Igneous Source Rocks Research Articles

SHRIMP U-Pb zircon ages of the Fuping Complex: Implications for Late Archean to Paleoproterozoic accretion and assembly of the North China Craton

The Fuping Complex is situated in the central part of the North China Craton and consists of four major lithological assemblages: Fuping tonalitic--trondhjemitic--granodioritic gneisses, Longquanguan augen gneisses, Wanzi supracrustal assemblage and Nanying granitic gneisses. SHRIMP U--Pb geochronology combined with U--Th and cathodoluminescence (CL) imaging of zircon enables resolution of magmatic and metamorphic events that can be directed towards understanding the late Archean to Paleoproterozoic history of the Fuping Complex. CL images reveal the coexistence of magmatic and metamorphic zircons in nearly all rock types of the Fuping Complex. The metamorphic zircons occur as either single grains or overgrowth (or recrystallization) rims surrounding and truncating oscillatory-zoned magmatic zircon cores, and are all characterized by nebulous zoning or being structureless, with extremely high luminescence and very low Th contents. These features make them distinct from magmatic zircons that are characterized by concentric oscillatory zoning, comparatively low luminescence and high Th and U contents. SHRIMP U--Pb analyses on magmatic zircons reveal that the tonalitic, trondhjemitic and granodioritic plutons of the Fuping gneisses were emplaced at 2523 ± 14 Ma, 2499 ± 10 Ma and 2486 ± 8 Ma, respectively; whereas the monzogranitic and granitic plutons of the Longquanguan augen gneisses were intruded, respectively, at 2510 ± 22 Ma and 2507 ± 11 Ma. Prismatic and oscillatory-zoned zircons dominate in the pelitic rocks of the Wanzi supracrustal assemblage and are interpreted as detritus from igneous source rocks. The concordant and discordant U--Pb ages of 2502 ± 5 Ma and 2507 ± 14 Ma obtained from two pelitic rock samples indicate these rocks must have been deposited no earlier than ∼2507 Ma ago. In addition, a zoned zircon grain in one pelitic rock sample has a near concordant age of 2109 ± 5 (1σ) Ma, which may provide a maximum depositional age for the Wanzi supracrustal rocks. SHRIMP results also reveal that granitic magmatism assigned to the Nanying granitic gneisses occurred over a protracted interval from ∼2077 ± 13 Ma to ∼2024 ± 21 Ma. The nebulously-zoned zircon grains and overgrowth/recrystallization zircon rims from different rocks yielded similar concordant ^207^Pb/^206^Pb ages in the range 1875 to 1802 Ma, interpreted as approximating the age of regional metamorphism of the Fuping Complex. Timing of primary zircon crystallization and regional metamorphism of the Fuping Complex is in general agreement with recent U--Pb zircon ion probe results for the Wutai and Hengshan Complexes that bound the Fuping Complex to the northwest. These areas are characterized by the emplacement of major granitoid bodies at around 2.50 Ga to 2.48 Ga ago, deposition of supracrustal rocks in the Paleoproterozoic, intrusion of Paleoproterozoic granitic bodies at 2.1 to 2.0 Ga, and regional metamorphism at 1.875 to 1.802 Ga. These data indicate that the Fuping and Hengshan Complexes do not represent an older crystalline basement to the Wutai Complex, as suggested in previous tectonic models but, together with the Wutai Complex, represent elements of a single late Archean to Paleoproterozoic magmatic arc system that has been subsequently tectonically disrupted and juxtaposed during the collision of the eastern and western North China blocks at ∼1.85 Ga, which resulted in the final assembly of the North China Craton.

