Felsic Suite Research Articles

Late Permian to Early Triassic mafic to felsic intrusive rocks from North Liaoning, North China: Petrogenesis and implications for Phanerozoic continental crustal growth

Zircon U–Pb dating, whole-rock major oxide, trace element and Sr–Nd–Hf isotopic data are presented for the Late Permian to Early Triassic Shijianfang batholith from Faku in North Liaoning Province, North China. Two main magmatic suites are documented: one, with an emplacement age of ca. 260 Ma, is mafic to intermediate in composition, forming small gabbroic to monzonitic intrusions; the other is felsic and quartz monzonitic to granitic in composition, with an emplacement age of ca. 250 Ma. Geochemically, the rocks from the mafic suite show strong enrichment of LILE and LREE and depletion in HFSE, and also have moderately depleted isotopic compositions, with I Sr ranging from 0.7049 to 0.7054, ε Nd( t) from + 2.72 to − 1.82 and zircon ε Hf( t) values from + 3.4 to + 7.1. These features suggest derivation from high degree partial melting of a subduction-related metasomatized lithospheric mantle source. The rocks from the felsic suite range from 66.7 to 77.2 wt.% SiO 2, and define a continuous chemical evolutionary trend, from less-differentiated low-silica members displaying LREE enriched patterns, no Eu anomalies and high Ba, Sr, Zr and Hf, to more-differentiated, high-silica members with higher alkalis, strong depletion in Ba, Sr and Ti, enrichment in HFSE, fractionated REE patterns and strong negative Eu anomalies. However, they exhibit indistinguishable isotopic compositions from the mafic rocks, with I Sr ranging from 0.7043 to 0.7062, ε Nd( t) from + 2.28 to − 0.55 and zircon ε Hf( t) values from + 3.6 to + 6.6. This suggests that the parental magma for the felsic suite originated from partial melting of mixed protoliths composed of juvenile basaltic underplate and ancient lower crustal materials. Subsequent fractional crystallization, with overprinting by magmatic hydrothermal fluids, can explain the geochemical variations of the felsic suite. The juvenile character of both lithospheric mantle and crustal rocks, as recorded by the Shijianfang batholith, suggests that the northern Liaoning block forms part of a Phanerozoic accretionary orogenic belt. This observation indicates that the Chifeng-Kaiyuan Fault likely represents the Mesozoic lithospheric boundary between the North China Craton and the Xing-Meng Orogenic Belt in northern Liaoning. The Shijianfang batholith, together with Late Permian to Middle Triassic adakitic and A-type rocks, mafic–ultramafic cumulates and alkaline intrusions from the neighboring areas, constitutes an important post-collisional to intra-plate anorogenic magmatic province within the continental interior of the newly amalgamated North China–Mongolian plate. The unique mafic and felsic coupling in the Shijianfang batholith provides a good proxy record for the multiple-step vertical continental crustal growth in the newly amalgamated continental interior during the Phanerozoic, triggered by continued magmatic underplating due to lithospheric delamination and hot asthenospheric upwelling within a post-collisional to post-orogenic extensional regime.

High-K granites of the Rum Jungle Complex, N-Australia: Insights into the Late Archean crustal evolution of the North Australian Craton

The Late Archean (c. 2.54–2.52 Ga) high-K granitoids of the Rum Jungle Complex, Northern Australia, display the igneous mineral assemblage of K-feldspar, quartz, plagioclase, biotite, and magnetite, and accessories such as zircon, monazite, titanite, allanite, apatite, and ilmenite. The granites underwent a variably severe greenschist facies alteration and associated deformation during the Barramundi Orogeny (1.88–1.85 Ga). The K-rich granitoids have variable compositions, mainly comprising syenogranite and quartz-monzonite. They can be subdivided into two major groups, (1) felsic granites and (2) intermediate to felsic granites, quartz-monzonites, and diorite. The felsic group (69–76 wt.% SiO 2) shares many features with typical Late Archean potassic granites. They are K- and LILE-rich and show marked depletion in Sr and Eu and the high field strength elements (HFSE), particularly Nb and Ti, relative to LILE and LREE. Compared to the average upper crust they have anomalously high Th (up to 123 ppm) and U (up to 40 ppm). The intermediate to felsic group (56–69 wt.% SiO 2) differs from the felsic group in having weakly lower Th and U but higher Mg#, Ti, Ba, Sr, Ni, Cr and REE, with a less pronounced negative Eu anomaly. This group displays well-defined trends in Harker diagrams, involving a negative correlation of Si with Sr, Ca, Na, and P whereas K, Rb, and Ba increase in the same direction, suggesting fractional crystallization of feldspar was more prominent than in the felsic suite. The mineralogical and geochemical characteristics of the felsic group are consistent with granite formation by intracrustal melting of plagioclase-rich igneous protoliths, probably of tonaltic to granodioritic composition, at moderate crustal levels. The intermediate to felsic granites, on the other hand, appear to be the products of mantle–crust interaction, possibly by melting of or mixing with more mafic igneous rocks. As evidenced by the presence of older inherited zircons crustal recycling of a pre-greenstone crust of the North Australian Craton of > 3.5 Ga played an important role in the formation of the Late Archean granites of the Rum Jungle Complex.

