Mechanical instabilities and physical accumulation of K-feldspar megacrysts in granitic magma, Tuolumne batholith, California, USA

Examination of K-feldspar megacrysts in the Tuolumne batholith (TB), Sierra Nevada, California, USA, indicates that they grew from melts, rather than metasomatically. During K-feldspar growth, local mechanical instabilities in the magma were common and resulted in the physical accumulation of K-feldspar megacrysts in schlieren tubes, troughs, irregular clusters, dikes, and small diapirs. Evidence favoring physical accumulation of concentrations of K-feldspar megacrysts includes a. clustering of megacrysts in much greater modal proportions than is likely from the magma composition, b. imbrication (tiling) of megacrysts, c. concentrations below schlieren layers with the layering draping around K-feldspars due to filter pressing, d. scattered to locally concentrated megacrysts in schlieren, together with microgranitoid enclaves ("mafic" enclaves) and xenoliths, e. dike-like and irregular "slumped" concentrations of megacrysts that in some places intrude other units with few to no megacrysts, and f. truncation of internal zoning in K-feldspars by fracturing or contact melting where megacrysts impinge on one another in clusters. The above features emphasize the widespread occurrence of instabilities and local flow within the TB, as well as the role that these processes play in changing the mineral proportions preserved in frozen magma chambers. An alternative suggestion that concentrations of megacrysts are formed by in situ growth is opposed by the evidence for physical accumulation, as well as the following observations: a. normal zoning of barium in the megacrysts; b. lack of molding of megacrysts around one another; c. mixing of megacrysts into microgranitoid enclaves ("mafic" enclaves); d. evidence for hydraulic equivalence between megacrysts and other minerals; and e. draping of schlieren layers around megacrysts.20 page(s