Chemical interrelationships in a low-pressure granulite terrain in Namaqualand. South Africa,and their bearing on granite genesis and the composition of the lower crust

The results of a chemical study of a suite of low-pressure granulite facies rocks in Namaqualand, South Africa, are reported. The area is underlain by augen gneisses and quartzites, which contain interlayered granular quartz-feldspar rocks (termed ‘granulites’) derived by extensive partial melting of the gneiss. The K/Rb ratio of the gneiss increases from 140 to 250 over a melting interval of 70%: the rate of increase being influenced by the presence of biotite. Simultaneously K/Ba and Rb/Sr decrease from 80 to 25 and from 4 to 0.3, respectively. The partial melts (granulites), which reflect, in part, a cumulate character, have similar K/Rb ratios to the parent gneiss (175) but larger K/Ba (238) and Rb/Sr (5) ratios, due to the retention of Ba and Sr in the residue.Three granites intrude the gneisses. One of these was produced by very advanced partial melting of the gneiss. Continuity of chemical composition suggests that the remaining two granites, although spatially separate, are comagmatic, and evolved by feldspar fractionation during ascent. Lower Sr⁸⁷/Sr⁸⁶ ratio coupled with enrichment of Ba, Sr and Rb in the parent magma of these granites relative to the country rocks precludes local derivation and indicates a lower crustal source rock of intermediate composition.The progressive increase in cafemic character of the gneisses, which is similar to that observed in world granulite terrains as a whole, coupled with intrusive granite which reflects reworking of the lower crust in the area studied, supports a partial melting model for the development of a lower crust of progressively more cafemic composition.     

The geochemistry and evolution of early precambrian mantle

Seven high-purity cumulate clinopyroxenes from 2.7 b.y. maficultramafic rock associations from the Abitibi belt, Superior Province, Canada, have been analyzed for major elements and K, Rb, Cs, Ba, Sr and ⁸⁷Sr/⁸⁶Sr ratio. Attempts to reconstruct the trace element patterns of the original parent magmas were partially successful; Sr contents (140 ppm), K/Rb (470) and K/Ba (16) ratios are similar to those of modern low-K island arc tholeiites. K/Cs ratios (2700) are significantly lower than island arc tholeiites (17,000) or oceanic island and oceanic ridge basalts (> 30,000); the presentday mantle seems to be more depleted in Cs than in Archean times. Initial Sr isotope ratios of the 7 Archean clinopyroxenes average 0.70114±13(2σ) with relatively little variation; this value is in good agreement with initial ratios published for felsic and mafic rocks of the same age, though the latter show much larger variations and uncertainties. The pyroxene Sr isotope data, in conjunction with data for rocks of other ages, defines the following simple model for mantle evolution:

1. starting with primordial Sr, a short period of relatively rapid ⁸⁷Sr/⁸⁶Sr growth, followed by Rb depletion;
2. a period between ≧ 3.5 b.y. and ～ 1.7 b.y. when closed-system Sr isotope evolution occurred (with Rb/Sr ～ 0.023);
3. development of large-scale Rb/Sr heterogeneities in the mantle at ～ 1.7 b.y., leading to a present-day mantle with ⁸⁷Sr/⁸⁶Sr ranging from 0.7023 to 0.7065 and Rb/Sr ranging from ～ 0 to 0.065.

     

Strontium isotope systematics of Amba Dongar and Sung Valley carbonatite-alkaline complexes,India: evidence for liquid immiscibility,crustal contamination and long-lived Rb/Sr enriched mantle sources