Geochemistry and geothermometry of volcanic rocks from Serra Branca, Iberian Pyrite Belt, Portugal

Volcanic rocks from Serra Branca, Iberian Pyrite Belt, Portugal, consist of calc-alkaline felsic and intermediate rocks. The latter are massive andesites, whereas the former include four dacitic to rhyolitic lithologies, distinguishable on spiderdiagrams and binary plots of immobile elements. Zircon thermometry indicates that two felsic suites may have formed from different magmas produced at distinct temperatures, with only limited fractionation within each suite. Alternatively, all the felsic rocks can be related through fractionation of a single magma if the lower zircon saturation temperature obtained for one suite merely results from Zr dilution, mostly reflecting silicification. The relatively high magma temperatures at Serra Branca ease the classification of felsic rocks based on their HFSE contents and also indicate volcanogenic massive sulfide deposit favorability. This contrasts with other areas of the Belt that register lower magma temperatures and are subsequently barren. However, magma temperatures may have not been high enough to cause complete melting of refractory phases in which HFSE reside during crustal fusion of an amphibolite protolith, implying difficult discrimination of tectonic environments for the felsic rocks. The intermediate rocks were possibly formed by mixing between basaltic magmas and crustal material, compatible with volcanism in an attenuated continental lithosphere setting.

Geochemistry, mineral chemistry and petrogenesis of a Neoproterozoic dyke swarm in the north Eastern Desert, Egypt

Dyke swarms traverse Neoproterozoic rocks in the Hawashiya region in the extreme northern part of the Eastern Desert of Egypt. They are a suite of basaltic andesite and andesite mafic dykes, and dacitic and rhyolitic felsic dykes. The mafic dyke suite is more abundant in the younger granites (577 ± 6 Ma) than in the older granitoids (614 Ma), in which the felsic dykes are the most common. The dyke swarms trend predominantly NE–SW, and the felsic dyke suite is older than the mafic dyke suite. Both dyke suites are calc-alkaline (alkaline dykes are rare) and are relatively poor in TiO2 and Nb but enriched in the incompatible elements and HFSE. The felsic dyke suite is enriched in REE and is strongly LREE fractionated relative to the mafic dyke suite. Although the Hawashiya dykes were emplaced at the end of the Neoproterozoic era in an extensional tectonic setting, they have geochemical characteristics that are consistent with a subduction-related regime. These chemical signatures were inherited from the lithospheric rocks that produced their host Hawashiya granitoids. The felsic dyke suite magma may be derived from crustal rocks (essential source component) by partial melting. The mafic dyke suite magma was generated from a lithospheric mantle and has undergone fractional crystallization of plagioclase, amphibole, clinopyroxene and magnetite, as documented by major and trace elements fractionation modelling.

Phase transitions and volumetric properties of cryolite, Na3AlF6: Differential thermal analysis to 100 MPa

Cryolite, Na3AlF6, is the most abundant aluminos uoride mineral in highly evolved felsic suites and their pegmatites, but its phase transitions and thermodynamic properties at elevated pressures are unknown. We used a simple modiÞ cation of the TZM pressure vessel to perform differential thermal analysis of cryolite at high pressures. Temperatures of the α-β transition are as follows: 559.30 ± 0.23 °C (1 atm), 562.10 ± 0.28 °C (47 MPa), and 567.33 ± 0.23 °C (101 MPa). Cryolite melting temperatures increase as follows: 1011.4 ± 0.2 °C (1 atm), 1019.2 ± 0.4 °C (50 MPa), and 1028.7 ± 0.4 °C (100 MPa). Both pressure-temperature relationships are linear: (dT/dp)α-β = 78.4 ± 8.4 °C/GPa and (dT/ dp)m = 174 ± 12 °C/GPa. Application of the Clapeyron relationship leads to the following volumetric changes: ΔVα−β = 0.089 ± 0.019 J/(mol·bar) and ΔVm = 1.49 ± 0.12 J/(mol·bar). Despite the signiÞ cant self-dissociation in the cryolite liquid, melting sensu stricto (without dissociation) dominates the heat and volumetric changes during melting in comparable amounts: 83.3 ± 6.7 % ΔHm and 68 ± 15 % ΔVm and suggests that the degree of dissociation has no signiÞ cant effect on the (dT/dp)m. Evaluation of previous and current volumetric data for cryolite polymorphs leads to Vβ,1284 = 8.49 ± 0.17 J/(mol·bar); coefÞ cients for the volumetric thermal expansion in the form of the third-order polynomial equation are: V298 = 7.080 ± 0.012 J/(mol·bar), a1 = (1.39 ± 0.20)·10 –4

