Calc-alkaline Suite Research Articles

Le volcanisme quaternaire d'Anatolie centrale (Turquie): association de magmatismes calco-alcalin et alcalin en domaine de convergence

Collision volcanism in Central Anatolia (Cappadocia) began at least in the late Miocene. Because of the North–South Arabian-Eurasian convergence since this period, the Anatolian block is displaced towards the West along the North and East Anatolian strike-slip faults. Kinematic reconstructions show that the East Anatolian Fault is both sinistral and convergent. As a consequence, the Anatolian block is currently being deformed. Quaternary volcanism in Central Anatolia is represented by several hundreds of monogenetic scoria cones, lava flows, maars, and domes as well as two strato-volcanoes, Hasan Dag and Erciyes Dag. The monogenetic volcanism is bimodal (basalts and rhyolites), whereas the stratovolcanoes exhibit a complete calc-alkaline suite, from basalts to rhyolites. Most of the igneous products are calc-alkaline. Basalts erupted mainly from the monogenetic cones, lava flows, and maars. Andesites are encountered in the strato-volcanoes as lava flows, domes, and nuees ardentes deposits. Dacites and rhyolites occur as ignimbrites and dispersed maars and domes. Volcanic events were recorded up to historical times. Some basalts from monogenetic edifices, contemporaneous with the calc-alkaline suite, exhibit mineralogical and geochemical features that are typical of intraplate alkaline suites, such as normative nepheline, alkali feldspars, and Ti and Cr-rich Cpx. Euhedral microlites of aluminous garnet, although rare, have been observed in basalts, rhyodacites, and rhyolites. This association of contemporaneous calc-alkaline and alkaline suites may be related to collision tectonics.

Geochemical and isotopic studies of syenites from the Yamato Mountains, East Antarctica: Implications for the origin of syenitic magmas

Voluminous syenites were intruded during the waning stage of the granulite facies metamorphism in the Yamato Mountains of East Antarctica. The area has been interpreted as part of a Cambrian continental collision zone with regional upper amphibolite to granulite facies metamorphism occurring during ca. 500–660 Ma period. Regardless of minor geochemical variations between different groups, all syenites are characterised by high K 20 + Na 20 (8–12%), K 20/Na 20 (∼2), Sr (800–3500 ppm), Ba (2000–8500 ppm), and comparatively high TiO 2, P 20 5, Zr, and light REES relative to I-type granites. They are significantly higher in Mg number (50–75) compared with typical calc-alkaline suites, igneous charnockites, or A-type granites and define a distinctive trend on an AFM (alkali-FeO tot-MgO) diagram. Their trace element distribution diagrams are characterised by pronounced enrichment in LIL and REES, large negative Nb and Ti anomalies, and no depletion in Sr or Ba relative to the neighbouring elements. In this regard, they closely resemble the ∼500 Ma post-tectonic mela-syenite to alkali basalt dikes widely occurring in East Antarctica. Such geochemical features are distinct from rift- or hotspot-related syenites, which are usually characterised by low K/Na ratios, negative Ba and Sr anomalies, and a lack of negative Nb anomalies. Initial isotopic compositions of the syenites are characterised by relatively low initial ε Nd values (−2.6 to −5.5) and high Sri ratios (0.7057–0.7088). Since the syenites are extremely enriched in Sr and Nd, such isotopic signatures are interpreted as reflecting the nature of the mantle source, rather than significant crystal contamination. Such isotopic signatures are also distinct from those of the rift- or hotspot-related syenites which are thought to be derived from depleted asthenospheric mantle. Considering the distinctive geochemical signatures of the Yamato syenites and their analogy to posttectonic alkaline mafic dikes in Antarctica, it is proposed that the syenites were generated by fractionation and magma mixing (with a crystal melt) of a Si-undersaturated alkali basaltic magma in a lower-crust magma chamber, followed by further crystal fractionation/cumulate-melt unmixing at middle to upper crustal levels (<5 kbars). Tectonically, it is proposed that the syenites were probably formed within the hinterland of the proposed Cambrian continental collision zone, with the parental magma being derived ultimately by partial melting of the metasomatised mantle wedge above the deepest part of the subduction zone. Similar models may also apply to the origin of some post-tectonic alkaline dikes in East Antarctica, with their sources being the continental lithospheric mantle previously modified by subduction-related processes.

Chlorites in a spectrum of igneous rocks: mineral chemistry and paragenesis

AbstractThe chlorite data presented are from four igneous complexes covering the compositional spectrum of igneous rocks (gabbro to granite) of orogenic and anorogenic settings. The four igneous complexes are; early orogenic gabbro-diorite-tonalite (D-T) suite, late orogenic granodiorite-adamellite (G-A) suite (both are calc-alkaline suites), high-alumina trondhjemite (TR), and anorogenic peralkaline granite (PGR).Chlorites in these igneous rocks show characteristic compositional fields. The Mg vs Fe plot provides the best discriminant, as data points define three compositionally different groups. Phases in the PGR are Fe-rich, siliceous, interlayered chlorite-smectite (Fe/Mg = 8.6), and differ significantly from those in the calc-alkaline D-T and G-A rocks which are Mg-rich chlorites (Fe/Mg = 0.6–0.8). The X-ray diffraction data for the peralkaline granite samples show superlattice reflections at approximately 31 Å (air-dried) and 34 Å (ethylene glycollated), thus suggesting the presence of an expandable (smectite-like) component in this interlayered (chlorite-smectite) phyllosilicate phase. Chlorites in the peraluminous TR rocks contain Fe/Mg values intermediate between the other two types (Fe/Mg = 1.3). Tetrahedral Al (AlZ) values are remarkably low (0–0.5) in phyllosilicates in the PGR, but vary from 1.9–2.5 in chlorites from the other suites. Yet, these chlorite groups with their generally low AlZ values are distinct from the more stable (type IIb) metamorphic chlorites. Sedimentary chlorites are somewhat similar, in their low AlZ values and metastable structural type, to chlorites in igneous rocks.In the calc-alkaline rocks, chlorite may have been formed at the expense of both biotite [biotite + 3M + 3H2O = chlorite + A], and calcic amphibole [2 Ca-amphibole + 6H2O + 5O2 + 1.8Al = 1 chlorite + 8SiO2 + A], where M = Fe, Mg, Al, and A = K, Na, Ca. The alteration of alkali amphibole in the peralkaline rocks may have produced interlayered chlorite-smectite via this reaction; [1 Na-amphibole + 7H2O + 2.5O2 + M = 1 chlorite-smectite + A]. The presence of such interlayered chlorite-smectite which typically form at low T (150–200°C) suggests that the region was not affected by any major reheating events, which is consistent with the nature of the feldspars.

