Cogenetic Suite Research Articles

Multiple sources for the Coastal Batholith of central Chile (31–34°S): geochemical and Sr–Nd isotopic evidence and tectonic implications

The Coastal Batholith of central Chile between 31 and 34°S represents a locus of long-lived (c. 200 Ma) plutonic activity. It is composed of four plutonic complexes: Santo Domingo, Limarı́, Papudo–Quintero and Illapel. These complexes, which occur as north-trending discontinuous belts that decrease in age eastward, were emplaced during four major episodes of arc magmatism. The Santo Domingo Complex (Carboniferous) comprises hornblende–biotite bearing tonalites and granodiorites with abundant mafic enclaves, and enclave-free granites. The association of enclave-bearing granitoids and enclave-free granites resulted from different degrees of mixing of crustal- and mantle-derived magmas. The Limarı́ Complex (Lower Jurassic) is a bimodal association of crustal leucogranites and mantle-derived gabbros. The lithology of the Papudo–Quintero Complex (Middle Jurassic) varies from hornblende–pyroxene diorite to biotite granite. The main rock types, namely tonalites and granodiorites, commonly contain mafic enclaves. The granitoids and the enclaves have remarkably similar REE patterns and are similar in Sr–Nd isotope composition, which suggest that they represent a cogenetic suite of mantle origin. The Illapel Complex (Cretaceous) consists of hornblende bearing tonalites, trondhjemites and granodiorites that resemble the rocks of TTG suites. The ε Nd values and initial 87 Sr/ 86 Sr ratios (Sr i) for the mafic rocks of the Carboniferous Complex ( ε Nd: −2.0 and −3.5; Sr i: c. 0.7057) differ markedly from those of the Mesozoic mafic rocks ( ε Nd: +1.9 to +5.4; Sr i: 0.7033 to 0.7039), the Cretaceous rocks being the most depleted with regard to Sr–Nd. A change from Carboniferous lithosphere-dominated sources to Mesozoic asthenosphere-dominated sources of the magmas that started in the Early Jurassic is recognized. Slab components can be recognized in the source of the Cretaceous rocks. The crustal source of the Lower Jurassic leucogranites was more isotopically depleted and refractory than that of the Carboniferous granites. On the other hand, no crustal participation in the origin of the Middle Jurassic and Cretaceous felsic granitoids is recognized; they are differentiated from mantle-derived magmas. Progressive removal of old lithospheric mantle during the Mesozoic due to a process of lithospheric delamination would explain the profound change in the subcrustal source, as well as the melting and modification towards a more refractory nature of the continental crust.

Primary lithological variation in the kaolinized St Austell Granite, Cornwall, England

Geological mapping of existing and redundant kaolin workings within the St Austell Granite has identified a suite of granitic rocks which show evidence of complex late-stage magmatic and hydrothermal processes. Coarse porphyritic biotite granites, like those which predominate in southwest England, occur much more widely than previously acknowledged, and are intruded by an apparently cogenetic suite of lithium-mica granites and tourmaline granites. The tourmaline granites characteristically exhibit very variable textures, with coarse quartz grains set within a fine grained, tourmaline-rich matrix. A highly evolved fine-grained tourmaline granite represents the most evolved of this suite. Topaz granites intrude the earlier granite varieties, and all are intruded by rhyolite porphyry dykes (elvans). Major and trace element chemical data suggest that the biotite granite–lithium-mica granite–tourmaline granite suite represents the product of crystallization of a granitic magma within which B (but not F) became progressively enriched until water saturation was achieved. Water exsolution effectively quenched any remaining granitic melt, resulting in the very variable textures shown by the tourmaline granites. The topaz granites are chemically distinct from their predecessors, showing marked enrichment in F, Li and P 2 O 5 (but not B). Instead of being products of differentiation of biotite granite magma, the topaz granite melts may have been derived separately in a later episode of partial melting of the same source. Kaolinization is widespread throughout the western part of the St Austell Granite, and deposits worked at present tend to be located in granite varieties other than biotite granite. The geochemical parameters used to distinguish the primary granite types (particularly Nb v. Zr and Ga–Nb–Zr plots) are sufficiently robust to permit the parent granitic rock type to be identified for heavily kaolinized material.

