Geochemistry of highly aluminiferous Vendian-Cambrian sediments of the Tunka Bald Mountains,East Sayan

This paper presents comprehensive geochemical characteristics of highly aluminiferous schists discovered in the Tunka Bald Mountains and associated with the Vendian-Cambrian cover of the Tuva-Mongolian Microcontinent. The Al₂O₃ content varies within a wide range, the maximum being up to 36.11 wt. %. A specific feature of the geochemical composition of these rocks is the simultaneously high contents of Cr (up to 1400 ppm), V (up to 2700 ppm), and Zr (up to 800 ppm). The origin has been interpreted, and the conditions in which the highly aluminiferous rocks formed have been considered. Based on geochemical data, it is assumed that the main origin could be weathering crusts formed after ultramafic and mafic rocks, with a felsic source playing a secondary role.     

Cordierite-anthophyllite rocks from north-west Queensland, Australia: metamorphosed magnesian pelites

Abstract Cordierite-anthophyllite rocks and related cordierite-rich, talc-rich and chlorite-rich rocks occur in the Rosebud Syncline, north-west Queensland, Australia, as part of a Proterozoic metasedimentary sequence. Field relations and rock compositions attest the sedimentary origin of these rather unusual metamorphic rocks. Their chemical composition is comparable to that of unmetamorphosed, alkali- and Ca-poor pelites, which are associated with some evaporite deposits. Other occurrences of cordierite-anthophyllite rocks have commonly been interpreted as metamorphosed chloritic alteration products derived from mafic or felsic volcanics. A comparative chemical study, using analyses of cordierite-anthophyllite rocks from such alteration zones and analyses of unmetamorphosed magnesian pelites, demonstrates the general chemical similarity between these two rock groups of entirely different origin. However, distinct differences in major element relations help to distinguish these two genetic groups. Particularly useful are Al₂O₃–FeO–MgO plots, in which evaporitic pelites occupy the Fe-poor side. The highly magnesian metamorphic rocks from the Rosebud Syncline fall entirely into the compositional field of evaporitic clays and shales. Furthermore, analyses of relatively immobile trace elements give supporting evidence for the sedimentary origin of these cordierite-anthophyllite rocks. The correlation with trace element ranges of clays and shales is very good. However, the correlation with trace element ranges of mafic and felsic volcanics is poor, and major discrepancies occur with Cr, Ni, Co, Nb, Sc, Th and Ti. Thus, the magnesian metamorphics of the Rosebud Syncline appear to be derived from evaporitic clays rich in magnesian clay minerals, such as palygorskite, sepiolite, chlorite or corrensite. The complete metamorphic rock assemblage of interlayered calcareous, aluminous and magnesian rocks is interpreted as a metamorphosed carbonate-evaporite-pelite sequence.     

Geochemistry of the Osnitsk-Mikashevichy volcanoplutonic complex of the Ukrainian shield

This paper addresses the geochemistry of intrusive (Osinitsk complex) and volcanic (Klesov Group) rocks of the Osnitsk-Mikashevichy volcanoplutonic belt (OMVPB) of the Ukrainian shield, which is an active continental margin existing approximately 1980–2000 Ma ago. The Osnitsk complex comprises a wide range of rocks, from ultrabasics to granitoids, and the Klesov Group is dominated by extrusive rocks of basaltic and rhyolitic compositions metamorphosed under epidote-amphibolite facies conditions. The Sr-Nd-Hf isotopic systematics (?Sr₁₉₉₀ from ?4 to +10, ?Nd₁₉₉₀ from ?0.6 to +2.3, and ?Hf1990 from 0.1 to 1.4) indicates a juvenile source for the OMVPB rocks. Geochemical data suggest independent origin of the rocks of gabbroid (SiO₂ < 60 wt %) and granitoid (SiO₂ > 60 wt %) series. The gabbroids are subdivided into pre- and post-granite groups on the basis of the higher contents of incompatible trace elements and lower contents of compatible elements in the post-granite rocks. The geochemical characteristics of the two groups of basic rocks indicate their formation in a convergent tectonic setting. The origin of the granitoid melts is attributed to the low-degree (eutectoid) melting of basic rocks at relatively low temperatures.     

