Grenvillian Research Articles

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).

Neoproterozoic metamorphic evolution in the Transangarian Yenisei Ridge: Evidence from monazite and xenotime geochronology

Chemical mapping and in situ dating of U-Th-rich minerals in zoned garnets from gneisses of the Garevka metamorphic complex were used to constrain multiple metamorphic events in the Transangarian Yenisei Ridge. The data provide supporting evidence for three distinct metamorphic stages. The first episode occurred as a result of the Grenville orogeny during the Late Mesozoic and Early Neoproterozoic (1050–850 Ma) and was marked by low-pressure zoned metamorphism and a metamorphic field gradient with dT/dH = 20−30°C/km typical of orogenic belts. At the second stage, the rocks experienced Late Riphean (801–793 Ma) syn-collisional medium-pressure metamorphism with a low metamorphic field gradient (dT/dH ≤ 10°C/km). The final stage evolved as a synexhumation dynamic metamorphism (785–776 Ma) with dT/dH ≤ 12°C/km and reflected rapid exhumation of rocks in shear zones. The sequence of collisional events within the western margin of the Siberian craton affected by the Valhalla orogen suggests that Siberia and cratons of the North Atlantic region were in close proximity to one another at about 800 Ma, which is supported by recent paleomagnetic reconstructions.

Timing and conditions of poly-phase metamorphism within the Twelve Mile Bay shear zone: implications for the evolution of mid-crustal decollement zones and western Grenville tectonics

The Twelve Mile Bay assemblage (TMBa) forms the high-strain interior of the Twelve Mile Bay shear zone (TMBsz), a major ductile decollement zone within the western Canadian Grenville orogen. Metasupracrustal gneiss within the TMBa preserves evidence for an early granulite facies (˜10–11 kbar and ˜840°C) metamorphism overprinted by amphibolite facies (˜5–7 kbar and ˜650°C) assemblages that define the high-strain shear zone fabric. U–Pb zircon ages for TMBa samples were determined by LA-ICP-MS. A low-strain amphibolite pod with partially preserved granulite facies assemblage and textures yielded an anchored discordia intercept of 1157 ± 11 Ma and 207Pb/206Pb weighted average of 1146 ± 10 Ma. Three higher strain samples with recrystallized amphibolite facies assemblages all yield younger ages, with 207Pb/206Pb weighted averages of 1125 ± 16, 1110 ± 8, and 1095 ± 17 Ma. Phase equilibrium modelling shows that up to 40 vol.% anatectic melt could have been produced in TMBa pelitic rocks during peak metamorphic conditions, and thus, much of the package likely would have been substantially weakened during the early stages of TMBsz development. Strain apparently continued to accumulate within the TMBa until ca. 1100 Ma, concurrent with pegmatite dike emplacement and hydration along the base of the overlying interior Parry Sound domain (iPSD), perpetuating TMBsz activity during cooling and exhumation to shallower crustal levels. Similarities between the TMBa and the upper parts of the basal PSD (bPSD), in terms of timing and conditions of metamorphism and shearing, as well as structural position relative to the overlying iPSD allochthon, indicate that these units are likely correlative. The composite bPSD–TMBa system appears to have contemporaneously localized strain within the middle orogenic crust during early to middle stages of Grenvillian collision, providing a petrologically constrained mechanism for the long distance transport of mid-crustal nappes predicted in thermal-mechanical models of continental collision for this area.

Mesoproterozoic intraplate magmatic ‘barcode’ record of the Angola portion of the Congo Craton: Newly dated magmatic events at 1505 and 1110 Ma and implications for Nuna (Columbia) supercontinent reconstructions

