Basic Magma Research Articles

Permo-Triassic hypabyssal mafic intrusions and associated tholeiitic basalts of the Kolyuchinskaya Bay, Chukotka (NE Russia): Links to the Siberian LIP

In order to test tectonic hypotheses regarding the evolution of the Arctic Alaska–Chukotka microplate prior to the opening of the Amerasian basin, we investigated rocks exposed near Kolyuchinskaya Bay, eastern Chukotka. Hypabyssal mafic rocks and associated basaltic flows enclose terrigenous sediments, minor cherts and limestones in pillow interstices. The hypabyssal mafic rock yields a U–Pb zircon age of 252 ± 4 Ma and indicates intrusion of basic magma at the Permo-Triassic boundary, contemporaneous with voluminous magmatism of the Siberian large igneous province (LIP). The lava flows and hypabyssal mafic rocks of the Kolyuchinskaya Bay region have trace elements, Sm–Nd and Rb–Sr isotope compositions identical to the tholeiitic flood basalts of the main plateau stage of the Siberian LIP, but differ from the latter in the major-element variations. We conclude that compositional variations in the hypabyssal rocks studied reflect their generation in an extensional environment that might be related to the Siberian super-plume activity at the time. Although the genetic and temporal links between intrusive mafic rocks and lavas are not well proved, compositional variations of the eruptive rocks still indicate their generation in an extensional environment.

Middle-proterozoic anorthosite–rapakivi granite complexes: An example of within-plate magmatism in abnormally thick crust: Evidence from the East European Craton

Large, bimodal, and multistage anorthosite–rapakivi granite complexes (ARGCs) of Middle-Proterozoic age (1.8–1.5 Ga) are distinct magmatic assemblages in the western part of the present-day European Craton. ARGCs formation commenced after stabilization of the Svecofennian orogen and relics of its abnormally thick crust occur in many places. In the eastern part of the Kola-Karelian domain and the Volga-Urals domain, where crustal thickness is normal ∼40 km, ARGCs are practically absent. Geological data evidence that melting processes occurred both in the crust and in the mantle during formation of the complexes. Geochemical peculiarities of ARGC rocks include enrichment in alkalis (mainly in K), Ti, Zn, Pb, Zr, Be, Sn, In, Y, Nb, Rb, F, Cu, W, Mo, sometimes Li and U. The negative ɛNd values and relatively high Th and Zn concentrations that are most frequently observed in the rocks imply that parental magmas were considerably contaminated by crustal components. According to the gravity and seismic profiles, ARGCs represent upper parts of large transcrustal magmatic systems, composed of alternating basic and silicic rocks layers. They are located over mantle uplift structures 10–20 km high, interpreted to represent former mantle plume heads. Origin of transcrustal systems was probably linked with penetrating sill-like bodies of Fe–Ti basalts, which after intruding into sialic crust, caused large-scale re-melting of the granitic material above them, forming two-layer magma chambers. Solidification of such chambers occurred in two stages: lower basic layers solidified first while heated from below, whereas the upper silicic layer crystallized later. Co-existing neighboring chambers led to gravitational instability, overturn, and mass redistribution in large volumes of crust. Furthermore, multistage formation of the ARGCs led to important petrological consequences: contamination of basic magmas by crustally derived SiO 2 and Al 2O 3, causing preferred precipitation of plagioclase and formation of plagioclase cumulates (anorthosites). Diffusion of Na into the basic melt and K into the silicic melt led to the appearance of potassic rapakivi granites. ARGCs on all Precambrian shields were developed in places with stabilized Paleoproterozoic orogens with atypical unusually thick crust compared with the Phanerozoic, and associated with large Mesoproterozoic belts of within-plate felsic volcanism. Under these conditions, the majority of mantle plume-derived mafic magmas were not able to reach the surface and were lost within the crust in the form of sill-like intrusions with zones of intense melting of a sialic roof. Evidently, ARGCs illustrate structure and processes that are not repeated in the Phanerozoic.

