Initial Sr Ratios Research Articles

The role of fractional crystallisation, crustal melting and magma mixing in the petrogenesis of rhyolites and mafic inclusion-bearing dacites from the Monte Arci volcanic complex (Sardinia, Italy)

Rhyolitic lavas and mafic inclusion-bearing dacites (MIBD) form the dominant products of the Monte Arci volcanic complex, one of the most active sites of volcanic activity in Sardinia during the Pliocene. The massif is composed of four distinct eruptive episodes (Phase 1: rhyolites; Phase 2: dacites and andesites; Phase 3: quartz-normative trachytes; Phase 4: mafic lavas ranging from subalkaline to mildly alkaline). Monte Arci magmatism has been characterized by open-system behaviour, with both mantle and crustal contributions and magma mixing. Although the mafic products are restricted to the latest stage of activity, mafic inclusions are quite common in many rhyolites and dacites. The mineral assemblages of the inclusions are dominated by plagioclase + orthopyroxene ± augite, with minor olivine, FeTi oxides and variable amounts of residual trapped liquid giving rise to a fine-grained groundmass. They represent blobs of magma entrained in a partly molten state and provide evidence of a basaltic contribution to the petrogenesis of their enclosing lavas, both as parental magmas or as a source of heat for partial melting of crustal rocks. Rhyolites are metaluminous to slightly peraluminous rocks with a wide range of SiO 2 contents (67–75%). They fall into two groups: (1) Mafic inclusion-bearing rhyolites (MIBR). These have initial 87 Sr 86 Sr ratios between 0.70526 and 0.70897 and commonly show relatively high values of Ti, Mg, Fe, Ca, Sr and compatible trace elements (Ni, Cr, V); cordierite-bearing, highly restitic, crustal xenoliths are rare. (2) Inclusion-free rhyolites (IFR). They include both porphyritic lavas (IFR1) and obsidians (IFR2, IFR3), which share low values of Ti, Fe, Mg, Ca, Sr and are strongly depleted in compatible trace elements. IFR1–2 have a marked LREE/HREE fractionation and a narrow range of 87 Sr 86 Sr ratios (0.70885–0.70972), whereas IFR3 have larger Sr isotopic ratios (0.71529–0.71553), low Th, Zr, Hf, LREE contents and low LREE/HREE. Nd isotopic composition is quite uniform for all rhyolite types, with relatively low 143 Nd 144 Nd ratios (0.51216–0.51228, ϵ Nd ranging between −9.3 and −6.9). MIBR appear to be the product of fractionation of subalkaline (tholeiitic) magmas + assimilation of a crustal component moderately enriched in 87Sr and/or with low Sr content. IFR1–2 may be derived from partial melting of moderately 87Sr-rich crustal lithologies with retention of garnet and plagioclase (+K-feldspar) in the restite, although AFC processes (followed by effective separation from the basalts) cannot be excluded. Obsidians IFR3 need a crustal source with higher 87 Sr 86 Sr ; their peculiar geochemical features most probably reflect the complex behaviour during partial melting of accessory minerals carrying REE, Th, Zr, Hf. Among the eruptive products of Phase 2, mafic inclusion-bearing dacites (MIBD) provide evidence of basalt-rhyolite mixing: (a) coexistence of glasses with SiO 2 content ranging from 64 to 72 wt.%; (b) a phenocryst assemblage including phases of basaltic and rhyolitic provenance; and (c) abundant crystal-rich inclusions related to a basaltic intrusion. The extrusion of these lavas has been preceeded by an explosive eruption of rhyolitic tuffs ( 87 Sr 86 Sr initial ratio=0.71097 ). The most satisfactory petrogenetic model for MIBD includes: (1) differentiation of subalkaline (tholeiitic) magma to a silica-poor and Ti-Fe-P rich liquid; (2) partial melting of wall-rocks yielding a rhyolitic magma segregated at the top of the basaltic reservoir; and (3) explosive eruption of the uppermost portion of the magma chamber followed by mixing of rhyolite with the underlying part of the system to form a hybrid dacite.

