Windows of metamorphic sulfur liberation in the crust: Implications for gold deposit genesis

Understanding the source of metamorphic sulfur is critical to clarifying the complete cycle of ore genesis, from source to sink, for several mineral deposit types. In this study, a mass balance approach and the thermodynamic computer programs Thermocalc and PerpleX were used to constrain the P-T range of pyrite breakdown to pyrrhotite (which liberates sulfur) in common metamorphic lithologies. The results suggest that most of the continental crust’s metamorphic sulfur is liberated in a relatively narrow temperature-pressure window corresponding to the terminal breakdown of chlorite at moderate to low pressures. This is because pyrite stability is controlled partly by temperature and pressure, and partly by the amount of H₂O present. During prograde metamorphism from the greenschist to the amphibolite facies, metamorphic H₂O is produced primarily through chlorite breakdown in mafic to pelitic bulk compositions. As temperature increases, more sulfur is required from pyrite to maintain equilibrium proportions of H₂O, H₂S and SO₂ in the fluid, and in addition, progressively more sulfur is required at lower pressures. At low temperatures, little sulfur is required by metamorphic fluid released during initial chlorite breakdown, whereas at higher temperatures coinciding with the terminal breakdown of chlorite, not only is more fluid present, but the fluid’s sulfur requirement has also increased dramatically. In this way, metamorphic dehydration drives pyrite breakdown and generation of sulfur-rich hydrothermal fluids at mesothermal conditions. Beyond the chlorite stability field there is minimal metamorphic fluid production, except at low pressures and high temperatures where muscovite can break down without causing melting; conditions that are a long way from typical crustal geotherms. However, deformation also plays a key role in pyrite breakdown. Without deformation, small amounts of fluid in chemical communication with individual pyrite grains will quickly acquire equilibrium concentrations of the sulfur species and minimal pyrite breakdown is necessary. Whereas during deformation, there may be a continuous fluid flux past pyrite grains, promoting ongoing sulfur liberation. In this way, periods of deformation may be the major sulfur-liberating episodes during a metamorphic cycle. Since hydrothermal fluids are inherently buoyant and consequently tend to migrate upwards and towards cooler temperatures through the crust, these results imply that orogenic gold deposits are most likely to form at lower-amphibolite to prehnite-pumpellyite facies conditions, and unlikely to form at higher temperatures. The pressure constraint on metamorphic sulfur liberation implies that tectonic settings that allow prograde metamorphism to follow low pressure P-T-t paths in an occasionally compressional or transpressional environment are necessary. Settings that promote extensive injection of felsic magma into a mid-crust that contains a significant proportion of pyritic carbonaceous metasediment are shown to be ideal for orogenic gold deposit genesis. Inverted back-arc basins are interpreted to be the most favourable of these.     

A catalytic delamination-driven model for coupled genesis of Archaean crust and sub-continental lithospheric mantle

