Abstract Dashwoods is a composite peri-Laurentian terrane in Newfoundland and forms the basement to the Early Ordovician to Silurian Notre Dame arc. The southern part of Dashwoods is characterized by paragneiss that is intruded by Early Ordovician to Late Silurian plutons and affected by polyphase Taconic to Salinic deformation and high-grade metamorphism. The crystalline basement of Dashwoods is not exposed and pre-Middle Ordovician paragneiss is investigated herein to constrain the provenance of Dashwoods. SHRIMP U–Pb zircon analysis of the paragneiss yielded metamorphic rims ranging from c. 500 to 395 Ma and abundant detrital grain cores ranging from c. 1853 to 546 Ma. The presence of abundant Tonian dates differentiates Dashwoods from the adjacent Humber Margin in Newfoundland, and Hebridean and Grampian terranes in the British Isles. The detrital provenance of Dashwoods is most similar to the Baie Verte Margin in Newfoundland, and Tyrone Complex and Dalradian Supergroup in Ireland. These data suggest that Dashwoods and the Baie Verte Margin originated near the Rockall promontory and were subsequently emplaced outboard of the Humber Margin by Ordovician to Carboniferous motion along the Baie Verte–Brompton Line.

Melilitite, nephelinite, basanite, and alkali basalt, along with phonolite differentiates, form the Freemans Cove Complex (FCC) in the south-eastern extremity of Bathurst Island (Nunavut, Canada). New 40 Ar/ 39 Ar chronology indicates their emplacement between ~ 56 and ~ 54 million years ago within a localized extensional structure. Melilitites and nephelinites, along with phonolite differentiates, likely relate to the beginning and end phases of extension, whereas alkali basalts were emplaced during a main extensional episode at ~ 55 Ma. The melilitites, nephelinites, and alkali basalts show no strong evidence for significant assimilation of crust, in contrast to some phonolites. Partial melting occurred within both the garnet- and spinel-facies mantle and sampled sources with He, O, Nd, Hf, and Os isotope characteristics indicative of peridotite with two distinct components. The first, expressed in higher degree partial melts, represents a relatively depleted component (“A”; 3 He/ 4 He ~ 8 R A , ε Nd i ~ + 3 ε Hf i ~ + 7, γ Os i ~ 0). The second was an enriched component (“B” 3 He/ 4 He < 3 R A , ε Nd i < – 1 ε Hf i < + 3, γ Os i > + 70) sampled by the lowest degree partial melts and represents carbonate-metasomatized peridotite. Magmatism in the FCC shows that rifting extended from the Labrador Sea to Bathurst Island and reached a zenith at ~ 55 Ma, during the Eurekan orogeny. The incompatible trace-element abundances and isotopic signatures of FCC rocks indicate melt generation occurred at the base of relatively thin lithosphere at the margin of a thick craton, with no mantle plume influence. FCC melt compositions are distinct from other continental rift magmatic provinces worldwide, and their metasomatized mantle source was plausibly formed synchronously with emplacement of Cretaceous kimberlites. The FCC illustrates that the range of isotopic compositions preserved in continental rift magmas are likely to be dominated by temporal changes in the extent of partial melting, as well as by the timing and degree of metasomatism recorded in the underlying continental lithosphere.

The 40Ar/39Ar dating method is among the most versatile of geochronometers, having the potential to date a broad variety of K-bearing materials spanning from the time of Earth’s formation into the historical realm. Measurements using modern noble-gas mass spectrometers are now producing 40Ar/39Ar dates with analytical uncertainties of ∼0.1%, thereby providing precise time constraints for a wide range of geologic and extraterrestrial processes. Analyses of increasingly smaller subsamples have revealed age dispersion in many materials, including some minerals used as neutron fluence monitors. Accordingly, interpretive strategies are evolving to address observed dispersion in dates from a single sample. Moreover, inferring a geologically meaningful “age” from a measured “date” or set of dates is dependent on the geological problem being addressed and the salient assumptions associated with each set of data. We highlight requirements for collateral information that will better constrain the interpretation of 40Ar/39Ar data sets, including those associated with single-crystal fusion analyses, incremental heating experiments, and in situ analyses of microsampled domains. To ensure the utility and viability of published results, we emphasize previous recommendations for reporting 40Ar/39Ar data and the related essential metadata, with the amendment that data conform to evolving standards of being findable, accessible, interoperable, and reusable (FAIR) by both humans and computers. Our examples provide guidance for the presentation and interpretation of 40Ar/39Ar dates to maximize their interdisciplinary usage, reproducibility, and longevity.

