Trondhjemite Plutons Research Articles

Re-initiation of plutonism at the Gondwana margin after a magmatic hiatus: The bimodal Permian-Triassic Longwood Suite, New Zealand

The Cambrian to Cretaceous Tuhua Intrusives, New Zealand, preserve an igneous record of Phanerozoic subduction and crustal growth at the margin of Gondwana. Within the Tuhua Intrusives, the coeval gabbroic and trondhjemitic intrusions of the c. 261–243 Ma Longwood Suite stand out as being isotopically more primitive and chemically distinct from all other New Zealand plutonic suites. We present new U-Pb crystallization ages, trace element analyses and Sr-Nd isotope compositions of the Longwood Suite. U-Pb SHRIMP zircon ages of 258.5 ± 2.5 Ma, 256.0 ± 1.8 Ma, 247.8 ± 2.7 Ma and 243.2 ± 2.4 Ma obtained from plutons on Ruapuke Island, and a dike at Bluff, affirm the restricted time range and expand the known areal extent of the Longwood Suite. Longwood Suite granitoids are I-type and sodic (K/Na < 0.4), with distinctive low Rb and Nb/Ta, flat rare earth element patterns (La/YbN < 10), unradiogenic 87Sr/86Sr(t) (0.7029 to 0.7032) and radiogenic ε143Nd(t) (+6.3 to +8.2), compared to the nearby, calc-alkaline, Late Triassic Darran Suite I-type plutons of the Tuhua Intrusives. Stable Nd isotope ratios of Longwood Suite samples are highly variable (δ146/144Nd = 233 ppm) compared to global plutonic rocks (δ146/144Nd = 44 ppm) and reflect the removal of phosphate minerals. Collectively, these geochemical characteristics are consistent with generation of the granitoids by shallow (garnet-absent) melting of an amphibolitic residue, from which we infer relatively thin lithosphere. The Longwood Suite has a maximum areal addition rate of 43 km2/Ma, substantially less than the subsequent plutonic suites when the magmatic arc was fully established. We suggest a petrotectonic model whereby Gondwana continental margin crust was tectonically underplated by Permian intra-oceanic island arc crust and mantle lithosphere, which subsequently melted to generate the isotopically primitive gabbro and trondhjemite plutons of the Longwood Suite.

Linking metamorphism, magma generation, and synorogenic sedimentation to crustal thickening during Southern Appalachian mountain building, USA

Abstract The nature of metamorphism, magma compositions, the spatial distribution of plutons, and foreland sediments reflect, in part, the character and thickness of continental crust. We utilized metamorphic pressure-temperature-time (P-T-t) paths, garnet Sm-Nd ages, zircon U-Pb ages, and pluton compositions to estimate paleocrustal thickness and temporal changes in crustal magma sources in the Blue Ridge of the southernmost Appalachians. Garnet Sm-Nd ages for amphibolite-facies metamorphic rocks range from 331 ± 4 to 320 ± 3 Ma. Low- and high-Sr/Y plutons that intruded these metamorphic rocks have zircon U-Pb ages of 390 ± 1 to 365 ± 1 Ma and 349 ± 2 to 335 ± 1 Ma, respectively. Therefore, garnet growth began during regional metamorphism synchronous with or shortly after intrusion of the youngest high-Sr/Y trondhjemite plutons. Phase diagram sections and thermobarometry indicate that garnet growth initiated at ∼5.8 kbar and 540 °C and grew during temperature increases of 60–100 °C and pressure increases of 2–3 kbar. The older, low-Sr/Y magmas are inferred to have been sourced in the crust at depths &lt;∼30 km, insufficient for garnet to be stable. However, the younger, high-Sr/Y magmas are inferred to have been sourced at &gt;30 km depths where garnet was stable. Hafnium isotopic compositions for all the plutons, but one, exhibit a range from negative initial εHf(i) to weakly positive initial εHf(i), indicating incomplete mixing of dominantly crustal sources. Our data require minimum crustal thicknesses of ∼33 km at 331 Ma; however, Alleghanian crustal thicknesses must have locally reached 39 km, based on crustal reconstruction adding the Alleghanian thrust sheet beneath the eastern Blue Ridge. We infer the presence of hot, tectonically thickened crust during intrusion of the early Alleghanian high-Sr/Y plutons and conclude that garnet growth and plutonism reflect a progressive increase in crustal thickness and depth of magma generation. The crustal thickening was synchronous with deposition of Mississippian to early Pennsylvanian sediments in the foreland basin of the Appalachian orogen between 350 and 320 Ma. This crustal thickening may have preceded emplacement of the Alleghanian thrust sheets onto the North American craton.