An isotopic study of the Ni-Cu-PGE-rich Wellgreen intrusion of the Wrangellia Terrane: Evidence for hydrothermal mobilization of rhenium and osmium

Re-Os, Sm-Nd, Rb-Sr, and oxygen isotope systematics of the Wellgreen intrusion in the Wrangellia terrane were investigated in an effort to deduce the origin of this mafic-ultramafic sill and its attendant Ni-Cu-PGE deposit. Radiogenic initial Os ratios (1.06–1.82) and Sr ratios (0.7044–0.7062), and heavy δ 18 O (7.3–7.9%.) suggest alteration of the intrusion by hydrothermal fluids that carried radiogenic 187Os and 87Sr from the surrounding country rocks. The great majority (>99%) of the Os, however, and by inference the other PGEs, derived from a mantle-derived magma that suffered little or no interaction with the crust prior to crystallization. Initial Nd isotope ratios are not as variable ( ϵ Nd( t) ranges from 2.02–4.49) and suggest that the rocks were derived from a light-rare-earth-element depleted mantle source. The Nd results, together with Os data from relatively undisturbed wehrlites, are compatible with either a plume ( Richards et al., 1991) or island arc ( Samson et al., 1990) model for Wrangellia. This study contributes to a growing body of evidence that documents the ready mobilization of Os in hydrothermal fluids. The potential effects of this mobility must be carefully evaluated prior to invoking crustal assimilation to explain variable and radiogenic Os initial ratios in layered intrusions.

2.0 Ga old pyroxenite-carbonatite complex of Hogenakal, Tamil Nadu, South India

The Hogenakal carbonatites form a series of discontinuous bodies within two pyroxenite dykes. Each body forms veins and lenses emplaced in pyroxenite. Based on field relationships, the sequence of formation is inferred to be pyroxenite first, syenite later, and carbonatites last. The carbonatitic rocks are classified into three types: mica-apatite-calcite (MAC) carbonatite, mica-pyroxene-apatite-calcite (MPAC) carbonatite, and carbonate mica (CM) pyroxenite. Two whole-rock mineral RbSr isochrons for MPAC carbonatites yield ages of 1.984±0.078 Ga and 1.994±0.076 Ga, respectively, which makes this carbonatite not only the oldest yet known in the Indian subcontinent but also one of the few oldest carbonatites known in the world. The carbonatites are poor in alkalies and rich in Sr and to a certain extent in Ba. REE abundances and LaYb ratios vary in the order pyroxenite < CM pyroxenite < MPAC carbonatite < MAC carbonatite; all rocks show LREE-enriched patterns with steep slopes. It is suggested that the pyroxenites represent intrusions of crystal mush formed by separation of pyroxenes from an ijolitic magma. Increase in the concentration of CO2 in such a magma led to the separation of a carbonatitic melt which subsequently intruded the pyroxenite. Mixing of this carbonatite melt with fragmented pyroxenite led to the formation of the MPAC carbonatite and the CM pyroxenite in which pyroxenite dominates over carbonatite. In the closing stages a second pulse of carbonatite was intruded which formed the MAC carbonatite. K-metasomatism is restricted essentially to phlogopitization of pyroxenes and K-feldspar. The association of albite/oligoclase-rich fenites with the carbonatite complex is indicative of NaK metasomatism. The near equality of the initial Sr ratios of the three samples analysed, together with their high Sr contents, argue against any significant crustal contamination. The initial ratio of 0.70169 must therefore represent the Sr composition of the mantle source of the ijolitic magma. This ratio corresponds to an ϵSr of −6.3 ± 0.6, which implies that the subcontinental mantle which gave rise to the Hogenakal pyroxenite/carbonatite was depleted in incompatible elements even prior to 2 Ga ago, presumably due to an earlier event of crustal formation.

Generation and distortion of Rb/Sr whole-rock isochrons - effects of metamorphism and alteration

