The Panafrican Stratoid Granites of Madagascar: Alkaline Magmatism in a Post-Collisional Extensional Setting

A major alkali province of late Panafrican age occupies centralMadagascar and takes the form of a thick sequence of ‘stratoid’(sheet-like)granites emplaced in a mid-crustal gneissic basement This alkalinemagmatism has been interpreted as a consequence of extensionaltectonics accompanying the collapse of the Mozambique belt.The rocks belong to three petrographic types: subsolvus granites,hypersolvus alkaline granites and syenites. Major and traceelement analyses have typical A-type characteristics. Two distinctmagmatic suites are recognized: a mildly alkaline suite includingall the subsolvus granites and a strongly alkaline suite includingthe hypersolvus alkaline granites and the syenites. We proposethat the mildly alkaline suite was derived from a granodioriticcrustal protolith. Some of the strongly alkaline granites andthe quartz syenites display low ¹⁸O isotopic signatures of around+6.The parental magmas for this suite are most probably of mantlederivation. The more evolved compositions are consistent withcrystal fractionation processes. Contemporaneous alkaline silicicplutonismoccurs in many parts of the Panafrican belt of Eastern Africa;however, sheet-like intrusions have rarely been described. Asa large-scale province, the nearest analogues of the stratoidgranites of Madagascar are the rapakivi granites of earlierProterozoic age in Scandinavia and Greenland. KEY WORDS: alkaline granite; Madagascar; Panafrican; pastcollisional magmatism ^*Corresponding author     

Post-orogenic and anorogenic A-type fluorite-bearing granitoids, Eastern Desert, Egypt: Petrogenetic and geotectonic implications

The present study focuses on four A-type fluorite-bearing granitic plutons in the Eastern Desert of Egypt which are classified into post-orogenic subsolvus (Homrit Waggat, 535 Ma; Homer Akarem, 541 Ma and Ineigi, 571 Ma) and anorogenic hypersolvus (Gabal Gharib, 476 Ma) granites. All the granites are Si- and alkali-rich and MgCaTi poor. Whereas both granite types appear relatively homogeneous in terms of most of their major and trace elements, they differ in that the subsolvus granites are depleted in TiO₂, FeO^*, Ba, Sr and Zr and enriched in Rb and Y with respect to the hypersolvus granites. The two granite types, however, can be distinguished more easily by their rare-earth element (REE) patterns. Chondrite-normalized REE patterns of the hypersolvus granite display a gull-wing shape, characterized by a large negative Eu anomaly and moderate-to-high LREE contents. Relative to the hypersolvus granite, subsolvus granite is depleted in LREE and more enriched in HREE contents. The increase of HREE in the subsolvus granite is presumably caused by F complexing during the late stage of its evolution. This is supported by the abundance of fluorite veins cross-cutting the subsolvus granite. The negative Eu anomalies in the subsolvus granite point to the role of feldspars as residual phase in the source, and as a crystallizing phase during magmatic differentiation.Field relations, textural, mineralogical and geochemical data of the post-orogenic subsolvus granite are consistent with its derivation from a parental basic magma through crystal-liquid fractionation of alkali feldspar, plagioclase, amphibole, FeTi oxides, titanite, zircon, monazite and allanite. Crystallization occurred in a water-enriched and rather oxidizing environment, as a result of which the entire suite has a transitional character between that of a post-orogenic and an anorogenic setting. On the other hand, the most credible mechanism for the origin of the anorogenic hypersolvus granite is partial melting of I-type granodiorite-monzogranite source rocks in the study area.     

