Ignimbrites of the Cerro Galan caldera, NW Argentina

The 35 × 20 km Cerro Galán resurgent caldera is the largest post-Miocene caldera so far identified in the Andes. The Cerro Galán complex developed on a late pre-Cambrian to late Palaeozoic basement of gneisses, amphibolites, mica schists and deformed phyllites and quartzites. The basement was uplifted in the early Miocene along large north-south reverse faults, producing a horst-and-graben topography. Volcanism began in the area prior to 15 Ma with the formation of several andesite to dacite composite volcanoes. The Cerro Galán complex developed along two prominent north-south regional faults about 20 km apart. Dacitic to rhyodacitic magma ascended along these faults and caused at least nine ignimbrite eruptions in the period 7-4 Ma (K-Ar determinations). These ignimbrites are named the Toconquis Ignimbrite Formation. They are characterised by the presence of basal plinian deposits, many individual flow units and proximal co-ignimbrite lag breccias. The ignimbrites also have moderate to high macroscopic pumice and lithic contents and moderate to low crystal contents. Compositionally banded pumice occurs near the top of some units. Many of the Toconquis eruptions occurred from vents along a north-south line on the western rim of the young caldera. However, two of the ignimbrites erupted from vents on the eastern margin. Lava extrusions occurred contemporaneously along these north-south lines. The total D.R.E. volume of Toconquis ignimbrite exceeds 500 km³.Following a 2-Ma dormant period a single major eruption of rhyodacitic magma formed the 1000-km³ Cerro Galán ignimbrite and the caldera. The ignimbrite (age 2.1 Ma on Rb-Sr determination) forms a 30–200-m-thick outflow sheet extending up to 100 km in all directions from the caldera rim. At least 1.4 km of welded intracaldera ignimbrite also accumulated. The ignimbrite is a pumice-poor, crystal-rich deposit which contains few lithic clasts. No basal plinian deposit has been identified and proximal lag breccias are absent. The composition of pumice clasts is a very uniform rhyodacite which has a higher SiO₂ content but a lower K₂O content than the Toconquis ignimbrites. Preliminary data indicate no evidence for compositional zonation in the magma chamber. The eruption is considered to have been caused by the catastrophic foundering of a cauldron block into the magma chamber.Post-caldera extrusions occurred shortly after eruption along both the northern extension of the eastern boundary fault and the western caldera margin. Resurgence also occurred, doming up the intracaldera ignimbrite and sedimentary fill to form the central mountain range. Resurgent doming was centred along the eastern fault and resulted in radial tilting of the ignimbrite and overlying lake sediments.     

Strontium isotope ratios in volcanic rocks from the South Sandwich Islands

⁸⁷Sr/⁸⁶Sr ratios in the island-arc tholeiite series from the South Sandwich Islands cluster about a value of 0.7040. There is no apparent correlation of strontium isotope values with any major chemical component or with Rb/Sr ratios. The uniformity of the⁸⁷Sr/⁸⁶Sr values is consistent with the fractional crystallization relationship previously proposed for this suite. Though higher than values reported for ocean-floor tholeiites they are not significantly different from ratios reported for calc-alkaline island-arcs.     

Strontium isotope composition in volcanic rocks from the Northeast Japan arc

In the Northeast Japan arc, a number of Quaternary volcanoes form a long, narrow belt, parallel to the Japan Trench. ⁸⁷Sr/⁸⁶Sr ratios were determined in 52 specimens of volcanic rocks from 27 volcanoes in the Northeast Japan arc area. The results reveal that the ratios change systematically in space. Decreasing ⁸⁷Sr/⁸⁶Sr ratios across the arc were confirmed over a wide area of Northeast Japan. In the same direction, increases in both Rb and Sr contents were also found. The regular trends are considered to be a strong constraint for elucidation of subduction-originated magma genesis at the Eurasia plate vs. Pacific plate boundary. In the northern region of the Northeast Japan arc, ⁸⁷Sr/⁸⁶Sr ratios in volcanic rocks along the volcanic front were almost constant (0.7038–0.7045) and slightly higher than those from the Izu-Ogasawara arc (0.7032–0.7038). This suggests that “interactions” between the Eurasia plate and the Pacific plate, and those between the Philippine Sea plate and the Pacific plate are slightly different. The southern region of the Northeast Japan arc, where the direction of the volcanic front bends from southward to westward, showed anomalously high ⁸⁷Sr/⁸⁶Sr ratios, reaching to 0.7077. This region coincides with the triple junction of the Eurasia, Pacific and Philippine Sea plates, suggesting “anomalous interaction” at the triple junction.     