Stikine terrane is one of the largest crustal blocks in the Cordillera, measuring more than 1100 km long and 250 km wide. In northern British Columbia it is well known for its large porphyry Cu-Au ±Mo-Ag deposits. Stikine terrane tapers northward, replaced by a broadening wedge of oceanic crustal rocks of the Cache Creek terrane, interpreted to have overthrust Stikine terrane reducing its exposed width to ~10 km at 60°N. This overthrust region is where Triassic-Jurassic magmatic belts with known porphyry deposits disappear, and it is mostly underlain by Triassic-Jurassic arc-derived clastic rocks of the Whitehorse trough. Overthrusting has traditionally been attributed to the northwest-trending King Salmon fault, which carries conspicuous Late Norian Sinwa Formation limestone in its hanging wall. On many terrane maps, the Sinwa Formation marks the western margin of the oceanic Cache Creek terrane. However, clast provenance, biochronology, conodont fossil fauna, and sedimentary facies carried by the King Salmon fault are inconsistent with this interpretation. Instead, these data suggest that rocks in the hangingwall of the King Salmon fault were deposited in the Triassic forearc of the Stikine terrane, isolated from the subducting Cache Creek oceanic lithosphere by an intervening trench. Thus, the King Salmon fault is not a terrane boundary, and although regionally important, it is but one of several faults that carry Sinwa Formation limestone. Complicating this simple tectonic picture are detrital zircons from one sample collected in the footwall of the King Salmon fault. They form a nearly unimodal population with a main peak at 242 Ma, an age unknown in Stikinia but common within volcanic and plutonic rocks of the Kutcho-Sitlika-Venables arc, which have historically been included in the Cache Creek terrane. If this provenance link is correct, it supports the Kutcho-Sitlika-Venables arc as a separate terrane, distinct from the Cache Creek, and juxtaposed with the Stikine forearc before the Bajocian (~173 Ma) juxtaposition of Cache Creek terrane. King Salmon and adjacent fault panels carry steep northeast plunging folds having southeast-dipping axial surfaces, consistent with a top to the north component of motion (or sinistral if originally steep) that may be related to a phase of deformation during latest Triassic Kutcho-Sitlika-Venables arc collision.

In southwest Yukon, the boundary between the Alexander terrane and Wrangellia corresponds with the Duke River fault. In this paper, we report on observations of the Duke River fault from four localities in southwest Yukon, and provide new constraints on (1) Permian regional metamorphism within the Alexander terrane, (2) Cretaceous ductile deformation along the Duke River fault, and (3) post-Miocene brittle deformation along the fault. Within these areas, the Duke River fault juxtaposes imbricated, pervasively foliated and folded greenschist-facies rocks of the Alexander terrane southwest of the fault against sub-greenschist-facies, less deformed rocks of Wrangellia. Multiple lines of evidence from this region indicate the Alexander terrane has been juxtaposed against Wrangellia along a southwest-dipping thrust fault. 40Ar/39Ar dates from muscovite, which grew during faulting or have been reset by motion along the Duke River fault, range from 79 to 105 Ma, suggesting that ductile movement along the fault is at least as old as Cretaceous (Albian to Cenomanian). This phase of faulting is interpreted as the local expression of Cretaceous shortening, which has been documented along the length and width of the Cordillera. Cretaceous structures along the Duke River fault are overprinted by brittle deformation that affects rocks as young as Miocene (or Pliocene?). The Duke River fault appears to be accommodating present-day transpression through uplift and reactivation of the thrust fault.