Source regions of granites and their links to tectonic environment: examples from the western United States

This review, in honor of Ilmari Haapala's retirement, reflects on lessons learned from studies of three granitic systems in western North America: (1) Mesoproterozoic samples from west Texas and east New Mexico; (2) Laramide granitic systems associated with porphyry-copper deposits in Arizona; and (3) granites of the Colorado Mineral Belt. The studies elucidate relationships amongst tectonic setting, source material, and magma chemistry. Mesoproterozoic basement samples are from two different felsic suites with distinct elemental and isotopic compositions. The first suite, the “plutonic province”, is dominantly magnesian, calc-alkalic to alkali-calcic, and metaluminous. It has low K 2O/Na 2O and Rb/Sr, and Nd model ages of 1.56 to 1.40 Ga. The second suite, the “Panhandle igneous complex”, is magnesian, metaluminous, alkalic, and is part of the Mesoproterozoic belt of magmatism that extends from Finland to southwestern United States. Samples from the Panhandle igneous complex demonstrate three episodes of magmatism: the first pulse was intrusion of quartz monzonite at 1380 to 1370 Ma; the second was comagmatic epizonal granite and rhyolite at 1360 to 1350 Ma. Both of these rock types are high-K to slightly ultra-high-K. The third pulse at 1338 to 1330 Ma was intrusion of ultra-high-K quartz syenite. Nd model ages (1.94 to 1.52 Ga) are distinct from those of the “plutonic province” and systematically older than crystallization ages, implying a substantial crustal input to the magmas. At the Sierrita porphyry-copper deposit in the Mazatzal Province of southeastern Arizona, trace element, Sr, and Nd isotopic compositions were determined for a suite of andesitic and rhyolitic rocks (67 Ma) intruded by granodiorite and granite. Isotopic composition and chemical evolution are well correlated throughout the suite. Andesite has the least negative initial ε Nd (−4.3) and lowest 87Sr/ 86Sr i (0.7069). It is also the oldest and chemically most primitive, having low concentrations of Rb, SiO 2, and high concentrations of transition elements. These parameters change through the system to the youngest unit (granite), which has the most negative ε Nd (−8.5), the highest 87Sr/ 86Sr i (0.7092), and is chemically most evolved. Correlation between chemical and Nd isotopic evolution probably resulted from a continuous process of progressive assimilation, in which mafic magmas invade and incorporate continental crust. Deposits in Arizona with ε Nd values more negative than the −8.5 of Sierrita lie in the older Yavapai province in the northwestern part of the state. The difference in the most negative epsilon Nd implies that Nd isotopic signature is sensitive to the age of the Precambrian domain. The granites from the Colorado Mineral Belt were emplaced during the transition from Laramide convergence to mid-Tertiary extension. Three different groups of granites are recognized. The first is Laramide and was formed during assimilation-fractional crystallization involving lower crustal mafic source materials; the second and third groups are mid-Tertiary and represent intracrustal melting of heterogeneous sources. This change in source regions and melt regimes in transition from convergence to extension is fundamental to the Mesozoic and Cenozoic evolution of western North America.

Geochemistry of Volcanic Rocks, Albernoa Area, Iberian Pyrite Belt, Portugal

Volcanic rocks from the Albernoa area essentially consist of calcalkaline quartz-feldspar-phyric coherent and hyaloclastic rhyodacites, and alkaline and tholeiitic basaltic rocks. Binary plots show that high-field-strength elements behaved as immobile elements, and allow for the identification of two felsic rock suites. Silica and alkali mobility, however, is reflected by compositional scatter on major-element diagrams: felsic rocks display rhyolitic to apparent andesitic compositions, and the mafic rocks display basaltic to apparent dacitic compositions. Silica and alkali mobility was focused along fracture networks and within the matrices of hyaloclastic breccias. Problematic classification of geotectonic setting for the felsic rocks is a reflection of anomalous high-field-strength element systematics; this probably results from a low temperature of crustal fusion, causing decreased solubility of the refractory phases in which these elements reside. The mafic rocks, however, evidently were generated in an extensional setting without involvement of subduction; the existence of apparent arc signatures was caused by crustal assimilation. This is compatible with volcanism in an attenuated continental lithosphere setting, due to strike-slip tectonics during oblique continental collision.

