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The margin off northwestern Morocco is a classic area for the study of the Atlantic style passive continental margins and evolution of the Central Atlantic Ocean. The margin was formed during the opening of the Central Atlantic around 180–200 Ma. Granitic rocks were discovered at depth along the margin edge of the Mazagan Plateau during the Deep Sea Drilling Program Leg 79 and were assumed to represent the Paleozoic basement of the Moroccan continental margin. The granites are peraluminous two mica granodiorites and yielded a zircon U–Pb age of ~ 34 Ma. The geochemistry of the granites suggests they were generated by partial melting of psammitic sedimentary rocks that also contained zircons with Paleocene–Cretaceous (~ 60–100 Ma), Late Triassic–Early Jurassic (~ 200 Ma) and Neoproterozoic (570–600 Ma) ages. After partial solidification, the granitic melt was squeezed/emplaced as a diapiric intrusion. This starved passive continental margin underwent Paleogene geodynamic rejuvenation which included extension, crustal thinning and crustal recycling.

Ordovician and Siluro-Lower Devonian magmatic rocks in the northern Appalachians south of the Iapetus suture are currently interpreted as distinct belts composed of multiple, small, peri-Gondwanan terranes that amalgamated during the sequential closures of Iapetus (latest Ordovician), the Tetagouche backarc basin (early Silurian), the Acadian seaway (Siluro-Devonian), and the Rheic Ocean (Devono-Carbonferous) (multiple terrane model). Here, the Siluro-Lower Devonian magmatic belts are shown to have slab failure affinities and together with the Ordovician arcs form paired belts parallel to the Iapetus suture, which suggests that they were emplaced along the common, peri-Avalonian margin during pre- and post-collisional processes. The Iapetan suture and the paired belts are inferred to repeat in Atlantic Canada due to dextral, strike-slip processes of mid-Late Devonian or younger age (terrane wreck model). In Newfoundland, the repetition is inferred to be the result of oblique, dextral offset of ca. 250 km. In the Quebec Embayment, the Iapetan paired magmatic belts are repeated twice in the limbs of a Z-shaped orocline related to oblique, dextral offsets of ca. 1200 km of the southern limb. Limited Siluro-Devonian paleomagnetic data indicate no paleolatitudinal differences across the Iapetus suture, however ca. 100° post-mid Silurian clockwise rotation is indicated for the middle fold limb; these data favour the terrane wreck model. The terrane wreck model results in a simple tectonic scenario of southerly subduction of Iapetus beneath a single ribbon continent (Avalonia sensu lato) that was subsequently deformed.

Voluminous bimodal volcanic rocks of the Silurian (ca. 422–420 Ma) Dickie Cove Group in the Ganderia domain of northern New Brunswick, Canada, are subaerial units that were deposited in an extensional setting, with the mafic types corresponding to continental tholeiites. Felsic rocks are rhyolites with calc-alkaline affinities. They exhibit geochemical characteristics that are typical of A2-type felsic magmas, such as enrichments in the incompatible elements Zr, Nb, and Y, as well as high FeO*/(FeO* + MgO) and Ga/Al ratios. Their εNd(t) values are positive (+0.7 to +3.4) but lower than those of the associated basalts. Saturation thermometry has yielded average zircon crystallization temperature estimates for the rhyolites that are well above 900 °C. The geochemical data indicate that the felsic melts were likely sourced from heterogeneous Neoproterozoic lower crust and generated by dehydration melting triggered by heat derived from underplated mafic magma. Parent melts of the rhyolites underwent fractional crystallization in a complex magma chamber prior to eruption. The Nd isotopic data suggest that the lower crust of Ganderia is similar to that of Avalonia in northern mainland Nova Scotia, and that the two microcontinents shared a common Neoproterozoic history and origin as continental blocks rifted from neighboring parts of Gondwana. The tectono-magmatic setting of the Dickie Cove Group volcanic rocks is interpreted as being related to Pridolian, post-Salinic relaxation and slab breakoff, which generated volcanism initially constrained within the Chaleur zone of the Chaleur Bay synclinorium, a large domain of the northern Appalachians. This was followed later in the Pridolian by extensional collapse and widening of the area of magmatic activity, which then prograded into the Tobique zone farther to the southwest.

The Meguma terrane is a unique unit of the Northern Appalachians as it is only identified in Nova Scotia. It was thrust over the Avalon terrane during the Early Devonian Acadian Orogeny. The Avalon and Meguma terranes are exotic to North America and likely originated along the margin of Gondwana. The precise relationship between the terranes is uncertain and very little is known about the basement rocks of each terrane. Hosted within the Late Devonian lamprophyric Popes Harbour dyke of the Meguma terrane are xenoliths of meta-sedimentary and meta-igneous rocks that are from the basement of the Avalon terrane. The xenoliths offer a glimpse into the nature of the lower crust of the Northern Appalachians. In this study, we present in situ zircon U-Pb age dates from a rare dioritic xenolith in order to assess its origin. The results show that the majority of zircons ages are between ~580 Ma and ~616 Ma with smaller groups at 750–630 Ma, ~2100 Ma, and

Table S1: Major and trace element analyses of rhyolitic rock samples from the Dickie Cove Group

In Mongolia, rare earth element (REE) mineralization of economic significance is related either to the Mesozoic carbonatites or to the Paleozoic peralkaline granitoid rocks. Carbonatites occur as part of alkaline silicate-carbonatite complexes, which are composed mainly of nepheline syenites and equivalent volcanic rocks. The complexes were emplaced in the Gobi-Tien Shan rift zone in southern Mongolia where carbonatites usually form dikes, plugs or intruded into brecciated rocks. In mineralized carbonatites, REE occur mainly as fluorocarbonates (bastnäsite, synchysite, parisite) and apatite. Apatite is also present in the carbonatite-hosted apatite-magnetite (mostly altered to hematite) bodies. Alkaline silicate rocks and carbonatites show common geochemical features such as enrichment of light REE but relative depletion of Ti, Zr, Nb, Ta and Hf and similar Sr and Nd isotopic characteristics suggesting the involvement of the heterogeneous lithospheric mantle in the formation of both carbonatites and associated silicate rocks. Hydrothermal fluids of magmatic origin played an important role in the genesis of the carbonatite-hosted REE deposits. The REE mineralization associated with peralkaline felsic rocks (peralkaline granites, syenites and pegmatites) mainly occurs in Mongolian Altai in northwestern Mongolia. The mineralization is largely hosted in accessory minerals (mainly elpidite, monazite, xenotime, fluorocarbonates), which can reach percentage levels in mineralized zones. These rocks are the results of protracted fractional crystallization of the magma that led to an enrichment of REE, especially in the late stages of magma evolution. The primary magmatic mineralization was overprinted (remobilized and enriched) by late magmatic to hydrothermal fluids. The mineralization associated with peralkaline granitic rocks also contains significant concentrations of Zr, Nb, Th and U. There are promising occurrences of both types of rare earth mineralization in Mongolia and at present, three of them have already established significant economic potential. They are mineralization related to Mesozoic Mushgai Khudag and Khotgor carbonatites in southern Mongolia and to the Devonian Khalzan Buregtei peralkaline granites in northwestern Mongolia.

