Distinct Magma Sources Research Articles

The magmatic evolution of the late Miocene laccolith–pluton–dyke granitic complex of Elba Island, Italy

Since late Miocene time, post-collisional extension of the internal parts of the Apennine orogenic belt has led to the opening of the Tyrrhenian basin. Extensive, mainly acidic peraluminous magmatism affected the Tuscan Archipelago and the Italian mainland during this time, building up the Tuscan Magmatic Province as the fold belt was progressively thinned, heated and intruded by mafic magmas. An intrusive complex was progressively built on western Elba Island by emplacement, within a stack of nappes, of multiple, shallow-level porphyritic laccoliths, a major pluton, and a final dyke swarm, all within the span from about 8 to 6.8 Ma. New geochemical and Sr–Nd isotopic investigations constrain the compositions of materials involved in the genesis of the magmas of Elba Island compared to the whole Tuscan Magmatic Province. Several distinct magma sources, in both the crust and mantle, have been identified as contributing to the Elba magmatism as it evolved from crust-, to hybrid-, to mantle-dominated. However, a restricted number of components, geochemically similar to mafic K-andesites of the Island of Capraia and crustal melts like the Cotoncello dyke at Elba, are sufficient to account for the generation by melt hybridization of the most voluminous magmas (c. εNd(t) −8.5, 87Sr/86Sr 0.715). Unusual magmas were emplaced at the beginning and end of the igneous activity, without contributing to the generation of these hybrid magmas. These are represented by early peraluminous melts of a different crustal origin (εNd(t) between −9.5 and −10.0, 87Sr/86Sr variable between 0.7115 and 0.7146), and late mantle-derived magma strongly enriched in incompatible elements (εNd(t) = −7.0, 87Sr/86Sr = 0.7114) with geochemical–isotopic characteristics intermediate between contemporaneous Capraia K-andesites and later lamproites from the Tuscan Magmatic Province. Magmas not involved in the generation of the main hybrid products are not volumetrically significant, but their occurrence emphasizes the highly variable nature of crust and mantle sources that can be activated in a short time span during post-collisional magmatism.

Lithospheric dynamics and mantle sources of alkaline magmatism of the Cenozoic West Antarctic Rift System

Lithospheric extension in the West Antarctic Rift System and the Ross Sea embayment is related to Cenozoic alkaline volcanism. Basanites and alkali basalts define the two endmembers of primary magmas with distinct petrographic, chemical and isotopic compositions. Basanites are generally more primitive and significantly more enriched in incompatible trace elements than alkali basalts. Parallel incompatible trace-element distribution patterns in both rock types but slightly different isotopic compositions suggest a derivation by different degrees of partial melting from different mantle sources. Sr-, Nd- and Pb-isotopes allow the identification of distinct magma sources: asthenospheric mantle, enriched lithospheric mantle and a HIMU plume component which is widespread beneath the entire area. Spatial and temporal variations in isotopic compositions suggest a relationship between the dynamics of lithospheric extension and changing mantle sources of related magmas. Geothermobarometry on mantle xenoliths documents a pressure–temperature–time evolution of the mantle lithosphere which is related to rifting, uplift and cooling of mantle below the Ross Sea Rift–Transantarctic Mountain transition. This paper reviews existing data and ideas but is biased towards the author's own working area, the Western Ross Sea.

Isotopic compositions of late Cenozoic volcanics from southeast Papua New Guinea: Evidence for multi-component sources in arc and rift environments

Transition from collisional to extensional tectonics in eastern Papua New Guinea (PNG) is reflected in cessation of arc-type volcanism and eruption of rifting-related transitional basalts. Pb, Sr and Nd isotopic compositions for 28 samples representing these volcanic suites and subordinate high-K trachytes of unknown origin reflect melting and mixing of three distinct magma sources. The arc volcanics have high La/Nb, low Ce/Pb and overabundances of LILE relative to the LREE consistent with melting of subduction-modified sources. 87 Sr 86 Sr and 143 Nd 144 Nd ratios are moderately heterogeneous ( ϵ Nd = + 2.8 to + 6.3) and plot in the upper OIB field. Pb isotopic ratios are similar to modern Pacific sediment. Melting of a depleted MORB-type mantle containing up to 1.5% of Pacific sediment can account for the isotopic trends. The modeled amount of sediment is higher than that generally observed in Pacific arcs but lower than that suggested for Indonesia. Rift basalts have 87 Sr 86 Sr and 143 Nd 144 Nd ( ϵ Nd = + 5.3 to + 7.8) intermediate between Pacific MORB and associated arc volcanics. 207 Pb 204 Pb and 206 Pb 204 Pb ratios form a mixing trend and define Pacific MORB and sediment as end-members. High La/Nb and low Ce/Pb are further evidence for an important influence of subduction-modified sources at mantle and/or crustal levels. 143 Nd 144 Nd and 87 Sr 86 Sr ratios in the high-K trachytes plot near those of the most enriched arc volcanics but low 206 Pb 204 Pb and high 208 Pb 204 Pb indicate an old source that had evolved with low 238 U 204 Pb and high Th/U. These characteristics are similar to those in inferred lower crust from northeast Australia and EM 1 mantle sources; however, high La/Nb and low Ce/Pb ratios suggest additional source modification. Moderately radiogenic 207 Pb 204 Pb ratios preclude a significant contribution from crustal material much older than 2 Ga. We suggest that the trachytes inherited their isotopic signatures either from lower crust that was recycled into the upper mantle possibly during Mesozoic rifting of the Australian craton, or from a yet not recognized ancient continental block within the Late Cretaceous basement of PNG.