Volcanism above a subducted extinct spreading center: a reconnaissance study of the North Luzon Segment of the Taiwan-Luzon Volcanic Arc (Philippines)

The aseismic ridge system of the South China Sea, extinct since about 17 Ma, subducts below the central part of the Taiwan-Luzon volcanic arc, North Luzon Segment (NLS). The buoyancy of the relatively young and warm crust of the ridge system may be responsible for the high elevation of the NLS. Volcanism in this part of the arc is characterized by the eruption of small volumes of intermediate to felsic magma (57–65% SiO 2). The eruption products from the volcanic front in the central part of the Cordillera form a low-K, calc-alkaline suite, characterized by high Sr and low HREE and Y contents. Some workers consider these characteristics to be the geochemical fingerprints of magmas generated by slab-melting. However, model calculations require the amount of residual garnet in the source to be small (⩽ 10%), in contrast to experimental work that indicates high amounts of residual garnet (⩾ 30%). Alternatively, the melts may have been generated by melting of amphibolitic lower crust. The magmas erupted in the eastern and southern parts of the segment are calc-alkaline suites with moderate to high K contents and Sr and Y contents typical for arc suites. The small volume and evolved composition of the erupted magmas may be related to the uplift of the NLS with the crust acting as a density filter, which prevents more mafic magmas from reaching the surface.

Geochemical and Nd isotopic systematics of granites from the Arunta Inlier, central Australia: implications for Proterozoic crustal evolution

Geochemical and SmNd isotopic results are reported for granites from the Proterozoic Arunta Inlier of central Australia. These, combined with new geochronological data for the granites (Zhao and Bennett, 1995), allow important constraints to be placed on Proterozoic tectonic and crustal evolution in central Australia. Granites from the Arunta Inlier can be divided into three major geochemical groups, a Calcalkaline-trondhjemitic Group (CAT), a High-heat-production Group (HHP), and a volumetrically significant Main Group. The CAT Group, which occurs only in the southern margin of the inlier, is characterised by high Na 2O, Na K , Sr, K Rb and Sr Y , and relatively low K 2O, Rb, Rb Sr , Th, U, REE, Nb and Y, analogous to calc-alkaline suites occurring in modern convergent plate margins. The HHP Group, which occurs mainly in the interior of the Arunta Inlier and is spatially associated with the Main Group, is characterised by high K, Rb, Th, U, Rb Sr and Rb Zr , and relatively low Sr, Ba, Na K , K Rb , Ba Rb , MgO, Cr and Ni. The Main Group, which occurs throughout the Arunta Inlier, is geochemically intermediate between the CAT and HHP groups. It is geochemically analogous to the 1880-1850 Ma old Barramundi Igneous Association recognised in other Proterozoic terrains of central Australia, and can be further subdivided into four age subgroups, 1820, 1780-1750, 1650 and 1615-1600 Ma, respectively. All three groups show similar negative Nb anomalies on the trace-element-normalized diagrams. Despite the geochemical diversity of the Arunta granites, no correlations between the Nd isotopic and geochemical signatures are observed. The ranges of initial ϵ Nd values and Nd-depleted mantle model ages ( T Nd DM)) for the three groups of granites overlap with each other. Overall, there are two groups of T Nd DM ages, the most common ranging from 2.33 to 1.96 Ga, and the other from 1.83 to 1.72 Ga. Although initial ϵ Nd values of the granites show large variations, there is a general trend for the ϵ Nd values to increase (i.e., become less negative) with decreasing crystallization ages. The large geochemical and isotopic variations of the Arunta granites reflect considerable heterogeneity in the sources of the granites and do not support a uniform, 2.3-2.1 Ga old Barramundi-type underplate source for the origin of the granites. The combined geochemical and Nd isotopic data suggest the sources of the granites contain at least two main components, an Palaeoproterozoic mantle-derived component and an older crustal component. The older crustal component was incorporated into the source region of the granites either through magma assimilation of crustal materials at lower crustal levels or more probably via sediment subduction and mantle wedge metasomatism. The increasing ϵ Nd values with decreasing crystallization ages may be a result of progressively increasing proportions of newly accreted island-arc materials being incorporated in the subducted sediments. Alternatively, a juvenile mantle-derived component may have been added to the source region of the younger granites. It is apparent that the Nd isotopic signatures in the granites has resulted from complex processes and neither simple mixing nor simple two-stage protolith models can as yet satisfactorily explain the observations. The petrogenesis of the three geochemical groups of granites can be interpreted in a plate-tectonics scenario. It is considered that the CAT Group was formed by fractionation of arc-type magmas and/or partial melting of arc-related intrusions or underplates in a subduction-related continental margin setting, whilst the formation of the Main Group may involve partial melting of fractionated and/or modified arc-related underplates, but not necessarily in a subduction environment. The younger HHP Group was probably generated by remelting of the coexisting Main Group during subsequent tectonothermal events.

