WIDESPREAD RECYCLING OF OLDER INTO YOUNGER MAGMATIC PHASES IN A LARGE MAGMA CHAMBER: THE TUOLUMNE BATHOLITH, SIERRA NEVADA, CALIFORNIA

A healthy debate continues about the maximum size(s) of upper crustal magma chambers and the degree to which processes operating within these chambers can affect final compositional and structural heterogeneity. In the ~1100 km² Tuolumne Batholith, Sierra Nevada (California), field, geochronologic and geochemical studies indicate that recycling of older phases of the batholith into younger phases occurred on scales ranging from single crystals, through tens of meters large “crystal-mush blocks,” to kilometer-scale domains along internal contacts. Sometimes the locations of (roof, main chamber, feeder system?) and processes by which recycling occurred remain unclear. But in two settings evidence of recycling is preserved at the presently exposed level of this batholith: (1) along well defined internal contacts between pulses; and (2) in numerous, widely distributed, but localized flow channels represented by troughs, tubes, and pipes.

A particularly spectacular example of the former occurs in the Sawmill Canyon area near the eastern margin of the batholith where the older Kuna Crest and Half Dome phases are getting removed and recycled into younger phases by stoping of “crystal mush blocks” and the formation of zones of complex flow channels, mingling, mixing, and local thermal and mechanical instabilities.

Examples of the latter processes are widely distributed throughout the batholith (we estimate that 1000s exist) and are represented by schlieren-bounded troughs, schlieren-bounded stationary and migrating tubes, and crystal accumulations or melt segregation in pipes. Most of these structures indicate that localized erosion, transport, and redeposition of crystals and melt leading to mingling and mixing remained active until fairly late in the crystallization of this body. The scale and widespread evidence of recycling indicates to us that vertically and/or horizontally extensive magma chamber(s) must have existed during the formation of this batholith.