The Cougar Gulch area near Coeur d’Alene, Idaho, is a newly recognized Paleoproterozoic to Archean basement occurrence located in the southern Priest River complex. Here, a structural culmination exposes deeper levels of the core complex infrastructure, similar to where Archean basement is exposed in the northern portion of the complex near Priest River, Idaho. At Cougar Gulch, the basement rocks are composed of a variety of granitic orthogneisses and amphibolite, which are unconformably overlain by a graphite-bearing orthoquartzite. The orthoquartzite is in turn overlain by the Hauser Lake Gneiss. The similarity of structure, metamorphic fabrics, and kinematics here and in the northern portions of the complex is consistent with the Cougar Gulch area being the southern continuation of the Spokane dome mylonite zone.Neoarchean amphibolites (2.65 Ga) have been identified as part of the basement sequence. These amphibolites had a basaltic protolith and can be distinguished geochemically from amphibolites found within the overlying Hauser Lake Gneiss (Mesoproterozoic, Lower Belt Group equivalent), which are metamorphosed Moyie sills. The Archean amphibolites have steeper REE (rare earth element) slopes and consistently higher REE values. Protoliths of the Paleoproterozoic orthogneisses (1.87–1.86 Ga) are calc-alkaline, “I-type” monzogranites and granodiorites, which exhibit subduction-related geochemical characteristics such as high LILE:HFSE (large ion lithophile element: high field strength element) concentrations, along with characteristic depletions in Nb, Ta, P, Ti, and Eu. A second distinctive geochemical unit of orthogneiss, the Kidd Creek tonalite, exhibits TTG (tonalite-trondhjemite-granodiorite) geochemical characteristics. The Kidd Creek tonalite has Sr/Y and La/Yb ratios, along with Y and HREE (heavy rare earth element) concentrations (no Eu anomalies) similar to Precambrian TTG compositions formed in subduction settings. Detrital zircon data from the orthoquartzite unit, along with characteristic graphite and its consistent stratigraphic level support correlation to the pre-Belt Gold Cup Quartzite in the northern part of the complex.

The Waterbury dome, located in the Rowe-Hawley zone in western Connecticut, is a triple window exposing three terranes: parautochthonous or allochthonous peri-Laurentian rocks in its lowest level 1, allochthonous rocks of the Rowe-Hawley zone in its middle level 2, and allochthonous cover rocks, including Silurian-Devonian rocks of the Connecticut Valley Gaspé trough, in its highest level 3. Levels 1 and 2 are separated by the Waterbury thrust, a fault equivalent to Cameron's Line, the Taconic suture in southwestern New England. Relict mesoscopic folds and foliation in levels 1 and 2 are truncated by a dominant D2 migmatitic layering and are likely Taconic. U-Pb zircon crystallization ages of felsic orthogneiss and tonalite, syntectonic with respect to the formation of S2, and a biotite quartz diorite that crosscuts level 2 paragneiss are 437 ± 4 Ma, 434 ± 4 Ma, and 437 ± 4 Ma, respectively. Level 3 nappes were emplaced over the Waterbury dome along an Acadian décollement synchronous with the formation of a D3 thrust duplex in the dome. The décollement truncates the Ky + Kfs-in (migmatite) isograd in the dome core and a St-in isograd in level 3 nappes, indicating that peak metamorphic conditions in the dome core and nappe cover rocks formed in different places at different times. Metamorphic overgrowths on zircon from the felsic orthogneiss in the Waterbury dome have an age of 387 ± 5 Ma. Rocks of all levels and the décollement are folded by D4 folds that have a strongly developed, regional crenulation cleavage and D5 folds. The Waterbury dome was formed by thrust duplexing followed by fold interference during the Acadian orogeny. The 40Ar/39Ar ages of amphibole, muscovite, biotite, and K-feldspar from above and below the décollement are ca. 378 Ma, 355 Ma, 360 Ma (above) and 340 (below), and 288 Ma, respectively. Any kilometer-scale vertical movements between dome and nappe rocks were over by ca. 378 Ma. Core and cover rocks of the Waterbury dome record synchronous, post-Acadian cooling.

