Felsic Units Research Articles

Gold Mineralisation in Chigargunta Area of the Kolar Schist Belt, South India- A Part of the Archean Greenstone Belt

Gold mineralisation is reported for the first time in ‘Champion gneiss’ (quartzo-feldspathic schist) a felsic unit,in the eastern sector of the Chigargunta area (Lat: 120 430300N, Long:780 150 000E) of the Kolar schist belt, South India, during 1979-80. Quartzo-feldspathic gneiss and hornblende schist are the predominant rock types of the area. These rocks are characterised by a strong pervasive foliation trending N50E –S50W to N200E - S200W with easterly dips of 70-85. In the northern part of the area the rocks are folded with the foliation which is axial planar. Lower to middle amphibolite facies metamorphism is widespread in the area. Gold mineralisation is localised along shear zones which are ductile to brittle in nature. These zones are parallel to subparallel to the trend of foliation in the host rock and are characterized by strong mylonitic fabric, profuse quartz venation and hydrothermal alteration. Pyrite and pyrrhotite are the dominant sulphides. Gold occurs in native form. This felsic hosted gold mineralisation termed as E-2 lode (for exploration purpose) is estimated to contain a reserve of 3.13 million tonnes averaging 4.7 g/t gold. Besides, this lode there are several loads ( E-1,E-3, etc) occur within mafic units as well as in felsic unit adjacent to E-2 lode with different reserves, within the schist belt. Epigenetic gold mineralisation shows a close temporal and spatial relationship to late Archean (2700-2500 m.y) crustal accretion, stabilisation and granulite formation in the South Indian Shield. The present work delineate, the nature of mineralization in felsic unitwithin the schist belt which was not considered as source rock of gold mineralization during the period and left untouched until the present work

Mafic enclaves in north of Urumieh plutonic complex: evidence of magma mixing and mingling, Sanandaj–Sirjan zone, NW Iran

The felsic granitoids in the north of Urumieh plutonic complex are associated with abundant mafic rocks as mafic microgranular enclaves (MMEs). Typically, mafic enclaves are gabbrodiorite and monzo-gabbro/diorite in composition, and they are hosted by quartz syenite. MMEs have variable size (from a few centimeters up to 4 m) and shapes (including rounded, ellipsoidal, discoidal, lenticular, tabular, and elongated). MMEs, structurally and petrographically are characterized by irregular, gradual, and sharp contact surfaces, finer-grained enclaves and areas enriched in mafic material, feldspar phenocrysts in enclaves, plagioclase zoning with resorbed rims, acicular apatite, blade-like biotite, and mafic clots, indicating mixing and mingling of co-existing mafic and felsic magmas with liquid–liquid relation. MMEs generally contain higher values of Fe2O3, MgO, CaO, TiO2, MnO, V, Co, U, Sr, Nb, Ta, Ti, P, and heavy earth elements (HREE) and lower values of Al2O3, Na2O3, K2O, Ba, Rb, Th, K, Hf, and Zr than their host, which are in consistent with occurrence of abundant ferromagnesian minerals in the enclaves. Geochemically, the mafic enclaves and host are metaluminous with medium- to high-K calc-alkaline nature and strongly enriched in long ionic lithophile elements (LILE) and light earth elements (LREE) compared with high field strength elements (HFSE) and HREE. Some major and trace elements form linear trends on harker diagrams, having approximately identical concentration of trace elements, similar mineralogical composition, and high Mg no. in the both enclaves and host, suggesting their co-genetic origin. Our investigation shows that the enclave-forming magma may have originated from the uppermost metasomatized asthenospheric mantle and formed from a basaltic parent magma. Consequently, felsic part might have been produced by fractional crystallization from enclave-forming mafic magma in lithospheric mantle, supported by large volume of the mafic mass relative to the felsic unit. Finally, coeval mafic and felsic magmas may have mingled and mixed at two distinct, but continuous periods at different levels (before and during ascent and emplacement) as a result of combined multistage interactions (least and slight hybridization stages), which are commonly represented by basic to intermediate or hybrid MMEs.