Geochemistry of calc-alkaline volcanic rocks from southeastern Iran (kouh-e-shahsavaran)

The Upper Tertiary to Quaternary volcanic complex of Kouh-e-Shahsavaran in southeastern Iran is composed of calc-alkaline rocks of island are type (high-alumina basalts, basic andesites, andesites and dacites) even though it was emplaced on the continental basement. The volcanic rocks of the complex are genetically related and were probably derived by low-pressure fractional crystallization of high-Al basalts. The anomalously high content of Sr in some rocks probably reflects an accumulation of plagioclase. The trace element data are consistent with the origin of the parental magma by partial melting of an “enriched” upper mantle peridotite.     

Helium isotope geochemistry of some volcanic rocks from Saint Helena

³He/⁴He ratios have been measured for olivine and clinopyroxene phenocrysts in 7–15 m.y. old basaltic lavas from the island of St. Helena. Magmatic helium was effectively resolved from post-eruptive radiogenic helium by employing various extraction techniques, includingin vacuo crushing, and stepwise heating or fusion of the powders following crushing. The inherited³He/⁴He ratio at St. Helena is 4.3–5.9 R_A. Helium isotope disequilibrium is present within the phenocrysts, with lower³He/⁴He upon heating and fusion of the powders following crushing, due to radiogenic ingrowth or to -particle implantation from the surrounding(U + Th)-rich lavas.

A single crushing analysis for clinopyroxene in a basalt from Tubuaii gave³He/⁴He= 7.1 R_A.³He/⁴He ratios at St. Helena and Tubuaii (HIMU hotspots characterized by radiogenic Pb isotope signatures) are similar to³He/⁴He ratios previously measured at Tristan da Cunha and Gough Island (EM hotspots characterized by low²⁰⁶Pb/²⁰⁴Pb). Overall, the HeSrPb isotope systematics at these islands are consistent with a mantle origin as contiguous, heterogeneous materials, such as recycled crust and/or lithosphere.³He/⁴He ratios at HIMU hotspots are similar to mantle xenoliths which display nearly the entire range of Pb isotope compositions found at ocean islands, and are only slightly less than values found in mid-ocean ridge basalts (7–9 R_A). This suggests that the recycled materials were injected into the mantle within the last 10⁹ yrs.     

Characteristics of late Cenozoic volcanism along the Archibarca lineament from Cerro Llullaillaco to Corrida de Cori, northwest Argentina

