Devonian Plutons Research Articles

Devonian plutons in the eastern Meguma terrane, Nova Scotia, Canada: zircon U–Pb, Lu–Hf, and O isotopic compositions, age, and petrogenetic implications

Abundant granitic plutons intruded the eastern Meguma terrane of Nova Scotia in the middle- to late Devonian. Less voluminous diorite–tonalite and gabbro intrusions are associated with the granitic plutons along the northern margin of the terrane adjacent to the Cobequid–Chedabucto fault zone. All plutons contain metasedimentary xenoliths, and the mafic plutons show magma mingling textures with their adjacent granitic plutons. New U–Pb zircon data from autocrystic zircon in 13 samples indicate coeval emplacement of mafic and granitic plutons between ca. 382 and 368 Ma. However, the zircon grains contain numerous inherited domains that range in age from Palaeoproterozoic to Devonian. These inherited ages correspond to detrital zircon U–Pb dates from the Cambrian to Ordovician metasedimentary host rocks. Zircon oxygen isotopic data (δ18O) are between +7.4 ± 0.2‰ and +9.3 ± 0.3‰ indicating significant involvement of the crust as the magma source or contaminant. If the high δ18Ozrn values are a result of contamination, the contaminant was likely the metasedimentary rocks of the Meguma terrane. Hafnium isotopic data from autocrystic zircon have εHf( t) between −6.0 ± 1.5 and +2.1 ± 2.5. The new zircon U–Pb, O, and Hf isotopic data from plutons in the eastern Meguma terrane are indistinguishable from published data from the South Mountain Batholith. The data suggest that Devonian magmatism in the Meguma terrane post-dated the main orogenic event that caused folding and regional metamorphism and involved the same magma source and/or contaminants throughout the terrane.

Evolution of Silurian to Devonian magmatism associated with the Acadian orogenic cycle in eastern and southern Newfoundland Appalachians: Evidence for a three-stage evolution characterized by episodic hinterland- and foreland-directed migration of granitoid magmatism

The migration and character of magmatism over time can provide important insights into the tectonic evolution of an orogen. We present evidence for three separate stages of compositionally distinct granitoid magmatism associated with the Acadian orogenic cycle in the eastern and southern Newfoundland Appalachians. The interpretations are based on new zircon U-Pb ages, geochemical data, and Sr-Nd-Hf-O isotopic data for 18 samples from 15 Silurian and Devonian granitoid plutons, combined with previously published data. The three stages outline hinterland- and foreland-directed migration trends and represent subduction (435−420 Ma), syncollision (415−405 Ma), and postcollision (395−370 Ma) settings in the Acadian orogenic cycle. The Silurian plutons (435−420 Ma) of the first stage consist mainly of quartz diorite, tonalite, granodiorite, monzogranite, and syenogranite, with high-K calc-alkaline and enriched Sr-Nd-Hf-O isotopic compositions (e.g., εNd[t] = −5 to −2; εHf[t] = −3 to −1; δ18O = +6‰ to +8‰). They are interpreted to record the subduction of oceanic lithosphere of the Acadian seaway that separated the leading edge of composite Laurentia, represented by the Gander margin, and Avalonia. Early Devonian plutons (415−405 Ma) of the second stage contain more voluminous monzogranite and syenogranite; they have calc-alkaline to high-K calc-alkaline features, adakite-like compositions, and more depleted Sr-Nd-Hf-O isotopic compositions (e.g., εNd[t] = −6 to 0; εHf[t] = +1 to +3; δ18O = +5‰ to +6‰). Plutons of this stage occur mostly to the northwest of the Silurian granitoids, indicating a regional-scale northwestward (hinterland-directed) migration of magmatism with a rate of >9 km/m.y. The migration is interpreted to have been related to the progressive shallow underthrusting of Avalonia beneath the Gander margin (composite Laurentia) at least as far as 90 km inboard. The Middle to Late Devonian plutons of the third stage (395−370 Ma) consist mainly of monzogranite, syenogranite, and alkali-feldspar granite, which are silica- and alkali-rich granites with large negative Eu anomalies. These rocks are concentrated along both sides of the Dover−Hermitage Bay fault zone, which represents the boundary between Avalonia and composite Laurentia, to the southeast of the Silurian and Early Devonian igneous rocks. This stage of magmatism represents a foreland-directed (retreating) migration. The Early Devonian and Middle to Late Devonian episodes of magmatism were separated by a gap between 405 Ma and 395 Ma and recorded an evolution from (high-K) calc-alkaline to alkaline compositions, ascribed to partial delamination of Avalonian lithospheric mantle in a postcollisional setting.

