Grenvillian Orogenesis Research Articles

Paleomagnetic Records From Pulsed Magmatism in the Southwestern Laurentia Large Igneous Province and Cardenas Basalt Support Rapid Late Mesoproterozoic Plate Motion

AbstractMafic intrusions, lava flows, and felsic plutons in southwestern Laurentia have been hypothesized to be associated with the emplacement of a late Mesoproterozoic (Stenian Period) large igneous province. Improved geochronologic data resolve distinct episodes of mafic magmatism in the region. The ca. 1,098 Ma main pulse of southwestern Laurentia large igneous province (SWLLIP) magmatism is recorded by mafic intrusions across southeastern California to central Arizona. A younger episode of volcanism resulted in eruptions that formed the ca. 1,082 Ma Cardenas Basalt, which is the uppermost unit of the Unkar Group in the Grand Canyon. With the updated geochronological constraints, we develop new paleomagnetic data from mafic sills in the SWLLIP. Overlapping poles between the Death Valley sills and rocks of similar age in the Midcontinent Rift are inconsistent with large‐scale Cenozoic vertical axis rotations in Death Valley. We also develop a new paleomagnetic pole from the ca. 1,082 Ma Cardenas Basalt (pole longitude = 183.9°E, pole latitude = 15.9°N, = 7.4°, N = 18). The new paleomagnetic data are consistent with the pole path developed from time‐equivalent rocks of the Midcontinent Rift, supporting interpretations that changing pole positions are the result of rapid equatorward motion. These data add to the record of Laurentia's rapid motion from ca. 1,110 to 1,080 Ma that culminated in collisional Grenvillian orogenesis and the assembly of Rodinia.

Magnetotelluric insights into the formation and reactivation of trans-crustal shear zones in Precambrian basement of the eastern U.S. Midcontinent

Three-dimensional inversion of regional long-period magnetotelluric (MT) data reveals the presence of two distinct sets of high-conductivity belts in the Precambrian basement of the eastern U.S. Midcontinent. One set, beneath Missouri, Illinois, Indiana, and western Ohio, is defined by northwest−southeast-oriented conductivity structures; the other set, beneath Kentucky, West Virginia, western Virginia, and eastern Ohio, includes structures that are generally oriented northeast−southwest. The northwest-trending belts occur mainly in Paleoproterozoic crust, and we suggest that their high conductivity values are due to graphite precipitated within trans-crustal shear zones from intrusion-related CO2-rich fluids. Our MT inversion results indicate that some of these structures dip steeply through the crust and intersect the Moho, which supports an interpretation that the shear zones originated as “leaky” transcurrent faults or transforms during the late Paleoproterozoic or the early Mesoproterozoic. The northeast-trending belts are associated with Grenvillian orogenesis and also potentially with Iapetan rifting, although further work is needed to verify the latter possibility. We interpret the different geographic positions of these two sets of conductivity belts as reflecting differences in origin and/or crustal rheology, with the northwest-trending belts largely confined to older, stable, pre-Grenville cratonic Laurentia, and the northeast-trending belts largely having formed in younger, weaker marginal crust. Notably, these high-conductivity zones spatially correlate with Midcontinent fault-and-fold zones that affect Phanerozoic strata. Stratigraphic evidence indicates that Midcontinent fault-and-fold zones were particularly active during Phanerozoic orogenic events, and some remain seismically active today, so the associated high-conductivity belts likely represent long-lived weaknesses that transect the crust.

Fluid Flow, Alteration, and Timing of Cu-Ag Mineralization at the White Pine Sediment-Hosted Copper Deposit, Michigan, USA