The Rb/Sr method is used for dating both rocks and minerals. Aliquots of a homogenized whole rock are called whole-rock samples. Two requirements have to be met for meaningful whole-rock isochrons: identical initial Sr ratios for all samples at the time of formation, and closed system conditions for the Rb/Sr ratio and Sr isotopic composition. Whole-rock isochrons especially those of metamorphic rocks may be disturbed. This study summarizes current knowledge on relevant problems, Magmatic processes generally produce initial isotopic homogeneity. However, heterogeneous isotopic composition of Sr is usually found in rocks derived from crustal-anatectic melts and hybrid magmas. Isotopic homogenization may also be the result of metamorphic overprinting. Greenschist-facies metamorphism, promoted by microtectonism in the presence of an abundant aqueous phase, may cause isotopic adjustments to take place over distances of hundreds of meters. On the other hand, under conditions of amphibolite facies metamorphism isotopic homogenization may be restricted to only a few millimeters or centimeters. Alteration of rocks may occur either by shallow (cold) or by intracrustal (hot) hydrous interaction. Sr-loss from rock specimens is observed in many places. Losses of mainly non-radiogenic Sr isotopes may result in the original age information being preserved. Losses of radiogenic 87 Sr may locally be proportional to the Rb/Sr ratios, thus preserving linear arrays of the analytical data in an isochron diagram. Such apparent isochrons are lines with a decreased slope. However, even severely altered rock samples will yield the real age of an intrusion or a phase of metamorphism if the chemical attack took place immediately after their formation. For precise dating the boundary conditions of the Rb/Sr whole-rock dating method rarely appear to be fulfilled. Nevertheless, it seems noteworthy that all reported Rb/Sr ages from altered rocks are lower than ages determined on fresh samples from the same rock. Therefore altered rock samples may yield minimum Rb/Sr ages

RbSr ages of Proterozoic kimberlites of India: evidence for contemporaneous emplacement

RbSr analyses of leached phlogopite macrocrysts from kimberlite pipes 1, 2, 5 and 7 in south India give concordant ages of 1091±20, 1092±15, 1093±20 and 1091±10 Ma, respectively, indicating contemporaneous emplacement of these pipes. The present results are in contradiction with earlier measurements. Groundmass mineral assemblages with low Rb Sr ratios give a tight set of initial Sr ratios at 0.7029±2, 0.7019±2, 0.7029±2 and 0.7030±4 suggesting derivation of the primary kimberlite magma from a relatively uniform and depleted mantle source. A similar age and initial Sr isotopic ratio of one sample of the Majhgawan lamproite from Central India imply that the Proterozoic kimberlite/lamproite activities in India, though widely separated in space, were nearly contemporaneous.

The Mozambique Belt in northern Mozambique: Neoproterozoic (1100–850 Ma) crustal growth and tectogenesis, and superimposed Pan-African (800–550 Ma) tectonism

Abstract The Precambrian basement of northern Mozambique is composed mainly of high-grade gneiss, granulite, migmatite and orogenic plutonic rocks emplaced and deformed during the Mozambican orogeny between 1100 and 850 Ma. Granulite nappes (Lurio Supergroup) were thrust over allochthonous supracrustal units (Chiure Supergroup), and both sequences were then thrust over an autochthonous migmatitic foreland (Nampula Supergroup). The granulite rocks are mostly of plutonic origin, and a model is proposed of an initial calc-alkalic enderbitic mass extracted from the mantle at ∼1050 Ma and emplaced in the lower crust; this enderbitic parent is considered as the source for successive alkali-rich melts which were emplaced synkinematically between 1050 and 850 Ma. The supracrustal units comprise marine metasedimentary rocks, and calc-alkalic and tholeiitic rocks associated with ultramafic-mafic sequences and synkinematic granite. The autochthonous foreland consists mainly of plutonic migmatite, showing low Sr initial ratios and originating from mantle-derived tonalitic rocks emplaced between 1100 and 950 Ma, associated with crustal-melt granite showing higher Sr initial ratios and dated as late as 850 Ma. Major synmetamorphic shearing and blastomylonitization, and regional thrusting to the east and southeast, are the main manifestations of the Mozambican orogeny. Early structures are preserved in the granulite complex and the migmatite foreland. Low-grade metasedimentary rocks (phyllite, sandstone, conglomerate and marble), with possible glacial affinities, uncomformably overlie the Mozambican basement and are correlated with the Katangan Supergroup. These rocks, together with the basement, were caught up at ∼538 Ma in Pan-African thrusts, transcurrent shear zones and folds in an intracontinental environment. These new data allow a reassessment of the entire Mozambique Belt, until now regarded as a Pan-African mobile belt involving variably remobilized cratons. The authors believe the belt to be a major fan-shaped collision zone between Kalahari craton and an Indo-Antarctic craton.

