Implications For Magma Sources Research Articles

The Conrad Rise Revisited: Eocene to Miocene Volcanism and Its Implications for Magma Sources and Tectonic Development

AbstractThe Conrad Rise (CR), located midway between Antarctica and the Southwest Indian Ridge (SWIR), remains one of the least explored submarine large igneous provinces (LIPs) in the Indian Ocean to date. Relying on only seafloor paleomagnetic records, early studies hypothesized that the formation of the CR occurred during the Late Cretaceous. Here, we present new geochemical and geochronological data, including Sr‒Nd‒Pb‒Hf isotopes and 40Ar/39Ar data. Our results indicate that the uppermost part of the CR (Ob and Lena seamounts) unexpectedly formed later than previously predicted, at approximately 40 Ma in an intraplate setting. Another small seamount north of the Ob seamount formed later, at 8.5 Ma. The isotopic composition of lava from the small seamount north of the Ob seamount overlaps with that commonly defined by the Indian plume component. Overall, the isotopic variations defined by the volcanic suite from the CR could be accounted for by a three‐component mixing model involving the common component, lower continental crust, and depleted mantle endmembers. The newly obtained 40Ar/39Ar ages imply that the CR volcanism might have been triggered by major regional plate reorganizations during the middle to late Eocene and the late Miocene, inducing the release of a small upwelling rising from the African large low‐velocity province.

Petrology and geochemistry of early Carboniferous volcanic rocks in the Xinyuan region of western Tianshan: Implications for magma sources

Understanding the sources and processes that produce fossil continental arc magmas is still a challenging problem. To address this problem, here we perform detailed petrological investigations as well as whole-rock geochemical and SrNd isotopic analyses on the early Carboniferous volcanic rocks in the Xinyuan region. These volcanic rocks mainly consist of calc-alkaline basalt, trachy-andesite, and rhyolite, and show subduction-related geochemical and isotopic signatures. Petrographic observations and new geochemical data suggest that the rhyolite was generated by partial melting of juvenile crustal materials, while the basalt and trachy-andesite might undergo a series of open-system magmatic processes, such as fractional crystallization, crustal contamination, and magma replenishment and mixing. The primitive basaltic rocks with Mg# values of ~70 were least affected by crustal-level magmatic processes, and they could be used to constrain sub-arc mantle processes. The Nb/Yb, Zr/Yb, and Ta/Yb ratios of these basaltic rocks are similar to those of E-MORB, suggesting a relatively fertile mantle source prior to subduction, while their high Ba/Th, Sr/Nd, and Th/Nd ratios suggest a significant contribution of slab-derived components to the pre-subduction mantle wedge. Geochemical modelling based on trace elements and SrNd isotopic ratios indicates that the pre-subduction mantle wedge was modified by ~0.5% subducted sediment-derived fluids and ~2% bulk sediments. About 10% partial melting of such enriched mantle source could produce the early Carboniferous primitive magmas in the Xinyuan region.

Late Paleozoic alkaline magmatism in Western Transbaikalia, Russia: Implications for magma sources and tectonic settings

This paper presents the results of geochronological (40Ar-39Ar, U–Pb SHRIMP II), petrological and geochemical studies of the Late Paleozoic complexes of alkaline rocks (Zimovechinsky, Tuchinsky and Koma) located within the Vitim Plateau (the western part of the Mongol-Okhotsk Orogenic Belt). The rocks were formed at 310–280 ​Ma. It is coeval with Late Paleozoic magmatism within the Central Asian Orogenic Belt. The εNd(T) values show large variations from −2.1 to +3.3 as well as the initial Sr(I) isotopic ratios from 0.7042 to 0.7138, that demonstrate strong isotopic heterogeneity of the magmatic source. The geochemical characteristics of the rocks show pronounced positive Pb and negative Ti, Zr–Hf anomalies that can be explained by involvement of the subducted component in primary melts. The rocks intruded in a setting of extension at the active continental margin of the Siberian Craton during subduction of Mongol-Okhotsk oceanic crust under the Siberian Craton.