Geologic mapping of Mesoproterozoic lithologies and foliations, and sensitive high-resolution ion microprobe (SHRIMP) U-Pb crystallization ages of 43 samples of orthogneisses and metagranitoids from the northern Blue Ridge establish new subdivisions: group 1 (1183–1144 Ma), group 2 (1143–1111 Ma), and group 3 (1078–1028 Ma). Protoliths of group 1 were metamorphosed at amphibolite- to granulite-facies conditions and strongly deformed between ca. 1153 and ca. 1144 Ma. Metagranitoids of groups 2 and 3 were emplaced continually for another 115 m.y. and display only local effects of diminishing deformation events. Ages of zircon overgrowths overlap temporally with igneous crystallization ages of group 3, but continued until ca. 960 Ma.

The Abitibi-Opatica terrane is defined to include the Abitibi granite-greenstone Subprovince and the Opatica granite-gneiss domain in southeastern Superior Province, Canada. We combine the geological, structural, geochronological, and geochemical knowledge base for the region, with new geochemical data for suites of granitic rocks, in order to establish a testable model for the geodynamic setting and the tectonomagmatic evolution of the Late Archean crust. The geochemistry of TTG orthogneiss and plutons are correlated, petrogenetically and temporally, with the published data and interpretations for the volcanic stratigraphy. The geochemistry of later granodiorite plutons is correlated with the crustal melting signatures of the youngest volcanic assemblage. Putting the geochemical data and interpretations into a framework with data on crustal structure, crustal thickness, and geochronology allows us to define the precollisional tectonomagmatic history of the Abitibi-Opatica terrane. A geodynamic-tectonic model is proposed, involving subduction of an ocean basin beneath an existing, magmatically active and partially differentiated oceanic plateau. The geochemical signature of “plume-arc interaction” is attributed to subduction that was initiated under the magmatically active oceanic plateau, in the presence of a still-active plume. The proposed plate tectonic model explains the presence of plume-type and subduction-type signatures in the volcanic stratigraphy, and in the TTG gneiss and plutons, and requires a single period of plate convergence and subduction that lasted for ~35 million years, ending in a tectonic collision event, ca. 2700 Ma. We propose that the interstratification of plume-type and subduction-type lavas, and the concomitant emplacement of TTG plutons with slab-melting characteristics, might be explained by the formation of a slab window in the subplateau subduction zone.

A well-defined axis of maximum dilation within the ca. 148 Ma Independence dike swarm is significantly offset across Owens Valley. Dilation by diking within the axis of maximum dilation is greater than 5%, commonly exceeds 10%, and locally ranges over 40%. Elsewhere in the swarm, dilation rarely ranges above 2%. The axis of maximum dilation steps ∼75–130 km rightward across Owens Valley, although the offset is difficult to measure precisely because the dike swarm is diffuse and intersects the valley at a relatively low angle. Comparison with other recently investigated geologic markers favors 65 ± 5 km of dextral offset since 83.5 Ma and perhaps an additional 10–65 km of offset prior to 83.5 Ma. Although Owens Valley is a locus of modern dextral slip, regional relations suggest that most of the 65 km of dextral displacement accumulated in Latest Cretaceous–early Paleogene (Laramide) time, when Cordilleran subduction was strongly right-oblique. Thermobarometric, structural, stratigraphic, and geochronologic evidence from the southern Sierra Nevada have previously been interpreted to reflect south-directed tectonic unroofing of deep-crustal rocks to form a metamorphic core complex during Laramide time. Large-magnitude Laramide right slip across Owens Valley thus may have been transferred southward into extension in the southern Sierra Nevada. Linked systems of late-Laramide to post-Laramide strike-slip faults and metamorphic core complexes have long been recognized in the plutonic-metamorphic core of the northern Cordillera. Recognition of this tectonic style in California suggests that it may have characterized most of the western Cordilleran orogen at this time.

Ca. 500 Ma orthogneisses and bimodal suites are widespread along the northern part of the Bohemian Massif (central European Variscides) and are interpreted to document intense magmatism during a continental break-up episode along the northern periphery of Gondwana. Based on geological setting, and geochemical and isotopic evidence, these felsic igneous rocks record the generation of: (1) magmas of pure or predominantly crustal derivation, represented by minor extrusives and much more voluminous orthogneisses similar to S-type granitoids; (2) subordinate magmas of exclusively mantle origin (ranging from within-plate alkali trachytes to oceanic plagiogranites) corresponding to felsic derivatives of associated basalts; and (3) magmas of hybrid origin, produced either as a result of large degrees of contamination of mantle-derived magmas ascending through the crust, or alternatively, generated by partial melting of mixed sources, such as interlayered sediments and mafic rocks or graywackes containing a juvenile component. The high-temperature dehydration melting process responsible for the generation of the most abundant rock-types necessitated the advection of mantle heat, in a context of continental lithosphere extension, as documented by broadly coeval basaltic magmatism at the scale of the igneous province. The large volumes of felsic magmas generated during the 500-Ma anorogenic event are interpreted to result from the combination of a hot extensional tectonic regime with the widespread availability in the lower crust of fertile lithologies, such as metagraywackes. This in turn reflects the largely undifferentiated nature of the crustal segment accreted some 50–100 m.y. earlier during the Cadomian orogeny.

