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To determine the emplacement age, petrogenesis, and geodynamic setting of the Xizhelimu diorite in Keyouzhongqi, Inner Mongolia of northeastern China, a detailed study of the petrography, geochronology, and whole-rock geochemistry has been conducted. Geological and petrographic studies show that the Xizhelimu diorite is zoned: the central lithofacies zone is composed of medium-fine-grained monzodiorite and quartz diorite, and the marginal lithofacies zone is fine-grained diorite. The zircon U–Pb dating results show that the ages of the central and marginal facies are 133.5 ± 1.9 and 133.4 ± 1.4 Ma, respectively. The whole-rock rare earth and trace element characteristics of the Xizhelimu diorite show an O-type adakite affinity. Combining the analysis of zircon Hf isotope composition (εHf(t) values of +7.7 to +10.0), the geochemical features of whole rock, and the results of partial melting modeling, we suggest that the parental magma of the Xizhelimu diorite was derived from the partial melting of altered oceanic crust mixing with subducting sediments at shallow depths. In the early stage of early Cretaceous, the Xizhelimu diorite originated in an extensional setting, mainly related to the closure of the western part of the Mongol–Okhotsk Ocean. The upwelling asthenospheric flow in this extensional setting induced partial melting of the paleo-oceanic crust to form the parental magma of the Xizhelimu diorite. [ABSTRACT FROM AUTHOR]

The 1.85 Ga Sudbury impact structure is considered a remnant of a peak-ring or multi-ring basin with an estimated original diameter of 150 to 200 km. The Offset Dikes are radial and concentric dikes around the Sudbury Igneous Complex (SIC) and are composed of the so-called inclusion-rich Quartz Diorite (IQD) and inclusion-poor Quartz Diorite (QD), and in some Offset Dikes, Metabreccia (MTBX). We carried out a detailed field and analytical investigation of MTBX from the Parkin Offset Dike in the North Range of the Sudbury structure. Our observations suggest that MTBX represents impact breccia that originally formed underneath the Main Mass of the SIC and that was subsequently contact-metamorphosed and entrained during the emplacement of the Parkin Offset Dike. The MTBX bears no resemblance to the QD and IQD in which it is hosted, but it does share many similarities with Footwall Breccia (FWBX), suggesting that the two shared a similar initial origin. A genetic relationship between MTBX and FWBX is also supported by whole rock geochemical analyses. [ABSTRACT FROM AUTHOR]

New geochemical and 40Ar/39Ar hornblende and biotite data from the Grenvillian Trenton Prong inlier provide the first constraints for the identification of lithotectonic units, their tectonic setting, and their metamorphic to post-metamorphic history. Gneissic tonalite, diorite, and gabbro compose the Colonial Lake Suite magmatic arc that developed along eastern Laurentia prior to 1.2 Ga. Spatially associated low- and high-TiO2 amphibolites were formed from island-arc basalt proximal to the arc front and mid-ocean ridge basalt-like basalt in a back-arc setting, respectively. Supracrustal paragneisses include meta-arkose derived from a continental sediment source of Laurentian affinity and metagraywacke and metapelite from an arc-like sediment source deposited in a back-arc basin, inboard of the Colonial Lake arc. The Assunpink Creek Granite was emplaced post-tectonically as small bodies of peraluminous syenogranite produced through partial melting of a subduction-modified felsic crustal source. Prograde mineral assemblages reached granulite- to amphibolite-facies metamorphic conditions during the Ottawan phase of the Grenvillian Orogeny. Hornblende 40Ar/39Ar ages of 935–923 Ma and a biotite age of 868 Ma record slow cooling in the northern part of the inlier following the metamorphic peak. Elsewhere in the inlier, biotite 40Ar/39Ar ages of 440 Ma and 377–341 Ma record partial to complete thermal resetting or new growth during the Taconian and Acadian orogens. The results of this study are consistent with the Trenton Prong being the down-dropped continuation of the Grenvillian New Jersey Highlands on the hanging wall of a major detachment fault. The Trenton Prong therefore correlates to other central and northern Appalachian Grenvillian inliers and to parts of the Grenville Province proper. [ABSTRACT FROM AUTHOR]