The results of a Sr isotopic study of coexisting alkaline silicate rocks and carbonatites of two Cretaceous alkaline complexes of India, Amba Dongar (Deccan Flood Basalt Province) and Sung Valley (Rajmahal–Bengal–Sylhet Flood Basalt Province) are reported. The overlapping nature of initial Sr isotopic ratios of alkaline rocks and carbonatites of both the complexes is consistent with a magmatic differentiation model. Modelling of initial ⁸⁷Sr/⁸⁶Sr variation in alkaline rocks of Amba Dongar is consistent with a process of crustal assimilation by the parent magma undergoing simultaneous fractional crystallization of silicate rocks and silicate–carbonate melt immiscibility. A maximum of ∼5% crustal contamination has been estimated for the parent magma of Amba Dongar, the effect of which is not seen in the Sr isotope ratio of carbonatites generated by liquid immiscibility. A two point Rb–Sr isochron of the Sung Valley carbonatites, pyoxenite and a phlogopite from a carbonatite yielded an age of 106±11 Ma, which is identical to the ⁴⁰Ar–³⁹Ar age of this complex. The same age for the carbonatites and the alkaline silicate rocks, similar initial Sr ratios and the higher Sr concentration in the former than the latter favour the hypothesis of liquid immiscibility for the generation of the Sung Valley. The higher initial ⁸⁷Sr/⁸⁶Sr ratio for these complexes than that of the Bulk Earth indicates their derivation from long-lived Rb/Sr-enriched sources.     

Rubidium,strontium content and strontium isotopic composition of strongly alkalic basaltic rocks from the Cape Verde islands

The Cape Verde islands are characterized by the presence of very strongly alkalic lavas. Cenozoic volcanics—covering the broadest compositional range present in the archipelago—and ranging from alkali-basalts to phonolites, associated with plutonic essexites and nepheline syenites, were analyzed for Sr isotopic compositions and concentrations in K, Rb and Sr. The close values of the Sr⁸⁷/Sr⁸⁶ ratios (ranging from 0.7029 to 0.7033) indicate a comagmatic origin for the different rock types; no correlation appears between the Sr isotopic composition and the K-content of the lavas, thus indicating that the lavas with high K₂O/K₂O + Na₂O ratio are generated from a primary magma by differentiation at shallow depths. The values of the Sr isotopic composition are distinctly lower than most values obtained for lavas of other oceanic islands. The origin of the magma type is discussed on the basis of these isotopic compositions and the K/Rb and Rb/Sr ratios: it is suggested that the primary magma has a nephelinitic composition and was formed by partial melting of a small fraction of undepleted mantle peridotite, containing phlogopite; the deeper part of the mantle where this nephelinitic magma generates would have a strontium isotopic ratio of about 0.703 and a Rb/Sr ratio lower than that of the upper part.     

Plume-related mantle source of super-large rare metal deposits from the Lovozero and Khibina massifs on the Kola Peninsula, Eastern part of Baltic Shield: Sr, Nd and Hf isotope systematics

The two world’s largest complexes of highly alkaline nepheline syenites and related rare metal loparite and eudialyte deposits, the Khibina and Lovozero massifs, occur in the central part of the Kola Peninsula. We measured for the first time in situ the trace element concentrations and the Sr, Nd and Hf isotope ratios by LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometer) in loparite, eudialyte an in some other pegmatitic minerals. The results are in aggreement with the whole rock Sr and Nd isotope which suggests the formation of these superlarge rare metal deposits in a magmatic closed system. The initial Hf, Sr, Nd isotope ratios are similar to the isotopic signatures of OIB indicating depleted mantle as a source. This leads to the suggestion that the origin of these gigantic alkaline intrusions is connected to a deep seated mantle source—possibly to a lower mantle plume. The required combination of a depleted mantle and high rare metal enrichment in the source can be explained by the input of incompatible elements by metasomatising melts/fluids into the zones of alkaline magma generation shortly before the partial melting event (to avoid ingrowth of radiogenic isotopes). The minerals belovite and pyrochlore from the pegmatites are abnormally high in ⁸⁷Sr /⁸⁶Sr ratios. This may be explained by closed system isotope evolution as a result of a significant increase in Rb/Sr during the evolution of the peralkaline magma.     

Strontium and hydrogen isotope geochemistry of fresh and metabasalt dredged from the Mid-Atlantic Ridge

Fresh basalt and metabasalt dredged from the Mid-Atlantic Ridge were studied for Na, K, Rb, Sr, and H₂O(+) contents, and strontium and hydrogen isotope ratios. Na, K, Rb, and Sr contents of these samples are within the range of those of oceanic tholeiite. H₂O(+) content, strontium, and hydrogen isotope ratios vary widely. The variation in water content of metabasalt is apparently related to the chlorite content. The metamorphic temperature was about 550 °C based on the estimated δD value of chlorite. There is positive linear relationship between water content and strontium isotope ratio. Based on this relationship, the variation of strontium isotope ratio of the metabasalt was interpreted as follows: complete exchange occurred between strontium in the chlorite portion of the metabasalt and strontium in sea water (⁸⁷Sr/⁸⁶Sr ratio=0.7090), while the original strontium (⁸⁷Sr/⁸⁶Sr∼0.7023) was retained in the non-altered portion of the basalts.     