Rapidity of orogenesis in the Paleoproterozoic Halls Creek Orogen, northern Australia; evidence from SHRIMP zircon data, CL zircon images, and mixture modeling studies

Combining U-Pb SHRIMP zircon geochronology with cathodoluminescence imaging enables the resolution of temporally closely-spaced geological events important for understanding tectonothermal processes in the Paleoproterozoic Halls Creek Orogen of northern Australia. The youngest zircon grains from a low-grade quartz-muscovite psammite of the Tickalara Metamorphics have a 207 Pb/ 206 Pb SHRIMP age of 1864+ or -4 Ma, defining a maximum depositional age for the unit. Zircon crystals from a high-grade garnet-biotite metapelite ( approximately 5-10 volume percent leucosome) from the same sequence are considerably more complex, and SHRIMP analyses form a single, large concordant group in the range approximately 1885 to 1830 Ma. The zircon crystals contain three distinct CL zoning patterns, and individual SHRIMP spots show a corresponding variation in 207 Pb/ 206 Pb ages. Concentric oscillatory-zoned zircon dominates pre-1850 Ma ages and is interpreted as detritus from igneous source rocks. Narrow structureless zircon rims infrequently overgrow and truncate the oscillatory zoning. These rims comprise most of the post-1850 Ma analyses and are inferred to be the product of uppermost amphibolite facies metamorphism, reflecting the interaction of a robust pre-existing zircon suite with a very limited melt volume. In addition, some zircon cores of uncertain geological affinity contain large areas devoid of oscillatory zoning with individual analyses clustering around 1850 Ma. Dividing the data into detrital and metamorphic suites solely on the basis of CL imaging yields an older group of 18 analyses ( 207 Pb/ 206 Pb age = 1867+ or -4 Ma) and a younger group of 10 analyses ( 207 Pb/ 206 Pb age = 1843+ or -4 Ma), not including five SHRIMP spots within areas of unzoned zircon. Mixture modeling of all 33 analyses in the post-1900 Ma data set resulted in a best-fit solution composed of two distinct components: (1) an older group of 19 analyses with an age of 1867+ or -4 Ma, and (2) a younger group of 14 analyses with an age of 1845+ or -4 Ma. These results suggest that the unzoned patches of zircon might be related to metamorphism rather than being cores. Importantly, the ages and proportions of populations predicted by mixture modeling are otherwise very similar to those derived from analysis of CL zoning patterns. These data imply that high-temperature metamorphism occurred in the metasedimentary rocks less than 25 my after the crystallization of the igneous source. Such rapid rates of erosion, deposition, and burial have rarely been proposed for Proterozoic rocks, despite evidence for analogous orogenic processes in the Mesozoic and Cainozoic on a comparable timescale. Careful evaluation of geological and geochronological data in Proterozoic provinces elsewhere may reveal similar patterns, with potential implications for the possible rates of Proterozoic orogenesis and crustal evolution.

Petrography and geochemistry of Holocene sands in the western Gulf of Mexico; implications for provenance and tectonic setting

ABSTRACT Petrographic and geochemical analysis of recent fluvial, beach, and dune sediments derived from the same source terrane was carried out in the western Gulf of Mexico to show the usefulness of the these three environments in determining the tectonic setting of the source. Petrographic analysis showed that dunes concentrate the less heavy minerals and quartz grains by means of the selectiveness of the wind as a transport agent. In contrast, the heavier grains, such as the rock fragments remain in the beach and fluvial environments because of the high transport energy. Beach and dune sands do not define a particular tectonic setting because of the selective wind action of the backshore of the beach and the dune environments and the less intense chemical weathering. The fluvial sands are the most representative in terms of interpretation of tectonic setting because they reflect the volcanic domain of the source rocks of the Trans-Mexican Volcanic Belt. The geochemical results of all three sedimentary environments reflect the calcalkaline character of the igneous source rocks of the Trans-Mexican Volcanic Belt. The samples fall in a continental island-arc-margin field with basaltic and andesitic source rocks. A correlation and factor analysis indicates an alkali-basalt source that contributes to the composition of the beach, dune, and fluvial sediments. The geochemical analysis of the sediments more accurately reflects the tectonic setting regardless of the depositional setting of the sediments.