Evaluation of Recent Tectonomagmatic Discrimination Diagrams and their Application to the Origin of Basic Magmas in Southern Mexico and Central America

Discrimination diagrams to decipher tectonic settings have been in use for nearly 40 years. Although old diagrams have been extensively used, the recent ones based on discriminant functions of ratio variables, with or without log-transformation, proposed during 2004–2010 for the discrimination of four tectonic settings of island arc, continental rift, ocean-island and mid-ocean ridge, were newly evaluated to show their high success rates of 57.3–100% and 58.5–100% for major-element and immobile-element based diagrams, respectively. For the continental arc of the Andes evaluated for its similarity to island arc, these four sets of diagrams showed success rates of 62.1–83.8%. These four sets of five diagrams per set were therefore used to infer tectonic setting of the Mexican Volcanic Belt (MVB), Los Tuxtlas volcanic field (LTVF), and Central American Volcanic Arc (CAVA). Using this approach, the MVB, especially its western, central and eastern parts, and the LTVF of Southern Mexico show a dominantly continental rift setting and the CAVA shows an arc setting. The west-central part of the MVB is consistent with dual tectonics of arc and rift. These results confirm the application of an unusual mantle upwelling rift-model for the Mexican on-land volcanism, whereas the conventional plate tectonic subduction model seems to be applicable for the CAVA from Guatemala to north-western Costa Rica.

The Plio-Quaternary magmatic feeding system beneath Gran Canaria (Canary Islands, Spain): constraints from thermobarometric studies

Abstract: A complete framework of chronostratigraphic, petrographic, geochemical and thermobarometric data allows a reinterpretation of the evolution of the Plio-Quaternary volcanism in Gran Canaria. Bulk-rock and mineral chemistry has been used to define P – T crystallization paths, based on clinopyroxene–melt thermobarometry. Mafic magmas mainly crystallized at mantle depths (17–22 km) with secondary fractionation at crustal levels (1.5–8.0 km), suggesting multistage magma ascent. Crystallization of evolved magmas took place at crustal depths (3–15 km). The complexity of the magmatic plumbing system increased in the last 1 Ma, as shown by the presence of reverse zoned clinopyroxenes in the lavas, with green diopside–hedenbergite cores surrounded by brown diopsidic rims. Petrographic and chemical features of such clinopyroxenes support a xenocrystic origin of the green cores, related to magma mixing or mingling processes between pre-existing colder evolved magmas and new batches of hot basic magmas. The evolution of the magmatic feeding system beneath Gran Canaria was affected by the long-term uplift of the island as a result of lithospheric flexure caused by loading of the neighbouring island of Tenerife. This uplift favoured the generation of giant landslides and nesting of the recent volcanic activity (the last 1 Ma) within landslide basins. The association of rifts, giant landslides and nested volcanism with a higher degree of magmatic differentiation is proposed to constrain the plumbing system model.

Evolution of acidity of hydrothermal fluids related to hydrolysis of chlorides

⎯ + HCl. The analysis of experimental data shows that, after separation from silicate melts, the supercritical fluid should be alkaline. But during skarn formation (at 800-600°C), because of weak dissociation of electrolytes, the fluids act as subneutral or alkalescent. During further cooling, the electrolyte dissociation grows, and the effective alkalinity of fluids rises, with their interaction with country rocks resulting in formation of Kfeld� spar and, later, albite metasomatites. A typical feature of ore mineralization related to skarns (Fe, B, W, Cu, Pb, Zn) and feldspathic metasomatites (Mo, W, Be, U), is its exclusively metasomatic formation, at complete absence of open space filling veins. After cooling of fluids to 450-300°C, HCl dissociates better than NaOH and KOH, and the fluids should become acid. But detailed studies of metasomatites and fluid inclusions has shown that simple cooling of alkaline fluids is not sufficient for formation of acidic metasomatites. Their for� mation is the result of country rocks interaction with the acid and silicadeficient condensates of a gas phase of heterogeneous subcritical fluids. It is proved by leaching of silica from the wall rocks during formation of acidic metasomatites and by close temporal relationships of greisens and other acidic metasomatites with the formation of typical open space filling veins. Such environment of long coexistence of gravity separated gas and liquid fluids was typical formation style of postorogenic veintype W and Sn deposits of calcalkal ine granitoids affiliation and Au, Ag, Cu, Zn, Pb, As, Sb and Hg deposits related to basic magmas of mantle origin. The ore deposition during this period proceeded only from a liquid phase of fluids and after formation of metasomatites of the same mineralization stage. Thus, the evolution of acidity of natural magmatogene fluids from subneutral and alkaline to acid and further again to lowtemperature alkaline and then neutral, which has been suggested by D.S. Korzhinskii (1953), was discovered as really operating process of hydrothermal mineralization. But there is a reliable chemical and geological explanation of the reason for such evolution, alternative to D.S. Korzhinskii's (1958) hypoth� esis of an "overtaking wave of acidic components" in a solution being filtered through the country rocks. And a typical hydrothermal mineralization can comprise both one or more "alkalinity waves" and two or more "acidity waves" created due to P ⎯T dependence of chloride hydrolysis and intermittent (multistage) ascent of fluids into the realm of ore deposition.