In the Angola portion of the Congo Craton, the only Proterozoic large igneous province (LIP) dated prior to this study was the 1380–1370Ma (Kunene Intrusive Complex and related units). U–Pb TIMS ages on baddeleyite from dolerite sills and gabbro-noritic dykes, has revealed two additional Mesoproterozoic intraplate events: at ca. 1505 and ca. 1110Ma, that are each proposed to be part of the plumbing system for LIPs. The identification of these three Mesoproterozoic magmatic events (ca. 1505, 1380, and 1110Ma) represent an initial magmatic ‘barcode’ for this portion of Congo Craton (and formerly connected São Francisco Craton), which can be compared with the magmatic ‘barcode’ record of other blocks to identify former nearest neighbors in the Precambrian supercontinent Nuna (also known as Columbia).Specifically, a 1502±5Ma U–Pb TIMS baddeleyite age has been obtained for the prominent Humpata dolerite sill which is part of a wider sill province in SW Angola portion of the Congo Craton. The combined presence of both 1505Ma and 1380Ma magmatism in the Congo–São Francisco reconstructed craton is a match with similar ages published for two intraplate magmatic provinces in northern Siberia and suggests a nearest-neighbor relationship in the supercontinent Nuna in which northern Siberia is juxtaposed adjacent to the western São Francisco portion of the reconstructed São Francisco–Congo Craton.In addition, a precise U–Pb TIMS baddeleyite age of 1110±2.5Ma was obtained for a prominent NNW–NNE trending gabbro-noritic (GN) dyke swarm in southeastern Angola, but this age is currently unknown in Siberia suggesting that the breakup of Congo–São Francisco Craton from Siberia happened earlier, perhaps in association with the 1380Ma event. This 1110Ma age is however, a precise match with that of the Umkondo large igneous province (LIP) of the Kalahari Craton, and also with mafic intraplate magmatism on other blocks such as the Bundelkhand Craton (India) and the Amazonian Craton. We provisionally consider these three cratons to have been nearest neighbors to the Congo–São Francisco Craton at this time and to have shared this 1110Ma magmatic event as a LIP node. There is also an age match with the early part of the Keweenawan event (in the interior of the Laurentia); however, on previously discussed paleomagnetic grounds the Keweenawan event is likely to have been distant and unrelated (and on the other side of the Grenville orogen).

Mid-Tertiary (25–21 Ma) lamprophyres in NW Mexico derived from subduction-modified subcontinental lithospheric mantle in an extensional backarc environment following steepening of the Benioff zone

The mid-Tertiary lamprophyre dike swarm (~8km×2.5km in size) from Hermosillo (Sonora, NW Mexico) has calc–alkaline characteristics and includes NNW-striking, amphibole-phyric spessartite (~85% of the swarm) and NNE-striking, phlogopite-phyric kersantite. The 40Ar/39Ar geochronology of amphibole and phlogopite gives overlapping plateau ages ranging from 25 to 21Ma. Although all the lamprophyres are enriched in incompatible elements and display negative Nb–Ta and Ti anomalies on the primitive mantle-normalized plots, kersantite has higher K/Na, La/Yb, P, Ti and incompatible trace elements (e.g., Zr) compared to spessartite. The lamprophyres have radiogenic Sr and Nd isotopic signatures (87Sr/86Sr ~0.7057–0.7065 and εNd ~−1 to −2.3) suggesting derivation from the subcontinental lithospheric mantle that was previously modified by subduction-related fluids. This mantle is similar to that beneath the southern Grenvillian orogen, which has younger TDM ages than the 1.6–1.7Ga TDM ages of the Caborca block. The lamprophyric magmas were generated at various mantle depths at the southwestern edge of North America. Intrusion of the lamprophyres was synchronous with extension that produced normal faults and core complexes with WSW-vergence. Extension occurred immediately following steepening of the Benioff zone, during which the magmatic arc migrated from east to west of Hermosillo, and the lamprophyres were intruded just behind the contemporaneous arc.

Nd isotope mapping of the Dysart gneiss complex: Evidence for a rifted block within the Central Metasedimentary Belt of the Grenville Province

More than 65 new Nd isotope analyses are presented for the vicinity of the Central Metasedimentary Belt Boundary Zone (CMBBZ) near Haliburton in the Grenville Province of Ontario, Canada. They fall into two well-defined groups, with distinct ranges of TDM model ages. These two groups comprise the CMBBZ with an average TDM age=1.46Ga, and rocks of the Central Metasedimentary Belt (CMB) with an average TDM age of 1.28Ga. However, a distinct block of crust within the CMB comprising the Dysart gneiss complex has an older model age signature (TDM age=1.40Ga) resembling the CMBBZ rather than the rift zone. The Dysart complex is surrounded by marble tectonites interspersed with elongate bodies of orthogneiss that yield young Nd model ages typical of the CMB. However, it is argued that the development of the Dysart complex as a distinct crustal block predates ductile deformation during the Grenville orogeny. Instead, the Dysart Block is thought to have rifted away from the Central Gneiss Belt to the northwest during the formation of the CMB as an ensimatic back-arc rift zone. Isotopic mapping of the Dysart block, along with the previously discovered Elzevir block, provides evidence for the geometry of the rifting process, and also has important implications for the structure of Grenville basement in eastern North America.