There is no consensus on the processes responsible for near-coeval formation of Archaean continental crust (dominantly tonalite-trondhjemite-granodiorite: TTG), greenstone belts dominated by komatiitic to tholeiitic lavas (KT), and sub-continental lithospheric mantle (SCLM). The Douglas Harbour domain (2.7-2.9 Ga) of the Minto Block, northeastern Superior Province, has two TTG suites, the western and eastern Faribault-Thury (WFT and EFT), with embedded KT greenstones. Tonalites of both suites have high light/heavy rare-earth element ratios (L/HREE), high large ion lithophile element (U-Th-Rb-Cs-La: LILE) contents, positive Sr-Pb anomalies, and negative Nb-Ta-Ti anomalies. Such typical Archaean TTG signatures are commonly explained by melting of subducted oceanic crust, but could also originate by melting the base of thick basaltic plateaux formed above mantle upwellings (plumes), leaving behind restites containing pyroxene, garnet, and rutile. Field relationships (in situ segregation veins), phase equilibria (hornblende stabilized at lower crustal pressure), petrography (corroded epidote and muscovite phenocrysts, rare plagioclase phenocrysts), and trace element models, all imply that FT tonalite to trondhjemite evolution reflects hornblende-dominated fractional crystallization, not partial melting of subducted crust. The geochemistry of parental FT tonalites can be modeled by 15-30% melting of FT tholeiitic metabasalts, with residues of eclogite, garnet-websterite, or hornblende-garnet websterite. A minor residual Ti-phase such as rutile is also needed to generate negative Ti-Nb-Ta troughs in the TTGs. However, large volumes of eclogitic restites complementary to TTG are not observed either at the base of Archaean crustal sections, or in the SCLM. Additional problems with slab-melting models include: (a) the rarity of lithologies and associations characteristic of active margins (ophiolites, andesites, blueschists, accretionary mélanges, molasse, flysch, high-pressure belts, and thrust-and-fold belts); (b) the need to deliver plume-derived KT melt through the slab; and (c) extracting enough TTG melt from a subducting slab in the time available (200-300 my). In the plateau-melting model, heat for crustal anatexis is supplied by ongoing KT magma derived from mantle upwellings. However, SCLM rocks differ from predicted 1-stage mantle melting residua; and the voluminous residual eclogites complementary to TTG generation somehow need to be removed. These two problems might solve one another if the dense crustal restites disaggregated and mixed into the underlying depleted mantle. Mantle melting slows upon exhaustion of Ca-Al-rich phases, with large temperature increases needed to extract more melt from harzburgite residua. Physical addition of delaminated crustal restites would refertilize the refractory mantle, allowing extraction of additional melt increments, and might explain the ultra-depleted and orthopyroxene-rich nature of the SCLM. A hybrid source composed of 10% eclogitic restite of EFT tonalite generation, mixed with harzburgitic residues from 25% melting of primitive mantle, yields model melts with trace element signatures resembling typical Munro komatiites. Variations in the mineralogy and geochemistry of the delaminated component might account for the diversity of komatiite types. Degassing of hornblende-rich delaminated restites would transfer LILE to surrounding depleted mantle and could generate boninites. Fusion of undepleted metabasalt sandwiched among denser restites could generate sanukitoids. Mantle melt pulses generated by catastrophic delamination events would underplate nascent TTG crust and trigger renewed crustal melting, followed by delamination of newly formed eclogitic restites, triggering additional mantle melting, and so on. I posit that delamination of crustal restites catalyzed multi-stage melting of the SCLM and maturation of the Archaean continental crust. Thus, Archaean crust and SCLM are genetically inter-linked, and both form above major mantle upwellings.     

Fluid inclusions in Scourian granulites from the Lewisian complex of NW Scotland: evidence for CO2-rich fluid in Late Archaean high-grade metamorphism

In this paper the first fluid-inclusion data are presented from Late Archaean Scourian granulites of the Lewisian complex of mainland northwest Scotland. Pure CO₂ or CO₂-dominated fluid inclusions are moderately abundant in pristine granulites. These inclusions show homogenization temperatures ranging from − 54 to + 10 °C with a very prominent histogram peak at − 16 to − 32 °C. Isochores corresponding to this main histogram peak agree with P-T estimates for granulite-facies recrystallization during the Badcallian (750–800 °C, 7–8 kbar) as well as with Inverian P-T conditions (550–600 °C, 5 kbar). The maximum densities encountered could correspond to fluids trapped during an early, higher P-T phase of the Badcallian metamorphism (900–1000 °C, 11–12 kbar). Homogenization temperatures substantially higher than the main histogram peak may represent Laxfordian reworking (≤ 500 °C, < 4 kbar). In the pristine granulites, aqueous fluid inclusions are of very subordinate importance and occur only along late secondary healed fractures. In rocks which have been retrograded to amphibolite facies from Inverian and/or Laxfordian shear zones, CO₂ inclusions are conspicuously absent; only secondary aqueous inclusions are present, presumably related to post-granulite hydration processes. These data illustrate the importance of CO₂-rich fluids for the petrogenesis of Late Archaean granulites, and demonstrate that early fluid inclusions may survive subsequent metamorphic processes as long as no new fluid is introduced into the system.     