Modern processing methods were applied to 3400 line-kilometres of legacy seismic data from Sabine Peninsula of Melville Island in the Canadian Arctic Islands. Post-stack reprocessing improved the imaging, allowing new insight into the following issues: the northern extent of lower Paleozoic source rocks, extensional structures and rock types in the upper Paleozoic succession, the timing of the gentle Drake Point Anticline; and the age and extent of igneous sills. The central part of Sabine Peninsula is underlain by a half-graben containing upper Paleozoic strata. The half-graben fill is intersected by just one well, but it likely contains Upper Carboniferous to Lower Permian strata. The two largest conventional gas fields in Canada (Drake Point and Hecla) are hosted in Mesozoic strata within a gentle anticline that partially overlies the half-graben. Previously, the Drake Point Anticline was interpreted to have been formed during Eocene time. We propose that 280 m of the 430 m of structural relief on the Drake Anticline formed in response to uplift at the axis of the anticline in the Early Cretaceous, as shown by thinning of the Lower Cretaceous Christopher Formation over the Drake Anticline. The remaining 150 m of structural relief formed by differential movement between the Marryatt Point Syncline and Drake Point Anticline after the Early Cretaceous. Early Cretaceous relief on the Drake Point Anticline means it was at least partially present at the time of maximum hydrocarbon generation in the Late Cretaceous.

The Annieopsquotch accretionary tract (AAT) comprises a thrust stack of Lower to Middle Ordovician arc and backarc terranes that were accreted to the composite Laurentian margin of Iapetus during the Middle to Late Ordovician. Geological relationships suggest that the constituent terranes of the AAT initially formed outboard of the composite Laurentian margin in an extensional arc that underwent multiple rifting episodes prior to its accretion. The initiation of AAT magmatism led to the development of Tremadocian to Floian supra-subduction zone ophiolites (481 to 477 Ma) with organized ridges indicated by the presence of well-developed sheeted dyke complexes. This spreading centre propagated through a fragment of Laurentian crust and separated it from the composite Laurentian margin. This Laurentian crust fragment then formed the basement to subsequent Floian to Darriwilian AAT arc magmatism. The Floian arc (473 to 468 Ma) underwent extensive rifting indicated by organized spreading in the Lloyds River backarc basin, which was floored by juvenile backarc ophiolitic crust (472 Ma). The establishment of the Darriwilian arc (467 to 462 Ma) was in part coeval with yet another stage of rifting. Darriwilian magmatism is characterised by significant along-strike variability, ranging from continental to primitive calcalkaline arc to tholeiitic backarc-like magmatism. The diversity of Darriwilian magmatism can be attributed to fragmentation and magmatic reworking of Laurentian-derived basement along strike in the same arc undergoing dis-organized spreading. The development of the AAT is interpreted to be similar to that of the modern Izu -Bonin Mariana arc in the western Pacific.

