Nathan L. Green

SLAB DEHYDRATION AND BASALT PETROGENESIS IN SUBDUCTION SYSTEMS INVOLVING VERY YOUNG OCEANIC LITHOSPHERE

Abstract Compositions of post-Miocene basalts erupted in the Garibaldi and Central America volcanic arcs exhibit significant correlations with the age of the subducted plate. In general, SiO 2 , Al 2 O 3 , CaO, V, and (Sr/P) N decrease and FeO, MgO, TiO 2 and Na 2 O increase as the age of the subducted plate decreases. Variations in CaO/Al 2 O 3 , SiO 2 , (Sr/P) N , and Ba are compatible with lesser slab input, and hence less hydrous melting conditions in the mantle wedge in segments of the arcs overlying the youngest oceanic lithosphere. This interpretation is supported by comparison with peridotite melting experiments, which suggest higher melt pressures and temperatures in the mantle wedge above very young oceanic lithosphere. These observations point to a model in which dehydration of the downgoing slab occurs at shallow depths in subduction systems involving oceanic lithosphere younger than about 20 Ma. Because young oceanic lithosphere is relatively warm, little post-subduction heating is required to produce metamorphic reactions that release slab volatiles. Geodynamic models indicate most volatile-liberating reactions will occur within the seismogenic zone in oceanic lithosphere younger than 20 Ma, thus limiting the volatile flux beneath the arc and encouraging drier, higher temperature and higher pressure melting conditions in the mantle wedge in comparison to typical arc systems. Liberation of volatiles in the downgoing plate is strongly dependant on the shear stress on the fault, but is predicted to occur within the seismogenic zone for shear stresses greater than ∼33 MPa. Similarly, early loss of volatiles is predicted over a wide range of convergence rates, plate dips, and convergence angles. These results are shown to be robust for realistic ranges of slab dip, convergence angle, and shear stress, suggesting that volatile-poor melt generation is a characteristic of modern and ancient arc systems that involve subduction of young oceanic lithosphere.

On the relationship between subducted slab age and arc basalt petrogenesis, Cascadia subduction system, North America

Abstract Olivine-normalized ≤2.0 Ma magnesian basalts erupted close to the volcanic axis of the Cascadia subduction system exhibit arc-parallel compositional variations compatible with a northward decrease in slab-derived components in the underlying mantle wedge. Inferred decreases in slab input correlate strongly with a systematic decrease in age of the oceanic crust along the convergent margin. Geochemical trends are most pronounced in southwestern British Columbia and northern Washington basalts, where the Garibaldi belt strikes obliquely to the trend of isochrons on the subducted plate. From southern Washington to northern California, High Cascades arc segments trend nearly parallel to subducted plate isochrons, and basalts show more subdued arc-parallel variations. These observations support a model in which temperature and depth of melting in the mantle wedge beneath the arc are influenced by the age of the subducted plate. Where the arc is underlain by relatively young and hot oceanic crust, only a minor amount of heating during subduction is required before dehydration reactions begin in the subducted plate. The slab loses much of its volatiles trenchward of the arc, and the volatile budget in the mantle wedge beneath the arc is relatively low. In the Garibaldi belt, which overlies very young oceanic crust, this produces progressively more alkalic basalts to the north. The High Cascades overlies older oceanic lithosphere which was cooler at the time of subduction. As a result, dehydration reactions are delayed and a greater amount of volatiles are released beneath the arc. This results in lower melt temperatures and higher degrees of melting, producing predominantly low-K tholeiite and LILE-enriched HFSE-depleted calc-alkaline basalt magmas.