Geochronology, Geochemistry, and Hf Isotopic Compositions of Monzogranites and Mafic-Ultramafic Complexes in the Maxingdawannan Area, Eastern Kunlun Orogen, Western China: Implications for Magma Sources, Geodynamic Setting, and Petrogenesis

This paper presents zircon U-Pb-Hf isotopic compositions and whole-rock geochemical data for monzogranites and mafic-ultramafic complexes of the Maxingdawannan area in the western end of the east Kunlun orogenic belt, western China. The data are used to determine the ages, petrogenesis, magma sources, and geodynamic setting of the studied rocks. U-Pb zircon dating indicates that monzogranites and gabbros of the complexes were emplaced at 399 and 397 Ma, respectively. The monzogranites are shoshonitic, with high SiO2, Al2O3 and total-alkali contents, and low TFeO, MgO, TiO2 and P2O5 contents. The mafic-ultramafic complexes are characterized by low SiO2 contents. The monzogranites display enrichment in light rare-earth elements (LREE) and large-ion lithophile elements (LILE), depletion in heavy REEs (HREE) and high-field-strength elements (HFSE), and negative Eu anomalies (Eu/Eu*=0.36-0.48). The mafic-ultramafic complexes are also enriched in LREEs and LILEs, and depleted in HREEs and HFSEs, with weak Eu anomalies (Eu/Eu*=0.84-1.16). Zircon eHf(t) values for the monzogranites and mafic-ultramafic complexes range from -6.68 to 1.11 and -1.81 to 6.29, with zircon model ages of 1 812-1 319 Ma (TDM2) and 1 087-769 Ma (TDM1), respectively. Hf isotopic data indicate that primary magmas of the monzogranites are originated from partial melting of ancient lower crust during the Paleo-Mesoproterozoic, with a juvenile-crust component. Primitive magmas of the mafic-ultramafic complexes are likely originated from a depleted-mantle source modified by slab-derived fluids and contaminated by crustal components. Geochemical data and the geological setting indicate that Devonian intrusions in the Maxingdawannan area are related to northward subduction of the ProtoTethys oceanic lithosphere.

The Overmaraat-Gol Alkaline Pluton in Northern Mongolia: U–Pb Age and Preliminary Implications for Magma Sources and Tectonic Setting

A new Wenlockian zircon U–Pb age (~426 Ma) of the Overmaraat-Gol nepheline syenite (foyaite, juvite) pluton in the SW Lake Hovsgol area (Northern Mongolia) prompts a long history of alkaline magmatism in the western Central Asian Orogenic Belt, exceeding the duration of the Devonian and Permian–Triassic events. The LILE and HFSE patterns of pluton samples analyzed by X-ray fluorescence (XRF) and inductively coupled plasma (ICP-MS) methods indicate intrusion in a complex tectonic setting during interaction of a mantle plume with accretionary-collisional complexes that previously formed on the active continental margin. As a result, the parent magma had a heterogeneous source with mixed mantle (PREMA and EM) and crustal components. This source composition is consistent with Nd–Sr isotope ratios of the Overmaraat-Gol alkaline rocks, from −0.1 to −1.2 εNd(t) and from ~0.706 to 0.707 87Sr/86Sr(t).

Miocene shoshonite volcanism in Sardinia: Implications for magma sources and geodynamic evolution of the central-western Mediterranean

In this paper we document the existence of a Miocene shoshonite (SHO) volcanism in Northern Sardinia (Anglona). This occurrence completes the spectrum of orogenic magmas related to the subduction process which developed from the Eocene along the Palaeo-European continental margin, in concert with the opening of the Ligurian–Balearic back-arc basin and southeastward drift/rotation of the Sardinia–Corsica continental block. K–Ar ages show that the oldest volcanics of the area are calcalkaline (CA) basalts and andesites (~21Ma), overlain by 19.7–18.4Ma-old more potassic products such as high-potassium calcalkaline (HK-CA) and SHO lavas. CA, HK-CA and SHO suites include basalts and differentiated lavas of andesite and latite composition, respectively, that (according to the PELE software modelling) represent ~40–45% residual liquid fraction after shallow fractional crystallization. Application of the “Arc Magma Simulator” software suggests that the generation of primary melts of the distinct suites may occur at similar degrees of partial melting (5–8%) and melting pressures (2–2.2GPa, ~60–70km depth) in the mantle wedge. By contrast, the potassic character of parental melts of CA, HK-CA and SHO suites is controlled by 1) the amount of subducted continental components (possibly terrigenous sediments) and 2) the pressure (depth) at which these metasomatic agents are released from the slab. Results suggest that the slab depth beneath the volcanic district increased from ~80–100 to 100–120km for CA and SHO magmas, respectively. Accordingly, the evolution from CA to SHO magmatism in the same plumbing system could be related to slab deepening and increase of the subduction angle of ~5–10° in the time span of 2–3Ma. This tectono-magmatic scenario conforms to the major anticlockwise rotation (~30°) event of the Sardinia block (between 20.5 and 18Ma). This geodynamic evolution preludes the development of the volcanism in the Apennine–Tyrrhenian domains, where the final collisional/post-collisional stages of subduction were characterized by accentuated slab retreat and roll back, inter-arc extension and eruption of highly potassic magmas in the frontal arc (Roman and Aeolian Provinces).