The Baie Verte Peninsula, western Newfoundland Appalachians, preserves evidence for Early to Mid Ordovician closure of the Taconic seaway, which led to obduction of the Baie Verte oceanic tract (BVOT) ophiolites onto the Laurentian (Humber) margin and Taconic orogenesis. The scarcity of Taconic radiometric ages (and predominance of Silurian (Salinic) data) from the Humber margin rocks (down-going plate) has been problematic, calling into question the intensity and existence of Taconic collisional orogenesis. 40Ar/39Ar and in situ U–Pb geochronology was undertaken on metamorphosed units from the Laurentian basement (Mesoproterozoic East Pond Metamorphic Suite), from the ca. 560 Ma Birchy Complex forming the leading edge of the Humber margin, and from the ca. 490 Ma ophiolitic rocks of the BVOT (Advocate Complex) in order to address this question. Our results confirm evidence of Taconic metamorphism along the Humber margin and at the base of the ophiolites. Ages obtained from the structural base of the Advocate Complex (481–465 Ma) are interpreted to reflect the timing of accretion and internal thickening of the ophiolite, whereas data from the underlying Birchy Complex (467–461 Ma) record the underthrusting and exhumation of the leading edge of the Humber margin along a subduction channel, penecontemporaneously with final obduction of the BVOT. A concordant ca. 465 Ma zircon age and REE data obtained from retrogressed eclogite of the East Pond Metamorphic Suite suggest that the parautochthonous Humber margin was locally subducted to eclogite-facies conditions during the Taconic collision and partly exhumed to amphibolite-facies conditions prior to a strong Silurian (Salinic) tectonometamorphic overprint.SOMMAIRELa péninsule de Baie Verte dans les Appalaches de l’ouest de Terre-Neuve a conservé des indices de la fermeture du bras de mer taconique, qui a mené à l’obduction des ophiolites de la bande océanique de Baie Verte (BOBV) sur la marge laurentienne (Humber) et à l’orogénèse taconique. La rareté des âges radiométriques taconiques (et la prédominance des données siluriennes (saliniques)) provenant des roches de la marge de Humber (i.e. la plaque subductée) a été problématique, mettant en question l’intensité et l’existence de la collision orogénique taconique. De la géochronologie 40Ar/39Ar et U–Pb in situ a été réalisée sur des unités métamorphisés provenant du socle laurentien (la Suite Métamorphique d’East Pond d’âge Mésoprotérozoïque), du Complexe de Birchy daté à ca. 560 Ma formant la partie frontale de la marge de Humber, and des roches ophiolitiques de la BOBV (Complexe d’Advocate) datée à ca. 490 Ma afin de confronter ce questionnement. Nos résultats confirment les indices de métamorphisme taconique le long de la marge de Humber et à la base des ophiolites. Les âges obtenus à la base structurale du Complexe d’Advocate (481–465 Ma) sont interprétés comme reflétant la période d’accrétion et d’épaississement interne de l’ophiolite, tandis que les données du Complexe de Birchy sous-jacent (467–461 Ma) enregistrent le sous-charriage et l’exhumation de la partie frontale de la marge de Humber au sein d’un chenal de subduction, de façon pénécontemporaine à l’obduction finale de la BOBV. Un âge concordant de ca. 465 Ma d’un zircon et les données de terres rares provenant d’une éclogite rétromorphosée de la suite métamorphique d’East Pond suggèrent que la marge de Humber parautochtone a été localement subductée à des conditions du faciès éclogitique durant la collision taconique et partiellement exhumée à des conditions du faciès des amphibolites précédant la forte surimposition tectonométamorphique silurienne (salinique).

Late Paleozoic assembly of the Alexander-Wrangellia-Peninsular composite terrane is recorded by two phases of regional deformation, metamorphism, and magmatism within basement complexes of the Alexander (Craig and Admiralty subterranes), Wrangellia, and Peninsular terranes in the Canadian and Alaskan Cordillera. New secondary ion mass spectrometry (SIMS) and chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) zircon U-Pb ages, whole-rock major-and trace-element and Nd-Sr isotope geochemical compositions, and geological field observations of late Paleozoic igneous rocks were used to identify the precise timing and significance of this tectonism in the Saint Elias Mountains region of southwestern Yukon and eastern Alaska. Middle to Late Pennsylvanian (301-307 Ma) igneous rocks, herein assigned to the Barnard Glacier suite, were preferentially emplaced along the Wrangellia-Craig subterrane boundary and mainly comprise syenitic plutons that intrude Paleozoic country rocks with evidence of Pennsylvanian or older (D1) deformation. We propose that Barnard Glacier suite magmatism was produced by a slab breakoff event after the consumption of a narrow backarc ocean basin and early Pennsylvanian collision between the Wrangellia-Peninsular arc and Craig subterrane passive margin. Early Permian (284-291 Ma) dioritic to granodioritic rocks, herein assigned to the Donjek Glacier suite, comprise the vestiges of an extensive magmatic system within the Craig subterrane of southwestern Yukon and southeastern Alaska. The available data suggest that the Donjek Glacier suite represents part of a short-lived, Early Permian arc that initiated along the outboard margin of the Craig subterrane-Wrangellia-Peninsular block after Pennsylvanian collision and slab breakoff. At two field localities in southwestern Yukon, Paleozoic country rocks with D1 fabrics are also intruded by sills and dikes of the Donjek Glacier suite that show evidence of ca. 285 Ma regional deformation and metamorphism (D2). Field evidence for Early Permian tectonism in the Saint Elias Mountains implies direct connections with coeval deformation and metamorphism in the Admiralty subterrane, a microcontinent in the Admiralty Island region of southeastern Alaska that developed separately from the Craig subterrane prior to the Early Permian. D2 tectonism was likely related to the entry of the Admiralty subterrane margin into the Early Permian subduction zone, which resulted in collision and final amalgamation of the Alexander-Wrangellia-Peninsular composite terrane. Our tectonic scenarios require the currently accepted configuration of the Alexander terrane (composite of the Craig and Admiralty subterranes) to have only existed after the Early Permian collision between the Admiralty subterrane and the previously assembled Craig subterrane-Wrangellia-Peninsular terrane. Biogeographic and other geological data suggest that the two-part assembly of the Alexander-Wrangellia-Peninsular composite terrane took place along a convergent margin to the north of the Cordilleran pericratonic arc terranes (Yukon-Tanana, Quesnellia, and others), in between the paleo-Pacific Ocean and paleo-Arctic Ocean realms, to the northwest of the supercontinent Pangea.The assembly of the Alexander-Wrangellia-Peninsular composite terrane might have been associated with the Early to Middle Permian subduction polarity flip recognized in the Cordilleran pericratonic realm, which led to the closure of a backarc ocean basin and Late Permian arc-continent collision along the western margin of North America.

