Partial Melting Of Mantle Wedge Research Articles

PETROGENESIS AND ORIGIN OF THE CHENAR GRANITOID STOCK, NW OF KERMAN, IRAN: EVIDENCE OF NEOTETHYS SUBDUCTION RELATED ARC MAGMATISM

The lower to middle Miocene Chenar granitoid stock, a part of Central Iranian volcanic belt, is intruded into Eocene volcano-sedimentary complex in northwestern Kerman in south Central Iran. The Chenar granitoid stock consists mainly of coarse to medium grain granodiorite and monzogranite with subordinate tonalite and syenogranite. Alkali granites as aplitic veins and quartz dioritic dykes frequently cut through the granitoid stock. Enclaves of various types and sizes (andesitic and microgranular mafic, few meters to several centimeters wide) also occur near the margin. The granitoid rocks represent convergent margin magmas enriched in large ionic lithophile elements (LILE) such as Rb, Ba, K, Ce and depleted in high field strength elements (HFSE) such as Y, Nb and Zr. The REE patterns have relatively smooth LREE enriched chondrite normalized REE profiles with (La/Yb)n between 4.28 to 14.66. A lack of Eu anomalies along with a continuous increase in the slope of LREE profile from tonalite to syenogranite exhibit a relatively low fractionated chondrite normalized pattern [(La/Yb)n] and also involvement of plagioclase in the melting processes. Small negative Eu anomaly in alkali granite is an indication of more differentiated rock type [(La/Yb)n=42.78] in Chenar granitoid stock. Geochemical data, various trace element discriminant diagrams, common igneous microgranitoid enclaves, and ocean ridge granite normalized multi-element diagrams indicate that the Chenar granitoid stock has characteristics of metaluminous to slightly peraluminous, calc-alkaline, I-type granite of a volcanic arc settings and is formed in an active continental margin environment. In this model, a NE-dipping subduction zone of Neotethys could have accounted for the arc volcanism of the Central Iran where felsic, Andean-type magmatic arc volcanics and plutonics were formed. As a result of subduction mafic arc magmas with significant fluids were produced following the dehydration of oceanic crust and partial melting of mantle wedge which in turn provoke partial melting of considerably metasomatised and enriched subcontinental lithosphere, leading to generation of siliceous magma. Its low-pressure crystal fractionation gave rise to generation of hydrous calc-alkaline magmas, represented in part by Chenar granitoid stock.

Petrogenesis of the tholeiitic basalt, calc-alkaline basaltic andesite and high magnesian andesite lava succession of the Oligo-Miocene Anamizu Formation in northeastern Noto Peninsula, central Japan.