Abstract White Pine, located in Michigan’s Upper Peninsula, is an archetypal sediment-hosted stratiform copper deposit. The Midcontinent rift system is one of only seven basins globally that host a giant sediment-hosted stratiform copper deposit. Despite many similarities with other deposits of this type, White Pine displays some important differences, including the late Mesoproterozoic age, a thick basalt sequence, an apparent lack of evaporites, and a lacustrine depositional setting. This study analyzes paleofluid flow related to the formation of White Pine and places a particular emphasis on structural and diagenetic fluid pathways. Most of the ore is located in a 30-m-wide zone spanning the Copper Harbor Formation red beds and the overlying Nonesuch Formation shales. Sedimentation of these units was accompanied by subtle synsedimentary faulting. Premineralization phases include calcite concretions and nodules, illite and hematite grain coatings, isopachous chlorite rims, emplacement of liquid petroleum (now pyrobitumen), and bleaching. Mineralization introduced native copper into the footwall sandstones and a zoned suite of native copper and sulfur-poor copper sulfide minerals across a migrating redox front in the overlying shales where copper minerals nucleated on authigenic and detrital chlorite grains. Postmineralization phases include quartz cement, calcite cement, and calcite veins that partially overlapped inversion of synsedimentary faults. Contrary to previous studies, we identified evidence for only one phase of mineralization. An Re-Os chalcocite age of 1067 ± 11 Ma places mineralization 11 to 17 m.y. after host-rock deposition. Sulfide δ34S values of –14.0 to 29.9‰ suggest an important contribution from sour gas and thermochemical sulfate reduction of seawater. Carbon (δ13C) and oxygen (δ18O) isotope compositions of five calcite generations range from –15.1 to –1.3‰ and 10.4 to 41.3‰, respectively, and record early meteoric pore water displaced by later seawater. White Pine is both a sediment-hosted stratiform copper deposit and a paleo-oil field. Synsedimentary faults controlled the sedimentological character of the upper Copper Harbor Formation, and together these imparted a strong control on fluid flow and later diagenetic processes. Early oxidized meteoric fluids were displaced by liquid petroleum and sour gas, which were in turn succeeded by metal-rich but sulfate-poor oxidized seawater. Burial compaction during deposition of the overlying Freda Formation drove fluids through White Pine due to its situation on a paleotopographic high near the basin margin. Mineralization occurred at ~125°C at depths of ~2.0 km and spanned incipient basin inversion related to the distal effects of Grenvillian orogenesis. The hightenor copper mineral assemblage is the product of an abundant supply of metal from basaltic volcanic detritus in the Copper Harbor Formation and low seawater sulfate concentrations in late Mesoproterozoic oceans. This demonstrates that viable sediment-hosted stratiform copper systems can form when a readily leachable metal source rock is present, even if hypersaline and sulfate-rich brines are not.

Crustal thickness of the Grenville orogen: A Mesoproterozoic Tibet?

AbstractThe Grenville Province on the eastern margin of Laurentia is a remnant of a Mesoproterozoic orogenic plateau that comprised the core of the ancient supercontinent Rodinia. As a protracted Himalayan-style orogen, its orogenic history is vital to understanding Mesoproterozoic tectonics and paleoenvironmental evolution. In this study, we compared two geochemical proxies for crustal thickness: whole-rock [La/Yb]N ratios of intermediate-to-felsic rocks and europium anomalies (Eu/Eu*) in detrital zircons. We compiled whole-rock geochemical data from 124 plutons in the Laurentian Grenville Province and collected trace-element and geochronological data from detrital zircons from the Ottawa and St. Lawrence River (Canada) watersheds. Both proxies showed several episodes of crustal thickening and thinning during Grenvillian orogenesis. The thickest crust developed in the Ottawan phase (~60 km at ca. 1080 Ma and ca. 1045 Ma), when the collision culminated, but it was still up to 20 km thinner than modern Tibet. We speculate that a hot crust and several episodes of crustal thinning prevented the Grenville hinterland from forming a high Tibet-like plateau, possibly due to enhanced asthenosphere-lithosphere interactions in response to a warm mantle beneath a long-lived supercontinent, Nuna-Rodinia.

A Laurentian cratonic reference from the distal Proterozoic basement of Western Newfoundland using tandem in situ and isotope dilution U-pb zircon and titanite geochronology

The Humber Margin of Newfoundland preserves the most distal exposures of Proterozoic basement in northeastern Laurentia. Age uncertainty has permitted a range of hypotheses for its origin and links to subsequent tectonic events. One hypothesis has proposed large-scale orogen-parallel displacement between basement blocks in western Newfoundland. The apparent absence of Grenville- (∼1250–950 Ma <i>sensu lato</i>) or Taconic-aged (∼480–450 Ma) magmatism or metamorphism on the Corner Brook Lake Block (CBLB), which are defining features of the Humber Margin, has been reconciled by restoring the CBLB to a pre-Taconic position in Labrador with &gt;400 km of post-Taconic dextral motion along the Humber River Fault. To test this model and better define the basement and Paleozoic rifted margin of North America, we conducted a geochronological study of the CBLB and the basement of the adjacent Humber Margin at Indian Head Range using tandem <i>in situ</i> and isotope dilution U-Pb zircon and titanite geochronology. These basement blocks, separated by the Humber River Fault, consist of <i>∼</i>1500 and <i>∼</i>1250 Ma protoliths, 1140 to 1135 Ma magmatism, 1000 to 970 Ma metamorphism, and <i>∼</i>607 Ma intraplate magmatism. These basement blocks are also overlain by similar late Ediacaran to Cambrian siliciclastic successions with similar detrital zircon age spectra. From this set of geological data, we conclude that the Humber River Fault did not accommodate significant orogen-parallel displacement. New basement ages and a revised compilation of detrital zircon ages from overlying rift-related deposits contribute to a geochronologic cratonic reference datum for western Newfoundland9s crystalline basement, whose protolith has a restricted age range from <i>circa</i> 1500 to 950 Ma. New age constraints for metasedimentary rocks are also used to document a 1250 to 1135 Ma succession at Indian Head Range and a ∼1000 Ma succession on the CBLB associated with Grenvillian orogenesis. Protracted late Grenvillian tectono-thermal events are inferred from cores and metamorphic overgrowths of ∼990 to 920 Ma detrital titanite in late Ediacaran conglomerate overlying CBLB basement.

