U-Pb isotopic evidence for the accretion of different crustal blocks to form the Lewisian Complex of northwest Scotland

Single zircon and titanite U-Pb SHRIMP data presented for tonalite-trondhjemite-granodiorite (TTG) suite gneisses and an ultramafic rock from the northern and central regions of the Lewisian Complex of northwest Scotland, show that protolith ages of tonalitic gneisses in the northern region (2800–2840?Ma) are significantly younger than those in the central region (2960–3030?Ma). Further evidence of a major (2490–2480?Ma) metamorphic event in the central region is documented by a metamorphic zircon associated with a granulite facies ultramafic body. A dioritic gneiss from the northern region has also been dated at c. 2680?Ma. The northern region therefore does not comprise reworked central region rocks and consequently the old models for the evolution of the Lewisian which were based upon this concept need replacing. It is instead proposed that two distinct crustal blocks, now the northern and central regions, were tectonically juxtaposed along a boundary corresponding to the Laxford Front. Juxtaposition would appear to have occurred in Proterozoic times, as it must have postdated the 2490–2480?Ma (?Inverian) metamorphism recorded only in the central region, and the emplacement of granite sheets restricted to the northern side of the boundary. The first recorded event common to both regions is resetting of titanite ages associated with c. 1750?Ma Laxfordian amphibolite facies metamorphism. Zircon inheritance in rocks of both regions is scarce. Within one zircon from the northern region a c. 3550?Ma core was found. This represents the oldest known material from the region.     

Age and tectonic setting of granitoid gneisses in the Eastern Desert of Egypt and south-west Sinai

Strongly deformed and locally migmatized gneisses occur at several places in the southern Eastern Desert of Egypt and in Sinai and have variously been interpreted as a basement to Pan-african (900 to 600 Ma) supracrustal and intrusive assemblages. A suite of grabbroic to granitic gneisses was investigated in the Hafafit area, which constitutes an I-type calc-alkaline intrusive assemblage whose chemistry suggests emplacement along an active continental margin and whose granitoid members can be correlated with the so-called Older Granites of Egypt.²⁰⁷Pb/²⁰⁶Pb single zircon evaporation from three samples of the Hafafit gneisses yielded protolith emplacement ages between 677 ± 9 and 700 ± 12 Ma and document granitoid activity over a period of about 23 Ma. A migmatitic granitic gneiss from Wadi Bitan, south-west of Ras Banas, has a zircon age of 704 ± 8 Ma, and its protolith was apparently generated during the same intrusive event as the granitoids at Hafafit. Single zircons from a dioritic gneiss from Wadi Feiran in south-west Sinai suggest emplacement of the protolith at 796 ± 6 Ma and this is comparable with ages for granitoids in north-east Sinai and southern Israel. None of the above gneisses is derived from remelting of older continental crust, but they are interpreted as reflecting subduction-related calc-alkaline magmatism during early Pan-african magmatic arc formation.     

Age and evolution of scheelite-hosting rocks in the Felbertal deposit (Eastern Alps): U-Pb geochronology of zircon and titanite

U-Pb isotope analyses of zircon and titanite extracted from different rocks of the Felbertal scheelite deposit yield the following information: (1) An age of 593±22 Ma (2) is obtained for zircon crystallization in the scheelite-bearing matrix of an eruption breccia in the western ore field. (2) Discordant zircons from an elongated, up to 8 m thick scheelite-rich quartzite body in the eastern ore field give an upper intercept age of 544±5 Ma. This quartzite contains a laminated, fine-grained scheelite mineralization. (3) Zircons from a small granitoid intrusion of the western ore field reveal an age of 336±16 Ma, and concordant titanites document an age of 282±2 Ma for Variscan amphibolite facies metamorphism. Both events, granitoid intrusion and later metamorphism caused ore re-mobilization, including the formation of yellowish fluorescent (molybdo-) scheelite porphyroblasts. (4) For a narrow lamprop-1hyric dike in the western ore field, a concordant titanite age of 283±7 Ma is obtained. This age is identical with the titanites from the amphibolite facies metamorphic intrusion. Tiny scheelite grains were tapped by the dike from pre-existing scheelite mineralizations in the truncated host rocks. (5) Alpine metamorphism at 31±4 Ma did not exceed lowermost amphibolite facies conditions, and it caused scheelite re-mobilization on a minor scale only, producing bluish fluorescent porphyroblasts in quartz veinlets and veins, as well as bluish fluorescent scheelite rims around older scheelite grains. Moreover, crosscutting Alpine fissure fillings show bluish fluorescent, inclusion-free scheelite. (6) The preservation of Variscan titanites, the absence of Alpine titanite growth, and the large degree of Variscan scheelite re-mobilization demonstrate that amphibolite facies metamorphism in the Felbertal area has a Variscan age. This result clearly documents Variscan tectono-metamorphism to be the dominant event, instead of the hitherto surmised Alpine metamorphism. This multi-stage evolution of the Felbertal ore bodies corroborates the view that tungsten deposits are conditioned by several succeeding thermal events, leading to a series of stages that ultimately produce high-grade scheelite concentrations. These high-grade ores predominately occur along shear zones of different age, accompanied by the formation of large volumes of low-grade scheelite mineralizations along host rock foliations and quartz veinlets and veins.     