Geochronological investigations of early Archean gneiss complexes by U-Pb dating methods are complicated by the presence of zircon populations that contain multiple ages of magmatic and metamorphic growth, and inherited components from premagmatic crust formation processes. In an early ion-microprobe study in the Saglek block, Nain Province, northern Labrador, identical ages for magmatism of 3.73 Ga were reported for both amphibolite- and granulite-grade orthogneisses, based on the oldest 207Pb/206Pb ages in two and three grains, respectively (Schiøtte et al., 1989). Here we present zircon ages for the same rock units as in the previous study of 3348 ± 7 Ma and 3634 ± 31 Ma, respectively, obtained by isotope-dilution thermal ionization mass spectrometry (ID-TIMS) methods, and based on the best quality grains from the dominant population of each rock. Zircon cores in the amphibolite-facies orthogneiss were separated from igneous overgrowths using a brief HF acid treatment and give ages ranging from 3700 to 3760 Ma, the age reported for magmatism in Schiøtte et al. (1989). The time of major metamorphism in the amphibolite-facies orthogneiss is recorded by both Pb loss in the magmatic zircons and by new zircon growth within remnants of cross-cutting metadiabase at 2702 ± 2 Ma. The age for the early stage of granulite-facies metamorphism was reported by Schiøtte et al. (1989) as 2766 ± 17 Ma compared to the age reported here at 2740 ± 4 Ma. The results of this comparative study are instructive for the interpretation of sequential inherited, magmatic, and metamorphic zircon components in a complex population and for devising sampling strategies based on simple geological relations. Our new results demonstrate the way different ages can be obtained for the same rock unit due to contrasting approaches to dating geological events, and it is suggested that the earlier published data require reinterpretation. The new data reported here add to the database on igneous activity in the region so that the formation of crustal components is documented at 3850–3800, 3750–3700, ca. 3630, 3348, and 3200 Ma. This crustal instability may reflect the lack of an early mantle keel that was not fully established until the 2740–2702 Ma episode of metamorphism and ductile flow. A thin-skinned protocrust with multiple ages of intrusion and metamorphism, possibly driven by plume-related heat from below, i.e., multistage crustal recycling rather than competent plate interactions, is suggested for the formation of these Archean gneisses.

Granites of the Batholith of Central Patagonia were emplaced into late Precambrian metamorphic basement rocks and Palaeozoic orthogneisses (here dated by Rb–Sr whole-rock errorchrons at 346 ±35 Ma and 267 ±27 Ma), and are in fault-contact with younger volcanic rocks, mostly andesites. Two main suites of granites are recognised: both are much younger than their previously-supposed Late Palaeozoic age. The Gastre Suite is composed predominantly of hornblende-biotite granodiorite and monzogranite, often slightly foliated, and has yielded a Rb-Sr whole-rock isochron age of 220 ±3 Ma. The Lipetrén Suite includes biotite monzogranite and leucogranite, grading into quartz-feldspar porphyries and felsites, and has been dated at 208 ±1 Ma. A minor granodioritic unit yields an apparently Middle Jurassic age of 172 ±15 Ma. Textural evidence and hornblende geobarometry confirm that these are high-level sub-volcanic intrusions. Metaluminous compositions are common in the Gastre Suite, but are subordinate to highly siliceous (>70% SiO2) and peraluminous varieties in the Lipetrén Suite. Despite this compositional bias, the granites are almost entirely calcalkaline and I-type, and have volcanic-arc rather than intraplate or collisional trace element characteristics. Initial 87Sr/86Sr ratios of 0·706 and εNdt values of −2·5 are also relatively primitive and are thought to indicate a juvenile crustal contribution (Nd “depleted mantle” model ages are less than 1,000 Ma). The Triassic “Gondwana” magmatic episode is thus not an expression of Permo-Triassic collision of an allochthonous Patagonian terrane with the rest of southern S America, but may be related to the initial stages of supercontinent rifting.