藏东吉塘变质杂岩体锆石U-Pb年龄、地球化学特征及其地质意义

藏东羌塘地块变质基底的时代归属和构造演化一直以来存在较大争议,沿龙木错-双湖缝合带东段出露的由酉西群和吉塘群组成的吉塘变质杂岩体成为解决这一问题的理想研究对象。本文对酉西群变质岩和吉塘群变质岩进行了系统的岩石学、岩相学、地球化学及锆石年代学研究。吉塘变质杂岩体的基底主要由中-新元古代的变沉积岩组成,其中酉西群岩性主要为云母绿泥钠长片岩,吉塘群则以黑云母二长片麻岩和黑云母斜长片麻岩为主,它们具有高Si O2、高Al2O3、富Fe OT、贫碱和贫钙的特征。微量元素均显示富集K、Rb、Th等大离子亲石元素(LILE),亏损Ti、Nb、Ta等高场强元素(HFSE),而Zr、Hf、Th、U等轻度富集则说明其源区更富集长英质组分而贫镁铁组分。碎屑锆石均为岩浆成因,年龄峰值范围为560～600Ma、808～865Ma、925～997Ma和1071～1119Ma,代表变质基底的年龄主要由泛非期和格林威尔造山期的构造-岩浆热事件所形成,在560Ma的沉积时代之前,源区可能发生有4次岩浆事件。地球化学特征和碎屑锆石组成指示酉西群和吉塘群原岩分别以泥砂质沉积岩和长英质粘土岩及砂砾岩等副变质岩为主。吉塘变质杂岩体所在的南羌塘地块与喜马拉雅地块和拉萨地块具有相近的物源区,故而认为南羌塘地块在中-新元古代期间可能属于冈瓦纳大陆的一部分。     

黑龙江西北部小古里河过钾质基性、超基性火山岩的发现

本文报道了黑龙江省五大连池裂谷带北端出露的一套超基性、基性的过钾质火山岩,通过详细的野外调查,确认了火山岩的喷发顺序,肯定了火山岩从基性岩向超基性岩的演变以及它们的K2O、TiO2含量不断提高的事实。在此基础上作者对超基性-过钾质岩石的岩浆成因提出了初步认识,认为它们不是岩浆结晶分异作用的产物,而是在裂谷演化构造背景下的一次独立的深源岩浆事件。本文将小古里河的过钾质火山岩和乌干达及意大利等世界上著名的钾质火山岩进行了比较。     

Petrogenesis of Monte Vulture volcano (Italy): inferences from mineral chemistry,major and trace element data

The paper presents major and trace element data and mineral compositions for a series of foiditic-tephritic to phonolitic rocks coming from Monte Vulture, Southern Italy, and investigates their origin, evolution and relationship with the other centres of the Roman province.Major and trace element variation in the foiditic to tephritic suite agrees with a hypothesis of evolution by simple crystal/liquid fractionation, whereas the early erupted phonolitic trachytes and phonolites have geochemical characteristics which do not support their derivation from tephritic magma by crystal fractionation. Foiditic and phonolitic rocks have mineral compositions which are interpreted as indicating magma mixing. However geochemical evidence shows that this process did not play an important role during the magma evolution.The Vulture rocks have compositional peculiarities such as high abundance of Na₂O, CaO, Cl and S, when compared with other Roman volcanics. Instead, the distribution of incompatible elements is similar to those of Roman rocks, except for a lower content of Rb and K, higher P and lower Th/Ta and Th/Nb ratios which are still close to the values of arc volcanics.The high contents of Na, Ca and of volatile components are tentatively attributed to the interaction of magma with aqueous solutions, rich in calcium sulphate and sodium chloride, related to the Miocene or Triassic evaporites occurring within the sedimentary sequence underlying the volcano. The distribution pattern of the incompatible elements is interpreted as indicative of magma-forming in a subduction modified upper mantle and of the peculiar location of M. Vulture.     

Origin of crystalline,cold desert salts in the McMurdo region,Antarctica

The distribution of crystalline salt minerals in deposits in the McMurdo region of Antarctica has been examined to study the origin of these salts. Sulphate, chloride, sodium and calcium salts are most frequently encountered. Salts containing chloride and sodium ions become less common away from the coast. Sulphate salts are more regularly distributed but tend to be related isotopically to sea water sulphate. Salts containing magnesium ion tend to exist mainly on substrates composed of basic igneous rocks, whereas calcium and carbonate salts are present on all rock types in the region.These distributions show that salts of marine origin are regionally and quantitatively most important but that chemical weathering of mafic materials in rocks and soils is also significant. However, biological, volcanic and hydrothermal processes are or have been active contributing to salts in local areas, that include penguin rookeries and eastern Taylor Valley, the summit area of Erebus Volcano, and subsurface rocks, respectively.     

