Composition Of Volcanic Rocks Research Articles

Pb-Sr-Nd Isotopic Systematics of Mantle-derived Rocks in the World

Based on more than 30000 Pb-Sr-Nd isotopic analyses of the basaltic rocks of different tectonic settings such as mid-ocean ridge, oceanic island, island arc and intra-plate environments both within continental and oceanic plates, which have been reported in literatures, the sources of magma for different types of basalts and the nature of the mantle are reviewed in this paper. The data suggest that mantle domains such as EM1, EM2 and HIMU that have unique isotopic compositions do not represent typical mantle but rather represent local heterogeneity in the mantle, possibly in the transitional zone between the upper and lower mantles. PREMA (previous mantle) is a common mantle domain for oceanic island basalts and inter-plate flood basalts. This type of mantle domain probably exists in the lower mantle. DUPAL-anomaly exists not only in the source region of mid-ocean basalts, but also in the source regions of oceanic island and island arc basalts, and flood basalts within oceanic plates where variations in different isotopes in the rocks are coupled. The isotopic systematics of flood basalts indicate that their sources include recycled crustal materials in the mantle, partial melts from lithosphere owing to decompressing melting, mantle-derived melts from the transition zone between the upper and lower mantles, and from the lower mantle with primitive or slightly depleted isotopic signatures. Pb isotopic systematics may provide unique evidence for the contribution of subducted materials. Pb isotopic compositions of volcanic rocks from the Andean arc suggest that these rocks are not related to the subduction of the Pacific oceanic plate beneath the Andes.

Li isotopic compositions of volcanic rocks from Logudoro, Sardinia island: Constraints on its EM1 source

Li isotopic compositions of volcanic rocks from Logudoro, Sardinia island: Constraints on its EM1 source

The role of slab melting in the petrogenesis of high-Mg andesites: evidence from Simbo Volcano, Solomon Islands

The petrogenesis of high-Mg andesites (HMA) in subduction zones involves shallow melting of refractory mantle sources or, alternatively, the interaction of ascending slab-derived melts with mantle peridotite. To unravel the petrogenesis of HMA, we report major, trace element and Sr–Nd–Hf–Pb isotope data for a newly found occurrence of HMA in the New Georgia group, Solomon Islands, SW-Pacific. Volcanism in the Solomon Islands was initiated by subduction of the Pacific plate beneath the Indian–Australian plate until a reversal of subduction polarity occurred ca. 10 Ma ago. Currently, the Indian–Australian plate is subducted northeastwards along the San Cristobal trench, forming the younger and still active southwestern Solomon island arc. However, a fossil slab of Pacific crust is still present beneath the arc. The edifice of the active volcano Simbo is located directly in the San Cristobal trench on top of the subducting Indian–Australian plate. Simbo Island lies on top of a strike-slip fault of the adjacent Woodlark spreading centre that is subducted beneath the Pacific plate. Geochemical and petrological compositions of volcanic rocks from Simbo are in marked contrast to those of volcanic rocks from islands north of the trench (mostly arc basalts). Simbo-type rocks are opx-bearing HMA, displaying 60–62 wt% SiO2 but rather primitive Mg–Ni–Cr characteristics with 4–6 wt% MgO, up to 65 ppm Ni, up to 264 ppm Cr and Mg# from 67 to 75. The compositions of the Simbo andesites are explained by a binary mixture of silicic and basaltic melts. Relict olivine phenocrysts with Fo88–90 and reaction-rims of opx also support a mixing model. The basaltic endmember is similar to back-arc basalts from the Woodlark Ridge. A slab melt affinity of the silicic mixing component is indicated by Gd(N)/Yb(N) of up to 2.2 that is higher if compared to MORB and other arc basalts from the Solomon Islands. 87Sr/86Sr, ɛNd and ɛHf values in the analysed rocks range from 0.7035 to 0.7040, +6.4 to +7.9 and +12 to +14.4, respectively. These values reveal the presence of the Indian–Australian mantle domain beneath Simbo (i.e. the Indian–Australian plate) and also beneath all other volcanic islands of the New Georgia group, which are located north of the San Cristobal trench. 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values (18.43–18.52, 15.49–15.55 and 18.13–18.34, respectively) confirm the presence of slab melts from the subducted Pacific plate beneath southern Simbo where the highest Gd(N)/Yb(N) ratios are reported. A spatial shift towards an Indian–Australian slab signature is observed when approaching the active San Cristobal trench on northern Simbo, reflecting the decreasing influence of slab melts from the old subducted Pacific plate.

