DECIPHERING CHAMBER GROWTH AND INTERNAL MAGMA CHAMBER PROCESSES USING MAGMATIC LOBES AS SNAPSHOTS OF PROCESSES DURING THE CONSTRUCTION OF THE TUOLUMNE BATHOLITH, SIERRA NEVADA

Increasingly precise geochronology and detailed geochemistry show that large, long lived continental arc batholiths are incrementally constructed. The resulting complexity of a batholiths evolution and internal processes at different stages of its hypersolidus duration is also implied from the relative timing of formation of diverse magmatic fabrics and internal contacts between pulses. Thus, the meaning of geochronology, geochemistry and field data from batholiths needs careful evaluation since they may represent a complex combination of pulses from different source regions and ages that potentially mingle or mix while internally fractionating.

An invaluable approach in analyzing composite batholiths is to examine the structure, geochemistry and geochronology of smaller, compositionally simpler magmatic lobes extending into the host rock away from the main batholith. Here, no interaction with other intruding pulses has occurred at the emplacement level. Thermal modeling further indicates that lobes are rapidly frozen “aliquots” of magma sampled during batholith construction. Thus, they are a) great geochemical “references” to understand processes in individual magma units, such as the number of internal pulses, length scales of fractionation crystallization, mingling, and mixing and b) preserved “snapshots” of processes prior to additional, longer duration processes during the construction of the main batholith.

The Tuolumne Batholith (TB) is a great test case, because it preserves lobes of each main compositional unit. Data of each TB lobe help a) understand internal processes in each unit in comparison to the main body and b) test the two major emplacement models for the TB: 1) incremental intrusion of a few, nested diapirs into a large, long lived magma chamber and 2) ephemeral intrusion of innumerable dikes that partly pass into laccoliths without forming a large magma chamber.

We are presently examining four of the lobes in the TB. For example, our remapping of the southern Half Dome lobe, mapped as equigranular granodiorite, established spectacular compositional zoning. Initial major and trace element distributions support fractional crystallization from marginal equigranular to porphyritic granodiorite into leucogranite in the lobe center, a zoning pattern similar to the main batholith.