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Mineralogical Magazine; December 1997; v. 61; no. 6; p. 743-764
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The Minastira peraluminous granite, Puno, southeastern Peru; a quenched, hypabyssal intrusion recording magma commingling and mixing

Daniel J. Kontak, and Alan H. Clark

Queen's University, Department of Geological Sciences, Kingston, ON, Canada

The Minastira granite, a c. 25 Ma subvolcanic plug of fine-grained granitic rock in the Cordillera Oriental of SE Peru, has preserved textures indicative of a history involving mixing of at least two magmas, a volumetrically dominant felsic component and a less voluminous mafic one. The felsic component is represented by variably fractured, altered and embayed crystals of quartz, feldspar, biotite with minor coarse-grained melt-and fluid-inclusion rich apatite, and possible cordierite (now a pseudomorphous Fe-Mg phase), whereas the mafic component is represented by calcic plagioclase. The process of magma mixing is reflected by: (1) ubiquitous sieved-textured plagioclase with complex textural relationships; (2) a large range in plagioclase compositions with reversals and spike patterns in profiles; (3) embayed and internally fractured (thermal shock?) quartz; (4) the rare occurrence of pyroxene coronas on quartz; and (5) textures within biotite suggestive of its incipient breakdown. The lack of mafic enclaves indicates that physico-chemical conditions of the mixing were conducive to homogenization (i.e. chemical diffusion) and a superficially homogeneous rock is now observed. The association of glomeroclasts of crystals originating from both the mafic and felsic end members and a quenched quartz-feldspar matrix indicate that the mixing occurred in an underlying magma chamber.

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