The Archibarca lineament is one of several NW–SE-trending transverse lineaments that cut across the Central Andes of Argentina and Chile. Central Andean, Late Miocene–Quaternary subduction-related volcanism is mainly restricted to a 50-km-wide arc forming the approximately N–S axis of the Cordillera, but extends along the transverse lineaments for up to 200 km to the SE. Lineaments are interpreted to be deep-seated, long-lived basement structures or anisotropies, which can control the localization of magmatism and, in some cases, magmatic–hydrothermal ore deposits (e.g., the Escondida porphyry Cu deposit, Chile). As a first step towards exploring the regional-scale controls on magmatism and related mineralization exerted by such structures, the styles of volcanism and near-surface hydrothermal activity along a segment of the Archibarca lineament in the Puna of northwest Argentina are described here. Volcanic structures have been mapped and sampled along a 50-km transect from Cerro Llullaillaco, a large medium-K dacitic Quaternary stratovolcano, to Corrida de Cori, a range of Pliocene–Pleistocene high-K andesitic vents. Apart from a southeastward increase in K content and the predominance of dacitic lavas at Cerro Llullaillaco, the geochemical affinity of late Cenozoic volcanic rocks varies little in time or space. This uniformity extends further SE to Cerro Galán, where published data closely match the results from the study area. In detail, trace element compositions reveal the localized (in both time and space) effects of crustal contamination (recognized as Th>10 ppm), and depth of fractionation (1/Yb>0.7 ppm⁻¹, reflecting garnet residue). Explosive volcanic rocks such as ignimbrites show the strongest indications of crustal contamination, whereas the Cerro Llullaillaco dacite lavas mostly record significant garnet fractionation. Other lavas from the Llullaillaco area, including one flow from Cerro Llullaillaco, do not show garnet control, suggesting that different batches of magma stalled and fractionated at different levels in a thick (60-km) crust prior to eruption. The youngest volcanism in the Corrida de Cori area is represented by olivine–phyric basaltic andesite cinder cones and flows. The ascent of these relatively primitive magmas appears to have been controlled by late Quaternary normal faults, which directly tapped deeply derived melts. The Corrida de Cori volcanic range has experienced intense fumarolic alteration with deposition of abundant sulfate and native sulfur (previously mined at Mina Julia). Deeper levels of hydrothermal alteration have been sampled by an ignimbrite erupted from Cerro Escorial, which, among other lithic clasts, contains numerous fragments of vein quartz. Fluid inclusions in this quartz record evidence for a boiling, high-salinity fluid, which may represent a link between a high temperature magmatic–hydrothermal system at depth (i.e., a porphyry-type system) and shallow-level fumarolic activity. An ignimbrite erupted from Cerro Escorial preserves textures such as internal wave forms between flow units and surface wave morphologies at its distal limits that indicate flow as a series of dense turbulent pulses, which interdigitated and interfered with one another. Lithic lag breccias occur near the base of the flow proximal to the vent, but no air-fall deposits are preserved, probably due to transport of ash far from the vent by strong, high-altitude winds.     

O, S, Sr, and Pb isotope variations in volcanic rocks from the Northern Mariana Islands: implications for crustal recycling in intra-oceanic arcs

Quaternary lavas from the Northern Mariana Islands have respective O- and S-isotope ranges ofδ¹⁸O = +5.7 to +6.6 (‰ SMOW) andδ³⁴S = +2.0to+20.7 (‰ CDT). Chemically evolved andesites and dacites with meanδ¹⁸O = +6.3 ± 0.2 are slightly enriched in¹⁸O with respect to unfractionated basalts of<53%SiO₂ with meanδ¹⁸O = +6.0 ± 0.1. This¹⁸O enrichment can be explained in terms of differentiation of parental mafic magmas havingδ¹⁸O values between +5.7 to +6.2‰ through closed-system crystal fractionation because the lavas from all nine islands of the arc define a coherentδ¹⁸OSiO₂ trend. The S-isotope composition of oxidized magmas is not modified extensively through the degassing of SO₂; therefore, the meanδ³⁴S value of ca. +11‰ for the Mariana lavas is considered to be representative of their source region.The enrichment of¹⁸O and³⁴S in Mariana Arc parental magmas relative to ocean floor basalts withδ¹⁸O ca. + 5.7‰ andδ³⁴S = ca.0.3‰ is attributed to the recycling of¹⁸O- and³⁴S-rich crustal components (sediment withδ¹⁸O = ca. + 25‰ and seawater sulfate withδ³⁴S = ca. +20‰ into the upper mantle source region for these arc magmas. This interpretation is consistent with enrichments of radiogenic Sr and Pb in the same lavas relative to ocean-floor basalts erupted either side of the arc, which are presumed to share a common upper mantle source. This enrichment is considered to reflect the mixing of two components, one having a typical upper mantle composition and the other having a more radiogenic character similar to that of western Pacific pelagic sediments.     

Strontium isotope composition of ophiolitic and related rocks,Drocea Mountains,Rumania