Geochemistry of late Archaean metagreywackes from the Western Dharwar Craton,South India: Implications for provenance and nature of the Late Archaean crust

Late Archaean metagreywackes of the Ranibennur Formation, Dharwar Supergroup, in the Dharwar–Shimoga schist belt of the Western Dharwar Craton (WDC) are texturally and mineralogically immature of the quartz-intermediate type. The SiO₂ content in them ranges from 60.58 to 65.26 wt.%. Chemical Index of weathering (CIW) values varies between 50 and 65. 4 indicating a low degree of chemical alteration of the provenance rocks. A high degree of correlation between K₂O and Al₂O₃ (r = ? 0.73) and low Rb/Sr ratios also suggest a low degree of alteration of provenance rocks. Abundances of transition group elements (Cr = 118–221; N = 89–154; V = 89–192 and Sc = 11–16 ppm) as well Zr (132–191 ppm) suggest a mixed mafic–felsic provenance for the metagreywackes. Low HREE and Y content, and low Tb/Yb ratios (0.23–0.41) suggest the presence of tonalite as an important component in the provenance areas. Values of Eu/Eu?(0.78) and Th/Sc (0.55) suggest that the granodioritic upper crust had evolved prior to serving as the provenance. Mixing calculations suggest 50–55 vol.% tonalite, 20–25 vol.% granite, 18–20 vol.% basalt and ~ 5 vol.% komatiite composition for the provenance. Geochemical characteristics of the Ranibennur metagreywackes suggest that sedimentary basin formed in the vicinity of a magmatic arc in a continental island arc setting, and the detritus were shed from the arc rock.     

The genesis of the Archaean Welcome Well volcanic complex,Western Australia

The Welcome Well volcanic complex east of Leonora, Western Australia, is interpreted to be the eroded remnant of an Archaean stratovolcano. Andesitic flows and intercalated mudflow deposits comprising the volcanic centre give way to coarse, poorly-sorted lithic wackes that were deposited in alluvial fans skirting the lower slopes or base of the subaerial volcanic edifice. These deposits are succeeded both laterally and vertically by fine-grained, subaqueous, turbiditic sediments that are intercalated with pillowed, tholeiitic basalts.There is a complete petrographic and geochemical gradation from porphyritic basalt through porphyritic andesite to porphyritic dacite. In general, the rocks show calc-alkaline patterns of elemental behaviour, consistent with fractionation of variable proportions of the modal minerals amphibole, plagioclase, clinopyroxene and Ti-magnetite. Among these minerals, amphibole appears to have assumed a major role in producing the geochemical characteristics of the high-Si andesites and dacites as evidenced by the behaviour of Zr, Nb, Y and REE. In order to account for the geochemical variability of the basalts and low-Si andesites, it is proposed that they differentiated from primitive basic parents which had a range of major and LIL element contents. The most plausible origin for the primary magmas involves shallow, hydrous melting of a LIL element-enriched mantle source over a significant pressure range.     

赣南江贝变质岩风化壳离子吸附型稀土矿床特征及成因

通过对赣南江贝稀土矿床进行岩相学、矿床学和地球化学研究,表明该矿床成矿母岩主要为寻乌岩组变质岩,岩性有变粒岩、片岩和片麻岩,其中云母片岩稀土元素含量最高,是最有利的成矿母岩,属轻稀土富集型母岩,是近些年新发现的1处中深变质岩风化壳离子吸附型稀土矿.矿区呈低缓丘陵地貌,风化壳平均厚度12.33 m,矿体主要赋存于全风化层中,矿体平均厚度4.68 m, SRF2 O3品位0. 035%~0. 122%,属离子吸附型稀土矿.稀土主要来源于易风化的水磷酸盐类矿物和稀土氟碳酸盐类矿物,风化壳化学蚀变指数CIA>90 %,表明了强烈的风化作用为矿床的形成创造了有利条件.会昌—安远—寻乌地区中深变质岩广泛分布,具有大型稀土矿成矿潜力.     