Using Physical Volcanology, Chemical Stratigraphy, and Pyrite Geochemistry for Volcanogenic Massive Sulfide Exploration: An Example from the Blake River Group, Abitibi Greenstone Belt,

An innovative approach to enhance volcanogenic massive sulfide (VMS) exploration in regions outside of mining camps is to first use physical volcanology with litho- and chemostratigraphy to establish the location of effusive centers in the volcanic units and use pyrite geochemistry in sulfide-bearing stratified intervals with whole-rock geochemistry in the underlying volcanic units to identify hydrothermal upflow zones. This methodology is illustrated by this study in the Archean Blake River Group within the Abitibi greenstone belt of Quebec and Ontario. The Blake River Group contains numerous VMS deposits, yet large segments remain underexplored, including the Hebecourt Formation which contains four tholeiitic units that range from basalt to rhyolite in composition. Effusive centers are located for three felsic units and subunits: (1) low Ti (porphyritic) subunit of the main rhyolite, (2) high Ti (aphyric) subunit of the main rhyolite, and (3) the upper rhyolite, and for a basaltic andesite unit. Stringer and disseminated Zn-Cu mineralization occurs within the flank breccia of the low Ti rhyolite dome. An inferred vent area for an overlying basaltic andesitic unit has also been identified in this area, illustrating the coincidence of hydrothermal upflow zones with volcanic vents. LA-ICP-MS analysis of pyrite grains from several sulfide-bearing stratified intervals indicates two broad areas of higher Cu, Zn, Au, and Ag contents. The eastern region corresponds to known volcanic vents and mineralization. The western region also indicates upflow of Cu-bearing hydrothermal fluids and corresponds to a possible effusive center for the high Ti subunit. The western region does not contain known mineralization at lower stratigraphic positions, but it has not been thoroughly explored.

Generation of early Archaean felsic greenstone volcanic rocks through crustal melting in the Kaapvaal, craton, southern Africa

High-potassium felsic volcanic rocks interlayered with basalt and komatiite in early Archaean greenstone sequences in the Barberton Greenstone Belt of South Africa and Swaziland, previously considered to be derived from melting of mafic precursors, are shown to be the result of melting of significantly older felsic crust. This is documented by a combination of SHRIMP zircon dating with Hf-in-zircon and whole-rock Lu–Hf and Sm–Nd isotopic data. Zircons from felsic rocks of the oldest Barberton unit, the 3.53 Ga Theespruit Formation, yielded predominantly negative εHf(t)-values, whereas whole-rock εHf(t)- and εNd(t)-data are slightly negative to slightly positive. Similar results were obtained for ca. 3.45 Ga felsic rocks in the Hoeggenoog and Noisy Formations higher up in the greenstone stratigraphy. These data rule out derivation of the felsic units from melting of basaltic precursors and favor a crustal source, most likely of TTG composition. The isotopic data are not compatible with an entirely oceanic origin of the Barberton greenstone sequences and favor a pre-greenstone basement beneath the volcanic rocks. Crustal melting of Eo- to Paleoarchaean lower crust probably generated the felsic volcanic rocks and is likely to have been responsible for gradual stabilization of the Kaapvaal craton.

Geological mapping of the Francistown area in northeastern Botswana by surface temperature and spectral emissivity information derived from Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) thermal infrared data

We identified geological units in the Francistown area in northeastern Botswana by using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) thermal infrared (TIR) data, which contains both surface temperature and spectral emissivity information. A scatter plot of ASTER L1B daytime TIR digital values and L2 daytime temperature indicates that in the ASTER L1B daytime TIR data, bands 10, 11, and 12 contain spectral emissivity and temperature information, whereas surface temperature dominates the spectral emissivity information in bands 13 and 14. Visual interpretation of the ASTER TIR false color composite (FCC) images generated by assigning red, green, and blue to band 14:band 12:band 10 using L1B daytime data allowed us to identify mafic-to-ultramafic units and quartz-rich felsic units. Mafic-to-ultramafic units such as gabbro, dolerite, and dunite appear white in ASTER L1B daytime TIR FCC images due to their high spectral emissivities in the 8–9μm region (bands 10 and 12) and high surface temperatures. Mafic-to-ultramafic units have higher surface temperatures than other geological units because they absorb more solar radiation due to their lower albedos and they have a lower thermal inertia. Quartz-rich felsic units such as granite and dry river sand appear reddish in the ASTER L1B daytime TIR FCC image because the spectral emissivity of quartz is lower in the 8–9μm region (bands 10 and 12) than in the 10–12μm region (band 14). Mafic-to-ultramafic and granitic units are important targets for mineral exploration because they are potential geological units to host or accompany mineralization. The proposed ASTER L1B daytime TIR FCC images can be prepared very simply and they provide valuable information for geological mapping and mineral exploration.