Petrography and Geochemistry of Holocene Sands in the Western Gulf of Mexico: Implications for Provenance and Tectonic Setting

ABSTRACT Petrographic and geochemical analysis of recent fluvial, beach, and dune sediments derived from the same source terrane was carried out in the western Gulf of Mexico to show the usefulness of the these three environments in determining the tectonic setting of the source. Petrographic analysis showed that dunes concentrate the less heavy minerals and quartz grains by means of the selectiveness of the wind as a transport agent. In contrast, the heavier grains, such as the rock fragments remain in the beach and fluvial environments because of the high transport energy. Beach and dune sands do not define a particular tectonic setting because of the selective wind action of the backshore of the beach and the dune environments and the less intense chemical weathering. The fluvial sands are the most representative in terms of interpretation of tectonic setting because they reflect the volcanic domain of the source rocks of the Trans-Mexican Volcanic Belt. The geochemical results of all three sedimentary environments reflect the calcalkaline character of the igneous source rocks of the Trans-Mexican Volcanic Belt. The samples fall in a continental island-arc-margin field with basaltic and andesitic source rocks. A correlation and factor analysis indicates an alkali-basalt source that contributes to the composition of the beach, dune, and fluvial sediments. The geochemical analysis of the sediments more accurately reflects the tectonic setting regardless of the depositional setting of the sediments.

Comparative study of yttrium and rare-earth element behaviours in fluorine-rich hydrothermal fluids

The mineral ‘fluorite’ is utilized as a probe to investigate the behaviour of the pseudolanthanide yttrium with respect to the lanthanides (rare-earth elements, REE) in fluorine-rich hydrothermal solutions. Hydrothermal vein fluorites are characterized by the close association of Y and REE, but in contrast to igneous and clastic rocks they show variable and nonchondritic Y/Ho ratios of up to 200. This suggests that Y and Ho, although similar in charge and size, may be fractionated in fluorine-rich medium-temperature aqueous fluids. In such solutions Y acts as a pseudolanthanide heavier than Lu. Y/Ho ratios of hydrothermal siderites are slightly below those of chondrites, suggesting that in (bi)carbonate-rich siderite-precipitating solutions Y may act as a Sm-like light pseudolanthanide. This indicates that Y-Ho fractionation is not a sourcerelated phenomenon but depends on fluid composition. Based on these results it is strongly recommended that discussions of normalized REE patterns in general should be extended to normalized Rare-Earth-and-Yttrium (REY) patterns (Y inserted between Dy and Ho), because the slightly variable behaviour of the pseudolanthanide yttrium with respect to the REE may provide additional geochemical information. Available thermodynamic data suggest a negative correlation between Y/Ho and La/Ho during migration of a fluoriteprecipitating hydrothermal solution. Cogenetic fluorites, therefore, should display either similar Y/Ho and similar La/Ho ratios, or a negative correlation between these ratios. This criterion may help to choose samples suitable for Sm-Nd isotopic studies prior to isotope analysis. However, in cogenetic hydrothermal vein fluorites the range of Y/Ho ratios is often almost negligible compared to the range of La/Ho ratios. This may be explained by modification of REE distributions by post-precipitation processes involving (partial) loss of a separate LREE-enriched phase. The presence of variable amounts of such an accessory phase in most fluorite samples is revealed by experiments employing stepwise incomplete fluorite decomposition. Fluorites derived from and deposited near to igneous rocks apparently display chondritic Y/Ho ratios close to those of their igneous source-rocks. However, a positive YSN anomaly is likely to develop as the distance between sites of REY mobilization and deposition increases.

The mobility of zirconium and other “immobile” elements during hydrothermal alteration