The volatile content of magmas from Arenal volcano, Costa Rica

We provide the first direct measurements of water in mafic melts from Arenal volcano, Costa Rica. Ion microprobe analyses of olivine-hosted melt inclusions (MI) from the prehistoric ET3 (AR-19) and ET6 (AR-16) tephra layers reveal high concentrations of volatile species: ∼ 1–4 wt.% H 2O, 50–300 ppm CO 2, and > 3000 ppm S and Cl. The MI with the highest water concentrations are the most mafic, and the dataset as a whole records a history of degassing coupled with fractionation and ascent from ∼ 2 kbar to 0.2 kbar. Arenal MI form two groups based on their Al, CO 2 and S contents. The ET3 high-Al MI were trapped at the highest pressure, are closest to equilibrium with their host olivines (∼ Fo 79), which are closest to equilibrium with the bulk-rock liquid. These MI are excellent candidates for Arenal parental liquids, and can generate most Arenal volcanic rock compositions by crystal accumulation (up to 30%), or crystal fractionation at a range of pressure (0.5–3 kb) and H 2O contents (0.5–3.5 wt.%). The new sulfur data reported here predict total sulfur output over the past ∼ 30 years from bulk basaltic andesite liquid that matches well spectroscopic estimates, and resolves the previously noted imbalance. MI from different ET3 and ET6 samples show different F/Cl, while most trace element ratios show a limited range similar to that of the host rocks. The high water content (4 wt.% H 2O) of Arenal basaltic magma is somewhat surprising given the weak subduction signal recorded geochemically (e.g., low 10Be and B). The Arenal MI data contribute to a positive correlation between primary water contents and Ba/La in Central American volcanoes, although further testing is required given the small number of data points, and the expectation that water and trace elements should have different sources in the subduction zone.

Mesoproterozoic rocks of Namibia and their plate tectonic setting

Two main Mesoproterozoic provinces occur in southern Namibia: (1) The high-grade Namaqua Metamorphic Complex (NMC) composed of a supracrustal sedimentary succession and interpreted as a passive margin sequence in the west of the Kalahari craton; (2) The Sinclair Group and its northeastern correlatives, composed of two main magmatic and metamorphic units, reflecting northeast-directed subduction, which started before 1.37 Ga and lasted until about 1.1 Ga. These two units were tectonically juxtaposed during the 1.1–1.03 Ga Namaqua orogenic event. The Kairab–Kumbis Metamorphic Complex comprises metasedimentary and metavolcanic rocks intruded by the 1.37 Ga arc-related Aunis tonalite. The mafic volcanic rocks from this complex have geochemical features of island arc calcalkaline basalts; they were emplaced and metamorphosed along an active margin before 1.37 Ga. The 1.2–1.1 Ga low-grade unmetamorphosed volcanic and immature sedimentary rocks of the Sinclair Group and its northwestern equivalents rest disconformably on the Kairab–Kumbis Complex. They occur in fault-bounded depocenters defining a regional arc-shape structure up to 100 km-wide and with a minimum length of 2000 km. The plate tectonic setting of this arc is best constrained by the composition of volcanic rocks from the ∼1.2 Ga Barby Formation and coeval granitoids; they comprise high-K calcalkaline rocks suggesting emplacement in an active continental margin setting. The final stage of this continental arc evolution is recorded in the <1.1 Ga tholeiites of the Opdam Formation. High Ti-content and flat REE-patterns in the tholeiites suggests an extensional event, whereas high Th/Ta and La/Nb ratios, low Ce/Pb values and negative anomalies for Nb–Ta suggest a subduction-related setting for the mantle source from which the mafic magmas were derived. Docking of continents led to the slab detachment, allowing interaction between the asthenospheric mantle and the mantle wedge enriched during the subduction process. The magmatic underplating related to this event induced the genesis of large-scale batholitic granitoid bodies in the NMC and a ∼1.1–1.0 Ga high-grade LP/HT metamorphism, with mineral assemblages indicating an anti-clockwise P–T–t path.