Trace element and isotopic fingerprints in HP–LT metamorphic rocks as a result of fluid–rock interactions (Ile de Groix,France)

Whole rock trace element and isotopic compositions of different HP–LT metamorphic rocks of the Ile de Groix were analysed to characterise geochemical fingerprints during subduction and exhumation in a late Palaeozoic HP metamorphic terrain. Massive metabasites of hydrothermally altered enriched mid-ocean ridge basalt (E-MORB) origin are in association with banded metabasic rocks of volcano-sedimentary origin and metapelites. Fluid-rock interactions that likely occurred during seafloor hydrothermal alteration and early subduction metasomatism increased δ¹⁸O values, as well as K₂O, Na₂O, MgO, and LILE contents and decreased CaO contents of metabasites. Most metabasites have retained their early-subduction and pre-HP trace element and isotopic composition, even for rocks metamorphosed to lower eclogite-facies P–T conditions. Micaschists also preserved apparent pelitic protolith trace element values and oxygen isotopic compositions. During retrograde metamorphism related to the exhumation, metabasites were rehydrated by fluids in equilibrium with the host rock compositions, which were likely derived from the basic rocks. This style of fluid–rock interaction formed a greenschist facies mineral assemblage. Metabasites that underwent pervasive alteration by seafloor hydrothermal and metasomatism processes prior to peak metamorphism, show greater effects of retrogression and albitisation, probably because they were richer in H₂O and Na₂O. The variety of metamorphic assemblages on the Ile de Groix is thus directly related to the pre-HP rock composition. The extent of retrogression in the western part of the Ile de Groix primarily reflects stronger metasomatic intensities than in the eastern part.     

Contrasting Pb isotopes of Archaean and Palaeoproterozoic sulphide mineralisation, Disko Bugt, central West Greenland

Sulphide separates from mineralisation in Archaean and Palaeoproterozoic host rocks of the Disko Bugt area, central West Greenland, have been analysed for trace elements and Pb isotopes. Isotopic compositions of lead from sulphide separates of Archaean supracrustal rocks show wide variations. Archaean semi-massive sulphides and sulphides separated from felsic metavolcanites yield an errorchron age of 2821 +77/−82, with a model μ₁ value of 7.36; this is comparable to the estimated age of the supracrustal rocks. The two most prominent mineralised sites, the Andersen and Eqip prospects, have their own unique Pb isotope pattern; the Andersen prospect is considered to represent the result of an upper crust of Palaeoproterozoic process. The sulphide separates of Palaeoproterozoic epigenetic mineralisation hosted in shear and fault zones in the supracrustal rocks has a common origin, e.g. linked to a metamorphic peak and/or hydrothermal alteration. Gold-bearing samples indicate a local origin for associated sulphides; no regional processes seem to be involved in the formation of the gold occurrences. Received: 17 March 1997 / Accepted: 8 July 1997     

The origin of magnesite deposits from the Polish Foresudetic Block ophiolites: preliminary δ13C and δ18O investigations

Twenty-three magnesite samples from vein deposits, found in ultramafic rocks related to ophiolites in the north-east Bohemian Massif, have been examined for carbon and oxygen isotope ratios. In these rocks two stages of antigorite serpentinization took place prior to magnesite precipitation. δ¹⁸O_SMOW in magnesites ranges from 24.5 to 27.9%o (corrected for acid fractionation factor) which leads us to conclude a meteoric origin of the ore-forming solution. This inference is further supported by δ¹³C values (?18.7 to ?4.4%o) which point to a biogenic origin of carbon in the magnesites.     

The Lac Cornu retrograded eclogites (Aiguilles Rouges massif,Western Alps,France): evidence of crustal origin and metasomatic alteration