In the pre-Silurian lithotectonic units of the northern Vermont Appalachians, the timing of orogenesis and tectonometamorphism has traditionally been ascribed to the combined effects of the Middle Ordovician Taconian orogeny and Middle to Late Devonian Acadian orogeny. However, numerous geochronological studies throughout the Northern Appalachians, including neighboring southern Quebec, have obtained Silurian and Early Devonian age data that document more or less continuous tectonometamorphic activity throughout the Ordovician Devonian. The structural and metamorphic evolution of northern Vermont can be separated into three regional phases, which are characterized by distinct structures, fabrics, and metamorphic parageneses. The first phase (D-1), associated with westward emplacement of various thrust slices leading to crustal thickening and regional metamorphism, and the second phase (D-2), characterized by bivergent structures and metamorphic overprint, have both been considered to be Taconian. The third phase, the structure and fabric of which are also observed in the Silurian Devonian rocks to the east, is considered to be Acadian. We present new step-heating and spot fusion Ar-40/Ar-39 geochronological data on amphibole and fabric-forming muscovite from samples taken across the Green Mountain anticlinorium, which, coupled with published data, provide improved age constraints on tectonometamorphism of D-1 (latest Cambrian to Middle Ordovician), D-2 (Silurian Early Devonian), and D-3 (Middle Devonian) events. By comparing structural and metamorphic characteristics, and now timing, these phases are interpreted to be correlative to the tripartite tectonometamorphic evolution documented in southern Quebec, and they further exemplify the along-strike diachronism of tectonism induced by the inherited irregular geometry of the Laurentian margin.

Newfoundland Section 2010 Spring Technical Meeting February 22–23, 2010 JOHN sON GEO cEN tr E, siGNA l Hi ll, st. J OHN ’s, NEWFOUND lAND Following our long-standing tradition, and in keeping with our sense of the absurd, the 2010 Spring Technical Meeting was once again held in the depths of the Newfoundland winter in the Johnson GEO CENTRE on scenic Signal Hill in St. John’s. The meeting featured a special session intended to highlight recent resource developments and the renewed interest in the energy potential of the province, especially as it relates to onshore and offshore petroleum, but also included energy minerals and other resources. A special session featured a series of talks related to the Geological Survey of Canada’s Targeted Geoscience Initiative (TGI) program in Newfoundland. For those specifically interested in the TGI Program and the geology of central Newfoundland, a post-conference workshop (organized by Geological Survey of Canada and Geological Survey of Newfoundland and Labrador) was held to discuss outcomes and future plans. In addition, a general session included papers on an eclectic range of topics, as is normally the case at these meetings. The 2010 meeting featured the third of the “Topical Geoscience Lecture” series, co-sponsored by the Professional Engineers and Geoscientists of Newfoundland and Labrador (PEG-NL). The speaker was Dr. Maurice Tivey of the Woods Hole Oceanographic Institute, who spoke on the scientific, technical, and jurisdictional challenges in the development of seafloor mineral resources. The Newfoundland and Labrador Section of the Geological Association of Canada is pleased to have once again hosted an interesting and diverse meeting, and we are equally pleased to see the abstracts published in Atlantic Geology. Our thanks are extended to all of the speakers and the editorial staff of the journal.