REE and Sr-Nd isotope geochemistry for Yixian fluorite deposit, western Liaoning Province, China, and its geological implications

Up to now, there were no systematic studies of geochemistry and isotopic age for the Yixian (义县) fluorite deposit, western Liaoning (辽宁) Province, China. Based on the analysis of metallogenic geological setting, we studied the REE, Rb-Sr and Sm-Nd isotopes. The chondrite-normalized REE patterns of fluorite are characterized by moderate LREE depletion (LREE/HREE=0.95–3.57, (La/Yb)N=0.08–2.84) and enrichment of Sr (146×10−6-596×10−6) and moderately positive Eu anomalies (δEu=1.10–1.34), which are similar to those of the host Mesoproterozoic carbonate rocks. The fluorite display (87Sr/86Sr) t =−0.708 5, (143Nd/144Nd) t =−0.511 785, and e Nd(t)=−12.8, which are similar to those of the host Mesoproterozoic carbonate rocks and volcanic rocks of Middle Jurassic Lanqi (蓝旗) Formation. The REE and Sr-Nd isotope geochemistry suggest that the source of the ore-forming material may be the volcanic rocks of Lanqi Formation and host carbonate rocks. The Sm-Nd isochron age of 154±14 Ma (MSWD=0.23) indicates that the Yixian fluorite mineralization nearly corresponds to the period of Lanqi Formation. Based on the integrated geological and geochemical studies, coupled with previous studies, we suggest that Yixian fluorite deposit formed in the extension setting of postcollisional stage and may be attributed to the partial melting of ancient basaltic rocks in the lower crust induced by underplating of basic magma and to the reaction between the F-rich ore-forming fluids and the host carbonate rocks.

Volcano destabilization by magma injections in a detachment

We report the discovery of a detachment fault within reach of structural investigation in a shield volcano (Piton des Neiges, La Reunion hotspot). The detachment, exposed by erosion, is located between the top of an extinct gabbroic magma chamber and a cover of debris avalanche deposits. The fault simultaneously concentrated ductile and brittle deformation toward the sea, hydrothermal alteration in the greenschist facies, and repeated sill injections of basic magmas. Our study of this exceptional detachment exposure suggests that two mechanisms participate in volcano flank failure: hydrothermal alteration, likely sustained by the heat of cooling sills, induces slow inter-intrusion deformation, whereas sill injections in the fault may trigger rapid cointrusive slip. This twofold process of destabilization might currently occur on some active volcanoes such as Piton de la Fournaise.

The significance of mafic microgranular enclaves in the petrogenesis of the Dehno Complex, Sanandaj–Sirjan belt, Iran

Quartz-monzodioritic and granodioritic enclaves occur in the Upper Cretaceous–Paleocene Dehno Complex, NW of Isfahan, Iran. They are hosted by hornblende-granodiorite and granodiorite–monzogranite, respectively. The presence of xenocrystic plagioclase, biotite, quartz, magnesio-hornblende, occasionally with relics of clinopyroxene (in quartz-monzodiorites), minor K-feldspar and the lack of peraluminous minerals dominate the mineralogy of these rocks and suggest a metaluminous source. The plagioclase in the quartz-monzodioritic enclaves has higher An-content than that of the respective plagioclase of their host (hornblende-granodiorite) indicating crystallization from more mafic magma. The quartz-monzodioritic enclaves generally contain higher values of Fe 2O 3, MgO, CaO, MnO, Na 2O, transition elements (Ni, Cr, Co, V), REE and lower values of SiO 2, K 2O, P 2O 5 and Zr than their host which are in consistent with occurrence of abundant ferromagnesian minerals in these rocks. Compare to their host, the granodioritic enclaves have higher amount of Fe 2O 3, MgO, MnO, TiO 2, P 2O 5, Rb, Zr, Y, Ni, Cr, Co and lower concentration of SiO 2, K 2O and Ba. Aside from hornblende-granodiorite and granodiorite–monzogranite, the Dehno Complex comprises leucogranodiorite and tourmaline-leuco granites. Geochemically, the rocks of the Complex are metaluminous to strongly peraluminous and are enriched in LILE (e.g. Rb, K, Sr, Ba) and depleted in HFSE (e.g. Nb, Ta, P, Ti, Zr, Hf) with medium to high-K calc-alkaline nature. These geochemical features are proposed for the volcanic arc granites emplaced in an active continental margin. Whole-rock and quartz δ 18O values for all rocks are high (9.7–14.4‰ and 10.1–14.9‰, respectively), suggesting the interaction of magma with crustal-derived components. The strongly peraluminous composition can be interpreted in terms of this hypothesis. All these mineralogical and geochemical features support the idea that the granitoid rocks of the Dehno Complex developed through variable degrees of mixing/mingling between a basic magma and a supracrustal melt during subduction of the Neotethys oceanic crust beneath Central Iranian microcontenent. The quartz-monzodioritic enclaves most likely represent the nearest composition to the original more mafic magma. The granodioritic enclaves result from the mixing/mingling of globules basic magma with host granitic magma.