Large ion lithophile elements in rocks from high-pressure granulite facies terrains

A comparison of K, Rb, Th and U concentrations in granulite facies rocks with those of unmetamorphosed common rock types shows that depletion of these elements in granulites is variable. K/Rb ratios for granulites are generally higher than unmetamorphosed rocks, but K/Rb ratios only reach extreme values when K < 1%. The covariation of K/Rb ratio with K concentration suggests that protolith composition, hence mineralogy, is very important in controlling the degree of Rb depletion in granulites. Felsic granulites exhibiting extreme K/Rb ratios are mainly Archean, reflecting the high abundance of low K felsic rocks in Archean terrains. The Scourian granulites of Scotland all have very high K/Rb ratios and cannot be considered to be representative of granulite facies terrains. It is impossible from this data set to state conclusively whether K is depleted in granulites; K/La ratios of granulites show complete overlap with igneous rocks. Th/U ratios in many granulites are greater than 4, indicating U loss relative to Th. Felsic granulites with low Th/U ratios also have high La/Th ratios, indicating that these granulites have been depleted in Th. The low Th/U ratios of these rocks may reflect retention of Th and U in resistant accessory phases.     

Tertiary basalts and peridotite xenoliths from the Hessian Depression (NW Germany), reflecting mantle compositions low in radiogenic Nd and Sr

A total of 17 alkali basalts (alkali olivine basalt, limburgite, olivine nephelinite) and quartz tholeiites, and of 10 peridotite xenoliths (or their clinopyroxenes) were analyzed for Nd and Sr isotopes. ¹⁴³Nd/¹⁴⁴Nd ratios and ⁸⁷Sr/⁸⁶Sr ratios of all basalts and of the majority of ultramafic xenoliths plot below the mantle array with a large variation in Nd isotopes and a smaller variation in Sr isotopes. The tholeiites were less radiogenic in Nd than the alkali basalts. Volcanics from the Eifel and Massif Central regions contain Nd and Sr, which is more radiogenic than that of the basalts from the Hessian Depression. Nd and Sr isotopic compositions of all rocks from the latter area, with the exception of one tholeiite and one peridotite plot in the same field of isotope ratios as the Ronda ultramafic tectonite (SW Spain), which ranges in composition from garnet to plagioclase peridotite. The alkali basaltic rocks are products of smaller degrees of partial melting of depleted peridotite, which has undergone a larger metasomatic alteration compared with the source rock of tholeiitic magmas. For the peridotite xenoliths such metasomatic alteration is indicated by the correlation of their K contents and isotopic compositions. We assume that the upper mantle locally can acquire isotopic signatures low in radiogenic Nd and Sr from the introduction of delaminated crust. Such granulites low in radiogenic Nd and Sr are products of early REE fractionation and granite (Rb) separation.     

Chemical and isotopic (Pb,Sr) zonation in a peraluminous granite pluton: role of fluid fractionation