The Mount Gharib A-type granite,Nubian Shield: petrogenesis and role of metasomatism at the source

The Mount Gharib peralkaline A-type complex (476±2 Ma), located in the Nubian Shield of Egypt, contains sodic-calcic to sodic amphiboles, accessory astrophyllite, zircon, fluorite, apatite, allanite, aenigmatite, elpidite(?) and ilmenite. This “within plate” hypersolvus suite is enriched in large-ion lithophile (LIL) and high field-strength (HFS) elements, and characterized by a fractionated REE pattern (Ce/Yb=49) and a significant negative Eu anomaly. A fine-grained acicular-amphibole-bearing roof facies shows further enrichment in the LIL and HFS elements. The suite was emplaced in a Pan-African granodiorite-adamellite host, which it locally metasomatized. The affected rocks contain hydrothermal albite, end-member arfvedsonite, astrophyllite, and levels of the LIL and HFS elements intermediate between those in the peralkaline granite and the roof facies. Trace element and isotopic modeling of this A-type granite, with its high initial ⁸⁷Sr/⁸⁶Sr value (0.7110), documents an active role of the lithosphere in magma generation. Lithospheric extension, expressed by regional dyke-swarms, was caused by cooling, fracturing and relaxation of the thin, newly formed Pan-African crust. Localized partial melting took place in an open system, possibly as a result of an influx of alkali-rich fluid derived from a sublithospheric source. Metasomatic reactions similar to those observed in the metasomatized wallrocks are considered to have played an important role just prior to the onset of anatexis and generation of the A-type melt.     

Geochemistry of phonolites and trachytes from the summit region of Mt. Kenya

Two suites of felsic eruptives and intrusives are represented in a set of samples from the summit region of the Plio-Pleistocene volcano, Mt. Kenya. Most of the samples are moderately or strongly undersaturated and have ⁸⁷Sr/⁸⁶Sr initial ratios in the range 0.70360–0.70368 (mean=0.70362). Members of this phonolitic suite are phonolites, nepheline syenites or kenytes and as a group they show a wide variation in TiO₂, FeO, P₂O₅, Sr, Ba, Zr and Nb. The minor and trace element geochemistry reflect variation in the nature of the parental basaltic magmas from which the phonolitic rocks evolved and variation in the crystal fractionation process in individual cases. Crystal fractionation involving plagioclase, alkali feldspar, clinopyroxene, olivine and magnetite is the process by which most of the phonolitic rocks evolved and variation in the relative proportions of these phases in individual cases has led to a broad spectrum of trace and minor element behaviour. The second suite of felsic samples is critically saturated and consists of trachytes showing either slight oversaturation or slight undersaturation with respect to SiO₂. This trachyte suite has lower initial ⁸⁷Sr/⁸⁶Sr ratios (mean=0.70355) and is derived from transitional alkalic basalts by low pressure (crustal) crystal fractionation involving feldspar, clinopyroxene, magnetite and olivine. The range in minor and trace element chemistry observed among the felsic rocks is a consequence of variation in the parental basalts which is related to mantle source variation and to the specific nature of the crystal fractionation process.     

Age,origin and evolution of the anorogenic complex of Evisa (Corsica): A K-Li-Rb-Sr study

The anorogenic complex of Evisa (Corsica) is made up of riebeckite hypersolvus granite and of albite-riebeckite-aegirine granite. Ten samples from the southern part of the complex provided Rb-Sr whole-rock isochrons for each facies. The ages of the two granites are indistinguishable at 246±7m.yr. corresponding to the Upper Permian. ⁸⁷Sr/⁸⁶Sr initial ratio (0.7034±0.0011) is in the mantle range of values and precludes any important crustal contamination. Li and Rb contents are controlled by the peralkaline fluid phase, reflected by deuteric changes. These alterations are weak in the hypersolvus facies but are obvious in the albite facies: replacement of preexisting perthite by albite, late precipitations of aegirine and fluorite, associated weak mineralization. The low value of ⁸⁷Sr/⁸⁶Sr initial ratio and the similarity of this ratio for both facies indicate that the fluid phase interacted with the crystallized rocks soon after the emplacement of the complex and provoked autometamorphic reactions without important external supplies.     