Basalt-basaltic andesite mixing at Mount Baker volcano, Washington, I. Estimation of mixing conditions

Abstract The Holocene Sulphur Creek basaltic andesite, which erupted from a small cinder cone on the southern flank of Mount Baker, locally contains 1–15 cm spheroidal to tongue-shaped inclusions of high-alumina basalt. Textural and chemical relationships indicate that the basalt was mixed with and quenched within the host lava, but that there was little or no homogenization of the two magmas. Both Sulphur Creek liquids had temperatures in excess of 1000°C, and mixing probably occurred at temperatures less than 1150°C at a pressure betweeen 0.5 and 2.0 kbar. Available evidence suggests that mixing of the two magmas did not result from simultaneous flow within the Sulphur Creek conduit, but rather occurred within a density-stratified magma chamber. The initial density contrast between basaltic and basaltic andesite liquids was determined by the thermal and compositional contrast across their interface, and the oxidation state, water content, and crystallinity of the two magma columns. The bulk density of the basalt was probably only slightly greater than that of basaltic andesite due to the high crystal content of the more-differentiated liquid. The basalt would not have had to reach water-saturation in order for the densities of the two liquids to become equal. Overturning of the magma chamber could have occurred without the requirement of volatile exsolution in the lower mafic layer.

Geochemistry and tectonic setting of basaltic volcanism, northern Coast Mountains, southeastern Alaska

Subgreenschist to amphibolite facies metavolcanic rocks found along the western flank of the Coast Plutonic Complex (western metamorphic belt) between Berners Bay and Endicott Arm, southeastern Alaska, are divided into three distinct sequences on the basis of age and the nature of interbedded metasedimentary rocks. These sequences crop out in linear belts, from west to east: the Gravina belt of Jurassic-Cretaceous age, the western Taku terrane of Permian to Triassic age, and the eastern Taku-Yukon-Tanana terrane of largely unknown age. Major, minor, trace element, and relict phenocryst chemistry are used to locate boundaries between the sequences and to provide clues to their origin. Variations in large ion lithophile element (LILE) and high field strength element (HFSE) abundances indicate that Gravina and eastern Taku magmas were derived through subduction-related processes. Pyroxene phenocryst compositions, LILE, light rare earth element, and HFSE enrichments in Gravina belt rocks are suggestive of high-K calc-alkaline to shoshonitic magmatism. Amphibolite facies metavolcanic rocks in the eastern Taku terrane are inferred to be island arc tholeiites. Western Taku metavolcanic rocks show little or no evidence of subduction-related genesis and resemble within-plate, possibly plume-related, basaltic magmas. Tectonic settings and magma sources inferred from metabasalt geochemistry and the associated stratigraphy of the three sequences are compatible with derivation of all the volcanic rocks in the western metamorphic belt and the Wrangellia terrane from a single Permian to Cretaceous arc complex. Initiation of arc tholeiite volcanism in the eastern Taku terrane possibly occurred during or prior to the Permian. Correlation of western Taku and Wrangellia rocks suggests a Triassic link between the Alexander and at least part of the Taku terrane, and nonsubduction-related basaltic volcanism prior to the Late Jurassic. Later rifting and/or changes in subduction geometry may have resulted in eruption of Gravina arc lavas into a marine basin, possibly floored by western Taku-Wrangellia rocks.

Fluorine geochemistry of quaternary volcanic rocks from Southwestern British Columbia: Some petrogenetic implications

Fluorine contents in 38 Quaternary volcanic rocks, representing calc-alkaline andesite eruptive groups from the Garibaldi Lake area, were determined by a selective ion-electrode method. A close relationship is evident between F abundance and the type of ferromagnesian phenocrysts present in the andesitic rocks. Hypersthene andesites have the lowest F contents (142–212 ppm), whereas hornblende-biotite andesites exhibit the highest F values (279–368 ppm); hornblende andesites have intermediate F contents (238–292 ppm). The hornblende-free Desolation Valley basaltic andesite has a lower F content than the hornblende-bearing Sphinx Moraine basaltic andesite (122 ppm versus 317–333 ppm).Different eruptive suites can be grouped on the basis of F differentiation patterns into (1) a hornblende-free lava series in which the F content of basaltic andesite is less than that of andesite, and (2) a hornblende-bearing lava series in which F contents remain constant or decrease slightly from basaltic andesite through dacite. Fluorine variation in the former series was controlled largely by fractionation of anhydrous minerals, whereas that in the latter was influenced by crystallization of amphibole, biotite and apatite.The distinctive F variation patterns of the two lava series appear to represent real differences in the volatile contents of Garibaldi Lake magmas. These different volatile concentrations may reflect varying degrees of magma-wallrock interaction, differences in the initial volatile contents of the primary magmas, or some combination of these factors.