Petrology of Calc-Alkaline Lavas at Volc n Ollag e and the Origin of Compositional Diversity at Central Andean Stratovolcanoes

Volcán Ollagüe (21°17'S) is a large stratovolcano that lies slightly east of the main axis of Quaternary Volcanoes in the Andean Central Volcanic Zone (CVZ). Euptive products range from basaltic andesite to dacite and define a high-K, calc-alkaline suite. This compositional range is similar to the collective compositional range of the other stratovolcanoes in the CVZ, and it provides a record of both early and late-stage differentiation processes operating at the stratovolcanoes. The volumetrically dominant andesitic and dacitic lavas are divided into four eruptive series on the basis of vent locations and petrography. In ascending stratigraphic order they are: the Vinta Loma, Chasca Orkho, post-collapse, and La Celosa series. Whole-rock compositions of the lavas are remarkably similar regardless of eruptive series. Variations in phenocryst assemblages and magmatic fo2 however, suggest differences in subliquidus volatile contents for magma chambers developed beneath the summit of the volcano versus those developed beneath the flanks. Basaltic andesite magmas are principally preserved as quenched inclusions within the andesitic and dacitie lava flows. Large ranges in isotopic ratios over a narrow compositional range indicate that the basaltic andesites were derived by crystal fractionation coupled with large amounts of crustal assimilation. Increasing Ce/Yb ratios with decreasing Yb contents further suggest that this initial stage of differentiation occurred at deep crustal levels where garnet was stable. Additional supporting evidence for differentiation in the deep crust includes isotopic and trace element compositions that indicate assimilation by the basaltic andesite magmas of a crust different from upper-crustal rocks exposed at present in the region. Whole-rock major and trace element trends of the dacitic lavas can be simulated largely by fractional crystallization of parental andesitic magma. The fractionating assemblages for the different eruptive series are consistent with the observed modes of the parent magmas. Small increases in Sr isotope ratios with increasing Rb contents indicate that the fractionating magmas also assimilated small amounts of wall rocks similar in composition to the upper-crustal basement to the volcano. Consideration of the chemical trends, mineral compositions, and eruptive history of Ollagüe rocks permits construction of a model for the evolution of shallow crustal magma chambers beneath the stratovolcanoes in the CVZ. At a relatively mature stage, the magma chambers may be compositionally, thermally, and density stratified. Temperatures estimated from Fe-Ti oxide and pyroxene thermometry for the chambers beneath Ollagüe range from 1000 to 790°C with increasing SiO2 from 59 to 67 wt.% in the upper reaches, and from 1150 to 1020°C with increasing SiO2 from 53 to 59 wt.% in the lower reaches. The occurrence of basaltic andesite magmatic inclusions within the intermediate lavas and the repeated eruption of monotonous composition andesitic magmas indicate that the shallow chambers are periodically replenished with parental basaltic andesite magmas. Ubiquitous, reversely zoned plagioclase and pyroxene phenocrysts in the lavas at Ollagüe suggest that convective cooling of the basaltic andesite releases buoyant derivative liquid that mixes with the overlying intermediate-composition body of the chambers. Further crystallization and differentiation of the intermediate magmas may take place in solidification zones at the boundaries of the magma chambers. If so, the return of residual liquid from the crystallizing margins and mixing with the interior are highly efficient such that magma differentiation can be modeled as a simple, homogeneous, fractional crystallization process.

Origin and regional significance of late Precambrian and early Palaeozoic granitoids in the Pan-African belt of Somalia

Granitoids within the Precambrian basement of north-eastern and southern Somalia are subdivided on the basis of geology, geochronology and petrology into three different assemblages. The post-kinematic assem- blage in north-eastern Somalia (~ 630 Ma) comprises granodiorites and granites which belong to a medium-K calc-alkaline suite. Average initial Sr, Nd and Pb isotopic ratios (Sri = 0.7048, end = --1.8, 2°6pb/2°4pb(i) = 17.704 and 2°Tpb/2°4pb(i)= 15.611) indicate that these melts were derived from a mantle or juvenile crustal source with only slight involvement of pre-existing crust as a contaminant. Two different assemblages are found in southern Somalia. The older assemblage is composed of crustal anatectic, synkinematic, parautochthonous grani- tes (~ 600 Ma) related to amphibolite facies retrogres- sion of an intensively reworked pre-Pan-African crust (Sr i = 0.7100, ~Na = --8.4, 2°6pb/2°4pb(i) = 15.403 and 2°7pb/2°4Pb(i) = 15.259). Thesemonzo-andsyenograni- tes are moderately potassic and peraluminous. The younger assemblage (~ 470 Ma) consists of post-kine- matic monzonites to syenogranites with A-type affinities. Initial Sr, Nd and Pb isotopic data for this metaluminous assemblage (Sri = 0.7114, end = --13.1, 2°6pb/2°4pb(i) = 16.913 and 2°TPb/2°4pb(i) = 15.512) indicate a signifi- cant lower crustal component but, however, also a mantle signature. The late Proterozoic to early Palaeozoic grani- toids in Somalia thus express contrasting regimes, charac- terized by strong juvenile input in the north, close to the Arabian- Nubian Shield, whereas intense crustal rewor- king with little addition of juvenile material prevailed in the south. Somalia was definitively not a cratonic area

Early calc-alkaline magmatism in the Mogollon-Data Volcanic Field, New Mexico, USA

The Mogollon-Datil Volcanic Field in southwest New Mexico was active between 40 and 20 Ma. It is situated c. 500 km from the western North American plate margin, and yet the early pre-30 Ma volcanic rocks comprise a high-K calc-alkaline suite ranging from basaltic andesites to dacites. All the samples are characterized by high and variable LILE and LREE contents, and LILE/HFSE ratios, and variable Nd and Sr isotope ratios of 0.51231–0.51250 and 0.7055–0.7065 respectively. In detail, three groups are recognized: Group 1 rocks have the lower SiO 2 , incompatible element abundances and 87 Sr/ 86 Sr ratios; Group 2 preserves the clearest evidence for open system differentiation, and Group 3 exhibits little variation in 87 Sr/ 86 Sr and the most coherent major and trace element arrays. The Group 2 rocks have been modelled in terms of simple binary mixing between selected Group 1 basaltic andesites and a higher silica crustal endmember, which appears to have been derived from the mid-lower crust in the presence of residual garnet. The Group 3 rocks, by contrast, are consistent with near closed system differentiation, modelled in three stages and involving Plag-Aug-O1-Mag and subsequently hornblende and K-feldspar. Published experimental data indicate that the differentiation of the parental basaltic magmas took place at elevated pressures of c . 4-5 kbar, and water contents, c . 2wt% H 2 O. It is argued that the parental magmas to these calc-alkaline suites were generated in a period of lithospheric extension, and that they were derived from within the continental mantle lithosphere, as inferred for mid- to Late Tertiary rocks elsewhere in the western US, with no significant contribution from contemporaneous subduction processes.