The Kola Alkaline Province of the CIS and Finland: Precise RbSr ages define 380–360 Ma age range for all magmatism

Twenty-one new RbSr mineral and biotite-whole rock isochrons are presented for intrusive rock complexes of the Palaeozoic Kola Alkaline Province in the CIS and Finland. Samples from the ultrabasic-alkaline and nepheline-syenite complexes Khibina, Lovozero, Afrikanda, Turiy Mys, Ozernaya Varaka, Sokli and Iivaara all yield ages in the very narrow range from 361.7 to 377.3 Ma, i.e. late Middle Devonian to Upper Devonian. These data represent crystallization rather than intrusion ages, but are nevertheless argued as marking very closely the time of intrusion. There is no indication of bimodal age distribution. The emplacement of the alkaline magmas is related temporally to the subsidence of the Kontozero Graben of the Central Kola Peninsula. Sr initial ratios of the agpaitic nepheline-syenites of the giant Khibina and Lovozero intrusions scatter between 0.703 and 0.704. Because of high Rb Sr ratios of the rocks, these isotope ratios exclude long-term pre-enrichment of alkalis in the sources of the magmas prior to segregation and intrusion, and thus episodic enrichment during earlier rifting events in the northern Baltic Shield. The magma formation may be related to wrench movements in the surroundings of the major Trollfjord-Komagelv Fault, which forms part of the Late Caledonian Arctic—North Atlantic Megashear System.

A survey of Rb‐Sr systematics of eucrites

Abstract— This paper surveys the experimental material on the Rb‐Sr systematics of eucrites accumulated in the literature for the last two decades. Among the meteorites studied, those representatives have been chosen for which the absence of disturbances of isotope systems seems reliably fixed (according to evidence from Rb‐Sr, Sm‐Nd, and U‐Pb isotope data). These data have been subjected to joint statistical treatment. For ordinary eucrites the age has been estimated as T = 4.548 ± 0.058 Ga, and initial Sr isotope ratio, Iord = 0.698925 ± 14. For cumulate eucrites, the initial Sr isotope ratio has been estimated as Icum = 0.698872 ± 14. The substantial difference in initial Sr isotope composition suggests that those two eucrite groups originate from distinct parent bodies.Joint treatment of the available Rb‐Sr data for angrites has been carried out for comparison, and the angrites initial Sr ratio has been estimated. On the basis of the estimates obtained formation intervals for the corresponding parent bodies have been calculated.

Cretaceous metamorphism and plutonism in the Santa Cruz Mountains, Salinian block, California, and correlation with the southernmost Sierra Nevada

New data on the ages of plutonism and metamorphism in the northern Santa Cruz Mountains, part of the enigmatic Salinian block of California, clarify the area9s geologic history. Uranium/lead dates on zircon, monazite, sphene, and apatite and initial Pb and Sr isotopic ratios provide a fingerprint for comparison to the rest of the Salinian block and to other terranes. The U/Pb zircon dates of plutonic rocks in the Santa Cruz Mountains fall into three groups. Emplacement ages of the two younger, undeformed groups are 91 Ma and between 99 and 103 Ma. Individual zircon analyses range from concordant to significantly discordant but have simple crystallization histories, coupled with either inheritance of Precambrian zircon or Pb loss. The older group of plutons includes recrystallized granite to well-foliated orthogneisses that share the regional fabric. Analyses from the older group are slightly discordant, and multistage histories that include inheritance, Pb loss, and homogenization of Pb complicate interpretation of their age. Emplacement ages for the older group are Early Cretaceous—greater than about 103 Ma and probably less than 130 Ma. Foliated and unfoliated plutons from the older group appear to bracket the age of Salinian deformation and metamorphism between about 103 and 130 Ma. Lower-intercept ages for detrital zircon from schist and quartzite further support a Cretaceous metamorphic age. The detrital zircon population contains a component at least 1.6 Ga, probably close to 1.7 Ga. Uranium/lead cooling ages measured on sphene and apatite are 96 to 93 and 93 to 88 Ma, respectively. This cooling history appears to be a response to intrusion and rapid cooling of the youngest plutons. Common Pb and Sr initial isotopic ratios are radiogenic compared with ratios of oceanic rocks and the western parts of the Cordilleran batholiths, but they are the least radiogenic in the Salinian block and fit the local geographic gradient of Pb and Sr initial ratios. Linear variation of Pb-isotope ratios between radiogenic wall rock and the least radiogenic rocks and correlation of inherited zircon with elevated Pb-isotope ratios suggest that plutons assimilated wall rock. Isotopic data and geologic histories of basement rocks in the northern Santa Cruz Mountains fit the geographic patterns documented in the rest of the Salinian block. More intriguingly, the Santa Cruz Mountains plutons are strikingly similar to parts of the southern Sierra Nevada batholith. Ages of plutonism and metamorphism, U/Pb zircon discordance systematics, petrography, and initial Sr isotopes match closely between the Santa Cruz Mountains crystalline rocks and units in the southernmost Sierra Nevada batholith. These data reinforce the long-standing view that the Salinian block and Sierra Nevada batholiths were adjacent during their formation. Prior to right-slip of about 310 km on the San Andreas fault, the probable westward continuation of the low-angle Rand fault system into the southern Sierra Nevada and the Salinian block appears to have offset these two crystalline suites about 125 km.