Lithology Identification of the North QiLian Belt by Surface Temperature and Spectral Emissivity Information Derived from ASTER TIR Data

Lithology units have been identified in northern Qilian belt by using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) thermal infrared (TIR)L1B data which contains both surface temperature and spectral emissivity information. As mafic-to-untramafic units have higher surface temperature than other geologic units, surface temperature images were generated to identify the dolerite and peridotite etc. Reference to analysis of thermal spectra characteristics of geological units from JHU spectral library, a new ASTER colored composite band ratio combination band 11/12: band 12/10: band 12/14 as R:G:B was applied successfully to distinguish quartz-rich felsic units and feldspathic rocks from the large meadow in the study area , with the band ratio and de-correlation techniques. To clarify the geological boundaries and remove the vegetation effect more effective, NDVI image and false color composite image composed of band 3: band 2 : band 1 visible to near infrared bands were also generated. The result indicated that combination of TIR FCC image, VNIR FCC image, temperature image and NDVI image for the study lithology mapping works well and matches the field geological

Timing and characteristics of the Archean subaqueous Blake River Megacaldera Complex, Abitibi greenstone belt, Canada

The Archean Blake River Group of the Abitibi greenstone belt represents a megacaldera complex which evolved over 8–11M.y. from approximately 2704 to 2696Ma. The early Misema Caldera developed from a series of amalgamated shield volcano complexes identified by the remnants of mafic dyke and sill systems. These remnants are found in the Jevis-Clericy, Montsabrais-Renault, Clifford-Tannahill and Colnet regions, where summit calderas are delineated by circular ring dyke structures. The secondary 330°-trending New Senator Caldera formed within the envelope of the Misema Caldera and exhibits a box-work graben-type structure. Finally, the felsic-dominated, 070°-striking Noranda Caldera, well known for its VMS endowment, represents the final collapse of the megacaldera complex.Deformation has overprinted the calderas, but structural patterns can be used in order to reconstruct the pre-existing volcanic architecture. Structures such as mafic ring dyke complexes and rhyolitic dome-flow complexes have nucleated fold geometries and synvolcanic fractures were reactivated within zones of ductile deformation. Precise U-Pb geochronological analyses were conducted in selected areas with specific emphasis on the Misema and New Senator calderas. Felsic volcanism progressed throughout the evolution of the megacaldera complex and intermediate to felsic units have been dated to limit this evolution. The Misema Caldera formed between 2704 and 2702Ma via the amalgamation of shield volcanoes that were probably active prior to 2704Ma. The New Senator Caldera was generated between 2702 and 2700Ma during paroxysmal felsic volcanism, followed by the collapse of the Noranda Caldera culminating between 2700–2696Ma. The Misema Caldera was generated by a gravitational stress field consistent with the formation of ring and radial dyke architecture, whereas the SE-trending New Senator Caldera is more compatible with a SW-trending principal compression direction related to oblique convergence in the Abitibi belt. The Noranda Caldera is interpreted as a NE rift structure that formed in the final stages of megacaldera evolution.

GEOCHEMISTRY OF THE EARLY CRETACEOUS KOEGEL FONTEIN ANOROGENIC IGNEOUS COMPLEX, SOUTH AFRICA

The Koegel Fontein Igneous Complex in the Western Cape Province of South Africa is a high-level granite-syenite intrusive suite with minor plugs and dykes of felsic and mafic composition. Age dating and field relations show that the felsic rocks intruded as two sequential series, beginning with syenites and the Kerskloof suite of felsic dykes (andesite-trachyandesite, trachyte-rhyolite) at about 144 Ma, followed by the Rietpoort granite pluton and quartz-porphyry dykes at about 134 Ma. The intrusion of the mafic dykes and plugs took place between the two felsic series. The early Cretaceous age of the Koegel Fontein associates its formation with Gondwana breakup and establishment of the volcanic rifted margin in southwest Africa. This paper reports results from the first geochemical study of the Koegel Fontein Complex. Two groups of mafic units are distinguished based on mineralogy, field characteristics and chemical composition. The tholeiitic group comprises basalt and basaltic andesites, the alkaline group ranges from basanite-tephrite to phono-tephrite and basaltic trachy-andesite. Distinctions of incompatible element concentrations (K, P, Ti) and ratios (Y/Nb, Zr/Nb) indicate the presence of two separate magma series. Regional dolerite dykes (Gariep dykes and others) associated with contemporary magmatism along the continental margin occur in and near the complex. These regional dolerites are much more homogeneous compositionally than the mafic dykes of Koegel Fontein. Compositionally, they closely resemble the less-differentiated tholeiite dykes. The felsic units at Koegel Fontein show geochemical and isotopic evidence for two magma series, which correspond to the two intrusive episodes suggested by geochronology and field relations. The first series comprises the volumetrically minor intrusions of syenites, the Kerskloof dykes and the Rooivleitjie granite, with high values (ɛNd(t) from −0.4 to −1.8) indicating a strong mantle input. Similarities of trace element signatures suggest an affinity with the alkaline mafic units. The second series of felsic magmas makes up the bulk of the Koegel Fontein complex, with the large Rietpoort granite pluton and quartz-porphyry dykes (ɛNd(t) = −4.8 to −6.9). These rocks may be related to the tholeiitic mafic series but their isotopic composition indicates a large crustal component. The granitic rocks at Koegel Fontein have geochemical characteristics regarded as typical for anorogenic (A-type) granites, including high contents of Y, Nb, U and Th. In this and in many other features, the Koegel Fontein complex resembles the contemporary intrusive complexes in the Damaraland of northwest Namibia. Like the Damaraland granites, the Koegel Fontein magmas are of mixed origin, and their isotopic composition reflect the proportion of mantle vs. crustal source contributions.

