DECIPHERING MAGMA CHAMBER PROCESSES IN THE TUOLUMNE INTRUSION AND ITS LOBES USING ELEMENT DISTRIBUTION MAPS OF SINGLE MINERALS

Modern geochemical and geochronologic studies are increasing awareness about incremental growth of magma chambers and time and length scales of magma chamber processes. It is now recognized that both volcanic and plutonic suites contain different crystal populations that record individual complex histories. This implies that whole rock geochemical analyses in fact contain information that is averaged over all minerals, and the detailed information, as retrieved from single mineral analyses, is lost.

Recent field, U/Pb zircon geochronology and geochemistry studies on the 95-85 Ma Tuolumne batholith, California, have shown ample evidence for extensive mixing between different magmas and internal magma chamber recycling of marginal units into younger within the long lived, main batholith. Its southern lobes are shorter lived and simpler magma bodies since their composition is attributed to fractionation of magma derived from a single source. Single K-feldspars were chosen to test this hypothesis at the crystal scale (e.g. are Half Dome crystals recycled into the Cathedral Peak, and do lobes represent one pulsing event) with electron microprobe element distribution maps.

The lobe K-feldspars show simple, concentrically zoned crystals with major and minor element distributions gradually varying from center to margin. In the main body, particularly megacrystic K-feldspars show these variations to be oscillatory repeating over several cycles. Oscillatory zoning is particularly dramatic in Ba distribution maps, where the individual zones start with the highest Ba concentrations at a sharp contact to the previous zone. In the K-feldspars of the Cathedral Peak, there may be a positive correlation between the amount of zones evolved and the complexity and longevity of the magma the crystals grew in.

We conclude that K-feldspars of the Tuolumne batholith show element distribution pattern that support the idea of compositionally simpler, short lived nature of the lobes, whereas results from the main chamber reflect the complexity of a more dynamic and longer lived magma mush, where recycling, mixing and replenishment took place. More single mineral analyses including other minerals from different locations of the Tuolumne promise additional information pertaining incremental growth and magma chamber processes.