Geology of the volcanic-hosted Brockman rare-metals deposit, Halls Creek Mobile Zone, northwest Australia. II. Geochemistry and petrogenesis of the Brockman volcanics

Lavas and subvolcanic intrusions of the 1.87 Ga Brockman volcanics comprise a cogenetic suite of alkaline,Qz-normative, metaluminous trachyandesites, trachytes and trachydacites/rhyolites. They are genetically related to the rare-metal-bearing “Niobium Tuff” which contains extreme enrichments in high-field-strength incompatible elements (av. 1660 ppm Y, 9700 ppm Zr, 3200 ppm Nb, 175 ppm Yb). Neodymium isotopic data indicate the Brockman parent magma was mantle-derived with eNd(initial) + 3, analogous to basaltic magmas generated in some modern intraplate “hot-spot” volcanic provinces. The geochemical evolution and incompatible element enrichments in the Brockman suite can be modelled by AFC processes involving extensive degrees of crystallization and progressive contamination of derivative magmas with granitic/ metasedimentary upper crust. The large degrees of crystallization required to derive the more differentiated members of the Brockman suite are best accommodated by a process of “liquid fractionation” resulting in internal compositional stratification of the magma chamber with extreme differentiates such as the Niobium Tuff forming a volatile-enriched “cap” in the magma chamber roof-zone. The high fluorine content of the Brockman magmas played a crucial role in enhancing rare-metal contents by increasing the efficiency of crystal-liquid separation and decreasing mineral-melt Kd's. There appears to be no special role for fluorine-rich fluids in generating the rare-metal enrichments. However, leaching of fluorocarbonate minerals by late hydrothermal solutions, rather than fractionation of a LREE-selective phase, caused marked LREE-depletion in the Niobium Tuff.

Accessory-mineral fractionation of rare-earth element (REE) abundances in granitoid rocks

Accessory minerals with large distribution coefficients ( K d's) for the REE have the potential to strongly affect the evolution of REE abundances during igneous differentiation (e.g., crystallization, melting, assimilation). A cogenetic suite of lavas (basaltic andesites through high-silica rhyolites) from the Batopilas area of Mexico, which preserve petrographic evidence for apatite and zircon saturation, is used to illustrate and confirm an inverse geochemical approach to recognizing early-liquidus accessory-mineral saturation. The transition in behavior of Zr from an incompatible trace element in basaltic andesites and dacites, to an essential structural constituent (ESC) in high-silica rhyolites, marks the onset of zircon crystallization. A previously published zircon saturation model provides liquidus temperatures that support the interpretation that three high-silica rhyolites represent zircon-saturated melts while one has excess zircon. These results contradict the previous study in which zircon fractionation had been argued against. The potential effect of zircon fractionation and accumulation on REE abundances in the samples is evaluated and contributes to an improved petrogenetic interpretation. A cogenetic suite of granitoid samples from the Saganaga tonalite, Minnesota-Ontario, North America, is characterized by apparently strongly compatible behavior of the REE. Apatite, zircon and sphene are the observed high-REE- K d accessory minerals. Forward and inverse modelling approaches provide no evidence that accessory-mineral fractionation contributed to the behavior of the REE during differentiation of the samples. The data instead suggest that the REE characteristics of the samples were already established by other petrogenetic processes, prior to the samples becoming saturated with the observed accessory minerals.

Variability of trace elements in basaltic suites

This paper discusses the distribution of trace element concentrations in cogenetic suites of lavas, based on the study of trace element histograms. For each histogram, the variability is calculated ( V = s/m, where m is the mean concentration and s its standard deviation). The study of two basaltic series, each specific of a single igneous process (fractional crystallization and partial melting), shows that trace element variabilities are directly related to bulk partition coefficients. Considering different trace element behaviour models, quantitative relationships are established between the variability and the descriptive parameters for the processes (bulk partition coefficients, mean degree of fractionation and related standard deviation). An important feature is that the variability is independent of the initial concentrations in the source. This method is applied for comparison to several basalt suites on which case studies of magmatic processes have been conducted. It is also applied to study the behaviour of trace elements in different geodynamical contexts (hotspot, mid-ocean ridge and arc volcanism). Finally, a set of numerical values of bulk partition coefficients is proposed that accounts for N-MORB genesis.