Quantitative genetic modelling of the Turkel anorthosite, Eastern Ghats Belt, Orissa, India

Principal components analysis of the anorthositic and adjoining felsic suite of rocks of the Turkel anorthosite complex suggests that these are unlikely to be comagmatic. This has been supported by δ 18O values of these rocks. In synoptic principal component space, the compositions of anorthosite, leuconorite and ferrodiorite of Turkel are compared with the chemistry of coexisting phases based on experimental studies on Proterozoic massif-type anorthosite and naturally occurring high-Al gabbros (HAG). Such studies, in congruence with the experimental observations, strongly indicate that the Turkel ferrodiorite represents the residual melt after separation of anorthosite and leuconorite. Major and minor element mass balance computations suggest that an ensemble of 15.32 wt.% anorthosite, 34.74 wt.% leuconorite and 29.71 wt.% ferrodiorite may have originated from a melt which was derived from a HAG magma through fractional crystallisation of 8.86 wt.% olivine and 11.37 wt.% Ca-poor clinopyroxene. Consistent with petrological observations, a small amount of zircon and apatite assimilation from the granitoid country rocks has been invoked to best explain the observed high concentration of Zr and REEs in the anorthositic suite of Turkel.

The Neoproterozoic Dubr intrusives, Central Eastern Desert, Egypt: petrological and geochemical constraints on the evolution of a mafic-felsic suite

The Neoproterozoic magmatism in the Dubr area took place in two suites namely: (1) a mafic suite of a gabbro-diorite complex and (2) a felsic suite of a granodiorite and monzogranite composition. Pressures estimated for the gabbro and diorite rocks show that they have originated over a wide range of pressures (1.5-6.5 kbar). The gabbro and diorite rocks predict crystallization temperatures ranging from 820 to 940 °C and 660 to 710 °C, respectively. The rare-earth element abundances of the Fe--tholeiite gabbro-diorite complex display fractionated {(La/Lu) n = 1.53-2.96} and LREE-enriched {(La/Sm) n = 1.12-2.62} patterns. The REE patterns of the gabbro-diorite complex suggest that it can be generated from a spinel-bearing mantle source. Transition metal contents of the gabbro-diorite complex are variable and low (Ni, 9-133 ppm; Cr, 20-325 ppm; Sc, 17.7-34 ppm), indicating that none of the samples are primitive. The gabbro-diorite complex may have formed via relatively high degree partial melting (25-30 %) of an enriched mantle source followed by fractional crystallization of olivine, Cr-spinel with minor clinopyroxene and plagioclase. Chemical evidences imply that crustal contamination is likely to be a minor factor in trace element variation of the gabbro-diorite complex. The granodiorite and monzogranite are calc-alkaline showing metaluminous affinity for the granodiorite and a slightly peraluminous nature for the monzogranite. They display trace element and REE characteristics of I-type granites formed in a subduction-related arc environment. The hornblende of the granodiorite and monzogranite yielded pressures of an average 3 kbar and 2.5 kbar, respectively. Amphibole-plagioclase geothermometer yielded crystallization temperatures ranging from 680 to 700 °C and 650 to 675 °C for the granodiorite and monzogranite, respectively. The co-linear trends on variation diagrams for the granodiorite and monzogranite indicate a genetic link between them through fractional crystallization. It appears likely that the restite-unmixing model is responsible for the petrological and chemical features of the granodiorite. The most probable source for the granodiorite is a dioritic rock. The monzogranite was derived as a result of crystallization of plagioclase, biotite, hornblende, quartz, ilmenite and magnetite phases from a granodioritic melt.

Geochemical and Oxygen Isotope Signature of Sea-FloorAlteration Associated with a Polydeformed and Highly Metamorphosed MassiveSulfide Deposit, Ruostesuo, Central Finland