Mineralogical changes occurring during the retrogression of Archaean gneisses from the Lewisian complex of NW Scotland

Gneisses, metamorphosed at granulite facies ca 2.7 Ga, were subsequently retrogressed to amphibolite facies during a prolonged period of retrogression, perhaps lasting as long as 200 m.y. The Scourie dykes were emplaced towards the end of this event. Localised Laxfordian shear zones further modified the mineral assemblages. The retrogression caused the production of a uniform $\text{plagioclase-hornblende}\text{- ±}\text{quartz}\text{±}\text{biotite}$ assemblage. A study of hornblende composition shows that it depends on metamorphic grade, host rock composition and paragenesis. The sequence of mineral assemblages suggests that retrogression took place on a falling temperature path, beginning at about $\text{650±50°}\text{C}$. Post-tectonic muscovite indicates that temperatures were still in excess of 500°C after the formation of Laxfordian shear zones. This indicates that the Lewisian complex was uplifted and cooled extremely slowly.     

Metasomatism and the crystallization of zircon megacrysts in Archaean peridotites from the Lewisian complex,NW Scotland

Zircon megacrysts are locally abundant in 1–40 cm-thick orthopyroxenite veins within peridotite host rocks in the Archaean Lewisian gneiss complex from NW Scotland. The veins formed by metasomatic interaction between the ultramafic host and Si-rich melts are derived from partial melting of the adjacent granulite-facies orthogneisses. The interaction produced abundant orthopyroxene and, within the thicker veins, phlogopite, pargasite and feldspathic bearing assemblages. Two generations of zircon are present with up to 1 cm megacrystic zircon and a later smaller equant population located around the megacryst margins. Patterns of zoning, rare earth element abundance and oxygen isotopic compositions indicate that the megacrysts crystallized from crustal melts, whereas the equant zircon represents new neocryst growth and partial replacement of the megacryst zircon within the ultramafic host. Both zircon types have U–Pb ages of ca. 2464 Ma, broadly contemporaneous with granulite-facies events in the adjacent gneisses. Zircon megacrysts locally form?>?10% of the assemblage and may be associated to zones of localized nucleation or physically concentrated during movement of the siliceous melts. Their unusual size is linked to the suppression of zircon nucleation and increased Zr solubility in the Si-undersaturated melts. The metasomatism between crustal melts and peridotite may represent an analog for processes in the mantle wedge above subducting slabs. As such, the crystallization of abundant zircon in ultramafic host rocks has implications for geochemistry of melts generated in the mantle and the widely reported depletion of high field strength elements in arc magmas.     

Alteration of the Damiao anorthosite complex in the northern North China Craton: Implications for high-grade iron mineralization

The Damiao type iron deposit is hosted in a typical Proterozoic anorthosite complex in the northern North China Craton. The types of ores in Damiao mainly comprise massive Fe ores, massive Fe–P ores, and disseminated Fe and Fe–P ores. The disseminated Fe and Fe–P ores formed by fractional crystallization are generally hosted in oxide-apatite gabbronorite and account for 70% of the proven reserve of the Damiao type iron ore. The massive Fe and Fe–P ores account for 30% of the proven reserve of the Damiao type deposit iron ore and generally occur as irregular dykes or veins filling vertical fractures of the previously consolidated anorthosite, showing typical features of hydrothermal mineralization. The contact between the massive orebodies and wall rocks is sharp and straight. The anorthosite comprises white and dark varieties, with the former resulted by the alteration of the latter that occurs as relicts. Petrographic observation and electron microprobe analyses show abundant Fe–Ti oxide inclusions in plagioclase which impart the dark color to the rock. The similar spider diagram patterns between fresh and altered plagioclase and between dark- and white-colored anorthosite imply a genetic relationship between the dark and white types. During the alteration of anorthosite, CaO and MgO were slightly decreased, the SiO₂, Al₂O₃ and Na₂O were significantly increased, and the TFe₂O₃ and TiO₂ were significantly decreased. The TFe₂O₃ and TiO₂ in the dark-colored anorthosite have a range of 4.86–12.18 wt.% and 0.37–1.65 wt.%, respectively. However, The TFe₂O₃ and TiO₂ in the white-colored anorthosite have a range of 1.67–3.1 wt.% and 0.14–0.31 wt.%, respectively. These features suggest that the alteration of the anorthosite led the Fe element by leaching from the dark-colored anorthosite at highly oxidized condition, and then precipitated within the fractures of the anorthosite, thus forming the massive Fe and Fe–P orebodies. Because the estimated amount of transported Fe is much more abundant than the proven ore reserve, we infer that there should be huge potential for prospecting Damiao type iron ores.     