Tectonic history of the Grenville-age Trenton Prong inlier, Central Appalachians, USA: evidence from SHRIMP U–Pb geochronology

New zircon U–Pb geochronologic data from the Grenville-age Trenton Prong provide information on the age of magmatism, timing of metamorphism, and post-metamorphic history of the inlier. Diorite gneiss (1318 ± 13 Ma) of the Colonial Lake Suite temporally correlates to magmatic arc sequences that formed along the eastern margin of Laurentia at &lt;1.4 Ga. Metasedimentary gneisses yielded detrital zircon ages of ca. 1319–1133 Ma and ca. 1370–1207, consistent with sediment derived from a similar local source of Laurentian affinity. A small population of zircon (either detrital or igneous in origin) in one sample yielded ages of ca. 1074–1037 Ma. Possible interpretations for their formation are explored. Ca. 1060 Ma overgrowths on zircon in the northern part of the inlier constrain the timing of granulite-facies metamorphism to the Ottawan phase of the Grenvillian Orogeny. The undeformed Assunpink Creek Granite (1041 ± 6 Ma) intruded country rocks as small bodies of late-orogenic syenogranite. It provides a minimum age for amphibolite-facies metamorphism and Ottawan orogenesis elsewhere in the inlier. Regionally, zircon rim ages of ca. 1010–960 Ma record continued thermal activity during the Rigolet phase of the orogen that resulted in migmatization of paragneiss at ca. 1004 Ma and juxtaposition of upper- and mid-crustal rocks during orogenic collapse. The lithologic ages and tectonic history of the Trenton Prong correlate to those in other Appalachian Mesoproterozoic inliers and parts of the Canadian Grenville Province, confirming it is not an exotic terrane that was accreted to eastern Laurentia during Grenvillian orogenesis.

Formation and evolution of the Eastern Kunlun Range, northern Tibet: Evidence from detrital zircon U-Pb geochronology and Hf isotopes

The Eastern Kunlun Range, as a high-elevation and granitoid-rich tectonic element in northern Tibet, records Paleozoic–Mesozoic amalgamation process of the East Asia continent and Cenozoic uplift of the Tibetan Plateau. However, Precambrian evolution of the Eastern Kunlun remains poorly understood and relations between Eastern Kunlun and adjacent terranes (e.g., Qaidam and Qilian) during the Phanerozoic accretion process are still highly controversial. We use detrital zircon U-Pb geochronological and Hf isotopic data of Proterozoic and Paleozoic metasedimentary rocks from the Eastern Kunlun Range, to reconstruct its origin and subsequent evolutionary history. Detrital zircons of the Proterozoic rocks are dominated by early–middle Neoproterozoic ages (700–1000 Ma), with two age peaks at ca. 800 Ma and ca. 920 Ma and εHf(t) values ranging from −10 to 5. The youngest detrital zircon ages (648–788 Ma) demonstrate that these investigated Proterozoic strata, which were previously mapped as Paleoproterozoic to Mesoproterozoic, were most likely deposited in the middle–late Neoproterozoic. Abundant 0.9–1.0 Ga detrital zircon crystals are consistent with those crystalline rocks of similar ages across the Kunlun-Qaidam and Qilian terranes, which are generally interpreted as the product of Grenvillian orogenesis. These findings support the hypothesis that these terranes were probably within a single continental landmass (named as KQQ block) during the Neoproterozoic. The high similarity of detrital zircon ages, Hf isotopes and Neoproterozoic lithostratigraphy between western Yangtze and KQQ blocks, supports a temporary connection of the KQQ block to western Yangtze in Rodinia supercontinent. Detrital zircons of the analyzed Paleozoic rocks are characterized by 390–490 Ma age populations. These results, in combination with published granitoids data of the northern Tibet, favor a scenario in which the Kunlun-Qaidam and Qilian terranes underwent separated subduction and accretion processes during the late Cambrian–Devonian, but together formed an upper plate to northward subduction of the Paleo-Tethys during the Permian–Triassic.