Relative contributions of crust and mantle to generation of Campanian high-K calc-alkaline I-type granitoids in a subduction setting, with special reference to the Harşit Pluton, Eastern Turkey

We present elemental and Sr–Nd–Pb isotopic data for the magmatic suite (~79 Ma) of the Harşit pluton, from the Eastern Pontides (NE Turkey), with the aim of determining its magma source and geodynamic evolution. The pluton comprises granite, granodiorite, tonalite and minor diorite (SiO2 = 59.43–76.95 wt%), with only minor gabbroic diorite mafic microgranular enclaves in composition (SiO2 = 54.95–56.32 wt%), and exhibits low Mg# (<46). All samples show a high-K calc-alkaline differentiation trend and I-type features. The chondrite-normalized REE patterns are fractionated [(La/Yb) n = 2.40–12.44] and display weak Eu anomalies (Eu/Eu* = 0.30–0.76). The rocks are characterized by enrichment of LILE and depletion of HFSE. The Harşit host rocks have weak concave-upward REE patterns, suggesting that amphibole and garnet played a significant role in their generation during magma segregation. The host rocks and their enclaves are isotopically indistinguishable. Sr–Nd isotopic data for all of the samples display I Sr = 0.70676–0.70708, ε Nd(79 Ma) = −4.4 to −3.3, with T DM = 1.09–1.36 Ga. The lead isotopic ratios are (206Pb/204Pb) = 18.79–18.87, (207Pb/204Pb) = 15.59–15.61 and (208Pb/204Pb) = 38.71–38.83. These geochemical data rule out pure crustal-derived magma genesis in a post-collision extensional stage and suggest mixed-origin magma generation in a subduction setting. The melting that generated these high-K granitoidic rocks may have resulted from the upper Cretaceous subduction of the Izmir–Ankara–Erzincan oceanic slab beneath the Eurasian block in the region. The back-arc extensional events would have caused melting of the enriched subcontinental lithospheric mantle and formed mafic magma. The underplating of the lower crust by mafic magmas would have played a significant role in the generation of high-K magma. Thus, a thermal anomaly induced by underplated basic magma into a hot crust would have caused partial melting in the lower part of the crust. In this scenario, the lithospheric mantle-derived basaltic melt first mixed with granitic magma of crustal origin at depth. Then, the melts, which subsequently underwent a fractional crystallization and crustal assimilation processes, could ascend to shallower crustal levels to generate a variety of rock types ranging from diorite to granite. Sr–Nd isotope modeling shows that the generation of these magmas involved ~65–75% of the lower crustal-derived melt and ~25–35% of subcontinental lithospheric mantle. Further, geochemical data and the Ar–Ar plateau age on hornblende, combined with regional studies, imply that the Harşit pluton formed in a subduction setting and that the back-arc extensional period started by least ~79 Ma in the Eastern Pontides.