Bimodal volcanism in a tectonic transfer zone: Evidence for tectonically controlled magmatism in the southern Central Andes, NW Argentina

This field-based and analytical laboratory study focuses on the genetic relationship between bimodal volcanic centres and fault types of an important tectonic transfer zone in the southern Central Andes, the NW–SE striking Calama–Olacapato–Toro (COT) volcanic belt. More specifically, tectono–magmatic relationships are examined for the 0.55 Ma Tocomar, the 0.78 Ma San Jerónimo and the 0.45 Ma Negro de Chorrillos volcanic centres in the Tocomar area (66°30 W–24°15 S). Structures of the COT volcanic belt, notably NW–SE striking strike-slip faults and NE–SW trending normal faults, accommodated differential shortening between major N–S striking thrust faults on the Puna Plateau. We present evidence that bimodal volcanism was contemporaneous with activity of these fault types in the COT volcanic belt, whereby eruption and composition of the volcanic rocks in the Tocomar and San Jerónimo–Negro de Chorrillos areas appear to have been controlled by the kinematics of individual faults. More specifically, rhyolitic centres such as the Tocomar are associated with normal faults, whereas shoshonitic–andesitic monogenetic volcanoes, e.g., the San Jerónimo and Negro de Chorrillos centres, formed at strike-slip dominated faults. Thus, the eruption of higher viscous rhyolite magmas appears to have been facilitated in tectonic settings characterized by horizontal dilation whereas ascent and effusive volcanic activity of less viscous and hot basaltic andesites to shoshonites were controlled by subvertical strike-slip faults. While the Tocomar rhyolites are interpreted to be derived from an anatectic crustal source, geochemical characteristics of the San Jerónimo and Negro de Chorrillos shoshonitic andesites are in agreement with a deeper source. This suggests that the composition of erupted volcanic rocks as well as their spatial distribution in the Tocomar area is controlled by the activity of specific fault types. Such volcano–tectonic relationships are also evident from older volcanic centres of the COT volcanic belt. This points to a close genetic relationship between bimodal volcanism and the upper-crustal deformation regime, whereby episodes of increased volcanic activity correlate with deformation episodes in the Central Andes.

Chemical Composition of Ores from the Takara Volcanogenic Massive Sulfide Deposit, Central Japan

Abstract. The Takara volcanogenic massive sulfide (VMS) deposit occurs in Miocene formation of the Misaka Mountain, the South Fossa Magna region, central Japan. The tectonic setting of the Misaka Mountain is reconstructed to be a part of the paleo Izu‐Ogasawara arc which collided with the Honshu arc and to form accreted body in the present position. The Takara deposit, therefore, is considered to have formed in the paleo Izu‐Ogasawara arc.The ores from the Takara deposit are classified into pyrite‐type ore, chalcopyrite‐type ore, and sphalerite‐type ore on the basis of chemical composition and their mineral assemblages. Some pyrite‐type ores are characterized by their high Au content. The Au content is hardly recognized in the chalcopyrite‐type and sphalerite‐type ores.The ores from the Takara deposit have intermediate bulk chemical composition between those from the Besshi‐type deposits and the Kuroko‐type deposits that are two representative VMS deposits. However, the bulk chemical composition is closer to that from the Kuroko‐type deposits. And moreover, chemical composition of tetrahedrite‐tennantite series minerals (tetrahedrite) is similar to that from the Kuroko‐type deposits. The bulk chemical composition (Cu, Zn, Co, Pb, and As contents) of ores is affected by the chemical composition of volcanic rocks associated with VMS deposits.

Experiments on magma mixing

The arrival of a new hot batch of magma changes the thermal and the chemical state of a volcanic plumbing system. Formation of hybrid magma by chemical dilution (liquid miscibility model) of different magma batches is generally accepted. Observations of drop‐like domains of less evolved (newer) magma in more evolved (host) magma point to the contribution of a mechanical mixing process (liquid immiscibility). The interplay of both mechanisms seems to control the time scale from the dormant to the active state of a volcano. We carried out laboratory experiments using melts from natural volcanic rock compositions with the aim to validate model calculations for shear rate dependent domain sizes. The results confirmed the model and also the importance of hydro‐mechanically mixing for chemical dilution. The key process, however, that is able to change the state of a magma reservoir on a critical time scale is hydrodynamic magma mixing.