Magmatic evolution of the Cerro Toro granite, a complex Ordovician pluton of northwestern Argentina

The Cerro Toro granitoids range in composition from granite to gabbro and enclose abundant igneous and metamorphic enclaves that are assimilated in varying degree. The host rocks were intensely heated by intrusion of the Cerro Toro pluton, with local formation of anatexites. Geochronologic determinations point to an initial Sr ratio of 0.70967 ± 0.00043 and an age of 456 ± 14 Ma (Ordovician). The geochemical and field relationships between the acidic and basic members point to strong interactions between different unconsolidated magmas, with an extreme local fluidity that resulted in a gneiss-like aspect—although without cataclasis. Geochemical evidence indicates that the Cerro Toro granite was emplaced in a magmatic arc, at relatively deep crustal levels, under conditions close to anatexia.

The Arburese igneous complex (SW Sardinia, Italy) — an example of dominant igneous fractionation leading to peraluminous cordierite-bearing leucogranites as residual melts

The Arburese massif (SW Sardinia, Italy) is a zoned shallow pluton emplaced in the frontal part of a nappe pile at the end of the Hercynian orogeny. Its main lithologies are (a) pyroxene-bearing biotite granodiorites, and (b) cordierite-bearing peraluminous leucogranites. They form a sequence of sharply bounded multiple intrusions. The leucogranites (LG) form the core of the intrusive complex and are younger than the granodiorites. Three types of granodiorites (GD 1, GD 2, GD 3) have been distinguished on the basis of their field relations and their pyroxene content. Dark-coloured enclaves are common in the granodiorites. Their compositions range from biotite-bearing norite to pyroxene-biotite quartzdiorite/quartz-monzodiorite. A small body of olivine-bearing quartz monzogabbro-norite (MG) forms an independent intrusion in the border zone of the igneous massif. It is crosscut by the granodiorite. The whole-rock sequence from MG to GD 3 is characterized by the same mafic assemblage Opx (±Cpx) + Bt + Ilm. It formed by dominant crystal/liquid fractionation processes from a mantle-derived olivine-bearing magma. From GD 3 to three types of leucogranites (LG 1, LG 2 and LG 3), the rocks are progressively enriched in Al reaching a peraluminous composition. In the leucogranites, cordierite makes its appearance among the primary mineral phases, though confined to the later products of crystallization. Remarkably, cordierite was also observed among the latest crystallization products in some granodiorites and even in some dark monzodioritic enclaves. Independent whole-rock age determinations were made on the MG to GD 3 series and on the leucogranites. These gave very similar values of 309 ± 19 and 304 ± 21 Ma, respectively. Some textural and mineralogical features common to both GD 3 granodiorite and the leucogranites (e.g., both contain Ti-biotites characterized by a common trend of Al- and Fe-enrichment) suggest a genetic linkage between these two rock-types. The extreme iron-rich compositions displayed by the mafic minerals (biotite, cordierite) of the leucogranites suggest that they crystallized from highly evolved melts. Petrochemical, trace element and REE distribution patterns strongly support a common origin for the whole igneous sequence, including the leucogranites. Strontium isotope compositions do not conflict with the above conclusion. The oxygen isotopic composition changes progressively with time of emplacement: δ 18O increases from + 7.6 in the granodiorites up to + 11.9 in the leucogranites. This probably reflects some degree of magma contamination due to assimilation of crustal material. The Sr and O isotope systematics reveal that the MGGD series is characterized by very small variations of the Sr ratio and by pronounced changes in δ 18O. On account of the rather high Sr initial ratios, the observed trend is interpreted as due to AFC (assimilation-fractional crystallization) processes of a source-contaminated parental magma. Modelling the evolution path from MG to LG in terms of AFC processes shows the following: (a) the assumed fractionation steps give realistic mineral assemblages compatible with the observed lithologies; (b) they are able to produce high proportions of both leucogranitic and granodioritic rocks (as observed in the field); and (c) they can produce a progressive increase of alumina up to the peraluminous compositions observed in the more evolved members. Good results were also obtained by modelling the behaviour of trace elements such as Rb, Sr and Ba, whose distribution is mainly governed by the major mineral phases. The geochemical fingerprint of the whole igneous sequence is typical of a K-rich calc-alkaline series. Only some rare “grey enclaves” in the GD 3 granodiorite and in the leucogranites display a typical shoshonitic character. Their presence probably reflects the open-system condition of the deep-seated magma reservoir.