Following our long-standing tradition, and in keeping with our sense of the absurd, the 2010 Spring Technical Meeting was once again held in the depths of the Newfoundland winter in the Johnson GEO CENTRE on scenic Signal Hill in St. John's.The meeting featured a special session intended to highlight recent resource developments and the renewed interest in the energy potential of the province, especially as it relates to onshore and offshore petroleum, but also included energy minerals and other resources.A special session featured a series of talks related to the Geological Survey of Canada's Targeted Geoscience Initiative (TGI) program in Newfoundland.For those specifically interested in the TGI Program and the geology of central Newfoundland, a post-conference workshop (organized by Geological Survey of Canada and Geological Survey of Newfoundland and Labrador) was held to discuss outcomes and future plans.In addition, a general session included papers on an eclectic range of topics, as is normally the case at these meetings.The 2010 meeting featured the third of the "Topical Geoscience Lecture" series, co-sponsored by the Professional Engineers and Geoscientists of Newfoundland and Labrador (PEG-NL).The speaker was Dr. Maurice Tivey of the Woods Hole Oceanographic Institute, who spoke on the scientific, technical, and jurisdictional challenges in the development of seafloor mineral resources.

Cees van Staal1, D. M. Chew2, Alex Zagorevski3, Tom Skulski3, Sebastien Castonguay4, Vicki McNicoll3, and Nancy Joyce3 1. Geological survey of Canada, 625 Robson Street, Vancouver, British Columbia V6B 5J3, Canada ¶ 2. Department of Geology, Trinity College Dublin, Dublin 2, Ireland ¶ 3. Geological Survey of Canada, 601 Booth St., Ottawa, Ontario K1A 0E8, Canada ¶ 4. Geological Survey of Canada, 490 de la Couronne, Quebec, Quebec G1K 9A9, Canada

The tectonic history of the central mobile belt in the Newfoundland Appalachians (Dunnage Zone) is paramount to understanding the distribution and tectonic setting of mineral deposits and, by extension, to determining the prospectivity of the various terranes. On a broad scale, the distinction between the coeval peri-Laurentian and peri-Gondwanan arc complexes in the Dunnage Zone has been well constrained in previous studies on the basis of stratigraphic, isotopic and structural contrasts. This eventually led to the recognition of multiple volcanic terranes developed by several accretionary episodes, with a complexity similar to that of the modern Southwest Pacific. On a finer scale, the along-strike variability within terranes has not been well constrained, as the resolution of the data was generally insufficient. Recent detailed mapping and sampling allowed recognition of two distinct, but coeval and kinematically-related Darriwilian arc sequences in the Annieopsquotch Accretionary Tract of central Newfoundland, namely the Buchans (ca. 467– 462 Ma) and Red Indian Lake (ca. 466–460 Ma) groups. The Buchans Group likely represents an incipient rift, while the Red Indian Lake Group preserves a much more advanced stage of arc rifting, locally indicated by eruption of non-arc volcanic rocks similar to modern advanced backarc systems. The majority of the VMS mineralization in the Buchans-Robert’s Arm belt occurs in the ca. 465 Ma volcanic rocks (i.e., Buchans Group and Crescent Lake formation, Roberts Arm Group). The Darriwilian volcanic rocks are locally built on sporadically exposed Early to pre-Darriwilian arc basement comprised of plutonic and volcano-sedimentary rocks (ca. 467 to 473 Ma) that also locally host important VMS mineralization. The Red Indian Lake and Buchans groups are interpreted as along-strike equivalents, formed upon distinct peri-Laurentian basement blocks. The current arrangement of these terranes is likely due to lateral juxtaposition during the middle Ordovician closure of the main tract of the Iapetus Ocean and the Early Silurian closure of the Exploits-Tetagouche back-arc basin. The recognition of significant strike-slip displacements in this and previous studies allows an improved understanding of how the Laurentian margin responded laterally both during its development and subsequent accretion of the periGondwanan terranes.