The Oligo-Miocene Anamizu Formation in the Ushitsu-Matsunami area consists of a lower volcano-sedimentary member and an upper volcanic member. The upper member is essentially composed of three lava series; tholeiite (basalt and basaltic andesite), calc-alkaline (basaltic andesite) and high-magnesian andesite (bronzite andesite), in ascending order. The tholeiitic basalt is further divided into K-poor and K-rich (K-feldespar bearing) types. The tholeiitic basaltic andesite has higher FeO*/MgO (∼3) than the others (1-1.5). The calc-alkaline basaltic andesite has higher Cr and Ni than tholeiitic basaltic andesite. Bronzite andesite contains higher MgO, Cr and Ni than common calc-alkaline andesite of the same SiO2 content (∼60 wt%), but lower than those of boninite and sanukite. All the rock series are depleted in HFSE (Nb and Ti) in comparison with N-MORB and OIB, suggesting typical subduction-related arc magmas. High Zr/Y ratios of the tholeiitic basalt resemble those of active continental margin magmas rather than island-arc magmas. The HFSE, Ni and Cr compositions indicate a progressive depletion or increasing degree of partial melting of the mantle wedge source in the order tholeiite basalt (basaltic andesite)→calc-alkaline basaltic andesite→bronzite andesite. Spinel in the three series shows different trends: Cr-poor (Cr# 0.49 to 0.54) in the tholeiitic basalt, fairly Cr-rich (Cr# 0.61) in the calc-alkaline basaltic andesite, and Cr-rich (Cr# 0.73) in the high-magnesian andesite. These trends indicate different mantle restites; lherzolite for the tholeiitic basalt and harzburgite for the calc-alkaline basaltic andesite and high-magnesian andesite, and increasing degree of depletion in the order as above. On the contrary, LILE and LREE exhibit a gradual enrichment of the source in the same order. Corresponding decrease of TiO2/K2O suggests that the enrichment has been due to addition of fluids derived from the descending slab. These data indicate that the K-poor tholeiitic basalt magma has been formed by partial melting of the lherzolitic upper mantle wedge under almost anhydrous conditions; whereas the K-rich tholeiitic basalt magma may have been produced by partial melting of the metasomatized lherzolitic mantle source under slightly hydrous conditions. The calc-alkaline and bronzite andesite magmas have been produced by partial melting of the hydrous, metasomatized, harzburgitic mantle wedge. The stratigraphy of the lava succession in the studied area (tholeiite basalt-basaltic andesite→calc-alkaline basaltic andesite→bronzite andesite) indicates that depletion and hydration (metasomatism) of the mantle source have progressed simultaneously.

Partial melting of mantle wedge and evolution of island arc crust

Evolution of the crust in the northeast Honshu arc is discussed on the basis of the melting experiments of a peridotite under hydrous conditions and the crustal compositions estimated from deep crustal xenoliths and seismic velocity data. The chemical compositions of melts formed by partial melting of a hydrous peridotite within the pressure‐temperature conditions for shallow parts of the mantle wedge (1.2–2.0 GPa) have been experimentally determined with the “sandwich experiment” technique. The melts range from olivine tholeiitic to magnesian andesitic compositions (SiO2 48–56 wt %; MgO 9–14 wt %) beneath the volcanic front in the northeast Honshu arc and are slightly less silicic beneath the coast of the Sea of Japan. The crustal compositions of the volcanic zone in the northeast Honshu arc estimated from deep‐seated crustal inclusions in volcanic rocks and the velocity structures of the crust are andesitic in the central zone and high‐alumina basaltic near the coast of the Sea of Japan. The compositions of possible primary melts formed in the mantle wedge are significantly more mafic than these crustal compositions. Consequently, if the major part of the crustal materials in the volcanic zone was formed by magmas generated in the mantle wedge, thick (up to 30 km) ultramafic cumulates including olivine pyroxenite must have been formed beneath Moho during fractional crystallization and would constitute the upper part of the mantle wedge. The volume of the mantle wedge consisting entirely of even fertile peridotite was not sufficient to produce crustal materials and cumulates in the volcanic zone by such processes in the northeast Honshu arc. A static mantle wedge model is, therefore, not satisfactory for the growth of the crust in northeast Honshu arc. A dynamic model is suggested in which the mantle materials have been supplied into the mantle wedge from the back arc side by a convective flow.

Some Geochemical Applications of a Semi-Empirical Approach for Estimating Distribution Coefficients of a Series of Cations between Liquid Aluminosilicates and their Crystallization Products

The distribution coefficients of alkali, alkaline earth, rare earth and a series of other elements between solid and liquid natural aluminosilicate systems can be estimated by a semi-empirical equation which can be derived by extension of Schuetze's incremental method for calculating oxygen isotope effects in silicate systems. This equation is used for investigating the partial melting of mantle wedge and subducted oceanic crust as potential sources of magmas giving rise to continental growth and for evaluating the history of igneous rocks. A series of geochemical criteria are derived for attributing igneous rocks to common parent magmas and for contributing to the estimation of the length of intrusive pathways. Particular attention is devoted to the evaluation of the geochemical behaviour of the rare earth elements.