Dating mineralization using several isotopic methods: an example from the South Mountain Batholith, Nova Scotia, Canada

Granitic plutons in the Meguma Terrane may be broadly divided into two suites: (1) an areally predominant, cogenetic suite of peraluminous granitoid rocks (tonalite-leucogranite); and (2) an areally subordinate, paraintrusive suite specialized in Sn-W-Mo-Cu-F-U-P-Zn-As which appears to have been produced by fluid interaction with either residual magma or crystallized rock. Granite-related mineralization is generally considered to have developed during terminal stages of magmatic crystallization that lead to metasomatism and hydrothermal alteration. Previous geochronological data have suggested that the main, cogenetic suite was emplaced at ∼ 350–375 Ma whereas some of the paraintrusive suite is markedly younger (∼ 250–350 Ma). The apparently large time interval between the intrusion of the cogenetic suite and some of the mineralization suggests that they are not coeval, and has led to the conclusions that the younger mineralization may be related to later magmatism and/or tectonothermal events. Rocks associated with two mineral occurrences in the South Mountain Batholith that had previously yielded anomalously young isotopic ages (Millet Brook and Westfield) have been analyzed for U, Pb and Ar isotopes in an attempt to resolve this dilemma. New geochronological data from the Millet Brook area indicate that intrusion of granodiorite at 377 +5 −3 Ma (nearly concordant monazite age) was closely followed by intrusion of most other phases all of which had cooled through ∼ 375°C (closure temperature for Ar in muscovite) by ∼ (377−373) ± 6 Ma ( 40 Ar 39 Ar plateau ages). New data from Westfield indicate that a paraintrusive porphyry was intruded at ∼ 385 ± 2 Ma (upper intercept of a chord through four, nearly concordant zircon fractions) and that greisen muscovite cooled through the closure temperature by 368.2 ± 6 Ma ( 40 Ar 39 Ar plateau age). These data indicate that a previously reported RbSr whole-rock isochron age of ∼ 270 Ma at Westfield reflect isotopic resetting that occurred below the blocking temperature for 40 Ar 39 Ar in muscovite (∼ 375°C). Clearly, further RbSr analysis is required to document this process. Other ∼ 250–300 Ma ages in the Meguma Terrane are recorded along the Tobeatic Fault zone. Away from the fault, ages increase upto ∼ 370 Ma, suggesting that the fault may have acted as an important pathway for fluids and heat that facilitated isotopic resetting. RbSr whole-rock and mineral systems may be affected after formation of an alteration assemblage associated with mineralization by the migration of Rb and its concentration in secondary minerals.

Collision-related granitoid magmatism and crustal structure of the Hunza Karakoram, North Pakistan

Abstract The Cretaceous to Miocene Karakoram Axial Batholith runs E-W across the middle Hunza Valley in north Pakistan and separates a dominantly sedimentary terrain to the north from a regional metamorphic terrain to the south. A strong phase of deformation and metamorphism associated with the c. 65–50 Ma collision of India with Asia affected the area. The southern part of the batholith and the metamorphic rocks to the south are affected by strong south-vergent thrusting which has brought deeper level, hotter rocks over higher level, cooler rocks so that metamorphic grade increases with structural height. In contrast, the northern part of the batholith is relatively undeformed and shows an intrusive contact with an andalusite-bearing thermal aureole. Two groups of post-collisional magmatic rocks can be recognised in the area. The first group, the Hunza dykes, occurs as a massive dyke complex intruding the early phase of the batholith as well as the sillimanite-bearing gneisses to the south. These dykes consist of a co-genetic suite of granodiorites, monzogranites and leucogranites which appear to be related by feldspar-biotite fractionation. They are volatile-depleted, non-minimum melts with unevolved crustal isotopic ratios and enriched Sr and Ba signatures similar to the Miocene Baltoro pluton further east. They are considered to represent lower-crustal magmas derived by dehydration melting of metasediment, probably promoted by intrusion of mantle-derived magmas. The second group is represented by the Sumayar pluton which intrudes the staurolite-grade metamorphic rocks and is a homogeneous, water-saturated minimum-melt leucogranite, with a major element composition similar to that of the Manaslu leucogranite of central Nepal. Abundant tourmaline and evolved Sr isotope ratios indicate a metapelitic source. Analogy with Manaslu suggests that this source was either fluxed by fluids to promote melting or produced melt under vapour-absent conditions via muscovite breakdown. In the case of the latter, low melt fractions require that an extensive area was tapped to generate a pluton the size of Sumayar. Consideration of the petrogeneses of the two groups of magmas allow constraints to be placed on the deeper crustal structure of the Hunza Valley.

