic compositions of these late Caledonian plutonic rocks on both sides of the Great Glen fault indicate that metasomatism and enrichment of the subcontinental lithospheric mantle beneath the Northern Highlands terrane occurred just prior to emplacement of late Caledonian plutons. Within the same terrane, mafic and felsic to intermediate rocks display similar trace-element and rare earth element concentrations compatible with models implying that fractionation of a mafic magma played an important role in generating the felsic to intermediate magmas. The onset of slab failure magmatism may have been diachronous along the length of the collision zone. If so, slab failure may have propagated laterally, possibly initiating where promontories collided.

Abstract During the 1970s, geologists considered that the Upper Ordovician Taconic Orogeny represented the collision of Laurentia with the Ammonoosuc arc, now largely exposed on the Bronson Hill anticlinorium. Subsequently, several researchers noted that magmatic rocks which intrude and overlie the Ammonoosuc arc are younger than the c. 455–451 Ma Taconic Orogeny. This led them to hypothesize that a Middle Ordovician collision was followed by westward-dipping subduction beneath the amalgamated Laurentian–Ammonoosuc zone to produce the younger arc rocks. In this model, the Taconic allochthons and foredeep were produced later in a retro-arc setting above westward-dipping subduction. However, those models prove inadequate due to the lack of ash beds, foredeep sedimentation and deformation on the Laurentian platform prior to the Upper Ordovician Taconic Orogeny. Here, we resolve the dilemma by recognizing that the magmatic rocks, which post-date the 455–451 Ma Taconic Orogeny, are not arc rocks but, instead, typical post-collisional slab-failure rocks as old as 450 Ma, with Sr/Y > 10, Sm/Yb > 2.5, Nb/Y > 0.4 and La/Yb > 10. Thus, in New England and western New York, the Upper Ordovician Taconic Orogeny represents the collision of the Ammonoosuc arc with Laurentia followed by slab failure of the descending plate.

ABSTRACT In the standard model, Cordilleran-type batholiths form beneath volcanic arcs in thickened crust, but our survey of modern and ancient continental arcs revealed most to be regions of normal to thinned crust, not zones of crustal thickening. This suggested to us that the standard batholithic paradigm is flawed. In order to better understand the batholiths, we explored (1) the 100–84 Ma La Posta and Sierran Crest magmatic suites of the Peninsular Ranges and Sierran batholiths, which formed after the 100 Ma Oregonian event due to closure of the Bisbee-Arperos seaway; (2) plutons and batholiths emplaced into the metamorphic hinterland of the 124–115 Ma Sevier event, which occurred in the Great Basin sector of the United States but, due to younger meridional transport, are now exposed in the Omineca belt and Selwyn Basin of Canada; and (3) Late Cretaceous–early Cenozoic intrusive rocks, such as the Coast, Idaho, and Boulder batholiths, which intruded a metamorphic hinterland during and after the Laramide event. The dominance of syn-to postdeformational emplacement and the distinctive slab failure–type geochemistry indicate that most, but not all, Cretaceous plutons within Cordilleran batholiths formed during and after arc-continent collision as the result of slab failure. We interpret whole-rock geochemistry, as well as radiogenic and stable isotopes, to indicate that slab failure magmas involve only minor amounts of crust and are derived mainly from plagioclase-absent melting of garnet-bearing rocks in the mantle. Some suites, such as the <100 Ma Oregonian Sierran and Peninsular Ranges batholiths, have evolved Nd and Sr isotopes compatible with old enriched subcontinental lithospheric mantle. The well-known 0.706 87/86 Sr i isopleth appears to separate rocks of Oregonian slab failure from rocks of older arc magmatism and is probably unrelated to any obvious crustal break; instead, it reflects involvement of old subcontinental lithospheric mantle in the slab failure magmas. To expand our findings we examined the geochemistry of Cenozoic slab window and Precambrian tonalite-trondhjemite-granodiorite suites and found them to share many similarities with the Cretaceous slab failure rocks. Because most Cretaceous plutons in the North American Cordillera appear to represent juvenile additions to the crust, we argue that substantial volumes of continental crust are formed by slab failure magmatism. Slab failure rocks, especially those emplaced within the epizone, are richly metalliferous and make excellent exploration targets.