Niobium and tantalum are two rare metals that have similar physical and chemical properties and occur together in nature. They are considered to be strategic and critical materials for the economy and national security of many industrial countries. Both elements are on the 2022 List of Critical Minerals of the USA as well as on the European Union’s List of Critical Raw Materials. They rarely substitute for common elements in rock-forming minerals but are essential components in a range of rare minerals, particularly oxides and subordinately silicates. The economically important minerals are oxides. The columbite-tantalite and pyrochlore-microlite groups are the most common Ta- and Nb-bearing minerals. In Mongolia, primary niobium and tantalum mineralization includes two main types. The first type is mineralization associated with alkaline to peralkaline granites, pegmatites and syenites whereas the second type is related to the lithium-fluorine-rich peraluminous granites and related rocks (pegmatites and ongonites). The host rocks of both types of mineralization are the fractionated felsic rocks, which contain the primary magmatic ore assemblages associated with fractionation of magma rich in rare metals. Both assemblages were subsequently overprinted by the late magmatic to hydrothermal fluids, which remobilized and enriched the original mineralization. The newly formed ore mineral assemblages display complex replacement textures. In the case of peralkaline felsic rocks the processes produced the mineralization of Zr, Nb, heavy REE, Y, U, Th and Ta whereas peraluminous Li-F felsic rocks contain mainly mineralization of Sn, W, Ta, Li, and Nb. Mongolia hosts several promising occurrences of both types of Nb-Ta mineralization. However, they have not yet been sufficiently explored. Currently, the most promising is the occurrence in the Devonian Khalzan Buregtei peralkaline granites in northwestern Mongolia, where Nb-Ta is associated with REE and Zr mineralization. Mesozoic carbonatites of southern Mongolia do not host significant Nb and Ta mineralization.

A thick succession of mafic upper Asbian volcanic rocks is exposed as cap rocks in the Magdalen Islands of eastern Quebec. This succession records peak stages of a Late Devonian to Early Ca...

Current interpretations of the geology of Cape Breton Island suggest that it exposes a complete cross-section of the Appalachians from Laurentia across Iapetan vestiges to Avalonia. Crucial to this view is the presence of ca. 1 Ga plutons, including anorthosites, which have been regarded as correlatives of Grenvillian basement, a correlation that overlooks the fact that Avalonia is also underlain by a ca. 1 Ga basement. We analyzed zircons from the Red River anorthosite (Blair River Complex, northwestern Cape Breton Island) previously dated as ca. 1.1 Ga: they yielded 421 ± 3 Ma intrusive ages with older ages between 865 ± 18 Ma and 1044 ± 20 Ma inferred to be either xenocrysts derived from the country rock or from the source. Implications of these data suggest that the accompanying low pressure granulite-amphibolite facies metamorphism of the Blair River Complex is either the root of a 440–410 Ma, magmatic belt produced during slab break-off or relict ca. 1 Ga basement. The Blair River Complex occurs in a NNE-SSW, sinistral positive flower structure that progresses upwards from a Neoproterozoic rifted arc through a low grade upper Ordovician-Silurian overstep sequence to amphibolite facies fault slices, capped by the low-pressure, granulite facies rocks (Blair River Complex). The correlation of Neoproterozoic, rifted arc units across most of Cape Breton Island suggests it represents the deformed northwestern margin of Avalonia intruded by a Silurian-Lower Devonian magmatic belt. As the geological record in the Blair River Complex is similar to both Grenvillian and Avalonian basements, its provenance is equivocal, however Pb isotopic data suggest the Blair River Complex has Amazonian (≈Avalonia) affinities. Thus, Cape Breton Island, rather than representing a complete cross-section of the Appalachian orogen, is part of pristine—deformed Avalonia with a positive flower structure exposing a cross-section of Avalonian crust.

The Cambrian–Ordovician metasedimentary rocks of the Meguma terrane (Canadian Appalachians) were extensively intruded by silicic plutons during Middle Devonian to Early Carboniferous times. Syn-plutonic but volumetrically minor mafic-ultramafic intrusions were also emplaced. In most localities, the silicic plutons and mafic-ultramafic intrusions do not appear to be petrogenetically related and are likely derived from different sources. The Attwoods Brook gabbronorite of SW Nova Scotia yielded an in situ zircon weighted-mean 206Pb–238U age of 357.9 ± 3.3 Ma that is within the uncertainty of the age of the neighbouring Wedgeport pluton (357 ± 1 Ma). The Wedgeport pluton is a rare example of a mantle-derived, peraluminous A-type granite within the Meguma terrane. The similar ages and Nd isotopes of the Attwoods Brook gabbronorite (εNd(t) = +1.1 to +4.0) and Wedgeport pluton (εNd(t) = +2.1 to +3.3) suggest the two intrusions are petrogenetically related. Fractional crystallization modelling demonstrates that a parental magma similar to the Attwoods Brook gabbronorite can produce residual silicic liquids that resemble the granites of the Wedgeport pluton, indicating that they could be members of the same intrusive complex. The emplacement of the gabbronorite and Wedgeport pluton occurred during a period of tensional plate stress that was contemporaneous with rifting of the Maritimes Basin that produced the Fountain Lake continental flood basalts and A-type granites of the Cobequid Highlands within the Avalon terrane. It is possible that the Early Carboniferous rocks of SW Nova Scotia are related to the rifted-related magmatism within the Maritimes Basin.

Early Carboniferous tholeiitic dikes in the New Carlisle area of Quebec, Canada, are abnormally enriched in high field strength elements (HFSEs), including rare earth elements. The enrichment is systematic and was apparently caused by an enhanced incompatibility of HFSEs during a prolonged episode of crystal fractionation. As a result, HFSE concentrations are up to one order of magnitude higher than those of typical mafic rocks. Very high F and Cl contents suggest that halogen complexing was largely responsible for the trace-element enrichment. The high halogen contents are in part accounted for by a subcontinental lithospheric mantle source that had previously been enriched in these elements by prolonged subduction. Additional Cl enrichment is interpreted to have occurred in a magma chamber that developed within porous and brine-rich country rocks of the upper crust. This conclusion is supported by the observation that HFSE-enriched mafic plutons in the same magmatic province occur in nonmetamorphosed upper-crustal rocks, suggesting high buoyancy and therefore high temperatures. Such evidence for high heat in the late Paleozoic magmatic system of eastern Canada corroborates previous studies suggesting that the transtensional basin in which it evolved was overriding a mantle plume at the time. In the case of the New Carlisle dikes, which are more than twice as enriched in incompatible trace elements as slightly older mafic rocks of the same magmatic system, the regional paleoenvironmental setting suggests that the associated upper-crustal magma chamber may have evolved in rocks with saltier pore water due to long-lasting evaporitic conditions at the surface.