Metabasic rocks interbedded in amphibolite facies supracrustal gneisses outcrop around Lac Carnu in the Aiguilles Rouges massif (Western Alps). The cores of the thickest boudinaged lenses are made up of eclogitic amphibolites grading outwards into amphibolites. The common assemblage is unzoned garnet + symplectitic clinopyroxebe + hornblende + plagioclase. In a slightly amphibolitized sample, minimum P and T conditions of equilibrium between garnet and omphacite inclusions are 780°C and 11 Kb. A polymetamorphic pre-Alpine evolution of the massif is thus demonstrated. Eclogitic amphibolites show variations in major element composition similar to the Skaergaard evolution in the Al₂O₃ ? FeO + Fe₂O₃ ? MgO triangle. Variation diagrams in which Zr is taken as differentiation index also indicate magmatic trends for Mn, Ti, P, Ni, Co, Y, V and Cr, Ca, K, Na, Sr, Rb and Si were mobile during the evolution of the rocks. The igneous trend can be described by a quantitative model of fractional crystallization in which a noritic assemblage separated in the initial stage. This provides evidence of crustal P, T conditions of differentiation for the original materoal and in situ evolution for the eclogites. A second group of amphibolites shows banded structure and transitional terms with the surrounding gneisses. A volcano-sedimentary origin is suggested though the process cannot be modelled. The rocks may result from complex interactions of magnetic, metasomatic and sedimentary processes. It is concluded that Lac Cornu metabasites were originally continental tholeiites, though several geochemical criteria tend to indicate an oceanic origin. This casts some doubt on the validity of these criteria, when applied to metamorphic rocks.     

Carbonatization and mobility of Ti,Y, and Zr in Ascot Formation metabasalts,SE Quebec

The middle Ordovician Ascot Formation of southeastern Quebec consists of greenschist facies metamorphosed silicic to mafic pyroclastic rocks and lava flows and associated metasediments. Chemical analyses of lavas reveal a preponderance of metarhyolites and metabasalts, together with some porphyritic rocks with intermediate SiO₂ contents. The metabasalts exhibit wide ranges in concentrations of TiO₂ (0.25–2.0 wt.°), Y (9–46 ppm), and Zr (5–135 ppm). The extent of the ranges, and unusual interelement ratios, suggest that the concentrations of these normally immobile elements have been affected by secondary processes.There is a strong correlation between trace-element concentrations and the degree of carbonatization of the metabasalts. Low carbonate rocks are severely depleted in Ti, Y, and Zr whereas high carbonate rocks are depleted in Y and Zr and enriched in Ti. The differing movement of Ti can be explained in terms of variable chemical potential due to the various carbonatization reactions affecting titaniferous phases. Overall mobility of these generally immobile/rd elements is attributed to high CO₂ levels in the fluid phase during metamorphism.Extrapolation of the two alteration trends to a common origin enables one to infer primary concentrations of the trace-elements. Primary inter-element ratios arrived at in this way are compatible with an island-arc origin for the Ascot Formation although TiO₂ concentrations are a little high (1.5 wt/%).     

The thermal and mineral springs of Aitoloakarnania Prefecture: function mechanism and origin of groundwater

The study area is located in the northwestern part of Greece, in Aitoloakarnania prefecture. In this region, where no volcanic activity exists, thermal springs such as Kremasta and Kokkino Stefani, well-known for their healing properties occur. The objective of this study was the investigation of these springs, as well as the study of the chemical composition and origin of water. Relationships between these springs were also examined. The geological setting of the area comprises sedimentary rocks of the Pindos, Gavrovo-Tripolis and Ionian geotectonic zones, deformed by orogenic movements followed by Neogene extensional tectonism. The thermal and mineral springs were classified into three main groups. The first group is characterized by Ca-HCO₃ water type and low water temperatures. It corresponds to the springs that are hosted in the Ionian zone and their possible enrichment in SO₄ is mainly attributed to the evaporites. The other two groups consist of alkaline thermal water mainly hosted in the formations of Gavrovo-Tripolis zone. In these two groups, the very strong reducing conditions that prevail are expressed by high amounts of NH₃ and H₂S. Moreover, Na, F, Li, Sr and Ba display elevated concentrations. The second includes mineral waters of (Ca)-Na-HCO₃ type that are depleted in calcium. Their residence time is rather long and they originate from deep water circulation through siliceous rocks. The third group includes thermal waters of Ca-Mg-Na-HCO₃ water type of higher water temperatures that reveal characteristics of deep circulation directly associated with the underlain limestones.     

Lithological Features of Precambrian Gold-Bearing Rocks: Evidence from the Ukrainian Shield

Based on traditional petrochemical and nontraditional mineralogical methods (accessory zircon generation analysis), specific features of the primary composition of strongly metamorphosed rocks from some Early Precambrian Au-bearing rocks of the Ukrainian Shield (US) were studied. The confinement of several gold ore occurrences to primarily sedimentary Late Archean rocks of the Ukrainian Shield has been established and the possibility of their chemogenic origin is considered. The joint analysis of plicative tectonics and metamorphism facies in the study area demonstrated that Au-bearing primarily sedimentary (chemogenic) rocks of the Khashchevatoe–Zaval'ev Formation of the Bug Group (AR₂) are confined to amphibolite-facies domains within tectonic (high-order synform) structures with a significant gold potential.     