Sr–Nd–Pb isotopic compositions of volcanic rocks around the Hishikari gold deposit, southwest Japan: implications for the contribution of a felsic lower crust

The Sr–Nd–Pb isotope compositions of Late Pliocene to Pleistocene volcanic rocks around the Hishikari mine in southern Kyusyu of Japan, one of the largest hydrothermal gold deposit in the circum-Pacific belt, were determined in order to elucidate their source characteristics and evolution and the characteristics of magma related to gold mineralization. The Hishikari volcanic rocks (HVR) are classified into the Kurosonsan group (2.4–1.0 Ma) in the northern area and the Shishimano group (1.7–0.5 Ma) in the southern area. Each group is composed of three subgroups of andesite and one subgroup of rhyodacite in a late-stage. In the Kurosonsan group, the 87Sr/ 86Sr, 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb increase, while the 143Nd/ 144Nd decrease with decreasing age. The evolution pattern is opposite to this in the Shishimano group. On the Sr–Nd and Pb isotope diagrams, the HVR show a mixing array between two isotopically homogeneous components: a depleted component with a MORB-like composition, and an enriched component with radiogenic Pb and Sr and unradiogenic Nd. Regional comparison of isotope data from the HVR with those of rocks in and around Japan indicates that (1) the depleted component originated from low-K and high-alumina basaltic magma upwelling from the mantle, whereas (2) the enriched component was derived from rocks in lower crust of granodioritic composition, which we considered to have developed in the eastern margin of the Eurasian subcontinental lithosphere. According to a model calculation, the assimilation of the lower crust into the HVR increased with time from 51% to 77% in the Kurosonsan group, whereas decreased from 68% to 57% in the Shishimano group. We suggest that the Shishimano rhyodacite, which crystallized in the later stages of the Shishimano group and is considered to have been responsible for gold mineralization, was formed by intense fractional crystallization from andesite magma with a depleted Sr, Nd and Pb isotopic signature.

Deep roots of the Messinian salinity crisis.

The Messinian salinity crisis--the desiccation of the Mediterranean Sea between 5.96 and 5.33 million years (Myr) ago--was one of the most dramatic events on Earth during the Cenozoic era. It resulted from the closure of marine gateways between the Atlantic Ocean and the Mediterranean Sea, the causes of which remain enigmatic. Here we use the age and composition of volcanic rocks to reconstruct the geodynamic evolution of the westernmost Mediterranean from the Middle Miocene epoch to the Pleistocene epoch (about 12.1-0.65 Myr ago). Our data show that a marked shift in the geochemistry of mantle-derived volcanic rocks, reflecting a change from subduction-related to intraplate-type volcanism, occurred between 6.3 and 4.8 Myr ago, largely synchronous with the Messinian salinity crisis. Using a thermomechanical model, we show that westward roll back of subducted Tethys oceanic lithosphere and associated asthenospheric upwelling provides a plausible mechanism for producing the shift in magma chemistry and the necessary uplift (approximately 1 km) along the African and Iberian continental margins to close the Miocene marine gateways, thereby causing the Messinian salinity crisis.

Geochemistry of high-Mg andesites and adakitic andesite from the Sanchazi block of the Mian-Lue ophiolitic melange in the Qinling Mountains, central China: Evidence of partial melting of the subducted Paleo-Tethyan crust.