Influence of pre-eruptive storage conditions and volatile contents on explosive Plinian style eruptions of basic magma

Sub-Plinian to Plinian eruptions of basic magma present a challenge to modeling volcanic behavior because many models rely on magma becoming viscous enough during ascent to behave brittlely and cause fragmentation. Such models are unable, however, to strain low viscosity magma fast enough for it to behave brittlely. That assumes that such magmas actually have low viscosities, but the rare Plinian eruptions of basic magma may in fact result from them being anomalously viscous. Here, we examine two such eruptions, the 122 B.C. eruption of hawaiitic basalt from Mt. Etna and the late Pleistocene eruption of basaltic andesite from Masaya Caldera, to test whether they were anomalously viscous. We carried out hydrothermal experiments on both magmas and analyzed glass inclusions in plagioclase phenocrysts from each to determine their most likely pre-eruptive temperatures and water contents. We find that the hawaiite was last stored at 1,000–1,020°C, whereas the basaltic andesite was last stored at 1,010–1,060°C, and that both were water saturated with ∼3.0 wt.% water dissolved in them. Such water contents are not high enough to trigger Plinian explosive behavior, as much more hydrous basic magmas erupt less violently. In addition, despite being relatively cool, the viscosities of both magmas would range from ∼102.2–2.5 Pa s before erupting to ∼104 Pa s when essentially degassed, all of which are too fluid to cause brittle disruption. Without invoking special external forces to explain all such eruptions, one of the more plausible explanations is that when the bubble content reaches some critical value the fragile foam-like magma disrupts. The rarity of Plinian eruptions of basic magma may be because such magmas must ascend fast enough to retain their bubbles.

The mafic rocks of Shao La (Kharta, S. Tibet): Ordovician basaltic magmatism in the greater himalayan crystallines of central-eastern Himalaya

In the Kharta area, east of Mount Everest, the Greater Himalayan Crystallines are significantly richer in mafic rocks than the surrounding areas, Sikkim–West Bhutan and Makalu–Cho Oyu. These rocks are lenses with a complex metamorphic history. The mafic lenses of Shao La, in the Greater Himalayan Sequence south of Kharta, are here considerated as dismembered dykes apparently escaped the Himalayan high-temperature metamorphism and only record a low-grade metamorphic event. They are calc-alkaline medium-K basalts to basaltic andesites, consisting of plagioclase (core 62% An and rim 55% An), augite (Wo 43–47En36 36–37Fs 16–20), hypersthene (Wo 1.6–3.3En 50–52Fs 46–48), and minor brown hornblende, biotite and ilmenite. They show strong enrichment in low ionic potential elements relative to high-field-strength elements, and only minor Ce and P enrichment with respect to MORB. Combined Sr–Nd systematics suggest contamination of a basic magma from a subcontinental mantle source with a small amount of crust (about 4 vol.%). This in turn indicates that the Shao La basalts and basaltic andesites have the geochemical fingerprint of a supra-subduction zone magma. U–Pb dating of zircon from one sample yielded an age of 457 ± 6 Ma for the crystallisation of the Shao La basic rocks, assigning them to the Cambro-Ordovician Bhimphedian orogenic event. The age and geochemical characteristics of the Shao La rocks are similar to those of the basic rocks of the Cambro-Ordovician Mandi pluton further west. This suggests the existence of an extensive supra-subduction zone magmatism along the Indian margin of Gondwana. Like the bimodal granite-gabbro magmatism in the Mandi-Kaplas area, the Shao La basic rocks are contemporaneous with the emplacement of granitic plutons in the Everest-Kharta area. This acid plutonism is interpreted as crustal melt triggered by the upwelling of metasomatised mantle in a back-arc setting. The age of basic and acidic plutonism in the Everest-Kharta area is evidence that the Bhimphedian Orogeny was still active as late as the Late Ordovician

Average composition of basic magmas and mantle sources of island arcs and active continental margins estimated from the data on melt inclusions and quenched glasses of rocks