Minor-element and Sr-isotope geochemistry of tertiary stocks,Colorado mineral belt

Rocks of the northeast portion of the Colorado mineral belt form two petrographically, chemically and geographically distinct rock suites: (1) a silica oversaturated granodiorite suite; and (2) a silica saturated, high alkali monzonite suite. Rocks of the granodiorite suite generally have Sr contents less than 1000 ppm, subparallel REE patterns and initial ⁸⁷Sr/ ⁸⁶Sr ratios greater than 0.707. Rocks of the monzonite suite are restricted to the northeast part of the mineral belt, where few rocks of the granodiorite suite occur, and generally have Sr contents greater than 1000 ppm, highly variable REE patterns and ⁸⁷Sr/⁸⁶Sr initial ratios less than 0.706.Despite forming simple, smooth trends on major element variation diagrams, trace element data for rocks of the granodiorite suite indicate that they were not derived from a single magma. These rocks were derived from magmas having similar REE patterns, but variable Rb and Sr contents, and Rb/Sr ratios. The preferred explanation for these rocks is that they were derived by partial melting of a mixed source, which yielded pyroxene granulite or pyroxenite residues.The monzonite suite is chemically and petrographically more complex than the granodiorite suite. It is subdivided here into alkalic and mafic monzonites, and quartz syenites, based on the textural relations of their ferromagnesian phases and quartz. The geochemistry of these three rock types require derivation from separate and chemically distinct magma types. The preferred explanation for the alkalic monzonites is derivation from a heterogeneous mafic source, leaving a residue dominated by garnet and clinopyroxene. Early crystallization of sphene from these magmas was responsible for the severe depletion of the REE observed in the residual magmas. The lower Sr content and higher Rb/Sr ratios of the mafic monzonites requires a plagioclase-bearing source.The Sr-isotope systematics of the majority of these rocks are interpreted to be largely primary, and not the result of crustal contamination. The positive correlation of Rb/Sr and ⁸⁷Sr/⁸⁶Sr ratios for the least fractionated samples indicate that the sources from which parent magmas of both the granodiorite and monzonite suites were derived are Precambrian in age.     

Two “S-type” granite suites with low initial 87Sr/86Sr ratios from the New England Batholith,Australia

Two “S-type” (pelitic) granite suites from the New England Batholith, N.S.W., have Upper Carboniferous ages, indicating that they predate by 40 m.y. the intrusion of hornblende biotite granites, and are the oldest plutons of the batholith. Mineralogically and geochemically both suites have “pelitic” characteristics, one suite containing an Al-rich biotite, muscovite and cordierite, the other an Al-rich biotite and rare pyrope-almandine garnet. Low initial ⁸⁷Sr/⁸⁶Sr ratios of 0.706 for both suites probably reflect the volcanoclastic nature and young age of the sedimentary source of these granites at the time of melting. The age of the suites coincides with the last stages of (Andean type?) volcanism along an andesite/dacite volcanic chain to the west, suggesting an origin for the “S-type” granitic magma by partial melting of deformed sediments marginal to a continental region.     

Age and origin of the Nigerian mesozoic granites: A Rb-Sr isotopic study

Sixty-four Rb-Sr and two K-Ar isotopic measurements from seven ring complexes in central Nigeria provide evidence for a systematic age trend along a 200 km zone ranging from 174±5 m.y. in the north to 154±4 m.y. in the south. A peak of anorogenic magmatism occurred in the Jos Plateau region about 164±4 m.y. ago. Although a small syenitetrachyte complex at Zaranda, near Bauchi, gives an age of 190±15 m.y., unpublished ages of 290–330 m.y. for the southern Niger ring complexes confirm the existence of an overall southerly decreasing age trend in the Niger-Nigeria province of West Africa. Isotopic measurements on two small, oversaturated syenite intrusions at Zaranda and Pankshin suggest that syenitic liquids had initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7048—not significantly different from the mantle range of values, but that related peralkaline silicic variants from the same complexes are depleted in total Sr and have higher ⁸⁷Sr/⁸⁶Sr initial ratios characteristic of the earth's crust. This variation of initial ⁸⁷Sr/⁸⁶Sr ratios in syenite-related granitic liquids of the peralkaline spectrum has also been noted at the Shere Hills, near Jos, and at Liruei, near Kano, and may be representative for all syenite-granite occurrences in the Nigerian Younger Granite province. Such isotopic variations in the initial ⁸⁷Sr/⁸⁶Sr ratio may be attributed to “crustal enrichment” of syenitic liquids whose source lies in the mantle. Coarse-grained, peraluminous biotite granites have consistently low initial ⁸⁷Sr/⁸⁶Sr ratios in the range 0.706–0.709 (similar to the ca. 600 m.y. Pan-African granites of the basement), and may represent further modifications of originally syenitic liquids in the crust, or the granites may have originated from an independent source within a “dioritio” lower crust. Although the magmatic trends show small variations in the initial ⁸⁷Sr/⁸⁶Sr ratio, much higher initial ratios are recorded in granites which have been modified within their roof zone by deuteric (autometamorphic) and/or metasomatic processes.     