Isotopic study (Sr, Nd, O and C) of lamprophyres and associated dykes from Tamazert (Morroco): crustal contamination processes and source characteristics

The late-magmatic episode in the alkaline Tamazert complex (Morocco) corresponds to emplacement of a network of dykes made of various lamprophyres, phonolites and syenites. The isotopic compositions of Sr, Nd, O and C were measured in order to constrain the source characteristics of this magmatism. Very intense crustal contamination is recorded in the isotopic ratios of those lamprophyres that are intruded within limestones country rocks. Isotopic compositions as high as 0.70677 for Sr, +17.45 for O and +0.14 for C are attributed to isotope exchange with limestones during emplacement and cooling of the dykes. Samples that are free of crustal contamination display homogeneous initial Sr ratios (0.7034), variable initial Nd ratios [ ε Nd(T) = +1.5to+6 ], and a primary δ 18O value estimated around +6.5. These data are interpreted in terms of metasomatic effects in a depleted mantle source.

Pan-African plutonism of the Damagaram inlier, Niger Republic

This paper presents the first petrological, geochemical and geochronological data on the basement granitoids of Damagaram (southernNiger Republic) which cut a supracrustal sequence subjected to Barrovian metamorphism in upper greenschist to amphibolite facies. The petrographic study points to the great variety of granitoids although they all belong to the high-K calc-alkaline series and are late-kinematic. Six plutonic units and their enclaves were studied in some detail, five of which yielded Rb-Sr isochrons with good alignment. All the ages obtained are Pan-African with a mean of 575±12 Ma . A striking feature is the wide range in Sr initial ratios (0.705 to 0.720) indicating complex and strong interactions with crust, which is reflected in the geochemistry of the enclaves and the chemical evolution of the micas. This is in contrast with the Iforas (Mali) next to the suture with the WestAfrican craton where Sr initial ratios of the calc-alkaline rocks are less than 0.705. The calc-alkaline granitoids of the Damagaram cannot be related to the far distant suture bordering the West African craton nor the older SE Aïr collision (680 − 670 Ma) nevertheless probably represented in the Damagaram by the thrust sheets of eastern Damagaram and further east by the outcrops of the Gazamni Group similar to the Proche-Ténéré-Molassic Group which extends to the east of Hoggar and Aïr. The solution we now envisage to explain the presence of 610−570 Ma old subduction-collision granitoids in Damagaram and central aïr is the existence of a Pan-African suture close to long. 7°E, the key area to solve this problem being in Nigeria or in central Hoggar.

Caractérisation pétrologique et géochimique du magmatisme ubendien du secteur de Pepa-Lubumba, sur le plateau des Marungu (Nord-Est du Shaba, Zaire). Signification géodynamique dans l'évolution de la chaîne ubendienne

Volcanic and plutonic rocks of Pepa-Lubumba area in the Marungu plateau (Zaire) are mainly represented by intermediate and acid members which contain low abundances in TiO 2 (0.3 – 1.3 wt%). All these rocks exhibit a high-K calc-alkaline affinity and belong to a calc-alkaline continental-margin series. The large proportion of frequently zoned (normal and reverse zoning) plagioclase, the presence of hydrous phases (tschermakitic and Mg-hornblende, Mg-biotite), the early crystallization of FeTi oxides indicate crystallization under high water pressure conditions, consistent with a calc-alkaline affinity. High abundances in K, Rb, Sr and Ba; lack of correlation between Rb and Ba, low values of Rb/Zr (0.5 – 0.9) in granitoids, high contents of LREE and HFSE (Th/Ta : 6 – 14), are characteristic of “Andinotype” magmas. A whole rock RbSr isochron gives an emplacement age for granitoids of 1861 ± 28 Ma (MSWD : 0.82), Sr initial ratio of 0.7026 and epsilon Sr i of + 4.7. PbPb isotopes systematics for the same rocks give the following ratio ranges : 206Pb/ 204Pb : 17.26 – 20.10; 207Pb/ 204Pb 15.52 – 15.85, 208Pb/ 204Pb : 37.2 – 41.80. The low Sr initial ratio indicates an upper mantle source; while the PBPb isotope ratios suggest that the mantellic liquids have been contaminated by crustal material en route to the upper levels. In the regional geologic context, this magmatism forms a part of an important plutovolcanic Ubendian (Early Proterozoic) complex which crops out in Western Tanzania, North-Eastern Zambia and Marungu plateau (Zaire) and contains two magmatic cycles which define a spatial and temporal zonation suggesting a geodynamic evolution model for the Ubendian belt comprising subduction-obduction-collision processes. This latter episode has been relayed by a relaxation phase marked by emplacement of a third tholeiitic basic plutonic cycle.

