MODELING PREDICTS THERMAL EVOLUTION, GEOCHEMICAL, TEXTURAL AND FIELD RELATIONS OF THE ZONED INTRUSIVE SUITES OF THE SIERRA NEVADA BATHOLITH, CALIFORNIA

Thermal modeling of pluton assembly via incremental addition of horizontal or vertical sheets yields the following general observations: 1) partial melting of wall rock is absent early in the history, but becomes more prevalent through time, 2) an amorphous zone of interconnected melt is formed, and 3) melt percentages averaged over the assembled body are typically low. The results suggest that thermal maturation of an incrementally assembled plutonic system can yield the geochemical and textural variations observed within intrusive suites of the Sierra Nevada batholith, and are consistent with geophysical observations from active magmatic regions. Field relations and geochronology in Cretaceous Sierran zoned intrusive suites suggest they were assembled incrementally through amalgamation of sheet-like intrusions over millions of years. Thermal models designed to generate plutons of the approximate dimensions, depth of emplacement and age range of the suites using HEAT 3D (Wohletz, 2007) yield several results that are insensitive to the orientations (horizontal or vertical) and sequence of intrusion of the sheets, and are consistent with data from the Sierran intrusive suites. 1) Early intrusions generate little or no partial melt in wall rocks; however, later pulses emplaced into regional elevated geothermal gradients (owing to earlier intrusions) yield significant melting of the host rock. This is consistent with Sr and Nd isotopic ratios from Sierran intrusive suites that evolve toward more crustal values from older marginal units to younger interior units, and helps to explain why rocks at pluton margins show less wall-rock contamination than rocks (apparently) far removed from the margins. 2) All models evolve toward a 3-D thermal structure that is amorphous despite being assembled via regular 2-D intrusions. Thus, the zone of interconnected melt developed in the model bears no relation to the size or shape of the increments from which it was built. 3) Although the models yield large regions of interconnected melt, the melt fraction is low, consistent with geophysical data from active magmatic zones that fail to image regions likely to contain more than 20% melt. In all models, the volume of the pluton containing melt at any time is small compared to the total volume of the pluton.