U-Pb monazite ages in amphibolite- to granulite-facies orthogneiss reflect hydrous mineral breakdown reactions: Sveconorwegian Province of SW Norway

In the Rogaland–Vest Agder terrain of the Sveconorwegian Province of SW Norway, two main Sveconorwegian metamorphic phases are reported: a phase of regional metamorphism linked to orogenic thickening (M1) and a phase of low-pressure thermal metamorphism associated with the intrusion of the 931 ± 2 Ma anorthosite-charnockite Rogaland igneous complex (M2). Phase M1 reached granulite facies to the west of the terrane and M2 culminated locally at 800–850 °C with the formation of dry osumilite-bearing mineral associations. Monazite and titanite U-Pb geochronology was conducted on 17 amphibolite- to granulite-facies orthogneiss samples, mainly from a suite of 1050 ⁺²/₋₈ Ma calc-alkaline augen gneisses, the Feda suite. In these rocks, prograde negatively discordant monazite crystallized during breakdown of allanite and titanite in upper amphibolite facies at 1012–1006 Ma. In the Feda suite and other charnockitic gneisses, concordant to slightly discordant monazite at 1024–997 Ma probably reflects breakdown of biotite during granulite-facies M1 metamorphism. A spread of monazite ages down to 970 Ma in biotite ± hornblende samples possibly corresponds to the waning stage of this first event. In the Feda suite, a well defined monazite growth episode at 930–925 Ma in the amphibolite-facies domain corresponds to major clinopyroxene formation at the expense of hornblende during M2. Growth or resetting of monazite was extremely limited during this phase in the granulite-facies domain, up to the direct vicinity of the anorthosite complex. The M2 event was shortly followed by cooling through ca. 610 °C as indicated by tightly grouped U-Pb ages of accessory titanite and titanite relict inclusions at 918 ± 2 Ma over the entire region. A last generation of U-poor monazite formed during regional cooling below 610 °C, in hornblende-rich samples at 912–904 Ma. This study suggests: (1) that monazite formed during the prograde path of high-grade metamorphism may be preserved; (2) that monazite ages reflect primary or secondary growth of monazite linked to metamorphic reactions involving redistribution of REEs and Th, and/or fluid mobilisation; (3) that the U-Pb system in monazite is not affected by thermal events up to 800–850 °C, provided that conditions were dry during metamorphism. Received: 9 January 1997 / Accepted: 15 April 1998     

Geochemistry and geochronology of the Mkhondo suite, Swaziland: evidence for passive-margin deposition and granulite facies metamorphism in the Late Archean of Southern Africa

The Archean Mkhondo suite in southern Swaziland is a multiply deformed succession of metasediments intruded with amphibolite dykes and sills and granitoid gneisses. Mineral and textural relationships indicate an early period of granulite facies metamorphism, followed later by amphibolite facies metamorphism. Geothermobarometry indicates maximum temperatures of 700–900°C and burial depths of 25–3 km. Paragneisses and biotite quartzites have LREE enriched patterns with small negative Eu anomalies, whereas white quartzites show variable REE patterns and low REE concentrations. BIF has slight LREE enrichment and Eu anomalies. Amphibolites have moderate LREE enrichment and depletions in Ta---Nb and P. Unlike many Archean granitoids, the Mkhondo granitoid gneisses are high in K and other LILE, have large negative Eu anomalies and are not depleted in HREE.SHRIMP isotopic analyses of detrital zircons from a biotite quartzite define a source age of 3600–3460 Ma. A deformed granitoid in tectonic contact with the Mkhondo suite yields a zircon evaporation mean age of 3192±5 Ma, which is interpreted as the age of emplacement. A zircon evaporation age of a granitic melt patch in paragneiss, as well as whole-rock and garnet Sm---Nd isotopic ages, suggest that the peak of high-grade metamorphism in the Mkhondo suite occurred at about 2750 Ma. This is the first evidence for Late Archean high-grade metamorphism in the southeastern Kaapvaal craton. The age data of this study restrict deposition of the Mkhondo suite to between 3.2 and 2.75 Ga.Mkhondo paragneisses are interpreted as shales with biotite quartzites as iron- and quartz-rich detrital sediments. Geochemical mixing calculations indicate that the sediment sources were composed of basalt (±komatiite), TTG and Eu-depleted granitoids. The Mkhondo assemblage may have been deposited along a passive continental margin or in a continental interior basin. The presence of minor BIF with positive Eu anomalies suggests minor hydrothermal input into the sedimentary basin. Intense chemical weathering was probably most important in production of the relatively pure quartz sands.     