Mesoproterozoic (Grenville-age) granitoids and supracrustal rocks in Kaokoland, northwestern Namibia

We identified and describe rocks of a late Mesoproterozoic Grenville-age igneous province and associated supracrustal sequence in northwestern Kaokoland, Namibia. These rocks are ca. 1176–1534Ma in age and mostly consist of pinkish granitic and trondhjemitic plutons that are unfoliated within the Palaeoproterozoic Epupa Metamorphic Complex but become progressively deformed farther west together with late Neoproterozoic rocks of the Pan-African Kaoko orogen. Furthermore, a supracrustal assemblage of clastic sedimentary rocks and felsic volcanic rocks, known as Okapuka Formation, unconformably overlies the Epupa gneisses and also became deformed in the west during the Kaoko orogeny. A zircon age of ca. 1320Ma for an Okapuka felsic schist is suggested to date volcanism and deposition of sediments in an intracontinental basin that may have extended into southern Angola where similar rocks of the Chela Group are exposed. Some granites have A-type chemical characteristics, and it is likely that granitoid magmatism and Okapuka basin formation in northwestern Kaokoland was related to crustal extension as also documented by emplacement of the 1385±8Ma Kunene Gabbro-Anorthosite Complex.

Paleo- to Mesoarchean TTG accretion and continental growth in the western Dharwar craton, Southern India: Constraints from SHRIMP U–Pb zircon geochronology, whole-rock geochemistry and Nd–Sr isotopes

A multidisciplinary study involving field, petrographic, SHRIMP U–Pb zircon/titanite ages, whole rock geochemical and Nd–Sr isotope data is presented for the Peninsular Gneisses and associated plutons forming core of the Dharwar craton. Two major periods (3350–3280Ma and 3230–3200Ma) of crustal growth through TTG accretion sub-contemporaneous with greenstone volcanism are documented. Elemental and Nd–Sr isotope data suggest that the TTGs originated by low and high pressure melting of heterogeneous mafic sources (thickened island arc or oceanic plateau crust) at different depths. Among the early accreted TTGs, magmatic protoliths of low-Al gneisses formed by low-pressure (10–12kbar) melting of a depleted mafic source at shallow levels (island arc type crust) with plagioclase+amphibole±ilmenite in the residue, whilst high-Al gneisses were probably produced by high pressure melting (14–18kbar) of a chondritic or a less depleted mafic source with garnet±amphibole±pyroxene+ilmenite in the residue (base of Island arc crust or thickened oceanic plateau). The 3230–3200Ma trondhjemitic plutons marking the second accretion event derived by higher pressure (14–16kbar) melting of a mafic source (island arc type crust with garnet±plagiocalse±hornblende+ilmenite) with a minor involvement of previously accreted TTGs. This plutonism was coeval with a major phase of crustal-scale diapiric overturn that had a major imprint on crustal reworking. A later magmatic episode at ca. 3100Ma produced granitic plutons derived from crustal melting and leucocratic gneisses derived from a depleted mafic source in the lower crust. Documentation of depleted shallow and deep mantle reservoirs during 3350–3280Ma magmatism implies massive differentiation of the mantle during an earlier episode of crust formation around 3800–600Ma, as substantiated by Nd model ages and published Pb-isotope data, as well as U–Pb detrital zircon ages with in situ Hf isotope data.

The Sierra de Cachi (Salta, NW Argentina): geological evidence about a Famatinian retro-arc at mid crustal levels

The Cachi mountain ranges, located in Salta (NW Argentina), form an inlier where Neoproterozoic and Paleozoic materials of the An­des crop out in a plutono-metamorphic dome surrounded by Mesozoic and Cenozoic sedimentary rocks. The pre-Mesozoic rocks record a regional metamorphism of low pressure and high temperature conditions, which reached partial melting conditions and was coeval with the intrusion of sheeted granite and trondhjemite plutons. The metamorphic evolution and the emplacement of the plutons took place in an extensional tectonic setting. New U-Pb zircon dating by TIMS and SHRIMP methods yields similar ages for the migmatization (479.7 ± 3.5 Ma) and the emplacement of granites (472.1 ± 11 Ma), constraining the extensional event to Early Ordovician times. We also report the finding of gabbro boulders, which also yield a similar U-Pb SHRIMP zircon age of 477.5 ± 3.9 Ma. So, we propose that the extensional event recognized in Sierra de Cachi took place in a retro-arc setting linked to the Famatinian subduction along the western border of Gond­wana. The shortening produced by younger east-verging folds evidences the superposition of a compressional stress field, probably related with the Ocloyic orogeny (Middle/Late Ordovician-Devonian).

