Geochemical discrimination of different magma series and their differentiation products using immobile elements

Analytical data for Sr, Rb, Cs, Ba, Pb, rare earth elements, Y, Th, U, Zr, Hf, Sn, Nb, Mo, Ni, Co, V, Cr, Sc, Cu and major elements are reported for eocene volcanic rocks cropping out in the Kastamonu area, Pontic chain of Northern Turkey. SiO2% versus K2O% relationship shows that the analyzed samples belong to two major groups: the basaltic andesitic and the andesitic ones. High-K basaltic andesites and low-K andesites occur too. Although emplaced on continental type basement (the North Anatolian Crystalline Swell), the Pontic eocene volcanics show elemental abundances closely comparable with typical island arc calc-alkaline suites, e.g. low SiO2% range, low to moderate K2O% and large cations (Cs, Rb, Sr, Ba, Pb) contents and REE patterns with fractionated light and almost flat heavy REE patterns. ΣREE and highly charged cations (Th, U, Hf, Sn, Zr) are slightly higher than typical calc-alkaline values. Ferromagnesian elements show variable values. Within the basaltic andesite group the increase of K%, large cations, ΣREE, La/Yb ratio and high valency cations and the decrease of ferromagnesian element abundances with increasing SiO2% content indicate that the rock types making up this group developed by crystalliquid fractionation of olivine and clinopyroxene from a basic parent magma. Trace element concentration suggest that the andesite group was not derived by crystal-liquid fractionation processes from the basaltic andesites, but could represent a distinct group of rocks derived from a different parent magma.

Geochemistry of eocene calc-alkaline volcanic rocks from the Kastamonu area, Northern Turkey

MOST volcanic rocks in modern island and continental arcs are probably derived from melting of the mantle wedge, induced by hydrous fluids released during dehydration reactions in the subducted lithosphere1. Arc tholeiitic and calc-alkaline basaltic magmas are produced by partial melting of the mantle, and then evolve by crystal fractionation (with or without assimilation and magma mixing) to more silicic magmas2—basalt, andesite, dacite and rhyolite suites. Although most arc magmas are generated by these petrogenetic processes, rocks with the geochemical characteristics of melts derived directly from the subducted lithosphere are present in some modern arcs where relatively young and hot lithosphere is being subducted. These andesites, dacites and sodic rhyolites (dacites seem to be the most common products) or their intrusive equivalents (tonalites and trondhjemites) are usually not associated with parental basaltic magmas3. Here we show that the trace-element geochemistry of these magmas (termed 'adakites') is consistent with a derivation by partial melting of the subducted slab, and in particular that subducting lithosphere younger than 25 Myr seems to be required for slab melting to occur.

Derivation of some modern arc magmas by melting of young subducted lithosphere

Tectonic setting of basic volcanic rocks determined using trace element analyses

The chemical composition of subducting sediment and its consequences for the crust and mantle