Pb and nd isotope and trace element constraints on the origin of basic rocks in an early proterozoic igneous complex,minnesota

A suite of early Proterozoic basic to granitic rocks exposed near St. Cloud, Minnesota are cut by northeast-trending basaltic dikes. Pb isotope data for all of these rocks overlap and plot about an 1800 Ma PbPb correlation line. The basic rocks, which are light REE enriched, have ϵ_Nd values between +0.4 and −4.8. Petrogenetic considerations suggest that the basic rocks were derived from a light REE enriched source which had an Fe/Mg ratio greater than that for pyrolite. The enrichment in Fe/Mg is probably a result of the addition of basic melts to the source. The light REE enrichment may have a mantle origin by the addition of mantle-derived, light REE enriched basic melts or fluids; or a sedimentary origin by the addition of light REE enriched fluids or melts derived from subducted sediments with an early Proterozoic provenance. In either case, the Nd isotopes suggest that the light REE enriched component existed since c. 2300 Ma. The igneous complex may have formed at a convergent margin. An anorthositic gabbro near Mora, has an ϵ_Nd of +5 indicating that it was derived from a mantle source with a history of light REE depletion. This gabbro may have been part of an early Proterozoic ocean crust.     

Chemical and isotopic evidence for widespread Eoarchean metasedimentary enclaves in southern West Greenland

New mapping, geochemistry and zircon U-Pb ion microprobe geochronology of pre-3750 Ma rocks from West Greenland was used to identify sedimentary protoliths in a problematic high-grade metamorphic terrane. Samples were collected from southernmost part of the Itsaq Gneiss Complex where Akilia association supracrustal rocks have previously been noted. Supracrustal lithologies include laterally continuous and variably deformed units of amphibolite, ultramafics and ferruginous quartz-pyroxene rocks. Oxygen isotope and mass-independently fractionated sulfur isotopes, immobile trace elements and rare earth element patterns are consistent with origin of quartz-pyroxene rocks as chemical sediments deposited in a marine hydrothermal setting. We describe a further supracrustal lithology: Garnet-bearing quartz-biotite schists with elevated oxygen isotope values (δ¹⁸O_SMOW ? +16‰) and mass-independently fractionated S isotopes consistent with a low-temperature aqueous sedimentary origin. In several enclaves, granitoid gneisses within low-strain limbs transect lithologic contacts and contain inclusions of surrounding rocks. This supports the interpretation that some orthogneisses were originally emplaced as igneous veins that cut supracrustal lithologies. Zircon geochronology on orthogneisses that preserve intrusive relationships confirms minimum ages of ca. 3750 Ma for the supracrustals and pooled [Th/U]_zircon and δ¹⁸O_zircon values of older zircon populations are consonant with igneous growth in the bulk composition of the host rocks. Low [Zr]_WR and high Zr saturation temperatures further minimize the possibility of zircon inheritance. A >3750 Ma age and chemical sedimentary origin for various Akilia association lithologies underscores the widespread occurrence of rocks of this kind beyond the type locality on Akilia (island) at the southern limit of the Itsaq Gneiss Complex.     

Carbonate xenoliths in La Palma: Carbonatite or alteration product?

Carbonate xenoliths containing olivine and rimmed by kaersutitic amphibole were collected in basaltic rocks of the Basal Complex of La Palma. The mineralogical composition and microscopic appearance may suggest a relationship with carbonatites in general, thus a major element, trace element and stable isotope study was conducted to investigate the origin of the carbonate formation. Based on electron microprobe analyses, the carbonate is calcite with up to 6.3 wt% MgO and 7.2 wt% SiO₂. The elevated SiO₂ content may suggest a melt origin for the carbonate. However, the C and O isotope compositions of the carbonate xenoliths (δ¹³C and δ¹⁸O around −1‰ and 13‰, respectively) are similar to those of calcite veins and amygdales in basaltic rocks of the Basal Complexes of La Palma and Fuerteventura and are interpreted as produced by fluid degassing and metasomatism by CO₂-H₂O fluid derived from mobilization of sedimentary material. Trace element contents determined by laser-ablation ICP-MS analyses support the assumed origin, thus, the relationship with carbonatitic melts can be excluded. Based on trace element compositions, the amphibole surrounding the xenoliths is not related genetically to the carbonate. The elevated SiO₂ content of the calcite can be attributed to submicron relics of pyroxene, thus, the use of this feature as an evidence for melt origin is questionable.     