Differential response of U-Pb systems in coexisting accessory minerals,Winnipeg River Subprovince,Canadian Shield: implications for Archean crustal growth and stabilization

The U-Pb isotopic systems of zircon, monazite, titanite and some apatite and the Pb isotopic composition of K-feldspar have been investigated in three areas of the Winnipeg River Subprovince (WRS) of the Superior Province, Canada, in order to define the timing of magmatic and metamorphic processes in this Archean gneissic-granitoid terrain.The new data together with published results define the following stages in the evolution of the WRS: (1) an extended period of early crustal growth punctuated by the episodic generation of tonalite. New ages include 3170+20/s-5 Ma, 2875+20/s-5 Ma and 2840+20/s-5 Ma for tonalitic gneisses at Cedar Lake, Kenora and Daniels Lake, respectively. (2) This early evolution was concluded by about 2760 Ma after emplacement of tonalite-granodiorite at Cliff Lake and was followed by a period of magmatic quiescence between about 2760 and 2710 Ma that contrasts with the intensive igneous activity characterizing the evolution of neighbouring greenstone belts. (3) A major episode of magmatism, deformation and metamorphism affected the Kenora and Daniels Lake areas between about 2710 and 2700 Ma. (4) A younger event caused deformation, metasomatism and amphibolite to granulite grade metamorphism at Cedar Lake and Daniels Lake at about 2680 Ma. (5) A subsequent, protracted period of low grade activity reset or (re-)crystallized titanite and apatite defining ages that scatter between about 2640 and 2520 Ma at Cedar and Daniels Lake but not in Kenora where titanite closed by about 2690 Ma. The 2680 Ma metamorphism may have been triggered in part by crustal thickening due to nappe thrusting but the subsequent period of lower grade activity requires the protracted addition of heat and/or fluids probably derived from magmatic and metamorphic processes continuing deep in the crust.The isotopic compositions of K-feldspars are relatively homogeneous and indicate mixing of Pb evolved in different reservoirs. The general enrichment in ²⁰⁷Pb with respect to normal terrestrial Pb reflects the protracted Archean evolution of the terrain.Now-coexisting minerals were formed and closed isotopically at different stages of the complex evolution and were selectively involved or excluded from isotopic equilibration with each other or with external systems such as hydrothermal fluids. This cautions against the indiscriminate interpretation of isotopic values obtained from whole rock systems in such complex terrains.     

The iron-rich suite from the Amîtsoq gneisses of southern West Greenland: early Archaean plutonic rocks of mixed crustal and mantle origin

A distinctive group of augen gneisses and ferrodiorites (termed the iron-rich suite) is a component of the early Archaean Amîtsoq gneisses of southern West Greenland. The iron-rich suite outcrops south of the mouth of Ameralik fjord in an area that underwent granulite facies metamorphism in the early Archaean. The iron-rich suite forms approximately 30% of the Amîtsoq gneiss of this area and occurs as sheets and lenses up to 500 m thick. The rest of the Amîtsoq gneisses are predominantly tonalitic-granodioritic, banded grey gneisses. Despite intense deformation and polymetamorphism, there is local field evidence that the iron-rich suite was intruded into the grey gneisses after they had been affected by tectonism and metamorphism. The banded grey gneisses are interpreted as 3,700 to 3,800 Ma old; U-Pb zircon ages from the iron-rich suite give concordia intercepts at circa 3,600 Ma.Coarse grained augen gneisses with microcline mega-crysts are the dominant lithology of the iron-rich suite. They are mostly granodioritic, grading locally into granite and diorite, and are generally rather massive, but locally have well-preserved layering or are markedly heterogeneous. Mafic components are commonly concentrated into clots rich in hornblende and biotite and containing apatite, ilmenite, sphene and zircon. Variation in the proportion of these clots is the main reason for the compositional variation of the augen gneisses. The ferrodiorites of the suite occur as lenses in the augen gneisses. Leucocratic granitoid sheets locally cut the iron-rich suite. The augen gneisses and ferrodiorites have geochemical characteristics in common, such as high Fe/Mg values and high contents of FeOt, TiO₂, P₂O₅, Zr, Y and total REE (rare earth elements).The iron-rich suite probably formed as follows:Heating of the lower crust adjacent to mantle-derived basic intrusions caused melting of the lower crust, giving rise to granodioritic magmas. Disruption of partially crystallised basic intrusions caused mixing of the crustal melts and the fractionated mantle melts to produce the augen gneisses with their high FeOt, TiO₂, P₂O₅, Zr, Y and total REE enrichment. Fragmented, crystallised parts of the basic intrusions gave rise to the ferrodiorite inclusions. These heterogeneous plutons rose to higher crustal levels where they crystallised as sheets and possibly were responsible for the local granulite facies metamorphism. The granitoid sheets that cut the iron-rich suite are interpreted as crustal melts of local origin.The iron-rich suite resembles Proterozoic rapakivi granite-ferrodiorite-norite (anorthosite) associations which form characteristic suites in late- to post-tectonic environments in recently thickened sial. The occurrence of this type of magmatism in the early Archaean is evidence of the complex, polygenetic nature of the oldest known continental crust.     