Development of zircon and other Zr phases in hydrothermal deposits indicates that Zr can be highly mobile in these systems. Mobility is most common in, but not restricted to, F-rich hydrothermal systems related to alkalic, F-rich igneous suites; these suites can range from peralkaline through metaluminous to peraluminous. A few examples are neither alkalic nor F rich. Three locations in the Trans-Pecos Magmatic Province, Texas, U.S.A., demonstrate this hydrothermal Zr mobility. All three igneous systems are alkalic and F rich but vary in alkali/Al ratios. Peralkaline rhyolites and trachytes in the Christmas Mountains contain as much as 2100 ppm Zr, mostly in aegirine or arfvedsonite; zircon is rare or absent. Fluorspar replacement deposits in limestone at contacts with the rhyolites contain as much as 38,000 ppm Zr, occurring as small, disseminated zircons. The deposits also are enriched in a variety of incompatible elements, including Be, rare-earth elements (REE), Y, Nb, Mo, Hf, Pb, Th and U. The Sierra Blanca intrusions, a series of mildly peraluminous, F-rich rhyolite laccoliths, contain as much as 1000 ppm Zr, mostly as zircon. Hydrothermal zircon occurs as overgrowths on magmatic grains, as veinlets connected to overgrowths, and in fluorspar replacement bodies in adjacent limestone. The highest Zr concentrations in fluorspar are ∼200 ppm. Metaluminous quartz monzonite from the Infiernito caldera contains 400–600 ppm Zr, mostly as zircon. Euhedral zircon in quartz-fluorite veins in the quartz monzonite indicates mobility of Zr. Zirconium concentrations in the veins are unknown, but the paucity of zircon suggests little Zr enrichment relative to the host. Zircon and, more rarely, zirconolite, occur in skarn in the Ertsberg District of Irian Jaya, Indonesia. Unlike in Texas, related igneous rocks are metaluminous, and the hydrothermal system was F poor. Worldwide, hydrothermal zircon, other Zr phases, and Ti- and Al-bearing phases occur in skarn, epithermal precious metal veins, volcanogenic massive-sulfide deposits and mylonites. We propose that differences in Zr mineralogy of igneous source rocks is an important factor in determining the availability of Zr to hydrothermal fluids. Although Zr concentrations in the Sierra Blanca and Christmas Mountains rhyolites are similar, Zr enrichment in fluorspar was much greater in the Christmas Mountains. We suggest that hydrothermal solutions could easily break down aegirine and arfvedsonite to release Zr, but that zircon was only moderately attacked. Therefore, far more Zr was available for transport and subsequent deposition in the Christmas Mountains than at Sierra Blanca. Availability of other trace elements probably is also governed by their mineral host. Although Zr mobility is most common in F-rich hydrothermal systems related to alkalic and F-rich igneous systems, mobility at Ertsberg may have been promoted by sulfate complexing.

Sedimentologic and palaeoclimatic classification of Cretaceous red beds in East Asia: a general view

Based on the results of lithostratigraphic, petrographic and geochemical studies, the Cretaceous continental red beds in East Asia are classified into three groups of different origins: red beds formed in hot and dry conditions, and those influenced by metamorphic and igneous source rocks in moderate and humid environments. In order to redden sediments, haematite must be very finely diffused in the matrices of mudstones, or coat detrital grains as a thin film. Haematite and/or its precursors are derived from metamorphic and igneous basement rocks in nearby source areas, or from syngenetic pyroclastic rocks. The pigmenting haematite forms by decomposition of silicate minerals in source rocks during weathering, and is also preserved as neoformed ferric oxides in depositional sites. For these processes, moderate and humid, and hot and dry climates are favourable, respectively. The present study reveals that reddened Cretaceous rocks in East Asia are formed by different processes operating in a range of palaeoclimatic conditions.

Relationship between sediments and their igneous source rocks using clay mineral multi-element chemistry: the Cenozoic lacustrine Anloua basin (Adamaoua, Cameroon)

The Cenozoic lacustrine basin of Anloua (Adamaoua, Cameroon), is filled with the weathering products of the surrounding rocks. In order to determine the origin of these sediments, multi-element, especially trace element, analysis of their < 2 μm fraction was undertaken. The chemical compositions of these fractions were compared with those of the rocks surrounding the basin by multivariate statistical analysis and also graphically. A principal component analysis allowed the most discriminant elements to be selected, and a hierarchical ascending classification (HAC) made with these elements divided the samples into groups. Some of these groups contain < 2 μm fractions of both the lake sediments and surrounding rocks. These sediments are determined as deriving by weathering from the surrounding rocks. Some of the other groups contain no samples from the surrounding rocks and their petrographic origin cannot be given. A graphic comparison of the relative amounts of the elements (determined by HAC) from the < 2 μm fractions of the various groups and the rocks surrounding the basin allowed attribution of a petrographic origin for the various sediments analysed by using the fit between the curves. The clay mineralogy, the amounts of Ti, Zr, and heavy minerals in the < 2 μm fractions and their distribution in the area studied, as well as the presence of angular quartzite pebbles in some sediments gives an indication of the paleogeography of the Anloua basin. There was deltaic input towards the north and a high energy input, possibly torrential, to the west.

