T56 What is a Magma Chamber? The Role of Sheets in the Assembly of Intrusions I

Sunday, 16 October 2005
1:30 PM - 5:30 PM

EVALUATING PLUTON GROWTH MODELS USING HIGH RESOLUTION GEOCHRONOLOGY: TUOLUMNE INTRUSIVE SUITE, SIERRA NEVADA, CA

Paper number: T56-11
Program: Technical Sessions
Time: 4:20 PM – 4:35 PM

Authors

Abstract

Recent studies indicate that magmatic intrusions may be constructed over several Ma from magma pulses of varying size and shape. Both the geometry of magma pulses and frequency of magma recharge control the thermal/rheologic state of an evolving magma body and its host rock, and geochronologic data are critical for resolving complex spatial and temporal patterns along internal magmatic contacts. The Tuolumne Intrusive Series (TIS) represents a classic example of a large (~1200 km2), compositionally zoned intrusion and provides a natural laboratory for the study of magmatic processes. Recent 85-95 Ma U/Pb ages from the southern TIS were explained by emplacement of sheeted magma pulses that young toward the center of the intrusion. Precise constraints on the geometry and thermal input of individual magma pulses, however, require more structural and geochronologic data. We present U/Pb zircon and 40Ar/39Ar biotite analyses from an E-W oriented transect across the northern TIS to gain a more complete picture of its magmatic and thermal evolution.

U/Pb analyses of single zircons using a thermal annealing/chemical abrasion pretreatment yield concordant clusters of data at ca. 86 Ma from two samples of the voluminous Cathedral Peak unit. These crystallization ages are younger than the published 88 Ma age from this unit in the southern TIS. The indistinguishable ages from samples ~4 km apart in the northern transect place constraints on the size of a potential Cathedral Peak magma chamber. Both samples also contain several 87-92 Ma zircon grains that we interpret as inherited from earlier phases of the TIS magma system and may indicate the collapse of early-formed internal contacts. Biotite 40Ar/39Ar ages from the TIS and host rock record cooling through ~340-390 °C and range from ca. 83 Ma to 85 Ma from the eastern and western margins of the TIS, respectively. These dates require rapid cooling (up to 400 °C/Ma) following emplacement of Cathedral Peak magma and necessitate an efficient heat transfer mechanism in the final stages of batholith construction. Our new results suggest that models which assume different shapes and sizes of magma pulses should be tested not only across transects but throughout the pluton and with geochronologic techniques yielding precise and accurate ages.