Strontium isotopic analyses are reported for ophiolitic and associated rocks of Mesozoic and Tertiary age from the Drocea Mountains. The samples and their average Sr⁸⁷/Sr⁸⁶ and Rb/Sr ratios, in order, include: (a) ultramafics (partly serpentinized or uralitized peridotite, peridotite-melagabbro). 0.7043, 0.106; (b) gabbros, dolerites and anemasites (including magnetite-bearing, quartz dolerite, hornblende and normal gabbros), 0.7032, 0.021; (c) basalts (amygdaloidal, hyalopyroclasite), 0.7030, 0.040; and (d) granophyric and albitic vein rocks, 0.7046, 0.058. Also analyzed were (e) basalt-spilites of a younger intrusive cycle 0.7042, 0.046 (f) banatites, 0.7064, 0.542. Two Quaternary volcanics were analyzed from outside the Drocea Mountains: (g) augite-hypersthene andesite from Mt. Gut?i, 0.7083, 0.247, and olivine basalt from Raco?, 0.7043, 0.056. The data for the ophiolite suite show highest Sr⁸⁷/Sr⁸⁶ and Rb/Sr ratios in the ultramafics which suggests a two-stage origin with the ultramafics derived from a more primitive mantle than the later gabbros and basalts. Initial Sr⁸⁷/Sr⁸⁶ ratios range from 0.7021 to 0.7045 in gabbro and basalt and 0.7035 to 0.7056 in peridotite which is well within the limits found in oceanic tholeiites and suggests an origin for the complex as a spreading oceanic ridge. Cross-cutting felsic granophyric and albititic rocks as well as the late-stage basalts (a) have relatively low Sr⁸⁷/Sr⁸⁶ and Rb/Sr ratios, (b) represent a small volume, and (c) are intimately related to the ophiolites. They appear to have developed largely by late-stage differentiation and fractional crystallization of a tholeiitic magma. The higher ratios for the banatites and andesite from Mt. Gut?i suggest that significant amounts of sialic crustal material were involved in their formation. The basalt from Raco? is from the vicinity of a deep fracture zone; its relatively low Sr⁸⁷/Sr⁸⁶ ratio suggests a direct link to a mantle source with little or no crustal contamination.     

Possible origin of K-rich volcanic rocks from Virunga,East Africa,by metasomatism of continental crustal material: Pb,Nd and Sr isotopic evidence

The isotopic compositions of Sr, Nd and Pb together with the abundances of Rb, Sr, U and Pb have been determined for mafic and felsic potassic alkaline rocks from the young Virunga volcanic field in the western branch of the East African rift system.⁸⁷Sr/⁸⁶Sr varies from 0.7055 to 0.7082 in the mafic rocks and from 0.7073 to 0.7103 in the felsic rocks. The latter all come from one volcano, Sabinyo. Sabinyo rocks have negative ε_Ndvalues ofε_Nd = ?10. Nd and Sr isotopic variations in the basic potassic rocks are correlated and plot between Sabinyo and previously reported [1] compositions (ε_Nd = +2.5;⁸⁷Sr/⁸⁶Sr≈ 0.7047) for Nyiragongo nephelinites. The Pb isotopic compositions for Sabinyo rocks are nearly uniform and average²⁰⁶Pb/²⁰⁴Pb≈ 19.4,²⁰⁷Pb/²⁰⁴Pb= 15.79–15.84,²⁰⁸Pb/²⁰⁴Pb≈ 41.2. The basic potassic rocks have similar²⁰⁶Pb/²⁰⁴Pb values but range in²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb from the Sabinyo values to less radiogenic compositions.Excellent correlations of⁸⁷Sr/⁸⁶Sr with Rb/Sr, 1/Sr and²⁰⁷Pb/²⁰⁶Pb for Sabinyo rocks suggest these to be members of a hybrid magma series. However, the nearly uniform Pb compositions for this series points to radiogenic growth of⁸⁷Sr in the magma source region following an event which homogenized the isotopic compositions but not Rb/Sr. The Rb-Sr age derived from the erupted Sabinyo isochron-mixing line is consistent with the ～500 Myr Pb-Pb age from Nyiragongo [1], which suggests that this event affected all Virunga magma sources. The event can again be traced in the Pb-Pb, Pb-Sr and Nd-Sr isotopic correlations for all Virunga rocks, including Nyiragongo, when allowances are made for radiogenic growth subsequent to this mixing or incomplete homogenization event. Inferred parent/daughter element fractionations point to a metasomatic event during which a mantle fluid invaded two lithospheric reservoirs: a +ε_Nd reservoir sampled by the Nyiragongo nephelinites and suggested to be the subcontinental mantle and a ?ε_Nd reservoir sampled by the mafic and felsic potasssic volcanism. Whether this ?ε_Nd reservoir is the crust, continental crustal material in the mantle or anomalous mantle cannot be decided from the data. The simplest answer, that this reservoir is the continental crust, seems to be at variance with experimental evidence suggesting a subcrustal origin for basic potassic magmas. Partial melting of the ancient metasomatised lithospheric domains and ensuing volcanism seems to be entirely a response to decompression and rising geotherms during rifting and thinning of the lithosphere.     

