Complex age and internal structure of zircon in the Boulder batholith, MT: petrography, CL and BSE imaging, and SHRIMP U-Pb geochronology

The Late Cretaceous Boulder batholith is composed of at least 14 plutons, ranging in composition from granodiorite to granite. Routine SHRIMP U-Pb geochronology of zircon from 15 samples indicates that 12 plutons have ages of about 81-73 Ma (±1%). These data were collected from oscillatory-zoned mantles. A few grains in several samples yielded ages that are younger (beyond analytical uncertainty) than the calculated zircon crystallization ages. These younger ages prompted detailed CL imaging and development of an analytical methodology to identify multiple episodes of zircon crystallization. Euhedral, prismatic zircon grains from 12 samples were mounted in epoxy and only minimally polished (with no grinding) to expose the outermost growth zones of the grains. Surfaces deemed homogeneous based on CL imaging (i.e. horizontally oriented zones) were analyzed by SHRIMP for U-Pb age. The dated grains were then ground and polished to about half-thickness. Guided by CL, the innermost horizontal zones were dated. This type of "depth profiling" has the advantage of producing a large data set from many grains (at least 16 grains per sample), rather than many analyses on just a few grains. By analyzing horizontally-oriented growth zones, the SHRIMP spot sampled domains only ~1-2 μm thick that preserve geochronologically discrete episodes of zircon crystallization. Results of the U-Pb analyses indicate that zircon crystallization in a supposedly simple igneous system may be considerably more complicated than has been documented. Many grains from most samples preserve evidence of resoprtion and multiple episodes of crystallization spanning several m.y. Paradoxically, the interiors of some grains are younger than their exteriors. For example, analyses of zircon from the Pulpit Rock monzogranite (PRm) yield the following ages: interior—77.0 ± 1.0 Ma (n=14), 72.4 ± 0.9 Ma (n=13); exterior—75.7 ± 0.7 Ma (n=17), 73.5 ± 1.3 Ma (n=3). Although this unusual textural relationship was originally considered to be an analytical artifact, additional analytical sessions on two different SHRIMPs confirm the original data set. In two 2x3" thin sections from PRm, 194 zircon grains with lengths >20 μm were identified by SEM auto-scanning. Of these, 18% are completely enclosed within major rock-forming minerals, mostly biotite (Type 1). The other 82% (Type 2) occur with magnetite, apatite, and titanite in boundary areas between coarse, rock-forming minerals. Type 1 grains are euhedral and exhibit simple concentric oscillatory zoning throughout. Type 2 grains are subhedral to euhedral, and typically contain oscillatory-zoned cores plus irregularly-zoned overgrowths. Both types of zircon have similar U and Th concentrations. Serial grinding and polishing (~2-3 μm removed per step), accompanied by CL imaging, of zircon from PRm shows that in some grains the outer, younger material extended inside to replace cores. Thus, resorption textures in these grains provide irrefutable proof of complex igneous processes, possibly including magma reservoir replenishment that spanned several m.y. The occurrence of multiple-age zircon in these igneous rocks constitutes evidence of multi-stage magma solidification. As such, the concept of "crystallization age" of a plutonic rock may be an oversimplification.