THE HISTORY OF TETHYS IN THE LATE PALEOZOIC-EARLY MESOZOIC AND THE RECONSTRUCTION OF PANGEA

The Terra Nova ultrapotassic igneous rocks of northeastern Brazil consist of two dike swarms (alkali-feldspar syenites to quartz syenites and alkali-feldspar granites) and one elongated E-W syenitic body (the Serra do Livramento pluton), which intruded metasediments of the Cachoeirinha-Salgueiro fold belt from 580 to 514 Ma. Mafic ultrapotassic syenite enclaves are recorded in the Serra do Livramento and Terra Nova shoshonitic plutons, both of which are cut by the dike swarms.

Mineralogically, Terra Nova ultrapotassic hypabyssal rocks resemble shoshonitic lamprophyres. Pyroxene is present in all facies; the clinopyroxenes are zoned, SiO₂ saturated, and Al₂O₃ poor (0.12 to 1.15%), and range from earlier diopside to late acmite. Amphiboles are characterized by high SiO₂ and low Al₂O₃ (0.20 to 2.00%) and TiO₂ (0.0 to 1.76%) contents; their compositions range from calcic to alkaline. The late amphiboles are riebeckite-arfvedsonites instead of K-richterites, as expected in ultrapotassic rocks, reflecting the early crystallization of K-feldspar.

The syenitic-facies rocks are mostly peralkaline, whereas the granites are metaluminous. The syenites have high concentrations of incompatible elements (Ba, Sr, and Rb) and light-rare-earth-element (LREE) concentrations lower than for typical ultrapotassic rocks, with chondrite-normalized Ce/Yb ratios of 10 to 20 and wide variation in the La/Ta ratios (40 to 250). The granites have lower incompatible-element contents and La/Ta ratios (20 to 60) than do the syenites.

Syenites from the dike swarm exhibit high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7106), whereas εNd, values for the ultrapotassic mafic enclaves range from -1.1 to -3.7, suggesting that the enclaves and the syenites have different sources.

Field evidence, combined with geochemical data, shows that the granites and the syenites alternate in space and time, suggesting that syenites and granites cannot be associated by either fractional crystallization or partial melting of the same source. The syenites probably represent partial melting of a metasomatized lithospheric mantle, modified by subduction-zone fluids and crustal metasediments during the early stages of a Brasiliano (Pan-African) collisional event. Low-degree partial melting of a metasomatized lower crust appears to be the source of the granites.     

Mineralogy, geochemistry and petrogenesis of the recent magmatic formations from Mbengwi, a continental sector of the Cameroon Volcanic Line (CVL), Central Africa

The Mbengwi recent magmatic formations consist of volcanics and syenites belonging to the same magmatic episode. Lavas form a bimodal basanite-rhyolite alkaline series with a gap between 50 and 62?wt.% SiO₂. Mafic lavas (basanite-hawaiite) are sodic while felsic rocks (trachyte-rhyolite-syenites) are sodi-potassic, slightly metaluminous to peralkaline. The geochemical and isotopic characteristics (0.7031?<?(⁸⁷Sr/⁸⁶Sr)_initial?<?0.7043; 1.03?<?ε_Ndi?<?5.17) of these rocks are similar to those of other rocks from the CVL. The main differentiation process is fractional crystallization with two trends of fractionation. Their Rb/Sr isochron age of 28.2?Ma, almost similar to 27.40?±?0.6?Ma?K/Ar age obtained in a trachyte from neighboring Bamenda Mountains system, precludes any local age migration of an hypothetic hotspot. Mafic lavas have OIB features displaying an isotopic signature similar to that of HIMU mantle source different from FOZO known as source of most parental magmas along the CVL.     

