Petrology of the Berlins Porphyry: A study of the crystallisation of granitic magma

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Abstract
The Berlins Porphyry, exposed in the lower Buller valley, South Island, New Zealand consists of many small plutons of dacite-granodiorite composition as well as a few surface flows. Stratigraphic evidence indicates emplacement and final solidification of the sub-surface plutons at shallow depths, certainly less than two kilometres. Mineralogy, phenocryst abundances, and total rock chemistry all show little variation and indicate that the plutons at present exposed are probably satellites from a larger sub-surface batholith. Many small intrusions and the margins of most of the larger plutons are composed of black glassy dacite (hyalodacite), which is inferred to represent the original magma chilled at the time of emplacement. Calculations using phenocryst and groundmass compositions suggest that T = 790°C, PH2O = 1.5 kbar, and fO2 = 1013.5 when the magma was crystallising at depth. The inner portion of most larger plutons is composed of microgranodiorite grading into granodiorite which completed its crystallisation under near surface conditions of low PH.O and PTotal. Some of the earlier phenocryst phases, in particular biotite and quartz, are texturally modified by reaction with the magma. K-feldspar is the last phase to crystallise and is interstitial to all other grains. The sequence of crystallisation that would be inferred from examination of the granodiorite alone differs slightly from that observed by tracing mineralogical changes from chilled margins into coarse-grained granodiorite, and indicates that caution is needed in interpreting granitic textures.