Failed rifting and fast drifting: Midcontinent Rift development, Laurentia’s rapid motion and the driver of Grenvillian orogenesis

Author(s): Swanson-Hysell, NL; Ramezani, J; Fairchild, LM; Rose, IR | Abstract: The late Mesoproterozoic was a time of large-scale tectonic activity both in the interior and on the margins of Laurentia-most notably the development of the Midcontinent Rift and the Grenvillian orogeny. Volcanism within the North American Midcontinent Rift between ca. 1109 and 1083 Ma, as well as other contemporaneous volcanism within Laurentia, has provided an opportunity to develop extensive paleomagnetic data sets spanning this time period. These data result in an apparent polar wander path (APWP) for Laurentia that goes from a high-latitude apex known as the Logan Loop into a swath known as the Keweenawan Track. A longstanding challenge of these data was the appearance of asymmetry between relatively steep reversed polarity directions from older rift rocks and relatively shallow normal polarity directions from younger rift rocks. This asymmetry was used to support an interpretation that there were large non-dipolar components to the geomagnetic field at the time. Recent data sets support the interpretation that this directional change was progressive and therefore a result of very rapid motion of Laurentia from high to low latitudes rather than a stepwise change across non-dipolar reversals. We present high-precision U-Pb dates from Midcontinent Rift volcanics that result in an improved chronostratigraphic framework for rift volcanics and unconformities that improves correlations as well as constraints on rift development. We use these dates in volcanostratigraphic context to temporally constrain a new compilation of Midcontinent Rift paleomagnetic poles. These paleomagnetic poles include new data from the North Shore Volcanic Group, Minnesota, USA and the Osler Volcanic Group, Ontario, Canada. The U-Pb dates constrain the rate of implied plate motion more precisely than has previously been possible. We apply a novel Bayesian approach to assess the rate of implied plate motion through inverting for paleomagnetic Euler poles. If the path is to be explained by a single Euler pole these inversions reveal that motion of the continent exceeded 27 cm/yr. The path is particularly well-explained by a model wherein there is continuous true polar wander in addition to rapid plate motion that changes direction and slows ca. 1096 Ma. Laurentia's movement from high to low latitudes resulted in collisional tectonics on its leading margin which could be associated with such a change in plate motion. We propose that upwelling of the Keweenawan mantle plume was associated with an avalanche of subducted slab material and associated downwelling that drove fast plate motion. This fast plate motion was followed by the Grenvillian orogeny from ca. 1090 to 980 Ma. Prolonged collisional orogenesis could have been sustained due to this strong convective cell that therefore played an integral role in the assembly of the supercontinent Rodinia.

Constraints on the timing and duration of orogenic events by combined Lu–Hf and Sm–Nd geochronology: An example from the Grenville orogeny

Grenville age basement rocks of the Virginia Blue Ridge Province preserve a record of over 150 million years of tectonism and crustal reworking associated with the construction of the supercontinent Rodinia. Although the igneous origins of these rocks have been the subject of much previous work, few studies have examined the complex metamorphic history of these rocks during the Grenville orogeny.To better constrain the timing and duration of metamorphism, we determined Lu–Hf and Sm–Nd garnet and U–Pb zircon rim dates of garnet-bearing rocks exposed in the Virginia Blue Ridge. Multiple periods of Mesoproterozoic metamorphism and deformation have been previously recognized with a major deformational event occurring before ∼1065 Ma and subsequent local episodes occurring at least until 1000 Ma. Zircon overgrowths and monazite dates suggest metamorphic events between ∼1050 and 960 Ma.Our new Lu–Hf garnet dates range from 1032 ± 3 to 1019 ± 3 Ma. New Sm–Nd dates, determined on the same garnet dissolutions, define a younger range, from 965 ± 6 to 949 ± 6 Ma. Our zircon dates are consistent with previously measured ages (∼1090–940 Ma) and often overlap with the Lu–Hf garnet dates from the same samples. There is a large and systematic difference (60–80 m.y.) between the garnet dates from the two isotopic systems. Although Sm–Nd dates slightly younger than those recorded by Lu–Hf are not uncommon, the large discrepancies between the two systems that we see here are unusual. The dated garnets are unzoned and yield Lu–Hf dates that coincide with the zircon overgrowth dates; Sm–Nd ages overlap with the younger monazite dates. We interpret the discrepancy in garnet dates as reflecting the difference in closure temperature between the two isotopic systems. Further, these data require slow cooling following large-scale crustal thickening after the assembly of Rodinia during Grenville orogenesis. These data illustrate that the use of the coupled Lu–Hf and Sm–Nd chronometers in garnet can provide important constraints on the conditions of metamorphism during past orogenic events.