Tectonic Transition from Passive to Active Continental Margin of Nenjiang Ocean: Insight from the Middle Devonian-Early Carboniferous Granitic Rocks in Northern Great Xing’an Range, NE China

Northeast China occupies the majority of the eastern Central Asian Orogenic Belt, which mainly consists of continental blocks and accretionary terranes. The Devonian was a tectonic quiet period in the NE China region due to a lack of tectono-magmatism, but the tectonic background of this period has been unclear, especially for the Hegenshan-Heihe Suture between Xing’an and Songliao accretionary terranes, which represents the Paleozoic Nenjiang Ocean (a branch ocean of the eastern Paleo-Asian Ocean). Here we report granitic rocks from the Woluohe area, Northern Great Xing’an Range, NE China, to constrain the tectonic process of the transition from the Devonian quiet period to the Early Carboniferous active tectonic period. Three granitic rock samples produce zircon U-Pb ages of 389 Ma, 368 Ma, and 351 Ma, belonging to the Middle and Late Devonian and Early Carboniferous, respectively. They have high Si, Al, K, and Na contents, but with low Mg, Fe, and Ti contents, together with positive Hf isotopic features and low molar Al2O3/(MgO+FeOT) ratios, we suggest that they were derived from partial melting of lower crustal igneous rocks. Meanwhile, the narrow major element variation at odd with the fractionation process and their negative Nb and Ta anomalies imply the obvious contribution of crustal. Comprehensive tectonic setting analysis shows all samples are in calc-alkali magmatic series with rightward fractionated REE and trace element patterns that are enriched in LREE and LILE and depleted in HREE and HFS, indicating a subduction-related magmatic arc setting. Considering the regional tectonic setting and the small scale of the Devonian plutons, we suggest a limited subduction tectonic setting during the quiet period of the northern Great Xing’an Range, which might indicate the beginning of an initial northwestward subduction of the Nenjiang Oceanic lithosphere beneath the Xing’an Accretionary Terrane in the Middle Devonian, accelerated subduction in the Late Devonian, and bidirectional subduction in the Early Carboniferous.

Dating initial crystallisation of some Devonian plutons in central Victoria and geological implications

Rather than being inherited as detrital grains, many of the rounded and/or embayed cores in zircon crystals from granitic rocks are probably antecrysts, formed early in the magmatic systems. Using laser-ablation, sector-field ICP-MS, we obtained internally consistent weighted-mean concordia ages of 375.3 ± 2.5 Ma and 376.9 ± 2.6 for early crystallisation of two samples from the previously undated Mount Disappointment pluton in the Melbourne Zone. Within uncertainty, our date of 376.4 ± 2.4 Ma for the Baringhup pluton of the Harcourt batholith is the same as the published and our new date for the Mount Alexander pluton, in the same batholith. Using the same techniques, we produced an improved date of 370.9 ± 6.5 Ma for the I-type Ercildoun Granite in the neighbouring Bendigo Zone of the Lachlan Fold Belt. We also obtained dates that confirm published ages for the Mount Bute Granite in the Stawell Zone, the Tynong pluton of the Tynong batholith in the Melbourne Zone and the Oberon pluton of the Wilsons Promontory batholith in the Bassian Zone. These dates confirm the Late Devonian (Fransian) age of most of this widespread plutonism, as well as the reported Emsian age of the Wilsons Promontory batholith. However, part of what has been mapped as the Mount Wombat pluton of the Strathbogie batholith could be significantly older than the rest. KEY POINTS Many rounded or embayed cores in zircon crystals are antecrysts rather than inherited detrital grains. The Mount Disappointment pluton is confirmed as Late Devonian in age, at 375.2 ± 2.5 Ma. At 378.2 ± 2.0 Ma, the Baringhup pluton of the Harcourt batholith is the same age as the Mount Alexander pluton, in the same batholith. Part of the Mount Wombat pluton in the Strathbogie batholith may be older than the rest of the pluton.

P–T–t Path of Unusual Garnet–Kyanite–Staurolite– Amphibole Schists, Ellesmere Island, Canada—Quartz Inclusion in Garnet Barometry and Monazite Petrochronology