Origin of the Transgressive granophyres from the Layered Series of the Skaergaard intrusion, East Greenland

The Transgressive granophyres are a swarm of silicic dikes and sills in the Layered Series of the Skaergaard intrusion that range in composition from ferrodiorites to granites. Although originally thought to be derived by differentiation of Skaergaard magma, accumulated evidence indicates that the origin of these granophyres is more complex and involves contributions from melted country rocks. In the present study it is shown that field relations are inconsistent with any genetic relationship between the Transgressive granophyres from the Layered Series and differentiated Skaergaard magma. Although new Sr isotope analyses confirm that these granophyres are at least partly derived by fusion of Archean country rock, field relations preclude derivation of this crustal component from partially melted gneissic xenoliths from the Skaergaard magma chamber or from melting along the contact between the gneisses and the Skaergaard pluton. Major- and trace-element compositions, together with Sr isotope ratios indicate that the Transgressive granophyres from the Layered Series represent a cogenetic suite of magmas related to each other primarily by a closed system process such as crystal fractionation, and that the parental magma that gave rise to these granophyres was formed beneath the Skaergaard intrusion by contamination of differentiated post-Skaergaard mafic magmas with partially fused Archean crust.

Petrological and geochemical investigations at the Cajon Pass Deep Drillhole

Chemical analyses of cores from the Cajon Pass Deep Drillhole confirm vertical lithologic diversity of the basement rocks. They are metaluminous and calcalkaline; weak major element correlations with wide‐ranging SiO2 suggest they are not a simple cogenetic suite. Quartz contents correlate with the thermal gradient profile, supporting conduction as the principal thermal transport mechanism. Abundances of heat producing elements are variable and somewhat lower than average crustal values. Cross‐calibration with other measures of Th, U and K are underway. Ages of surface granitoids and core 17 are late Cretaceous; other plutonic rocks appear older but late Mesozoic. Initial lead isotopic signatures of the plutonic rocks change dramatically with depth in the hole.

Crustal Influences in the Petrogenesis of the Naivasha Basalt--Comendite Complex: Combined Trace Element and Sr-Nd-Pb Isotope Constraints

The bimodal Naivasha complex (central Kenya) comprises 2 suites of transitional basalts and 7 chemostratigraphic groups of comendites. The early basalt series (EBS) predates the Group 1 comendites with the later series (LBS) erupted between Groups 5 and 6. Basalts from both suites are notable for their relatively radiogenic 207Pb/204Pb isotope ratios which are higher than in the majority of ocean island basalt (OIB, Zindler & Hart, 1986), and 87Sr/86Sr ratios more radiogenic than basalts from northern Kenya. Both basalt suites exhibit systematic trace element and isotopic variations which appear related to greater assimilation of Proterozoic amphibolite facies crust by the chemically more evolved rock types. Their mantle source regions show evidence of residual plagioclase and have a 'Dupal'-like OIB trace element and Pb-Sr-Nd isotope signature (Hart, 1984). A contribution from the sub-continental lithosphere is proposed in basalt genesis.The seven comendite groups have distinct trace element and isotope systematics. Hydration of comendite glass causes significant changes in Sr and Pb isotope ratios. In terms of their Sr-Nd isotope relationships the unaltered comendites could be derived from the basalts by an assimilation-fractional crystallization (AFC) process dominated by the fractional crystallization of feldspars. However, the Pb systematics clearly demonstrate that the basalts and comendites are not part of a cogenetic suite. Chemical variations within individual comendite groups are predominantly the result of fractional crystallization of the observed phenocryst assemblages (i.e. alkali feldspar dominated) and minor crustal interaction. The majority of the chemical and isotopic differences between Groups 1-7 cannot be explained by fractional crystallization and appear to represent crustal melts derived from close to the interface between Pan African basement and the overlying Miocene-Holocene volcanoclastic rocks, at approximately 6 km depth (KRISP working group, 1987).Halogens play a fundamental role in the petrogenesis of the comendites (Cl+F<1·7 per cent) permitting small degree melts of low viscosity to be extracted from the crust and causing the breakdown of minor phases e.g, zircon. These factors explain the extreme enrichment of certain incompatible trace elements (Zr<2500, Nb<700) in the comendites and coupled with the retention of zircon in the source of the halogen poor comendites (Group 1<0·6 per cent Cl+F) result in notable fractionation among the HFSE (Zr/Nb 1·5-5·5). Halogens may be concentrated in the source region from the surrounding crust by the presently active hydrothermal system. Each of the chemostratigraphic comendite groups is chemically distinct, implying that partial melting of the heterogeneous crust is on a limited scale and that no extensive magma chambers exist beneath Naivasha.