Architecture of the Chugach accretionary complex as revealed by detrital zircon ages and lithologic variations: Evidence for Mesozoic subduction erosion in south-central Alaska

Subduction erosion is an important process at convergent margins but evidence in the geologic record is scarce because the involved materials are typically lost into the mantle. Detrital zircon dating of accretionary complex sediment allows us to document episodic growth that can be linked to possible triggers. Detrital zircons (n = 2508) from the Mesozoic Chugach accretionary complex, southern Alaska, have U-Pb ages that record progressive subduction accretion punctuated by two periods of tectonic erosion. Lithology and maximum depositional ages permit division of the Chugach accretionary complex into four main units. The oldest, the blueschist-greenschist unit, represents partially subducted sediment associated with a Jurassic oceanic arc, with subduction erosion from 180 to 170 Ma. The end of this erosion period is dated by the oldest maximum depositional age of the Potter Creek assemblage, a <156–169 Ma unit consisting of chert, argillite, and volcanic rocks. A trondhjemite pluton that intrudes the forearc was caused by ridge subduction at 125 Ma and resulted in a second period of subduction erosion lasting until 104 Ma. The end of Aptian–Albian erosion was marked by deposition of massive sandstone and conglomerate of the 101–91 Ma McHugh Creek assemblage. This influx of clastic sediment is interpreted to have occurred in response to the collision of Wrangellia with North America. This event and the erosional event preceding it steepened the forearc region, allowing mass wasting of forearc crust into the trench, filling it by 89 Ma, the oldest maximum depositional age of the Valdez Group flysch. Clasts in conglomerate include granodiorite from the basement of the Talkeetna arc dated from 199 ± 3 Ma to 179 ± 3 Ma, and sandstone clasts with maximum depositional ages of 100 Ma. From 89 Ma to at least 72 Ma, the Valdez Group flysch was deposited via turbidite fans onto the oceanic crust beyond the trench and accreted as imbricate thrust slices that retained coherent bedding. The source for detrital zircons in the Potter Creek assemblage is likely a Middle–Late Jurassic oceanic arc, possibly the Talkeetna arc. The abundance of zircons from ash fall tuffs is consistent with easterly winds and suggests the Chugach accretionary complex was south of latitude 25°N in the Late Jurassic. The dominant source for the Albian McHugh Creek assemblage and the Upper Cretaceous Valdez Group flysch was likely the arc associated with the Coast Mountains batholith. Jurassic (ca. 165 Ma) zircons in the McHugh Creek assemblage could have been derived from exhumed plutons, or may be second-cycle zircons derived from the Potter Creek assemblage. The first appearance of Proterozoic and Archean zircons in the Valdez Group records the breakdown of topographic barriers formed by the accreted arc terranes as rivers encroached into continental North America by the middle Late Cretaceous.

Granite emplacement by crustal boudinage: example of the Calmayo and El Hongo plutons (Córdoba, Argentina)

AbstractThis study deals with the structure and emplacement of the Calmayo and El Hongo trondhjemite plutons (Famatinian belt of Córdoba, Argentina). It provides structural data from the granites and the country rocks and a study of the magnetic fabric in the plutons. New U/Pb geochronological data yield intrusion ages of 512.1 ± 3.4 Ma and 500.6 ± 4.5 Ma for the Calmayo and El Hongo plutons respectively. The El Hongo massif and the southern part of the Calmayo trondhjemite preserve magmatic structures, whereas the northern domain of Calmayo shows the imprint of solid‐state deformation. The main foliation in the country rocks outlines a boudin‐like pattern at the map scale and the granites are located along boudin necks, suggesting that the emplacement of these trondhjemite plutons was linked to large‐scale boudinage of the country rocks.

TTGs in the making: Natural evidence from Inyoni shear zone (Barberton, South Africa)