Abstract Garnet–kyanite–staurolite assemblages with large, late porphyroblasts of amphibole form garbenschists in Ordovician volcaniclastic rocks lying immediately south of the Pearya terrane on northernmost Ellesmere Island, Canada. The schist, which together with carbonate olistoliths makes up the Petersen Bay Assemblage (PBA), displays a series of parallel isograds that mark an increase in metamorphic grade over a distance of 10 km towards the contact with Pearya; however, a steep, brittle Cenozoic strike-slip fault with an unknown amount displacement disturbs the earlier accretionary relationship. The late amphibole growth, probably due to fluid ingress, is clear evidence of disequilibrium conditions in the garbenschist. In order to recover the P–T history of the schists, we construct isochemical phase equilibrium models for a nearby garnet–mica schist that escaped the fluid event and compare the results to quartz inclusion in garnet (QuiG) barometry for a garbenschist and the metapelitic garnet schist. Quartz inclusions are confined to garnet cores and the QuiG results, combined with Ti-in-biotite and garnet–biotite thermometry, delineate a prograde path from 480 to 600°C and 0.7 to 0.9 GPa. This path agrees with growth zoning in garnet deduced from X-ray maps of the spessartine component in garnet. The peak conditions obtained from pseudosection modelling using effective bulk composition and the intersection of garnet rim with matrix biotite and white mica isopleths in the metapelite are 665°C at ≤0.85 GPa. Three generations of monazite (I, II and III) were identified by textural characterization, geochemical composition (REE and Y concentrations) and U–Pb ages measured by ion microprobe. Monazite I occurs in the matrix and as inclusions in garnet rims and grew at peak P–T conditions at 397 ± 2 Ma (2σ) from the breakdown of allanite. Monazite II forms overgrowths on matrix Monazite I grains that are oriented parallel to the main schistosity and yield ages of 385 ± 2 Ma. Monazite III, found only in the garbenschist, is 374 ± 6 Ma, which is interpreted as the time of amphibole growth during fluid infiltration at lower temperature and pressure on a clockwise P–T path that remained in the kyanite stability field. These results point to a relatively short (≈12 Myr) Barrovian metamorphic event that affected the schists of the PBA. An obvious heat source is lacking in the adjacent Pearya terrane, but we speculate it was large Devonian plutons—similar to the 390 ± 10 Ma Cape Woods granite located 40 km across strike from the fault—that have been excised by strike-slip. Arc fragments that are correlative to the PBA are low grade; they never saw the heat and were not directly involved in Pearya accretion.

Protracted intra- and inter-pluton magmatism during the Acadian orogeny: evidence from new LA-ICP-MS U-Pb zircon ages from northwestern Maine, USA

Devonian granitoid plutons comprise a major part of the bedrock of northwestern Maine representing the magmatic expression of the Acadian orogeny in this part of the northern Appalachian orogen. They are petrographically diverse with minerals characteristic of both I- and S-type granites, in some cases within the same intrusion, and some are compositionally zoned. New LA-ICP-MS ages presented here elucidate the timing and duration of this magmatism. The earliest phase of granitoid magmatism began around 410–405 Ma with the emplacement of the Flagstaff Lake Igneous Complex, and the presence of contemporaneous mafic rocks suggests that mantle-derived magmas were also produced at this time. Late Devonian ages, ca. 365 Ma, for many intrusions, such as the Chain of Ponds and Songo plutons, reveal that magmatism continued for 45 million years during which compositionally diverse I- and S-type magmas were produced. In addition, there is evidence that intrusive activity was prolonged within some plutons, for example the Rome-Norridgewock pluton and the Mooselookmeguntic Igneous Complex, with 10–15 myr between intrusive units. The new ages suggest a break in magmatism between 400 Ma and 390 Ma apparently separating Acadian magmatism into early and late pulses. The production of lower crustal I-type magmas appears to have been concentrated later, ca. 380–365 Ma, although several S-type granitoids were also emplaced during this period. These Late Devonian plutons display abundant zircon inheritance with ages around 385 Ma, which suggests that the crust was experiencing enhanced thermal perturbations during this extended timeframe. The new data for granitoid plutons in northwestern Maine are consistent with tectonic models for other parts of Ganderia which propose initial flat slab subduction followed by slab breakoff and delamination.

Petrology and geochronology of the San Martín de los Andes batholith: Insights into the Devonian magmatism of the North Patagonian Andes

Evidence is growing that a belt of Devonian plutonism is present in the Northern Patagonian segment of the present-day Andes. This plutonism formed along the paleo-Pacific margin of the Gondwana supercontinent. New whole rock chemistry and SHRIMP U–Pb zircon data for plutonic rocks of the San Martín de los Andes batholith (southwestern Argentina) provide insights into the magmatic and tectonic evolution of the southwestern Gondwana margin during the middle Palaeozoic. The San Martín de los Andes batholith was formed between 390 and 374 Ma and intruded into a Lower Paleozoic metamorphic basement that had been previously affected by Late Ordovician-middle Silurian deformation. Most of the batholith is composed of metaluminous, calc-alkaline tonalites and granodiorites, with subordinate volume of peraluminous garnet-bearing granites. Discrete and disperse gabbroic and dioritic rocks hosted in tonalites and granodiorites occur as either microgranular enclaves or syn-magmatic dykes. Tonalitic, granodioritic and granitic rocks develop transitional contacts among them revealing coetaneous building up throughout the plutonic suite, but when interpreting their chemical variability, they are not strictly comagmatic rocks. Collectively, field, petrographic, and geochemical data of the San Martín de los Andes batholith are interpreted to reflect concurrence of crystal fractionation, crustal assimilation and magma mixing in driving petrological diversification. Yet, gabbroic and dioritic rocks in the San Martín de los Andes batholith display geochemical characteristics of calc-alkaline primitive basalts that concur with what is being hypothesized to be a magmatic arc zone for the Devonian period. This study discusses how to interpret the evolution of the San Martín de Los Andes batholith together with other geodynamic models such as the Chaitenia magmatic arc.