Crust and mantle contributions to granite genesis — An example from the Variscan batholith of Corsica, France, studied by trace-element and NdSrO-isotope systematics

The Corsican batholith, formed during the Variscan orogeny, was studied with particular emphasis on the respective roles played by crustal- and mantle-derived material in granite genesis. Major- and trace-element data identify: the various types of magma and their genetic relationships; the magmatic processes that gave rise to the observed rocks; and the primitive liquids. OSrNd-isotope systematics constrain the origin of primitive liquids. Trace-element models provide data on the various source materials and their possible geodynamic setting. The evolution of two main calc-alkaline plutonic associations is compared: an older high-MgK association (U 1) with both ultrapotassic mafic and silicic rocks, the latter ranging from monzonite to leucosyenogranite, and a younger calc-alkaline composite association (U 2) involving a mafic cumulate sequence and granodiorite to leucomonzo-granite. Both types of granite are accompanied by mafic units that were non-cogenetic with surrounding granitic melts. Trace-element calculations indicate that a primitive liquid of monzodioritic composition gave rise to the U 1 MgK granite by fractional crystallization, amphibole and titanite playing a major role, which, with increasing crystallization, gave decreasing REE contents without a strong negative Eu anomaly. The U 2 calc-alkaline suite was formed by fractional crystallization involving feldspar and LREE-rich minerals, i.e. monazite, from a liquid of monzogranitic composition. Field and petrographical data identify magma mingling in both U 1 and U 2, between mafic and silicic rocks, but geochemical data only indicate how magma-mixing processes led to U 2 granodiorite. Geochemical modelling shows that a single protolith of calculated graywacke composition yielded the two associations under different melting conditions: the high-MgK monzodiorite melt was formed after partial melting of a 30% non-modal batch of granulite-facies metamorphic protolith (low P H 2O ), but the calc-alkaline monzogranite was formed by the same process in an amphibolite-facies source of similar composition (higher p H 2O ). The primitive granite magmas of both associations show the same crustal characteristics, i.e.: Sr i = 0.706–0.707; ϵ Nd ( t) = −4.3 to −2.2 and δ 18 O = +7 to +8‰ . This provides evidence that composition of both U 1 and U 2 granite melts probably was controlled by physico-chemical fusion rather than protolith-composition parameters. The U 1 ultrapotassic mafic rock is thought to be of mantle origin, i.e. a deep source containing phlogopite (±garnet). Zone refining led to a significant increase of incompatible trace-element contents during ascent of the magma. Mantle-crust interaction lowered the La/Yb and 143 Nd 144 Nd ratios, but the 87 Sr 86 Sr ratio increased. Nevertheless, interaction with associated U 1 MgK granite was not important and the crustal component is thought to be different from the associated granitic magma. In U 2, the mafic cumulate sequence, clearly of mantle origin, has E-MORB characteristics and seems the result of 10% non-modal partial melting of a heterogeneous mantle of spinel or amphibole peridotite without garnet. Some interaction occurred with the associated granitic melt. The E-MORB character of the mafic rock indicates that a large part of the batholith, and at least the U 2 magmatic association, was generated under post-collisional extensional conditions.

The Guerrero suspect terrane (western Mexico) and coeval arc terranes (the Greater Antilles and the Western Cordillera of Colombia): a late Mesozoic intra-oceanic arc accreted to cratonal America during the Cretaceous

The Guerrero suspect terrane, composed of Late Jurassic-Early Cretaceous sequences, extends from Baja California to Acapulco and is considered to be coeval with the late Mesozoic igneous and sedimentary arc sequences of the Greater Antilles, the West Indies, Venezuela and the Western Cordillera of Colombia. These sequences represent the remnants of an arc which accreted to the North American and northern South American cratons at the end of the Cretaceous. In western Mexico, the arc sequences built on continental crust consist of high-K calc-alkaline basalts, andesites and rhyolites enriched in LREE with abundant siliceous pyroclastic rocks interbedded either with Aptian-Albian reefal limestones or red beds. They do not show magmatic changes during the arc development. In contrast, the arc sequences built on oceanic crust show an evolution with time. Arc activity began with the development of depleted low K-tholeiitic mafic suite (Guanajuato igneous sequence), followed first by mature tholeiitic basalts and then by calc-alkaline olivine basalts interbedded with micritic limestones and radiolarian oozes of Early Cretaceous age. At the end of the arc growth, during Aptian-Albian times, calc-alkaline pillow basalts and and esites poured out in the volcanic front while shoshonitic olivine basalts extruded in the back arc. The tholeiitic and shoshonitic mafic rocks as well as the calc-alkaline lavas are mildly enriched in LREE, Y and Nb and show high ϵ Nd ratios, typical of oceanic arcs. In contrast, the calc-alkaline mafic suite enriched in LREE, Y and Nb exhibits lower ϵ Nd ratios suggesting that it was derived by the partial melting of a mantle source contaminated either by Paleozoic subducted sediments or old source enrichments (OIB). The Cretaceous arc rocks of the Greater Antilles, interbedded with and/or capped by Aptian-Albian limestones, the Cretaceous andesites of northern Colombia, the Cretaceous tholeiitic and calc-alkaline volcanic rocks of Venezuela, and the Cretaceous volcano-plutonic arc assemblage of Tobago share a similar magmatic evolution with the western Mexican oceanic arc. The tholeiitic plutono-volcanic assemblage of Tobago, depleted in LREE and characterized by high ϵ Nd values is similar to the Guanajuato volcano-plutonic sequence of Mexico, considered to represent the pristine stage of the arc. The mature tholeiitic sequences exposed in the proto-Caribbean arc show flat to moderately enriched LREE patterns like those of the Guerrero terrane. However, felsic plutonic and volcanic rocks prevail in the Caribbean. Calc-alkaline suites, accompanied locally by shoshonitic lavas, characterize the end of arc magmatic activity in both places. Thus, the geochemical features of the Late Jurassic-Cretaceous arc series of the Guerrero terrane and the proto-Caribbean are consistent with the following plate tectonic model. The Guerrero terrane and the proto-Caribbean probably belonged to the same intra-paleo-Pacific arc system the development of which was related to the subduction of oceanic basins fringing the North and northern South American cratons. This subduction zone was WSW dipping. While subduction was going on, these magmatic arcs drifted, moved closer to the North and South American cratons, and finally collided with the American borderlands at different periods during the Cretaceous. The late Mesozoic Guerrero and proto-Caribbean arc sequences show striking similarities with the Miocene calc-alkaline lavas dredged from the Banda Ridges, the North Marianas Seamount Province, and the Halmahera and Philippine arcs. We suggest that the diverse but mostly submarine segments of this late Mesozoic intra-Pacific arc rimmed the North and South American cratons as much as these Tertiary arcs rim Southeast Asia.