Thermochronology of the Mont Laurier terrane, southern Canadian Grenville Province, and its bearing on defining orogenic architecture

Convergence during pre-Grenvillian accretion led to crustal thickening and juxtaposition of shallowly southeast-dipping imbricated terranes of the Central Metasedimentary Belt in Ontario and its equivalent to the north in Québec known as the Mont Laurier terrane (MLT). This investigation focuses on the Otter Lake region of the southern MLT, which is composed of a Mesoproterozoic back-arc sequence metamorphosed to upper amphibolite/granulite facies (at conditions of 6±1kbar and 650–700°C). Notable peak assemblages include Fo-bearing marbles and Sil+Kfs migmatites, with little to no retrogression. The terrane is divided into the Marble (west) and Quartzite (east) domains, separated by the east-dipping Heney shear zone (HSZ). LA-ICPMS U-Pb geochronology on select migmatites from both domains yielded bimodal U-Pb zircon and monazite age populations at ca. 1230Ma and 1180–1140Ma, with mineral ages from the leucosomal portions of the migmatites populating the younger age bracket, and implying that the metamorphism and anatexis took place during the accretionary Shawinigan orogeny. Directly east of the HSZ, 40Ar/39Ar hornblende and phlogopite ages are ca. 1145Ma and ca. 1150Ma, respectively, whereas to the west in the Marble Domain hornblende ages are markedly younger (ca. 980–950Ma). Rapid cooling of the Quartzite Domain at the end of the Shawinigan orogeny together with the lack of evidence for any metamorphic overprinting in the southern MLT during the long-lived Grenvillian orogeny (1095–980Ma) is consistent with this region being part of an “orogenic lid” or suprastructure. Post-Grenvillian cooling of Marble Domain rocks through 500°C nearly 200m.y. after cooling of the Quartzite Domain suggests a protracted middle crustal residence of the Marble domain under thermally elevated conditions, implying that portions of the orogenic lid remained warm (above 500°C) during the Grenville orogeny. Moreover, the disparate cooling ages on either side of the HSZ provide evidence for important late displacement across the HSZ comparable to that proposed for terrane-bounding shear zones such as the Robertson Lake and Bancroft shear zones. These structures may be an expression of vertical orogenic collapse or lateral stretching of the orogenic lid. Biotite cooling ages are uniformly <900Ma and are similar to the majority of biotite 40Ar/39Ar ages reported across the southern Grenville Province, indicative of orogen-wide thermal stabilization by ca. 900Ma.

MINERALOGY, CLASSIFICATION, AND TECTONIC SETTING OF THE GRANITIC PEGMATITES OF NEW YORK STATE, USA

Pegmatites in New York State are largely restricted to areas of exposed Precambrian basement rocks, including the Adirondack Mountains and the Hudson Highlands. In the Adirondacks, they were emplaced into rocks that were metamorphosed during the Grenville orogeny to either the upper amphibolite facies (Adirondack Lowlands) or the granulite facies (Adirondack Highlands). Their mineral assemblages range from simple to complex, with localized occurrences of: (a) Be-, Al-, and B-rich species such as beryl, chrysoberyl, sillimanite, dumortierite, and tourmaline-supergroup minerals, (b) rare-element- and rare-earth-element-bearing minerals such as columbite-(Fe), uranopolycrase, fergusonite-(Y), uraninite, xenotime-(Y), and monazite-(Ce), (c) Ca-, F-, and K-dominant amphiboles, (d) phosphates (fluorapatite, isokite, wagnerite), (e) sulfides (bismuthinite, molybdenite), and (f) tungstates (scheelite). The pegmatite bodies from southern New York State and from some of the southern Adirondack locations display mineralogical zoning, typically with a quartz-rich core. Post-emplacement metamorphic features are observed in pegmatites in the northwestern Adirondack Lowlands, but are negligible or obscured in most of those from the southern and eastern Adirondacks. On the basis of the metamorphic grade of the host rocks, all New York pegmatites belong to the Abyssal class, whereas on the basis of mineralogy and inferred tectonic setting, they show affiliations to the NYF and mixed NYF–LCT pegmatite families. Pegmatites in the Adirondack Lowlands are related to the calc-alkaline arc magmatism of the Antwerp–Rossie granite suite and yield U–Pb zircon crystallization ages that indicate intrusion during the late Shawinigan orogeny (~1195 Ma); currently there are no known pegmatites related to the Ottawan and Rigolet orogenies in the Lowlands. Pegmatites in the Highlands yield U–Pb zircon crystallization ages that correspond to: (a) emplacement during the late Elzevirian (~1222 Ma), and metamorphism during the late Shawinigan (~1178 Ma) orogenies; (b) emplacement during the early (1098 Ma) and mid to late Ottawan orogeny (~1062–1025 Ma); (c) intrusion during the early Rigolet orogeny (1009–1003 Ma), and finally, (d) intrusion related to a thermal pulse at 949 Ma, possibly associated with the Cathead Mountain leucocratic dike swarm (935 ± 9.2 Ma). Few, if any, granitic pegmatites were intruded between 1178 and 1098 Ma in the Adirondack Highlands. The main pegmatite bodies in the Adirondack Highlands occur in association with extensional A-type granites such as the Hawkeye and Lyon Mountain Granite suites. These rocks were emplaced in response to orogenic collapse following the Ottawan orogeny. At present, relatively little can be concluded about the tectonic setting of pegmatites in southern New York.

