RETHINKING HOW WE MAP AND DATE PLUTONS: JOHN MUIR INTRUSIVE SUITE OF THE SIERRA NEVADA BATHOLITH

Cretaceous zoned intrusive suites of the Sierra Nevada batholith were each assembled over 8-10 m.y. The suites share similar zircon systematics and sequences of textural development linked to thermal maturation during incremental assembly. Field and U-Pb zircon data from the John Muir Intrusive Suite illustrate these features well.

Assembly of the Muir Suite began with intrusion of the Inconsolable, McDoogle, and Rock Creek plutons at ~95 Ma, after which the Lamarck Granodiorite (94-92 Ma) was assembled, punctuated by two intervals of high magma flux. Steep internal contacts require that the earlier flux of magmatism included multiple discrete pulses. Remaining units in the Muir Suite, including the Lake Edison (~91 Ma), Round Valley Peak (~89 Ma) and the Mono Creek (~87 Ma) plutons, preserve only gradational contacts defined largely by the gradual development of porphyritic to megacrystic textures. Zircon systematics become progressively more complex through time. The oldest units preserve homogeneous zircon populations with well-defined ages. Younger units preserve zircons that spread along concordia, consistent with either incorporation of zircons inherited from older portions of the system or in situ progressive growth of zircon between initial crystallization and “magmatic death.” The progressions through time of 1) sharp to gradational contacts, 2) equigranular to megacrystic textures, and 3) simple to complex zircon systematics is now documented in both the Muir and Tuolumne intrusive suites, and is suggested by existing data for the Whitney Suite.

We interpret these to reflect the thermal maturation of an incrementally assembled plutonic system. Early intrusions were emplaced into relatively cool wall rocks, and thus cooled below the solidus quickly and preserve internal contacts, equigranular textures, and simple zircon growth histories. Later intrusions intruded into progressively hotter environments, thus erasing contacts between increments via recrystallization while promoting megacryst growth and protracted zircon crystallization. This interpretation predicts that contacts in many intrusive suites are the result of thermal and textural maturation of the plutons and not of discrete pulses of magma.