The Davis Lake pluton (DLP, ~800 km²) of southwestern Nova Scotia, Canada, part of the large peraluminous South Mountain batholith of ca. 380 Ma (U/Pb zircon, Ar/Ar mica), consists of granite and subordinate topaz–muscovite leucogranite that hosts greisen tin-base metal mineralization. A new Pb–Pb isochron age for leucogranite from the most evolved part of the DLP indicates a crystallization age of 378±3.6 Ma, coincident with other radiometric ages of the DLP (Rb–Sr, Re–Os, Pb–Pb). The intrusion displays a compositional zonation defined by lead and strontium isotopic ratios, as well as some major elements (e.g., Si, F), incompatible trace elements (e.g., Li, Rb, Ta, U, Sn), and elemental ratios (e.g., K/Rb and Nb/Ta). The greisens and the leucogranites that host them are characterized by extreme radiogenic compositions for Pb and Sr, and their chemical-isotopic trends are extensions of the trends displayed by the less evolved granites. The covariations of the isotopic ratios with several major and trace elements and elemental ratios as well as the Pb–Pb and Rb–Sr isochrones indicate that all phases of the intrusion originated from a homogeneous parental magma. The granitoid magma underwent extensive fractional crystallization of feldspars, minor biotite and accessory minerals (monazite, apatite and zircon) in a compositionally zoned magma chamber that was subsequently accompanied by fluid fractionation, during which time the internally derived fluorine-rich fluids modified the Rb/Sr, U/Pb and Th/Pb ratios, leading to distinct variations of ⁸⁷Sr/⁸⁶Sr, ²⁰⁶Pb/²⁰⁴Pb, ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb isotopic ratios. These data therefore document the evolution of a granitic magma through magmatic (i.e., crystal fractionation), orthomagmatic (i.e., crystal-fluid fractionation) and hydrothermal (i.e., fluid fractionation) stages that culminated in the formation of a tin-base metal deposit. The Pb isotope data also constrain the source region for the DLP as being Avalonian basement that, by inference, must underlie much of the Meguma Terrane.Editorial responsibility: T.L. Grove     

Petrogenesis of basement rocks of the upper rhine region elucidated by rubidium-strontium systematics

Initial ⁸⁷Sr/⁸⁶Sr ratios have been plotted against time of formation for various rock-types in the Schwarzwald and Vosges basement areas. These data exhibit strong positive correlations between the two variables. The Schwarzwald data define two distinct trend lines on the diagram, for the time span 500 to 250 Ma ago. The first comprises the data from orthogneisses, diatexites and pre-tectonic granites and documents the Sr isotope evolution in the crust underlying the Sehwarzwald. This region of the crust had a ⁸⁷Rb/⁸⁶Sr ratio of about 1. The other is delineated by data points from the post-tectonic plutons. These form a band corresponding to the development of closed systems with ⁸⁷Rb/⁸⁶Sr ratios of between 10 and 20 as indicated by the slope of the band. The second trend is interpreted as resulting from the formation of large, stable, deep-seated magma chambers formed by segregation of anatectic melts during a phase of tectonism 330Ma ago. The high ⁸⁷Rb/⁸⁶Sr ratios of these magmas arose by fractional crystallization, in response to tectonic decompression, during the coalescence of the presumedly water-saturated melts. Data for rocks from the Vosges, taken from the literature, form a single development line. Its slope corresponds to a ⁸⁷Rb/⁸⁶Sr ratio of about 5 and the line is analogous to the second trend line defined by the Schwarzwald data. The differences in the Rb/Sr systematics between the two areas can be explained as resulting from the deeper level of erosion in the Vosges. An important implication for Rb/Sr studies is that co-magmatic rocks can have very different initial ⁸⁷Sr/⁸⁶Sr ratios so that discrepancies in this ratio between rock-types cannot be used to divide plutons into genetic suites. On the other hand essential information is contained in the Rb/Sr systematics of orogenic magmatic rocks, such as the Variscan plutons in the Schwarzwald, that cannot be obtained from geochemical and petrological studies alone.     

Petrology and geochemistry of hawaiite lavas from Crater Flat,Nevada

Hawaiite-type lavas were erupted in three cycles (3.7, 1.2, and 0.3 M.y.) at Crater Flat, Nevada. The compositions of all three cycles, considered together, form a straddling alkalic series as defined by Miyashiro, in which the less evolved basalts plot near the normative olivine-diopside divide and the more evolved basalts project into hypersthene or nepheline fields. Fractionation modeling based on the oldest cycle allows the removal of olivine, clinopyroxene, and amphibole to arrive at the more evolved hawaiite compositions. In general, fractionation of phlogo-pite or feldspar is limited by the fractionation modeling and by Eu/REE relations. In detail, all hawaiites within one cycle (3.7 M.y.) need not be derived from a single parent magma. Varied parentage is more evident between cycles although all cycles are consistently of hawaiite composition. Basalts of the youngest two cycles are generally enriched in trace elements. Superimposed on this enrichment is a lack of Rb variation, leading to Rb/Sr ratios far lower than required to generate the high ⁸⁷Sr/⁸⁶Sr ratio (0.707) typical of basalts in this region. The very low Rb/Sr ratios limit processes that may lead to trace-element enrichment during magma evolution (cyclic recharge of a fractionating magma chamber). Decreased fractions of mantle melting leaving phlogopite in the residuum or an earlier event of metasomatic transport from phlogopite-bearing mantle rocks into a phlogopite-absent mantle assemblage might explain the observed trace-element enrichment with low Rb/Sr.     