Instability of the southern Canadian Shield during the late Proterozoic

Cratons are generally considered to comprise lithosphere that has remained tectonically quiescent for billions of years. Direct evidence for stability is mainly founded in the Phanerozoic sedimentary record and low-temperature thermochronology, but for extensive parts of Canada, earlier stability has been inferred due to the lack of an extensive rock record in both time and space. We used 40Ar/39Ar multi-diffusion domain (MDD) analysis of K-feldspar to constrain cratonic thermal histories across an intermediate (∼150–350 °C) temperature range in an attempt to link published high-temperature geochronology that resolves the timing of orogenesis and metamorphism with lower-temperature data suited for upper-crustal burial and unroofing histories. This work is focused on understanding the transition from Archean–Paleoproterozoic crustal growth to later intervals of stability, and how uninterrupted that record is throughout Earth's Proterozoic “Middle Age.” Intermediate-temperature thermal histories of cratonic rocks at well-constrained localities within the southern Canadian Shield of North America challenge the stability worldview because our data indicate that these rocks were at elevated temperatures in the Proterozoic. Feldspars from granitic rocks collected at the surface cooled at rates of <0.5 °C/Ma subsequent to orogenesis, seemingly characteristic of cratonic lithosphere, but modeled thermal histories suggest that at ca. 1.1–1.0 Ga these rocks were still near ∼200 °C – signaling either reheating, or prolonged residence at mid-crustal depths assuming a normal cratonic geothermal gradient. After 1.0 Ga, the regions we sampled then underwent further cooling such that they were at or near the surface (≪60 °C) in the early Paleozoic. Explaining mid-crustal residence at 1.0 Ga is challenging. A widespread, prolonged reheating history via burial is not supported by stratigraphic information, however assuming a purely monotonic cooling history requires at the very least 5 km of exhumation beginning at ca. 1.0 Ga. A possible explanation may be found in evidence of magmatic underplating that thickened the crust, driving uplift and erosion. The timing of this underplating coincides with Mid-Continent extension, Grenville orogenesis, and assembly of the supercontinent Rodinia. 40Ar/39Ar MDD data demonstrate that this technique can be successfully applied to older rocks and fill in a large observational gap. These data also raise questions about the evolution of cratons during the Proterozoic and the nature of cratonic stability across deep time.

He Grenville Orogenesis Recorded by Monazite from the Paragneiss of North Qaidam UHP Metamorphic Belt, Western China

He Grenville Orogenesis Recorded by Monazite from the Paragneiss of North Qaidam UHP Metamorphic Belt, Western China

The Grenvillian orogeny in the Altun–Qilian–North Qaidam mountain belts of northern Tibet Plateau: Constraints from geochemical and zircon U–Pb age and Hf isotopic study of magmatic rocks

Numerous Neoproterozoic magmatic and metamorphic events in the Altun–Qilian–North Qaidam (AQNQ) region record Grenvillian orogenesis and amalgamation of the supercontinent Rodinia. However, the tectonothermal regimes responsible for these Neoproterozoic events and the assumed position of the AQNQ in Rodinia remain controversial. Zircon U–Pb age data show that the orthogneiss and paragneiss/schist of the AQNQ experienced concurrent magmatism and metamorphism at 895–925Ma. Zircon Lu–Hf isotopic data indicate that the gneisses in the AQNQ have εHf (0.9Ga) values and tDM2 (Hf) model ages ranging from −5.6 to +3.9 and 1.4 to 1.9Ga. These data suggest that the early Neoproterozoic magma in the AQNQ was predominately derived from a late Paleoproterozoic–early Mesoproterozoic crustal source between 1.4 and 1.9Ga, marking an important episode of crustal growth in the AQNQ. The Neoproterozoic magmatism is geochemically characterized by (1) high SiO2, K2O, and low P2O5; (2) A/CNK ratios >1.0, ranging from 1.03 to 1.09; (3) enrichment in Rb, Th and U, and depletion in Ba, Nb, Ta, Sr, Ti, and Eu. Based on the geochemical resemblance to high-K calc-alkaline I-type granite and zircon Lu–Hf isotope signatures, the Neoproterozoic magmatism in the AQNQ was probably derived from ancient mafic-intermediate igneous rocks in an active continental margin. The Neoproterozoic tectono-magmatic–metamorphic history of the AQNQ, directly associated with the South China block (SCB) and the Tarim block (TB), indicates that the AQNQ and the TB coexisted as a single block in the early Neoproterozoic, which was temporarily connected to the SCB to the north or west in Rodinia during the late stages of the Grenvillian orogeny (950–900Ma).