Geochemical Signatures of Karlad Lake Sediments,North Kerala: Source Area Weathering and Provenance

In the present study, the lake floor sediments of the Karlad lake, located at higher elevation in Wayanad region of north Kerala, were analyzed for textural characteristics, organic matter, calcium carbonate, major oxides and trace elements. This study was carried out to infer the chemical composition, provenance and intensity of chemical weathering of the source rocks in the lake catchment area. Textural studies signify that lake floor sediments are predominantly as clays (38.75%) followed by sand (36.36%) and silt (25.19%) fractions. The C/N ratio of the lake sediments signify that the sediments are both autochthonous and allochthonous in origin. The major oxides average content reveals the order of abundance as follows; SiO₂ > Al₂O₃ > Fe₂O₃ > TiO₂ > MgO > CaO > K₂O > P₂O₅ > Na₂O > MnO. Moreover, the various weathering indices such as Chemical Index of Alteration (CIAAvg. 93.5%), PlagioclaseIndex of Alteration (PIA- Avg. 95.6%) and Chemical Index of Weathering (CIW- Avg. 95.76%) suggest an intense chemical weathering of the source area. The A-CN-K diagram is also corroborating the same. Various provenance discrimination diagrams reveal that the sediments are derived from the mafic source rocks.     

Protoliths of Paleoproterozoic calciphyres from the Irkut block (Sharyzhalgai uplift of the Siberian craton): composition and origin

The paper presents data on high-grade silicate–carbonate rocks (calciphyres) from the Irkut block (Sharyzhalgai uplift, southwestern Siberian craton). Their origin and age were determined from the rock characteristics, U–Pb dating, REE content, and Hf isotope composition of zircon. The calciphyres occur both as independent section fragments and as interbeds within Paleoproterozoic garnet-bearing and high-alumina (cordierite- and sillimanite-bearing) gneisses. They were produced by metamorphism of terrigenous-carbonate sediments. The terrigenous sediments range in maturity from arenites and wackes to argillaceous rocks; this is consistent with the reconstruction of the sedimentary protoliths of paragneisses, which are predominant in the metasedimentary rocks. The petrogeochemical features of the calciphyres, their LREE enrichment relative to “pure” carbonate rocks, and a distinct Eu anomaly were inherited from the terrigenous component of calc-silicate sediments. The Nd model age (2.4–2.7 Ga) of the calciphyres and the value T_Hf(DM-2st) = 2.5–3.0 Ga for zircon from these rocks indicate that carbonate accumulation was accompanied by the supply of terrigenous material, which formed during the erosion of Archean and Paleoproterozoic crust. Zircon from the calciphyres is similar to metamorphic zircon in REE patterns and Th/U ratios. It might have been of detrital origin and then recrystallized during high-temperature metamorphism. Terrigenous-silicate rocks were metamorphosed at ca. 1.87 Ga. This is close to the previous age estimates for the terrigenous rocks metamorphism (1.85–1.86 Ga) and the age of baddeleyite from apocarbonate metasomatic rocks (1.86 Ga).     

The origin of the acidic and basic rocks of the Tichka Massif,Morocco, based on rare earth elements

The Tichka Massif is one of many late-Paleozoic massifs emplaced in northwestern Africa. It consists of granitic pods surrounded by a complex assemblage of dioritic and gabbroic rocks. Previous workers have suggested an independent origin for the granitic and gabbroic rocks and this study evaluates this suggestion using REE data.The Ce/Yb ratio versus Ce plot for the gabbro shows a large Ce/Yb variation for small changes in Ce, whereas the granites show a linear increase of Ce/Yb with Ce. In the granites, the samples with the highest Ce/Yb ratio have the lowest SiO₂, whereas in the gabbros the Ce/Yb ratio is lowest in the low SiO₂ rocks. The diorites show a tightly clustered pattern.Based on the REE data the granite-diorite-gabbro sequence cannot be related by fractional crystallization. Partial melting of a garnet bearing mantle is consistent with the gabbroic trends. Partial melting of a crustal source with small amounts of apatite and abundant hornblende can produce the granitic rocks.