Subduction zone metamorphism – pioneering contributions from the Alps

Circum-Pacific-type high-pressure (HP) metamorphic belts occupy contractional sutures marked by intensely deformed, imbricated nappes that verge seaward. Mineral assemblages reflect recovered depths of ∼25–50 km for mafic and felsic crustal units carried down on an underlying oceanic plate; such units recrystallized prior to disengagement and exhumation of slices of chiefly low-density rock. Low-grade parts of Alpine-type subduction complexes are similar to regurgitated circum-Pacific-type HP belts, but coesite ± microdiamond inclusions in tough, unreactive host minerals show that lithospheric underflow and continental collision involve partial recovery of ultra-high-pressure (UHP) rocks from depths of 100–140 km. Associated garnet peridotites from the Central Alps and several other collision sutures display exsolution lamellae and intergrowths suggesting the former existence of majoritic garnet and additional phases requiring formation at >300 km, attesting to earlier, unrelated mantle upwelling, or to the resurrection of profoundly subducted and back-reacted complexes. HP and UHP terranes return to mid-crustal levels mainly driven by buoyancy. Times of deep-seated storage and rates of exhumation are short; mineral parageneses, intervals of HP–UHP recrystallization and nappe structures reflect the two-way, relatively rapid transport of now-exposed subduction complexes. More fully than any other areas, integrated mineralogic-geochemical-structural studies of the Alps have illuminated relationships between metamorphic petrology and convergent plate tectonics. HP–UHP transformations of mafic and quartzofeldspathic rocks initially formed at, or near, the Earth's surface, imbricate structures of subduction complexes including their prograde metamorphic zonations + retrograde parageneses, association with deep-seated UHP mantle rocks and speedy ascent were first documented in the Alps.

Cumberland batholith, Trans-Hudson Orogen, Canada: Petrogenesis and implications for Paleoproterozoic crustal and orogenic processes

Large volume, plutonic belts, such as the ∼ 221,000 km 2, ca. 1.865–1.845 Ga Cumberland batholith (CB) of the Trans-Hudson Orogen in Canada, are major components of Paleoproterozoic orogenic belts. In many cases, they have been interpreted as continental arc batholiths. The petrogenesis and tectonic context of the CB and implications for crustal growth and recycling are interpreted herein based on a 900 km geochemical-isotopic (Nd–O) transect across it and into granitoid plutons within bounding Archean cratons in central and southern Baffin Island. The mainly granulite grade CB, emplaced over an age span of between 14 and 24 Ma, consists mainly of high-K to shoshonitic monzogranite and granodiorite, but also includes low- and medium-K granitoid rocks. Metaluminous to slightly peraluminous compositions and δ 18O (VSMOW) values (+ 6 to + 10‰) indicate derivation from infracrustal (I-type) sources. ε Nd 1.85 Ga signatures (− 12 to − 2) of both mafic and felsic units suggest a dominance of evolved sources. Isotopic signatures in the interior of the CB (− 2 to − 7) are more radiogenic than those within Archean domains in central (− 8 to − 15) and southern (− 5 to − 19) Baffin Island. The isotopic transect is interpreted as ‘imaging’ an accreted microcontinental block (Meta Incognita) and bounding Archean cratons. The CB includes granites of arc, within-plate (A-type) and post-collisional affinity and volumetrically minor mafic rocks with both arc and non-arc features. (La/Yb) CN and Sr/Y values range from < 1 to 225 and < 1 to 611, respectively. In these respects, some CB granitoid rocks resemble Paleozoic adakitic granites, interpreted as partial melts of greatly thickened crust within post-collisional settings, such as Tibet. Thus, the CB likely encompasses various non-consanguineous magmatic suites generated at deep- to mid-crustal depths. Although CB granitoid rocks undoubtedly had important crustal sources, it is hard to assess the relative contribution of mantle-derived magmas. The CB is best interpreted as a post-accretion batholith resulting from large-scale lithospheric mantle delamination followed by the upwelling of hot asthenospheric mantle leading to voluminous crustal partial melting. Contributors to crustal instability which may have facilitated such delamination included: (a) a collage of recently assembled small cratons underlain by hot, weak lithosphere with mantle-depth structural breaks within this segment of the Trans-Hudson Orogen; (b) the gabbro-eclogite phase transformation, and (c) a greatly thickened crustal section (> 60 km), as evidenced by adakitic granites.

