Abstract:

The Late Devonian, high-level, post-orogenic Mt Disappointment pluton was emplaced as a nested laccolith. Field observations and magnetic fabric (anisotropy of magnetic susceptibility) data show that the monzogranitic to granodioritic magmas entered the pluton from a central, linear feeder. The first set of magmas solidified to a non-megacrystic unit and the second to a unit carrying K-feldspar megacrysts. Emplacement was accommodated partly by uplift of the overburden on brittle marginal faults. The magmas were produced through mid- to deep crustal (<600 MPa) partial melting of a source containing both ‘igneous' and ‘sedimentary' rocks, possibly a package of volcaniclastic and aluminous metagreywackes. The spectrum of magma compositions was not formed through any differentiation or magma mixing process. At the source, the melt compositions varied as a result of mixing between liquids derived from the two main source rocks, with variations in the amount of plagioclase component that entered the melt. The degree of entrainment of peritectic orthopyroxene and ilmenite appears to have controlled the Fe, Mg and Ti contents of the magma batches. A variety of evidence suggests that magma withdrawal from the protolith was rapid and episodic. The magma batches so formed were emplaced into the growing pluton essentially without further chemical evolution.

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