PETROGENETIC EVOLUTION OF S-TYPE GRANITES IN THE SANTA CATALINA METAMORPHIC CORE COMPLEX: EVIDENCE FROM AGE AND CHEMICAL MAPPING OF ZIRCON

Zircons from S-type granitic sills of the Santa Catalina metamorphic core complex in southern Arizona display a complex growth history. LA-ICP-MS spot analyses of zircon grains from the lowest member, the Seven Falls Sill, show an apparently continuous crystallization period of over 20 million years, with two phases that appear igneous and two that appear metamorphic based on CL images and U/Th values. This apparent growth history is problematic given the moderately shallow level of emplacement.

We reconstruct the growth history of these complex zircon crystals by generating high-resolution maps of U-Th-Pb age and trace and rare-earth element mapping. Our analyses are conducted using a Teledyne Iridia excimer laser system equipped with HDIP software and a Thermo Element2 HR-ICP-MS. Preliminary analyses reveal strong zonation in Nb, Yb, Hf, Pb, Th, and U, as well as patterns in Sm, Eu, Gd, and Ti that correlate with age domains. Ongoing method refinements are focused on increasing the sensitivity for analysis of low-abundance isotopes and developing software that allows for spatial and statistical analysis of the age and chemical patterns.

Our study aims to answer critical questions, such as whether zircon growth was continuous or episodic, and what role monazite (and perhaps other phases) played in controlling the chemistry of zircon. We also aim to identify trace and rare-earth elements that can help determine whether zircon growth was igneous or metamorphic. Additionally, we will investigate what new insights Raman spectroscopic analysis may provide toward understanding complex mineral textures and zones. Results from our study should provide new insights into the origin of S-type granites and new techniques for reconstructing the petrogenesis of complicated zircon crystals.