The Martian surface as imaged, sampled, and analyzed by the Viking landers

The two Viking landers touched down on the surface of Mars in July and September of 1976. Viking Lander 1, renamed the Mutch Memorial Station (MMS), continued to operate for 3 Martian years in Chryse Planitia. The landing site is a rolling, sparsely cratered plain. Impact crater size‐frequency distributions demonstrate that only meters of vertical erosion have occurred over the lifetime of the surface. The site exhibits bedrock exposures, numerous rocks excavated during cratering events, indurated fine‐grained sediment (blocky material) mixed with dust and lithic fragments, and aeolian drifts that overlie the blocky material and which probably accumulated during global dust storms. Lander 2 operated for approximately 2 Martian years in Utopia Planitia, a region subjected to volcanic, impact, aeolian, and periglacial phenomena. Lander 2 is located west of the 100‐km‐diameter crater Mie, on or near the distal end of the continuous ejecta deposits. Polygonal ground suggests that desiccation or freeze‐thaw processes have occurred. The Lander 2 site exhibits numerous rocks excavated from craters and exposed by erosion, set in a matrix of crusty to cloddy sediment. Rare aeolian drifts overlie the crusty to cloddy material. Lander multispectral imaging of sediment exposures at both sites is best matched with iron‐rich, claylike weathering products called palagonites. The spectral characteristics are probably governed by thin deposits of dust from global and local dust storms. Darkest rocks have reflectance values that match iron‐rich igneous rocks, although most rocks are covered with indurated coatings of the palagonitelike products. Chemical compositions of sediments at the two landing sites are strikingly similar, suggesting an aeolian origin. The compositions suggest an iron‐rich igneous source rock and are matched by an assemblage of clays and salts. Samples of blocky materials were found to have about 50% more Cl and SO3 than other samples, implying cementation by salts. The magnetic component in the sediment is silt‐sized or finer and may be maghemite or ultrafine‐grained hematite. No indigenous organic compounds were found. The materials were found to be highly reactive, releasing O2 when H2O was added, oxidizing organic nutrients on board the landers and fixing atmospheric CO2. Clay‐ to silt‐sized particles were inferred for drift and crusty to cloddy materials. No terrestrial samples have been found that replicate all the attributes of the Viking samples. Approximately 10 µm of H2O ice accumulated during both winters at Lander 2 and each time evaporated over approximately 200 Martian days. Thin layers of bright red dust from global and local dust storms accumulated at both sites during the first 2 Martian years. During the third winter, strong winds generally scoured the MMS site, leaving behind a darker aeolian lag deposit. Bedrock at both sites formed billions of years ago, and rocks have been generated and weathered slowly over geologic time, but aeolian sediments accumulate, become cemented, and are subsequently eroded over time spans of millions of years or less. The lifetime of the sedimentary deposits at the landing sites may be governed by the 105‐ to 106‐year variations in climate due to quasi‐periodic changes in obliquity, eccentricity, and spin axis orientation.