The Allendale Lake stock is a representative intrusion of the Eocene alkaline igneous province in south-central British Columbia (Canada), which forms the northwestern edge of the Montana-Wyoming alkaline province. The shallow-seated intrusion (~53 Ma), about 4.5 km long and 2 km wide, is composed predominately of feldspar-phyric biotite-clinopyroxene-amphibole-bearing monzonitic/syenitic rocks. The feldspar forms distinct rhomb-shaped phenocrysts, which contain abundant exsolution of perthite and antiperthite. The Allendale Lake rocks are silica-undersaturated with SiO2 ranging from 52 to 63 wt% and high contents of alkalis, particularly K2O (4.5–6.5 wt%), typical of shoshonitic rocks. The rocks show a distinct enrichment of large-ion-lithophile elements relative to heavy rare earth elements and high-field strength elements. Their isotopic compositions are highlighted by high negative ƐNd(t) values (−4.5 to −5.5), Neoproterozoic Nd model ages (750–900 Ma) and high but relatively uniform initial 87Sr/86Sr ratios (~0.706). The rocks were generated by fractional crystallization under oxidizing conditions. The parent magma, a hydrous alkaline basaltic melt, was formed by partial melting of an amphibole-phlogopite-bearing peridotite of the subduction-modified subcontinental lithospheric mantle. The mantle source underwent Neoproterozoic and subsequently Cenozoic metasomatic enrichment events. The alkaline volcanic rocks of the Penticton Group (Marron Formation) from the nearby Eocene Challis-Kamloops volcanic belt are volcanic correlatives of these intrusions. The Eocene alkaline rocks in the south-central part of the British Columbia are related to continental arc rifting.

The evolution of the mantle source beneath the Tepla-Barrandian (TBB) and the adjacent southern part of the Saxo-Thuringian (S-STB) and northern part of the Moldanubian (N-MB) blocks of the Bohemian Massif (Czech Republic) is tracked from the Late Neoproterozoic to the Cenozoic in order to examine the coupling between the crust and underlying continental lithospheric mantle (CLM) during the Late Paleozoic Variscan orogeny. In the TBB, Late Neoproterozoic mafic rocks have highly radiogenic Nd but moderately radiogenic Sr isotopes and were derived from a spinel peridotite mantle. These within-plate rocks were emplaced in an intra-oceanic back-arc basin. Early Paleozoic (Ordovician to Early Devonian) mafic rocks are rift/extension-related, enriched in LREE and Nb relative to HREE and were probably derived from a garnet peridotite mantle source. Compared to Neoproterozoic basalts, their ƐNd(t) values are slightly lower but TDM model ages are indistinguishable suggesting derivation from a deeper portion of the same mantle source. Thus, the weak deformation and low-grade metamorphism associated with the Cadomian orogeny did not significantly decouple the crust from the underlying mantle source beneath the TBB. Early Paleozoic rift-related mafic rocks of the S-STB and N-MB have comparable isotopic signatures indicating a similar mantle source. Late Paleozoic mafic magmatism occurred during the extensional collapse phase of the Variscan orogen, in the aftermath of the collision between Gondwana and Laurussia. The TBB and S-STB basaltic rocks are within-plate, and transitional between alkaline and tholeiitic compositions but show a relative depletion of Nb. Their isotopic characteristics contrast with older basalts in that they have negative ƐNd(t) values, high initial 87Sr/86Sr ratios and older TDM model ages (1050 to 1300 Ma). Late Paleozoic mantle-derived potassic-ultrapotassic magmas occurring as lamprophyre dikes and small volume high Mg K intrusions in N-MB, S-STB and TBB have similar isotopic characteristics suggesting that all these Late Paleozoic rocks were derived from an old CLM contaminated by fluids or silicic melts derived from subducted Precambrian crustal material or alternatively, the region was underthrust by Gondwanan mantle during the Variscan collision. Cenozoic volcanic rocks in the Bohemian Massif are local representatives of the regionally extensive Cenozoic European volcanic province. The rocks are within-plate, alkaline basalts enriched in LREE and Nb and were derived from garnet peridotite mantle. They are significantly more juvenile than Carboniferous-Early Permian mafic rocks, with higher ƐNd(t) values (typically +3 to +5), lower initial 87Sr/86Sr ratios and significantly younger TDM age. These data suggest significant input from upwelling asthenospheric mantle, implying that at least a part of the Permian CLM mantle was re-fertilized.

The Triassic volcanic rocks of Wrangellia erupted at an equatorial to tropical latitude that was within 3000 km of western North America. The mafic and ultramafic volcanic rocks are compositionally and isotopically similar to those of oceanic plateaux that were generated from a Pacific mantle plume-type source. The thermal conditions, estimated from the primitive rocks, indicate that it was a high temperature regime (TP > 1550 °C) consistent with elevated temperatures expected for a mantle plume. The only active hotspot currently located near the equator of the eastern Pacific Ocean that was active during the Mesozoic and produced ultramafic volcanic rocks is the Galápagos hotspot. The calculated mantle potential temperatures, trace elemental ratios, and Sr–Nd–Pb isotopes of the Wrangellia volcanic rocks are within the range of those from the Caribbean Plateau and Galápagos Islands, and collectively have similar internal variability as the Hawaii-Emperor island chain. The paleogeographic constraints, thermal estimates, and geochemistry suggests that it is possible that the Galápagos hotspot generated the volcanic rocks of Wrangellia and the Caribbean plateau or, more broadly, that the eastern Pacific (Panthalassa) Ocean was a unique region where anomalously high thermal conditions either periodically or continually existed from ~ 230 Ma to the present day.

Middle to Upper Ordovician volcanic rocks in the Arisaig area of Nova Scotia, Canada, constitute the only known record of volcanism in West Avalonia during that interval. Hence, they have been extensively studied to test paleocontinental reconstructions that consistently show Avalonia as a drifting microcontinent during that period. Identification of volcanic rocks with an intermediate composition (the new Seaspray Cove Formation) between upper Darriwilian bimodal volcanic rocks of the Dunn Point Formation and Sandbian felsic pyroclastic rocks of the McGillivray Brook Formation has led to a reevaluation of magmatic relationships in the Ordovician volcanic suite at Arisaig. Although part of the same volcanic construction, the three formations are separated by significant time-gaps and are shown to belong to three distinct magmatic subsystems. The tectonostratigraphic context and trace element contents of the Dunn Point Formation basalts suggest that they were produced by the high-degree partial melting of an E-MORB type source in a back-arc extensional setting, whereas trace element contents in intermediate rocks of the Seaspray Cove Formation suggest that they were produced by the low-degree partial melting of a subduction-enriched source in an arc setting. The two formations are separated by a long interval of volcanic quiescence and deep weathering, during which time the back-arc region evolved from extension to shortening and was eventually onlapped by arc volcanic rocks. Based on limited field constraints, paleomagnetic and paleontological data, this progradation of arc onto back-arc volcanic rocks occurred from the north, where an increasingly young Iapetan oceanic plate was being subducted at an increasingly shallow angle. Partial subduction of the Iapetan oceanic ridge is thought to have subsequently generated slab window magmatism, thus marking the last pulse of subduction-related volcanism in both East and West Avalonia.