New Sr, Nd, and Pb isotopic data from plutons in the northern Great Basin: Implications for crustal structure and granite petrogenesis in the hinterland of the Sevier thrust belt

The influence of tectonic setting and age on the variation of isotopic signatures of granitic plutons in the northern Great Basin has, in general, not been apparent from previous investigations. Although Elison et al. pointed out isotopic differences between Jurassic and younger plutons near the 0.706 Sr isopleth, and Farmer and DePaolo noted that there might be a difference between Cenozoic vs. Mesozoic plutonic rocks in the eastern part of the northern Great Basin, neither of these studies revealed the remarkable correlation between isotopic signature, age, and tectonic setting shown by our expanded Sr, Nd, and Pb isotopic data base. Jurassic-Early Cretaceous plutons in the northern Great Basin have a limited range of Sr and Nd isotopic values that cluster near bulk earth. All but one of these plutons have ϵ Nd values less negative than -7 despite their location both to the west and east of the ϵ Nd = -7 line. Construction of Sr 0.706 and ϵ Nd = -7 isotopic boundaries is virtually impossible for plutons of this age range. In contrast, Upper Cretaceous peraluminous granites east of the ϵ Nd = -7 line have very negative ϵ Nd values and high initial Sr ratios, and they appear to represent essentially pure crustal melts. The data favor a model that equates generation of these plutons via crustal thickening associated with the Sevier thrust belt. Cenozoic plutons appear to be mixtures of mantle and crustal reservoirs, and their isotopic systematics, along with those of the Late Cretaceous age plutonic suite, define a previously unrecognized, approximately east-west-trending crustal boundary between predominantly Archean crust to the north and predominantly Proterozoic crust to the south. The isotopic data from the Jurassic-Early Cretaceous plutonic suite do not reflect the presence of this boundary, suggesting that the isotopic systematics of this plutonic suite may not have been controlled by the same variations in crustal and/or mantle lithospheric structure at depth.

A petrologic comparison of Triassic plutonism in the San Gabriel and Mule Mountains, southern California

Triassic magmatism in the southwest U.S. Cordillera forms a semicontinuous magmatic arc extending from northwestern Nevada to southeastern California. Quartz monzodioritic and quartz monzonitic rocks and associated diorites and granites are widespread in southeastern California, and we suggest that these rocks represent exposure of a structurally deeper part of the Triassic arc, where it was emplaced into comparatively thick Proterozoic crust. Elemental and isotopic data suggest that Triassic quartz monzodiorites and quartz monzonites in the Mule and San Gabriel Mountains were derived from a relatively undepleted, nonradiogenic mafic lithospheric source, with virtually no upper crustal interaction. Very limited data for associated Triassic(?) diorites indicate a wide range in composition and a surprisingly radiogenic isotopic signature. Younger Triassic(?) granites record a strong geochemical signature of interaction with continental crust, including inherited zircon and high initial Sr ratios but comparatively less radiogenic Pb isotopic compositions. The major and trace element geochemistry of Late Triassic plutonic rocks in southeastern California is similar in many respects to alkalic components of the Triassic arc in the Mojave Desert. However, contemporaneous rocks farther north have a calc‐alkalic signature, perhaps reflecting the variation in age and composition of lithosphere across which the Triassic arc was constructed.