Tectonic models for the origin of regional transverse structures in the Grenville Province of SW Quebec interpreted from regional gravity

Analysis of regional gravity data over the Grenville Province of SW Québec, Canada, has identified hitherto unrecognized transverse deep-crustal structures, in addition to lineaments that correspond to mapped upper crustal, orogen-parallel tectonic boundaries. Lineaments that trend NW–SE, i.e. transverse to the orogen, are interpreted as reactivated Palaeoproterozoic faults in Archaean basement that are controlled by structures in Archaean sub-crustal lithospheric mantle (SCLM). A prominent lineament (the Mont-Laurier lineament) that extends from the SE Superior Province across the SW Grenville Province is identified on both gravity and seismic tomographic imagery, the latter confirming it represents a lithospheric-scale structure. Baltica is placed at the Laurentian margin in tectonic reconstructions, and the Mont-Laurier lineament correlates with the extension towards the Superior Craton of the Belomorian Province between the Karelia and Kola cratons. It is proposed that this structure developed as a normal fault zone, and was subsequently reactivated as a transfer fault, during two orthogonal Palaeoproterozoic rifting events. The ensuing irregular basement geometry subsequently influenced sedimentation and deformation during the Grenvillian orogenic cycle and the Mont-Laurier lineament again acted as a transfer fault zone during Grenvillian orogenesis and post-orogenic collapse. Identification of tranverse structures has implications in understanding the architecture of this part of the Grenville Province. The role of pre-existing basement structures and rigid SCLM on the formation and preservation of long, linear structures transverse to an orogen throughout a complex sequence of rifting, back-arc basin formation, collisional orogenesis, and post-orogenic collapse in the Grenville Province of SW Quebec is also documented in New Guinea, with which comparisons are made.

Grenville-age orogenesis in the Qaidam-Qilian block: The link between South China and Tarim

The link between the South China and Tarim blocks is a key issue in defining the tectonic framework of China. We report here that rocks from the Paleozoic (Caledonian) ultrahigh-pressure metamorphic (UHPM) belt also record a Grenville-age orogenic event in the Qaidam-Qilian block that lies between the South China and Tarim blocks. This event is marked by the presence of a juvenile Grenville-age continental margin that had been subducted to and exhumed from mantle depths of 100–200km at the present-day location of northern Tibetan Plateau. The magmatic and metamorphic ages of 900–1000Ma recorded in the gneisses provide direct evidence for this Grenville-age orogeny that extends northwestward from the Yangtze block, to the Qaidam-Qilian block, and to the Tarim block. This Grenville-age orogen along the North Qaidam UHPM belt (1) represents the link for the once South China–Qaidam-Qilian–Tarim continent, which, we name here as the “South-West China United Continent (SWCUC)”, had existed before the India–Asia collision, (2) sets a framework for precise reconstruction of Supercontinent Rodinia, and (3) presents an example of multi-epoch tectonic recycles, represented by the Neoproterozoic Grenvillian orogenesis and the Early Paleozoic Caledonian orogenesis.

Basin Evolution and Unconformity-Related Uranium Mineralization: The Camie River U Prospect, Paleoproterozoic Otish Basin, Quebec

The Paleoproterozoic Otish Basin, Quebec, hosts several uranium prospects that until recently remained underexplored and poorly understood. In this study, the Camie River U prospect, which shows similar characteristics to high-grade unconformity-related U deposits, is the focus of an integrated basin analysis in the western Otish Basin. Conglomerate and sandstone of the Indicator Formation, which were deposited in at least six depositional sequences, were affected by insignificant early diagenetic compaction and cementation. This allowed the formation of regional peak diagenetic aquifers, which became muscovite altered due to interaction with fluids having δ 18O and δ 2H values similar to those of seawater-influenced basinal brines at 250°C. U mineralization at Camie River occurred at 1721 ±20 Ma based on a 207Pb/206Pb date obtained by laser ablation of uraninite, which coincides with a phase of the Otish Gabbro intrusion that has been dated at ca. 1730 Ma. The intrusive event promoted circulation of U-bearing basinal brines, triggering U mineralization at several locations in the western Otish Basin. Interaction of basinal brines with the Otish Gabbro produced coarse-grained chlorite, tourmaline, and phengitic muscovite, which decreased the fluid-conducting capabilities of diagenetic aquifers and resulted in fault zone- and fracture-dominated fluid flow. Subsequent fluid alteration events produced limited U remobilization, sulfides, sudoite, and siderite between ca. 1670 and 1410 Ma based on mineral paragenesis and 40Ar/39Ar dates of muscovite. Metamorphic fluids having high δ 18O values and temperatures around 300°C accompanied 1.2 to 1.0 Ga Grenville orogenesis and subgreenschist-grade metamorphism in the Otish Basin but were present at low water/rock ratios at Camie River and therefore produced little alteration. Post-Grenville uplift of the Otish Basin likely produced late, low-temperature alteration minerals that have been influenced by recent meteoric water, suggesting that the fault zones and fractures the minerals occupy remain as preferential fluid-flow pathways to the present day. Radiogenic Pb and the characteristic trace elements Mo + W + Nb have also preferentially dispersed from the mineralization along fault zones, fractures, and depositional sequence boundaries, and can be used to explore for Camie River-style U mineralization.