Provenance Characteristics of Detrital Opaque Fe-Ti Oxide Minerals

ABSTRACT We have investigated petrogenetic characteristics of detrital opaque Fe-Ti oxide minerals (DOPQ) in sand samples collected from Holocene streams draining exclusively igneous and metamorphic source rocks in the Rocky Mountain region under a semi-arid climate, and from the Appalachian Mountains under a humid climate. Our sampling strategy allows us to compare and contrast the properties of detrital mineral grains not only from igneous and metamorphic source rocks, but also those from dissected-arc and recycled-orogen provenances. Reflected-light petrography including extensive modal analysis and electron probe microanalysis of polished grain mounts were the two principal methods of data gathering. Between 25% and 50% of all DOPQ show oxidation exsolution and/or other lamellar intergrowth textures, i.e., they are polymineralic grains. Lamellar intergrowth texture is present in about twice as many grains derived from igneous source rocks as from metamorphic source rocks. Lamellae thinner than 2 µm are predominant in grains derived from dissected-arc provenance, whereas lamellae thicker than 10 µm are predominant in grains derived from recycled-orogen provenance. Exsolution lamellae along {111} planes of magnetite (three directions) and presumably along {0001} planes of ilmenite (one direction) characterize dissected-are and recycled-orogen provenance, respectively. Detrital ilmenites derived from the igneous source rocks studied show a wide range of TiO2 co tent with a mode around 47%, whereas those from the metamorphic source rocks show a tight cluster around 52% TiO2 content. DOPQ derived from metamorphic source rocks contain < 0.5% MgO, whereas those from igneous rocks may contain up to 1.2% MgO. Both MgO and MnO are low, 0.3% and 2.5%, respectively, in DOPQ derived from dissected-arc provenance, but may be relatively high, up to 1.2% and 8.0%, respectively, in grains derived from recycled-orogen provenance. Whereas no single character of DOPQ may be diagnostic of provenance, discriminant function analysis of the complete set of data show that collectively they are diagnostic in 95% to 100% of cases. We recommend the use of 1) presence of exsolution texture, 2) minimum lamellae widths and the number of crystallographic orientation of the lamellae, and 3) chemical composition including minor elements of DOPQ as important properties for provenance identification. This should supplement and refine existing criteria for provenance determination.

Geochemical signature of provenance in sand-size material in soils and stream sediments near the Tobacco Root batholith, Montana, U.S.A.

Trace-element geochemistry of sandstones are being used to determine provenance. We have conducted preliminary and limited experiments to determine to what extent daughter sands retain the geochemical signature of parent rocks. Six sets of first-order stream sediments, soils from adjacent slopes, and a variety of parent rocks were collected from southwestern Montana, U.S.A. Sampling in a low-relief area ensured that climate and residence time of soils on slopes could be eliminated as variables. Sand-size fractions of stream sediments and soils, and the corresponding parent rocks (granodiorite, quartz monzonite, granite gneiss, biotite-tonalite gneiss and amphibolite) were analyzed for most major elements and selected trace elements. Petrologic modal analysis of the parent rocks and the 0.25–0.50-mm fraction of each sand was done to monitor major mineralogic control, if any, on chemical compositions of the samples. Our data show that the abundances of the Si and Al in sediments do not discriminate provenance. Abundances of Ca, Mg, Fe and Ti may broadly distinguish between sands derived from metamorphic and igneous source rocks, at least in the area studied. Differences in abundances of the Ba and Th, and the ratio of La/Lu between granitic, tonalitic and amphibolitic parent rocks are preserved in the daughter sediments that we studied. However, the size of the Eu anomaly in the REE patterns of different daughter sediments is not diagnostic of parent rocks. Abundances of Co and Sc distinguish between sediments derived from felsic and mafic rocks. A better provenance discrimination is obtained if the ratios La/Sc, Th/Sc, La/Co, Ba/Sc and Ba/Co are used. Petrologic modal data show that mineral contents and chemical compositions of parent rocks are compatible with each other. The chemical composition of the sands may be roughly correlated to the petrological modal data but the abundances of some minor and trace elements of sediments cannot be inferred from modal mineralogy. This is expected because these elements may concentrate in accessory minerals and/or may weather out into aqueous or clay mineral fractions; it is also compatible with conclusions of previous studies that some of these elements do not reside in sand-size fractions of siliciclastic sediments.

The New England Batholith: constraints on its derivation from Nd and Sr isotopic studies of granitoids and country rocks