Despite the consensus that TTGs, the main constituents of the Archaean continental crust worldwide, originated by partial melting of garnet-bearing amphibolites, natural evidence is scarce. A large variety of Archaean amphibole-rich rocks, including migmatitic amphibolites and hornblende-rich cumulates, was exhumed as a tectonic melange in the Inyoni shear zone (ISZ) near Barberton (South Africa), likely at the time of emplacement of the 3.2Ga Nelshoogte–Badplaas plutons. This unique collection provides the opportunity to investigate partial melting of garnet-amphibolites, as well as the differentiation processes occurring in the TTG magmas en route to the surface.The ISZ migmatitic amphibolites are characterized by quartz-plagioclase leucosomes in equilibrium with garnet, amphibole, titanite±epidote. Garnet compositions are characterized by high almandine and grossular contents. Actually, the leucosomes and neosomes likely lost part of their melt component and mesosomes may have been also modified towards restite-rich compositions due to melt segregation. Restite-bearing rocks are very iron-rich according to their high contents in garnet. Besides, the Mg-rich compositions of the hornblende cumulates and their REE distribution patterns resemble those of the nearby TT (tonalite–trondhjemite) plutons.Thermobarometric estimates from the ISZ migmatitic rocks cluster in the range 720–800°C and 1.1–1.2GPa for the melting reaction and the derived geothermal gradient is ca 17–22°C/km. These conditions are consistent with either water-present melting or, more likely, water-absent epidote-dehydration melting. Such a geothermal gradient is therefore assigned to the genesis of large volumes of medium-pressure TTG magmas, that will likely form at greater depth. Unmelted garnet-bearing amphibolites correspond to slightly lower conditions.The magmatic cumulates crystallized at ca 0.6GPa. The cumulate rocks evidence that the TTG parental magmas en route to the surface experienced fractional crystallization of a large amount of amphibole before crystallizing as tonalite–trondhjemite plutons at slightly shallower depths (ca 0.5GPa).

Origin of the volcanic complexes of La Désirade, Lesser Antilles: Implications for tectonic reconstruction of the Late Jurassic to Cretaceous Pacific-proto Caribbean margin

La Désirade Island is located on the hanging wall of the present-day Lesser Antilles subduction zone and consists of a suite of Mesozoic igneous rocks capped by Neogene limestone. The basement suite contains Kimmeridgian to Tithonian (~ 153–145 Ma) mafic lava flows and pillow basalts overlain by felsic flows and breccias and intruded by a Mid-Berriasian (~ 144 Ma) trondhjemite pluton and intermediate to felsic dykes. The mafic rocks form a ~ 300 m thick sequence which trace element geochemistry reveals to contain, in stratigraphic order: (1) tholeiites with a very weak subduction signature; (2) calc-alkaline and tholeiitic arc rocks containing pelagic and terrigenous sediment slab-related components and (3) arc tholeiites with a minor subduction signature. The mantle wedge source was depleted and did not contain a significant plume-related component. The overlying felsic rocks show similar trace element patterns and incompatible trace element ratios to the mafic units. Factors such as pelagic sedimentary deposition and re-working, low eruption rates and the presence of MORB-like and felsic rocks are best explained by an origin at a back-arc spreading ridge. This back-arc was most likely in the proto-Caribbean (Colombian Marginal) seaway and was related to east-dipping Andean/Cordilleran subduction. Other sites in the Greater Antilles and Central America older than the ~ 135 Ma westward acceleration of North America appear to corroborate a latest Jurassic–Early Cretaceous east-dipping arc system. The preservation of La Désirade in the fore-arc of the present Antilles arc is consistent with Mid-to-Late Cretaceous inception of west-dipping subduction along the former back-arc axis which had previously given rise to La Désirade.

Age constraints on Late Paleozoic evolution of continental crust from electron microprobe dating of monazite in the Peloritani Mountains (southern Italy): another example of resetting of monazite ages in high-grade rocks

In situ monazite microprobe dating has been performed, for the first time, on trondhjemite and amphibolite facies metasediments from the Peloritani Mountains in order to obtain information about the age of metamorphism and intrusive magmatism within this still poorly known sector of the Hercynian Belt. All samples show single-stage monazite growth of Hercynian age. One migmatite and one biotitic paragneiss yielded monazite ages of 311 ± 4 and 298 ± 6 Ma, respectively. These ages fit with previous age determinations in similar rocks from southern Calabria, indicating a thermal metamorphic peak at about 300 Ma, at the same time as widespread granitoid magmatism. The older of the two ages might represent a slightly earlier event, possibly associated with the emplacement of an adjacent trondhjemite pluton, previously dated by SHRIMP at 314 Ma. No evidence for pre-Hercynian events and only a little indication for some monazite crystallization starting from ca. 360 Ma were obtained from monazite dating of the metasediments, suggesting either a single-stage metamorphic evolution or a significant resetting of the monazite isotope system during the main Hercynian event (ca. 300 Ma). Rare monazite from a trondhjemite sample yields evidence for a late-Hercynian age of about 275 Ma. This age is interpreted as representing a post-magmatic stage of metasomatic monazite crystallization, which significantly postdates the emplacement of the original magmatic body.