Isotope characteristics of submarine lavas from the Philippine Sea: implications for the origin of arc and basin magmas of the Philippine tectonic plate

Igneous rocks from the Philippine tectonic plate recovered on Deep Sea Drilling Project Legs 31, 58 and 59 have been analyzed for Sr, Nd and Pb isotope ratios. Samples include rocks from the West Philippine Basin, Daito Basin and Benham Rise (40–60 m.y.), the Palau-Kyushu Ridge (29–44 m.y.) and the Parece Vela and Shikoku basins (17–30 m.y.). Samples from the West Philippine, Parece Vela and Shikoku basins are MORB (mid-ocean ridge basalt)-like with 87Sr/ 86Sr= 0.7026−0.7032 , 143Nd/ 144Nd= 0.51300−0.51315 , and 206Pb/ 204Pb= 17.8−18.1 . Samples from the Daito Basin and Benham Rise are OIB (oceanic island basalt)-like with 87Sr/ 86Sr= 0.7038−0.7040 , 143Nd/ 144Nd= 0.51285−0.51291 and 206Pb/ 204Pb= 18.8−19.2 . All of these rocks have elevated 207Pb/ 204Pb and 208Pb/ 204Pb compared to the Northern Hemisphere Regression Line (NHRL) and have δ 207Pb values of 0 to +6 and δ 208Pb values of +32 to +65. Lavas from the Palau-Kyushu Ridge, a remnant island arc, have 87Sr/ 86Sr= 7032−0.7035 , 143Nd/ 144Nd= 0.51308−0.51310 and 206Pb/ 204Pb= 18.4−18.5. Unlike the basin magmas erupted before and after them, these lavas plot along the NHRL and have Pb-isotope ratios similar to modern Pacific plate MORB's. This characteristic is shared by other Palau-Kyushu Arc volcanic rocks that have been sampled from submerged and subaerial portions of the Mariana fore-arc. At least four geochemically distinct magma sources are required for these Philippine plate magmas. The basin magmas tap Source 1, a MORB-mantle source that was contaminated by EMI (enriched mantle component 1 [31]) and Source 2, an OIB-like mantle source with some characteristics of EMII (enriched mantle component 2 [31]). The arc lavas are derived from Source 3, a MORB-source or residue mantle including Sr and Pb from the subducted oceanic crust, and Source 4, MORB-source or residue mantle including a component with characteristics of HIMU (mantle component with high U/Pb [31]). These same sources can account for many of the isotopic characteristics of recent Philippine plate arc and basin lavas. The enriched components in these sources which are associated with the DUPAL anomaly were probably introduced into the asthenosphere from the deep mantle when the Philippine plate was located in the Southern Hemisphere 60 m.y.b.p.

Uranium in oceanic rocks: DSDP Leg 37

Fission-track mapping of uranium concentrations in basalts recovered from Leg 37 of the Deep Sea Drilling Project indicates that secondary enrichment or redistribution of the element is confined to specific zones of altered basalt. Primary magmatic variation in uranium concentration corresponding to major and trace-element variation within the basalts can also be discerned. Uranium, strontium, and other trace element values for the basalts fall into two distinct groupings, providing clear indication that two separate magmas were involved in the formation of the upper oceanic crust at site 332. Different degrees of partial melting of upper mantle material is the most probable mode of formation of the two distinct magma sources, which were tapped repeatedly and alternately to form the observed sequence of basalts encountered during the drilling of hole 332B.

Archaean mafic and ultramafic rock associations in the Eastern Goldfields region, Western Australia

Recent studies in Archaean volcanic belts of the Eastern Goldfields region of Western Australia have led to the development of a general picture of Archaean volcanic activity in this area. Three volcanic associations are recognized: a felsic volcanic-porphyry-clastic and volcanogenic sediment association, a pillow basalt-dolerite association and an ultramafic association. The ultramafic association consists of high-Mg basalts and zoned ultramafic lenses, both with textures indicative of high level intrusion or extrusion, and numerous small layered sills. The association commonly forms the lower parts of individual volcanic sequences, and is overlain by basalt. In some areas felsic volcanic rocks terminate this sequence. It is believed that the mafic and ultramafic associations represent two distinct magma sources. Elements of similarity between these stratiform belts and more recent oceanic crust are discussed.