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McMahon, Timothy Paul 1994 Pliocene Intrusions in the Ertsberg (Gunung Bijih) Mining District, Irian Jaya, Indonesia: Petrography, Geochemistry, and Tectonic Setting, PhD Dissertation, The University of Texas at Austin.

    © Timothy Paul McMahon, 1994. Use of any part of this thesis for any purpose must be acknowledged.


At least sixteen Pliocene (3-4 Ma) intrusions crop out in the Ertsberg Mining District of Irian Jaya, Indonesia. Most of the intrusions are small ($<$4 km$/sp3$), hypabyssal dikes and plugs, but one (the Ertsberg Intrusion) may be as large as 10-20 km$/sp3$. The intrusions are divided into a high-K group of latites, trachytes and trachydacites, and a low-K group of andesites and dacites. Intrusions range from equigranular (Ertsberg Intrusion) to porphyritic (others), and plagioclase is the dominant mineral in all intrusions. Most intrusions contain amphibole and biotite, and most high-K intrusions contain diopsidic pyroxene as well. Differences in mineralogy and texture were controlled by the amount of dissolved water in the magmas during crystallization and the amount of crystallization prior to emplacement at shallow crustal levels. All intrusions have arc geochemical signatures, with high LILE, and Nb and Ti depletions. Compared to high-K intrusions, low-K intrusions generally have higher La/Th and Ba/Th ratios, but similar Zr/Hf ratios. Both low- and high-K magmas were derived from a single lower crustal magma chamber, but low-K intrusions were more affected by assimilation of lower crust. Differences in the type of ore deposits associated with the Ertsberg Intrusion (skarns) and Grasberg Complex (porphyry Cu) can be related to magmatic evolution. Emplacement of a large body of crystal-poor magma which cooled slowly (Ertsberg) allowed formation of peripheral calc-silicate skarns which were later mineralized. In contrast, periodic tapping of (alternately) magmas and fluids from the cupola of a deeper magma chamber led to porphyry copper mineralization (Grasberg). In Papua New Guinea, a belt of 10-20 Ma magmatic centers lies to the north of a belt of $/leq$7 Ma centers, which is along strike from the Ertsberg District. There is little evidence for the older belt in Irian Jaya. A tectonic model is presented in which this older belt is the product of south-dipping subduction. Younger magmatism in eastern New Guinea represents the last vestiges of this subduction, while that in Irian Jaya is a result of lithospheric delamination.


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