Sm-Nd and Rb-Sr isotopic and geochemical systematics in Phanerozoic granulites from Fiordland,southwest New Zealand

Sm-Nd and Rb-Sr isotopic analyses are reported for granulite facies orthogneisses from Fiordland southwest New Zealand. Whole-rock samples define a Rb-Sr isochron age of 120±15 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70391±4. _Nd values (at 120 Ma) show a relatively wide range of from –0.4 to 2.7 indicating decoupling of Sr-Nd isotope systems. Associated ultramafic rocks have initial ⁸⁷Sr/⁸⁶Sr ratios of from 0.70380 to 0.70430 and _Nd values of from 0.1 to 3.0. The different initial ratios suggest that the various intrusions, although contemporaneous, were not derived through fractionation of a single parent magma. A metasedimentary enclave incorporated during emplacement of the granulitic rocks preserves a Proterozoic isotopic signature with a measured _Nd(0) value of –10.2, ⁸⁷Sr/⁸⁶Sr ratio of 0.73679 and a T ^Nd provenance age of 1490 Ma. The Rb-Sr whole rock age of the granulites is the same as obtained from recent U-Pb zircon dating (Mattinson et al. 1986) and is interpreted as the time of magmatic emplacement and essentially contemporaneous granulite facies metamorphism. Rb-Sr and Sm-Nd analyses of mineral systems indicate that the terrain had cooled below 300° C by 100 Ma providing further evidence that high grade metamorphism was of exceptionally short duration.Unmetamorphosed leucogabbros from the Early Cretaceous Darran Complex of eastern Fiordland have significantly higher _Nd values (3.9 to 4.6) and slightly lower ⁸⁷Sr/ ⁸⁶Sr (0.70373 to 0.70386) than the western Fiordland granulites. This indicates that the western and eastern Fiordland complexes are not correlative although both have geochemical similarities to Phanerozoic calc-alkaline island-arc suites. The Fiordland granulites are LREE enriched (La_N/ Yb_N=12 to 40) and have trace element characteristics (e.g. high K/Rb and low Rb/Sr ratios) typical of many Rb-depleted Precambrian granulite terrains. The Fiordland trace element trends, however are attributed to magmatic, not metamorphic processes, reflecting the character of the Early Cretaceous magma sources. The range of _Nd values, but uniform initial ⁸⁷Sr/⁸⁶Sr of the western Fiordland granulites is consistent with derivation of the parent Early Cretaceous magmas at least in part from a LREE enriched, low Rb/Sr protoliths of mid-to late-Paleozoic age. Partial melting of this protolith occurred during or immediately preceding a period of great crustal thickening culminating in rapid thickening of existing crust by 20 km following emplacement of the granulitic rocks. The rapid crustal thickening was probably a consequence of a collisional event in which an Early Cretaceous magmatic arc was over-ridden by one or more thrust sheets.     