Ages of pre-rift basement and synrift rocks along the conjugate rift and transform margins of the Argentine Precordillera and Laurentia

New geochronologic data from basement rocks support the interpretation that the Argentine Precordillera (Cuyania) terrane was rifted from the Ouachita embayment of the Iapetan margin of Laurentia. New data from the Ozark dome show a range of ages in two groups at 1466 ± 3 to 1462 ± 1 Ma and 1323 ± 2 to 1317 ± 2 Ma, consistent with existing data for the Eastern Granite-Rhyolite province and Southern Granite-Rhyolite province, respectively. Similarly, a newly determined age of 1364 ± 2 Ma for the Tishomingo Granite in the Arbuckle Mountains confirms previously published analyses for this part of the Southern Granite-Rhyolite province. Along with previously reported ages from basement olistoliths in Ordovician slope deposits in the Ouachita embayment, the data for basement ages support the interpretation that rocks of the Southern Granite-Rhyolite province form the margin of Laurentian crust around the corner of the Ouachita embayment, which is bounded by the Ouachita rift and Alabama-Oklahoma transform fault. In contrast, both west and east of the corner of the Ouachita embayment, Grenville-Llano basement (approximately 1325–1000 Ma) forms the rifted margin of Laurentia. New U/Pb zircon data from basement rocks in the southern part of the Argentine Precordillera indicate crystallization ages of 1205 ± 1 Ma and 1204 ± 2 Ma, consistent with previously reported ages (approximately 1250–1000 Ma) of basement rocks from other parts of the Precordillera. These data document multiple events within the same time span as multiple events in the Grenville orogeny in eastern Laurentia, and are consistent with Grenville-age rocks along the conjugate margins of the Precordillera and Laurentia. Ages from one newly analyzed collection, however, are older than those from other basement rocks in the Precordillera. These ages, from granodioritic-granitic basement clasts in a conglomerate olistolith in Ordovician slope deposits, are 1370 ± 2 Ma and 1367 ± 5 Ma. These older ages from the Precordillera are consistent with indications that the Iapetan margin in the Ouachita embayment of Laurentia truncated the Grenville front and left older rocks of the Southern Granite-Rhyolite province (1390–1320 Ma) at the rifted margin. Chronostratigraphic correlations of synrift and post-rift sedimentary deposits on the Precordillera and on the Texas promontory of Laurentia document initial rifting in the Early Cambrian. Previously published data from synrift plutonic and volcanic rocks in the Wichita and Arbuckle Mountains along the transform-parallel intracratonic Southern Oklahoma fault system inboard from the Ouachita embayment document crystallization ages of 539–530 Ma. New data from synrift volcanic rocks in the Arbuckle Mountains in the eastern part of the Southern Oklahoma fault system yield ages of 539 ± 5 Ma and 536 ± 5 Ma, confirming the age of synrift volcanism.

A non‐collisional, accretionary Sveconorwegian orogen

Terra Nova, 25, 30–37, 2013AbstractThe late Mesoproterozoic Sveconorwegian orogen in southwest Baltica is traditionally interpreted as the eastward continuation of the Grenville orogen in Canada, resulting from collision with Amazonia, forming a central part in the assembly of the Rodinia supercontinent. We challenge this conventional view based on results from recent work in southwest Norway demonstrating voluminous subduction‐related magmatism in the period 1050–1020 Ma, followed by geographically restricted high‐T/medium‐P metamorphism between 1035 and 970 Ma, succeeded by ferroan magmatism over large parts of south Norway in the period 990–920 Ma. This magmatic and metamorphic evolution may be better understood as reflecting a long‐lived accretionary margin, undergoing periodic compression and extension, than continent–continent collision. This study has implications for Grenville–Sveconorwegian correlations, comparisons with modern continental margins, Rodinia reconstructions and how we recognize geodynamic settings in ancient orogens.

