Petrogenesis of high-Mg andesites : an experimental and geochemical study with emphasis on high-Mg andesites from Cape Vogel, PNG

A detailed petrographic, geochemical and experimental study has been undertaken on selected high-Mg and ites from Cape Vogel, PNG, the Bonin Islands and the Mariana Trench. These rocks are chemically and mineralogically unusual; however,they are important to our understanding of the Earth's mantle since they represent primary magmas, and occur within an island-arc environment. Mineralogically these volcanic rocks are characterized by very magnesian orthopyroxene (and/or clinoenstatite) (Mg80 _ 92), clinopyroxene, magnesian and chrome-rich spinel, and occasionally olivine (Mg85 _ 92). Petrographic and chemical criteria show that orthopyroxene is the major fractionating phase. Comparison of pyroxene phenocryst composition with host magma composition demonstrates an extremely sensitive and regular relation. The occurrence of protoenstatite ( now clinoenstatite) is limited to low pressures by the sensitivity of the pyroxene compositions to Ca-content of the melt (normative Di/Di+Hy) and the position of the appropriate orthoenstatite to protoenstatite inversion reaction. Experimental studies were undertaken on selected compositions in the Mg-rich corner of the pyroxene quadrilateral. These studies were done in the hope that if the effects of Fe/Mg sUbstitution and of CaMgSi20G solid solution on the protoenstatite to orthoenstatite inversion were more fully known it might be possible to limit the extrusion temperatures of clinoenstatite-bearing lavas. These studies did not closely constrain the conditions of origin of the high-Mg andesites , because the major determinant of protoenstatite appearance was found to be magma composition rather than magma temperature. The results of this study clarify our understanding of the pyroxene stability relations at low pressure. In particular, the demonstration that there is a high temperature stability field of orthoenstatite denies the existence of a stable invariant point de fined by the reactions OE + PE + 01, PE + DI t. PI and OE + DI \t: PI in the system CaMgSi20G-Mg2Si20 6 . New phase relations consistent with the experimental findings of this and other studies for the Mg-rich corner of the pyroxene quadrilateral are presented. Combination of experimental liquidus studies on high-Mg andesites and the controls on the occurrence of protoenstatite and olivine suggest conditions of origin of T > 1200°C and P < 5-10 kb. Water is essential to derive these melts from a peridotite source but the earlier view that water-saturated or near-saturated melting of peridotite is necessary is inconsistent with the observed mineralogy and results of the experimental studies. ().I.MA) Typically the high-Mg andesites HMA have Si02 - 56-58% MgO - 11-16% CaO/Al203 < 0.6 Ti02 < 0.6% Mg/Mg+Fe - 67-80 Ni - 200-400 ppm and Cr - 600-1200 ppm. The incompatible element abundances in the HMA are generally less than lOx chondrit1c or primitive mantle values. The peridotite source region for these volcanics is refractory as reflected in the Gr-rich compositions of the spinels high Mg-numbers low CaO/Al203 and low incompatible element abundances for the bulk rocks. However the incompatible trace element ratios and patterns are inconsistent with derivation from a refractory source and a complex series of enrichment events have been superimposed on the refractory source. These enrichment events have given the HMA a distinctive geochemical signature characterized by low Ti/Zr (<60) and light rare earth element enrichment. Two groups of high-Mg andesites are recognized on the basis of their rare earth patterns and the relatlonship between the rare earths and Ti Zr and Nb. Type-C I-IMA occur at all the localities studied and have a concave rare earth element pattern. There is no correlation of Ti/Zr and Zr/Nb ratios with La/Yb in type-C I-IMA. Type-E HMA are recognized only at Cape Vogel PNG and are characterized by light rare earth enrichment which has an inverse correlation with Ti/Zr and Zr/Nb ratios. Detailed comparisons between high-Mg andesites and other high-Si02 high-MgO liquids demonstrates that these rocks define a petrogenetic group characterized by derivation from peridotite sources at P < 15 kb and T - 1100-1400°C. Of the group of high-Si02 high-MgO liquids for which experimental liquidus data are available only the HMA require water for derivation from a peridotite source. While the high-Si02 high-MgO liquids define a broad petrogenetic group there are nonetheless significant geochemical and petrographic differences between these rocks which preclude their derivation from a coJIDilon peridotite source. The location of known high-Mg andesites favours their origin in island-arc petrogenetic processesalthough their place in the complex interactions of island-arc volcanism back-arc basin creationand trench formation remains uncertain."