Based on the generalization of data on melt inclusions and quenched glasses, the average compositions of subduction (island arc and active continental margin settings) basic magmas were estimated. The main geochemical features of the average composition of these magmas are significant depletion in Nb and Ta, less significant depletion in Ti, Zr, and Sm, and enrichment in Cl, H2O, F, and P in the primitive mantlenormalized patterns. The average normalized contents of moderately incompatible HREE in these magmas are close to those in the basic magmas of other geodynamic settings. Subduction basic magmas exhibit negative correlation of Li, Y, Dy, Er, Yb, Lu, and Ti contents with MgO content. Most of incompatible elements (Nb, Ta, U, Th, LREE) do not correlate with MgO, but correlate with each other and K2O. Variations in element contents are related to crystallization differentiation, magma mixing, and possibly, participation of several sources. The water content in the island arc basic magmas varies from almost zero value to more than 6 wt %. Most compositions are characterized by weak negative correlation between H2O and MgO contents, but some compositions define a negative correlation close to that in magmas of mid-ocean ridges (MOR). Considered magmas demonstrate distinct positive correlation between MgO content and homogenization temperature, practically coinciding with that of MOR magmas. Modeling of phase equilibria revealed widening of crystallization field of olivine in the magmas of subduction zones compared to MOR magmas. This can be related to the high water content in subduction magmas. Simultaneous liquidus crystallization of olivine and clinopyroxene in subduction magmas occurs at pressure approximately 5 kbar higher than that of MOR magmas. Based on the average ratios of trace element to K2O content, we determined the average compositions for subduction magma sources. Relative to depleted mantle, they are enriched in all incompatible elements, with positive anomalies of U, Rb, Ba, B, Pb, Cl, H2O, F, and S, and negative anomalies of Th, K, Be, Nb, Ta, Li, Nd, Pb, and Ti. A general elevated content of incompatible elements indicates a reworking of the rocks of mantle wedge by fluids and melts that were released from the upper layers of subducted plate.

Nd–Sr–Hf–O isotope provinciality in the northernmost Arabian–Nubian Shield: implications for crustal evolution

Multi-isotope study including whole-rock Nd–Sr, single zircon Hf, and SIMS δ18O analyses of zircons sheds light on magma sources in the northernmost Arabian–Nubian Shield (ANS) during ~820–570 Ma. Reconnaissance initial Nd and Sr isotope data for the older rocks (~820–740 Ma) reaffirms previous estimates that early crustal evolution in this part of the shield involved some crustal contamination by pre-ANS material. Prominent isotope provinciality is displayed by post-collisional calc-alkaline and alkaline igneous rocks of ~635–570 Ma across a NW-SE transect across basement of the Sinai Peninsula (Egypt) and southern Israel. Silicic rocks of the NW-region are characterized by lower εNd(T)–εHf(T) and higher Sri and δ18O compared with rocks of the SE-region, and the transition between the regions is gradual. Within each region isotope ratios are independent of the extent of magma fractionation, and zircon cores and rims yield similar δ18O values. Comparison with southern segments of the ANS shows that the source for most ~635–570 Ma rocks can be modeled as the isotopically aged lower-intermediate crust in the ANS core (SE-region) and its northern, more contaminated ANS margins (NW-region). Nevertheless, Nd–Sr isotope enrichment of the lithospheric mantle is indicated by some basic magmas of the NW-region displaying the most enriched Nd–Sr isotope compositions. Comparison of Nd and Hf depleted mantle model ages for rocks of the SE-region may indicate that crustal formation events in the ANS geographical core took place at 1.1–1.2 Ga and were followed by crustal differentiation starting at ~0.9 Ga.

Age and source material of the Kingash ultramafic-mafic massif, East Sayan

The REE distribution patterns and Nd whole-rock and mineral isotope ratios of the Kingash ultramafic-mafic massif enabled us to propose a multistage history for its evolution at 1410 and 875 Ma. These stages reflect the magmatic evolution of the Siberian paleocontinent margin during the Late Precambrian. The age of metamorphism of the massif during collision and accretion in the Early Paleozoic (∼500 Ma) was obtained based on a Sm-Nd mineral isochron from rheomorphic veined albitite. The Nd and Sr isotopic compositions of rocks from the Kingash massif suggest mantle sources for picritic and basic magmas, which are thought to have originated by mixing of different proportions of depleted (PREMA or DM) and enriched (EM) melts. The initial isotope ratios of the parental melts transformed during interaction with Sr-rich material from the host metasedimentary complexes.

Iron in plagioclase in the Bushveld and Skaergaard intrusions: implications for iron contents in evolving basic magmas