Late Caledonian dyke-swarms of northern Britain: Spatial and temporal intimacy between lamprophyric and granitic magmatism around the Ross of Mull pluton, Inner Hebrides

Numerous sheet intrusions in Iona and western Mull (Scotland) range from basic calc-alkaline lamprophyres (∼45% SiO2;mg number 65–75%; Thornton-Tuttle differentiation index [DI] 45%), through intermediate (microdioritic) compositions (malchites) to microgranitic porphyries (∼70% SiO2; DI 85%). Well-defined major element trends, remarkably consistent normalized multielement patterns, and strong linear correlations between homologous elements (e.g. Sc-V-Fe-Ti, Mg-Ni-Cr) suggest that these sheets form one cogenetic suite. Although Rb-Sr systems have been disturbed by Archaean country-rock contamination, isotopic signatures and field evidence suggest that sheet emplacement bracketed that of the 414 Ma Ross of Mull granitoid pluton: a few late sheets cut the pluton; exceptionally well-developed multiple and composite sheets provide clear evidence forsimultaneity of lamprophyric and granitic magmas; and an early group of sheets appears to be metamorphosed by the pluton. The composite sheets comprise lamprophyric borders around thicker microgranitic quartz-feldspar-porphyries, each rock-type carrying xenocrysts or xenoliths of the other. Mutual contacts mark wide discontinuities in major element composition, and sites of local hybridisation, yet close trace element similarities strongly imply the lamprophyres and porphyries are cogenetic. Least-squares modelling further shows that fractionation of actual phenocrysts in one lamprophyric margin could reproduce the porphyry with which it is in intimate contact. The most primitive (mg>70%) lamprophyres combine very high K, Rb, Th and Ba with MORB-like contents of Cr, Ni, Ti, Y, and Sc, implying a substantial mantle-derived component, and in some cases near-primary status, but not precluding some crustal participation. Biotite- and hornblende-lamprophyres are locally heteromorphic. Discontinuities in some chemical trends, field evidence, and initial87Sr/86Sr ratios (∼0.706), imply more complex interactions of fractionation and hybridisation with lamprophyric parent magmas, plus crustal contamination, in the genesis of the intermediate and microgranitic rocks. The association between lamprophyric and granitic magmatism in Iona/western Mull is not therefore fortuitous but genetic. The lamprophyres have an undoubted role in the genesis not merely of the microgranitic minor intrusions but probably of the granite pluton itself.

Assimilation-fractional crystallization of Polvadera Group rocks in the northwestern Jemez Volcanic Field, New Mexico

Pliocene Polvadera Group rocks in the northwestern Rio Grande rift-marginal portion of the Jemez Volcanic Field record the rapid transition from weakly alkaline Lobato Basalt magmatism (48–52% SiO2; 7.9 Ma) through calc-alkaline Lobato andesite and dacite (53–64% SiO2) and Tschicoma dacite-rhyodacite magmatism (63–69% SiO2; 7.4 Ma). Petrologically, Lobato andesite and dacite and Tschicoma dacite-rhyodacite represent a cogenetic suite of differentiates (the La Grulla Plateau or LGP suite) distinctive from the bulk of Polvadera Group rocks including Tschicoma andesite. Increasing (87Sr/86Sr)O ratios with differentiation within the LGP suite from 0.7051 (54% SiO2) to 0.7064 (68% SiO2), trace element variations, and disequilibrium mineral assemblages suggest open system differentiation involving 87Sr-enriched upper crust. A likely parental magma is the voluminous Lobato Basalt ((87Sr/86)O= 0.7043–0.7050) which was erupted predominantly earlier and to the east toward the rift axis. The best model for petrogenesis involves bulk assimilation of locally wide-spread Proterozoic (1.4–1.6 Ga) upper crustal granite by fractionally crystallizing Lobato Basalt. Assimilation-fractional crystallization (AFC) modeling of Sr-isotope and trace element variation (DePaolo 1981) indicates that ∼40% crystallization of Lobato Basalt accompanied by ∼10% addition of granite reproduces the observed geochemical and Sr-isotopic diversity. Neither magma mixing, nor mafic recharge have complicated the magmatic evolution of the LGP suite. Crustal thickness and/or retarded tectonism could have facilitated conditions necessary for evolution by AFC to occur within the upper crust.