Major and trace element and Nd and Pb isotopic compositions of volcanic rocks from the Sanchazi (SCZ) block of the Mian-Lue ophiolitic melange in the Qinling Mountains, central China were analyzed to provide insights into the subduction processes during the Late Paleozoic along the margins of the paleo-Tethyan ocean. All igneous rocks from the SCZ block show characteristics typical of arc volcanic rocks such as depletion in high field-strength elements (HFSE) and enrichment in large-ion-lithophile elements (LILE) relative to normal mid-ocean ridge basalts (N-MORB). Included among these arc rocks are high-Mg andesites (HMAs) and an adakitic andesite. The adakitic andesite has a steep REE pattern, low Y and Yb, and high La/Yb values, which is similar to those of typical adakites derived by partial melting of subducted basaltic crust. The HMAs from the SCZ block also have adakitic affinities except that they have lower La/Yb (∼10) and Al2O3 (<15 wt.%). Both the adakitic andesite and HMAs have higher MgO, Ni, and Cr contents than the normal andesites and basalts. The age-corrected Nd-Pb isotopic compositions of the HMAs and adakitic andesite show an enriched isotopic signature relative to the MORB-type rocks in the Mianlue area, suggesting that the former probably contain components from sediments or continental crust materials. The data reveal that the adakitic andesite and HMAs in the SCZ block were most probably produced by partial melting of an eclogitic oceanic crust and/or subducted sediments, followed by interaction of the melt with the overlying mantle wedge, and finally by a possible contamination of the melt by the crustal materials. Our result infers that seafloor spreading was active in the paleo-Tethyan ocean, and an ancient island arc system was present along the proto-Qinling area in the Late Paleozoic. The paleo-Tethys ocean separated the South China and North China blocks in Late Paleozoic. The final integration between above two blocks is suggested to have occurred in the Triassic.

Petrogenesis of the volcanic suite of Bouvet ya (Bouvet Island), South Atlantic

Whole rock and mineral compositions of volcanic rocks collected during the Norwegian Polarsirkel expedition (1978/79) to the volcanic island of Bovetøya (close to the Bouvet Triple Junction) are discussed and compared with previously published data from the island. The rock types, hawaiite, benmoreite, and peralkaline trachyte and rhyolite (comendite) are related to each other by crystal fractionation processes. The trace element and radiogenic isotope signatures displayed by the Bouvetøya rocks are those of a moderately enriched oceanic island suite. On several isotope plots Bouvetøya rocks fall on or close to mixing lines between the enriched EM-1 and HIMU mantle components. Mixing between depleted morb mantle (DMM) and enriched components is not likely. Thus, Bouvetøya displays a typical plume signature.

Methodology for the study of melt inclusions in Cr-spinel, and implications for parental melts of MORB from FAMOUS area

Melt inclusions trapped in Cr-spinel in mid-ocean ridge picrites (FAMOUS area, Atlantic Ocean) were studied using a heating stage and analyzed with both electron and ion microprobes. This new technique can provide information on the most primitive magma compositions, olivine-spinel-melt compositional relationships, crystallization temperature and geochemical diversity of parental melts. The study of melt inclusions in spinel can be applied to different high-Mg suites to show the effects of shallow level fractionation, mixing and contamination that usually affect the composition of volcanic rocks and glasses.

Age and Sr isotopic composition of volcanic rocks in the Maricunga Belt, Chile: implications for magma sources

K-Ar and 40Ar/ 39Ar dating of volcanic rocks from the Maricunga belt of north-central Chile indicate that igneous activity took place throughout most of Miocene time at various places in the 150 by 30 km belt. No migration patterns of volcanism appear in the Miocene rocks of the belt. Volcanic activity ceased by the end of the Miocene. All the Miocene volcanic rocks studied are calcic andesites to dacites with about 62% SiO 2, 18% A1 2O 3, 4% Fe 2O 3 (total), and 2% K 2O. Initial 87Sr/ 86Sr (Sr i) values fall into two groups, one of lower values around 0.7050 to the west and the other of higher values around 0.7060 to the east. It is postulated that the two Sr i groups reflect two adjoining coherent lower-crustal magma sources of possibly different age and subtly different composition that form part of the western edge of the South American craton.

Chemical and Isotopic Variations in Volcanic Rocks from the Rungwe Province: Constraints on the Development and Scales of Source Heterogeneity Beneath the African Western Rift

The chemical and isotopic compositions of volcanic rocks erupted in continental rifts reflect contributions from the asthenosphere, the continental lithospheric mantle, and the continental crust. In particular, enriched or metasomatized subcontinental lithosphere may be an important source for alkaline marie volcanism (Lloyd and Bailey, 1975; Hawkesworth et al., 1990; Rogers et al., 1992); however, the chemical composition of this reservoir is not well characterized and is likely to be spatially heterogeneous. Both experimental and petrological evidence suggests that small volume CO2and/or H20-rich magmas can affect the chemistry and mineralogy of the