新疆尉犁县风屏北山片麻杂岩地质特征及原岩成因探讨

对风屏北山片麻杂岩进行了同位素年龄确定,采用SHR IMP放射性同位素测年,确定同位素定年结果为2 484±20 Ma。根据岩石化学、稀土、微量元素等特征,对片麻杂岩进行了分析研究,认为该片麻岩原岩为I型花岗岩,经区域变质作用形成。     

A Metamorphosed, Early Archaean Chromitite from West Greenland: Implications for the Genesis of Archaean Anorthositic Chromitites

An early Archaean (>3·81 Ga) chromitite–ultramaficlayered body from the Ujaragssuit nunât area, west Greenland,may represent the Earth’s oldest chromitite. The layeredbody occurs as a large xenolith (800 m x 100 m) entrained withintonalitic gneisses and preserves primary igneous layering andtextures. New Re–Os and Pb–Pb isotope results supportthe view that it has been metamorphosed twice, in the earlyand late Archaean at 3·75 Ga and 2·8 Ga. Mineralchemistry and textures indicate that the chromite compositionspreserve two different evolutionary trends. There is a mainmagmatic trend in which Cr/(Cr + Al) ratios remain relativelyconstant but in which there is strong enrichment in Fe³⁺, Fe²⁺and Ti with progressive differentiation. This trend is a compositeof magmatic-liquidus, magmatic-cooling and subsolidus re-equilibrationprocesses. A second trend is defined by chromites from harzburgitesin the upper part of the layered body. These chromites showmagmatic replacement textures in which Fe-rich chromites arealtered to aluminous chromites. Chromites showing magmatic replacementtextures are thought to have formed by reaction with a late,interstitial melt during the solidification of the layered body.The close association between the Fe³⁺–Cr-chromites ofthe main trend and Al-rich chromites of the type found in otherArchaean megacrystic anorthosites suggest a magmatic-geneticrelationship between the two types of chromite. We propose thatanorthositic chromites form in an Fe-rich basaltic melt derivedfrom a komatiitic, boninitic or basaltic parent magma throughreaction between the melt and early-formed Fe-rich chromite. KEY WORDS: chromite; anorthosite; Archaean; Greenland; Re–Os     

Geochemical evolution of Archaean granulite-facies gneisses in the Vestfold Block and comparisons with other Archaean gneiss complexes in the East Antarctic Shield