The recently discovered Dongyang low-sulphidation epithermal Au deposit is located in Fujian Province. The Au mineralization hosted in rhyolite porphyry and the Lower Jurassic Nayuan Formation continental volcanic rocks is considered to be related to intermediate–acidic intrusions (rhyolite porphyry, quartz diorite porphyry, and dacite porphyry). The zircon U–Pb ages of these samples are 160.3 ± 0.8, 156.6 ± 0.8, and 154.1 ± 0.6 Ma, respectively, and the sulphide Rb–Sr isotope isochron age is 152.4 ± 1.7 Ma, indicating a temporal link between porphyry emplacement and Au mineralization. Porphyries are enriched in large-ion lithophile elements and light rare earth elements and slightly depleted in heavy rare earth elements and high field strength elements, belong to the high-potassium calc-alkaline or shoshonitic series, and show characteristics of volcanic arcs or active continental margins. Moreover, the εHf(t) values of the porphyries range from −11.6 to −7.6, −11.2 to −5.4, and −9.8 to −4.6, respectively. The 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb values of the ore minerals are from 38.417 to 38.406, 15.603, and from 18.179 to 18.175, respectively. Pb and Hf isotopes indicate that the intermediate–acidic magma related to mineralization originated from Mesoproterozoic lower crust. Consequently, these data favour an intermediate–acidic magma origin for Au mineralization in the Dongyang deposit. Integrating new and published data on the tectonic evolution, we suggest that the rhyolite porphyry, quartz diorite porphyry, dacite porphyry, and associated Au mineralization in the Dongyang deposit formed at 160–152 Ma in an extensional environment related to subduction of the Palaeo-Pacific Plate beneath the South China block. [ABSTRACT FROM AUTHOR]

Evidence for exotic terranes in the central Appalachian Piedmont Province is fragmented between central Virginia, northern Maryland, and southeastern Pennsylvania. Here we present laser ablation-inductively coupled plasma mass spectrometry data from detrital zircon that support the presence of an exotic terrane in this region. U–Pb dating of detrital zircon from new samples of the Storck quartzite (central Virginia) and the Hoods Mill rocks (northern Maryland) confirms the presence of a major peak at ca. 630–610 Ma in these units. These ages are consistent with derivation from Gondwana, but not Ediacaran Laurentia. Further, modern εHf values of five of the ca. 670–580 Ma grains in these samples are inconsistent with derivation from the few plutons of this age in Ediacaran Laurentia. The Loch Raven Schist and a metasedimentary xenolith in the Wilmington Complex contain a smaller proportion of ca. 670–580 Ma grains than the Storck quartzite and the Hoods Mill rocks, but more such grains than in sediment derived from Ediacaran Laurentia, so we tentatively conclude that these two units also received sediment from Gondwana. Detrital zircon ages from the Piney Run Formation, Pleasant Grove Schist, Prettyboy Schist, and Wissahickon Formation allow sediment provenance solely in Ediacaran Laurentia. We also present new zircon spot U–Pb and Lu–Hf isotopic data from western Newfoundland plutons for comparison with these types of data from the detrital zircon. Intrusion ages of the Steel Mountain Anorthosite, Disappointment Hill Tonalite, and Round Pond Granite are 608 ± 12, 600 ± 8, and 590 ± 9 Ma, respectively. None of these units was derived entirely from the depleted mantle. [ABSTRACT FROM AUTHOR]

The Whistle offset dike is a radial dike linked to the ca. 1850 Ma Sudbury Igneous Complex (SIC) in Sudbury, Ontario. It is linked through the funnel-shaped Whistle embayment which formed as a depression along the floor of a superheated impact melt sheet represented by the present-day SIC. The dike consists of metabreccia, quartz diorite, and inclusion-bearing quartz diorite, emplaced in older Archean gneisses and intrusions. Metabreccia has a fine-grained, igneous-textured matrix surrounding Archean basement clasts. It overlaps in major-element compositions (MgO, CaO, SiO2) with the two quartz diorites. The three have similar chondrite-normalized rare-earth element patterns characterized by pronounced negative slopes, i.e., (La/Yb)N = 17-29, and a weak negative Eu anomaly. This suggests that they originated from the same source melts. Metabreccia has higher high-field-strength element (HFSE) ratios, i.e., Gd/Yb, Ce/Yb, Zr/Y, and lower model initial 207Pb/204Pb isotopic ratios than the two quartz diorites, suggesting that it assimilated more Archean rocks with higher HFSE ratios and lower 207Pb/204Pb ratios. It formed as SIC melts mixed with melted basement rocks at the SIC-basement contact. This produced a strongly contaminated, inclusion-bearing melt, which drained downward into an offset structure or fault, entrained more fragments plucked from the walls of the fault, and crystallized as metabreccia. Less contaminated quartz diorite was then emplaced in the still-viscous metabreccia during reactivation of the fault caused by isostatic readjustment of the gravitationally unstable crater. More melting, assimilation of clasts, and mixing of SIC melts at the SIC-basement contact produced inclusion-bearing quartz dioritic melts that were emplaced last along the Whistle offset fault. [ABSTRACT FROM AUTHOR]