Zircon and monazite geochronology in the Palmer zone of transpression, south-central New England, USA: Constraints on timing of deformation, high-grade metamorphism, and lithospheric foundering during late Paleozoic oblique collision in the Northern Appalachian orogen

Abstract Middle to late Paleozoic high-angle (45°) oblique convergence between Laurentia and composite Avalon terranes resulted in crustal shortening of ∼5:1 across the Central Maine and Bronson Hill zones of the southern New England Appalachians (USA). The Palmer zone of transpression illustrates the midcrustal expression of magmatism, metamorphism, and ductile deformation that developed in response to oblique convergence in the apparent absence of subduction. Secondary ion mass spectrometry zircon U-Pb and monazite Th-Pb ages, supplemented by zircon laser-ablation–inductively coupled plasma–single-collector mass spectrometry U-Pb ages, expand the geochronology constraining the evolution of the Palmer zone of transpression system and potential models for transcurrent collisional tectonics. Ordovician, Silurian, and Early Devonian plutons, and regional high-grade metapelitic country rocks that comprise the pre-transpressional crustal infrastructure are the same age as lithologic equivalents to the north and northeast along the orogen that did not experience high-angle oblique convergence. All plutons within the Palmer zone of transpression and adjoining areas were deformed (flattened, boudinaged, attenuated, folded, and/or ductilely sheared) by the regional transpression system. The most widespread magmatic event in the study area, which is not observed elsewhere in the Northern Appalachians, is intrusion of a diorite-tonalite suite at 370–360 Ma, the oldest rocks of which contain granulite-facies mineral assemblages and all of which are deformed. Leucopegmatites that intrude pelitic paragneisses, which likely formed in response to high-grade metamorphism and which are all deformed, are 370–355 Ma in age. All metapelitic paragneisses contain foliations and lineations that are synkinematic with retrograde garnet + K-feldspar → sillimanite + biotite assemblages and fabrics that formed during vertical and lateral crustal escape in response to extreme shortening. Contemporaneous diorite-tonalite magmatism and regional high-grade metamorphism are interpreted to reflect regional-scale heating of the crust preceding inception of the transpressional system. Transpressional deformation of both plutons and paragneisses indicates shortening commenced after crustal thermal conditions peaked (ca. 355 Ma). Monazite in paragneiss, the formation of which in most samples is linked texturally to formation of transpressional fabrics, yielded a continuum of Th-Pb ages of ca. 360–330 Ma, indicating transpression continued after peak thermal conditions. The extreme crustal shortening of the Bronson Hill and Central Maine zones from Maine to southern New England, pre-transpressional magmatism, and high-grade metamorphism overprint an Acadian (ca. 400–370 Ma) magmatic-metamorphic infrastructure that occurred in the absence of subduction. The transpressional system exhibits all predicted thermal, magmatic, deformation, metamorphic, and exhumation/erosion characteristics of mantle lithospheric foundering (delamination, detachment, or drip) in response to extreme lithospheric shortening and vertical stretching at ca. 375–370 Ma, leading to advection of heat to the lower continental crust and attendant magmatic and metamorphic responses over the time span 370–355 Ma.

Petrogenesis and tectonic setting of Late Devonian I-type granitic plutons in the Kekesala area, Chinese western Tianshan: implication for tectonic evolution of the North Tianshan Ocean

ABSTRACT Massive Late Palaeozoic magmatic rocks are well developed along the northern margin of the Chinese western Tianshan. Previous studies mainly focused on the Carboniferous to Permian magmatism, and the Devonian magmatic rocks were rarely reported. Thus, this study presents new zircon U-Pb dating, whole-rock geochemistry and Lu-Hf isotopic data for the Late Devonian granitic plutons from the Kekesala area of the Chinese western Tianshan, aiming to elucidate their ages, petrogenesis, and tectonic settings. The Kekesala granitic plutons comprise granodiorites, monzogranites and syenitic granites. Zircon U-Pb dating results show they were emplaced in Late Devonian (ca. 372 Ma). Geochemically, the granodiorites and monzogranites are characterized by high-K calc-alkaline I-type series, and have low Ni and Cr, and weakly positive εHf(t) values, interpreted to be predominantly derived from partial melting of a lower crust with minor addition of mantle-derived materials; The syenitic granites with medium-K calc-alkaline features, exhibit high Sr and low Y and Yb, likely originated from partial melting of a subducted slab involving some mantle peridotites. The Kekesala I-type granitic plutons provide robust evidence for the existence of a subduction regime for the northern part of the Chinese western Tianshan in the Late Devonian. Based on the studied results, together with available data, we conclude that initial subduction of the North Tianshan Ocean began at least in the Middle Ordovician and terminated at the beginning of the Late Carboniferous rather than in the Middle Silurian.