Sr and Nd isotope composition of the metamorphic, sedimentary and ultramafic xenoliths of Lanzarote (Canary Islands): Implications for magma sources

The lavas produced by the Timanfaya eruption of 1730–1736 (Lanzarote, Canary Islands) contain a great many sedimentary and metamorphic (metasedimentary), and mafic and ultramafic plutonic xenoliths. Among the metamorphosed carbonate rocks (calc-silicate rocks [CSRs]) are monomineral rocks with forsterite or wollastonite, as well as rocks containing olivine ± orthopyroxene ± clinopyroxene ± plagioclase; their mineralogical compositions are identical to those of the mafic (gabbros) and ultramafic (dunite, wherlite and lherzolite) xenoliths. The 87Sr/ 86Sr (around 0.703) and 143Nd/ 144Nd (around 0.512) isotope ratios of the ultramafic and metasedimentary xenoliths are similar, while the 147Sm/ 144Nd ratios show crustal values (0.13–0.16) in the ultramafic xenoliths and mantle values (0.18–0.25) in some CSRs. The apparent isotopic anomaly of the metamorphic xenoliths can be explained in terms of the heat source (basaltic intrusion) inducing strong isotopic exchange ( 87Sr/ 86Sr and 143Nd/ 144Nd) between metasedimentary and basaltic rocks. Petrofabric analysis also showed a possible relationship between the ultramafic and metamorphic xenoliths.

Regional geochemical and isotopic variations of northern New England plutons: Implications for magma sources and for Grenville and Avalon basement-terrane boundaries

Examination of 36 Devonian to Pennsylvanian plutons within and bordering the Central Maine terrane of north-central New England shows that the plutons display regional variations in mineral and bulk-rock compositions and in oxygen and sulfur isotopic values. These variations principally correspond to the regional structural trends of the Central Maine terrane. Plutons located on that terrane9s southeast flank adjacent to the Massabesic Gneiss complex, on the terrane9s northwest flank adjacent to the Bronson Hill anticlinorium, and in the northeastern part of the Northeast Kingdom batholith of Vermont have biotites with relatively low Al concentrations (≤3.0 cations p.f.u. [per formula unit]), feldspars with higher An contents (up to An 55 ), and lower δ 18 O (7‰–9‰) and higher δ 34 S (average of 0‰) bulk-rock values compared to plutons within the central parts of the Central Maine terrane. These characteristics are most compatible with metaigneous or metavolcaniclastic sources. As shown by others, the plutons of Vermont and northwestern Maine preserve Grenvillian signatures whereas those of southeastern New Hampshire (specifically, the granite at Milford, New Hampshire) and Maine (Sebago, Phillips) have Avalonian signatures. Within the interior of the Central Maine terrane of New Hampshire and western Maine and in the Connecticut Valley trough of Vermont are distinct regions characterized by peraluminous plutons with high δ 18 O values (>11‰), low δ 34 S values (−25‰), Al-rich biotites (>3.3 cations p.f.u.), and An-poor plagioclases ( 30 ). These data are most consistent with metasedimentary sources, or, at a minimum, the plutons were heavily contaminated by Central Maine terrane or Connecticut Valley trough metasedimentary rocks. These peraluminous plutons do not have the unique signatures of the underlying Precambrian basements. Several metaluminous plutons in the Central Maine terrane along the New Hampshire–Maine border separate two regions of peraluminous plutons. These metaluminous plutons have biotite and isotopic compositions that are similar to those of the plutons along the flanks of the Central Maine terrane, indicative of less Paleozoic metasedimentary input compared to the peraluminous plutons in the interior of the Central Maine terrane. The metaluminous plutons also have trace element characteristics identical to those of plutons in Vermont and northwestern Maine that have a known Grenville basement-terrane source. We suggest that the metaluminous plutons also had sources in the underlying Grenville basement and that the Grenville-Avalon boundary cuts diagonally across northern New England. Because the plutons with Grenvillian signatures are adjacent to plutons with Avalonian signatures—the Sebago batholith and Phillips pluton—we suggest that there is no Medial New England basement in this region of New England. Some peraluminous, two-mica plutons have trace element and stable isotopic characteristics that are compatible with partial melting of metasedimentary rocks, the components of which were deposited in an anoxic basin such as the Smalls Falls Formation of the Central Maine terrane, but many other plutons have compositions that are inconsistent with this model. We suggest that the compositions of many New Hampshire Plutonic Suite plutons indicate greater heat input than that available from U-enriched metasedimentary rocks and instead require emplacement of mafic magmas from the mantle or lower crust.

Regional geochemical and isotopic variations of northern New England plutons: Implications for magma sources and for Grenville and Avalon basement-terrane boundaries

Regional geochemical and isotopic variations of northern New England plutons: Implications for magma sources and for Grenville and Avalon basement-terrane boundaries

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.