XRF and Raman Characterization of Antimonite

Two type Antimonite (stibnite) ore samples were analyzed by polarized energy dispersive X-ray fluorescence (PEDXRF) spectrometry and by confocal Raman spectrometry (CRS) in this study. The samples were collected from two different localities (İzmir-Ödemiş and Bursa-Uludağ zones from Turkey) according to the sampling methods. The first locality hosted within the metamorphic basement and the second locality hosted within the felsic intrusive rock units. The goal of this paper is to compare two different stibnite samples from various regions through PEDXRF and CRS with the optical characteristic features under the ore microscope. Ore microscopy studies reveal that the İzmir-Ödemiş stibnite is associated mainly with calcite and the Bursa-Uludağ stibnite is associated with celestite and quartz as gangue minerals. Two different colored species were observed in the stibnite of the studied samples. Ore microscopy results, CRS with the PEDXRF results show that the stibnites from the two different localities have different mineral associations, different chemical compositions, and different physical properties.

Partial melting and melt segregation in footwall units within the contact aureole of the Sudbury Igneous Complex (North and East Ranges, Sudbury structure), with implications for their relationship to footwall Cu–Ni–PGE mineralization

We performed detailed field and drill core mapping of partial melting features and felsic rocks (footwall granophyres, FWGRs) representing segregated and crystallized partial melts within the contact aureole of the Sudbury Igneous Complex (SIC) in the 1.85 Ga Sudbury impact structure. Our results, derived from mapping within the North (Windy Lake, Foy, Wisner areas) and East Ranges (Skynner, Frost areas) of the structure, reveal that partial melting was widespread in both felsic and mafic footwall units up to distances of 500 m from the basal contact of the SIC. Texturally and mineralogically, significant differences exist between rocks formed by partial melting within and between localities. In general, however, melt bodies are dominated by different quartz-feldspar intergrowths (e.g. granophyric, graphic) and miarolitic cavities up to 5 cm in diameter. Major and trace element compositions of Wisner and Frost FWGRs imply that they crystallized from melts dominantly derived from partial melting of felsic Levack Gneiss and Cartier granitoid rocks, as well as from gabbroic rocks only at Frost. These results accord with our observations on in situ partial melting features and crystallized melt of microscopic scale in both felsic and mafic rocks. We conclude that partial melting occurred at a pressure of 1.5 ± 0.5 kbar and at temperatures up to 750°C in the Wisner area and up to 900°C in the Frost and Windy Lake areas. Segregations of partial melt into veins and dikes are present in all localities, and were promoted by deformation of the Sudbury structure in the Penokean orogeny as indicated by dominant strike directions. Whereas veins and dikes reflect brittle conditions during melt migration, sheared melt pods in the Sudbury breccia matrix indicate ductile conditions during their crystallization. Our results suggest a close genetic association of partial melting, melt segregation, and hydrothermal processes responsible for remobilization of Cu–Ni–PGE sulphides into and within the SIC footwall.