40Ar/39Ar age constraints on deformation and mineralization, Rosebery Zn-Pb-Cu and Mount Lyell Cu deposits, Tasmania, Australia

The ⁴⁰Ar/³⁹Ar dating of alteration muscovite from the Rosebery Zn-Pb-Cu and Mount Lyell Cu deposits, Tasmania, Australia, has determined a succession of deformation events which occurred from 400-378 Ma, and comprises the Devonian Tabberabberan Orogeny. The dates from Rosebery range from 400-390 Ma, are a minimum age for mineralization, indicate the time of deformation, and provide a maximum age limit for granitoid emplacement in the vicinity of the deposit. The ages from the Mount Lyell field range from 400-378 Ma, are a minimum for mineralization, and date cleavage development. The North Lyell Cu mineralization, which was probably broadly coeval with deformation, may have formed at 400 Ma. All pre-Devonian alteration micas in the Rosebery and Mount Lyell areas have been recrystallized or reset. The Tabberabberan deformation in western Tasmania was broadly contemporaneous with widespread crustal shortening in southeastern Australia, as established from the dating of alteration minerals associated with deformation-related precious and base metal deposits.     

U–Pb geochronology of gneisses and granitoids from the Danish island of Bornholm: new evidence for 1.47–1.45 Ga magmatism at the southwestern margin of the East European Craton

The Danish island of Bornholm is located at the southwestern margin of the Fennoscandian Shield, and features exposed Precambrian basement in its northern and central parts. In this paper, we present new U–Pb zircon and titanite ages for granites and orthogneisses from 13 different localities on Bornholm. The crystallization ages of the protolith rocks all fall within the range 1,475–1,445 Ma (weighted average ²⁰⁷Pb/²⁰⁶Pb ages of zircon). Minor age differences, however, may imply a multi-phase emplacement history of the granitoid complex. The presence of occasional inherited zircons (with ages of 1,700–1,800 Ma) indicates that the Bornholm granitoids were influenced by older crustal material. The east–west fabric observed in most of the studied granites and gneisses, presumably originated by deformation in close connection with the magmatism at 1,470–1,450 Ma. Most titanite U–Pb ages fall between 1,450 and 1,430 Ma, reflecting post-magmatic or post-metamorphic cooling. Granitoid magmatism at ca. 1.45 Ga along the southwestern margin of the East European Craton has previously been reported from southern Sweden and Lithuania. The ages obtained in this study indicate that the Bornholm magmatism also was part of this Mesoproterozoic event.     

Monazite dating of granitic gneisses and leucogranites from the Kerala Khondalite Belt,southern India: implications for Late Proterozoic crustal evolution in East Gondwana

Two stages of granitic magmatism occurred during the Pan-African evolution of the Kerala Khondalite Belt (KKB) in southern India. Granitic gneisses were derived from porphyritic granites, which intruded prior to the main stage of deformation and peak-metamorphism. Subsequently, leucogranites and leucotonalites formed during fluid-absent melting and intruded the gneiss sequences. Monazites from granitic gneisses, leucogranites and a leucotonalite were investigated by conventional U-Pb and electron microprobe dating in order to distinguish the different stages of magma emplacement. U-Pb monazite dating yielded a wide range of ages between 590–520 Ma which are interpreted to date high-grade metamorphism rather than magma emplacement. The results of this study indicate that the KKB experienced protracted heating (>50 Ma) at temperatures above 750–800 °C during the Pan-African orogeny. The tectonometamorphic evolution of the study area is comparable to southern Madagascar which underwent a similar sequence of events earlier than the KKB. The results of this study further substantiate previous assertions that the timing of high-grade metamorphism in East Gondwana shifted from west to east during the Late Proterozoic.     