The Garzón Massif, Colombia-a new ultrahigh-temperature metamorphic complex in the Early Neoproterozoic of northern South America

The Garzon Complex of the Garzon Massif in SW Colombia is composed of the Vergel Granulite Unit (VG) and the Las Margaritas Migmatite Unit (LMM). Previous studies reveal peak temperature conditions for the VG of about 740 °C. The present study considers the remarkable exsolution phenomena in feldspars and pyroxenes and titanium-in-quartz thermometry. Recalculated ternary feldspar compositions indicate temperatures around 900–1,000 °C just at or above the ultra-high temperature–metamorphism (UHTM) boundary of granulites. The calculated temperatures range of exsolved ortho- and clinopyroxenes also supports the existence of an UHTM event. In addition, titanium-in-quartz thermometry points towards ultra-high temperatures. It is the first known UHTM crustal segment in the northern part of South America. Although a mean geothermal gradient of ca 38 °C km−1 could imply additional heat supply in the lower crust controlling this extreme of peak metamorphism, an alternative model is suggested. The formation of the Vergel Granulite Unit is supposed to be formed in a continental back-arc environment with a thinned and weakened crust behind a magmatic arc (Guapoton-Mancagua Gneiss) followed by collision. In contrast, rocks of the adjacent Las Margaritas Migmatite Unit display “normal” granulite facies temperatures and are formed in a colder lower crust outside the arc, preserved by the Guapoton-Mancagu Gneiss. Back-arc formation was followed by inversion and thickening of the basin. The three units that form the modern-day Garzon Massif, were juxtaposed upon each other during collision (at ca. 1,000 Ma) and exhumation. The collision leading to the deformation of the studied area is part of the Grenville orogeny leading to the amalgamation of Rodinia.

Deciphering the early history of a multiply reactivated fault using strain analysis in an adjacent fold: Evidence for early brittle-plastic slip on the Picuris-Pecos fault, New Mexico, USA

Determining the earliest history of a multiply reactivated fault is difficult, particularly when the rock record is incomplete and brittle deformation reorients strain markers. We present a method that incorporates strain analysis of an adjacent fold, inferred deformation temperatures, and published thermochronology to find the initiation age of the north-striking Picuris-Pecos fault of the southern Rocky Mountains (USA). The Picuris-Pecos fault has accommodated at least 37 km of dextral slip. The older east-trending Hondo syncline (New Mexico) is refolded adjacent to the fault and has a map pattern consistent with dextral drag. We analyzed quartz grain shapes, deformation mechanisms, and fractures within the Hondo syncline to determine if the refold is genetically related to the Picuris-Pecos fault. Nine calculated grain-shape ellipsoids predate refolding and are rotated about a vertical axis subparallel to strike of the refold limbs. Healed fractures and semibrittle microstructures are most abundant in the hinge of the refold and adjacent to the fault. We interpret refolding and slip on the Picuris-Pecos fault to have initiated during unroofing through the brittle-plastic transition (∼250–310 °C). Regional thermochronologic studies suggest that the rocks of the Hondo syncline were at this temperature range during the Grenville orogeny. The refold is probably a fault-propagation fold that predated throughgoing faulting, or possibly formed during dextral drag on the Picuris-Pecos fault. Refolding in the Picuris Range and the Truchas Mountains accounts for at least 2.6 km of the total dextral shear on the fault.

Where did the lower Paleozoic rocks of Yucatan come from? A U–Pb, Lu–Hf, and Sm–Nd isotope study