Rocks from the Blair River inlier of Northern Cape Breton Island (Nova Scotia, Canada) have been correlated with either the Grenville basement of eastern Laurentia or the accreted Avalon terrane. Additional zircon U-Pb dates of spatially associated anorthositic dykes (425.1±2.2 Ma) and a metagabbro (423.8±2.5 Ma) from the Fox Back Ridge intrusion of the Blair River inlier reveal Late Silurian emplacement ages. Their contemporaneity suggests that they may be members of a larger intrusive complex. The anorthositic rocks have high Eu/Eu∗ values (>2.5), and bulk compositions are similar to the mineral compositions of labradorite (An50-70) and andesine (An30-50). The metagabbro is compositionally similar to alkali basalt and does not seem to have been affected by crustal contamination (Nb/U>24; Th/NbPM≤1.1) although it was metamorphosed. The high Tb/YbN (1.8-1.9) ratios suggest that the parental magma of the metagabbro was derived from a garnet-bearing peridotite. Fractional crystallization and mass balance calculations indicate that the anorthositic rocks can be derived by mineral accumulation from a mafic parental magma similar in composition to the metagabbro of this study. The Late Silurian ages suggest that the rocks were emplaced into the Avalon terrane after the closure of the Iapetus Ocean but before Early Devonian (415-410 Ma) sinistral transpression.

Ordovician megacrystic granitoids intrude the Neoproterozoic-Paleozoic Acatlan Complex of southern Mexico, which consists of rift-passive margin, clastic rocks and rift-related igneous rocks. Most of the Neoproterozoic-Ordovician rocks were folded at greenschist-amphibolite facies metamorphism, however some rocks underwent polyphase deformation, underthrusting under high-pressure metamorphism followed by extrusion during the Late Devonian-Carboniferous (365–330 Ma). Most of our new Nd isotopic data for Ordovician granitoids have ƐNd(T) ranging between −4.5 and −7.2 with TDM ages of 1.3–1.6 Ga that are similar to most published data from such Ordovician plutons: ƐNd(T) of −0.2 to −6.6 and TDM ages of 1.2–1.8 Ga. These values are comparable to the range shown by the Oaxacan Complex, which confirms the conclusion reached using zircon U-Pb isotopic data indicating an Oaxacan source. One exception is the Ordovician Teticic granitoid that has ƐNd(T) of −4.8 to −5.8 and TDM ages of 2162–2946 Ma. The combination of ca. 1.5 and 2–3 Ga TDM ages is also recorded in northwestern Amazonia, which is consistent with previous correlations of Oaxaquia with Amazonia. The transfer of Oaxaquia from Amazonia to southern Laurentia occurred during the amalgamation and breakup of Pangea, where the suture was roughly parallel to their relative motion: this resulted in dextral strike-slip faulting generally attributed to the Mojave-Sonora megashear.

The Late Devonian Corner Bay pluton is a part of the Sitkoh alkaline plutonic complex on the Chichagof Island, southeastern Alaska, which is located in the Alexander terrane of the North American Cordillera. The intrusion, about 12 km long and 4 km wide, is composed predominantly of amphibole-biotite-bearing syenitic/ monzonitic rocks with minor alkaline gabbros and has a U-Pb zircon age of ~367 Ma interpreted as the crystallization age of the pluton. The rocks are metaluminous and have silica contents ranging from 47 to 63 wt% with a silica gap at 52–58 wt% and have high contents of alkalis, typical of shoshonitic rocks. They are rich in large-ion-lithophile elements (including Rb, Sr, Ba, U) and their isotopic composition is characterized by relatively uniform ƐNd(t) values (+3.4 to +3.7), Neoproterozoic Nd depleted mantle model ages (650–700 Ma) and initial 87Sr/86Sr ratios (~0.704). The rocks were generated by fractional crystallization of alkali basaltic magma formed by partial melting of an amphibole-phlogopite-bearing peridotite of the sub-arc lithospheric mantle. The mantle-source underwent a Neoproterozoic metasomatic enrichment event and does not show a contribution from juvenile Devonian mantle indicating that rifting associated with the magma emplacement was of limited extent as it did not replace the old lithospheric mantle. Several geothermobarometers document emplacement of the parental magma at mid-crustal levels (8.5–10 kb) and polybaric crystallization from ~1200 °C to solidus. Crystallization of amphibole took place at ~880-980 °C and 2.4–5.5 kb. Subsequently the rocks underwent recrystallization (feldspars: 170 Ma – 40Ar/39Ar age) and re-equilibration (biotite: 129 Ma (40Ar/39Ar) at ~600-650 °C and 2 kb). Zircon, rutile and apatite saturation temperatures provide consistent results (~800 °C). The intrusion is a part of a metallogenic province of rare metals, which stretched across the islands in the southeastern most part of Alaska.

The compositionally bimodal volcanic rocks of the Eocene–Miocene Masset Formation from Queen Charlotte basin, Haida Gwaii, British Columbia, Canada, underlie an area greater than 5000 km 2 where their exposed sections are up to 1.6 km thick. The suite of mafic and felsic rocks (dacites and rhyolites) that erupted closely spaced in time, in both submarine and subaerial conditions, was associated with significant crustal extension and thin continental crust (~ 19–24 km thick), with volcanism persisting for ~ 35 Ma (from 46 to 11 Ma). Predominant mafic types (mafic:felsic ~ 2:1) are moderately enriched mid-ocean-ridge-like basalts that were derived by a partial melting of a heterogeneous spinel peridotite source. Felsic rocks are plagioclase-phyric, two pyroxene-bearing, mainly peraluminous types which have Nd, Pb and Sr isotopic compositions overlapping those of basalts including high positive Ɛ Nd(t) values (up to >+6). The chondrite-normalized REE patterns show light REE enrichment but flat heavy REE along with a variable negative Eu anomaly. Mineralogy, major and trace elements, Nd–Sr–Pb isotopic data and model calculations using MELTS are consistent with a derivation of felsic rocks from the basalts by fractional crystallization. The intercalation of basaltic and felsic rocks suggests the existence of separate, simultaneously active plumbing and feeder systems and relatively stable magma chamber(s) to generate large volumes of differentiated felsic magmas by fractional crystallization. The Masset rocks provide an example for the generation of felsic magmas of bimodal volcanic suites during rifting along a thinned continental margin.