Igneous rocks of the Brook Street Terrane, New Zealand: Implications for Permian tectonics of eastern Gondwana and magma genesis in modern intra‐oceanic volcanic arcs

The Brook Street Terrane of South Island, New Zealand, is a remnant of a primitive intra‐oceanic arc system of Permian age. The terrane consists largely of volcanogenic sequences that contain plagioclase‐ and clinopyroxene‐phyric basalts, high‐MgO ankaramite dikes, and basaltic to andesitic volcaniclastic and sedimentary rocks. Dacites and rhyolites are relatively rare. Intruding the sequences are thick dolerite dikes, trondhjemite plutons, and numerous small cumulate complexes. The cumulate complexes contain early‐formed olivine‐ and clinopyroxene‐rich ultramafic cumulates overlain by anorthite and hornblende‐bearing gabbros. There is convincing geological evidence to support earlier interpretations of a direct correlation between the Brook Street Terrane and the Gympie Terrane of Queensland, and we present geochemical data to support correlation with the Teremba Terrane of New Caledonia. It is likely that these terranes are exposed sections of an extensive island‐arc system that was active in the Pacific in Permian times. Dislocation of the arc probably occurred during accretion to the Gondwana margin and subsequent Gondwana breakup. The major and trace element geochemistry of a range of mafic dikes and flows from along the terrane precludes significant geochemical variation along the terrane and shows that the majority of magmas were primitive island‐arc tholeiites. The Bluff Complex is an exception and may have formed in a back‐arc or arc‐rift environment. The geochemistry, petrology, and field relations indicate that most of the intrusive and volcanic rocks are directly related products of upper crustal magmatic differentiation. Primary magma types include high‐MgO ankaramites and trondhjemites that are suggested to have formed by partial melting of lower crustal clinopyroxene‐rich cumulates and gabbros, respectively. The parental ankaramites fractionated to form the bulk of the Brook Street Terrane, including the mafic‐ultramafic cumulates and evolved melts of high‐Al basalt to andesite composition. The Brook Street Terrane is an excellent analogue for modern island‐arc systems and allows for the evaluation of magmatic processes that operate at the subvolcanic level of arcs. The wide distribution of ankaramites in the Brook Street Terrane indicates that parental magmas in island arcs may be more primitive than is currently recognised. Furthermore, partial melting of arc lower crustal cumulates before delamination may be crucial to the development of arcs and the evolution of the continental crust.

A Thrust Ramp Model for Gold Mineralization at the Archean Trondhjemite-Hosted Tarmoola Deposit: The Importance of Heterogeneous Stress Distributions around Granitoid Contacts

The trondhjemite-hosted Tarmoola gold deposit is situated in the Leonora district in the Eastern Goldfields province and is the largest granitoid-hosted gold deposit in the Yilgarn craton of Western Australia (>100 t Au). The massive trondhjemite pluton intrudes a northwest-striking, shallowly northeast-dipping, supracrustal sequence that comprises greenschist facies metamorphosed basalt, komatiite, chloritic siltstones, and a volcani-clastic unit. Several east-northeast-striking, quartz diorite dikes cut the trondhjemite and supracrustal sequence. Major ore zones (>1 g/t Au) occur (1) within the trondhjemite as discontinuous west-northwest-striking zones, (2) along the steeply dipping, eastern trondhjemite contact in the supracrustal and trondhjemite rocks, and (3) subparallel to the shallowly dipping, western trondhjemite margin in supracrustal rocks. Ore zones in the trondhjemite are controlled by west-northwest-striking, steeply south-west- and northeast-dipping, conjugate, quartz-carbonate veins that contain minor pyrite, chalcopyrite, sphalerite, galena, gold, and rare scheelite. Gold-bearing veins in supracrustal rocks have a similar vein mineralogy and timing to that of the auriferous veins in trondhjemite. Deformation at Tarmoola occurred during progressive, regional east-west-directed shortening. The first deformation event (D 1 ) caused early barren, isoclinally folded, quartz-carbonate veins and a pervasive foliation in supracrustal rock to be aligned subparallel to the steeply dipping eastern margin of the pluton. During the gold mineralization event (D 2 ), the trondhjemite pluton acted as a competent body within ductile supracrustal country rocks. Subhorizontal, northwest-southeast shortening caused auriferous conjugate veins in trondhjemite and the reactivation of the eastern contact and a preexisting subparallel foliation in the supracrustal rock. The reverse movement of supracrustal rock along the sheared western trondhjemite contact caused the formation of west-dipping, gold-bearing shear zones in the overlying supracrustal sequence. The reactivation of the eastern trondhjemite contact caused the development of low mean stress zones adjacent to the contact. Hy-drothermal ore fluids were focused into these regions to form ore zones. The third deformation event (D 3 ) produced brittle-ductile shear zones, quartz-carbonate veins, and brittle faults that offset lithological contacts and gold-bearing veins.