1 What is "Typical Calcalkaline Andesite"?.- 1.1 Introduction.- 1.2 Definition of Orogenic Andesite.- 1.3 Magma Series Containing Orogenic Andesites.- 1.4 Overview.- 2 The Plate Tectonic Connection.- 2.1 Spatial Distribution of Active Orogenic Andesite Volcanoes.- 2.2 Initiation of Subduction.- 2.3 Cessation of Subduction.- 2.4 Collisions.- 2.5 Reversal of Subduction Polarity.- 2.6 Forearc and Transform Fault Volcanism.- 2.7 Anomalously Wide Volcanic Arcs.- 2.8 Andesites Clearly Not at Convergent Plate Boundaries.- 2.9 Conclusions.- 3 Geophysical Setting of Volcanism at Convergent Plate Boundaries.- 3.1 Topography, Gravity, Heat Flow, and Conductivity.- 3.2 Crustal Thickness, Structure, and Age.- 3.3 Upper Mantle Beneath the Forearc, Volcanic Arc, and Backarc Regions.- 3.4 Dipping Seismic Zones (Benioff-Wadati Zones) and Underthrust Lithosphere.- 3.5 Partial Melting and Magma Ascent Beneath Volcanic Arcs.- 3.6 Magma Chambers Beneath Orogenic Andesite Volcanoes.- 3.7 Conclusions.- 4 Andesite Magmas, Ejecta, Eruptions, and Volcanoes.- 4.1 Characteristics of Andesite Magma.- 4.1.1 Temperature.- 4.1.2 Density.- 4.1.3 Rheology.- 4.1.4 Miscellaneous Properties and Applications.- 4.2 Andesite Rock, Eruption, and Edifice Types.- 4.3 Variations in Magma Composition During and etween Historic Andesite Eruptions.- 4.4 Variations in Rock Composition During Evolution of Stratovolcanoes.- 4.5 Conclusions About Andesite Magma Reservoirs.- 4.6 Stress Fields and Volcano Spacings Within Volcanic Arcs.- 4.7 Relationships Between the Timing of Arc Volcanism and Plate Movements.- 4.8 Magma Eruption Rates at Convergent Plate Boundaries.- 4.9 Relative Proportions of Andesite.- 5 Bulk Chemical Composition of Orogenic Andesites.- 5.1 Rock Analyses: Significance, Averages, and Representative Samples and Suites.- 5.2 Major Elements.- 5.2.1 Silica Contents and Harker Variation Diagrams.- 5.2.2 Alkalies.- 5.2.3 Iron and Magnesium.- 5.2.4 Titanium.- 5.2.5 Aluminum and Calcium.- 5.2.6 Phosphorous.- 5.2.7 CIPW Normative Mineralogy.- 5.3 Volatiles.- 5.3.1 Water.- 5.3.2 Carbon Dioxide.- 5.3.3 Sulfur.- 5.3.4 Halogens.- 5.3.5 Oxygen.- 5.4 Trace Elements.- 5.4.1 The K-Group: Rb, Cs, Ba, and Sr.- 5.4.2 REE Group: Rare Earth Elements Plus Y.- 5.4.3 The Th Group: Th,U, and Pb.- 5.4.4 The Ti Group: Zr, Hf, Nb, and Ta.- 5.4.5 The Compatible Group: Ni, Co, Cr, V, and Sc.- 5.4.6 The Chalcophile Group: Cu, Zn, and Mo.- 5.4.7 Trace Element Systematics.- 5.5 Isotopes.- 5.5.1 Strontium.- 5.5.2 Lead.- 5.5.3 Neodymium.- 5.5.4 Inert Gases.- 5.5.5 U-Disequilibrium.- 5.5.6 Oxygen.- 5.5.7 Synthesis of Isotope Data.- 5.6 Comparison with Andesites Not at Convergent Plate Boundaries.- 5.7 Geochemical Distinctiveness of Volcanism at Convergent Plate Boundaries.- 5.8 Conclusions: Chemical Diversity of Orogenic Andesites.- 6 Mineralogy and Mineral Stabilities.- 6.1 Plagioclase.- 6.2 Pyroxenes.- 6.3 Amphibole.- 6.4 Olivine.- 6.5 Oxides.- 6.6 Garnet.- 6.7 Other Minerals.- 6.8 Inclusions in Orogenic Andesites.- 6.9 Mineral Stabilities in Andesite Magma.- 6.10 Trace Element Equilibria Between Minerals and Melt.- 6.11 Conclusions.- 7 Spatial and Temporal Variations in the Composition of Orogenic Andesites.- 7.1 Variations in Magma Composition Across Volcanic Arcs.- 7.2 Variations in Magma Composition Along Volcanic Arcs.- 7.3 Effects of Plate Convergence Rate on Magma Composition.- 7.4 Relationships Between Compositions of Orogenic Andesites and Adjacent Oceanic Crust.- 7.5 Changes in the Composition of Orogenic Andesites During Earth History.- 8 The Role of Subducted Ocean Crust in the Genesis of Orogenic Andesites.- 8.1 Characteristics of Subducted Ocean Crust Beneath Volcanic Arcs.- 8.2 Circumstantial Evidence of Slab Recycling in Arc Volcanism.- 8.3 Are Orogenic Andesites Primary Melts of Subducted Ocean Floor Basalt? No.- 8.4 The Sediment Solution.- 8.5 IRS Fluids and Maxwell's Demons.- 8.6 Conclusions.- 9 The Role of the Mantle Wedge.- 9.1 Characteristics of the Mantle Wedge.- 9.2 Circumstantial Evidence that Arc Magmas Originate Within the Mantle Wedge.- 9.3 Are Orogenic Andesites Primary Melts of Only the Mantle Wedge? Rarely.- 9.4 Fluid Mixing, Metasomatism, and Demonology in the Mantle Wedge.- 10 The Role of the Crust.- 10.1 Circumstantial Evidence for Crustal Involvement in Orogenic Andesites.- 10.2 Crustal Anatexis.- 10.3 Crustal Assimilation.- 11 The Role of Basalt Differentiation.- 11.1 General Arguments for and Against Differentiation.- 11.2 Roles of Plagioclase, Pyroxenes, and Olivine.- 11.3 Role of Magnetite and the Plagioclase-Orthopyroxene/Olivine-Augite-Magnetite (POAM) Model.- 11.4 Role of Amphibole.- 11.5 Role of Garnet.- 11.6 Role of Accessory Minerals: Apatite, Chromite, Sulfides, Biotite.- 11.7 Role of Magma Mixing.- 11.8 Role of Other Differentiation Mechanisms.- 11.9 Differentiation Processes Leading to Andesites in Anorogenic Environments.- 12 Conclusions.- 12.1 Andesite Genesis by POAM-Fractionation: the Most Frequent Mechanism.- 12.2 Some Outstanding Problems Requiring Clarification.- 12.3 Origin of Tholeiitic Versus Calcalkaline Andesites.- 12.4 Origin of Across-Arc Geochemical Variations.- 12.5 Epilog.- References.