The comparative strontium isotopic composition of alkaline rocks: New data from Southern Portugal and East Africa

Rb-Sr whole rock isochrons for the Monchique and Mt. Kenya complexes, together with a series of ⁸⁷Sr/⁸⁶Sr ratios for 7 other alkaline suites, reveal limited initial ratio variation within the oceanic basalt range. Such variation is ubiquitous in alkaline suites and renders interpretation of their Sr behaviour highly subjective; this has led to major interpretive inconsistencies between previous studies. A consideration of wall-rock reaction, responsible for a few anomalous ratios in the present data, is believed to reconcile many of these inconsistencies; the subjective element can also be minimised by using the total of available data as a basis for comparative interpretation instead of interpreting individual sets of data absolutely. This approach shows available data to be remarkably unified, implying less diversified petrogenesis than is apparent from the sum of interpretations for individual alkaline complexes. In particular, Sr data lend little support to various models invoking crustal participation in the origin of nephelinites, nepheline syenites, ijolites or peralkaline oversaturated rocks, but suggest that these generally are straight-forward derivatives of mantle liquids. The consanguinity of most alkaline complexes is also confirmed.     

Fe isotope systematics of coexisting amphibole and pyroxene in the alkaline igneous rock suite of the Ilímaussaq Complex,South Greenland

The Ilímaussaq intrusion, South Greenland, provides an exceptional test case for investigating the changes of stable Fe isotope fractionation of solidus phases with changes in the Fe³⁺/∑Fe ratio of an evolving melt. The intrusion comprises a sequence of four melt batches that were fed from the same parental alkali basaltic magma. Differentiation produced cumulate rocks that range from augite syenite (phase I) over peralkaline granite (phase II) to agpaitic syenites (phases IIIa and IIIb). Fe³⁺/∑Fe ratios in amphiboles increase substantially from phase I to phase II and III rocks and mark a major change in the parental magma composition from augite syenites to peralkaline granites and agpaitic syenites. Before this transition, olivine, clinopyroxene, and amphibole in augite syenite, the most primitive rock type in the Ilímaussaq Complex, have a uniform Fe isotope composition that is identical to that of the bulk of igneous crustal rocks and approximated by the average isotopic composition of basalts (δ^56/54Fe_IRMM-014 = 0.072 ± 0.046‰). After the transition, amphiboles in the peralkaline granites and agpaitic syenites yield significantly heavier Fe isotope compositions with δ^56/54Fe_IRMM-014 values ranging from 0.123 to 0.237‰. Contamination of the Ilímaussaq magma by ongoing crustal assimilation as cause for this increase can be excluded on the grounds of Nd isotope data. Large-scale metasomatic overprint with an external fluid can also be dismissed based on amphibole O and Li isotope systematics. Rather, the increase towards heavy Fe isotope compositions most likely reflects the change in chemical compositions of amphiboles (calcic in augite syenite to sodic in the agpaitic syenites) and their Fe³⁺/ΣFe ratios that mirror changes in the chemical composition of the melt and its oxygen fugacity. A sensitive adjustment of equilibrium Fe isotope fractionation factors to amphibole ferric/ferrous ratios is also supported by beta-factors calculated from Mössbauer spetroscopy data. Comparison of the measured isotope fractionation between clinopyroxene and amphibole with that predicted from Mössbauer data reveal Fe isotope systematics close to equilibrium in augite syenites but Fe isotopic disequilibrium between these two phases in phase IIIa agpaitic syenites. These results are in agreement with O and Li isotope systematics. While amphiboles in all Ilímaussaq lithologies crystallized at temperatures between 650 and 850 °C, textural evidence reveals later clinopyroxene crystallization at temperatures as low as 300–400 °C. Therefore, isotopic equilibrium at crystallization conditions between these two phases can not be expected, but importantly, subsolidus reequilibration can also be dismissed.     