Thermochronometry and palaeomagnetism of the Archaean Nelshoogte Pluton, South Africa

SUMMARY 207Pb/206Pb single-grain zircon, 40Ar/39Ar single-grain hornblende and biotite, and 40Ar/39Ar bulk-sample muscovite and biotite ages from the Nelshoogte trondhjemite pluton located in eastern Transvaal, South Africa, show that this granitoid had a protracted thermal history spanning 3213±4 Ma to about 3000 Ma. Whole-rock 40Ar/39Ar ages from cross-cutting dolerite dykes indicate that these were intruded at about 1900 Ma. There is no evidence of this or other, later events significantly aVecting the argon systematics of the minerals from the pluton dated by the 40Ar/39Ar method. The pluton has a well-defined palaeomagnetic pole which is dated at 3179±18 (2s) Ma by 40Ar/39Ar dating of hornblende. This pole (18°N, 310°E, A 95 =9°) yields a palaeolatitude of 0°, significantly diVerent from other Archaean poles from the Kaapvaal Craton. The palaeolatitude diVerence implies that there was significant apparent polar wander during the Archaean. A second, overprinting magnetization seen in the pluton is also seen in the lower-Proterozoic dolerite dykes, and is consistent with other lower-Proterozoic (2150‐1950 Ma) poles for southern Africa.

Plumes and hotspots as sources of greenstone belts

Greenstone belts that are older than 0.18 Ga formed under circumstances that generate a buoyant, garnet-poor root. Because they do not generate enough depletion in the lithospheric mantle, juvenile arcs involving average temperature Early Precambrian mantle were not a significant source of Early Precambrian greenstone belts. Instead, Early Precambrian greenstone belts originated from mantle plumes and hotspots. We focus on the origin of highly deformed, MORB-like greenstone belts that are underlain by tonalitic to trondhjemitic plutons and surrounded by metamorphosed, accretionary wedge-like sediments. These greenstone belts could form as the consequence of subducting a ridge segment within 500 km of a hotspot located beneath the continent, as has happened on the Taitao Peninsula in Chile. The tonalitic to trondhjemitic plutons result from partial melting of oceanic crust thickened by extra melt derived from the hotspot. The MORB-like rocks result from decompressive partial melting within the slab window produced by the subducting ridge. The stratigraphy and tectonic setting of the recently emplaced Taitao ophiolite are a possible Phanerozoic analog of some MORB-like Archean and Early Proterozoic greenstone belts.

Tectonic-magmatic stages of shield evolution: the Pan-African belt in northeastern Egypt

The Arabian-Nubian shield illustrates an example of plate tectonics during the Pan-African orogenic event (ca. 950-550 Ma); it is considered one of the most remarkable Proterozoic shield areas yet known. Although there is now a general agreement that the shield has evolved by a series of magmatic arcs and terrain accretion, the evolution of its northernmost part exposed in northeastern Egypt remains controversial. Two contrasting tectonic models have been proposed for the evolution of this region: regional crustal extension and magmatic-arc regimes. There is no convincing evidence of older (Archean) sialic materials within the region, although the region is flanked by Archean crust. The Neoproterozoic crustal components exposed in northeastern Egypt preserve a record for its evolution. A large gabbro-diorite-tonalite complex (GDT, 881 Ma) emplaced during the early stage of the Pan-African orogeny is geochemically primitive, exhibits a low initial 87Sr 86Sr ratio (0.704) and shows trace-element characteristics of oceanic-arc-related lavas. The GDT complex was produced (by fractionation) from a mantle-derived tholeiitic magma formed within an island-arc tectonic environment. A synorogenic stage characterized by extensive volcanism that produced the Dokhan volcanic suite (DVS, 620 Ma) forms a continuous basalt to rhyolitic ignimbrite calc-alkaline magma series, exhibiting features of arc-related volcanism, with REE profiles similar overall to Andean andesites and ignimbrites. The Dokhan suite was produced in a continental-arc setting. A granodiorite-adamellite-leucogranite composite batholith (552 Ma) emplaced during the late stage of the Pan-African orogeny exhibits typical features of I-type complexes, trace element traits of volcanic-arc granites, and is also interpreted to have been formed in an Andean-type setting. A trondhjemite pluton was also formed at this late orogenic stage, by partial melting of GDT host rocks at depth. Cooling and relaxation of the newly formed Pan-African crust caused extensive fracturing, which was followed by intrusion of NE-SW- and NNW-SSE-trending dyke swarms at 493 Ma. This episode marks a fundamental inversion from orogenic compressional to anorogenic extensional processes around the Neoproterozoic-Paleozoic boundary. Anorogenic magmatism was locally associated with such extension-induced structures. The Mount Gharib peralkaline granite (476 Ma), for example, exhibits trace element traits of A-type, within-plate granites, and a high initial 87Sr 86Sr ratio (0.711), and was formed in a rift environment. Thus, the region is characterized by lengthy episodes of discontinuous subduction (∼ 880-550 Ma), during which it evolved to form a primitive crust by early Pan-African island arcs and late Pan-African continental arcs. This was followed by an even longer period of crustal extension (∼ 550-90 Ma) that produced localized anorogenic magmatism.