Lithogeochemistry of the Collahuasi porphyry Cu–Mo and epithermal Cu–Ag (–Au) cluster, northern Chile: Pearce element ratio vectors to ore

Abstract Lithogeochemical vectors to ore-bodies are obscured by the variety of lithological units and the alteration events that have affected them. In the magmatic-hydrothermal context, Pearce element ratio (PER) analysis can be a cost-effective lithogeochemical technique that identifies and eliminates non-hydrothermal sources of variation, thus permitting the definition of material transfer related only to hydrothermal alteration. Dedicated PER diagrams for mafic and felsic lithologies in the world-class porphyry Cu–Mo and epithermal Cu–Ag (–Au) cluster of the Collahuasi district, I Region, northern Chile, effectively model background variability and discriminate between fresh or propylitically altered rocks and those with a hydrolytic alteration overprint. Furthermore, PER diagrams allow for the definition of an alteration index (AI) that quantifies the degree of metasomatic exchange during hydrolytic alteration of a particular rock. Plots of different PER values against the AI show that potassium enrichment during hydrolytic alteration was followed by potassium, calcium and sodium depletion as a result of the destruction of feldspars during sericitic and later argillic alteration. Although PER plots record no mass transfer processes involving major elements during propylitic alteration, the integrated AI for both mafic and felsic units spatially defines the major alteration centres within the district.

Petrology and geochemistry of the I-type calc-alkaline Qorveh Granitoid Complex, Sanandaj-Sirjan Zone, western Iran

This study examines the geochemistry of the metaluminous Eocene–Oligocene I-type calc-alkaline Qorveh Granitoid Complex (QGC), western Iran. The complex was emplaced in a Sanandaj-Sirjan Zone (SSZ) convergent setting which resulted from subduction of Neo-Tethyian oceanic crust below Central Iran. Three main units have been identifi ed within the QGC based on fi eld observations, mineralogical and geochemical characteristics. Mafi c intrusions consist of diorites, followed by felsic units that include granodiorites and granites. The diorites are characterized by SiO2 contents between 48 and 54 wt %, low abundances of incompatible elements (Ba, Nb, La and Th) relative to enriched mantle, but consistent with values for average middle crust. In addition, they have Al2O3/(MgO + FeO) ratios from 0.98 to 1.54 and molar CaO/(MgO + FeO) ratios from 0.59 to 0.71 and were probably derived from a mafi c crustal source. The geochemical features combined with the high volume of the granitoid rocks are inconsistent with an origin via differentiation of mantle-derived basaltic parent magma and assimilation. The granodioritic and the granitic rocks show moderate values of molar Al2O3/(MgO + FeO) and molar CaO/(MgO + FeO) suggesting an origin involving dehydration melting of a metagreywacke source. Geochemical data on REEs, Y, Rb and Sr in the latter units indicate that amphibole and plagioclase were the major fractionating phases during magma segregation.

Evolution and architecture of a large felsic Igneous Province in western Laurentia: The 1.6 Ga Gawler Range Volcanics, South Australia

The Gawler Range Volcanics (GRV) of South Australia are an intracontinental, subaerial Large Igneous Province (LIP) formed during supercontinent assembly in the Mesoproterozoic. Like many LIPs, the GRV are intraplate, were erupted over a geologically short time (∼ 2 million years), and are dominated by voluminous lavas. However, the GRV are overwhelmingly dominated by felsic units. The GRV are essentially undeformed and have not been deeply buried so that their original textures are exceptionally well preserved. Furthermore, they are well exposed in very gently northward and eastward dipping sections up to 4 km thick, allowing assessment of the facies architecture and evolution of this felsic volcanic LIP. The evolution of the GRV can be clearly separated into two main stages. Initial eruptions at numerous volcanic centres produced small to moderate volume, geochemically distinct, felsic lavas and lava domes, together with ignimbrites and minor mafic and intermediate lavas, forming a sequence 0.5 to 3 km thick. Volcanic activity in this lower sequence varied from effusive to explosive and was not much different in style or products from Phanerozoic felsic volcanic provinces. However, the second stage produced at least three voluminous felsic units, each of which represents about 1000–3000 km 3 of magma. Moreover, the dominance of evenly porphyritic textures and lack of pyroclastic textures (pumice, shards, broken crystal, lithic clasts) in these units suggests that they were erupted effusively and flowed as lavas. Each of these felsic lavas are generally dominated by a single uniform composition, and commonly mingled with a subordinate and compositionally distinct lava.

New constraints on source location and emplacement mechanisms of extensive felsic units in the Gawler Range Volcanics, South Australia

Eucarro Rhyolite is one of three voluminous (>675 km3 ) units in the upper part of the Gawler Range Volcanics. Eight stratigraphic sections through the Eucarro Rhyolite were sampled for anisotropy of magnetic susceptibility (AMS). The unit has AMS characteristics similar to those of lavas. AMS foliations (Kmax/Kint plane) consistently strike northwest and are very steeply dipping. Seven sections show a change from steep southwesterly dips near the base to vertical or northeasterly dips in the middle and near the top. Furthermore, the AMS pattern implies outflow from a source located to the southwest of present outcrops. In the remaining section, the AMS pattern implies outflow from the northeast. The opposing flow directions may be delineating the boundary between adjacent lava flow lobes. This study suggests that felsic lavas can have dimensions comparable with those of mafic lavas.