Identification of lateral inhomogeneity of arc basement by reconstructing the Late Devonian arc belt in the southwestern Central Asian Orogenic Belt

A reconstruction of the Late Devonian arc belt in the West Junggar and adjacent Kazakhstan regions is crucial for understanding the architecture of the southwestern part of the Central Asian Orogenic Belt. Here, we report zircon U–Pb age, elemental, and Sr–Nd isotopic data for the firstly recognized Late Devonian pluton in the central West Junggar terrane (NW China) to constrain the component of the inner arc belt (IAB) of the Kazakhstan Orocline. Dioritic rocks from the pluton, dated at ∼368 Ma, are calc-alkaline and display negative Nb–Ta–Ti anomalies and positive εNd(t) values ranging from +6.6 to +7.0, implying a juvenile arc setting. Such a Late Devonian arc may thus represent the eastern part of the northern limb of the IAB, but it was built on early Paleozoic accreted terranes, which is distinct from a Precambrian continental basement for the coeval arcs in the central and southern parts of IAB. This difference could be attributed to the lateral inhomogeneity of arc basement, similar to the Aleutian arcs.

Geochemistry of intrusive rocks on the Prescott Peninsula, central Massachusetts, USA: Implications for late detachment faulting within the Ordovician Taconian volcanic arc

The Prescott Peninsula contains two intrusive bodies: the Cooleyville Gneiss (449 Ma) and the Packard Gabbro (407 Ma), which cuts the Cooleyville. These are centrally located in the Bronson Hill Anticlinorium, a complex structural belt in western New England that includes metamorphosed rocks of the Ordovician Taconian arc plutonic complex (TAPC), and associated metamorphosed volcanic rocks. TPAC gneisses (locally 454–442 Ma) are overlain by stratified units including the Ordovician Ammonoosuc Volcanics (locally 453 Ma), Partridge Formation (locally 449 Ma), and several Silurian and Devonian units. The Cooleyville Gneiss and Packard Gabbro are both calc-alkaline and have similar, arc-associated geochemical anomalies expressed in multi-element diagrams. These units are chemically distinct from several regionally exposed Devonian plutons, but are almost identical to plutonic rocks of the TAPC. The Cooleyville Gneiss cuts the Ammonoosuc Volcanics and Partridge Formation, but the TAPC does not, at least over extensive regions. Additionally, the Ammonoosuc Volcanics and Partridge Formation are each in contact with the TAPC in different areas: Partridge Formation locally to the east and west, and Ammonoosuc Volcanics in a central belt. A multi-part model explains these relationships: 1) The Cooleyville Gneiss is part of the TAPC, having intruded the Ammonoosuc Volcanics and Partridge Formation east of the main arc axis. 2) Following arc collision with Laurentia, extensive detachment faulting decapitated the TAPC and Cooleyville pluton, sliding them and adjacent units west and down. 3) A second detachment carried part of the Ammonoosuc and Partridge farther west, completing the observed contact geographic pattern. 4) Following isostatic readjustment, erosion, and cooling, Silurian and Devonian units were deposited, in part unconformably on the TAPC. 5) Early Acadian (Devonian) subduction beneath composite Laurentia produced extensive central New England magmatism, and reactivated remnant Taconian lithospheric mantle beneath the TAPC, producing the Packard Gabbro and age-equivalent volcanics in the nearby Erving Formation. Detachment faulting is common in arc and collisional settings. This model of late Taconian detachment faulting appears to solve a long-standing problem understanding the TAPC contact with overlying Ordovician units.

