Petrogenesis of microcline megacrysts from Precambrian granitic rocks of the Dixon-Penasco area, northern New Mexico
— Philip E. Long and W. C. Luth

The origin of potassium feldspar megacrysts in granitic rocks has been a long-standing problem in petrology. Interpretations of their origin largely fall into two groups: (1) the megacrysts are porphyroblasts which grew after the bulk of the rock solidified (e.g., Dickson, 1968; Emmermann, 1968, 1969; Mehnert, 1969), or (2) the megacrysts are phenocrysts which crystallized directly from a melt as a primary phase (e.g., Kerrick, 1969; Nemec, 1975). Unfortunately, the petrographic criteria that have been employed to distinguish these two possibilities are largely inconclusive. It has been argued, for example, that the presence of megacrysts in the country rock adjacent to a granitic intrusive body is evidence that the megacrysts in the granite itself did not grow from a melt because those in the country rock apparently did not. Such observations, however, are only permissive since they do not demand that the megacrysts in the granite be porphyroblasts. An alternative explanation might be that the megacrysts in the country rock grew from a vapor phase that coexisted with a crystallizing magma. On the other hand, the sub-parallel orientation of tabular megacrysts has been given as evidence of a magmatic origin, the alignment supposedly caused by flow in the magma. An alternative explanation here might be deformation of the granite during subsolidus growth of the megacrysts. Other field and petrographic criteria suffer from similar uncertainties.

 

In view of this situation, some investigators have examined the chemistry of potassium feldspar megacrysts in hopes of finding a definitive answer to their mode of origin. Most of these studies have emphasized the distribution of Ba. For example, Dickson (1968) studied megacrysts from the Papoose Flat pluton, lnyo Mountains, California by staining for Ba; Kerrick (1969) and Kuryvial (1976) used microprobe analyses for Ba; and Emmermann (1968, 1969) and Nemec (1975) used bulk analysis for Ba. Emmermann (1968, 1969) and Dickson (1968) concluded that the megacrysts they studied formed as porphyroblasts, whereas Kerrick (1969), Nemec (1975) and Kuryvial (1976) concluded that the megacrysts they examined were phenocrysts.

 

This disagreement has prompted us to conduct a detailed study of Ba zoning in megacrysts from two Precambrian granitic bodies in northern New Mexico, and to perform preliminary experiments producing Ba-zoned alkali-feldspar crystals from a synthetic granitic melt at high temperatures and pressures.

 

In this study, megacrysts from the Puntiagudo granite porphyry and the Periasco quartz monzonite in the Dixon-Periasco area, northern New Mexico, were analyzed for Ba by continuous microprobe traverses from center to edge. This study was done as part of a larger study of the granitic and pegmatitic rocks of the Dixon-Penasco area (Long, 1974, 1976) in which particular attention was paid to late-stage and metamorphic processes in these rocks. Distinguishing between an igneous and a metasomatic origin for these megacrysts is important because it has a significant effect on our estimate of the relative importance of metasomatic processes in the area.