Metamorphism and near‐trench plutonism during initial accretion of the Cretaceous Alaskan forearc

Cenozoic uplift of the eastern Aleutian forearc has exposed the tectonic join between the Jurassic crystalline hanging wall of the arc and the oceanic assemblages of the footwall accreted during Cretaceous and Tertiary subduction, the Border Ranges fault system (BRFS). The early stages of accretion along the BRFS in the Knik River area were characterized by high‐temperature ductile thrusting and metamorphism at low to moderate P/T (P from 300–700 MPa, Tmax about 600°C). Peak metamorphic conditions at the currently exposed levels can be explained by heat carried to the area by tonalite and trondhjemite plutons. Generation of tonalite and trondhjemite melts within the Cretaceous forearc, however, requires a heat source within or near the BRFS. Ridge subduction cannot be ruled out as the heat source; however, simple thermal models presented herein indicate that the temperatures required for anatexis within the forearc can be explained by transient heating on the fault by shear heating of dry lithologies during the earliest stages of thrust motion. Water released from subducted oceanic crust would result in generation of partial melts in the hanging wall if shear heating is significant or if the initial thermal gradient within the hanging wall is high. Further accretion of oceanic sediment would quench the system and temperatures rapidly evolve to the high P/T conditions of normal subduction zones. Thermochronologic data from this study show that the duration of high‐T‐low P/T conditions was short, of the order of about 15 m.y. or less. Exhumed forearcs here and elsewhere commonly record a period of low to moderate P/T conditions during subduction initiation in which shear heating likely played an important role.

An overview of the relationship between granitoid intrusions and gold mineralisation in the Archaean Murchison Province, Western Australia

In the Archaean Murchison Province of Western Australia, granitoid batholiths and plutons that intruded into the ca. 2.7–2.8 Ga and ca. 3.0 Ga greenstone belts can be divided into three major suites. Suite I is a ca. 2.69 Ga monzogranite-granodiorite suite, which was derived from anatexis of old continental crust and occurs as syn-tectonic composite batholiths over the entire province. Suite II is a trondhjemite-tonalite suite (termed I-type) derived from partial melting of subducted basaltic crust, which intruded as syn- to late-tectonic plutons into the greenstone belts in the northeastern part of the province where most of the major gold deposits are situated. One of the Suite II trondhjemite plutons has a Pb−Pb isochron age of 2641±36 Ma, and one of the structurally youngest tonalite plutons has a minimum Pb−Pb isochron age of 2630.1±4.3 Ma. Suite III is a ca. 2.65–2.62 Ga A-type monzogranite-syenogranite suite which is most abundant in the largely unmineralised southwestern part of the province. Gold deposits in the province are mostly hosted in brittle-ductile shear zones, and were formed at a late stage in the history of metamorphism, deformation and granitoid emplacement. At one locality, mineralisation has been dated at 2636.8±4.2 Ma through a pyritetitanite Pb−Pb isochron. Lead and Sr isotope studies of granitoids and gold deposits indicate that, although most gold deposits have initial Pb isotope compositions most closely similar to those of Suite II intrusions, both Suite I and Suite II intrusions or their source regions could have contributed solutes to the ore fluids. These preliminary data suggest that gold mineralisation in the Murchison Province was temporally and spatially associated with Suite II I-type granitoids in the northeastern part of the province. This association is consistent with the concept that Archaean gold mineralisation was related to convergent-style tectonic settings, as generation of both Suite II I-type granitoids and hydrothermal ore fluids could have been linked to the dehydration and partial fusion of subducted oceanic crust, and old sialic crust or its anatectic products may also contribute solutes to the ore fluids. Integration of data from this study with other geological and radiogenic isotope constraints in the Yilgarn Block argue against direct derivation of gold ore fluids from specific I-type granitoid plutons, but favour a broad association with convergent tectonics and granitoid magmatism in the late Archaean.