Geochemistry and petrology of the jequie granulitic complex (Brazil): an archean basement complex

The Jequie granulitic complex is part of the extensive high-grade metamorphic terrain located within the Sao Francisco craton of northeastern Brazil. Some Jequie rocks appear to have been formed in the middle Archean ( 3.1 Ga) from preexisting sialic crust. The dominant mineral composition of these rocks is quartz-microcline-plagioclase-hyperstene and occurs over an extensive area ( 2,000 km²).Scattered enrichment of normal granites with many minor elements (e.g. Rb, Y, Zr, Nb, Ba, REE), and the non-depletion of other elements (e.g. Cs, U), normally considered mobile during granulite facies metamorphism, must lead either to the reconsideration of regional metasomatism subsequent to granulite facies metamorphism, or at least raise some doubts about common wisdom concerning the distribution of heat-producing elements at depth. The region includes large-scale thrust structures, which could play a part in influencing high-level emplacement of the rocks and their regional metasomatism and structures.     

SHRIMP monazite and zircon geochronology of high-grade metamorphism in New Zealand

Ion microprobe dating of zircon and monazite from high-grade gneisses has been used to (1) determine the timing of metamorphism in the Western Province of New Zealand, and (2) constrain the age of the protoliths from which the metamorphic rocks were derived. The Western Province comprises Westland, where mainly upper crustal rocks are exposed, and Fiordland, where middle to lower crustal levels crop out. In Westland, the oldest recognisable metamorphic event occurred at 360–370 Ma, penecontemporaneously with intrusion of the mid-Palaeozoic Karamea Batholith (c. 375 Ma). Metamorphism took place under low-pressure/high-temperature conditions, resulting in upper-amphibolite sillimanite-grade metamorphism of Lower Palaeozoic pelites (Greenland Group). Orthogneisses of younger (Cretaceous) age formed during emplacement of the Rahu Suite granite intrusives (c. 110 Ma) and were derived from protoliths including Cretaceous Separation Point suite and Devonian Karamea suite granites. In Fiordland, high-grade paragneisses with Greenland Group zircon age patterns were metamorphosed (M1) to sillimanite grade at 360 Ma. Concomitant with crustal thickening and further granite emplacement, M1 mineral assemblages were overprinted by higher-pressure kyanite-grade metamorphism (M2) at 330 Ma. It remains unclear whether the M2 event in Fiordland was primarily due to tectonic burial, as suggested by regional recumbent isoclinal folding, or whether it was due to magmatic loading, in keeping with the significant volumes of granite magma intruded at higher structural levels in the formerly contiguous Westland region. Metamorphism in Fiordland accompanied and outlasted emplacement of the Western Fiordland Orthogneiss (WFO) at 110–125 Ma. The WFO equilibrated under granulite facies conditions, whereas cover rocks underwent more limited recrystallization except for high-strain shear zones where conditions of lower to middle amphibolite facies were met. The juxtaposition of Palaeozoic kyanite-grade rocks against Cretaceous WFO granulites resulted from late Mesozoic extensional deformation and development of metamorphic core complexes in the Western Province.     

Latest precambrian (Cadomian) zircon ages,Nd isotopic systematics and P-T evolution of granitoid orthogneisses of the Erzgebirge,Saxony and Czech Republic