Solution behavior of +4 cations in high silica melts: petrologic and geochemical implications

The saturation surfaces of rutile (TiO2), zircon (ZrSiO4), and hafnon (HfSiO4) were determined in anhydrous, peraluminous, high silica liquids of the system SiO2-Al2O3-Na2O-K2O as functions of silica concentration at 1,400° C in air. The saturation concentrations of TiO2, ZrO2, and HfO2 in rutile, zircon, and hafnon-saturated liquids, respectively, decrease smoothly and gradually as functions of increasing silica concentration. Thermodynamic analyses of the data demonstrate that the activity coefficients of TiO2, ZrO2, and HfO2 increase smoothly and gradually as silica concentration is increased from 67 wt-% to 80 wt-%, and that changes in SiO2 of 1 or 2 wt-% result in small changes in the saturation concentrations and activity coefficients of +4 cations. Because the solution behavior of +4 cations in highly siliceous liquids (>75 wt-% SiO2) is predictably different than in less siliceous liquids (70 to 75 wt-% SiO2), classification of highly-siliceous igneous rocks on the basis of silica concentration alone should not be interpreted to mean that their solution chemistry differs significantly from that of less siliceous rocks. The results of this study are compared with other studies of +4 cation solution behavior. From this it is concluded that variations in liquid compositions observed in cogenetic suites of high silica rhyolites cannot cause the observed changes in +4 cation concentrations. Thus, even if a large change in solution behavior of +4 cations is inferred from the large variations in their concentrations, it cannot be due to changes in bulk composition of the parental liquid. In addition, the similarity in the solution behavior of Zr and Hf seen in this study suggests that their solution mechanisms are similar. It is thus unlikely that liquid-state processes can fractionate one with respect to another, and variations in Zr/Hf ratios in suites of extrusive rocks are likely due to crystal-liquid equilibria, e.g., zircon fractionation.

A reconnaissance Rb-Sr, Sm-Nd, U-Pb, and K-Ar study of some host rocks and ore minerals in the West Shasta Cu-Zn district, California

The Copley Greenstone, Balaklala Rhyolite, and Mule Mountain stock in the West Shasta Cu-Zn district, California, have Rb-Sr, Sm-Nd, U-Pb, and K-Ar systematics that indicate they are a cogenetic suite of ensimatic island-arc rocks about 400 m.y. Pervasive alteration and mineralization of these rocks, for the most part, was syngenetic and the major component of the mineralizing fluid was Devonian seawater. K-Ar ages of quartz-sericite concentrates from ore horizons and Rb-Sr systematics of a few rock and ore specimens record a later thermal and mineralizing event in the district of about 260 m.y. Contamination of some rocks with pelagic sediments is indicated by the Sm-Nd data.

The timing of ore formation in Southeast Missouri; Rb-Sr glauconite dating at the Magmont Mine, Viburnum Trend