Petrogenesis of Hercynian peraluminous granites from the Calabrian Arc, Italy

The Capo Rasocolmo (CR) and Villa San Giovanni (VSG) granitoids, in the Southern Calabrian Arc (CLA), form post-tectonic unzoned intrusions in migmatitic paragneisses. They are strongly peraluminous (two-mica aluminosilicate-bearing), showing significant small-scale mineralogical and chemical heterogeneities with compositions ranging from leucotonalite to monzogranite. The CR and VSG granitoids are characterized by SiO 2 contents varying from 67 to 75%, low mafics (TiO 2 + FeO t + MgO < 3.9%) AND HIGH AL 2 O 3 (A/CNK = 1.20-1.40) with low Rb and high Ba contents and strongly fractionated REE patterns (La n /Yb n = 34 and Yb n = 3.7, on average). They show broad trends in major and trace element compositions which define linear regressions on variation diagrams. The granitoids have an «S»-type signature in terms of their enclaves, mineralogy and zircon typology, whereas their geochemical characteristics are similar to those of late- to post-collision granites. Initial Sr isotopic ratios (0.7094-0.7107) are lower than that of the regionally exposed basement rocks and similar to that of the coeval hybrid calc-alkaline granitoids; ∈ NdI values range from -6.7 to -9.1. Petrographic and geochemical evidence indicate a crystal-liquid unmixing process as the dominant mechanism involved in granitoid evolution. A two-stage model was tested on the CR granitoids, involving fractional crystallization plus assimilation followed by partial solid-liquid segregation during the emplacement of the granite magma; the model accounts very well for the observed geochemical and isotopic variations. The ultimate origin of the CR and VSG granitoids investigated in the present study. A direct derivation from shale-like protoliths, suggested by field data, mineralogy, is consistent with REE patterns and experimental melting data, but is not supported by Ba-Rb decoupling and the Sr isotopic data. A comparison with the classical S-type granites of the Lachlan Fold Belt also excludes an S-type source for the CR and VSG granitoids. A direct derivation of the CR and VSG granitoids from a coeval calc-alkaline suite, through low pressure fractionation processes, is not consistent with the geochemical trends and REE patterns. An origin from a distinct calc-alkaline magma batch, involving both crustal and subcrustal components, is favoured as the most realistic petrogenetic model

Volcanological and petrological evolution of Volcan Tata Sabaya, SW Bolivia

Tata Sabaya (19°08'S 68° 32'W; 5430 m a.s.l) is located on the northern shores of Salar de Coipasa on the Altiplano of Bolivia. It is a 1700-m-high symmetrical composite cone; one of two Holocene centers in a chain of edifices trending E-W from the main N-S-trending volcanic arc of the Andean Central Volcanic Zone (CVZ). Four main stages are envisaged for the evolution of the edifice. Stage 1: formation of a pyroclastic “shield” possibly around a central dome complex. Stage 2: formation of an andesitic composite cone on top of the early pyroclastic sequence. Stage 3: collapse of a 120° sector of the southern flank of the volcano which produced a prominent debris avalanche deposit covering ∼300 km 2 of the Salar de Coipasa to the south. Collapse was induced by inflation of the edifice by an input of fresh magma into the sub-volcanic magma chamber. Stage 4: flows and domes which have since healed the edifice. Tata Sabaya shows a restricted range of whole-rock compositions, most units fall in the range 60–62.5% SiO 2 with a general evolution to the more silicic stage 4 lavas through time. The typical phenocryst assemblage is plagioclase > hornblende > hypersthene > augite > oxide. Biotite is found in the stage 4 lavas and is dominant over pyroxene. Hornblende and biotite are thought to be a late-stage sub-liquidus products. By far the most mafic compositions at Tata Sabaya are the hornblende and plagioclase inclusions in two of the stage 4 domes (∼ 52% SiO 2). Macroscopic, microscopic, and chemical evidence for magma mixing (mixed phenocryst and matrix populations, resorption features, mixed glass compositions, contraindicative temperature indicators etc.,) are found throughout Tata Sabaya. The whole-rock chemistry bears out that magma mixing was a dominant process and that it was responsible for buffering the composition of the magma in an open magmatic system. Despite being located 35km to the east of the main arc front the rocks of Tata Sabaya are compositionally similar to those from other centers in the northern (17–19°S) segment of the CVZ. The high K 2O, calc-alkaline suite is characterised by LREE and extreme LILE enrichments (up to 1800 ppm Ba and Sr). 87Sr/ 86Sr (0.7056–0.7068) and 143Nd/ 144Nd (0.51217–0.51227) are similar to other CVZ lavas; this is regarded as a baseline signature derived during deep crustal contamination. This is corroborated by Pb-isotope data which indicates influence of the low 206Pb/ 204Pb Precambrian basement that is thought to underlie the northern portion of the CVZ. Modification of the isotope ratios during open system processes at shallow level is also indicated. The most extreme incompatible element enrichments are found in the quenched mafic inclusions. These characteristics have been reported from other Holocene mafic lavas elsewhere in the northern CVZ, and may represent a distinct recent, magmatic episode.