Single zircons from two orthogneiss complexes, the Grey Gneiss and Red Gneiss, the lowermost tectonic units in the Erzgebirge, were dated. The grey Freiberg Gneiss is of igneous origin and has a ²⁰⁷Pb/²⁰⁶Pb emplacement age of 550±7 Ma. A quartz monzonite from Lauenstein contains idiomorphic zircons with a mean ²⁰⁷Pb/²⁰⁶Pb age of 555±7 Ma as well as xenocrysts ranging in age between 850 and 1910 Ma. Red gneisses from the central Erzgebirge contain complex zircon populations, including numerous xenocrysts up to 2464 Ma in age. The youngest, idiomorphic, zircons in all samples yielded uniform ²⁰⁷Pb/²⁰⁶Pb ages between 550±9 and 554±10 Ma. Nd isotopic data support the interpretation of crustal anatexis for the origin of both units. _Nd(t) values for the grey gneisses are –7.5 and –6.0 respectively, (mean crustal residence ages of 1.7–1.8 Ga). The red gneisses have a wider range in _Nd(t) values from –7.7 to –2.8 (T _DM ages of 1.4–1.8 Ga). The zircon ages document a distinct late Proterozoic phase of granitoid magmatism, similar in age to granitoids in the Lusatian block farther north-east. However, Palaeozoic deformation as well as medium pressure metamorphism ( 8 kbar/600–650° C) are identical in both gneiss units and distinguish these rocks from the Lusatian granitoids. The grey and red gneisses were overthrust by units with abundant high-pressure relicts and a contrasting P-T evolution. Zircon xenocryst and Nd model ages in the range 1000–1700 Ma are similar to those in granitoid rocks of Lusatia and the West-Sudetes, and document a pre-Cadomian basement in parts of east-central Europe that, chronologically, has similarities with the Sveconorwegian domain in the Baltic Shield.     

U-Pb ages of zircons from meta-igneous and meta-sedimentary rocks of the Sierra de Guadarrama: implications for the Central Iberian crustal evolution

U-Pb data of zircons from various gneisses of the eastern Sierra de Guadarrama, Central Spain, make it possible to reconstruct the pre-Hercynian and Hercynian history of this area: (1) upper intercept ages (2400 Ma for meta-igneous and 2000 Ma for meta-sedimentary rocks) of discordias defined by data of anhedral zircon fractions document the existence of Early Proterozoic crust; (2) lower intercept minimum ages of anhedral zircon fractions show radiogenic lead loss at about 540 Ma due to a thermal event leading to volcanism and emplacement of granitoid rocks into Precambrian crust: growth of euhedral zircons is possibly related to this event; (3) lower intercept minimum ages of about 380 Ma defined by anhedral zircon fractions in meta-sedimentary rocks prove an Early Hercynian event. According to present knowledge on the metamorphism in the Sierra de Guadarrama this event could be explained in terms of a Barrow-type medium-pressure metamorphism. The inferred Cadomian igneous event relates the geological history of Central Spain with that of western Africa to the south and Brittany to the north. Furthermore, similarities of the crustal evolution in the area studied and other internal zones of the Hercynian belt (Moldanubian zone, French Central Massiv) are confirmed. The Early Hercynian event for the first time affected all the rocks of the area together. The pre-Hercynian evolution of the two complexes is different and the present association of the basement rocks may be explained by thrusting or in terms of Early-Hercynian nappe transport.     

Iron- and manganese-rich minor intrusions emplaced under late-orogenic conditions in the proterozoic of South Greenland

Three small intrusions in Ketilidian gneisses near Julianehaab comprise sheets and veins of olivine-magnetite-grunerite or magnetite-amphibole rocks partly surrounded by garnetiferous hornblende-biotite granitoid rock. The latter skin locally widens out into diffuse bodies of fayalite-orthopyroxene-quartz syenite or monzonite and biotite granite, which show layering similar to that resulting from gravity settling of crystals. Near the intrusions the country rocks lose their foliation and have been partially melted. Intrusion probably occurred at the close of regional metamorphism 1,750–1,780 m.y. ago, just prior to emplacement of the rapakivi granite suite of South Greenland. The mafic minerals of the intrusions are markedly enriched in iron and, in the case of olivine, orthopyroxene, grunerite and garnet, in manganese as well: olivine Fa₉₀Te₅Fo₅; orthopyroxene (inverted pigeonite) Ca₂Fe₇₇Mn₆Mg₁₅; calciferous amphiboles are typically hastingsitic; biotites generally have Fe/Fe+Mg ratios of 0.8; garnets are almandine-grossularite-spessartine mixtures; essentially pure magnetite is the dominant oxide mineral and ilmenite is only moderately manganiferous. Crystallization of the mafic rocks appears to have followed the trend of the quartz-fayalite-magnetite buffer curve from perhaps 800°C to <550°C at pressures, calculated from thermodynamic considerations, of 4 to 5 kb. However, the presence of Mn makes estimates of pressure and temperature uncertain. Comparison with other late- to post-orogenic intrusions—the South Greenland and Finnish rapakivi granite suites, the Labrador adamellite complex and the Pikes Peak batholith of Colorado—reveals both similarities and differences, particularly with respect to mineral parageneses, depth of emplacement and manganese enrichment.     