Seven Bonneterre and Davis Formation glauconite samples from the Magmont mine area, Viburnum Trend, southeast Missouri, yield a 359 + or - 22-m.y. Rb-Sr isochron with an initial 87 Sr/ 86 Sr ratio of 0.7234 + or - 0.0273 (2 sigma). Gangue calcite from the ore zone has an 87 Sr/ 86 Sr ratio of approximately 0.7112. The isochron age for the Magmont glauconite samples is about 30 percent younger than the age of their Cambrian host rocks.The Rb/Sr ratios for Magmont glauconite samples are typical of many early Paleozoic glauconites but distinctly lower than ratios for glauconites from unmineralized localities in basinal Bonneterre facies, some distance from the Vibrunum Trend.Because the Rb-Sr data from Magmont glauconite samples define an isochron, it appears that the glauconites behaved as a cogenetic suite which was isotopically homogenized 359 m.y. ago. It is very unlikely that episodic or incomplete loss or exchange of radiogenic strontium, or the addition of rubidium, would result in the formation of an isochron. Addition of common strontium (mixing) will lower and produce a greater spread in Rb/Sr ratios but will not significantly change the calculated Rb-Sr isochron age for the glauconites. Because the total thickness of stratigraphic cover in the southeast Missouri region has never exceeded a kilometer, it is unlikely that the anomalously young glauconite isochron age resulted from heating associated with deep burial. Mississippi Valley-type ore fluids offer a hot, chemically reactive medium which could promote isotopic homogenization in glauconites. Therefore, the 359-m.y. age probably reflects a real geologic event, most likely the time of ore formation in southeast Missouri.A review of other attempts at dating early Paleozoic glauconites from the midcontinental United States indicates that all Rb-Sr and K-Ar glauconite ages are 10 to greater than 30 percent younger than their known stratigraphic ages. These glauconites are from Paleozoic formations on the stable craton, and again, resetting due to burial metamorphism is unlikely. Because there was no immediately obvious explanation for the discordant young ages, early Paleozoic glauconites were simply considered to be unsuitable geochronometers. It is suggested that the anomalously young radiometric ages for early Paleozoic glauconites record disturbances in the Rb-Sr and K-Ar isotopic systems, the result of Mississippi Valley-type fluid migration in the midcontinent region.

Case studies on the origin of basalt

This paper presents a method for the systematic trace element modelling of a cogenetic suite of lavas. It is based on the geochemical inversion technique of Allegre and coworkers and utilizes the variations in the trace element concentrations of the lavas to calculate initial concentrations and source mineralogy. We reduce this inversion to a simple, step-by-step procedure: (1) correcting for fractional crystallization; (2) testing the inferred primary melt compositions for consistency with a model of equilibrium partial melts (with constant partition coefficients) formed from identical sources; (3) estimating the proportions of mineral phases entering the melt; (4) computing concentrations and bulk partition coefficients in the initial source relative to the concentration of a common reference element; (5) estimating relative mineral abundances in the source. Except for the fractionation correction, the calculations are done element by element using a direct analytic solution. For the purpose of comparison we apply this method to the same set of data used by Minster and Allegre (1978), a suite of lavas from Grenada (lesser Antilles) originally analyzed by Shimizu and Arculus (1975). The results of both methods agree well for the source abundances of the light REE, whereas the heavy REE abundances are shown to be poorly constrained by the data. Both methods require residual clinopyroxene and garnet in the source, but the ratio of these minerals is not well constrained. We are unable to reproduce the shape of D0 pattern (=bulk partition coefficients of the initial source) given by Minster and Allegre. The reason for this cannot be evaluated without repeating their calculations in detail. The set of data from Grenada is useful for comparison of the methods only, because it is now known from isotopic data that the samples are not truly cogenetic. Possibly better suited sets of samples for petrogenetic modelling are presented in parts II and III of this series.

U-Pb isotope ages of granitic plutons near Cananea, Sonora

The Cananea crops out in the Cananea mining district, which includes some of the most important known copper deposits in Mexico. The area is in northern Sonora, approximately 100 km southeast of the twin cities of Nogales, Arizona-Sonera, on the international border.By means of isotopic analyses of U-Pb systems in cogenetic suites of zircon from two different phases of the pluton, its interpreted age has been firmly established at 1,440 + or - 15 m.y. Cananea granite, actually a quartz monzonite, is lithologically characteristic of a major Precambrian suite of anorogenic, consanguineous plutons of porphyritic granodiorite to granite. Constituents of this previously recognized series, which crop out throughout the southwestern U. S. and into northern Mexico, consistently yield zircon radiometric ages within the interval 1,425-1,475 m.y.Major younger intrusions in and nearby the Cananea district include Cuitaca granodiorite and Chivato monzodiorite. These bodies yield apparent zircon ages of 64 + or - 3 and 69 + or - 1 m.y., respectively. Although more precise geochronologic relationships between young plutons, the adjacent layered sequence, and cross-cutting, brecciated, quartz porphyry plugs are needed, correlation exists between major plutonism and mineralization.The Precambrian age of the Cananea granite precludes its chronological association with the younger mineralized plugs and should be taken into account in future planning of exploration.