Characteristics of internal contacts in the Tuolumne Batholith, central Sierra Nevada, California (USA):: Implications for episodic emplacement and physical processes in a continental arc magma chamber

Internal contacts along the eastern margin of the Tuolumne Batholith, central Sierra Nevada, California, and structures preserved along these contacts, are highly variable; the contacts range from relatively sharp, to gradational boundaries, to sheeted zones, to very complex boundaries formed by multiple processes. Fractional crystallization, kilometer-scale mixing within broad transition zones, voluminous magmatic stoping along sharp contacts, and downward return flow (and/or margin collapse) of older magma units were important large-scale processes along these contacts during chamber construction. In contrast, sheeting, extensional cracking and diking represent only second-order, small-scale complexities. Formation of the most complex zones along internal contacts resulted from the interaction of the sequential emplacement of different units with the irregular geometry of these contacts, which often resulted from removal of earlier phases by stoping. Our results indicate that multiple processes are likely during emplacement of large magma bodies within one another and that it is unlikely that evidence for all internal processes during batholith construction are preserved. We also argue that fairly large magma chambers existed in this batholith and thus that large accumulations of magma may exist in upper crustal chambers for significant periods of time.