The broadly accepted hypothesis for the development of the segmented Cordilleran orogen above a long-lived eastwardly dipping subduction zone is at odds with many critical observations. Therefore, I explore an alternative collisional model in which the western edge of North America was partially subducted to the west beneath the Rubian ribbon continent. The collision of the two initially led to the localized Sevier fold-thrust belt and later to the more extensive Laramide deformational event. The Rubian ribbon continent was assembled piece by piece, but at 160 Ma, two previously assembled blocks, Sierrita and Proto-Rubia, collided. Proto-Rubia formed during the Mississippian by collision of the Roberts Mountain allochthon with the Antler margin, a Neoproterozoic–Paleozoic passive margin of unknown provenance. Additions at 260–250 Ma included Yukon-Tanana–Slide Mountain terranes and the Golconda allochthon. Sierrita formed during the Middle Jurassic between ~170 and 160 Ma when several east-facing arcs, including the Smartville, Slate Creek–Lake Combie, and Hayfork, were amalgamated on the western side of the Sierran–Black Rock arc just prior to and at about the same time as it collided with the western margin of Proto-Rubia to the east. Consequent slab failure generated an arc-parallel suite of postcollisional intrusions, including the Independence dike swarm and the bimodal, alkaline Ko Vaya suite. New eastward subduction beneath the western margin of Rubia started sometime between 159 Ma and ~130 Ma. The Sevier phase of the Cordilleran orogeny began at ~125 Ma when a promontory located in the Great Basin segment of the North American craton was pulled into the westward-dipping subduction zone that existed on the Panthalassic side of the Rubian superterrane. The entry of the margin into the trench formed the Sevier fold-thrust belt and led to accretion of exotic megathrust sheets to western North America. During(?) and after the collision, most of the Rubian superterrane migrated southward relative to North America. To the west at ~100 Ma, a dextral transpressional collision led to the closure of the Gravina-Nutzotin-Dezadeash-Gambier basin(s) in Canada and Alaska, the accretion of the Alisitis arc in Baja California, and the closure of a now cryptic basin within the Sierra Nevada. Post-collisional plutonic suites, such as the La Posta and the Sierran Crest may have been caused by slab failure. At around 80 Ma, North America started to migrate southward, and this led to the collision of the entire Rubian ribbon continent, which extended from the Alaskan sector at least to northern South America, with the outboard margin of North America during the Laramide phase of the Cordilleran orogeny. The resultant shutdown of subduction along both margins led to (1) termination of Cordilleran-type magmatism, (2) exhumation of Franciscan blueschists within accretionary complexes along the western side of Rubia, (3) emplacement of a linear belt of slab-failure magmatism within the Sonora-Mojave region and the then adjacent Coast plutonic belt, and (4) oblique northward migration of Rubia. The oblique convergence between Rubia and North America created a region of thick-skinned deformation within the Great Basin segment south of the Orofi no fault and thin-skinned folding and thrusting in the Alaskan, Canadian, and Sonoran sectors. Prior to their northward migration, the previously amalgamated terranes presently located within the Canadian Cordillera were located several thousand km farther south and joined at their south end with the northern end of the Sonoran sector. The collision was followed by linear, orogen-parallel regions of extensional collapse and exhumation. Final collapse of the thickened collision zone to form the Basin and Range province occurred in the Great Basin and Sonoran sectors where the North American craton had been pulled beneath the Rubian superterrane. New easterly directed subduction started beneath the amalgamated collision zone at ~53 Ma and has continued to the present.

The North American portion of the Cordilleran orogen extends continuously from Alaska to southern Mexico, and from east to west over much of its length the orogen comprises an easterly vergent fold-thrust belt, a complexly deformed metamorphic hinterland that collapsed gravitationally, and an interlaced mosaic of exotic terranes. Although most models for the development of the Cordilleran orogen invoke Late Jurassic–Cretaceous intraplate, backarc shortening above an eastwardly dipping subduction zone, a simple collisional model in which the leading edge of North America was subducted to the west, beneath a segmented, arc-bearing microcontinent, better fits the data. During the early Mesozoic, Panthalassic Ocean crust was subducted westward beneath a ribbon continent named Rubia, where it created a generally low-standing continental arc. At about 124 Ma, the widespread deposition of intraformational gravels and conglomerates atop the passive margin marked the passage of the North American shelf over the outer bulge of the trench and its entry into the subduction zone. Loading by the “bulldozed” and thickened accretionary wedge—as well as the overlying eastern edge of the Rubian ribbon continent—depressed the lithosphere to create the Cretaceous foredeep, which migrated eastward during progressive convergence. As the westernmost edge of North America was subducted, the dewatering of slope-rise and rift deposits abruptly created voluminous melts that rose to thicken and assimilate the overlying exotic crust, where they formed Cordilleran-type batholiths. Owing to the difficulty of subducting an old craton, convergence slowed to a halt by 80–75 Ma, causing the shutdown of Cordilleran-type magmatism, and finally, during the Maastrichtian, break-off of the North American plate. The first segment to fail was likely the Great Basin segment, located south of the Lewis and Clark lineament and north of the Sonoran segment. There, slab failure rates were apparently slow enough that there was considerable lithospheric necking, and so slab-failure magmas were prevented from rising into the overriding plate. The diachronous break-off caused a catastrophic stress inversion in both upper and lower plates. Released from its oceanic anchor, the partially subducted edge of the North American craton rose rapidly, causing its stress regime to change from extensional to compressional, which, along with continued convergence, generated the thick-skinned Laramide deformation. Uplift and gravitational collapse of the overlying Rubian plate formed the linear belt of Paleocene-Eocene metamorphic core complexes within the orogenic hinterland. In the Canadian segment, located north of the Lewis and Clark line, the Coast plutonic complex was uplifted rapidly as asthenosphere rose through the torn lower-plate lithosphere to invade Rubia with a 1500-km linear belt of break-off–generated magmas. Within the Sonora segment to the south, break-off magmatism was also prevalent. Both the Canadian and Sonoran segments have abundant porphyry copper mineralization temporally and spatially associated with the break-off magmas, which suggests a genetic link between slab failure and porphyry copper mineralization. By 53 Ma, eastwardly dipping subduction of Pacific Ocean crust was generating arc magmatism on the amalgamated Cordilleran collision zone in both the Canadian and Sonoran segments. Oceanic schists, such as the Orocopia-Pelona-Rand, were formed in the ocean basin west of Rubia and accreted during initiation of the new easterly dipping subduction zone. A major transform fault, called the Phoenix fault, connects the Sevier fold-thrust belt at the California-Nevada border with that in eastern Mexico and separates the Great Basin and Sonoran segments. It juxtaposes the Sierra-Mojave-Sonora block alongside the Transition Zone of the Colorado Plateau. Cordilleran events affected the subsequent development of western North America. For example, the structural Basin and Range Province appears to coincide with the region where exotic allochthons sit atop North American crust in both the Great Basin and Sonoran segments. Also, within the triangular Columbia embayment, large segments of Rubia appear to have escaped laterally during the Cordilleran orogeny to create a lithospheric “hole” that was later filled by basalt of the Columbia River and Modoc plateaux.