Isotopic evidence for crustal contamination in the Karroo rhyolites of Swaziland

RbSr and Pb isotopic data are reported for some of the Karroo volcanics from Swaziland. Linear arrays of Karroo data are found in the RbSr and Pb/Pb isochron diagrams. Certain of these linear arrays, if interpreted as isochrons, give ages considerably in excess of the age normally accepted for the Karroo and must be interpreted as “erupted isochrons”. It is argued that these “erupted isochrons” represent mixing lines resulting from contamination of mantle-derived magma by continental crust. It is unlikely that the RbSr “erupted isochron” has any age significance because the Rb/Sr ratios were probably fractionated at the time or eruption, both during the contamination event and during subsequent fractionation. “Erupted isochrons” in the Pb/Pb isochron diagram are much more likely to have time significance because relatively recent U/Pb fractionation events do not significantly affect the Pb isotopic compositions. The Pb/Pb “erupted isochron” has an apparent age of 3260 ± 60 m y. This age is very similar to that of the Kaapvaal craton on which the volcanics rest and it is argued that the “erupted isochron” approximately dates the formation of the Kaapvaal craton.Six Pb isotope analyses of separated feldspar crystals are also reported. Three of these were demonstrably out of isotopic equilibrium with the magma at the time of eruption. This shows that isotopic heterogeneities on the scale of a few millimetres may be preserved between crystals and their host magma.The nature of the crust which contaminated the magma is considered and it seems most likely that the magma was contaminated by heterogeneous crust of uniform age.     

Stable isotope compositions and water contents of boninite series volcanic rocks from Chichi-jima, Bonin Islands, Japan

Measurements of stable isotope compositions and water contents of boninite series volcanic rocks from the island of Chichi-jima, Bonin Islands, Japan, confirm that a large amount (1.6–2.4 wt.%) of primary water was present in these unusual magmas. An enrichment of 0.6‰ in¹⁸O during differentiation is explained by crystallization of¹⁸O-depleted mafic phases. Silicic glasses have elevated δ¹⁸O values and relatively low δD values indicating that they were modified by low-temperature alteration and hydration processes. Mafic glasses, on the other hand, have for the most part retained their primary isotopic signatures since Eocene time. Primary δD values of −53 for boninite glasses are higher than those of MORB and suggest that the water was derived from subducted oceanic lithosphere.     

Heterogeneous initial Sr isotope compositions of highly evolved volcanic rocks from the Main Ethiopian Rift,Ethiopia

Many individual mineral Sr isotope studies have revealed the complex evolution of highly evolved rocks in upper crustal magmatic systems, casting doubts on the meaning of whole-rock Sr isotopes in such samples. In this paper, whole-rock Sr isotope measurements were replicated (three to 13 times) on six highly evolved peralkaline rhyolites from the Main Ethiopian Rift (MER) to appraise their internal heterogeneity and, thus, the significance of such data. These rocks were all fresh samples of pumice, obsidian and lava. Their maximum Sr contents and ages were 15 μg/g and 1.7 Ma, respectively. Significant small-scale heterogeneities of both Sr isotopes and Rb and Sr contents were observed in most samples, although not necessarily associated with petrological characteristics suggesting possible inheritance processes. Only two, almost crystal-free, obsidian give fairly homogeneous Sr isotope ratios. These results outline the ambiguity of a single whole-rock Sr isotope determination on highly evolved peralkaline rocks, especially when no simultaneous accurate determination of the Rb/Sr is performed. They also suggest that the limitations of Sr whole-rock analyses are not restricted to phenocryst-rich samples as phenocryst-poor obsidian and almost aphyric pumices and lava are also concerned. These data further underscore that unreplicated whole-rock Sr isotope measurements should always be used with great caution in the petrogenetic modeling of highly evolved rocks. However, multiple determinations of whole rock ⁸⁷Sr/⁸⁶Sr in the same samples, combined with other geochemical and isotopic data, may provide constraints on the shallow level evolution of these magmas. It is suggested that selective upper crustal contamination and/or interactions with halogen-bearing hydrous fluids, typical of evolved peralkaline magmas, were probably involved in the late magmatic evolution of these MER rhyolites. The more pervasive character of fluid interaction processes would probably better account for the small-scale association of uncontaminated and contaminated signatures in a single sample. Thus, even fresh samples may have their Rb–Sr isotopic system significantly modified by fluid interactions, not as a secondary process but at the late magmatic stage.     