Massive sulfide deposits in the Vihanti-Pyhasalmi district, central Finland, are hosted by a highly deformed and metamorphosed bimodal volcanic sequence. Alteration zones that envelop these deposits are composed of cordierite + orthoamphibole ± garnet-bearing rocks (referred to as COR rocks) belonging to upper amphibolite to granulite facies. Outcrops of mafic and felsic COR rocks (MCOR and FCOR) in the vicinity of the Ruostesuo Zn-Cu deposit define mappable layers with distinctive metamorphic mineral assemblages: orthoamphibole + cordierite ± plagioclase ± quartz (M1COR); garnet + orthoamphibole + cordierite ± plagioclase ± quartz (M2COR); quartz + cordierite + biotite ± orthoamphibole ± cummingtonite ± garnet ± plagioclase (F1COR); and quartz + cordierite + garnet + sillimanite (F2COR). These rocks are interlayered with, or grade into, typical amphibolites (i.e., hornblende + plagioclase) and quartz + plagioclase-bearing rocks that are interpreted as least altered mafic and felsic metavolcanic rocks, respectively. Whole-rock geochemical data for least altered metavolcanic rocks and COR rocks at Ruostesuo define three mafic suites (primitive basalts, basaltic andesites, and ferrobasalts) and two felsic suites (R1 and R2 rhyolites). Using a combination of high field strength element ratios and rare earth element profiles, COR rocks can be assigned to all five volcanic suites. Calculated mass changes for the majority of exposed COR rocks at Ruostesuo are similar to those reported for chlorite + quartz assemblages found within the inner core of alteration pipes located below some sea-floor massive sulfide deposits. Relative to least altered precursors, mafic COR rocks have gained Fe, Mg ± S and lost variable amounts of Si, Ca, Na, K, Rb, Ba, Sr, and Eu, which resulted in an average net mass loss of about 10 percent. The bulk composition of most altered mafic COR rocks (near total loss of Ca, Na, K) reflects the proportion of chlorite to quartz within the premetamorphic alteration assemblages and the composition of the chlorite. Mass-change results for altered mafic rocks indicate that basalts at Ruostesuo were altered by an evolved, Fe-rich hydrothermal fluid at high water/rock ratios. Mass changes for COR rocks of the R2 felsic volcanic suite are similar to those of the mafic rocks (enriched in Fe ± Mg and depleted in Ca, Na, K, Rb, Ba, Sr, and Eu) except for significant additions of Si that resulted in net mass gains of 20 to 30 percent. Felsic COR rocks of the R1 volcanic suite show gains in K, Rb, and Ba that are attributed to the formation of premetamorphic sericite during an alteration stage dominated by hydrothermal seawater. In comparison to least altered metavolcanic rocks, mafic and felsic COR rocks are depleted in 18O, with {Delta}18O values 2 to 5 per mil lower than precursor rocks. Estimated temperatures of alteration were 270° to 350°C. These data indicate that the signature of a high-temperature (~300°C) sea-floor hydrothermal upflow zone is preserved within upper amphibolite to granulite facies rocks at Ruostesuo. The stratigraphic boundary between strongly chloritized mafic volcanic rocks and silicified felsic volcanic rocks marks a transition from net mass losses associated with hydrothermal fluid-dominated alteration to net mass gains associated with seawater-dominated alteration. Silicification at or near the boundary between mafic and felsic volcanic rocks may have been the result of conductive cooling or mixing with seawater from above. In the absence of textural or crosscutting relationships indicative of a younging direction, a stratigraphic model is proposed on the basis of the above geochemical constraints. Altered basalts at Ruostesuo represent the exposed base of a chloritic stockwork zone that passes upsection into altered felsic volcanic rocks that host massive Zn-Cu sulfides. This stratigraphic model has significance for the interpretation of other massive sulfide deposits in the Vihanti-Pyhasalmi district and exploration strategies that couple chemostratigraphy with mass-change results. More significantly, the preservation of premetamorphic oxygen isotope values for strongly metamorphosed and locally migmatitic COR rocks indicate that oxygen isotope mapping is a viable exploration tool in the district and in other mineralized upper amphibolite to granulite facies terranes worldwide.

Host-Rock and Structural Controls on the Nature and Timing of Gold Mineralization at the Holloway Mine, Abitibi Subprovince, Ontario

The Holloway mine, a mesothermal gold deposit, is located within the western Abitibi subprovince, Ontario. Mineralized zones that contain microscopic gold within pyrite are hosted by a sequence of metakomatiites, mafic to felsic metavolcanic, and metasedimentary rocks. There were two mineralization events, however the early one was the main event in terms of the amount of gold mineralization. This event was associated with albitization, silicification, chloritization, and sericitization in addition to pyritization. The later mineralization is associated with sericitization and pyritization. Carbonate and sericite alterations are associated with both mineralizing events. Results of U/Pb zircon age dating of rocks within the sequence constrains the main mineralization event to ~2672 Ma. Ore lenses of the Holloway deposit are enveloped by 080°-striking, steeply south dipping zones of intense deformation and fabric development within the Porcupine-Destor deformation zone. The shear zones are characterized by a strong foliation and, locally, an associated extension lineation. The ore lenses are interpreted to represent lithons that were formed by boudinage of the more competent mineralized zones, which are pervasively albitized. Analysis of geochemical data demonstrates that the REE, Th, Nb, Hf, Ti, Fe, and Al were largely immobile with respect to the albite, quartz, chlorite, sericite, and hematite alterations. Locally preserved textures such as abundant varioles and spherulites indicate that the host metavolcanics were relatively evolved rocks. Geochemical data show they have an iron enrichment trend typical of tholeiitic mafic to felsic suites and that they were derived through fractional crystallization of parent basalt. Because of their high Fe/Mg ratios the rocks reacted with hydrothermal solutions carrying gold, presumably as thio complexes, to form pyrite, which scavenged the gold.