Nd and Sr isotopic compositions are reported for the granitic suites which comprise the late Palaeozoic to earliest Mesozoic New England Batholith of eastern Australia. Some of the granitic suites are typically I-type in their mineralogy, chemistry and isotopic compositions, implying a derivation from igneous (infracrustal) source rocks, whereas other suites have characteristics consistent with a derivation from a protolith which was predominantly sedimentary and relatively felsic (S-types). The I-type granitoids of the Nundle Suite have ϵ Nd values (+3.3 to +6.1) that are amongst the most primitive yet documented for a relatively felsic (SiO 2 ~ 65%) plutonic suite and these values imply a derivation from either a depleted upper mantle source or, more probably, a complex source region involving both volcanic-arc rocks and detrital material. Their compositions are distinctly more primitive than those of the New England Super-Suite which constitutes the Permian ‘core’ of the batholith. This extensive Super-Suite (comprising granitoids traditionally designated as I-type) has an overall range in initial Nd and Sr isotopic compositions of −1.7 to +4.6 and 0.70458 to 0.70624 respectively, although the majority of plutons have initial Nd isotopic compositions which fall into a very narrow range (+1.0 ±1.5 ϵ units). This limited range is remarkable considering the extreme lithological diversity and range in chemical composition of the analysed samples (SiO 2 47%–74%) and implies a source region of considerable volume having reasonably uniform isotopic compositions but variation in chemistry. A similarly uniform source isotopically is also indicated for the S-type granitoids of the Carboniferous Hillgrove Suite and Carboniferous-Permian Bundarra Suite with initial ϵ Nd values of +0.8 to +2.3 and initial 87Sr 86Sr compositions of 0.70474 to 0.70577 showing only limited ranges. Five pelites, three ‘felsic’ and four ‘mafic’ greywackes, representing typical country rocks from different stratigraphic levels have initial ϵ Nd values (−1.7 to +6.6) and initial 87Sr 86Sr compositions (0.70378 to 0.70585) which essentially mirror the compositional variation in the granitoids. A chemical, mineralogical and isotopic bimodality in these sediments indicates two very distinctive sources, one felsic (rhyodacitic) the other relatively mafic (andesitic), which were separated spatially and temporally in many but not all areas of early- to mid-Palaeozoic New England. A model is presented in which the S-type granitoids are derived from a predominantly felsic source, i.e. pelites and ‘felsic’ greywackes, whereas some of the granitoids belonging to the New England Super-Suite may have been derived from source rocks consisting of both felsic and mafic sedimentary components. The distinction between many S-type and I-type granitoids in New England is unclear for two principal reasons; (a) because the granitoids and their respective source rocks are relatively young geologically so that their isotopic systems have not evolved to any considerable extent, and (b) because of the intrinsic igneous chemical compositions of any sedimentary component that may be involved in their genesis.

Ultrametamorphism and granitoid genesis

A model is presented to explain the geochemical and mineralogical characteristics of granitoids and their inclusions. The product of ultrametamorphism is melt + residuum, both of which may move en masse to the site of crystallization.The nature of the source material can be deduced from studies on the granitoids and their inclusions; based on studies of the Lachlan belt of southeastern Australia we recognize granitoids derived from metasedimentary rocks (S-types) and those derived from igneous source rocks (I-types).The straight-line variation diagrams of most granitoid suites is explained by progressive separation of residuum (= restite) and melt. It is shown that some granitoid suites consist of minimum melt + residuum whereas others represent the crystallization of “nonminimum” melts + residuum.Residuum is recognized as metasedimentary xenoliths in S-types and as mafic hornblende-rich xenoliths in I-types. Xenocrystal material is more difficult to recognize petrographically. We suggest that the complexly zoned and twinned plagioclases so characteristic of orogenic rocks are modified residuum. These and xenoliths are absent in granitoids which have largely crystallized from a melt such as those of the Tuolumne Suite of the Sierra Nevada.Mafic minerals of granitoids whether residuum or crystallization products are mostly equilibrium assemblages. Relict mafic phases from the source do not persist and hence P-T conditions of magma generation cannot be deduced.

An Evaluation of the Provenance of Terrigenous Sediments from Offshore Vancouver Island

Modern sands and gravels from Barkley Sound, Vancouver Island, are mainly composed of igneous lithic fragments, present in sediments coarser than medium sand, and of andesine plagioclase, present in finer size fractions. Heavy mineral assemblages are dominated by hornblende, epidote, opaque minerals, and chlorite. This sediment mineralogy is mainly controlled by the composition of the source area, and to a lesser extent, by hydraulic processes of the depositional environment; effects of weathering at source and river transport are minimal. Sources include Mesozoic, eugeosynclinal basalts and andesites, late Mesozoic, post-geosynclinal granodiorite and diorite plutons, and Cretaceous to (?) late Pleistocene sediments which themselves were ultimately derived from the aforementioned igneous sources.Relict sands and gravels on the continental shelf adjacent to Barkley Sound, are similar in composition to the modern sediments and have been derived from similar igneous and sedimentary source rocks. However, relative to modern sands, relict sands have a greater variety of minor constituents, particularly heavy minerals, which in addition to hornblende, epidote, and opaque minerals, include garnet, orthopyroxene, and clinopyroxene. It is suggested the more varied mineralogy is due to: (1) a more extensive source area; and (2) introduction of additional sediment from outside the study area by modern bottom currents.

