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


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Jenner, George Albert 1982 , 'Petrogenesis of high-Mg andesites : an experimental and geochemical study with emphasis on high-Mg andesites from Cape Vogel, PNG', PhD thesis, University of Tasmania.

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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
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 processes,although their place in the complex
interactions of island-arc volcanism, back-arc basin creation,and trench
formation remains uncertain.

Item Type: Thesis - PhD
Authors/Creators:Jenner, George Albert
Keywords: Andesite, Petrogenesis
Copyright Holders: The Author
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Copyright 1982 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1983. Includes bibliographical references

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