The evolved, iron-rich rocks of the tholeiitic Bushveld and Skaergaard intrusions are similar in containing cumulus magnetite, ilmenite, plagioclase, clinopyroxene, apatite and olivine, and also orthopyroxenes/pigeonite in Bushveld. Here, we evaluate their liquid evolution trends using the total iron content in plagioclase determined by electron microprobe analyses. To aid this analysis a revised mass balance model for the liquid evolution of Skaergaard is presented. For plagioclase in the Upper Zone of Skaergaard it was previously demonstrated that total FeO increases from ~0.25 to ~0.45 wt% with differentiation and correlates inversely with An% [100 × Ca/(Na + Ca)]. The reverse trend is observed in two recently published datasets for Bushveld, showing that total FeO in plagioclase decreases upward through the magnetite-bearing Upper Zone from ~0.30 to ~0.15% and from ~0.40 to ~0.25% in the western and northern limbs, respectively, and correlates positively with An%. The partition coefficient of total iron between plagioclase and magma increases with oxidation and polymerisation in the liquid. Although Bushveld formed under slightly more oxidizing conditions than Skaergaard, differences in the partition coefficients cannot explain the two observed trends. We therefore conclude that the differentiation trends of the liquids subsequent to magnetite saturation were fundamentally different. The inferred liquid composition for Bushveld contained about 15 wt% total FeO at the level of magnetite-in, which is slightly less than the total FeO content of the subsequent cumulates. In contrast, the Skaergaard liquid contained more total FeO than the ensuing cumulates. As a result, in Bushveld residual liquids total FeO decreased after magnetite saturation, whereas in Skaergaard the residual liquids continued to become enriched in iron. This conclusion is corroborated by simple mass balance calculations between modelled residual liquids and extracted cumulate rocks. Despite the mineralogical similarities of evolved iron-rich rocks of Skaergaard and Bushveld, their liquid evolution trends were very different, and generalizations about the extent of iron enrichment in tholeiitic magmas should be avoided.

Early post-collisional granitic and coeval mafic magmatism of medium- to high-K tholeiitic affinity within the Neoproterozoic Southern Brazilian Shear Belt

The post-collisional stage of the Brasiliano/Pan-African Orogenic Cycle in the Garopaba region, southern Brazil, began at 630–620 Ma by the emplacement of large granitic massifs with associated mafic rocks. The Paulo Lopes Suite (PLS) is composed of metaluminous to slightly peraluminous biotite-hornblende Paulo Lopes and Garopaba granitoids, and by the Silveira Gabbro, containing augite, pigeonite and orthopyroxene. Mafic microgranular enclaves and mutual intrusive relations attest to the coeval character of mafic and felsic magmas. The PLS intruded during regional transcurrent shearing, and microstructures indicative of high-temperature solid-state deformation are described in the mafic and granitic rocks. Mafic rocks are medium- to high-K tholeiitic and show mineralogical and geochemical features, including isotopic ones that are compatible with magmas from sources modified by previous lithospheric subduction, such as OIB-EM1. The PLS granitic magmas are interpreted as evolved from a mafic magma that was compositionally equivalent to the Silveira Gabbro, contaminated by crustal melts or by more complex mechanisms; hence, they may be considered as granitic magmas of tholeiitic affinity. Transcurrent shearing seems to be essential for promoting the generation of mantle-derived magmas and their interaction with crustal melts, with consequent generation of abundant granitoids. Basic magmas have resulted from melting of heterogeneous mantle reservoirs, and their highly evolved character is indicated by their low contents of MgO, Ni and Cr. The compositional similarity of PLS granitoids, particularly the Garopaba Granitoids, with A-type granitoids discriminates them from the medium- to high-K granitoids of mature magmatic arcs, and this difference probably reflects the tholeiitic character of their primary magmas, typical of the early magmatic stages of post-collisional settings in southern Brazil. Trace element and isotopic geochemical data indicate subduction-modified (OIB) mantle sources for the mafic magmas, which is coherent with the post-collisional character of this magmatism.

Os anfibolitos do complexo costeiro na região de São Sebastião, SP

Geochronological, petrographic and lithochemical data have been used to characterize the crustal evolution and the thermochronology of the Costeiro Complex, Sao Paulo State, Brazil. This part of the Ribeira Fold Belt is composed of paraderived rocks to the East, and orthoderived rocks to the West, which are organized along a ductile shear zone forming a positive flower structure whose axis runs ENE-WSW. The main amphibolite occurrences are found within predominant garnet-paragneisses along this axis. The age of crystallization of the intrusive basic magmas which originated the amphibolites was 580 Ma (U-Pb SHRIMP). They formed tabular intrusions or secondary magma chambers, that locally still preserve continental tholeiitic geochemical characteristics. The close proximity of the basic magmatism and the metamorphic peak is evident from the 570 Ma, U-Pb age determination of the overgrowths on detritic zircon crystals in the paragneisses. This suggests a tectonic environment where the basic magmas intruded a back-arc sedimentary basin established over continental crust. The source areas were probably rocks differentiated from the mantle between the Paleoproterozoic and Neoproterozoic.