Petrogenesis of the Middle Devonian syenites in the Miyun–Huairou Region of Beijing and its tectonic implications

Three Devonian magmatic rocks have been identified from the mid‐western part of the Yanshan Thrust‐and‐Fold Belt (YTFB). Here, we present new geochronological, Sr–Nd–Hf isotopic, and whole‐rock chemical data. Our new LA‐ICP‐MS U–Pb dating recognized syenite plutons of ca. 385 Ma, which include Laogongying, Shicheng, and Qiaomaiyu, respectively, around the Yunmeng Mountain in the Miyun–Huairou area of Beijing. Rocks from three plutons are characterized by low SiO2 and high alkali contents, especially high K2O, and they show syenite and foid syenite compositions. They display slight–middle light REE enrichment, no negative or positive Eu anomalies, strong depletion in Rb, Th, U, Nb, Ta, P, Zr, Hf, and Ti, enrichment in Ba, K, and Sr, low contents of Y and Yb, and high Sr/Y ratios. They have a relatively narrow range of isotopic compositions with initial 87Sr/86Sr ratios from 0.708203 to 0.710477, εNd(t) values from −12.26 to −13.28, and εHf(t) values from −19.92 to −17.96. This study shows the Middle Devonian intrusions were produced by fractional crystallization of a parental magma, which was derived by interaction between mantle‐derived alkaline magmas of a type I‐enriched mantle and pre‐existing continental crust. Tectonic background was likely emplaced in a back‐arc extension environment related to southward subduction of the Palaeo‐Asian oceanic plate or during the cessation of the subduction. Feldspar geothermometer and aluminium‐in‐hornblende barometry studies of the Middle Devonian plutons yielded emplacement temperature of ~700°C and pressure of ~6.5 kbar, indicating the emplacement depths of about 24 km. Combined with previous geobarometry results on the Middle Devonian plutons within the Inner Mongolia Palaeo‐uplift (IMPU) on the northern margin of the NCC, the strong uplift and exhumation the IMPU were identified during Middle Devonian to Late Triassic times. The exhumation was more than 4–5 km than those of the IMPU, resulting in widely absent of Palaeozoic strata within the YTFB.

A Late Devonian Magmatic Link between Rhode Island and Nova Scotia

AbstractSeveral Late Devonian plutons across southeastern New Hampshire, coastal Maine, and north-central Massachusetts have within-plate compositions distinct from the 400–410 Ma calc-alkaline plu...

Geochronology and lithogeochemistry of granitoid rocks from the central part of the Central plutonic belt, New Brunswick, Canada: implications for Sn-W-Mo exploration

The central part of the Central plutonic belt in New Brunswick is underlain by numerous plutons of calc-alkaline, foliated and unfoliated granite that intrude Cambrian to Early Ordovician metasedimentary rocks. U-Pb (zircon) dating demonstrates that granites range in age from Middle Ordovician to Late Devonian, although most are late Silurian to Early Devonian. An age of 467 ± 7 Ma has been obtained on the foliated McKiel Lake Granite, whereas unfoliated intrusions yield ages of 423.2 ± 3.2 Ma (Bogan Brook Granodiorite), 420.7 +1.8/-2.0 Ma (Nashwaak Granite), 419.0 ± 0.5 Ma (Redstone Mountain Granite), 416.1 ± 0.5 Ma (Beadle Mountain Granite), 415.8 ± 0.3 Ma (Juniper Barren Granite), 409.7 ± 0.5 Ma (Lost Lake Granite), and 380.6 ± 0.3 Ma (Burnthill Granite). All plutons exhibit mixed arc-like and within-plate geochemical signatures, although the Redstone Mountain and Burnthill granites are dominantly of within-plate type. Trace element data reveal a close overall geochemical similarity between Ordovician and Silurian – Devonian plutons, indicating that all were generated by partial melting of the same crustal source. Late Silurian to Early Devonian plutons mainly comprise biotite and/or muscovite-bearing, peraluminous granite and are considered prospective for granophile-element mineralization. All plutons contain Sn well in excess of the granite global average abundance, and several contain average tin values comparable to productive stanniferous granites elsewhere. The Burnthill, Lost Lake, Beadle Mountain, and Nashwaak granites are geochemically most evolved and enriched in Sn and W. The Burnthill Granite in particular has experienced late-stage hydrothermal processes that have resulted in local enrichments of these elements.

Au-bearing gabbro-tonalite-granodiorite-granite plutons of the urals: Age, geochemistry, and magmatic and ore evolution

Gabbro-tonalite-granodiorite-granite (GTGG) plutons productive for gold are suprasubduction intrusive bodies formed at an Andean-type active continental margin 410–380, 365–355, and 320–290 Ma ago. The Devonian plutons are situated in the southeastern marginal continental zone, whereas the Carboniferous plutons occur in the northwestern zone. All GTGG plutons are mantle-crustal. Their formation started with hydrous basic magmatism and was accompanied by such magmatism up to the final stage. Mantle-derived amphibole gabbro and diorite experienced partial melting (anatexis) in the lower crust under a pressure of 6–10 kbar, giving birth to the tonalite-granodiorite members of the GTGG series. The latter, in turn, were involved in anatexis with the formation of adamellite and granite, immediately accompanied by hydrothermal gold mineralization. The multistep anatexis is the main petrogenetic process responsible for the gold resource potential of GTGG plutons. In the process of anatexis occurring under high fluid saturation, gold was repeatedly removed from rocks into fluid, facilitating its concentration in ore deposits.