Detrital zircon grains from Lower Paleozoic sedimentary rocks in the Yucatan Peninsula have an age distribution characterized by major probability peaks at ~1.0, ~1.2, and ~1.5Ga (Martens et al., 2010). Here, we present new Lu–Hf data (MC–ICPMS) paired with U–Pb ages (ID-TIMS) for additional zircon grains from the same rocks. This analytical approach yields precise information about the time and geochemical environment of zircon growth, which in turn helps to distinguish between different crustal source regions that just happen to host zircon populations of similar age. In addition, single zircon grains from granitoids that intruded the sedimentary rocks were dated by U–Pb laser ablation MC–ICPMS and ID-TIMS, and their Hf-isotope compositions were determined by solution MC–ICPMS. The zircon data are complemented by Sm–Nd analyses of the sedimentary and igneous whole rocks.The Yucatan Peninsula, which forms part of the Maya block of Central America, includes lower Paleozoic rocks in the Maya Mountains of Belize. The pre-Mesozoic geologic history of the Maya block is related to the evolution of the mid-Proterozoic basement in Mexico (Oaxaquia), other Paleozoic peri-Gondwanan terranes, and the Pan-African–Brasiliano type basement of Florida. The initial 176Hf/177Hf values of ~1.0Ga detrital zircon grains lie on a crustal evolution trajectory similar to that defined by older, ~1.2 to ~1.5Ga grains. This trajectory is consistent with those that would be produced by crustal reservoirs that separated from the depleted mantle between 1.70 and 2.05Ga. However, some grains have significantly less radiogenic 176Hf/177Hf(t), indicating influence from even older cratonic crust. Zircon grains from granitoids that intruded the Early Paleozoic sedimentary rocks of the Yucatan Peninsula yielded Late Silurian to Early Devonian (~415–400Ma) crystallization ages. More radiogenic Hf isotope ratios indicate anatexis of a crustal reservoir that is distinct from that of the Early Paleozoic sedimentary rocks. The Sm–Nd systematics of whole rock samples further support the results from the Hf isotopes in zircon grains. The data suggest a more continental provenance for the sedimentary rocks from Yucatan as compared to typical ca. 1.3 to 1.0Ga outcrops in southern and central Mexico. The results indicate that sediments were shed either from mid-Proterozoic complexes of NW Amazonia or from similar continental sequences that were thrust over Oaxaquia during the Grenville orogeny and subsequently eroded in the early Paleozoic. Integrating the data into a new model for early Paleozoic times, the southern Maya block is inferred to have formed during the opening of the Rheic Ocean along the western margin of Amazonia adjacent to Oaxaquia. The paucity of Ediacaran (Pan African–Brasiliano) signatures implies that before the Silurian, the southern Maya block evolved geographically separated from NW Yucatan and Florida, where Pan African–Brasiliano crystalline rocks have been reported.

Mafic and ultrapotassic rocks from the Canyon domain (central Grenville Province): geochemistry and tectonic implications

The Canyon domain and the Banded complex in the Manicouagan area of the Grenville Province preserve a record of magmatic activity from ∼1.4 to 1 Ga. This study focuses on 1.4–1.2 Ga mafic rocks and 1 Ga ultrapotassic dykes. Geochemistry and Sm–Nd isotopic signatures were used to constrain the origin of these rocks and evaluate the changing role of the mantle with time and tectonic setting from the late evolution of the Laurentian margin to the Grenvillian orogeny, in the Manicouagan area. The mafic rocks include layers inferred to represent flows, homogeneous bodies in mafic migmatite, and deformed dykes, all of which were recrystallized under granulite-facies conditions during the Grenvillian orogeny. In spite of the complexities inherent in these deformed and metamorphosed mafic rocks, we were able to recognize suites with distinctive geochemical and isotopic signatures. Integration of this data along with available ages is consistent with a 1.4 Ga continental arc cut by 1.2 Ga non-arc basalts derived from depleted asthenospheric mantle, with varied degrees of crustal contamination and inferred to represent magmatism in an extensional environment. The 1 Ga ultrapotassic dykes postdate the Grenvillian metamorphism. They are extremely enriched in incompatible elements, have negative Nb anomalies, relatively unradiogenic Sr-isotopic compositions (initial 87Sr/86Sr ~ 0.7040) and εNd –3 to –15. Some dykes have compositional characteristics consistent with derivation from the mantle, ruling out crustal contamination as a major process in their petrogenesis. The most likely source region for the ultrapotassic dykes is a metasomatized subcontinental lithospheric mantle, with thermal input from the asthenosphere in association with post-orogenic delamination.

THE PETROGENESIS OF THE GARNET MENZERITE-(Y) IN GRANULITE FACIES ROCKS OF THE PARRY SOUND DOMAIN, GRENVILLE PROVINCE, ONTARIO