The Neoarchean Pukaskwa batholith consists of pre-, syn-, and post-tectonic phases emplaced over an interval of 50 million years. Pre-tectonic phases are broadly synvolcanic and have a high-Al tonalite-trondhjemite-granodiorite (TTG) affinity interpreted to reflect derivation by partial melting of basaltic crust at lower crustal or upper mantle depths. Minor syn-tectonic phases slightly post-date volcanism and have geochemical characteristics suggesting some involvement or interaction with an ultramafic (mantle) source component. Magmatic emplacement of pre- and syn-tectonic phases occurred in the midcrust at paleopressures of 550-600 MPa and these components of the batholith are thought to be representative of the midcrust underlying greenstone belts during their development. Subsequent to emplacement of the syntectonic phases, and likely at approximately 2680 Ma, the Pukaskwa batholith was uplifted as a structural dome relative to flanking greenstone belts synchronously with ongoing regional sinistral transpressive deformation. The driving force for vertical tectonism is interpreted to be density inversion (Rayleigh-Taylor-type instabilities) involving denser greenstone belts and underlying felsic plutonic crust. The trigger for initiation of this process is interpreted to be an abrupt change in the rheology of the midcrust attributed to introduction of heat from the mantle attendant with slab breakoff or lithospheric delamination following the cessation of subduction. This process also led to partial melting of the intermediate to felsic midcrust generating post-tectonic granitic phases at approximately 2667 Ma. We propose that late density inversion-driven vertical tectonics is an inevitable consequence of horizontal (plate) tectonic processes associated with greenstone belt development within the Superior Province. Le batholite néoarchéen de Pukaskwa comprend des phases pré-, syn- et post-tectoniques mises en place durant une période de 50 Ma. Les phases pré-tectoniques sont largement synvolcaniques et elles ont une affinité Al et TTG (« tonalite-trondhjemite-granodiorite ») élevée qui est interprétée comme étant le reflet de la dérivation par fusion partielle d'une croûte basaltique à des profondeurs de croûte inférieure ou de manteau supérieur. Des phases syntectoniques mineures sont légèrement plus tardives que le volcanisme et elles ont des caractéristiques géochimiques qui suggèrent une certaine implication ou interaction avec une composante de source ultramafique (manteau). La mise en place magmatique des phases pré- et syntectoniques a eu lieu dans le milieu de la croûte à des paléopressions de 550-600 MPa et ces composantes du batholite seraient représentatives des ceintures de roches vertes sous-jacentes, au centre de la croûte durant leur développement. Après la mise en place des phases syntectoniques, et probablement vers 2680 Ma, le batholite de Pukaskwa a été soulevé en dôme structural par rapport aux ceintures de roches vertes de part et d'autre, et ce, de manière synchrone avec la déformation transpressive régionale senestre en cours. La force motrice derrière le tectonisme vertical serait une inversion de densité (instabilités de type Rayleigh-Taylor) impliquant les ceintures de roches vertes plus denses et la croûte felsique plutonique sous-jacente. Le déclencheur pour l'initiation de ce processus est interprété comme étant un changement abrupt dans la rhéologie de la croûte moyenne attribué à l'introduction de chaleur du manteau auxiliaire à la rupture de la dalle ou à la délamination lithosphérique suivant la cessation de la subduction. Ce processus a aussi mené à une fusion partielle de la croûte moyenne intermédiaire à felsique générant des phases granitiques post-tectoniques vers 2667 Ma. Nous proposons que la tectonique verticale poussée par l'inversion de densité tardive soit une conséquence inévitable des processus tectoniques horizontaux (plaques) associés au développement de ceintures vertes dans la Province du Supérieur. [ABSTRACT FROM AUTHOR]