An inversion approach to assimilation and fractional crystallisation processes

A numerical inversion program has been developed to investigate isotope and trace element variations in suites of related rocks in terms of assimilation and fractional crystallisation processes. Major element compositions constrain the degree of fractionation, and the program may be used to calculate the isotope and trace element composition of either the crustal contaminant, or of the parental magmas once the contaminant is known. The ratio of the rate of assimilation to the rate of fractional crystallisation, and the bulk partition coefficients, can be varied with changes in magma composition. The approach is illustrated with a suite of calc-alkaline rocks from the Cerro Galan centre in NW Argentina, and a suite of continental flood basalts from the southern Paraná in Brazil. Both exhibit striking increases in ⁸⁷Sr/⁸⁶Sr with increasing SiO₂ consistent with progressive contamination during differentiation within the continental crust. In detail, both suites contain a subset of samples which have undergone relatively little contamination, and a subset characterised by relatively high SiO₂ and ⁸⁷Sr/⁸⁶Sr reflecting larger crustal contributions. Individual samples within the more contaminated subset can be modelled in terms of assimilation with fractional crystallisation (AFC) processes, although no clear progressively contaminated liquid lines of descent are preserved. At both Cerro Galan and the southern Paraná, the associated dacites and rhyodacites appear to have similar compositions to the crustal contaminants. Thus the inversion program has been used to investigate the amount of contamination, the relationship between the amount of contamination and the amount of crystals removed by fractional crystallisation, and the isotope and trace element contents of the parental magmas prior to AFC. The latter are estimated to have had ⁸⁷Sr/⁸⁶Sr=0.7055 and ¹⁴³Nd/¹⁴⁴Nd=0.51256 at Cerro Galan, and 0.7090 and 0.51226 respectively in the southern Paraná.     

Early‐Cretaceous Syenites and Granites in the Northeastern Tengchong Block,SW China: Petrogenesis and Tectonic Implications

Whole-rock major and trace element and Sr-Nd isotopic data, together with zircon LA ICPMS in-situ U-Pb and Hf isotopic data of the syenites and granites in the Tengchong Block are reported in order to understand their petrogenesis and tectonic implications. Zircon U-Pb data gives the emplacement ages of ca. 115.3±0.9 Ma for syenites and 115.7±0.8 Ma for granites, respectively. The syenites are characterized by low SiO_2 content(62.01–63.03 wt%) and notably high Na_2O content(7.04–7.24 wt%) and Na_2O/K_2O ratios(2.02–2.10), low MgO, Fe_2O_3 T and TiO_2, enrichment of LILEs(large-ion lithophile element) such as Rb, Th, U, K, and Pb) and obvious depletion HFSE(high field strength element; e.g. Nb, Ta, P, and Ti) with clearly negative Eu anomalies(d Eu=0.53–0.56). They also display significant negative whole-rock εNd(t) values of-6.8 and zircon εHf(t) values(-9.11 to-0.27, but one is +5.30) and high initial ~(87) Sr/~(86) Sr=0.713013. Based on the data obtained in this study, we suggest that the ca. 115.3 Ma syenites were possibly derived from a sodium-rich continental crustal source, and the fractionation of some ferro-magnesian mineral and plagioclase might occur during the evolution of magma. The granites have high SiO_2 content(71.35–74.47 wt%), metaluminous to peraluminous, low Rb/Ba, Rb/Sr, and Al_2O_3/(MgO+FeOT+TiO_2) ratios and moderate(Al_2O_3+MgO+FeOT+TiO_2) content. They show low initial ~(87) Sr/~(86) Sr(0.703408 to 0.704241) and εNd(t) values(-3.8 to-3.5), plotted into the evolutionary trend between basalts and lower crust. Hence, we suggest that the granites were derived from the melting of mixing sources in the ancient continental crust involving some metabasaltic materials and predominated metasedimentary greywackes. Together with data in the literatures, we infer that the Early Cretaceous magmatism in the Tengchong block was dominated by magmas generated by the partial melting of ancient crustal material, which represent the products that associated to the closure of Bangong-Nujiang Meso-Tethys.     