Northwestern Palmer Land is situated within the Mesozoic magmatic arc of the Antarctic Peninsula. Three temporally distinct granitoid groups, of lower Paleozoic, Jurassic, and Cretaceous ages, have been identified in this region. They are all calc-alkaline, I-type granitoids, ranging from quartz-diorites to granites, have high LIL/HFS element ratios, and their analyses all fall in the volcanic arc granite field when plotted on granite discrimination diagrams. The Paleozoic group comprises orthogneisses that have undergone high-grade metamorphism. The Jurassic group is variably foliated and partially recrystallized. It is subdivided into high-K metagranitoids and a smaller subgroup of low-K metatonalites, which are restricted to the western margin. The Cretaceous granitoids, which are undeformed, are subdivided on the basis of petrography into granophyric granites and nongranophyric granitoids. The orthogneisses, high-K metagranitoids, and nongranophyric granitoids are characterized by increasing LIL elements (particularly K and Rb) with increasing SiO2 and an initial increase followed by a decrease in the HFS elements (Zr, Nb, and, to a lesser extent, Y). Variable degrees of partial melting or fractional crystallization could have produced the mafic to intermediate compositions, but plagioclase, K-feldspar, and minor phase fractionation are most likely responsible for the geochemical trends in the intermediate to acid compositions. In contrast the metatonalites show no enrichment in either the LIL or the HFS elements but still have high LIL/HFS element ratios. Their origin is uncertain, although they appear to have been derived from the same source as the high-K metagranitoids, but probably evolved under higher PH2O and fO2 conditions. The Cretaceous granophyric granites are more enriched in both the LIL elements and the HFS elements at high SiO2 levels than the nongranophyric granitoids. These are interpreted to have differentiated from the nongranophyric granitoids by the process of filter-pressing. Rb-Sr isotope data for all the groups suggest there is no correlation between initial 87Sr/86Sr ratios and time. The values lie in the range 0.705 to 0.707, suggesting that all the groups are predominantly mantle-derived but may contain a significant crustal component. The orthogneisses are part of the extensive Paleozoic plutonism that occurred along the length of the Pacific margin of Gondwanaland. Within the context of the Antarctic Peninsula as a whole, the Jurassic granitoids are geographically more restricted and geochemically more variable than the Cretaceous granitoids. The latter form a geochemically uniform group of plutons that occur throughout the Antarctic Peninsula.

Major-element chemical analyses of 130 samples of high-grade plagioclase gneiss, together with geologic data, suggest that metadacitic gneisses of the late Proterozoic Avalon and Putnam-Nashoba terranes include four extrusive and two intrusive units formerly mapped as Monson Gneiss and Middletown Formation. Evidence for an igneous rather than sedimentary protolith for these gneisses comes from the mineralogic and chemical homogeneity, and the consistency of the chemical variations within individual bodies of gneiss with fractional crystallization processes. Evidence for the extrusive origin of some units comes from interlayered contacts with metasediments and metarhyolitic (alaskitic) and metatholeiitic (amphibolitic) volcanics, and from a 20- to 50-m-scale chemical cyclicity, typical of modern ash-flow tuffs. Evidence for an intrusive origin of two bodies comes from abundant amphibolite and calc-silicate xenoliths, from a massive structure lacking compositional layering, and from kilometer-scale chemical zoning or homogeneity. The identification of some units as lithodemic orthogneisses indicates that intrusion as well as extrusion led to the lithologic sequences in the Avalon terrane of southeastern Connecticut. It also precludes the lithostratigraphic correlation of these orthogneisses with paragneisses. Given this geochemical and geologic evidence, we believe that previously advanced stratigraphic arguments that major recumbent folds relate gneiss bodies now interpreted to be intrusive and extrusive should be reexamined. We believe that the part of the Avalon terrane consisting of metavolcanic rocks may be a right-side-up volcanic pile, and that major recumbent folding may not be present.