Volcanic, sedimentary and tectonostratigraphic environments of the ∼3.46 Ga Warrawoona Megasequence: a review

The Warrawoona Megasequence comprises a ∼3.46 Ga assemblage of tholeiitic and calc-alkaline volcanic rocks interlayered with cherty sedimentary rocks. Deposition occurred in a range of shallow- to deeper-water environments ranging from the shoreline to distal deposits in sediment starved basins. Volcanism provided the dominant, or only, source of clastic sediment. Sedimentary rocks also contain evaporites, probable stromatolites, other evidence for the existence of microbial life and possible evidence of meteorite impact. The tectonostratigraphic assemblage and its contained facies associations are most similar to those developed in younger volcanic-arc, or near-arc settings. The compositions of volcanic rocks are also comparable with those of younger tholeiitic and calc-alkaline suites developed above subduction zones. The calc-alkaline suite has a complex petrogenesis which probably involves mantle-derived basaltic melts, intermediate to silicic melts derived from subducted mafic crust, magma mixing and fractional crystallization. Intense hydrothermal alteration and a distinctive association of metal deposits are also compatible with the interpretation that the Warrawoona Megasequence contains examples of early Archaean volcanic-arc and near-arc assemblages.

Origin of the 3500-3300 Ma calc-alkaline rocks in the Pilbara Archaean: isotopic and geochemical constraints from the Shaw Batholith

Calc-alkaline plutonic rocks, intruded at ∼3450Ma, comprise a major component of the Shaw Batholith in the Archaean east Pilbara Block, Western Australia. New whole-rock Pb isotopic geochronology confirms the extent of these rocks, but a minor plutonic phase is dated at 3338±52 Ma and represents a second plutonic event of the same age as much of the nearby Mt Edgar Batholith. The Sm-Nd isotopic systematics of the ∼3450Ma rocks imply their derivation from a heterogeneous source, which probably included a slightly older crustal component as well as a depleted mantle component. The 3338±52 Ma pluton includes components derived from crustal sources older than 3600 Ma. The geochemistry and SmNd isotopic systematics of these rocks are consistent with crustal growth in the early Archaean from upper mantle sources as depleted as the modern upper mantle. The Shaw Batholith calc-alkaline suites exhibit very similar chemical trends on variation diagrams to modern calc-alkaline plutonic rocks which can be modelled by a combination of mixing and fractionation. A suite collected from outcrops displaying prominent igneous layering shows distinct geochemical trends which can be modelled by differentiation into a component enriched in ferromagnesian minerals, principally hornblende, and possibly sphene, magnetite and epidote, and into a leucocratic component containing quartz, plagioclase and K-feld-par. These Archaean calc-alkaline plutonic rocks, in common with rocks from many other Archaean calc-alkaline provinces, exhibit very fractionated REE patterns with depleted HREE contents, a feature considered to result from equilibrium with garnet at depth in lower crustal regions. The geochemistry of the Pilbara Archaean calc-alkaline rocks is identical to the subset of modern continental-margin calc-alkaline plutonic rocks with fractionated REE patterns, such as those from the central and eastern Peninsular Ranges Batholith, western USA. The tectonic setting in which the Archaean calc-alkaline rocks formed is still not known. This reflects both uncertainty associated with the petrogenesis and environments of modern calc-alkaline rocks, as well as the limited knowledge of the precise timing and relationships of plutonic, depositional and tectonic events in the Pilbara Archaean.

Intra-oceanic settings of the western Mexico Late Jurassic-Early Cretaceous arc sequences. Implications for the Pacific-Tethys geodynamic relationships during the Cretaceous

The Guerrero suspect terrane composed of Late Jurassic-Early Cretaceous sequences, extends from Baja California up to Acapulco and is considered to be coeval with the Late Mesozoic igneous and sedimentary arc sequences of the Greater Antilles, Venezuela and Western Cordillera of Colombia. New geological, petrological and geochemical data from central and southern Mexico, led us to propose a new model for the building of the Alisitos-Teloloapan arc. This arc, partly built on the Pacific oceanic lithosphere and partly on continental fragments, could be related to the subduction of an oceanic basin - the Arperos basin - under the Paleo-Pacific plate. This subduction was dipping southwest. At the beginning of the magmatic activity of the oceanic segment of this arc, depleted tholeiitic basalts were emitted in a submarine environnement below the CCD. While subduction was going on, the arc magmas evolved from LREE depleted tholeiites to slightly LREE enriched tholeiites and then, to calc-alkaline basalts and andesites enriched in LREE and HFSE. Concurrently, the arc sedimentary environment changed from deep oceanic to neritic with the deposition of Aptian-Albian reefal limestones, at the end of the arc building. In the continent-based segment, the arc magmas are exclusively differentiated calc-alkaline suites depleted in HREE and Y, formed of predominantly siliceous lavas and pyroclastic rocks, emitted in a sub-aerial or shallow marine environment. Thus, taking into account this above mentioned model, the Cretaceous volcanic series, accreted to the margins of cratonal America, in Colombia, Venezuela, Greater Antilles and Mexico, could be related to the same west-south-west dipping subduction of oceanic basins, fringing the North and South American continental cratons and connected directly with the inter-American Tethys. While the subduction was proceeding, this magmatic arc drifted towards the North and South American cratons and finally, collided with the continental margins at different periods during the Cretaceous.