Anorogenic metaluminous and peraluminous granite plutonism in the mid-proterozoic of Wisconsin,USA

A magmatic gap from 1.82 to 1.76 b.y. in the Lake Superior region represents the transition from synorogenic calc-alkaline igneous activity of the Penokean Orogeny to anorogenic potassic granophyric granite and ignimbrite. This paper deals with the petrogenetic evolution of 1.76 b.y. granites which represent a major change in source material and conceivably tectonic setting. Although perhaps related to a termination of the Penokean Orogeny by melting of a tectonically thickened crust during collision, these post-Penokean granites may represent the initial appearance of anorogenic, potentially rift-related igneous activity that was widespread throughout North America during late Precambrian time.These post-Penokean granites are too iron-rich and Al-poor to be considered calc-alkaline, a compositional feature shared with most anorogenic igneous activity of continental regions. Within this suite in central and northern Wisconsin, regional differences in composition indicate at least two different granite magma types: one a metaluminous suite of biotite and biotite-hornblende granite and a peraluminous suite of two-mica granite. The systematic compositional differences (Al, Fe/Mg, Ba/Sr, REE) in the two magma suites are likely the result of small differences in residue mineralogy and/or source composition. In general, the degree of fusion was small (10%) and probably of relatively young Penokean material. Both suites have a range of composition due to feldspar dominated fractional crystallization. Removal of the accessory minerals apatite, zircon, and allanite resulted in the REE depletion with differentiation of the two-mica granites.The granites intruded into the upper levels of the crust, and the appearance of primary celadonitic muscovite and subsolvus alkali feldspars (silicic members only) in the two mica granites indicate crystallization at depths of 10–11 km. The biotite granites contain both hypersolvus and subsolvus members and are intruded at depths less than 6 km with the more shallow members generating major volumes of ignimbrite. As a marked departure from the characteristics of most anorogenic granites, these melts crystallized at fairly oxidizing conditions (higher for the two-mica suite) as reflected in the composition of biotite, predominance of magnetite over ilmenite, and early appearance of the Fe-Ti oxides in the crystallization sequence.     

Geochemical and Isotopic (O, Nd, Pb and Sr) Constraints on A-type Granite Petrogenesis Based on the Topsails Igneous Suite, Newfoundland Appalachians

The voluminous, bimodal, Silurian Topsails igneous suite consistsmainly of ‘A-type’ peralkaline to slightly peraluminous,hypersohnis to subsolvus granites with subordinate syenite,onzonite and diabase, plus consanguineous basalts and highsilicarhyolites. _Nd(T) values from the suite range from –1.5to +5.4; most granitoid components exhibit positive _Nd(T) values(+1.1 to +3.9). Granitoid initial ⁸⁷Sr/⁸⁶Sr and most ¹⁸ O valuesare in the range expected for rocks derived from mantle-likeprotoliths (0.701–0.706 and +6 to +80/). Restricted ²⁰⁷Pb/²⁰⁴Pbvariation is accompanied by significant dispersion of ²⁰⁶Pb/²⁰⁴Pband ²⁰⁸Pb/²⁰⁴Pb. Superficially, petrogenesis by either direct(via fractionation from basalt) or indirect (via melting ofjuvenile crust) derivation from mantle sources appears plausible.Remelting of the granulitic protolith of Ordovician are-typegranitoids can be ruled out, because these rocks exhibit negative_Nd(T) and a large range in ²⁰⁷Pb/²⁰⁴Pb. Geochemical and isotopicrelationships are most compatible with remelting of hybridizedlithospheric mantle generated during arc-continent collision.A genetic link is suggested among collision-related delaminationor slab break-off events and emplacement of ‘post-erogenic’granite suites. A-type granites may recycle previously subductedcontinental material, and help explain the mass balance notedfor modern arcs. However, they need not represent net, new,crustal growth. KEY WORDS: A-type granites; juvenile crust; isotopes; Newfoundland ^*Telephone: (613) 995-4972. Fax: (613) 995-7997. e-mail: jwhalen{at}gsc.emr.ca     

Volatile control of contrasting trace element distributions in peralkaline granitic and volcanic rocks