One of the defining characteristics of the basaltic rocks from the Early Jurassic Eastern North America (ENA) sub-province of the Central Atlantic Magmatic Province (CAMP) is the systematic compositional variation from South to North. Moreover, the tectono-thermal regime of the CAMP is debated as it demonstrates geological and structural characteristics (size, radial dyke pattern) that are commonly associated with mantle plume-derived mafic continental large igneous provinces but is considered to be unrelated to a plume. Mantle potential temperature (T P) estimates of the northern-most CAMP flood basalts (North Mountain basalt, Fundy Basin) indicate that they were likely produced under a thermal regime (T P ≈ 1450 °C) that is closer to ambient mantle (T P ≈ 1400 °C) conditions and are indistinguishable from other regions of the ENA sub-province (T Psouth = 1320–1490 °C, T Pnorth = 1390–1480 °C). The regional mantle potential temperatures are consistent along the 3000-km-long ENA sub-province suggesting that the CAMP was unlikely to be generated by a mantle plume. Furthermore, the mantle potential temperature calculation using the rocks from the Northern Appalachians favors an Fe-rich mantle (FeOt = 8.6 wt %) source, whereas the rocks from the South Appalachians favor a less Fe-rich (FeOt = 8.3 wt %) source. The results indicate that the spatial-compositional variation of the ENA basaltic rocks is likely related to differing amounts of melting of mantle sources that reflect the uniqueness of their regional accreted terranes (Carolinia and West Avalonia) and their post-accretion, pre-rift structural histories.

The Ediacaran–Ordovician Meguma Supergroup was thrust over Avalonia basement prior to the intrusion of post-Acadian, ca. 370 Ma, S-type granitic batholiths. This has led to two main hypotheses regarding the original location of the Meguma terrane, a continental rise prism bordering either NW Africa or Avalonia. On the other hand, the pre-Acadian, ca. 440 Ma Brenton pluton has yielded the following U/Pb LA-ICP-MS zircon data: (1) 448 ± 3 Ma population peak inferred to be the intrusive age and (2) ca. 550 and 700 Ma inherited ages common to both Avalonia and NW Africa. In contrast, Hf isotopic analyses of zircon yielded model ages ranging from 814 to 1127 Ma with most between 940 and 1040 Ma: such ages are typical of Avalonia and not NW Africa. The ages of the inherited zircons found within the Brenton pluton suggest that it was probably derived by partial melting of sub-Meguma, mid-crustal Avalonian rocks, upon which the Meguma Supergroup was deposited. Although Avalonia is commonly included in the peri-Gondwanan terranes off NW Africa or Amazonia, paleomagnetic data, faunal provinciality, and Hf data suggest that, during the Ediacaran-Early Cambrian, it was an island chain lying near the tropics (ca. 20–30 °S) and was possibly a continuation of the Bolshezemel volcanic arc accreted to northern Baltica during the Ediacaran Timanide orogenesis. This is consistent with the similar derital zircon population in the Ediacaran–Cambrian Meguma Supergroup and the Dividal Group in northeastern Baltica.

The Acadian Orogeny of the Northern Appalachians was caused by accretion of the peri-Gondwanan terranes Avalonia and Meguma to the eastern margin of Laurentia during the Devonian. The lithotectonic relationship between Avalonia and Meguma prior to accretion is uncertain. Radioisotopic dating of detrital zircons from metasedimentary granulite xenoliths from the structural basement to the Meguma terrane indicates that Avalonia and Meguma were proximal and likely contiguous as they transited the Rheic Ocean. The zircon ages range from the Cryogenian to Late Silurian with a minor Paleoproterozoic peak. Mesoproterozoic zircons are also identified and, coupled with the Ordovician to Silurian zircons, distinguish the rocks from those of the Meguma terrane. Furthermore, three distinct metamorphic events are identified at 399.0 ± 2.1 Ma, 376.9 ± 1.6 Ma, and 353.8 ± 3.3 Ma. We conclude that the granulite facies metamorphism experienced by the metasedimentary rocks occurred 10 to 20 million years after deposition of their protoliths during the initial stages of the Acadian Orogeny whereas the younger events are related to syn- and post-collisional episodes. The implication is that Avalonia and the Meguma terrane jointly transited from Gondwana.

Understanding the formation and evolution of Precambrian greenstone belts is hampered by gaps in the rock record and the uncertainty of the tectonic regime that was operating at the time. Thus identifying a modern analogue of a Precambrian greenstone belt can be problematic. In this paper we present geological, geochemical and petrological evidence outlining the case for Haida Gwaii (British Columbia, Canada) as a modern example of a greenstone belt. Haida Gwaii is comprised of two rift-related volcano-sedimentary sequences. The older (Early Triassic) Karmutsen volcanic sequence consists of subaqueous ultramafic-mafic volcanic rocks that are capped by marine carbonate and siliciclastic rocks. The younger (Paleogene) Masset bimodal volcanic sequence consists of tholeiitic and calc-alkaline basalt along with calc-alkaline silicic volcanic and intrusive rocks that are capped by epiclastic sandstones. The Karmutsen and Masset volcanic rocks have indistinguishable Sr-Nd-Pb isotopes demonstrating they were derived from a similar mantle source. Some of the Masset calc-alkaline rocks are compositionally similar to magnesian andesites (SiO2 = 56–64 wt%; Mg# = 0.50–0.64) that are typical of subduction-related Archean greenstone belts. We show that the calc-alkaline signature observed in the bimodal sequence of the Masset Formation is likely due to fractional crystallization of a tholeiitic parental magma under relatively oxidizing (ΔFMQ + 0.7) conditions indicating that a calc-alkaline signature is not prima facie evidence of a subduction setting. Given the geological and geochemical evidence, Haida Gwaii represents one of the best analogues of a modern subduction-unrelated Archean greenstone belt.