The Vestfold Block, like other Archaean cratons in East Antarctica and elsewhere, consists predominantly of felsic orthogneiss (Mossel and Crooked Lake gneisses), with subordinate mafic granulite (Tryne metavolcanics) and paragneiss (Chelnok supracrustals). Two major periods of continental crust formation are represented. The Mossel gneiss (metamorphosed about 3,000 Ma ago) is mainly of tonalitic composition, and is similar to much of the roughly coeval Napier Complex in Enderby Land. The Crooked Lake gneiss was emplaced under high-grade conditions about 2,450 Ma ago and comprises a high proportion of more potassic rocks (monzodioritic and monzonitic suites), as well as tonalite and minor gabbro and diorite. Both Mossel and Crooked Lake gneisses are depleted in Y and have moderate to high Sr, Ce/Y, and Ti/Y, consistent with melting of a mafic source (?subducted hydrated oceanic crust) leaving major residual hornblende (± garnet). Most Crooked Lake gneisses are more enriched in incompatible elements (P, Sr, La, Ce, and particularly Rb, Ba, and K) than Mossel gneisses, suggesting derivation from a more enriched mafic source. The Vestfold Block contains few orthogneisses derived by melting of older felsic crustal rocks, in marked contrast to the Archaean Napier Complex and, in particular, southern Prince Charles Mountains. Both Mossel and Crooked Lake tonalites are strongly depleted in Rb, K, Th, and U, and have very low Rb/Sr and high K/Rb; more potassic orthogneisses are depleted in Th, U, and, to lesser extents, Rb. Tryne metavolcanics are depleted in Th and Rb, but appear to have been enriched in K (and probably Na), possibly during early low-grade alteration.     

The crust beneath the Polish Sudetes: evidence from a gneiss xenolith in Tertiary basanite from Paszowice

Xenoliths found in basaltic lavas provide useful information about the materials of the Earth crust and mantle sampled by the lava on its way to the surface, and they help to extend our interpretations based on geophysical data. A garnet-biotite-hornblende-gneiss xenolith (min. ca. 40 cm across) was drilled at a depth of ca. 18 m in a Tertiary basanite flow at Paszowice village near Jawor, in the easternmost part of the Kaczawa Mountains, central Sudetes (SW Poland). Based on petrography, geochemistry and thermobarometry, the xenolith is interpreted to have been formed at deep crustal levels from a metasedimentary rock. The maximum PT conditions recorded in the gneiss (T 700–740oC, P ca. 10 kbar) indicate that the main metamorphic event took place at a depth of 25–30 km, thus corresponding to the present-day lower crustal level defined on geophysical constraints. The gneiss xenolith contains four zircon populations, as revealed by U-Pb SHRIMP analyses: (1) 1.7 – 2.1 Ga, (2) 547 – 623 Ma, (3) 473 – 494 Ma, and (4) 354 – 437 Ma. The zircons of the youngest group (4) are interpreted to have originated during a metamorphic episode, at ca. 374±3 Ma, corresponding well with the HT-MP major metamorphic event in the neighboring Góry Sowie gneisses and migmatites. A range of features (e.g. the good preservation of the xenolith, lack of significant chemical and thermal interaction with the lava) suggest that the gneiss was sampled by the rising lava on the final path of the magma to the surface, probably from a depth of less than ca. 10 km. That means that the upper crust below this region (underlying the low-grade metamorphic Kaczawa Complex) contains gneisses originally derived from deep-crustal levels, and similar to those locally exposed in the neighborhood (e.g. in the Góry Sowie Massif to the south-east). This interpretation extends our knowledge about the structure and composition of the crust in the eastern part of the European Variscides, which otherwise is inferred only from geophysical data.     

Role of underthrust oceanic crust in the genesis of a Fijian calc-alkaline suite

A test of the proposition that calc-alkaline magmas are direct partial melts of underthrust oceanic crust is presented. It involves numerical evaluations of whether or not the major and trace element and isotopic composition of a Fijian calc-alkaline rock suite is consistent with these rocks representing unfractionated partial melts of oceanic crust at high pressures. Experimental data for one of the samples constrain the calculations.When compositions of liquidus minerals at 27 kb are combined with compositions of the volcanic rocks, close approximations can be made to the composition of oceanic crust only if the degree of partial melting is between 20% (dacite) and 45% (basaltic-andesite), and if accessory minerals are refractory phases. Concentrations of elements such as K, Rb, Sr, Pb, Th, and U, and 87Sr/86Sr ratios in the Fijian suite can be satisfactorily explained only if the parental material consisted of altered rather than fresh ocean floor basalt. Sediments are not likely to have been involved. Concentrations of Na, Ni, Co, Cr, Sc, V, the REE, Y, Zr, Hf, and Nb cannot be explained unless, or in some cases even if, several accessory phases are partially refractory. Therefore, partial melting of underthrust lithosphere does not seem likely to produce magmas with the composition of at least one quite typical calc-alkaline suite.     