Petrology of volcanic rocks from anjouan,comores archipelago

Anjouan is an extinct and substantially denuded volcanic island, belonging to the Comores Archipelago, a chain of volcanic islands in the northern Mozambique Channel. Volcanism was probably controlled by a complex regional stress pattern, closely implicated with the recent geotectonic evolution of East Africa and the western Indian Ocean. On Anjouan a lengthy period of shield construction was succeeded by fissure-controlled eruptions forming three extended peninsulas to the north, south and west. A ‘rejuvenescent’ phase of activity erupted lavas on to an erosional surface formed during a period of quiescence following build-up of the main volcanic edifice. Lavas of the shield-building stage comprise ankaramites, oceanites and olivine basalts, with minor development of hawaiites and trachytes. The fissure controlled eruptions are also mainly basaltic although generally more alkaline, while rejuvenescent lavas comprise basanites and a significant proportion of derivatives following a trend towards phonolite. Coarse-grained xenoliths are found in lavas of the latter two phases, while a gabbroic intrusion (the ‘Tatinga Intrusion’) is exposed in the centrally-situated Cirque de Bambao in the vicinity of N’Tingui (1595 m). The main chemical trends and petrographic characteristics of the Anjouan lavas are related to the eruptive sequence.     

Pure forsterite in Nyiragongo lavas: evidence for subsolidus oxidation of volcanic rocks

Some volcanic rocks from Nyiragongo volcano in the Democratic Republic of Congo contain highly oxidized olivine crystals.These olivines crystals are made of two phases,dark olivine on backscattered electron images of pure forsterite composition and grey Mg-poor areas made of olivine and iron-rich oxides.Calculation of the initial composition confirms that they are primary olivine with late separation of two different olivine compositions.Pure forsterite is enriched in SiO₂ but contain...     

Petrogenesis and tectonic implications of Early Cretaceous high-K calc-alkaline volcanic rocks in the Laiyang Basin of the Sulu Belt, eastern China

Abstract Early Cretaceous high‐K calc‐alkaline volcanism occurring in the Laiyang Basin north of the Sulu high‐pressure to ultrahigh‐pressure (HP‐UHP) Metamorphic Belt, eastern China, comprises a wide spectrum of rock types, ranging from trachybasalts to trachydacites. The basaltic–andesitic rocks erupted at 107–105 Ma, spanning an SiO₂ range of 50.1–59.6% and an MgO range of 2.6–7.2%, and are characterized by large ion lithophile element (LILE; e.g. Ba and K) and light rare earth element (LREE) enrichment, high field strength element (HFSE) depletion and highly radiogenic Sr but non‐radiogenic Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr(i) = 0.70750–0.70931; ?_Nd(t) = ?17.9 ? ?15.6). The geochemical similarities between these rocks and the earlier Sulu Belt lamprophyres suggest that both types of mafic rocks were derived from similar mantle sources with LILE and LREE enrichment. Thus, the Wulian–Qingdao–Yantai Fault that separates the two terranes at the surface should not be considered as a lithospheric boundary between the North China and Yangtze blocks. The felsic lavas erupted at 93–91 Ma, spanning an SiO₂ range of 61.6–67.0% and an MgO range of 1.1–2.6%, and show a trace element geochemistry similar to the basaltic rocks, but with higher radiogenic Sr and even lower Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr(i) = 0.70957–0.71109; ?_Nd(t) = ?19.1 ? ?17.5), similar to I‐type granitoids in the Sulu Belt. A crustal origin was proposed to explain their compositions (which are comparable to those of experimental slab melts), the >10 Ma eruption interval and the compositional gaps in some elements (e.g. P, Ti and Sr) between them and the older basaltic–andesitic rocks. These melts were derived from predominant metaigneous protoliths containing mafic accumulative counterparts of the basaltic–andesitic and/or lamprophyric magmas. The extensive extrusion of Early Cretaceous high‐K calc‐alkaline rocks in the Laiyang Basin favored an extensional regime in response to the progressive attenuation of the thickened lithosphere and orogenic collapse, as reflected in the development of the basin from a foreland basin (before the end of the Jurassic period) to a fault basin (since the Early Cretaceous period).     