Late-orogenic Sveconorwegian massif anorthosite in the Jotun Nappe Complex,SW Norway,and causes of repeated AMCG magmatism along the Baltoscandian margin

The age and tectonometamorphic history of massif anorthosite in the Jotun Nappe Complex, SW Norway, were investigated by zircon and titanite U–Pb ID-TIMS. The anorthosite contains sparse zircons showing complex U–Pb systematics reflecting events dated at 965 ± 4 and 913 ± 2 Ma, and a pronounced Caledonian metamorphic overprint. The oldest age is interpreted as the protolith age of the massif anorthosite. We propose that the Jotun anorthosite is related to 970–960 Ma magmatism in the Western Gneiss Region and coeval, orogen-perpendicular extension. Conversely, a 930 Ma high-grade metamorphic event in the Jotun Nappe Complex and the related Lindås Nappe is likely related to formation of the autochthonous ca. 930 Ma Rogaland anorthosite complex. We suggest that the two late- to post-orogenic AMCG events reflect two instances of lithospheric foundering below the orogen separated by ca. 20–30 my. The 913 ± 2 Ma metamorphic episode appears to date a heating event restricted to the outermost edge of the Western Gneiss Region. Leucosome formation in high-grade gneisses geographically close to the Jotun anorthosite is dated at 892 ± 4 Ma and suggested to reflect CO₂-rich (?) fluid flux along shear zones.     

Sm−Nd dating of multiple garnet growth events in an arc-continent collision zone,northwestern U.S. Cordillera

Integrated petrologic and Sm–Nd isotopic studies in garnet amphibolites along the Salmon River suture zone, western Idaho, delineate two periods of amphibolite grade metamorphism separated by at least 16 million years. In one amphibolite,P–T studies indicate a single stage of metamorphism with final equilibration at 600°C and 8–9 kbar. The Sm–Nd isotopic compositions of plagioclase, apatite, hornblende, and garnet define a precise, 8-point isochron of 128±3 Ma (MSWD=1.2) interpreted as mineral growth at the metamorphic peak. A⁴⁰Ar/³⁹Ar age for this hornblende indicates cooling through 525°C at 119±2 Ma. In a nearby amphibolite, garnets with a two-stage growth history consist of inclusion-rich cores surrounded by discontinuous, inclusion-free overgrowths. Temporal constraints for core and overgrowth development were derived from Sm–Nd garnet — whole rock pairs in which the garnet fractions consist of varying proportions of inclusion-free to inclusion-bearing fragments. Three garnet fractions with apparent ages of 144, 141, and 136 Ma are thought to represent mixtures between late Jurassic (pre-144 Ma) inherited radiogenic components preserved within garnet cores and early Cretaceous (128 Ma) garnet overgrowths. These observations confirm the resilience of garnet to diffusive exchange of trace elements during polymetamorphism at amphibolite facies conditions. Our geochronologic results show that metamorphism of arc-derived rocks in western Idaho was episodic and significantly older than in arc rocks along the eastern margin of the Wrangellian Superterrane in British Columbia and Alaska. The pre-144 Ma event may be an expression of the late Jurassic amalgamation of marginal oceanic arc-related terranes (e.g., Olds Ferry, Baker, Wallowa) during the initial phases of their collision with North American rocks. Peak metamorphism at 128 Ma reflects tectonic burial along the leading edge of the Wallowa arc terrane during its final penetration and suturing to cratonic North America.     

Geochemistry and Sr-Nd isotopes of amphibolite dykes of northern Victoria Land, Antarctica

Mafic dykes cutting the gneisses and migmatites in the Deep Freeze Range high-grade metamorphic complex of northern Victoria Land, Antarctica, have undergone strong recrystallization and deformation during amphibolite-facies metamorphism. Metamorphic mobility mostly affected the large-ion lithophile elements (LILE). Rare Earth (RE) and the high field-strength elements (HFSE) were essentially immobile during metamorphism. Together with the major-element geochemistry, this suggests primary characteristics of evolved tholeiitic magmas and mafic cumulates. No precise ages of intrusion are available for the dykes, but geological evidence suggest emplacement during the time interval 800 to 900 Ma. The Rb-Sr isotopic system in some of the dykes were also variably affected by a later thermal event, probably coincident with the time of amphibolite metamorphism, ca. 500–550 Ma ago. This event can be correlated with the Ross Orogeny in the Transantarctic Mountains. Nd isotopes and trace element abundances indicate that the dykes were derived by different degrees of partial melting and fractionation of heterogeneous sub-continental lithospheric mantle. The Nd isotopic compositions range from depleted to enriched signatures (ε^Nd computed back to 850 Ma = +4.5 to −11.61), and are coupled to different trace element normalized patterns characterized by a slight positive Nb anomaly in the former case to a strong negative Mb anomaly for the latter samples. On isotopic and chemical ground the depleted signature of the mantle source resembles that reported for E-type MORB. The nature of the enriched components cannot be uniquely stated; nevertheless, on the basis of isotopic and geochemical data, it could be represented by sediments recycled into the sub-continental mantle or by crustal contamination during underplating of mafic magmas, or a combination of the two processes.     