Orogenic Andesites and Plate Tectonics

In order to better constrain the paleogeographic evolution of south China we measured Sm-Nd and Rb-Sr isotopic compositions for 23 Mesozoic granites that crop out throughout the area. Tightly grouped neodymium depleted mantle model ages (1.4 ± 0.3 Ga) suggest the region is underlain by relatively homogeneous Proterozoic crust and fail to define crustal provinces. Neither the isotopic nor geologic data suggest that a Mesozoic suture exists. However, granites possessing anomalously high Sm (>8 ppm) and Nd (>45 ppm) concentrations, relatively high initial epsilon neodymium (−4 to −8), and high but variable initial 87Sr/86Sr (0.759 to 0.713) form a northeast trending zone that coincides with two prominent Mesozoic basins. Southeast of the zone lie the majority of Mesozoic intrusives and Upper Triassic to Lower Cretaceous extensional basins found in south China. Mesozoic paleomagnetic poles are well clustered northwest of the zone. Pre-Cretaceous poles southeast of it are discordant with respect to those from the northwest. The only recognized tectonostratigraphic terrane in south China lies southeast of the zone. The terrane is bordered by a northeast trending sinistral fault that was active in the Mesozoic. Other faults in south China have similar attitudes, ages, and sense of shear. Together, the observations suggest that the Mesozoic tectonic regime in south China consisted of strike-slip activity plus concomitant rifting as terranes or fragments of similar crust were transported north along sinistral faults. The zone, defined by the granites enriched in Nd and Sm, demarcates displaced terranes to the southeast from relatively stable land to the northwest.

Isotopic and paleomagnetic constraints on the Mesozoic tectonic evolution of south China

Rare earth elements and the island arc tholeiitic series

[1] We propose that across-arc differences in the geochemistry of Izu-Bonin arc magmas are controlled by the addition of fertile-slab fluids to depleted mantle at the volcanic front, and residual-slab fluids to fertile mantle in the back arc without slab melting or contemporaneous back arc spreading. The arc consists of a volcanic front, an extensional zone, and seamount chains (the Western Seamounts) that trend into the Shikoku Basin. Each province produces a distinct suite of arc-like volcanic rocks that have relative Nb depletions and high ratios of fluid-mobile elements to high field strength elements. The volcanic front has the lowest concentrations of incompatible elements and the strongest relative enrichments of fluid-mobile elements (high U/Nb, Ba/Nb, Pb/Zr, Th/Nb, 206Pb/204Pb, ɛNd, and 87Sr/86Sr). A fluid derived from both sediment and altered oceanic crust explains most of the slab-related characteristics of the volcanic front. The Western Seamounts and some of the extensional zone rocks have lower ɛNd, 87Sr/86Sr, 206Pb/204Pb, Ba/Th, and U/Th; moderate Ba/Nb and U/Nb; and similar or higher Th/Nb and Th/Nd. Although the lower ɛNd and higher Th/Nd tempt a sediment melt explanation, a lack of correlation between the strongest sediment proxies, such as ɛNd, Th/Nb, and Ce/Ce*, precludes sediment melts. The subduction component for the Western Seamounts is probably a fluid dehydrated from a residual slab that was depleted in fluid-mobile elements beneath (as well as trenchward of) the volcanic front. This depleted fluid is added to elementally and isotopically more enriched mantle beneath the Western Seamounts.

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Across-arc geochemical trends in the Izu-Bonin arc: Contributions from the subducting slab

Available geochemical and field data suggest that three different periods of igneous activity have occurred on Viti Levu. Rocks of the first period are “island arc tholeiites”; of the second, a calc-alkaline series; and of the third, shoshonites. Each period has a geochemical distinctiveness. The temporal sequence also corresponds to a spatial zonation analogous to that observed in many currently active island arcs. K2O and the larger trace elements increase south to north and with time, whereas iron enrichment and Na2O/K2O ratios decrease. Initial Sr 87/86 ratios average about 0.7041 in rocks of all periods. If Viti Levu magmas were genetically related to underthrust oceanic crust, as is argued for comparable ones of modern island arcs, then the island must have drifted or been rotated to its present position within the last 5–10 m.y.

James B. Gill

Papers

Orogenic Andesites and Plate Tectonics

Isotopic and paleomagnetic constraints on the Mesozoic tectonic evolution of south China

Rare earth elements and the island arc tholeiitic series

Across-arc geochemical trends in the Izu-Bonin arc: Contributions from the subducting slab

Geochemistry of Viti Levu, Fiji, and its evolution as an island arc