南极中山站区高级片麻岩中蠕英石的形成

南极中山站区(拉斯曼丘陵)高级片麻岩中多发育蠕英石即斜长石+石英,蠕英石的出现表明岩石中往往同时存在斜长石和钾长石.通常还有一些相关现象与蠕英石相伴,如矿物颗粒周围的斜长石±石英边、钾长石边.蠕英斜长石比主斜长石牌号更高和更低的现象均可发生,而且蠕英斜长石的成分比主斜长石变化范围更大.蠕英石的形成主要涉及长英质矿物的变化,不仅涉及到钾长石的分解,可能还与钾长石的出溶、石英、黑云母的分解,甚至斜长石的出溶和分解有关.析出的组分对先存矿物(主要是钾长石)溶蚀和交代,同时出现蠕英石.变形有利于蠕英石的形成,但不是必要条件,长英质岩石中蠕英石的形成意味着降压过程的发生.形成蠕英石的组分主要源于体系自身,而不是外部环境.结合与相关结构的对比和分析,蠕英石的产生主要是长英质组分晚期分异的结果由于K,Na(Ca)的分异,Na(Ca)(2)+沉淀,K+组分活动,SiO2亦可有一定的迁移;释放、迁移的组分更偏碱性.这种分异是岩石内部的成分和结构调整的产物,但相关组分活动的空间非常有限,一般限于若干个矿物颗粒的尺度.蠕英斜长石的形成可能代表着斜长石-石英之间的一种共结,并对新生斜长石的成分起到一种缓冲作用,其过程更接近溶液而不是岩浆性质,同时表示除H2O外各种组分活动的基本结束.     

Petrology of a xenolith of ferro-aluminous gneiss from the Nain complex

Several gneissic xenoliths occur in granite which has been intruded along the contact between an anorthosite body and gneisses in the Nain complex, Labrador. One of these xenoliths is a ferro-aluminous gneiss with ovoids, 1–3 cm in diameter, consisting of an intimate mixture of fine, granular cordierite-olivine-spinel-orthopyroxene-plagioclase-ilmenite-pyrrhotite enclosed by poikilitic grains of garnet and orthoamphibole. Biotite occurs as both granular and poikilitic grains. The textures indicate a two-stage contact metamorphic history characterized by the simplified reaction: cordierite+olivine+spinel+plagioclase +orthopyroxene→garnet+orthoamphibole Although, on the basis of texture, the reactants represent the early stage assemblage and the products represent the final stage, the distribution coefficient data for Fe-Mg indicate that, except for orthopyroxene, all of the minerals were stable and in equilibrium during the final stage. The coexistence of cordierite and olivine indicates that pressure was less than 3 kbar. The phase relationships agree well with the experimental work of Hsu and Burnham (1969) on the almandine-pyrope join at 2 kbar and indicate a temperature of 800° C for the first stage and 615° C for the second stage of contact metamorphism. This close agreement also suggests that the ovoids at one time may have been garnet porphyroblasts that were decomposed during the first stage. These data correlate well with the field relations: the first-stage, high-temperature effects were likely caused by the intrusion of the anhydrous anorthositic body and the second-stage, low-temperature effects by the invasion of the volatile-rich granite.     