Zircon U–Pb ages and tectonic implications of Paleozoic plutons in northern West Junggar, North Xinjiang, China

North Xinjiang, Northwest China, is made up of several Paleozoic orogens. From north to south these are the Chinese Altai, Junggar, and Tian Shan. It is characterized by widespread development of Late Carboniferous–Permian granitoids, which are commonly accepted as the products of post-collisional magmatism. Except for the Chinese Altai, East Junggar, and Tian Shan, little is known about the Devonian and older granitoids in the West Junggar, leading to an incomplete understanding of its Paleozoic tectonic history. New SHRIMP and LA-ICP-MS zircon U–Pb ages were determined for seventeen plutons in northern West Junggar and these ages confirm the presence of Late Silurian–Early Devonian plutons in the West Junggar. New age data, combined with those available from the literature, help us distinguish three groups of plutons in northern West Junggar. The first is represented by Late Silurian–Early Devonian (ca. 422 to 405 Ma) plutons in the EW-striking Xiemisitai and Saier Mountains, including A-type granite with aegirine–augite and arfvedsonite, and associated diorite, K-feldspar granite, and subvolcanic rocks. The second is composed of the Early Carboniferous (ca. 346 to 321 Ma) granodiorite, diorite, and monzonitic and K-feldspar granites, which mainly occur in the EW-extending Tarbgatay and Saur (also spelled as Sawuer in Chinese) Mountains. The third is mainly characterized by the latest Late Carboniferous–Middle Permian (ca. 304 to 263 Ma) granitoids in the Wuerkashier, Tarbgatay, and Saur Mountains. As a whole, the three epochs of plutons in northern West Junggar have different implications for tectonic evolution. The volcano-sedimentary strata in the Xiemisitai and Saier Mountains may not be Middle and Late Devonian as suggested previously because they are crosscut by the Late Silurian–Early Devonian plutons. Therefore, they are probably the eastern extension of the Early Paleozoic Boshchekul–Chingiz volcanic arc of East Kazakhstan in China. It is uncertain at present if these plutons might have been generated in either a subduction or post-collisional setting. The early Carboniferous plutons in the Tarbgatay and Saur Mountains may be part of the Late Paleozoic Zharma–Saur volcanic arc of the Kazakhstan block. They occur along the active margin of the Kazakhstan block, and their generation may be related to southward subduction of the Irtysh–Zaysan Ocean between Kazakhstan in the south and Altai in the north. The latest Late Carboniferous–Middle Permian plutons occur in the Zharma–Saur volcanic arc, Hebukesaier Depression, and the West Junggar accretionary complexes and significantly postdate the closure of the Irtysh–Zaysan Ocean in the Late Carboniferous because they are concurrent with the stitching plutons crosscutting the Irtysh–Zaysan suture zone. Hence the latest Late Carboniferous–Middle Permian plutons were generated in a post-collisional setting. The oldest stitching plutons in the Irtysh–Zaysan suture zone are coeval with those in northern West Junggar, together they place an upper age bound for the final amalgamation of the Altai and Kazakhstan blocks to be earlier than 307 Ma (before the Kaslmovian stage, Late Carboniferous). This is nearly coincident with widespread post-collisional granitoid plutons in North Xinjiang.

Geophysical modeling of Devonian plutons in the southern Gulf of St. Lawrence: implications for Appalachian terrane boundaries in Maritime Canada

A prominent positive magnetic anomaly spans the 100 km distance between Prince Edward Island and Cape Breton Island in the southern Gulf of St. Lawrence. The anomaly occurs in an area of complex structure where Appalachian terrane boundaries are poorly resolved because of thick late Paleozoic sedimentary cover. Analysis of the magnetic anomaly led to the interpretation that it is produced by four separate, approximately circular, source bodies aligned along the northwesterly trend of the anomaly. Seismic data, physical property measurements, and magnetic and gravity anomalies were used to further investigate the anomaly sources through forward modeling techniques. The four source bodies have densities and magnetic susceptibilities compatible with dioritic to granitic compositions. Modeling also suggests that basement to the north of the plutons has higher density and susceptibility than basement to the south, and hence the source bodies are interpreted as plutons emplaced along the boundary between Ganderian composite terranes to the north and the Ganderian Brookville – Bras d’Or terrane to the south. This interpretation suggests that the Ganderia–Avalonia boundary is located farther south, and shows the need for re-evaluation of the location and role of the Canso fault in offsetting terranes between Cape Breton Island and southern New Brunswick.