Evidence for post-1200 Ma — pre-Grenvillian supracrustal rocks in the Pinware terrane, eastern Grenville Province at Battle Harbour, LabradorThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.

U–Pb zircon geochronological investigations at Battle Harbour, in the Pinware terrane, eastern Grenville Province show that a cross-bedded psammitic rock from Battle Island was deposited after 1200 Ma and prior to Grenvillian orogenesis, as part of a package of psammitic, semi-pelitic and calc-silicate rocks. This represents a major finding as no previous data had indicated any supracrustal-rock deposition after ca. 1500 Ma in the interior eastern Grenville Province. The age is comparable to that obtained from supracrustal units of minor extent elsewhere in the interior Grenville Province, indicating that change in crustal level in these regions was modest during Grenvillian orogenesis. Grenvillian metamorphism at amphibolite facies occurred at 1030 ± 4 Ma, based on evidence from a concordant mafic rock within the supracrustal assemblage that is interpreted to be a sill. Grenvillian orogenesis at Battle Harbour was accompanied by abundant pegmatite injection over an extended period, as indicated by cross-cutting relationships and various states of deformation displayed by the pegmatites. An amazonite-bearing pegmatite has an age of 1024 ± 3 Ma. An even younger, cross-cutting pegmatite contains only inherited zircon grains.

New York–Alabama lineament: A buried right-slip fault bordering the Appalachians and mid-continent North America

The New York–Alabama (NY-AL) lineament, recognized in 1978, is a magnetic anomaly that delineates a fundamental though historically enigmatic crustal boundary in eastern North America that is deeply buried beneath the Appalachian basin. Data not in the original aeromagnetic data set, particularly the lack of any information available at the time to constrain the southern continuation of the anomaly southwest of Tennessee, left the source of the lineament open to conjecture. We use modern digital aeromagnetic maps to fill in these data gaps and, for the first time, constrain the southern termination of the NY-AL lineament. Our analysis indicates that the lineament reflects a crustal-scale, right-lateral strike-slip fault that has displaced anomalies attributed to Grenville orogenesis by ∼220 km. Palinspastic restoration of this displacement rearranges the trace of the Grenville belt in southern Rodinia and implies only passive influence on later-formed Appalachian structures. The precise timing of dextral movement on the NY-AL structure is not resolvable from the existing data set, but it must have occurred during one of, or combinations of, the following events: (1) a late, postcontractional (post-Ottawan) stage of the Grenville orogeny; (2) late Neoproterozoic to Cambrian rifting of Laurentia; or (3) right-slip reactivation during the late Neoproterozoic–Cambrian rifting of Laurentia, or during Appalachian movements. Our palinspastic reconstruction also implies that the host rocks for modern earthquakes in the Eastern Tennessee Seismic Zone are metasedimentary gneisses, and it provides an explanation for the spatial location and size of the seismic zone.

Whole-rock Pb and Sm-Nd isotopic constraints on the growth of southeastern Laurentia during Grenvillian orogenesis

The conventional view that the basement of the southern and central Appalachians represents juvenile Mesoproterozoic crust, the final stage of growth of Laurentia prior to Grenville collision, has recently been challenged. New whole-rock Pb and Sm‑Nd isotopic data are presented from Mesoproterozoic basement in the southern and central Appalachians and the Granite-Rhyolite province, as well as one new U-Pb zircon age from the Granite-Rhyolite province. These data, combined with existing data from Mesoproterozoic terranes throughout southeastern Laurentia, further substantiate recent suggestions that the southern and central Appalachian basement is exotic with respect to Laurentia. Sm-Nd isotopic compositions of most rocks from the southern and central Appalachian basement are consistent with progressive growth through reworking of the adjacent Granite-Rhyolite province. However, Pb isotopic data, including new analyses from important regions not sampled in previous studies, do not correspond with Pb isotopic compositions of any adjacent crust. The most distinct ages and isotopic compositions in the southern and central Appalachian basement come from the Roan Mountain area, eastern Tennessee–western North Carolina. The data set indicates U-Pb zircon ages up to 1.8 Ga for igneous rocks, inherited and detrital zircon ages >2.0 Ga, Sm-Nd depleted mantle model (T DM ) ages >2.0 Ga, and the most elevated 207 Pb/ 204 Pb observed in southeastern Laurentia. The combined U-Pb geochronologic and Sm-Nd and Pb isotopic data preclude derivation of southern and central Appalachian basement from any nearby crustal material and demonstrate that Grenville age crust in southeastern Laurentia is exotic and probably was transferred during collision and assembly of Rodinia. These new data better define the boundary between the exotic southern and central Appalachian basement and adjacent Laurentian Granite-Rhyolite province.