The Boothia mainland region of the north-central Rae domain is underlain by remnants of a Neoarchean volcano sedimentary sequence dismembered by two regionally extensive Neoarchean high-potassium granitoid suites with rare occurrences of a structurally interleaved, Paleoproterozoic sedimentary cover sequence. The granitoids and their gneissic equivalents are dominated by variably deformed and metamorphosed I-type, metaluminous, polyphase, commonly porphyritic to augen, biotite ±hornblende monzogranite, and subordinate granodiorite, with rare tonalite. New geochronological results, the first for this area, demonstrate that the widespread Neoarchean granitoid plutonism is dominantly 2.61-2.59 Ga, with a less prominent 2.66 Ga plutonic event. The age of zircon recrystallization suggests that ca. 2.60 Ga Archean metamorphism and fabric development (S1) affected the 2.66 Ga plutons prior to or contemporaneously with intrusion of the voluminous ca. 2.6 Ga suite. εNd(t) for the ca. 2.61-2.59 Ga suite range from 1.4 to -1.9, overlapping with the ca. 2.66 Ga suite that range from 1.4 to 1.5. The Nd isotopic data, coupled with the presence of inherited ca. 2.65, 2.70, and 2.85-2.90 Ga zircon, suggests recycling of older, Neoarchean to Mesoarchean crust in the formation of these suites. Metaplutonic rocks preserve Paleoproterozoic deformation (F4 and F5) and amphibolite-facies metamorphism, sporadically recorded in zircon rims that formed at 1.81 Ga. This event strongly reoriented the Neoarchean fabrics in metaplutonic rocks, generally without the development of a new coaxial Paleoproterozoic fabric, and we attribute this strain and metamorphism to the Hudsonian orogeny. La région terrestre Boothia du domaine Rae centre-nord repose sur des vestiges d'une séquence volcano-sédimentaire datant du Néoarchéen; cette séquence est démembrée par deux suites granitoïdes d'échelle régionale, à teneur élevée en potassium et possédant de rares occurrences d'une séquence structurellement intercalée d'une couverture sédimentaire datant du Paléoprotérozoïque. Les granitoïdes et leurs équivalents gneissiques sont dominés par un monzogranite à déformation variable, métamorphisé, de type I, métalumineux, polyphasé, souvent porphyrique à oeillé et à biotite hornblende avec une granodiorite subordonnée et une rare tonalite. De nouveaux résultats géochronologiques, les premiers pour ce secteur, démontrent que le plutonisme granitoïde néoarchéen étendu date surtout de 2,61 à 2,59 Ga, avec un événement plutonique moins important vers 2,66 Ga. L'âge de la recristallisation du zircon suggère que, vers 2,60 Ga, le métamorphisme archéen et le développement de la fabrique (S1) aient affecté les plutons âgés de 2,66 Ga avant ou en même temps que l'intrusion de la volumineuse suite vers 2,6 Ga. Les valeurs εNd(t) pour la suite datant de ~ 2,61 à 2,59 Ga varient de 1,4 à -1,9, chevauchant la suite datant ~2,66 Ga, dont les valeurs εNd(t) varient de 1,4 à 1,5. Les données d'isotopes Nd, jumelées à la présence de zircons hérités dont les âges sont ~2,65, 2,70 et 2,85-2,90 Ga, suggèrent un recyclage de l'ancienne croûte néoarchéenne à mésoarchéenne pour la formation de ces suites. Des roches métaplutoniques ont préservé la déformation paléoprotérozoïque (F4 et F5) ainsi que le métamorphisme au faciès des amphibolites, retrouvé sporadiquement dans les bordures des zircons qui se sont formées à 1,81 Ga. Cet événement a grandement réorienté les fabriques néoarchéennes dans les roches métaplutoniques, généralement sans le développement d'une nouvelle fabrique coaxiale paléoprotérozoïque; nous attribuons cette déformation et ce métamorphisme à l'orogenèse hudsonienne. [ABSTRACT FROM AUTHOR]

The 2.95–2.82 Ga quartzofeldspathic gneisses and granitoids in the Bighorn, western Owl Creek, and northeastern Wind River uplifts in the central Wyoming Province include low-K tonalite–trondhjemite–granodiorite (TTG) and high-K granodiorite–granite (GG) rocks. Both types of granitoids were intruded contemporaneously, although TTGs are more abundant in the older gneisses. The TTG suite consists of calcic to marginally calc-alkalic rocks that straddle the boundaries between metaluminous and peraluminous and between ferroan and magnesian compositions. Rare-earth element (REE) patterns of these rocks may be highly fractionated with low heavy rare-earth element (HREE) contents and modest to absent Eu anomalies but may also be less strongly HREE depleted. These rocks do not represent first-generation continental crust: most have unradiogenic Nd and radiogenic 207Pb/204Pb isotopic compositions that require the incorporation of isotopically evolved sources. The GG suite has compositions that are transitional between Archean TTG and modern, continental margin calc-alkalic rocks. The GG suite is characterized by higher alkali contents relative to CaO than the TTG suite and higher K/Na ratios but exhibits a similar range in REE patterns. The Nd, Sr, and Pb isotopic compositions of the GG suite are slightly less variable but lie within the range of those of the TTG suite. We interpret them as having a source similar to that of the TTG, perhaps forming by partial melting of preexisting TTG. The shift from TTG-dominated to GG-dominated continental crust was a gradual transition that took place over several hundred million years. Clearly subduction-related calc-alkalic magmatism is not recognized in the Wyoming Province prior to 2.67 Ga. [ABSTRACT FROM AUTHOR]