Isotopic connections between basement rocks exposed in the St. Francois Mountains and the Arbuckle Mountains, southern mid-continent, North America

Two distinct crustal provinces have been identified in the southern mid-continent based on U–Pb crystallization ages. Both contain large volumes of undeformed granite and rhyolite, with minor amounts of metamorphic rock and mafic intrusions. The Eastern Granite-Rhyolite province is characterized by felsic rocks with crystallization ages of 1,470 ± 30 Ma and exposures are restricted to the St. Francois Mountains in southeastern Missouri. Similarly, the Southern Granite-Rhyolite (SGR) province is characterized by felsic units with ages of 1,370 ± 30 Ma with primary exposures in the eastern Arbuckle Mountains of southern Oklahoma. Within the SGR province three magmatic pulses can be identified starting at 1,400, 1,370, and 1,340 Ma. Although the crystallization ages are different, the Sm–Nd isotopic signatures are similar for the units exposed in these areas as well as the buried basement in between. Depleted mantle model ages for rocks within the Arbuckle Mountains range from 1,530 to 1,430 Ma with ɛNd(t) values of +3.2 to +4.1 while units of the St. Francois Mountains range from 1,550 to 1,430 Ma and +4.5 to +4.7. Comparison of Sm–Nd isotopic data also indicate similarities between the 147Sm/144Nd and 143Nd/144Nd ratios for the rock units in these areas suggesting a common source.

Preservation/exhumation of ultrahigh-pressure subduction complexes

Ultrahigh-pressure (UHP) metamorphic terranes reflect subduction of continental crust to depths of 90–140 km in Phanerozoic contractional orogens. Rocks are intensely overprinted by lower pressure mineral assemblages; traces of relict UHP phases are preserved only under kinetically inhibiting circumstances. Most UHP complexes present in the upper crust are thin, imbricate sheets consisting chiefly of felsic units ± serpentinites; dense mafic and peridotitic rocks make up less than ∼ 10% of each exhumed subduction complex. Roundtrip prograde–retrograde P– T paths are completed in 10–20 Myr, and rates of ascent to mid-crustal levels approximate descent velocities. Late-stage domical uplifts typify many UHP complexes. Sialic crust may be deeply subducted, reflecting profound underflow of an oceanic plate prior to collisional suturing. Exhumation involves decompression through the P– T stability fields of lower pressure metamorphic facies. Scattered UHP relics are retained in strong, refractory, watertight host minerals (e.g., zircon, pyroxene, garnet) typified by low rates of intracrystalline diffusion. Isolation of such inclusions from the recrystallizing rock matrix impedes back reaction. Thin-aspect ratio, ductile-deformed nappes are formed in the subduction zone; heat is conducted away from UHP complexes as they rise along the subduction channel. The low aggregate density of continental crust is much less than that of the mantle it displaces during underflow; its rapid ascent to mid-crustal levels is driven by buoyancy. Return to shallow levels does not require removal of the overlying mantle wedge. Late-stage underplating, structural contraction, tectonic aneurysms and/or plate shallowing convey mid-crustal UHP décollements surfaceward in domical uplifts where they are exposed by erosion. Unless these situations are mutually satisfied, UHP complexes are completely transformed to low-pressure assemblages, obliterating all evidence of profound subduction.

The Mesoarchean emergence of modern-style subduction

Well-preserved volcanic sequences span the Paleoarchean to Neoarchean evolution of the Pilbara Craton, in northwestern Australia. This region provides the best physical evidence bearing on the stage of Earth's history when modern-style tectonic processes began. Paleoarchean assemblages in the eastern nucleus of the craton (the 3.51–3.24 Ga Pilbara Supergroup) show few features that can reasonably be interpreted as evidence for modern-style subduction processes. Incompatible trace element-enriched felsic volcanic horizons show geochemical evidence for the interaction between mafic magmas and crust, but this evidence, on its own, can equally well be interpreted in terms of either a subduction-enriched mantle source or local and limited assimilation of felsic crust into the voluminous tholeiitic magmas that dominate the Pilbara Supergroup. Viewed in context within the thick autochthonous and consistently upward-younging Pilbara Supergroup, these felsic units are most likely related to the same plume-dominated processes that formed the basalts that dominate the supergroup. It is very unlikely that modern-style plate tectonic processes played any role in the Paleoarchean evolution of the Pilbara Craton, although some form of non-uniformitarian (e.g. flat) subduction process may have operated. In stark contrast, the Mesoarchean units of the West Pilbara Terrane and the late-tectonic basins that cover that boundary between the West and East Pilbara Terranes, show clear evidence for modern-style convergent margin processes. Igneous rocks in this belt, which flanks the old eastern cratonic nuclei, have enriched geochemical signatures that cannot be accounted for by crustal contamination. This region is also characterised by a linear magmatic and structural fabric, by the presence of lithologically and geochronologically exotic belts, and by the presence of a broad belt of isotopically more juvenile crust. The collective strength of these arguments provides compelling evidence that a modern-style oceanic arc fringed the East Pilbara Terrane at 3.12 Ga and accreted to that terrane by 2.97 Ga. These assemblages mark the minimum age for the birth of modern-style plate subduction process.