Revised correlations of critical units in south-eastern Connecticut contradict earlier stratigraphic correlations that led to the interpretation of major recumbent fold napping in the Avalon terrane. The revisions are required in part by the discrimination between lithodemic and lithostratigraphic units. We find no compelling evidence that schistose Putnam-Nashoba rocks exist within the Avalon terrane. On the contrary, we interpret these blastomylonitic schists and gneisses within the Avalon terrane around the Lyme dome as ductile fault rocks, derived from an Avalonian granodioritic orthogneiss. Rocks originally mapped as Brimfield Formation (Merrimack terrane) west of Old Saybrook are shown to correlate with the Middletown Formation of the Bronson Hill terrane. This new correlation makes all plagioclase gneisses of the Killingworth dome isolated from rocks of other terranes by the Middletown Formation, and provides evidence against the assignment of any plagioclase gneisses south of the Bronson Hill terrane near the Killingworth dome to the Avalon terrane. In contrast to the interpretation of fold napping, we propose that a major fault nappe exists in the Avalon terrane. The fault zone separates rocks of the Lyme dome from overlying allochthonous rocks of the Waterford Complex in the “Selden Neck block,” and fault splays isolate paragneisses of unknown age. Splays of the fault extend west, where they cut an extension of the Honey Hill fault zone, narrow slivers of Merrimack terrane rocks, and units in the Bronson Hill terrane. Important deformation in the zone probably occurred from middle Carboniferous to middle Permian, although earlier deformation is possible. Subsequently, the entire zone was folded around the Lyme dome. The tuning of the deformation suggests that it was related to the Hercynian assembly of Pangaea.

The central Karakoram can be divided into three main tectonic units from north to south: a northern Karakoram terrane, the Karakoram batholith, and the Karakoram metamorphic complex. In the Baltoro Glacier region the Karakoram magmatism includes intrusive suites that predate and postdate the India-Eurasia collision. The oldest subduction-related phases include Jurassic hornblendite to biotite monzogranite of the Hushe complex, and Cretaceous (ca. 82 to 75 Ma) hornblende-biotite metagranitoids of the Muztagh Tower unit, all of which were deformed during the India-Kohistan-Karakoram collision. Volumetrically dominant is a postcollisional granite, the Baltoro Plutonic Unit (BPU), which consists of biotite monzogranites to two-mica ± garnet leucogranites and pegmatite-aplites of mildly peraluminous affinity. The BPU represents the youngest magmatic phase of the composite Karakoram batholith with a U-Pb age of 21 ± 0.5 Ma and K-Ar mica cooling ages ranging from 11.7 to 5.25 Ma. The Masherbrum migmatite complex (MMC), one of a series of such complexes along the southern margin of the batholith, immediately predates the BPU. Leucocratic dikes that cross-cut the MMC yield an Rb-Sr age of 14.1 ± 2.1 Ma and K-Ar ages of 17 to 10 Ma. The BPU is interpreted as a crustal melt ultimately derived from deep crustal levels and may not be related to leucogranite generation associated with the migmatite terrain to the south. Petrogenesis of the BPU is fundamentally different from that of the High Himalayan granites and may involve a degree of selective mantle contamination. Four major metamorphic-deformation phases can be distinguished in the central Karakoram. The earliest, M1, is represented by low-pressure andalusite-staurolite–bearing assemblages that are spatially associated with igneous components of the Hushe complex of Jurassic age. The dominant thermal event was a widespread Barrovian-type metamorphism (M2), which was syntectonic to the main deformation and overprinted M1 assemblages. M2-related structures are cut by the 37.0 ± 0.8 Ma Mango Gusar two-mica granite pluton. M2 kyanite-garnet-plagioclase-quartz-muscovite-biotite-staurolite assemblages indicate minimum pressure temperature (P-T) conditions of 550°C and 5.5 kbar (550 MPa). Thermal effects related to intrusion of the BPU constitute M3. Along its northern margin at Mitre Peak, the assemblage andalusite-cordierite-chlorite-biotite-muscovite-quartz-plagioclase indicates a maximum pressure of 3.75 kbar (375 MPa, ≃ 12.5-km depth). Along its southern margin at Paiyu, the presence of granitic melt pods with sillimanite, muscovite, plagioclase, and quartz indicates a minimum pressure of ca. 3.5 kbar (350 MPa) and a temperature 75° higher than local M2 assemblages. The replacement of kyanite by sillimanite and the appearance of granitic melt pods approaching the BPU along the Baltoro Glacier transect, may be an M3 overprinting of M2. M4 (<5 Ma) is a syntectonic retrogressive metamorphism along the hanging wall of the Main Karakoram Thrust—a breakback thrust responsible for the recent uplift of the Karakoram. Structural culminations of midcrustal rocks occur in the K2 and Broad Peak areas within Carboniferous-Lower Cretaceous sediments of the Gasherbrum Range. Metamorphism of the K2 gneiss (dominantly biotite-hornblende-K-feldspar orthogneiss) occurred during middle to Late Cretaceous time. Pegmatite dikes dated as 70 to 58 Ma (K-Ar-mica) cut the gneisses.