Emplacement and deformation history of the western margin of the Idaho batholith near McCall, Idaho: Influence of a major terrane boundary

Cretaceous plutons of the western margin of the Idaho batholith were emplaced along and to the west of the major terrane boundary separating middle Proterozoic and Paleozoic continental rocks from mostly Mesozoic accreted oceanic-arc terranes of the Blue Mountain Province. This boundary is marked by a change in the lithology of pendants and inclusions within the batholith. Plutons form two newly named complexes of igneous and metamorphosed igneous rocks. The Hazard Creek Complex, emplaced west of the boundary between the oceanic arc and the continental margin, consists primarily of a series of variably deformed and metamorphosed quartz diorite to trondhjemite plutons. The Little Goose Creek Complex, which intruded the boundary between the oceanic arc and the continental margin, is primarily porphyritic granodiorite to granite orthogneiss. A preliminary U-Pb age of 111 Ma for this porphyritic orthogneiss is a minimum age for the formation of the oceanic-arc-continent boundary. The plutonic rocks were deformed both during and after emplacement in response to east-west compressive stresses. Cretaceous deformation was localized along the boundary between the accreted terranes and the continental margin and is interpreted to have occurred after the formation of this boundary. The major deformation of the Hazard Creek Complex occurred during its emplacement. The dominant fabric in the Little Goose Creek Complex is due to subsolidus ductile deformation. The localization of two deformation events along the pre-existing boundary between the accreted terranes and the continental margin suggests that a terrane boundary may form a long-lasting, crustal flaw.

An isotopic study of the Island Lake Greenstone Belt, Manitoba: crustal evolution and progressive cratonization in the late Archean

Three new U–Pb zircon ages are reported for units within the Island Lake Greenstone Belt in northern Manitoba: the Jubilee Island dacite (2761 ± 12 Ma), the Chapin Bay tonalite (2748 ± 3 Ma), and the Wassagomach tonalité (2778 ± 5 Ma). Rb–Sr data suggest a mild metamorphic event about 2736 ± 67 Ma ago. Basalts within the basal Hayes River Group (2860–2900 Ma) appear to be derived from a time-averaged, Nd-depleted mantle with εNd values between +0.78 and +2.1. Lower εNd values (&lt; +0.31) are found in volcanic and intrusive rocks of the same age, implying contamination by or remelting of preexisting crust to form the more felsic derivatives. εNd values of early tonalitic, trondhjemitic, and granitic plutons (2729–2900 Ma) decrease from +0.31 to −1.8 as the plutons become younger, and are indicative of a period of dominantly intracrustal recycling processes in the Island Lake region. It is suggested that this trend records the evolution of the Island Lake region through extensive intracrustal melting and recycling from a variably contaminated volcanic arc to a small mature craton. Uplift associated with the plutonism resulted in sedimentation of the Island Lake Group (2729–2749 Ma), followed by renewed granitic plutonism of the Late Intrusive Suite (2699–2729 Ma). εNd values of these units range from +3 to −3, suggesting renewed interaction of depleted mantle with the Island Lake crust. This possibly resulted from magmatism associated with the accretion of the Island Lake Terrane to the rest of the Superior Province.

Tectonic implications of isotopic variation among Jurassic and Early Cretaceous plutons, Klamath Mountains

Jurassic and Early Cretaceous plutons in the Klamath Mountains province of California and Oregon were intruded both before and after thrusting during the Nevadan orogeny (ca. 155-148 Ma). Plutons intruded before Nevadan thrusting range from calc-alkalic to calcic and display a range of compositions that changes consistently with time. Among this group, Rb, K 2 O, and initial 87 Sr/ 86 Sr (Sr i ; range 0.7034 to 0.7047) decrease and ϵ Nd (range 2.0 to 4.8) increases in younger plutons. Within individual plutons, Sr i varies considerably, whereas ϵ Nd is nearly constant. The Sr and Nd isotopic data typically plot to the right of the mantle array. δ 18 O values in these plutons are greater than +8.0‰. Tonalitic and trondhjemitic plutons intruded after Nevadan thrusting are typically calcic, contain low Rb and K 2 O contents, and have δ 18 O less than +8.0‰. These plutons have a narrow range of SR i (0.7033-0.7036) and ϵ Nd (4.7 to 7.4); values that plot in the mantle array. The isotopic compositions of the pre-Nevadan plutons requires assimilation of metasedimentary rocks or mixing with crustal melts at deep-crustal levels. The deep-crustal component is not exposed in the province and was presumably truncated during Nevadan thrusting. Post-Nevadan plutons show little evidence of assimilation of metasedimentary crust but display trace-element evidence (low K 2 O, Rb, and rare-earth-element concentrations) consistent with an origin by partial melting of young ophiolitic rocks underthrust during the Nevadan orogeny.