Auriferous mineralization in the Sakoli Group, central India, with particular reference to sulphide-bound gold

Gold mineralization in the Pular-Parsori-Thutanbori belt of the Sakoli Group in central India is hosted predominantly by a felsic volcanic suite of rocks and is associated with quartz-sulphide ±carbonate veins. Both free and submicroscopic or invisible forms of latticebound gold occur. The free gold is found within the shear zone, as either disseminated or fracture-controlled grains within sulphides. Major opaque minerals in this gold-bearing belt include arsenopyrite, pyrrhotite, pyrite, chalcopyrite, sphalerite, galena, molybdenite, magnetite, hematite and scheelite. A three-stage paragenesis been established for these minerals.Mineral chemical analyses of the sulphides reveal the presence of sulphide-bound gold within the belt—the first time such occurrences have been identified in central India. Quantitative spot analyses of individual grains of sulphide indicate high concentrations of lattice-bound gold, which scanning electron micrographs show to be heterogeneously distributed. The substitution of Fe3+ by Au3+ in the lattice structure of the sulphides is postulated as the probable mechanism by which the gold became concentrated.

Geochemistry and Nd isotopes of the François Lake plutonic suite, Endako batholith: host and progenitor to the Endako molybdenum camp, central British Columbia

The Endako low-F granodiorite-type porphyry Mo deposit is hosted by the Triassic to Eocene Endako batholith, which comprises five temporally distinct plutonic suites, only one of which is mineralized. Pre-mineralization suites range in composition from diorite to granodiorite. The synmineralization Jurassic–Cretaceous François Lake suite includes two granodiorite- to monzogranite-bearing subsuites. Postmineralization phases include the Eocene Sam Ross Creek monzogranite. The batholith spans a silica range of 44–80 wt.% and consists of metaluminous to slightly peraluminous, low- to high-K, I-type granitoids; the Sam Ross Creek phase is an A-type granite. Positive ε Nd(T) values (+1.1 to +7.2) indicate derivation predominately from juvenile source materials, but with variable input from an older crustal component. Evidence suggests generation of older plutonic suites in a juvenile arc-type setting and younger K-rich felsic suites via recycling of juvenile arc crust without significant mantle-derived contributions. Three distinct Mo-deposition events in the Endako camp are linked to repeated generations of oxidized, highly evolved monzogranitic phases (pre-ore dykes, aplitic Nithi and Casey intrusions) belonging to both François Lake subsuites. Late pre-ore dykes with "Casey-like" geochemical signatures, along with massive unmineralized Casey intrusions near the Endako deposit, could reflect repeated injections from an underlying magma chamber that remained molten during the youngest Mo-deposition event. A genetic link may exist between the Sam Ross Creek phase, a pluton with Climax-type granite characteristics, and Eocene kaolinite alteration in the Endako deposit. Also, potential exists for Eocene-age Climax-type Mo mineralization within the Endako mining camp.

Plutonism in the Variscan Odenwald (Germany): from subduction to collision

Latest Devonian to early Carboniferous plutonic rocks from the Odenwald accretionary complex reflect the transition from a subduction to a collisional setting. For ∼362 Ma old gabbroic rocks from the northern tectonometamorphic unit I, initial isotopic compositions (eNd=+3.4 to +3.8;87Sr/86Sr =0.7035–0.7053;δ18O=6.8–8.0‰) and chemical signatures (e.g., low Nb/Th, Nb/U, Ce/Pb, Th/U, Rb/Cs) indicate a subduction-related origin by partial melting of a shallow depleted mantle source metasomatized by water-rich, large ion lithophile element-loaded fluids. In the central (unit II) and southern (unit III) Odenwald, syncollisional mafic to felsic granitoids were emplaced in a transtensional setting at approximately 340–335 Ma B.P. Unit II comprises a mafic and a felsic suite that are genetically unrelated. Both suites are intermediate between the medium-K and high-K series and have similar initial Nd and Sr signatures (eNd=0.0 to –2.5;87Sr/86Sr=0.7044–0.7056) but different oxygen isotopic compositions (δ18O=7.3–8.7‰ in mafic vs 9.3–9.5‰ in felsic rocks). These characteristics, in conjunction with the chemical signatures, suggest an enriched mantle source for the mafic magmas and a shallow metaluminous crustal source for the felsic magmas. Younger intrusives of unit II have higher Sr/Y, Zr/Y, and Tb/Yb ratios suggesting magma segregation at greater depths. Mafic high-K to shoshonitic intrusives of the southern unit III have initial isotopic compositions (eNd=–1.1 to –1.8;87Sr/86Sr =0.7054–0.7062;δ18O=7.2–7.6‰) and chemical characteristics (e.g., high Sr/Y, Zr/Y, Tb/Yb) that are strongly indicative of a deep-seated enriched mantle source. Spatially associated felsic high-K to shoshonitic rocks of unit III may be derived by dehydration melting of garnet-rich metaluminous crustal source rocks or may represent hybrid magmas.