栖霞牙山花岗岩及其暗色包体地球化学特征及成因研究

栖霞牙山花岗岩体形成于中生代早白垩世（118 Ma）,其岩石类型以花岗岩和花岗闪长岩为主,岩体中发育大量的暗色闪长质微粒包体。通过对牙山花岗岩及其暗色包体地球化学特征研究表明,包体围岩w（SiO₂）=65.5%~68.82%,铝饱和度（A/CNK）为0.89~1.03<1.1,为准铝质钙碱性Ⅰ型花岗岩;暗色包体具有较低w（SiO₂）值（54.82%~60.89%）、高w（TFe₂O₃）值（6.11%~8.15%）、高w（MgO）值（3.57%~5.19%）的特征。稀土元素配分模式图显示二者皆为轻稀土富集的右倾型曲线,微量元素蛛网图中二者均表现为富集Ba、K等大离子亲石元素,亏损Ta、Nb、Ti等高场强元素,具有大陆地壳的特征。暗色包体与寄主岩石的Sr同位素初始比值（⁸⁷Sr/⁸⁶Sr）_i分别为0.709 29~0.709 58和0.709 21~0.709 71,应为同源岩浆的产物。两阶段Nd模式年龄（T_2DM）分为2 291 Ma-2 391 Ma和2 208 Ma-2 353 Ma,表明可能是古元古界陆壳物质部分熔融的产物。Pb同位素特征显示牙山岩体的原始岩浆以下地壳为主,w（Nb）/w（Ta）值介于下地壳与原始地幔之间,表明可能受到幔源物质的影响,包体中大量磷灰石呈针状结晶状态,进一步暗示存在岩浆混合作用。综上并结合区域构造背景认为,牙山岩体为早白垩世中晚期起源于火山弧环境的壳源特征花岗岩,形成过程中存在幔源物质的加入,它的形成与太平洋板块的俯冲作用密切相关。     

Development of heterogeneities in the subcontinental Mantle: Evidence from the Ferrar Group,Antarctica

Tholeiitic lava flows (Kirkpatrick Basalts) and dolerite sills and dikes (Ferrar Dolerites) of the Jurassic Ferrar Group from Antarctica and dolerite sills from Tasmania, Australia are characterised by initial strontium isotope ratios ranging from 0.7089 to 0.7153. The mean and standard deviation of 85 analyses is 0.7115±0.0012. Some of the scatter in the initial ⁸⁷Sr/⁸⁶Sr ratios can be attributed to sample inhomogeneity, analytical uncertainties and sample alteration. The published major element data show well-defined trends that are consistent with an evolution by fractional crystallization. Recognition of a parental magma is difficult due to the fractionated nature of the rocks. Trace element analyses, particularly the rare earth elements (REE) support a differentiation model. Compared to mid-ocean ridge basalts, Ferrar Group rocks are enriched in light REE. Kirkpatrick Basalts from the central Transantarctic Mountains show significant correlations between initial ⁸⁷Sr/⁸⁶Sr ratios and major elements only for SiO₂ and CaO. The general lack of strong correlation is the basis for rejecting the possibility of wholesale contamination by sialic material as a possible cause of the high ⁸⁷Sr/⁸⁶Sr ratios. Selective contamination of the basaltic magmas is a possibility and cannot be completely discounted. It would probably involve a fluid phase in order to transport and mix the light REE, Rb, ⁸⁷Sr, and other elements. By analogy with selective contamination of ocean ridge basalts by sea water it is difficult to envisage a similar process acting on magma emplaced in a non-marine environment. Because of the elevated values of the initial ⁸⁷Sr/⁸⁶Sr ratios, their similar average value over 2,500 km and the large volume of magma involved (4× 10⁵ km³) a mantle origin for the high Sr ratios is preferred. Models to account for the enrichment of Rb and light REE in the Antarctic mantle during or prior to the Jurassic include:

1. addition of continental material from a Palezoic Mesozoic subduction zone;
2. metasomatism of volatile elements from the lower mantle; and
3. evolution of a mantle with a high Rb/Sr ratio.

     

Age and origin of early Proterozoic dolerite dykes in South-West Greenland

Samples of three dolerite dykes from South-West Greenland give a pooled weighted mean Rb-Sr whole-rock isochron age of 2,130±65 Ma. Initial ⁸⁷Sr/⁸⁶Sr ratios are 0.70155±0.00018 for one of the dykes (Naujat) and 0.70277±0.00012 for the other two (Torssut and Akinaq). A fourth dyke (Qaqarssuaq) did not yield an isochron. Torssut samples have significantly lower initial ²⁰⁷Pb/²⁰⁴Pb ratios than samples from the Naujat dyke, indicating that the Torssut dyke has been affected by crustal contamination. Samples of the dykes vary widely in chemical composition because of an uneven distribution of different primocryst phases throughout the dykes, and because of variable degrees of fractionation of the magma. For two of the dykes some constraints on the composition of the parent magma have been inferred from trace element data. Higher concentrations of K, Rb, Ba, Sr, Ce and perhaps Nb (at comparable levels of magmatic fractionation) in the dykes with higher Sr_i are probably related to crustal contamination of their parent magmas. Two of the dykes probably intruded from zoned magma chambers. The marginal zones of these dykes crystallised from a strongly fractionated magma, whereas the centres of the dykes consist of more primitive material with large proportions of olivine and plagioclase primocrysts. The central part of Torssut is less strongly contaminated with crustal Pb than the border zones. A more detailed analysis of the Pb-isotopic composition in the Torssut dyke shows that contaminant lead must have been derived from rocks in the deeper crust considerably older than the late Archaean gneisses which occur at the surface.     