Upper Paleozoic (~ 365 Ma) mafic and intermediate volcanic rocks of the Piskahegan Group constitute a subordinate part of the Mount Pleasant caldera, which is associated with a significant polymetallic deposit (tungsten–molybdenum–bismuth zones ~ 33 Mt ore with 0.21% W, 0.1% Mo and 0.08% Bi and tin–indium zones ~ 4.8 Mt with 0.82% Sn and 129 g/t In) in southwestern New Brunswick (Canada). The epicontinental caldera complex formed during the opening of the late Paleozoic Maritimes Basin in the northern Appalachians. The mafic and intermediate rocks make up two compositionally distinct associations. The first association includes evolved rift-related continental tholeiitic basalts, and the second association comprises calc-alkaline andesites, although both associations were emplaced penecontemporaneously. The basalts have low Mg# ~ 0.34–0.40, smooth chondrite-normalized REE patterns with (La/Yb)n ~ 5–6, primitive mantle-normalized trace element patterns without noticeable negative Nb–Ta anomalies, and their ɛNd(T) ranges from + 2.5 to + 2.2. The basalts were generated by partial melting of a transition zone between spinel and garnet mantle peridotite at a depth of ~ 70–90 km. The calc-alkaline andesites of the second association have chondrite-normalized REE patterns that are more fractionated, with (La/Yb)n ~ 7–8.5, but without significant negative Eu anomalies. Compared to the basaltic rocks, they have lower ɛNd(T) values, ranging from + 0.5 to + 1.9, and their mantle-normalized trace element plots show negative Nb–Ta anomalies. The ɛNd(T) values display negative correlations with indicators of crustal contamination, such as Th/La, Th/Nb and SiO2. The andesitic rocks are interpreted to have formed by assimilation–fractional crystallization processes, which resulted in the contamination of a precursor basaltic magma with crustal material. The parent basaltic magma for both suites underwent a different evolution. The tholeiitic basalts experienced shallow-seated fractional crystallization and evolved along a tholeiitic trend of “early iron” enrichment (non-oxic conditions). The contaminated magma of the second association followed a calc-alkaline fractionation trend of “no iron” enrichment (oxidizing conditions) characterized by a high PH2O and PO2 environment at the mid-crust levels. The Piskahegan Group, which is associated with an important polymetallic mineral deposit, differs from the numerous non-mineralized rift-related volcanic suites of the regional Upper Devonian to Lower Carboniferous successions in the Maritimes Basin by the presence of a significant amount of coeval calc-alkaline andesite, which may be an indicator of potential mineralization.

Upper Silurian to Lower Devonian volcanic rocks of the Gander Zone, from the northern mainland Appalachians of northern New Brunswick, occur in the Chaleur Bay Synclinorium which forms the southeastern part of the Middle Paleozoic Matapedia cover sequence. These rocks, which are parts of shallow marine to subaerial sequences (Dalhousie, Dickie Cove and Tobique groups), were erupted in a continental rift environment between ca. 422 and 407 Ma. The volcanic rocks are mostly bimodal with mafic types corresponding predominantly to continental tholeiites inferred to be generated by partial melting of subcontinental lithospheric mantle (SCLM). The mafic rocks of all three groups have age-corrected ɛNd values ranging from + 3.4 to + 5.3, and depleted model mantle ages ranging from 0.65 to 0.95 Ma that are interpreted to represent mantle enrichment ages associated with ancient Neoproterozoic subduction. This Neoproterozoic SCLM shows no contributions from juvenile Silurian mantle, suggesting that rifting was of limited extent and did not result in the replacement of the old SCLM by upwelling juvenile asthenosphere beneath the rift. These Nd isotopic data do not support the generation of the volcanic rocks by slab break-off, which would have likely introduced a juvenile asthenospheric mantle source for some of the Silurian–Devonian basaltic rocks. The ranges in ɛNd values and depleted mantle ages in northern New Brunswick are similar to those recorded in penecontemporaneous mafic lavas in Avalonia suggesting that the Neoproterozoic SCLM was common to both Avalonia and Ganderia.

The Neoproterozoic–Paleozoic Acatlan Complex and the Mesozoic Ayu Complex of southern Mexico consist of clastic rocks and rift-related igneous rocks inferred to have originated along a rifted passive margin in southwestern Mexico, either in an inactive (Neoproterozoic–Ordovician) or an active (Devonian–Carboniferous and Triassic–Early Jurassic) tectonic setting. The latter formed on the inner margin of a backarc basin. These passive margin rocks were partly underthrust beneath the Acatlan Complex, into which they were subsequently extruded: extrusion was synchronous with the backarc basin development. Thus, (i) the Neoproterozoic–Ordovician rocks underwent underthrusting, high-pressure metamorphism, and extrusion during the Late Devonian–Carboniferous (365–330 Ma); (ii) the Carboniferous rocks underwent underthrusting, amphibolite facies metamorphism, and extrusion during the Permian and Triassic; and (iii) the Triassic–Lower Jurassic rocks underwent underthrusting, amphibolite facies metamorphism, and extrusion during the Jurassic. Nd isotopic data from tholeiitic mafic rocks on either side of the HP extrusion zone reveal that both were underlain by similar peri-Rodinian subcontinental lithospheric mantle in the Neoproterozoic–Ordovician, which was supplemented in the Devonian–Carboniferous and Triassic–Early Jurassic by a juvenile depleted mantle source. The alternation of underthrusting and backarc rifting accompanied by extrusion may be related to flattening and steepening of the Beniof zone, respectively.

Post-collisional volcanic rocks associated with the Variscan orogeny are preserved in relicts of Late-Paleozoic volcano-sedimentary basins in the Bohemian Massif (central Europe). They include both the Late Carboniferous calc-alkaline and the Early Permian tholeiitic suites, which were emplaced in an extensional intracontinental setting. Both suites were derived from an enriched continental lithospheric mantle and underwent early fractionation of mafic silicate minerals. A second period of fractionation occurred within the crust. Modeling indicates that the calc-alkaline rocks fractionated at a greater depth (0.55 GPa), under oxidizing (ΔFMQ +1) and hydrous conditions whereas the tholeiitic rocks fractionated at shallow depth (0.1 GPa), under moderately oxidizing (ΔFMQ +0.7) and drier conditions. During the Variscan orogeny, the lithospheric mantle under the Bohemian Massif underwent wholesale modification, where the mantle was contaminated by subducted Precambrian sediment admixture. The Carboniferous-Permian mantle-derived rocks typically have negative ɛNd(t) and Precambrian depleted mantle model ages (TDM) compared to the equivalent Early Paleozoic rocks of the Bohemian Massif, which have positive ɛNd(t) and younger TDM. Subsequently, prior to the Cenozoic, the lithospheric mantle under the whole Bohemian Massif was rejuvenated as the Cenozoic mantle-derived rocks have positive ɛNd(t) and young TDM.

The Late Devonian South Mountain Batholith (SMB) of Nova Scotia is the largest batholith of the northern Appalachians. The peraluminous granitic rocks range from biotite granodiorite to leucogranite. Samples collected from a drill core of the Scrag Lake granodioritic pluton of the western SMB are chemically homogeneous from the surface to a depth of ~ 1425 m. The homogeneous composition implies that the granodiorite was derived from a relatively homogeneous source and that country rock assimilation was not an important source for the parental magma. Equilibrium partial melt modeling of underlying sub-Meguma granulite rocks indicates that they are the primary source rocks of the granodiorites. We suggest that mantle-derived magmas intruded the lower crust and induce large-scale melting of the granulite basement rocks to produce the granodiorites. Fractional crystallization of the granodiorites plus assimilation of Meguma Supergroup metasediments likely produces the silica-rich rocks of the SMB. The cause of mantle melting is uncertain however it may be related to the transitioning of the northern Appalachians from a position above the deep mantle Pacific large low shear velocity province (LLSVP) to a higher shear velocity region of the mantle.