Compositional Differences between Felsic Volcanic rocks from the Margin and Center of the Northern Main Ethiopian Rift

Pliocene felsic rift margin and Quaternary rift center volcanic rocks from the northern Main Ethiopian Rift (MER) exhibit contrasts in major and trace element contents and Sr-Nd isotopic ratios. Quaternary rift center felsic volcanic rocks are mainly peralkaline trachytes and rhyolites, whereas Pliocene felsic rift margin volcanic rocks are represented by benmoreites, weakly peralkaline trachytes and rare rhyolites. Most of the felsic rift margin volcanic rocks have greater Al2O3, K2O, Nb, Zr, Rb, and Sr, and lesser CaO, Zr/Nb, and CaO/Al2O3 than rift center volcanic rocks. These contrasts may have been inherited from differences in the compositions of their parental basic magmas, which were produced by variable degrees of partial melting. In both series, the felsic volcanic rocks generally have higher initial Sr- isotopic (0.7038-0.7073) ratios than their basic equivalents (0.7035-0.7046). Nd- isotopic ratios of most felsic rift center samples (0.5129-0.5126) are similar to their associated basic volcanic rocks. In contrast, the Nd-isotopic ratios (0.5128-0.5124) of felsic rift margin volcanic rocks are commonly lower than their companion basic volcanic rocks (0.512806-0.512893), and are relatively lower than rift center equivalents. The elemental and Sr-Nd isotopic compositions of the volcanic rocks suggest that fractional crystallization from differing basic parents accompanied by a limited assimilation (AFC) was the dominant process controlling the genesis of the MER felsic volcanic rocks. Keywords: Ethiopia; Northern Main Ethiopian Rift; Bimodal Volcanism; parental difference; Sr-Nd Isotopes, Fractional Crystallisation

PETROGRAPHICAL AND GEOCHEMICAL PROPERTIES OF PLAGIOGRANITES AND GABBROS IN GULEMAN OPHIOLITE

Petrographical and geochemical properties of gabbros and plagiogranites of Guleman ophiolite are determined. It was concluded that gabbros can be basic rocks on subduction zone and plagioclase rich leu- cocratic rocks (plagiogranite) are differentiation products of fractional crystallization of a basic magma in the magma chamber.

Water contents of pyroxenes in intraplate lithospheric mantle

Water contents of clinopyroxene and orthopyroxene in mantle peridotites from various xenolith occurrences in intraplate settings (both oceanic and continental) were determined by Fourier Transform Infrared Spectroscopy (FTIR). Samples from the following localities were studied: Sal Island (Cape Verde Archipelago); Baker Rocks and Greene Point (Northern Victoria Land, Antarctica); Panshishan and Lianshan (Subei Basin, Eastern China). They represent well-known localities where detailed petrographical and geochemical studies have already been carried out or areas which are currently under investigation. The water incorporated in these pyroxenes is low (cpx, 37–399 ppm; opx, 9–166 ppm) or very low (as in Greene Point, Antarctica; cpx, 5–16 ppm; opx, 9–16 ppm). Within each population there is no clear correlation with melting parameters such as MgO contents in any single mineral. Results are compared with the available literature data on water contents in mantle pyroxenes, which include peridotites from on-craton (hosted by kimberlitic-type magmas) and off-craton (hosted by alkaline basic magmas), as well as subarc mantle settings. The “relatively dry” (cpx, 140–528 ppm; opx, 38–280 ppm) sub-arc mantle xenoliths (Peslier et al. , 2002) are shown to be wetter than the intraplate (off-craton) xenoliths. Cratonic mantle pyroxenes are only represented by a few determinations on garnet peridotites and eclogite from Kaapvaal and Colorado Plateau. They record the highest water contents (cpx, 342–1012 ppm; opx, 180–491 ppm) so far measured in mantle pyroxenes from various tectonic settings. Despite the limited data set, the indication that the cratonic mantle is strongly hydrated is compelling. Rehydration of cratonic mantle may be related to plate subduction ( i.e. , Colorado Plateau) or alternatively to metasomatic enrichment ( i.e. , Kaapvaal Craton). However, assuming that the water content was initially very low, metasomatic events in intraplate settings, similar to those presented here, would not be able to produce a significant increase in pyroxenes water content. According to our own data and those reported in the literature, it appears that substantial rehydration may instead occur at convergent plate margins.