Palaeomagnetism of middle Ordovician volcanic rocks from Quebec

This palaeomagnetic study is centered on agglomerates and volcanic rocks from the western margin of the Appalachian belt in the Drummondville-Actonvale-Granby area, Quebec (long.: 72°30′W, lat.: 46°00′N). It involves a total of 36 oriented samples (111 speciments) distributed over eleven sites. Both thermal and AF cleaning techniques were used to isolate residual remanent components. The dispersion of the directions is slightly reduced after AF cleaning and thermal treatment.The palaeopole position obtained is 191°E, 6°N (d_m = 14°, d_p = 7°) after thermal treatment and 164°E, 19°N (d_m = 11°, d_p = 6°) after AF cleaning. The polarity of most of the sites (two exceptions) are reversed. The thermal-treated data appear to be relatively stable and an approximate value of the primary magnetization is extracted from them. The palaeopole obtained does not lie close to the tentatively defined position of the Cambrian and Ordovician poles from rocks of the North American plate; it is located near the Upper Cambrian and Lower Ordovician poles from eastern Newfoundland and the Lower Ordovician pole from the Caledonides in Europe.     

Garibaldi group volcanic rocks of the salal creek area, southwestern British Columbia: Alkaline lavas on the fringe of the predominantly calc-alkaline garibaldi (cascade) volcanic arc

The Salal Creek area, at the north end of the main group of vents for the Quaternary Garibaldi (Cascade) Volcanic Belt, southwestern British Columbia, was the site of several small eruptions of mafic lava during the past 1 Ma. In contrast to the calc-alkaline character of all other parts of the Garibaldi Belt and the geographically nearly coincident Miocene and older Pemberton Volcanic Belt, the Salal Creek area Quaternary lavas are predominantly alkaline basalt and hawaiite with typical alkaline volcanic petrography, chemistry, and fractionation trends. Trace elements Ti-Zr-Y show within-plate character for the suite. As for other Garibaldi Belt volcanic rocks, Rb is low, Rb/Sr very low, and ⁸⁷Sr/⁸⁶Sr ratio is low, averaging 0.7032. The oxygen isotopic composition average, ¹⁸O = 5.9, is normal for mantle-derived volcanic rocks.This distinct change in magma type at the end of a volcanic are may be the consequence of a smaller degree of melting, melting at a slightly greater depth than calc-alkaline magma production, or a descending-plate edge effect.Ponded flows and pillow-palagonite accumulations indicate that several Salal Creek area eruptions occurred in proximity to ice which filled major valleys during pre-Wisconsin glacial periods.     

Potassium-argon radioages of karroo volcanic rocks from Lesotho

The geology of Lesotho is relatively simple: the overall structure being that of a large shallow basin of Karroo sediments and volcanics. The rocks analysed in this study were collected in December 1966. The principal objectives of the study were (a) to date the inception of Karroo volcanism and (b) to arrive at an estimate of the time-span represented by the lava section along the Bushman’s Pass road east of Maseru. The date at which volcanism began in Lesotho is important because of recent discoveries of early mammalian fossils in underlying beds; in relation to the Phanerozoic Time-Scale and to the K-Ar age pattern found in the Karroo dolerites of South Africa byMcDougall (1963). Cox andHornung (1966) have suggested that the fractionation stage reached by Karroo magmas may depend upon either the height of the magma column or the time that elapsed since the beginning of the volcanic episode. An estimate of the time-span of volcanism along the Bushman’s Pass section is of interest because of the extensive palaeomagnetic work done on these rocks at the Bernard Price Institute of Geophysical Research in Johannesburg. The paper contains the results of triplicate conventional total degassing whole rock K-Ar age determinations on 8 Drakensberg lavas and on 8 Karroo dolerite sills and dykes. The analysed rocks are described petrographically and the age pattern obtained from them is discussed in relation to the age and petrological information available from other Karroo igneous rocks. It is concluded that Karroo volcanism began in Lesotho around 187 m.y. and that « Karroo » intrusive activity continued intermittently until at least 155 m.y. ago. Some possible geological and petrological implications of these conclusions are outlined.     