Geochemistry of subduction-related mafic to felsic volcanic rocks of the late Archean Wawa greenstone belts, Superior Province, Canada

The late Archean (2.7 Ga) Schreiber-Hemlo and White River-Dayohessarah greenstone belts are characterized by tectonically juxtaposed volcanic and siliciclastic sedimentary rocks, which were collectively intruded by the syn- to post-kinematic, high-Al, high La/Ybn tonalite-trondhjemite-granodiorite (TTG) plutons. The volcanic rocks are composed of two distinct types of sequences: (1) komatiitic to tholeiitic basalts ocean plateau sequences and (2) mafic to felsic volcanic arc sequences. Based on major and trace element abundances, mafic to felsic, arcrelated volcanic rocks of the Schreiber-Hemlo and White River-Dayohessarah greenstone belts are divided into three major suites: (1) mafic to intermediate tholeiitic flows, (2) mafic to intermediate calc-alkaline flows, and (3) felsic calc-alkaline flows. All these suites share positively fractionated REE patterns, and negative anomalies of Nb and Ti. However, some intermediate and felsic volcanic rocks have positive Zr and Hf anomalies. Fractionated REE patterns, and negative Nb and Ti anomalies with respect to the neighbouring REE are all consistent with a juvenile oceanic island arc origins a metasomatized mantle wedge source for the tholeiitic suite. In comparison to tholeiites, the mafic to intermediate calc-alkaline suite is distinct in terms of: (1) enrichment in MgO, Th and LREE, Cr, and Ni; (2) higher Al2O3/TiO2 ratios; (3) more fractionated REE patterns; (4) more pronounced negative Nb and Ti anomalies; and (5) less fractionation between HREE and Sc-V, suggesting deeper and more primitive mantle source compositions for the calc-alkaline suite than the tholeiitic counterpart. Felsic rocks are defined by the enrichment of Zr and Hf, with respect to MREE, and correlate negatively with Sc abundances, suggesting garnet + amphibole ± clinopyroxene residual in the source. The inferred presence of garnet and clinopyroxene in the source is consistent with an independent argument for ecologitic residues in the slab. The existence of overlapping fields between mafic, intermediate, and felsic suites on trace element ratio diagrams suggest that processes controlling the production of these volcanic rocks were more complex than simple slab and/or wedge melting. This complexity may have resulted from a mixture of slab and wedge melts, second stage melting, magma mixing, fractional crystallization, partial equilibration with sub-arc wedge peridotite, crustal contamination, or some combination. Higher MgO, Ni, and Cr contents at a given Mg# in the basalts and andesites are consistent with higher geothermal gradients at Archean subduction zones than Phanerozoic counterparts.

Crustal assimilation in the Burnt Lake metavolcanics, Grenville Province, southeastern Ontario, and its tectonic significance

Three intimately interbedded suites of volcanic rocks are identified geochemically in the Burnt Lake area of the Belmont Domain in the Central Metasedimentary Belt, and their petrogenesis is evaluated. The Burnt Lake back-arc tholeiitic suite comprises basalts similar in trace element signature to tholeiitic basalts emplaced in back-arc basins formed in continental crust. The Burnt Lake continental tholeiitic suite comprises basalts and andésites similar in trace element composition to continental tholeiitic sequences. The Burnt Lake felsic pyroclastic suite comprises rhyolitic pyroclastics having major and trace element compositions that suggest that they were derived from crustal melts. Rare earth element models suggest that the Burnt Lake back-arc tholeiitic rocks were formed by fractional crystallization of mafic magmas derived by approximately 5% partial melting of an amphibole-bearing depleted mantle, enriched in light rare earth elements by a subduction component. The modelling also suggests that the Burnt Lake continental tholeiitic rocks were formed by contamination – fractional crystallization of mixtures of mafic magmas, derived by ~3% partial melting of the subduction-modified source, and rhyolitic crustal melts. These models are consistent with the suggestion that the Belmont Domain of the Central Metasedimentary Belt formed as a back-arc basin by attenuation of preexisting continental crust above a westerly dipping subduction zone.