Stawell gold deposit: a key to unravelling the cambrian to early devonian structural evolution of the western Victorian goldfields

Major 440 Ma orogenic-gold deposits in the western Victorian goldfields formed during east – west shortening but have markedly different structural complexity. These deposits occur in: (i) a Cambrian Delamerian basement block that was substantially reworked and reactivated during the Lachlan Orogeny (Stawell); and (ii) Ordovician turbidites deformed solely by Lachlan-aged deformation (Bendigo, Ballarat, Castlemaine). This produced different structural histories prior to mineralisation, although gold deposits have been localised at the top of regional domal culminations. At Stawell, the 440 Ma gold event reactivated a strike-change along a pre-existing Cambrian fault system above a major lithospheric boundary. In the Bendigo Zone, 440 Ma orogenic-gold deposits have a trend oblique to the dominant structural grain and parallel to the western edge of an inferred crystalline basement block (the Selwyn Block). This gold trend is parallel to metamorphic field gradients and pluton age boundaries, which suggests an underlying basement control on the localisation of these orogenic-gold deposits, even though the exact basement architecture is still unresolved. Major variations in the regional stress fields occurred between 425 and 370 Ma, with large gold deposits [>62 t (2 million ounces) endowments] forming at ca 380 – 370 Ma. These events are not deposit-scale structural anomalies as they also regionally affect overlying cover sequences (e.g. the Grampians Group). Gold deposits that formed in the 425 – 400 Ma period have small endowments, but introduce a marked amount of structural and mineralogical complexity to the gold province. The 425 – 400 Ma period preserved at Stawell records sinistral wrenching associated with gold mineralisation, southeast-directed faulting, intrusion-related gold mineralisation and extensive high-level Early Devonian plutonism.

Variable hangingwall palaeotransport during Silurian and Devonian thrusting in the western Lachlan Fold Belt: Missing gold lodes, synchronous Melbourne Trough sedimentation and Grampians Group fold interference

The western margin of the Lachlan Fold Belt contains early ductile and brittle structures that formed during northeast‐southwest and east‐west compression, followed by reactivation related to sinistral wrenching. At Stawell all of these structural features (and the associated gold lodes) are dismembered by a complex array of later northwest‐, north‐ and northeast‐dipping faults. Detailed underground structural analysis has identified northwest‐trending mid‐Devonian thrusts (Tabberabberan) that post‐date Early Devonian plutonism and have a top‐to‐the‐southwest transport. Deformation associated with the initial stages of dismemberment occurred along an earlier array of faults that trend southwest‐northeast (or east‐west) and dip to the northwest (or north). The initial transport of the units in the hangingwall of these fault structures was top‐to‐the‐southeast. ‘Missing’ gold lodes were discovered beneath the Magdala orebody by reconstructing a displacement history that involved a combination of transport vectors (top‐to‐the‐southeast and top‐to‐the‐southwest). Fold interference structures in the adjacent Silurian Grampians Group provide further evidence for at least two almost orthogonal shortening regimes, post the mid‐Silurian. Overprinting relationships, and correlation with synchronous sedimentation in the Melbourne Trough, indicates that the early fault structures are mid‐ to late‐Silurian in age (Ludlow: ca 420–414 Ma). These atypical southeast‐vergent structures have regional extent and separate significant northeast‐southwest shortening that occurred in the mid‐Devonian (‘Tabberabberan orogeny’) and Late Ordovician (‘Benambran orogeny’).

Timing relationships between felsic magmatism and mineralisation in the central Victorian gold province, southeast Australia

New SHRIMP U–Pb data from major felsic intrusives and dykes confirm that initial gold emplacement in the central Victorian gold province was metamorphic‐related, pre‐dating the emplacement of Devonian intrusions in the western Lachlan Fold Belt by as much as 80 million years. Importantly, together with previously published geochronological constraints, these data strongly suggest that no genetic link exists between the main phase of ‘metamorphism‐related’ orogenic gold mineralisation and magmatism in the central Victorian gold province. A subsequent ‘intrusive‐related’ phase of gold mineralisation, often accompanied by elevated copper, tungsten, molybdenum or antimony, formed synchronously with, or post‐dated, the emplacement of Devonian plutons and dykes, and thus is spatially (if not genetically) related to melt generation at depth. These timing relationships differ from those found in many other orogenic gold districts in which ore emplacement is closely associated, spatially, with felsic magmatism. The introduction of metamorphically derived fluids well before magmatism is seen to reflect variations in the timing of peak metamorphic conditions at different crustal levels. The extensive development of melt in this environment is thought to occur in response to the transfer of thermal energy from the mantle into the overlying, thickened crust in a Cordilleran‐type setting, which is dominated by episodic subduction‐accretion processes. The new SHRIMP U–Pb data also demonstrate that many of the major granites throughout the central Victorian gold province have a composite intrusive history and are the product of multiple tectonomagmatic events.