Understanding the development of island arcs that accreted to the North American craton is critical to deciphering the complex geological history of the Canadian Cordillera. In the case of the Hazelton arc (part of the Stikine terrane, or Stikinia) in northwestern British Columbia, understanding arc evolution also bears on the formation of spatially associated porphyry Cu–Au, epithermal, and volcanogenic massive sulfide deposits. The Hazelton Group is a regionally extensive, long-lived, and exceptionally thick Upper Triassic to Middle Jurassic volcano-sedimentary succession considered to record a successor arc that was built upon the Paleozoic and Triassic Stikine and Stuhini arcs. In central Stikinia, near Terrace, British Columbia, the lower Hazelton Group (Telkwa Formation) comprises three volcanic-intrusive complexes (Mt. Henderson, Mt. O’Brien, and Kitselas) that, at their thickest, constitute almost 16 km of volcanic stratigraphy. Basal Telkwa Formation conglomerates and volcanic rocks were deposited unconformably on Triassic and Paleozoic arc-related basement. New U–Pb zircon ages indicate that volcanism initiated by ca. 204 Ma (latest Triassic). Detrital zircon populations from the basal conglomerate contain abundant 205–233 Ma zircons, derived from regional unroofing of older Triassic intrusions. Eleven kilometres higher in the section, ca. 194 Ma, rhyolites show that arc construction continued for >10 million years. Strata of the Nilkitkwa Formation (upper Hazelton Group) with a U–Pb zircon age of 178.90 ± 0.28 Ma represent waning island-arc volcanism. Telkwa Formation volcanic rocks have bimodal silica concentrations ranging from 48.1 to 62.8 wt.% and 72.3 to 79.0 wt.% and display characteristics of subduction-related magmatism (i.e., calc-alkaline differentiation with low Nb and Ti and high Th concentrations). Mafic to intermediate rocks form a differentiated suite that ranges from high-Al basalt to medium- to high-K andesite. They were derived from hydrous melting of isotopically juvenile spinel lherzolite in the mantle wedge and from subsequent fractional crystallization. Compared to basalts and andesites (εNd = +5 to +5.5), rhyolites have higher positive εNd values (+5.9 to +6.0) and overlapping incompatible element concentrations, indicating that they are not part of the same differentiation suite. Rather, the rhyolites formed from anatexis of arc crust, probably caused by magmatic underplating of the crust. This study documents a temporal and spatial co-occurrence of Hazelton Group volcanic rocks with a belt of economic Cu–Au porphyry deposits (ca. 205–195 Ma) throughout northwestern Stikinia. The coeval relationship is attributed to crustal underplating and intra-arc extension associated with slab rollback during renewed or reconfigured subduction beneath Stikinia, following the demise of the Stuhini arc in the Late Norian.

The Jurassic Bokan Mountain complex (BMC), composed of arfvedsonite and/or aegirine-bearing peralkaline A-type granitic rocks, is a circular body about 3 km in diameter located in southeastern Alaska. Like many other highly fractionated granitic bodies, the BMC granites were affected by late magmatic or post-magmatic processes, which, however, did not modify the contents of major elements. The granitic rocks are distinctly enriched in high-field-strength elements (HFSEs), rare earth elements (REEs), Y, Th, and U but depleted in Ba, Sr, and Eu and have high positive ɛNd(T) values. Unlike the variations in the major elements, Sr and Ba, which can be accounted by fractional crystallization, the abundance of REE, Y, HFSE, U, and Th (the elements which are hosted in accessory phases) were modified by F-rich hydrothermal fluids. The BMC hosts significant rare metal mineralization related to the late-stage crystallization history of the complex involving late magmatic and/or post-magmatic fluids. The min...

Volcanic rocks of the Jurassic Iskut River Formation (IRF) in northwestern British Columbia (Canada) host several volcanogenic massive sulfide (VMS) deposits, including the exceptionally high-grade Eskay Creek Ag–Au–Cu–Pb–Zn deposit. The IRF comprises voluminous pillow basalt (>90%), minor rhyolite, and sedimentary rock of late Early to early Middle Jurassic age, filling a series of sub-basins along a 300 by 50 km north-trending belt. Two geochemically distinct types of tholeiitic basalts interfinger; both resemble back-arc basin basalts formed from the melting of asthenospheric and sub-arc mantle sources. Group 2 basalts are more enriched in light rare-earth elements, Ba, K, Sr, Th, and U, and have lower positive εNd values than group 1 basalts (+3.2 to +6.3 versus +6.9 to +8.4, respectively). The compositional differences between group 1 and group 2 basalts are interpreted to result from crustal contamination in group 2. Group 1 basalts are most common in the southern part of the IRF belt where they are closely associated with the Eskay Creek, Bonanza, and Hidden Creek (Anyox) VMS deposits. Group 2 basalts are most abundant in the northern half of the belt and are not associated with exploited mineral deposits. The lack of crustal contamination in group 1 basalts indicates that they formed from rapidly ascending magma in an advanced rift setting and were associated with high heat flow that drove hydrothermal circulation. Group 1 and group 2 basalts are reliably discriminated by Ta/Th 2.5 in the latter. This geochemical criteria can therefore be used as an exploration tool to identify VMS permissive sub-basins and (or) stratigraphy in the IRF.

article i nfo The Early Permian Panjal Traps of northern India represent a significant eruption of volcanic rocks which occurred during the opening of the Neotethys Ocean. Basaltic, basaltic-andesites, dacitic and rhyolitic rocks collected from Guryal Ravine and Pahalgam show evidence for subaerial and subaqueous eruptions indicating that they are contemporaneous with the formation of a shallow marine basin. The major and trace element geochemistry of the basalts is consistent with a within-plate setting and there are basalts which have high-Ti (TiO2 N 2.0 wt.%) and low-Ti (TiO2 b 1.8 wt.%) compositions. The 'high-Ti' basalts are similar to OIB whereas the 'low-Ti' basalts are similar to continental tholeiites. The identification of 'high- and low-Ti' basalts within the Panjal Traps is analogous to other large igneous provinces (e.g. Karoo, Deccan, Parana, Emeishan). The Sr-Nd isotopic values (eNd(T) = −5.3 to +1.3; ISr = 0.70432 to 0.71168) of both types of basalts overlap indicating that the rocks may have originated from the same ancient subcontinental lithospheric (i.e. EMII-like) mantle source (TDM = ~2000 Ma). The two groups of basalts can be modeled by using a primitive mantle source and different degrees of partial melting where the high-Tirocksare producedby ~1% partial melting of a spinel peridotite source where- as the low-Ti rocks are produced by ~8% partial melting. Trace elemental and isotope modeling indicatesthat some of the basalts assimilated ≤ 10% crustal material. In contrast, the basaltic-andesites are likely formed by mixing be- tween basaltic magmas and crustal melts which produced rocks with higher SiO2 (~55 wt.%) content and enriched isotopic signatures (eNd(T) = −6.1; ISr = 0.70992). The Panjal Trap volcanism was likely due to partial melting of the SCLM within a passive extensional setting related to the rifting of Cimmeria from Gondwana. Contemporaneous volcanic and plutonic granitic rocks throughout the Himalaya are probably not petrogenetically related but are likely part of the same regional tectonic regime.