Apatite solubility in peraluminous liquids: Experimental data and an extension of the Harrison-Watson model

New experimental data at 750–1000°C, 2–5 kbar, and for peraluminous melt compositions show a dramatic increase of apatite solubilities and phosphorus melt contents when compared to metaluminous or peralkaline compositions ( Harrison and Watson, 1984). The formation of alumino-phosphate units in peraluminous melts qualitatively explains our data. A new apatite solubility model, incorporating the data for the peraluminous region, is presented. Application of this model to examples of peraluminous felsic suites is discussed.

The petrogenesis of Tertiary microgranites and granophyres from the Slieve Gullion Central Complex, NE Ireland

Granophyres and microgranites of the Tertiary central intrusive complex of Slieve Gullion, NE Ireland are intimately associated with appreciably larger volumes of mafic rocks whose compositions extend between basalt and basaltic andesite. These felsic intrusions show near minimum melt compositions and are characterized by trace element abundances and ratios consistent with evolution by crystal fractionation within a suite of granite magmas. Rb, Cs, Th and U all increase while Sr and Ba and Eu decrease with differentiation, suggesting an important feldspar control on the development of this felsic suite. All rocks are enriched in light rare earth elements relative to heavy rare earth elements and show negative Eu-anomalies (Eu/Eu* = 0.5–0.05), the extent of the anomaly increasing with differentiation. Isotopic ratios of Sr and Nd ( 87 Sr/ 86 Sr 57.6Ma = 0.70983–0.71044 and 143 Nd/ 144 Nd = 0.512379–0.512384) rule out an origin through straightforward crystal fractionation of basaltic magmas, and the Nd isotopes are distinct from those of the Scottish Hebridean centres such as Skye. The δ l8 O and δD values on whole rock and mineral separates show evidence for interaction between the granitoids and a meteoric groundwater system. However, the extent of the interaction is not as extreme as that in the Skye granitoids. We interpret the granites as end products of advanced crystal fractionation of mantle-derived mafic liquids which interacted with lithosphere. The isotopic distinction between our data and those from the Hebridean islands are a further argument for lithospheric provinciality in the NW British Isles.

Geochemistry of Siluro-Devonian Tobique volcanic belt in northern and central New Brunswick (Canada): tectonic implications

Siluro-Devonian volcanic rocks of the northwestern mainland Appalachians are found mainly in the Tobique belt of New Brunswick where they consist predominantly of bimodal mafic–felsic suites erupted in a continental-rift environment. The axis of the Tobique rift trends north-northeast – south-southwest, obliquely to the regional northeast–southwest trend of the Appalachians. These geometric relationships are interpreted as being the result of rifting in a sinistral shear regime produced during emplacement of the Avalon terrene. The basaltic rocks are continental tholeiites and transitional basalts derived from a heterogeneous upper-mantle source that was enriched in incompatible elements relative to the primordial mantle. The mantle source was probably affected by the subduction processes.

Geochemical and Geochronological Signatures in Australian Proterozoic Igneous Rocks

Summary Most igneous activity in Australian Proterozoic domains is related to major orogenic events, or else to crustal extensional events in which there is an earlier volcanogenic rift phase overlain by a non-volcanic thermal subsidence phase. The igneous suites are either predominantly mafic, felsic or bimodal, and there are no major suites of intermediate igneous rocks. With time, the amount of igneous material decreases volumetrically in the Proterozoic sedimentary sequences and there are also distinct compositional changes. Depleted tholeiites are more common in the early Proterozoic sequences, whereas enriched tholeiites and alkaline rocks are dominant after 1800 Ma. Felsic igneous rocks are rare in the earliest Proterozoic sequences, but dominate in the period 1880-1400 Ma. They are also compositionally time controlled, and certain major magmatic felsic suites at c . 1860, 1790, 1740, 1670, 1500 Ma have distinct chemical parameters which are common to most domains. All the major suites of felsic igneous rocks show enrichment in K, Rb, Th, U, La, and Ce relative to Phanerozoic analogues. The felsic rocks are predominantly I-types, derived from Proterozoic infracrustal rather than Archaean precursors; rare S-types also appear to have Proterozoic sources. Our preferred model has the felsic igneous rocks derived from underplated sources which are progressively accreted to the base of the crust, as a result of a series of extensional episodes. There are no direct compositional equivalents of modern subduction-related igneous sequences suggesting that subduction as it is known today was not a dominant process in the production of the igneous suites. There are also no recognizable ophiolites, ocean-floor crust, or other evidence of a Wilson cycle.