Isotope geochemistry and origin of calc-alkaline lavas from a caledonian continental margin volcanic arc

Analyses for major and trace elements, including REE, and Sr, Nd and Pb isotopes are reported from a suite of Siluro-Devonian lavas from Fife, Scotland. The rocks form part of a major calc-alkaline igneous province developed on the Scottish continental margin above a WNW-dipping subduction zone. Within the small area (ca. 15 km²) considered, rock types range from primitive basalts and andesites (high Mg, Ni and Cr) to lavas more typical of modern calc-alkaline suites with less than 30 ppm Ni and Cr. There is a marked silica gap between these rocks (< 62%) and the rare rhyolites (> 74%), yet the latter can be generated by fractional crystallization from the more mafic lavas. In contrast, variation in incompatible element concentrations and ratios in the mafic lavas can not be generated by fractional crystallization processes. Increasing SiO₂ is accompanied by increasing Rb, K, Pb, U and Ba relative to Sr and high field strength elements, increasing LREE enrichment and increasing _Sr calculated at 410 Ma, and by decreasing HREE, Eu/Eu*, Sm/Nd and _Nd (410). _Nd and _Sr are roughly anticorrelated and have more radiogenic compositions than the mantle array, in common with data reported elsewhere from this part of the arc. The correlation extrapolates up to cross the mantle array within the composition field of the contemporary MORB source, and extrapolates down towards the probable compositional range of Lower Palaeozoic greywackes, which may form the uppermost 8 km of the crust, or may be supplied to the source by subduction. One sample, however, lies within the mantle array, and closely resembles lavas from northwestern parts of the arc, where a mantle source with mild time-integrated Rb/Sr and LREE enrichment has been inferred. The lavas have relatively high initial ²⁰⁷Pb/²⁰⁴Pb for their ²⁰⁶Pb/²⁰⁴Pb, a feature which has been interpreted elsewhere as the result of incorporation of a sediment component into arc magmas. The systematic changes with increasing SiO₂ in isotopic and chemical parameters can be explained by mixing of a greywacke-derived component with depleted mantle. The various possible mixing mechanisms are discussed, and it is considered most likely that mixing occurred in the mantle source through greywacke subduction. The bulk of the Rb, K, Ba and Pb in the lavas is probably recycled from the crust, whereas less than some 40% of the Sr and Nd is recycled. The calc-alkaline chemical trends are solely a function of mixing with the sediment component.     

Transition from mildly-tholeiitic to calc-alkaline suite: the case of Chichontepec volcanic centre, El Salvador, Central America

The Chichontepec volcano is a Plio-Pleistocene composite volcano that erupted lavas ranging from high-alumina basalts to dacites. It experienced a caldera-forming paroxysmal eruption during the early Pleistocene. Pre-caldera lavas are mildly tholeiitic and they evolved mainly by low pressure crystal fractionation, notwithstanding the fact that most mafic lavas (low-MgO high-alumina basalts) retain traces of polybaric evolution. Conversely, post-caldera lavas, which are mainly pyroxene andesites, are clearly calc-alkaline, having evolved by open-system crystal fractionation. Sr–Nd isotopic data and trace elements characteristics indicate that the same mantle source was involved in the petrogenesis of these series. Modelling the AFC process showed that it did not play any role in the petrogenesis of these rocks; a crystal fractionation model is considered to be more relevant. A slight variation in the fractionating assemblage could have caused the transition from an early mildly tholeiitic trend to a late calc-alkaline one. Mineralogical evidence, mass-balance calculations and elemental chemistry support this hypothesis, assuming that the greater amount of pyroxene on the liquidus is at the expense of plagioclase; this would have prevented the trend in iron enrichment.