Indentor tectonism in the eastern Grenville Province

The easternmost Grenville Province behaved in a structurally distinct manner from the remainder of the eastern Grenville Province (EGP) during Grenvillian orogenesis (1085–985 Ma). Geochronological, structural and geophysical data are used to demonstrate that, in contrast to frontal-thrust ramp tectonics in western and central Labrador, Grenvillian orogenesis in eastern Labrador mostly involved a dextral strike-slip, lateral-ramp regime. It is also argued that allochthonous terranes in the western part of the EGP were displaced 150 km northwest, relative to their eastern counterparts during the late Grenvillian. Whereas central and western Labrador and adjacent Quebec can be successfully modeled in terms of frontal, collisional tectonism, utilizing channel-flow models that have been applied to the Himalayan–Tibetan tectonic system, eastern Labrador cannot. The point where frontal-thrust ramp tectonics gives way to a lateral-ramp regime is interpreted as an indentor corner. In contrast to the Himalayan situation where India is being subducted under Asia, the indenting crust in the EGP was thrust over pre-Grenvillian Laurentia. The lateral-ramp concept has important implications with respect to Rodinia reconstructions between eastern Laurentia, western Baltica and northern Amazonia.

Proterozoic southward accretion and Grenvillian orogenesis in the interior Grenville Province in eastern Labrador: Evidence from U–Pb geochronological investigations

This study is a fascinating example, from the eastern Grenville Province, of unraveling the triple challenges inherent in the geochronological investigation of any multi-orogenic region, namely inheritance, emplacement and subsequent metamorphism. The existence of hitherto unknown pre-Labradorian crust within allochthonous Grenvillian terranes is demonstrated; formerly unrecognized spatial relations between Labradorian and Pinwarian crust are outlined; and a new perception of mid-Grenvillian magmatic and metamorphic events is presented. The pre-Labradorian crust has an age range of 1800–1770 Ma and consists of calc-alkaline plutonic rocks intruding earlier supracrustal assemblages containing Archean and early Paleoproterozoic inherited zircons. Evidence is provided from dated enclaves and inheritance in younger rocks that similar-aged pre-Labradorian crust once existed farther south. The geochronologically defined geographic extent of early Labradorian crust having a 1680–1655 Ma age range is expanded. This crust is intruded by the mid-Labradorian Mealy Mountains Intrusive Suite, for which new results confirm its 1650–1630 Ma time of emplacement. Pinwarian orthogneiss, dated between 1513 and 1496 Ma and previously unknown in the region, is now recognized to be common. Sufficient data exist to define, for the first time, a boundary between Labradorian and Pinwarian orthogneiss (cryptic in the field), establishing the presence of Pinwarian-aged crust to the southwest, lacking Labradorian inheritance except close to the boundary. A belt of layered mafic, anorthositic and monzogranitic intrusions is situated close to the boundary. Current data favour a Pinwarian age for emplacement of these rocks. Mid-Grenvillian plutonism is demonstrated by 1043 Ma ages from two K-feldspar megacrystic granodiorite bodies. They are situated on either flank of a crustal-scale, northeast-trending, northeast-closing fold. The fold, which is 40 km wide and at least 160-km long and exposes Pinwarian orthogneiss in its core, post-dates emplacement of the 1043 Ma intrusions. The fold probably formed at the peak of Grenvillian tectonism in the region, between 1030 and 1015 Ma, and certainly before intrusion of weakly deformed granitoid rocks at 992 Ma. Final activity included the emplacement of late- to post-Grenvillian granitoid plutons between 964 and 951 Ma. A pluton having a 951 Ma age is the youngest known in the eastern Grenville Province. Cooling and stabilization was completed by ca. 940 Ma. Broader implications of the results, in conjunction with existing data, are that: (i) the northeasternmost part of the eastern Grenville Province was only peripherally affected by Grenvillian orogenesis, (ii) an east–northeast offshore extrapolation of the Grenville front as a major tectonic feature does not exist, (iii) high-grade metamorphic mineral assemblages in the Groswater Bay and Hawke River terranes are pre-Grenvillian, rather than an eastward expression of a Grenvillian high-pressure belt, and (iv) the north shore of the eastern St. Lawrence estuary represents across-orogen exposure, rather than orogen-parallel as previously supposed.