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Description
Since James Hutton first recognized that magmas intrude their host rocks, geologists have been plagued by a fundamental room problem; namely, what happens to the material displaced by the pluton. Field, geochemical, and structural studies from the contact aureole of the Middle Jurassic Emigrant Gap composite pluton, northern Sierra Nevada, California, indicate that a variety of mechanisms were active to facilitate space during pluton emplacement into the upper crust. From these studies, it is suggested that: (1) ductile deformation within the contact aureole can only produce a maximum of 20% of the space currently occupied by the pluton, (2) preliminary REE modeling indicates that assimilation of < 20 %of pelitic mudstones from the Shoo Fly Complex at the final site of emplacement can occur during pluton emplacement, (3) stoping played a significant role during the emplacement of the pluton and may account for as much as 48% of the space occupied by the pluton, and (4) mass transfer and volume losses of at least 12% can occur during contact metamorphism. The results of the mass transfer and volume loss analysis also pose interesting ramifications regarding the nature of contact metamorphism. Although conventionally interpreted to be an isochemical and constant volume phenomenon, these data indicate that (a) mass balance equations in conjunction with low-solubility reference-frame elements such as Ti and Al can be used to demonstrate major-and trace-element mass transfer during contact metamorphism of approximately -10%, (b) aureole mass losses correspond to volume strains within the aureole of -12%, and (c) changes in mass of major rock-forming elements such as Si (-16%) within the aureole, indicate that contact metamorphism is not an entirely isochemical and constant volume phenomenon. The above data and observations indicate that multiple structural and metamorphic processes may be active during the emplacement of magmas into the shallow crust. Yet, following the works of Hutton and Read, the quantification of these processes further suggests that the space problem is alive and well.