Geochemistry and origin of metabasites from the granulite-gneiss complex of the Angara-Kan block,southwestern Siberian craton

In this paper, we present data on major and trace elements in highly metamorphosed mafic rocks from the granulite-gneiss complex of the Angara-Kan block (southwestern Siberian craton), identify igneous protoliths of the metabasites, and assess the mobility of elements during metamorphism. Two types of rocks with different geologic relations and compositions were recognized. Garnet-bearing two-pyroxene granulites (Cpx + Pl + Grt + Opx) occur as sheet- and boudin-like bodies, which were folded and deformed with their host paragneisses. Dikes, which in most cases underwent only brittle deformation, are composed of metabasites characterized by the assemblage Cpx + Hbl + Pl + Grt. The major element compositions of igneous protoliths for the mafic granulites and metabasite dykes correspond to variously differentiated basaltic magmas. The protoliths of the metabasites are depleted in K₂O, LILE, Zr, Nb, and LREE and were derived from a depleted mantle source. The major and trace element compositions of the dike metabasites are similar to those of low-K tholeiitic basalts of oceanic island arcs. Continental intraplate basalts derived from an enriched mantle source are possible igneous protoliths for the mafic granulites enriched in Ba, LREE, Nb, Ta, Zr, and Hf. It is assumed that lower Rb, Th, and U contents in the mafic granulites compared with continental flood basalts, high K/Rb and La/Th, and moderate Th/U ratios reflect the loss of Rb, Th and U during granulite-facies metamorphism.     

Geochemical variations in Andean basaltic and silicic lavas from the Villarrica-Lanin volcanic chain (39.5° S): an evaluation of source heterogeneity,fractional crystallization and crustal assimilation

At 39.5° S in the southern volcanic zone of the Andes three Pleistocene-recent stratovolcanoes, Villarrica, Quetrupillan and Lanin, form a trend perpendicular to the strike of the Andes, 275 to 325 km from the Peru-Chile trench. Basalts from Villarrica and Lanin are geochemically distinct; the latter have higher incompatible element abundances and La/Sm but lower Ba/La and alkali metal/La ratios. These differences are consistent with our previously proposed models involving: a) a west to east decrease in an alkali metal-rich, high Ba/La slab-derived component which causes an across strike decrease in degree of melting; or b) a west to east increase in the contamination of subduction-related magma by enriched subcontinental lithospheric mantle. Silicic and mafic lavas from the stratovolcanoes have overlapping Sr, Nd and O isotopic ratios. Silicic lavas also have geochemical differences that parallel those of their associated basalts, e.g., rhyolite from Villarrica has lower La/Sm and incompatible element contents than high-SiO₂ andesite from Lanin. At each volcano the most silicic lavas can be modelled by closed system fractional crystallization while andesites are best explained by magma mixing. Apparently crustal contamination was not an important process in deriving the evolved lavas. Basaltic flows from small scoria cones, 20–35 km from Villarrica volcano have high incompatible element contents and low Ba/La, like Lanin basalts, but trend to higher K/Rb (356–855) and lower ⁸⁷Sr/ ⁸⁶Sr (0.70361–0.70400) than basalts from either stratovolcano. However all basalts have similar Nd, Pb and O isotope ratios. The best explanation for the unique features of the cones is that the sources of SVZ magmas, e.g., slab-derived fluids or melts of the subcontinental lithospheric mantle, have varying alkali metal and radiogenic Sr contents. These heterogeneities are not manifested in stratovolcano basalts because of extensive subcrustal pooling and mixing. This model is preferable to one involving crustal contamination because it can account for variable Sr isotope ratios and uniform Nd and Pb isotope ratios among the basalts, and the divergence of the cones from across-strike geochemical trends defined by the stratovolcanoes.     