Anomalous enrichments of Zr (>500 ppm), Zn (> 100 ppm), Nb (>25 ppm), Y (>60 ppm), Th (>20 pm), U (> 5 ppm), LREE (>230 ppm) and HREE (>35ppm), and high Rb/Sr (>5) characterize peralkaline granites, in contrast to their peraluminous and calc-alkaline equivalents. Within the peralkaline suite, comenditic and pantelleritic volcanics exhibit two- to five-fold increases in the concentrations of these trace elements over comagmatic granites. These cannot be explained by crystal- liquid fractionation processes, and require the evolution of a sodium-enriched fluid. Corresponding trace element increases in the granites in areas of alkali metasomatism support this argument, and reflect the partial confinement of this volatile phase within the high-level magma chambers. REE studies in particular might eventually allow an evaluation of the role of Cl⁻ versus F⁻ and CO₃-complexing in the evolution of the volatile fluid.     

The Koloa Volcanic Suite of Kauai, Hawaii

The Koloa lavas of the post-erosional volcanic suite of Kauaivary in composition from melilitite to alkali olivine basalt.⁸⁷Sr/⁸⁶Sr varies from {small tilde}0.7030 to 0.7033, the rangebeing similar to that of other Hawaiian post-erosional suites.The chemical composition within single lava flows is variableand forms major-element trends that are different from the mainKoloa trend. The main trend can be related to increased degreesof partial melting during decreasing pressure. The combinedevidence from Sr and Nd isotopes and rare earth elements (REEs)could suggest that an ocean island basalt (OIB) plume was infiltratedby a carbonate-rich melt with a high concentration of Sr andREEs derived from the mid-ocean ridge basalt (MORB) source.However, the Pb-isotope systematics of the Hawaiian islandsdo not suggest that a MORB source was involved during the generationof the posterosional lavas. Instead, we suggest that the Hawaiiantholeiitic and nephelinitic magma suites are derived from azoned plume with a low ⁸⁷Sr/⁸⁶Sr core and a high ⁸⁷Sr/⁸⁶Sr concentricmargin. The trends of the single flows and their variation inisotope ratios suggest that they formed from magmas accumulatedmainly from the central core of the zoned plume.     

Geochemistry and petrogenesis of the marginal zone of the Mount Lowe Intrusion,central San Gabriel Mountains,California

The marginal zone of the Mount Lowe Intrusion (MLI) is composed of a mineralogically and geochemically distinctive suite of quartz poor, alkali-enriched monzodiorites and quartz monzodiorites. Low initial ⁸⁷Sr/⁸⁶Sr, Rb, and Rb/Sr coupled with high Ab/Or, K/Rb, Ba and Sr suggest that feldspathic continental crust did not contribute significantly to the magma. The inferred original magma composition is most consistent with experimental data for about 20% batch modal melting of eclogite or quartz eclogite of altered oceanic tholeiite composition. The high absolute Ba and Sr abundances require in addition an enriched component, perhaps derived from dehydration processes within a subducted slab. At the level of exposure, the range of observed compositions resulted from fractionation of hornblende, plagioclase, sphene, apatite and magnetite. Fractionation resulted in formation of a peraluminous, Mn — enriched residual liquid which crystallized garnet. The grandite — enriched composition of the garnet is inferred to be due to high pressure (ca. 6.7 kb), high crystallization, relative to typical plutonic garnets. The composition of the marginal zone of MLI has many chemical similarities to early Mesozoic plutonism in the Cordilleran continental margin magmatic arc of western North America.     

Palaeomagnetic and RbSr isotopic evidence for the age of the Särna alkaline complex,western central Sweden

The stable palaeomagnetic pole of peralkaline syenites from the Särna intrusion lies close to the mean Upper Carboniferous pole for stable Europe and to other poles of similar age from the Baltic Shield. This age evidence is in agreement with an Rb-Sr 8-point mineral isochron for a single whole-rock sample of $\text{287±14}$ m.y. (2σ). The isochron age and the pole position are believed to reflect the emplacement and cooling of the body and a comparison is drawn with other events of similar age in the Baltic Shield. Variation of initial ⁸⁷Sr/⁸⁶Sr ratios of whole-rock samples from 0.7041 to 0.7070 (at 287 m.y.) is considered to be caused by contamination of an ultimately mantle-derived magma by radiogenic strontium from crustal material.