Tectono-stratigraphic terranes in Archaean gneiss complexes as evidence for plate tectonics: The Nuuk region,southern West Greenland

Prior to 1970 grey gneiss complexes were interpreted as partially-melted sedimentary sequences. Once it was recognised from the Nuuk region that they comprised calc-alkaline igneous complexes, it was understood that such complexes world-wide were dominated by TTG (trondhjemite-tonalite-granodiorite) initially found to have juvenile Sr, Nd and, subsequently, Hf isotopic signatures. Between 1970 and 1985 the Nuuk region gneiss complex was interpreted by the non-uniformitarian ‘super-event’ model of crust formation which proposed occasional but extensive crust formation, with craton-wide correlation of granulite facies metamorphism and deformational phases. The igneous rocks formed in a late- Meso- to early Neoarchaean super-event engulfed crust formed in an Eoarchaean super-event. Mapping and reinterpretation at Færingehavn showed there are three TTG gneiss domains, each with different early accretionary, metamorphic and tectonic histories, separated by folded meta-mylonites. This established the key feature of the tectono-stratigraphic terrane model; that each terrane has an early intra-terrane history of crust formation, deformation and metamorphism, upon which is superimposed a later deformation and metamorphic history common to several terranes after they were juxtaposed. Remapping and >250 U-Pb zircon age determinations have refined the geological evolution of the entire Nuuk region, and has confirmed at least four main crust formation events and two collisional orogenies with associated transient high pressure metamorphism within clockwise P-T-t loops. Via independent corroborative studies the tectono-stratigraphic terrane model has been accepted for the Nuuk region and, through the discovery of similar relations across other gneiss complexes, its mode of evolution is found to be applicable to Archaean high-grade gneiss complexes worldwide. The TTG and mafic components that dominate each terrane have geochemistry interpreted to indicate subduction-related magmatism at convergent plate boundaries. Each terrane is thus dominated by juvenile additions to the crust. Intra-terrane sedimentary rocks show near unimodal age distributions in contrast to those near the boundaries which are more diverse and complex. The combined geochronological, metamorphic and structural evidence of convergence of these terranes leading to collisional orogeny, this indicates that plate tectonic processes operated throughout the Archaean.     

Pb isotopic composition of late Archaean granites and the extent of recycling early Archaean crust in the Slave Province, northwest Canada

Initial Pb isotopic compositions have been determined for potassium feldspar from ca. 2.58 to 2.62 Ga plutonic rocks in the southern and central Slave Province of northwestern Canada to evaluate the extent of recycling of ancient crust within the province. Large differences in initial Pb compositions were measured which correlate with geographical areas of the province. Plutons in the east-central part of the province have initial compositions only slightly more radiogenic than estimated mantle values (²⁰⁷Pb/²⁰⁴Pb 14.8–14.9), and were dominantly deruved from juvenile crustal sources. In contrast, plutons in the Point Lake and western Contwoyto Lake areas of the western Slave Province have radiogenic compositions (²⁰⁷Pb/²⁰⁴Pb 15.1–15.2), and indicate significant recycling of pre-3.5 Ga crust. The Pb data support previous interpretations, based on Nd isotopes, for a major isotopic boundary in the central part of the province. Granites from the southern part of the province, near Yellowknife, have intermediate compositions which indicate: (1) the age of the protolith to the granitoids in the Yellowknife area is younger than at Point Lake, but older than in the eastern Slave; or (2) the granitoids in the Yellowknife area contain a mixture of an older Point Lake-type component and younger crust. The absence of pre-3.2 Ga crust in the Yellowknife area and lack of evidence for pre-2.8 Ga inherited zircons in the Yellowknife granitoids favour the former possibility. Evidence for recycling of ancient crustal sources, such as the Acasta Gneiss, is limited to a relatively small area of the west-central part of the province, suggesting that Acasta aged, or derived, crust is not widespread in the province. The marked regionality of isotopic composition may reflect a basement in the western part of the province which is itself a collage of crust of different age, being younger (ca 3.2-2.9) in the south, relative to the Point Lake region (3.9-3.2 Ga).     

Multiple crust reworking in the French Armorican Variscan belt: implication for the genesis of uranium-fertile leucogranites

International Journal of Earth Sciences - Muscovite peraluminous granites (MPGs) form by partial melting of the continental crust and can be related to metalliferous deposits such as tin, tungsten,...