Garnet-pyroxene thermometry and barometry in the Scourie granulites,NW Scotland

Garnets and pyroxenes from granulites ranging in composition from trondhjemitic to ultramafic were analysed with the electron probe in order to test current geothermometric and geobarometric models. A consistent pressure and temperature estimate based on garnet-pyroxene equilibria shows that the peak of metamorphism was at $\text{820±50°}\text{C}$ and 11 kb and implies a minimum crustal thickness of 30 km and a maximum geothermal gradient of 25–28°C, km^?1, at 2700 Ma in the Scourie area. These results are in contrast to earlier more extreme P-T estimates of $\text{1150±100°}\text{C and}\text{15±3}\text{kb}$.     

RbSr chemical and isotopic response of gneisses in late Archean shear zones of the Limpopo mobile belt,southern Africa

This note reports on the Rb, Sr and K chemistry and Sr isotope composition of sheared gneisses from the Limpopo mobile belt, an Archean high-grade terrain in southern Africa. Retrogression during shearing appears to have shifted the chemical and isotopic composition of the gneisses away from those of the surrounding unsheared granulites towards compositions characteristic of certain late Archean granites within the mobile belt and on the adjacent Zimbabwe craton. These results bear on the issues of RbSr dating of high-strain events and of granite genesis in the Limpopo belt.     

Weak compositional zonation in a silicic magmatic system: Incesu ignimbrite,Central Anatolian Volcanic Province (Kayseri – Turkey)

The Central Anatolian Volcanic Province (CAVP), one of four major volcanic provinces in Turkey, plays a significant role in the interpretation of the tectonic evolution of Central Anatolia. The CAVP developed within a complex collisional system involving the African, Arabian and Eurasian plates during the Miocene. The volcanism exhibits complicated variations in mineralogical, petrological and geochemical compositions resulting from post-collisional lithospheric dynamics. The Incesu ignimbrite has 5–20 m thick and covers an area of ～7800 km². It is composed of three stratigraphic levels. The lower level (LL) shows blackish brown and glassy welded structure. The middle level (ML) is a well-welded, reddish pink in color and has large amounts of fiamme. The upper level (UL) is grayish pink, weakly welded and has rock fragments of different compositions. The Incesu ignimbrite is composed of plagioclase (oligoclase, andesine) + pyroxene (augite, clinoenstatite) + opaque minerals and low amount of amphibole, biotite and quartz. Eutaxitic texture is dominant in ML and LL samples; these levels are more strongly and contain more flattened pumice fragments and volcanic glass shards than in the UL. A sharp color contrast defines the contact between LL and ML.Major, trace and rare earth element of the Incesu ignimbrite, characterized by their rhyolite, rhyodacite–dacite composition, medium–high K, calcalkaline and peraluminous nature, show fractional crystallization primarily controlled by plagioclase, clinopyroxene, magnetite, ilmenite and titanomagnetite. Sr and Nd isotopic ratios of Incesu ignimbrite display isotopic variations between the ignimbrite levels; they exhibit a limited range in ⁸⁷Sr/⁸⁶Sr (0.7043–0.7049) and ¹⁴³Nd/¹⁴⁴Nd (0.512716–0.512760). The Sr–Nd isotopic ratio of Incesu ignimbrite reveals an age of 3 Ma. The ignimbrite evolved through fractional crystallization and crystal contamination of the parent magma derived from Ocean Island Basalt (OIB) like magma. This suggestion is supported by the AFC modeling based on the trace elements and Sr isotope data.Variation of several major oxide concentrations (Fe₂O₃, TiO₂, CaO and K₂O), trace element concetrations (V, Sr, Cs and Rb) and trace element ratios (Ba/Rb, Sr/, K/Sr, K/Nb, Rb/Sr, Rb/Y and Rb/Nb) versus SiO₂ concentration show the magma chamber that generated the Incesu ignimbrite was compositionally zoned. All geochemical and Sr–Nd isotpic datas can be interepreted to be the result of a subduction related source.