Detrital modes of new zealand graywackes

Graywacke, used here for any dark indurated sandstone, and argillite are the dominant pre-Tertiary rocks underlying much of New Zealand. The strata, mainly Permian to Jurassic, can be divided into two facies: (1) the western marginal facies of orderly, fossiliferous strata exposed in the New Zealand marginal syncline; and (2) the eastern axial facies of strongly folded and faulted, and partly schistose, rocks that occupy the keel of the New Zealand geosyncline. The two facies are in fault contact along the length of New Zealand. Each facies is 15–20 km thick, and records deposition, probably on oceanic crust, at a rate ≈125 ± 25 m/million years (m.y.) for ≈150 m.y. The western marginal facies apparently was deposited on a rapidly subsiding shelf that was positioned as a major topographic bench between an oceanic trench, site of sedimentation of the turbidites of the eastern axial facies, and a volcano-plutonic arc, probable provenance of the detritus in the two facies. The two facies were metamorphosed at low grade under relatively high pressures and low temperatures, probably in the sub-trench belt of paired metamorphic belts, with the provenance region being the sub-arc belt of relatively low pressure and high temperature. The graywackes of the western marginal facies apparently record a progressive erosional modification of the volcano-plutonic provenance, from a volcanic terrane that shed detritus into lower horizons of the sequence to a plutonic terrane that shed detritus into higher horizons of the sequence as dissection bit into the igneous source rocks of the arc. The most common graywackes of the eastern axial facies are more quartzose than those of the western marginal facies, and may have received plutonic detritus that bypassed the shelf between the arc and the trench. Feldspatholithic “volcanic” graywackes are poor in quartz, rich in pyriboles, bear mainly plagioclase feldspar, and contain mostly volcanic rock fragments. Lithofeldspathic “plutonic” graywackes are relatively high in quartz and mica, bear K-feldspar as well as plagioclase, and contain abundant metamorphic as well as volcanic rock fragments. Analogous paired suites of graywackes may be common in the circum-Pacific region.

Paleocurrents and Origin of Huronian Lorrain Formation, Ontario and Quebec: ABSTRACT

The Lorrain Formation crops out in 3 areas between Sault Ste. Marie, Ontario, and Ville-Marie, Quebec--Bruce Mines, Whitefish Falls, and Cobalt. It conformably overlies the Gowganda Formation in each area; in the Cobalt area it also locally overlies Archean granite. The Lorrain is overlain by the Gordon Lake Formation. In ascending order, the Lorrain Formation consists of arkose, subarkose, jasper-bearing orthoconglomerate, and orthoquartzite. In the Bruce Mines area the Lorrain is 8,300 ft thick and is divided into 5 members (A-E); in the eastern areas the Lorrain is only 5,000 ft thick and is divided into 3 members (lower, middle, and upper). In each area, mineralogic and textural maturity increases upward; grains in immature arkose at the base are subrounded and 44% feldspar, and grains in supermature orthoquartzite at the top are well rounded and 95-100% quartz. Chronologically, the depositional environments suggested are shallow-water marine, lacustrine, delta fringe, overshelved beach, and high-energy beach. Currents flowing south and southeast deposited most of the sediments; however, in jasper-bearing conglomerate of the Bruce Mines area, cross-bedding and pebble composition indicate a 90° variance in transport direction, and an additional source on the east. Abundant potash feldspar and lack of metamorphic and sedimentary rock fragments in the lower members suggest a plutonic provenance. Subsequent deposition of jasper, chert, and quartz-rich detritus indicates erosion and reworking of Precambrian jaspilites and igneous source rocks similar to those now exposed in the Canadian shield. End_of_Article - Last_Page 850------------