Cretaceous to Cenozoic volcanism in South Korea and in the Sea of Japan: magmatic constraints on the opening of the back-arc basin

Abstract The major element, trace element, and radiogenic isotope compositions of volcanic rocks in the back-arc area of the eastern Eurasian continental margin provide insight into the nature of the mantle wedge and constrain the magmatic evolution of the Japan Sea back-arc basin linked to its tectonic history. Different phases of post-Early Cretaceous volcanic activity are identified along the Korean margin and in the Japan Sea. Volcanic rocks from Korea include (1) Cretaceous and early Cenozoic calc-alkaline lavas of a volcanic arc at an active margin, and (2) Pliocene and Quaternary intraplate flood basalts and volcanic islands of alkaline composition. Japan Sea volcanic rocks consist of (1) early Cenozoic andesite flows of a remnant arc in the Yamato Bank, (2) early Miocene basalts of the Japan Sea basin basement, which share compositional characteristics of island arc tholeiites, continental rift tholeiites and back-arc basin basalts, (3) late Miocene seamounts of tholeiitic and mildly alkaline compositions, and (4) Pliocene and Quaternary alkaline volcanic islands. Geochemically, these rocks belong to three broad magmatic groups: (1) an arc-related, calc-alkaline group of a continental, Andean margin type, which prevailed prior to the opening of the Japan Sea between the Cretaceous and early Miocene, (2) continental rift tholeiites and back-arc basin basalts, formed during the rifting stage in the early Miocene, and (3) an intraplate alkaline group similar to OIB, erupted later during spreading, between late Miocene and Holocene times. Trace element and Sr, Nd and Pb isotopic compositions of selected samples show that the sources of magma Group 1 calc-alkaline lavas and magma Group 2 tholeiitic lavas included varying contributions of two main mantle components: an Indian Ocean MORB-like depleted mantle source (DMM) and an enriched mantle component similar to EM II. The latter component could represent DMM contaminated by subducted oceanic sediments incorporated into the lower lithosphere during the long-lived subduction of west Pacific crust. During the opening of the Japan Sea back-arc basin, the relative proportion of the DMM component dramatically increased between the rifting and spreading stages. It is also necessary to postulate a third component present in the sources of the Group 3, post-opening alkaline lavas, perhaps enriched mantle of EM I composition, which may also have resided in the subcontinental lithospheric mantle.

北海道北部，利尻火山のデイサイト質溶岩ドーム群のＫ‐Ａｒ年代

The dacitic lava dome group (DLDG), located on the NNW and SSE flanks of Rishiri volcano, northern Hokkaido, has been considered to be the Tertiary basement or early volcanic products of the Rishiri volcano. Newly obtained K-Ar ages of the rocks from the DLDG are 0.133±0.008 Ma (Peshimisaki lava dome), 0.078±0.005 Ma (Fujino lava dome) and <0.1 Ma (Ishiyama lava dome). These ages suggest that the DLDG are the products of flank eruptions during the early stage of activity of Rishiri volcano, and that the structure of the volcanic eddifice must be revised from those of previous studies. In addition, the chemical compositions of volcanic rocks from adjacent regions has changed from low-K tholeiitic during the Tertiary to alkalic (high Na20) during the Quaternary.

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.

Compositional diversity in late Cenozoic mafic lavas in the Rio Grande rift and Basin and Range province, southern New Mexico