The once-contiguous North Atlantic craton (NAC) is crosscut by the Labrador Sea that opened during the Early Cenozoic after extensive Mesozoic continental rifting and removal of cratonic mantle. This large-scale structural change within the cratonic lithosphere was followed at about 150 Ma by the cessation of ultrapotassic and potassic-to-carbonatitic magma production, which had prevailed throughout much of the NAC history. At Aillik Bay, a sequence of olivine lamproites (1374.2+/-4.2 Ma, 2 sigma), aillikites/carbonatites (590-555 Ma), and nephelinites (141.6+/-1.0 Ma, 2 sigma) erupted through the southern NAC edge on the present-day Labrador Sea margin. Links between these alkaline magma types with diverse petrogeneses as a consequence of large-scale processes in the lithospheric mantle over a period of 1200 Myr are demonstrated utilizing their Sr-Nd-Hf-Pb isotope compositions.The Mesoproterozoic olivine lamproites are characterized by unradiogenic Nd (epsilon(Nd(i)) = -8.4 to -5.4), Hf (epsilon-(Hf(i)) = -11 to -7.8), and Ph (Pb-206/Pb-204((i)) = 14.2-14.8) but moderately radiogenic Sr isotope compositions (Sr-87/Sr-86((i)) = 0.7047-0.7062) fingerprinting long-term enriched cratonic mantle, which must have reached to depths of more than 150 kin at this time. In contrast, Neoproterozoic carbonate-rich aillikites and carbonatites have fairly radiogenic Nd (epsilon(Nd(i)) = 0-1-1.8), Hf (epsilon(Hf(i)) = -0.9 to +2.6), and Pb (Pb-206/Pb-204((i)) = 17.5-18.8) but unradiogenic Sr isotope compositions (Sr-87/Sr-86((i)) = 0.7033-0.7046) that point to the involvement of convective upper mantle material during melting. Simple binary mixing calculations coupled with the observation that carbonate-rich magmatism prevailed for over 30 Myr in the area imply a complex pattern of lithosphere-asthenosphere interaction at depths between similar to 180 and 140 km. The Cretaceous nephelinites have slightly unradiogenic Nd (epsilon(Nd(i)) = -4 to -1.4), moderately radiogenic initial Sr-87/Sr-86 (0.7044-0.7062), but initial epsilon(Hf) (-3.3 to +1.4) similar to the aillikites and highly radiogenic Pb (Pb-206/Pb-204((i)) = 19.1-20.2) isotope compositions. Their sodic mafic alkaline nature reflects partial melting at a higher level of the cratonic mantle tapping metasomatic components that had been introduced during the >30 Myr of Neoproteroz-oic aillikite/carbonatite magmatism.The new Ar-40/Ar-39 age and Sr-Nd-Hf-Pb isotope data, along with petrological arguments, suggest that at least 30 km of the cratonic mantle beneath the southern NAC edge had been replaced by the hotter upwelling asthenosphere between ca. 550 Ma, when a thick diamond-bearing lithosphere was present, and 150 Ma. This lithospheric thinning presumably occurred shortly prior to Cretaceous continental rifting in response to enhanced plate-tectonic stresses focused at this zone of persistent lithospheric weakness. It appears, however, that the recurrent volatile-rich alkaline magmatism and associated mantle metasomatism played an important role in destroying the structural integrity of the cratonic mantle thereby aiding the subsequent lithosphere thinning. (C) 2007 Elsevier B.V. All rights reserved.