The Early Jurassic (ca. 177 Ma) Bokan Mountain granitic complex, located on southern Prince of Wales Island, southernmost Alaska, cross-cuts Paleozoic igneous and metasedimentary rocks of the Alexander terrane of the North American Cordillera and was emplaced during a rifting event. The complex is a circular body (~ 3 km in diameter) of peralkaline granitic composition that has a core of arfvedsonite granite surrounded by aegirine granite. All the rock-forming minerals typically record a two-stage growth history and aegirine and arfvedsonite were the last major phases to crystallize from the magma. The Bokan granites and related dikes have SiO2 from 72 to 78 wt.%, high iron (FeO (tot) ~ 3–4.5 wt.%) and alkali (8–10 wt.%) concentrations with high FeO(tot)/(FeO(tot) + MgO) ratios (typically > 0.95) and the molar Al2O3/(Na2O + K2O) ratio

this volume is an outcome of a symposium entitled “amalgamation and Breakup of Pangea in the americas” held in Duncan’s honor in conjunction with the Cordilleran Section Meeting of the Geological Society of america on March 29–31, 2012 in Queretaro, Mexico. the symposium was co-sponsored by IGCP 597 (Origin and Evolution of Pangea) and IGCP 574 (Bending and Bent Orogens, and Continental Ribbons) and featured 16 contributions from friends, colleagues, current and former students, and one from his son, Fraser. Many of the papers presented at Queretaro are represented in this volume, supplemented by others from friends and colleagues who could not make that meeting but wished to contribute to this commemorative volume (Fig. 1). Duncan Keppie’s CV reveals a record of sustained excellence and productivity for nearly 40 years with the publication of more than 260 papers in refereed journals, over 40 book chapters, and a host of government publications, maps and memoirs, and the editorship of 7 books or journal special issues. But those of us who have known Duncan Keppie as a friend and scientific colleague know that these numbers only begin to tell the story. Duncan has been an excellent motivator and innovator and has an infectious enthusiasm for the earth sciences. His papers are not just a documentation of the evolution of various orogenic belts, although that in itself is a significant accomplishment. they also document fundamental processes, the principles, and practices of which have been applied to the structural evolution of orogenic belts throughout the world and continue to provide a theoretical framework for the understanding of the evolution and growth of continental crust. One of his many gifts, and a major strength in his research, is an unparalleled ability to grasp the essence of concepts and synthesize geological data from a very wide range of fields and from seemingly disparate pieces Duncan keppie: an appreciation and a celebration

The convergence between the Indian plate and the southern margin of the Eurasian continent created an active continental margin from Late Jurassic until about 40 Ma ago, which then evolved to form the Himalaya and the Tibetan Plateau during the continental collision stage. Post-collisional magmatism in southern Tibet, north of the Yarlung Zangbo Suture Zone (YZSZ) has been active since 45 Ma and is related to normal faulting and extensional tectonism. To date no such magmatism was reported within the YZSZ itself. This paper reports on the discovery of Miocene shoshonites within the YZSZ. They are significant because the magma traveled, at least in part, through oceanic crust, thus limiting interaction with the continental crust to the mid-crustal level and which affected the post-collisional magmatic rocks occurring in the northern part of the subduction system. In addition, xenoliths and xenocrysts of crustal origin in these rocks constrain the nature of metamorphic rocks underlying the YZSZ at mid-crustal level. The geochemical signatures of the shoshonitic rocks, including Nd and Sr isotope systematics, indicate derivation from a garnet-bearing middle continental crustal source. Crustal imprint complicates modeling of the petrogenetic processes which occurred prior to mid-crustal ponding of the magma which took place between 11 and 17 Ma at depths of 40 to 50 km. The significant role of crustal contamination raises serious concerns about models proposed for similar magmatic activity elsewhere in the Himalaya and the Tibetan Plateau.

The Jurassic Dashibalbar granitoid pluton (300 km 2 ) crops out in the Triassic North-Gobi rift of central Mongolia, just south of the 230 to 195 Ma Khentei batholith. The granitoids are shallow-seated dominantly, amphibole-bearing alkali feldspar granite that contain quartz-syenite/syenite enclaves. They are all composed of megacrystic mesoperthite, quartz, Ca-Na amphibole altered to biotite and rarely with pyroxene cores, magnetite and ilmenite. The pluton yielded a concordant U-Pb zircon age of 186 1 Ma, which is similar to a published 189 3M a 40 Ar/ 39 Ar amphibole age, and indicates rapid cooling through ca. 550 °C. This age is ca. 10 my younger than the 196 4 Ma age of the bimodal volcanic complex intruded by the pluton. The volcanic complex is composed of augite-phyric transitional basalt and rhyolite/ comendite. Both basalts and rhyolites/comendites are evolved within-plate varieties with positive Nd(t) ( 2.5) values. The granitoids are evolved alkaline, A-type granites and quartz-syenites/syenites that are enriched in light REE's, but show a distinct depletion in Eu, Sr and Ba, indicative of feldspar fractionation. The data are consistent with derivation of the granites from the syenites by an assimilation- fractional crystallization process involving a silicic crustal contaminant. The granitic rocks have Nd(t) values of 0.8 to 1.2, which are slightly lower than Nd(t) values 1.3 to 1.6 in the syenites, although both have similar TDM model ages (800-970 Ma). The 800 Ma model ages of the basalt and rhyolite/comendite are comparable to those of the intrusion and enclaves. The compositions of all these rocks, including Nd(t) and T DM , are within the range of A-type granites and volcanic complexes of the Early Mesozoic Mongolian-Transbaikalian igneous province. The results suggest deriva- tion of a parent magma of the granitoids and felsic volcanic rocks from underplated, enriched, Neoproterozoic mantle-derived basaltic rocks in the lower crust, whereas the Dashibalbar basalts were derived from Neoproterozoic subcontinental lithospheric mantle; Neoproterozoic megablocks crop out in adjacent parts of the Central Asian Orogenic Belt. Melting of lower crust and subcontinental lithospheric mantle implies a rising heat source. Although such a heat source is consistent with both rifting and passage over the Mongolian mantle plume, only the latter explains the west-to-east migration of the magmatism and rifting.