Proterozoic mafic–ultramafic intrusions in the Arunta Region, central Australia: Part 1: Geological setting and mineral potential

Proterozoic mafic–ultramafic intrusions of the Arunta Region record a protracted period of magmatism during the evolution of this geologically complex and tectonically long-lived terrane in central Australia. New U–Pb zircon geochronology data highlight the episodic emplacement of the mafic–ultramafic systems. Five major events of dominantly tholeiitic mafic magmatism have been recognised at ∼1810 Ma, ∼1780 Ma, ∼1690 Ma, and ∼1635 Ma, and a much younger event of probably early Palaeozoic age. A sixth event at ∼1135 Ma has alkaline-ultramafic affinities. Chondrite- and mantle-normalised multi-element patterns of rocks with melt-like compositions have refined the correlations of the magmatic systems indicated by the geochronological framework. Continental margin environments (e.g. subduction or back-arc related) are generally indicated for the magmatic events at ∼1810 Ma, ∼1780 Ma, and ∼1635 Ma, whereas the mafic magmatism at ∼1690 Ma and in the early Palaeozoic is more characteristic of intracontinental settings (e.g. extensional rifts). The intrusions occur in proximity to major province-wide faults. Differential vertical movements along these faults provide the opportunity to examine geological processes at crustal depths ranging from ∼5 km to 25 km (2 kb to 8 kb). The intrusions form large homogeneous mafic granulite bodies, granulite bodies interfolded with felsic units, contaminated gabbroic sheets, stacked sequences of high-level doleritic sills, small pods and laterally extensive sheets of amphibolite, and rare plug-like ultramafic bodies. Metamorphic overprints range from granulite to sub-amphibolite facies, with a concentration of high-grade mafic granulite bodies in the central Arunta Region attributed to deeper levels of crust uplifted north of the Redbank Thrust. Mafic rocks in the intrusions of high-metamorphic grade are dominantly two-pyroxene mafic granulites with high clinopyroxene to orthopyroxene ratios and variable amounts of minor hornblende, biotite, quartz, and garnet. In contrast, gabbroic rocks in the lower-grade intrusions of the western Arunta Region contain more orthopyroxene, alkali feldspar, quartz, and Fe–Ti oxides. LREE-enrichment trends with decreasing 147Sm/ 144Nd ratios and initial ɛ Nd values (+1.5 to −4.7) indicate that felsic crustal contamination processes were particularly important during the evolution of the latter intrusions. Incompatible trace-element trends show that the Arunta intrusions fall into two major geochemical groups that for the first time highlight geographical differences in mineral prospectivity. 1. A S-rich group (∼1200 to 300 ppm S: Andrew Young Hills intrusion, Mount Hay Granulite, Mount Chapple Metamorphics) from the western and central Arunta regions that has potential for basal Ni–Cu–Co-sulphide associations. 2. A relatively S-poor (<300 ppm S), slightly more primitive group (Attutra Metagabbro, Mordor Complex) from the eastern Arunta Region that has greater potential for stratabound PGE-sulphide associations. Some potential also exists for structurally controlled hydrothermal deposits of Cu–Au ± PGEs ± Ag ± Pb spatially associated with mafic–ultramafic rocks (Riddock Amphibolite). The major challenges for finding massive Ni–Cu–Co sulphides in the lower parts of prospective S-saturated intrusions (e.g. Andrew Young Hills, Mount Hay Granulite) are to determine the pre-deformational geometries of the bodies and to locate favourable mineralised environments, such as embayments in basal contacts and feeder conduits, concealed by thin Cainozoic alluvial deposits.