The recent discovery of menzerite-(Y), a garnet containing essential Y, in Mesoproterozoic supracrustal granulites from the southwestern Grenville province has prompted an investigation into the petrogenesis of these rocks, and their Y + HREE-bearing phases, through a high-grade metamorphic cycle. A majority of samples from the supracrustal sequence have bulk compositions that are similar to that of average Proterozoic andesitic and basaltic volcanic rocks. Light REE concentrations in samples from the menzerite-(Y)-bearing locality are similar to that of average upper continental crust, whereas HREE concentrations are considerably higher. Extrapolating from the REE concentrations in detrital zircon cores and bulk Zr content, primary zircon could have contributed only ~6% Y and ~14% Yb to the whole-rock budget. Thus another phase, most likely xenotime, was the main carrier of Y + HREE in the precursor and the source of these constituents for menzerite-(Y). Microstructural relations indicate that menzerite-(Y) crystallized early, in equilibrium with oligoclase, ferrosilite, quartz, clinopyroxene, and iron oxides, then broke down during partial melting, and finally was overgrown by Y + HREE-enriched euhedral almandine as the rocks underwent further heating and burial. Phase-equilibrium modeling constrains conditions for menzerite-(Y) stability to pressures less than 8.5 kbar and temperatures between 550 and 780 °C (at pressures above 5 kbar) for the bulk composition of the host rock. The new constraints on P-T conditions of prograde and peak metamorphism, when combined with other recently acquired data, define a counterclockwise P-T-t path for the PSD that records post-depositional burial of magmatic arc rocks to lower crustal depths between ca. 1250 and 1160 Ma. Peak temperature conditions were followed by a period of isobaric cooling to subsolidus conditions (by ca. 1145 Ma) and subsequent nappe-style transport (at ca. 1100 Ma) onto the Laurentian margin during the Grenville orogeny.

Wilson Cycles, Tectonic Inheritance, and Rifting of the North American Gulf of Mexico Continental Margin

The tectonic evolution of the North American Gulf of Mexico continental margin is characterized by two Wilson cycles, i.e., repeated episodes of opening and closing of ocean basins along the same structural trend. This evolution includes (1) the Precambrian Grenville orogeny; (2) formation of a rift-transform margin during late Precambrian opening of the Iapetus Ocean; (3) the late Paleozoic Ouachita orogeny during assembly of Pangea; and (4) Mesozoic rifting during opening of the Gulf of Mexico. Unlike the Atlantic margins, where Wilson cycles were first recognized, breakup in the Gulf of Mexico did not initially focus within the orogen, but was instead accommodated within a diffuse region adjacent to the orogen. This variation in location of rifting is a consequence of variations in the prerift architecture of the orogens. The Appalachian-Caledonian orogeny involved substantial crustal shortening and formation of a thick crustal root. In contrast, the Ouachita orogeny resulted in minimal crustal shortening and thickening. In addition, rather than a crustal root, the Ouachita orogen was underlain by the lower plate of a relatively pristine Paleozoic subduction system that is characterized by a shallow mantle. A finite element model simulating extension on the margin demonstrates that this preexisting structure exerted fundamental controls on the style of Mesozoic rifting. The shallow mantle created a strong lithosphere beneath the orogen, causing extension to initiate adjacent to, rather than within, the orogen. On the Atlantic margins, the thick crustal root resulted in a weak lithosphere and initiation of extension within the interior of the orogen. Major features of the modern Gulf of Mexico margin, including the Interior Salt Basin, outboard unextended Wiggins arch, and an unusually broad region of extension beneath the coastal plain and continental shelf, are direct consequences of the prerift structure of the margin.

Mesoproterozoic syntectonic garnet within Belt Supergroup metamorphic tectonites: Evidence of Grenville-age metamorphism and deformation along northwest Laurentia

Northern Idaho contains Belt-Purcell Supergroup equivalent metamorphic tectonites that underwent two regional deformational and metamorphic events during the Mesoproterozoic. Garnet-bearing pelitic schists from the Snow Peak area of northern Idaho yield Lu–Hf garnet-whole rock ages of 1085±2Ma, 1198±79Ma, 1207±8Ma, 1255±28Ma, and 1314±2Ma. Garnet from one sample, collected from the Clarkia area, was micro-drilled to obtain separate core and rim material that produced ages of 1347±10Ma and 1102±47Ma. The core versus rim ages from the micro-drilled sample along with the textural and spatial evidence of the other Lu–Hf garnet ages indicate two metamorphic garnet growth events at ~1330Ma (M1) and ~1080Ma (M2) with the intermediate ages representing mixed ages. Some garnet likely nucleated and grew M1 garnet cores that were later overgrown by younger M2 garnet rims. Most garnet throughout the Clarkia and Snow Peak areas are syntectonic with a regional penetrative deformational fabric, preserved as a strong preferred orientation of metamorphic matrix minerals (e.g., muscovite and biotite). The syntectonic garnets are interpreted to represent one regional, coeval metamorphic and deformation event at ~1080Ma, which overlaps in time with the Grenville Orogeny. The older ~1330Ma ages may represent an extension of the East Kootenay Orogeny described in western Canada. These deformational and metamorphic events indicate that western Laurentia (North America) was tectonically active in the Mesoproterozoic and during the assembly of the supercontinent Rodinia.