Metapelites of the parautochthonous Gagnon terrane at the footwall of the high-pressure (high-P) belt in the Manicouagan area (central Grenville Province) preserve an impressive textural record of partial melting reactions, mainly in polymineralic inclusions within garnet. The dominant textures were developed within the pressure–temperature (P–T) field of the continuous dehydration melting reaction biotite + kyanite (or sillimanite) + plagioclase + quartz = garnet + K-feldspar + melt, with sillimanite instead of kyanite in the southern part of the footwall. Inferred P–T paths have a hair-pin form in the range of 750–850 °C and 1000–1500 MPa for the kyanite-bearing rocks. Monazite crystallization ages are consistently late Grenvillian, either 995 or 985 Ma, and one sample contains monazite of both ages. Two of these samples also contain inherited monazite with ages of 1738 ± 5 and 1719 ± 30 Ma, indicative of an earlier metamorphic event. Tonalite and diorite from the same area yield Archean zircon crystallization ages and titanite ages of 961 ± 3 and 956 ± 4 Ma, the youngest in the Manicouagan region. The late Grenvillian metamorphism was of comparable intensity but ~50 Ma younger than in the overlying high-P belt in this area and therefore seems to be unrelated to the emplacement of the latter over the Parautochthonous Belt, as previously suggested. Rather, this younger metamorphism suggests a reactivation of the footwall by underthrusting of the Gagnon terrane during the waning stages of convergence, and a link with a major coeval post-tectonic thermal event farther south in the hinterland. [ABSTRACT FROM AUTHOR]

Pre-Silurian bedrock units played key roles in the early Paleozoic history of the Maine–Quebec Appalachians. These units represent peri-Laurentian material whose collision with the craton deformed the Neoproteozoic passive margin and initiated the Appalachian mountain-building cycle. We present new field, petrological, geochronological, and geochemical data to support the following interpretations related to these units. (1) The Boil Mountain Complex and Jim Pond Formation do not represent part of a coherent ophiolite. (2) Gabbro and tonalite of the Boil Mountain Complex intruded the Chain Lakes massif at ca. 477 Ma. (3) The Skinner pluton, an arc-related granodiorite, intruded the Chain Lakes massif at ca. 472 Ma. (4) The Attean pluton, with a reconfirmed age of ca. 443 Ma, is unrelated to Early Ordovician orogenesis. (5) The most likely timing for the juxtaposition of the Jim Pond Formation and the Boil Mountain Complex was during regional Devonian deformation. These interpretations suggest that the Boundary Mountains were once part of a series of arcs extending at least from central New England through Newfoundland. [ABSTRACT FROM AUTHOR]

In southeastern Alaska, granodiorite–tonalite plutons of the Admiralty–Revillagigedo belt intruded the Jurassic–Cretaceous Gravina belt along the eastern side of the Alexander terrane around 90 Ma. These plutons postdate some deformation related to a major contractional event between the previously amalgamated Wrangellia and Alexander terranes and the previously accreted terranes of the North American margin. We studied the aureole mineral assemblages of these plutons near Petersburg, Alaska, determined pressure and temperature of equilibration, and examined structures that developed within and adjacent to these plutons. Parallelism of magmatic and submagmatic fabrics with fabrics in the country rock indicates synchroneity of pluton emplacement with regional deformation and suggests that magma transport to higher crustal levels was assisted by regional deformation. Replacement of andalusite by kyanite or sillimanite indicates crustal thickening soon after pluton emplacement. Regional structural analysis indicates the crustal thickening was accomplished by thrust burial. Thermobarometric analyses indicate the aureoles reached near-peak temperatures of 525 to 635 °C at pressures of 570 to 630 MPa. Consideration of the rate of thermal decay of the aureoles suggests that burial was rapid and occurred at rates around 5 to 8 mm/year. Structural observations indicate there was contractional deformation before, during, and after emplacement of the 90-Ma plutons. Initial exhumation of the Admiralty–Revillagigedo belt in the Petersburg area may have occurred along a thrust west of the pluton belt within the Gravina belt. [ABSTRACT FROM AUTHOR]