Archean shoshonites from the Abitibi greenstone belt, Chibougamau (Québec, Canada): geochemistry and tectonic setting

Abstract Shoshonitic and calc-alkaline lava flows, dykes and sills of the Opemiska Group in the Chibougamau region of the northeastern part of the Archean Abitibi greenstone belt (Quebec, Canada) were emplaced in a sedimentary basin prior to the major phase of Archean deformation in the Chibougamau region (ca. 2695–2700 Ma). Petrography and geochemistry delineate feldspar-phyric (SiO 2 ca. 52–57%) and amphibole-pyroxene-phyric (SiO 2 ca. 51–53%) shoshonitic members; the spatially related calc-alkaline rocks include basalts, basaltic andesites and rare andesites (SiO 2 ca. 51–59%). Many of the shoshonitic and calc-alkaline rocks were affected by the accumulation of clinopyroxene (±amphibole) and plagioclase. The distribution of trace elements in the shoshonitic and calc-alkaline suites suggests that the parental magmas were derived by variable degrees of melting of a heterogeneous upper mantle source containing at least three components in a spinel stability field. Mantle-normalized trace element plots show patterns with distinct LILE and LREE enrichment accompanied by Nb, Ta and Ti depletion characteristic of magmas generated by subduction-related processes. Modern analogues such as those from the Aeolian Islands in the Mediterranean Sea have strikingly similar geochemical signatures supporting the inference of Archean plate subduction. The magmas were emplaced in a marginal or backarc basin of a mature island arc during extension in an overall convergent tectonic regime prior to major N-S Archean deformation.

Geochemistry of the Cambrian volcanic-hosted massive sulfide-rich Mount Read Volcanics, Tasmania, and some tectonic implications

The Cambrian Mount Read Volcanics belt in western Tasmania hosts five major gold-rich massive sulfide deposits and remains the focus of intense exploration activity and geologic research. A geochemical study of least altered lavas and shallow intrusions, with emphasis on basaltic to andesitic compositions, was carried out to determine the primary magmatic affinities and tectonic setting of eruption of the Mount Read Volcanics, to aid in internal correlations within the belt, and to provide starting-material estimates for mineralization-related alteration studies.Three calc-alkaline suites (one extending to shoshonitic compositions) and two tholeitiic suites have been distinguished within the Mount Read belt. Suite I is voluminous and includes the Eastern sequence, Central Volcanic Complex, Tyndall Group, the intrusive quartz-feld-spar porphyries and granitoids, and the andesitic lavas of the Que-Hellyer footwall sequence. Suite I andesites are transitional, medium to high K calc-alkaline rocks ((La/Yb) N = 5-12, avg 8.1) and are the least light REE-enriched calc-alkaline lavas in the Mount Read Volcanics.Suite II comprises intrusive and extrusive, often hornblende-porphyritic andesites and dacites mainly from the upper part of the southern Central Volcanic Complex. They are more P 2 O 5 and light REE enriched ((La/Yb) N = 10-26, avg 16.7) than suite I and have high K calc-alkaline affinities.Suite III includes basaltic and andesitic lavas from the Que-Hellyer hanging-wall sequence in the northern part of the belt, the Lynch Creek basalts near Queenstown, and intrusive basalts in the Howards Plains area northwest of Queenstown. Suite III rocks show a striking compositional range, from low TiO 2 (0.4-0.5%), low P 2 O 5 (<0.1%) basalts with (La/Yb) N values from 8 to 12 and almost flat heavy REE patterns, to low TiO 2 (0.4-0.8%) but strongly to exceptionally P 2 O 5 and light REE-enriched basalts with up to 350 times chondritic La and (La/Yb) N values up to 34. The latter are regarded as shoshonites.Tholeiitic pillow basalts of the Henty fault wedge, and the closely related dikes of the Henty dike swarm, constitute suite IV: these have TiO 2 contents from 1.0 to 1.6 percent, very low Nb contents (<3 ppm), and only slightly light REE-enriched REE patterns ((La/Yb) N = 1.4-3.4). They are considered to be most similar to suprasubduction zone basalts erupted during the early phase of arc splitting and back-arc basin development and indicate a period of tension and rifting of the Mount Read belt in post-Central Volcanic Complex but pre-Tyndall Group time.Suite V is constituted by the Miners Ridge basalts and includes low TiO 2 (<0.7%) basalts with strong light REE depletion ((La/Yb) N <0.5) and higher TiO 2 (to 2.6%) lavas with only slight light REE depletion ((La/Yb) N = 0.9). Suite V basalts are tentatively correlated with rift tholeiite sequences associated with the Crimson Creek Formation, and therefore, are regarded as part of the pre-Mount Read Volcanics basement.The three calc-alkaline suites occur in a general stratigraphic order (with suites I and II being more or less coeval near Queenstown); they indicate a magmatic evolution from transitional medium to high K rocks through high K rocks, to strongly enriched shoshonitic rocks. Closest modern analogues are postcollisional volcanics in regions of arc-continent collision (e.g., East Papua and the Roman province).The occurrence of boninite-derived chromites (probably derived from the allochthonous mafic-ultramafic complexes outcropping just west of the Mount Read belt) in two sedimentary units within the Mount Read Volcanics sequence, below suite III basalts, supports a postcollisional origin for at least the major part of the Mount Read Volcanics. Other similar postcollisional sequences, particularly if formed in a submarine setting, must be considered highly prospective for VHMS deposits.