A reappraisal of the Lewisian Gneiss Complex: geochronological evidence for its tectonic assembly from disparate terranes in the Proterozoic

New U-Pb single-zircon geochronology undertaken on tonalitic gneisses, granite sheets, migmatites and metasediments from the Lewisian Gneiss Complex on the mainland and the northern part of the Outer Hebrides, NW Scotland, have been used to test the correlation of so-called Laxfordian events across the complex from the Outer Hebrides to the mainland, and the current model for the evolution of the complex as a whole. The study has revealed that the granite sheets originated in two quite different melting events. Those on the mainland at Loch Laxford are ca. 1,855 Ma old whereas those on Harris and Lewis, with which they are presently correlated, are ca. 1,675 Ma old. Grey gneisses associated with granites on the south side of Loch Laxford are confirmed to belong to the 'northern region'. A migmatitic grey gneiss on Harris has given a protolith age of ca. 3,125 Ma, the currently oldest recognised in the complex. Detrital zircons in the Leverburgh and Langavat belts range in age from 2,780 to 1,880 Ma and unequivocally demonstrate deposition in the Palaeoproterozoic. The granulite facies metamorphism in this block is dated from zircon overgrowths at ca. 1,880 Ma. The Laxford Shear Zone which separates the northern and central regions is interpreted to have evolved post-1,860 Ma, during amphibolite facies metamorphism accompanying deformation which took place at ca. 1,740 Ma in both regions. On Harris, the Langavat-Finsbay shear zone developed after 1,675 Ma when a ca. 1,880-Ma granulite facies Proterozoic arc was juxtaposed against amphibolite facies Archaean rocks to the north. Therefore, the shear zones which bound tectonic blocks in the Lewisian Complex evolved at different times and can be interpreted as terrane boundaries. The new data confirm that the Lewisian Complex was not constructed from one contiguous piece of Archaean crust reworked in the Proterozoic but was progressively assembled from several discrete terranes during the Proterozoic. Accordingly, the former regional divisions of the Lewisian Complex are here renamed as follows. On the mainland, the northern region is called the Rhiconich terrane, and the central region the Assynt terrane. On the Outer Hebrides, the Archaean gneisses of Lewis and the northern part of Harris comprise the Tarbert terrane, whereas the newly accreted Proterozoic blocks are called the Roineabhal terrane in Harris and the Niss terrane in the north on Lewis. Wider correlations show that the geology of the Outer Hebrides has more in common with East Greenland than mainland Scotland on the eastern side of the Minch Fault.     

The augen gneisses of the Peloritani Mountains (NE Sicily): Granitoid magma production during rapid evolution of the northern Gondwana margin at the end of the Precambrian

The medium- to high-grade polymetamorphic basement rocks of the Peloritani Mountains, northern Sicily, include large volumes of augen gneiss of controversial age and origin. By means of a geochemical and SHRIMP zircon study of representative samples, the emplacement age of the original granitoid protoliths of the augen gneisses and the most likely processes and sources involved in that granitoid magmatism have been determined. U–Pb dating of three samples from widely spaced localities in the Peloritani Mountains yielded igneous protolith ages of 565 ± 5, 545 ± 4 and 545 ± 4 Ma, respectively. These late Ediacaran/early Cambrian ages are much older than was previously assumed on geological grounds, and are typical of the peri-Gondwanan terranes involved in the geodynamic evolution of the northern Gondwana margin at the end of the Avalonian–Cadomian orogeny. Major and trace element compositions and Sr–Nd isotopic data, in combination with zircon inheritance age patterns, suggest that the granitoid protoliths of the Sicilian and coeval Calabrian augen gneisses were generated by different degrees of mixing between sediment- and mantle-derived magmas. The magmas forming the ca. 545 Ma inheritance-rich granitoids appear to have had a significant contribution from partial melting of paragneiss that is the dominant rock type in the medium- to high-grade Peloritanian basement. The closeness of the inferred deposition age of the greywacke protoliths of the paragneisses with the intrusion age of the granitoids indicates rapid latest Precambrian crustal recycling involving erosion, burial, metamorphism to partial melting conditions, and extensive granitoid magmatism in less than ca. 10 Ma.