Discussion: A unifying model for the Torridon Group (early Neoproterozoic), NW Scotland: Product of post-Grenvillian extensional collapse, by GE Williams and J Foden

The Torridonian succession in NW Scotland is a well-exposed sequence of sedimentary rocks that record the early Neoproterozoic geological evolution of Laurentia. An excellent review by Stewart (2002) summarised the observations on the three main 'Torridonian' units: the Stoer, Sleat and Torridon groups. The Torridon Group, deposited between 1000 – 950 Ma, was initially interpreted as a rift basin ( [Stewart, 1982] and [Williams, 2001]), but a number of studies ( [Rainbird et al., 2001], [Kinnaird et al., 2007] and [Krabbendam et al., 2008]) have now concluded that it represents a foreland basin positioned in front of the Grenville Orogen (1200 – 960 Ma). In a recent paper in Earth Science Reviews, Williams and Foden (2011) propose, instead, that the Torridon Group was deposited in an ‘extensional collapse basin’. We question this conclusion and take this opportunity to highlight aspects of their new interpretation that we find equivocal.

Spatio-temporal evolution of the Satpura Mountain Belt of India: A comparison with the Capricorn Orogen of Western Australia and implication for evolution of the supercontinent Columbia

Reconstruction of the Neoproterozoic supercontinent Rodinia shows near neighbour positions of the South Indian Cratons and Western Australian Cratons. These cratonic areas are characterized by extensive Paleoproterozoic tectonism. Detailed analysis of the spatio-temporal data of the Satpura Mountains of India indicates presence of at least three episodes of Proterozoic orogeny at ∼2100–1900Ma, ∼1850Ma and ∼1650Ma, and associated basin development and closing. A subdued imprint of the Grenville orogeny (∼950Ma) is also found in rock records of this Mountain Belt. The Capricorn Orogen of Western Australia also shows three episodes of orogeny: Opthalmian–Glenburgh Orogeny (2100–1950Ma), Capricorn Orogeny (∼1800Ma) and Mangaroon Orogeny (∼1650Ma), and basin opening and closing related to these tectonic movements. These broad similarities suggest their joint evolution possibly in a near neighbour position during Paleoproterozoic Era. In view of juxtaposition of the Western Australia along the east coast of India, at the position of the Eastern Ghats, during Archean, it is suggested that the breaking of this Archean megacraton at ∼2400Ma led to northward movement of the broken components and formation of the Satpura–Capricorn Orogen (at ∼2100 and ∼1800Ma) due to the collision of cratonic blocks with the pre-existing northern cratonic nuclei of India and Western Australia. This is also the time of formation of the supercontinent Columbia. A phase of basin opening followed the ∼1800Ma event, followed by another phase of collisional event at ∼1600Ma at the site of the Satpura–Capricorn Orogen. Subsequent evolutions of the Satpura and the Capricorn Orogens differ slightly, indicating separate evolutional history.

The structure of the Glenelg-Attadale Lewisianoid Inlier and its relationship to the Moine Thrust Zone

Synopsis The Glenelg-Attadale Lewisianoid Inlier lies adjacent to the Moine Thrust Zone. The inlier is surrounded and divided into Eastern and Western units by Moinian metasediments. The Eastern Unit was emplaced against the Western Unit in the Grenvillian Orogeny. Moinian sediments were deposited unconformably on both units in the Neoproterozoic. During the Late Neoproterozoic Knoydartian and Lower Palaeozoic Caledonian orogenies both the inlier and the overlying Moinian rocks were deformed and metamorphosed in the amphibolite facies. Deformation affected particularly the western and eastern margins of the inlier and a strip of infolded Moinian rocks between Eastern and Western units, with the formation of blastomylonites and isoclinal folding (F1). Above a major décollement in the Moine strip and along the western margin of the inlier, the rocks were folded by easterly plunging reclined (F2) folds. This phase of folding affected mainly the Lewisianoid rocks of the Eastern Unit and the adjacent Moine sediments of the Mountain Belt. Deformation was associated with the formation of a large-scale mushroom interference structure and the extrusion of the inlier northwestwards, towards the foreland, marked by a dominant ESE stretching lineation. In Avernish, reclined F2 folds are cut by randomly-oriented hornblende porphyroblasts, which are altered to biotite and smeared out in localized mylonitic shear zones similar to the mylonites in the Moine Thrust Zone. If these mylonitic shear zones were formed contemporaneously with the mylonites in the Thrust Zone, folding and thrusting events superimposed on the mylonites, must have occurred later than events in the Mountain Belt.