A general review of volcanogenic massive sulphide deposits in China

Volcanogenic, massive sulphide deposits in China have, for the past forty years, been an important source of Cu, Zn, Pb and sulphur, and of a number of associated elements such as Au, Ag and Co. The areal distribution of the deposits is determined by the different geological settings in which they occur and which range in time from Archean to Triassic. Host-rock lithologies comprise felsic, mafic and ultramafic metavolcanic rocks, with or without subsidiary, intercalated metasedimentary rocks; four groups of ore associations, based on chemical composition, have been distinguished: Cu, CuZn, CuFe and PbZnCu. Only one deposit of economic significance (Hongtoushan) has been recognised so far in an Archean greenstone belt. This ore is hosted by metafelsic volcanic rocks of a calc-alkaline suite. The Proterozoic was an important period for the formation of two groups of volcanogenic massive sulphide deposits in China. Early Proterozoic CuFe deposits, such as Dahongshan and Lalachang, are predominantly distributed in graben-like depressions within cratons. Late Proterozoic CuZn deposits, e.g., Xiqiu and Liujiaping, occur mainly along the edges of the craton. The ores of all Proterozoic deposits are hosted by quartz keratophyre. The most important metallogenic epoch for this type of ores in China was the Lower Palaeozoic and is represented by a number of deposits situated in the northern Qilian Caledonian orogenic belt. Within this belt, the various groups of deposits can be related to specific plate-tectonic settings. CuZn or PbZnCu ores occur in quartz-keratophyric tuffite, with or without intercalated sediments, of island-arc environments (Houyanshan, Zheyaoshan, Xiaotieshan); Cu ores are hosted by spilite, generated probably in back-arc basin settings (Honggou); while CuZn ores presumably occur in metabasalts of oceanic ridge or off axis settings (Yendonggou). Volcanogenic, massive sulphides are relatively unimportant in the Late Palaeozoic, both with regard to size and numbers, but a rather unusual and important CuZn deposit, hosted by ultramafic rocks (Deerni) is ascribed to this epoch. An important PbZnCu deposit (Gacun) that is comparable with the Kuroko deposits was formed in the Early Mesozoic Yidun orogenic belt, which is considered to represent a mature arc environment. The ores are hosted by rhyolites within calc-alkaline volcanites of dominantly andesitic and rhyolitic composition. The very important sedex-type stratabound and stratiform base-metal deposits in China seem to have occurred mainly at times when the volcanogenic massive sulphide deposits became relatively unimportant. For instance, the Early Proterozoic, Early Palaeozoic and Early Mesozoic, are the main periods for the formation of volcex-type massive sulphides, while the Middle Proterozoic and Middle-Late Palaeozoic are most important for sedex-type, stratiform base-metal sulphides.

Petrochemistry and geotectonic setting of the Shalair granite, NE Iraq

The igneous history of the (U. Cretaceous-Miocene) Shalair granite of NE Iraq shows great similarity to the post Caledonian Cheviot granite of NE England. The Shalair granite comprises at least one igneous cycle (made up of two rock units) starting with basic ganodiorite and including a late porphyritic igneous unit. It is a high level pluton made up of a number of intrusive phase intruding a pile of andesite of a similar composition to the bulk of the plutonics. The whole represents a multicomponent ring dyke, whose top is near the present level of erosion. Geographically the Shalair granite forms one of a group of granitic intrusion all lying to the East of the Zagros suture. They provide a strong evidence of close links in space and time with the Shalair granite. At Shalair a whole spectrum of igneous rocks from calc-alkaline to alkaline affinities could have formed by subduction followed by a collisional stage. The subduction of the Arabian plate under the Iranian plate was active during U. Cretaceous-Paleocene producing calc-alkaline suites cropping at Sirstan, Aulan, Mishao and Laladar. Eventually the subduction process stopped by continental collision, most probably during the Late Eocene-U. Miocene forming a special kind of S-granite cropping out at Damamna.

Distinctive REE patterns for tholiitic and calc-alkaline magma series co-occurring at Adatara volcano, Northeast Japan.

Rare earth element (REE) concentrations were precisely determined by inductively coupled argon plasma/atomic emission spectrometry (ICP) for 9 tholeiitic and 13 associated calc-alkaline rocks from Adatara volcano, located at the volcanic front of Northeast Japan arc. The tholeiitic samples commonly display nearly flat REE patterns, whereas the calc-alkaline samples characteristically show concave light REE-enriched patterns. In both suites, the REEs, with the exception of Eu, are positively correlated to varying extents with K2O and other incompatible elements. Eu concentrations in the calc-alkaline suite do not vary significantly, resulting in significant negative Eu anomalies. In the tholeiitic suite, chondrite-normalized (Ce/Yb)CN ratios are low and show little variation (1.54–1.80) over a significant range in SiO2 content (52–62 wt%). In contrast, (Ce/Yb)CN in the calc-alkaline suite show much greater variation (2.17 to 2.94) over a smaller range in SiO2 content (56 to 62 wt%) and increase systematically with SiO2. REE data for the tholeiitic suite are consistent with a crystal fractionation model that successfully reproduces the observed major and trace-element variations. A similar fractionation model is compatible with major-element and some trace-element variations for the calc-alkaline suite, but results in excessively high heavy REE contents, and fails to reproduce the elevated (Ce/Yb)CN ratios for the most acidic samples. The preferential enrichment of light REEs may reflect additional processes such as: (1) assimilation, (2) incorporation of a differentiated liquid (or evolved magma) that has undergone fractionation of heavy REE-rich minerals, and (3) mixing of primary magma derived by a small degree of partial melting from a common (amphibolitic or hornblende gabbroic) source. Although the tholeiitic and calc-alkaline suites possess distinct 87Sr/86Sr, Rb/Ba and Zr/Nb ratios, different magma sources are not required to produce the distinct REE patterns of the coexisting suites. An equilibrium batch partial melting model can reproduce both the flat (tholeiitic) and concave (calc-alkaline) REE patterns from a common peridotitic source by larger (10–15%) and smaller (5%) degrees of partial melting, respectively. Therefore, both tholeiitic and calc-alkaline suites could have been derived from a common lherzolitic magma source, provided that the calc-alkaline suite underwent incorporation (assimilation) of isotopically distinct (lower crust-derived) materials prior to evolution.