The Lundy granite: a geochemical and petrogenetic comparison with Hercynian and Tertiary granites

AbstractNew chemical data show that the two main granite types (G1 and G2) cannot be discriminated, but that microgranite sheets/dykes (G3) are significantly different and more evolved, largely as a result of biotite, accessory mineral, and plagioclase fractionation. The Lundy granite is similar to other Tertiary granites of Scotland and Ireland, in age, setting, possible high-temperature mineralogy, relationship to basic magmatism, and REE patterns. These features and a highly evolved chemistry suggest derivation from an unexposed more ‘primitive’ granite that, in turn, had a basaltic parentage. However, similarities with the nearby S-type Hercynian granites, such as high aluminium saturation index (and normative corundum), high trace alkali, Nb, and F contents, low Zr, and high initial Sr ratio suggest a significant crustal component. The problem is resolved by proposing either mixing of silicic magma derived by strong fractionation of basaltic magma with anatectic magma from a pelitic/semi-pelitic crustal source, or fractionation of basaltic magma heavily contaminated by assimilated crustal material. Both origins would yield the high REE contents and fiat REE patterns of a ‘primitive’ granite magma. Fractionation, perhaps of hornblende initially, and later, of biotite and accessory minerals together with feldspars, would produce the small volume of highly fractionated Lundy granite.

Discussion on Rb‐Sr Isochron Ages of Granitoids in the Hengduan Mountains1

Abstract Based on the Rb‐Sr isotopic study of the granitoids in the Hengduan Mountains, the classification and geologic significance of whole‐ rock Rb‐ Sr isochrons as well as the factors influencing homogenization of the isotopic systems are discussed. Usually, there is no good linear correlation of isochrons for diorites and alkali‐rich intrusive rocks (including alkaline rock and alkalic granite). But by means of the external isochron of monominerals from the rocks, isochrons with good linear correlation as well as ages and Sr initial ratios with a high confidence can be obtained. In order to obtain a satisfactory isochron, the discriminant parameters should be calculated from the Rb / Sr ratios and estimated ages of samples. Only those that meet the requirements can be used as the Rb‐Sr isochrons. The quality of an isochron should be judged from three factors, i.e. correlation coefficient, MSWD and homogenization degree.

Crustal evolution of the Hercynian belt of Western Europe: Evidence from lower-crustal granulitic xenoliths (French Massif Central)

Granulite-facies lower-crustal xenoliths scavenged by the Tertiary basaltic pipes of Bournac and Roche Pointue (Massif Central, France) have been analysed for major and trace elements and radiogenic (Sr, Nd) isotope ratios. They display wide variations in major elements, rare-earth elements and isotopes ( 87 Sr 86 Sr = 0.7041–0.7216 ; ϵ Nd = −11.9 to +1.5), which tend to correlate with the three different rock types encountered: basic and acidic meta-igneous and metasediments. The basic liquids are of calc-alkaline affinity, and cumulates are abundant. There is a gap in silica content between the basic and acid meta-igneous xenoliths. The isotopic compositions suggest mixing of old pre-existing continental crust (represented by the metasediments) and mantle-derived basic melts which represent juvenile addition to the crust during the Hercynian orogeny. The basic meta-igneous rocks have been contaminated during their emplacement into the crust whereas the surrounding metasediments have suffered partial fusion and contamination by the same liquid, producing the Hercynian granitoids which have ϵ Nd and ϵ Sr initial ratios identical to those of the lower-crustal xenoliths. The residue from melting may be represented by the acid meta-igneous rocks, some of which have positive Eu anomalies. These results indicate that an important basaltic underplating event occurred during the evolution of Hercynian orogenic belt, although the exact timing of the underplating requires further constraint.

The Caledonian Heilhornet Pluton, north-central Norway: geological setting, radiometric age and implications for the Scandinavian Caledonides

The Heilhornet Pluton is a medium-sized ( c . 50 km 2 ), sub-circular, discordant intrusion which penetrates the thrust zone separating the Helgeland Nappe Complex from paragneisses in the Vestranden region, north-central Norway. The intrusion consists of biotite ± hornblende granite and ranges in composition from 61.5 to 72.5 wt.% Si0 2 The initial Sr-ratio ( c . 0.7070) shows that crustal sources have been involved in the petrogenesis of the granite. A U-Pb zircon age of 444 ± 11 Ma for the pluton indicates that juxtaposition of the nappe complex with metasedimentary rocks in Vestranden took place prior to or during the late Ordovician. A consequence of this is that the general correlation of the metasedimentary rocks in Vestranden with the Seve Nappes must be revised. Final eastward thrusting of the complex, together with a yet unknown part of the Vestranden supracrustal sequence, probably took place during the middle Silurian to early Devonian, Scandian, orogenic event.