This study reports major- and trace-element compositions of late Cenozoic (5 Ma and younger) mafic volcanic rocks from southern New Mexico. Stratigraphic sampling is reported for two of the volcanic fields, the Potrillo and the Jornado del Muerto volcanic fields; reconnaissance sampling, for an additional seven fields. These are among the first chemical analyses for some of these volcanic fields, and they provide insight into the dynamics of melt generation in this continental rift zone. The lavas fall into two groups: (1) an alkaline suite of basanite, alkali basalt, and trachybasalt with low SiO2 and high abundances of incompatible elements (TiO2, Nb, Rb, and Sr); (2) a subalkaline suite of subalkali basalts with high SiO2 and low abundances of the incompatible elements. Compositions of volcanic rocks from the Geronimo volcanic field, the Taos Plateau volcanic field, and the transition zone in central New Mexico are similar to those reported in this study, confirming that these are general characteristics of lavas throughout the region. For many of the large fields, the volcanic rocks belong exclusively to one compositional group or the other. Furthermore, there appear to be no patterns to the distribution of these compositional groups; for example, the Potrillo volcanic field, which is composed exclusively of alkaline lavas, and the Jornado del Muerto volcanic field, which is dominantly subalkaline, both occur in the axis of the Rio Grande rift. We see no evidence for evolution in melt chemistry during the past 5 m.y. In the Potrillo volcanic field, for which we have the best documentation of stratigraphic changes in composition, incompatible elements increase upsection and are correlated with decreasing Mg number, suggesting that magmas underwent differentiation before their final ascent. Phenocrystic olivine and plagioclase, which are liquidus phases at low pressure, imply a shallow depth for this crystallization event. Processes responsible for chemical divergence between the alkaline and subalkaline groups occurred at an early stage of magmatic evolution. Correlation between incompatible-element enrichment and silica undersaturation suggests that variable degrees of partial melting played a significant role in the generation of the two lava types.

Massive sulfide deposits of the Noranda area, Quebec. III. The Ansil mine

The Ansil massive sulfide deposit occurs at the contact of the underlying Northwest Rhyolite and the overlying Rusty Ridge Andesite, in the lower part of the Central Mine sequence of the Blake River Group. The orebody, which is roughly ellipsoidal in outline and up to 200 m × 150 m across, contained reserves of 1.58 Mt of massive sulfide grading 7.2% Cu, 0.9% Zn, 1.6 g/t Au, and 26.5 g/t Ag. Production began in 1989. Least-altered host rocks are low-K basaltic andesites and low-K rhyolites. These rocks have Zr/Y ratios of ~5 and LaN/YbN ratios of ~2.3, typical of tholeiitic volcanic rocks, although their major-element chemistry is transitional between tholeiitic and calc-alkaline volcanic rocks.The Ansil deposit, which dips ~50° east, is a single orebody comprising two main massive sulfide lenses (up to ~35 m thick) connected laterally via a thinner blanket of massive sulfides, with thin discontinuous but conformable massive magnetite units at the base and top of the orebody. Sulfide ore consists of massive to banded pyrrhotite–chalcopyrite. In the downplunge lens, up to 10 m of massive magnetite are capped by up to 10 m of massive sulfide. Finely banded cherty tuff, with sphalerite–pyrite–chalcopyrite, forms a discontinuous fringe to the deposit.The two main lenses of massive sulfide have the highest contents of Cu, Ag, and Au and are thought to have formed in areas of major hydrothermal input. Altered feeder zones contain either chlorite + chalcopyrite + pyrrhotite ± magnetite, or chlorite + magnetite ± sulfides. Footwall mineralization forms semiconformable zones ~5–10 m thick that directly underlie the orebody and high-angle pipelike zones that extend at least 50 m into the footwall. Ti–Zr–Al plots indicate that almost all altered footwall rocks were derived from a homogeneous rhyolite precursor. Hanging-wall andesites were also altered. Despite some severe alteration, all initial volcanic rock compositions can be readily identified, and thus mass changes can be calculated. Silica has been both significantly added or removed from the footwall, whereas K has been added except in feeder pipes. Oxygen-isotope compositions up to at least 50 m into the hanging wall and footwall are typically depleted in δ18O by 2–6‰. These rocks have gained Fe + Mg and lost Si. Altered samples in general range from light-rare-earth-element (REE) depleted to light-REE enriched, although some samples exhibit little REE modification despite strong alkali depletion. Mineralized volcanic rocks immediately below the orebody are enriched in Eu (as are some Cu-rich sulfides in the orebody).Contact and petrographic relations generally suggest that the main zone of massive magnetite formed by replacement of cp–po-rich sulfides, although local relations are ambiguous. Magnetite formation may reflect waning hydrothermal activity, during which fluids mixed with seawater and became cooler and more oxidized. Cu-rich feeder pipes that cut magnetite-rich footwall indicate a renewal of Cu-sulfide mineralization after magnetite deposition. Chloritic zones with disseminated sulfides occur up to a few hundred metres above the orebody, attesting to continuing hydrothermal activity.