TRACE-ELEMENT AND AGE TRENDS IN ZIRCON FROM A SINGLE BIMODAL DIKE: INSIGHTS INTO LARGER MAGMATIC PROCESSES

Recent studies of trace elements (TE) in zircon aid to unravel complex processes that occur during evolution and emplacement of large igneous bodies such as the Toulomne Intrusive Suite in eastern California. In order to shed light on similar processes that may be occurring on a larger scale, we investigated the response of zircon—with respect to its U-Pb date and TE signature—to the emplacement of a single "Holstein" dike, a Cretaceous mafic dike that intruded and mingled with its Jurassic host (Hurd Peak orthogneiss).

The 57 grains from within the margins of the dike fall into 4 age groups: 1) Pre-175 Ma inherited grains (n = 4); 2) ca. 175 Ma Hurd Peak (n = 22); 3) 114–101 Ma (n = 14); 4) ca. 95 Ma grains representing dike emplacement (n = 20). Duplicate analyses from individual grains indicate few differences in core vs. rim age, suggesting limited resorption of old, pre-Cretaceous zircon during injection of the dike. We interpret these two distinct groups of zircon grains as representing the two separate, mingling melts within the margin of the dike; 1) the mafic, ca. 95 Ma portion, and 2) the felsic entrained melted portion of the 175 Ma Hurd Peak.

Cretaceous analyses can be further subdivided by TE abundance: 1) moderate Hf, lower U/Yb, high Ti; 2) variable Hf, high U/Yb, low Ti; 3) strongly contaminated (hydrothermal crystallization?). The first two groups show a trend in Ti vs. U/Yb grains that is typical of Sierran arc zircon; high-Ti (i.e., hot) zircon crystallizes early, with low U/Yb, followed by later crystallization of lower-Ti, higher U/Yb zircon. However, Hf content is not consistent with this trend; although high-Ti, low-U/Yb (early crystallizing) zircon has more moderate Hf values as expected, the low-Ti, high-U/Yb (late crystallizing) zircon is variable with both high and low Hf values, suggesting a separate controlling process. We envision two possible scenarios: 1) resorption of earlier crystallized zircon in the mafic portion of the dike would reset the Hf, but yield U/Yb and Ti values consistent with the evolving magma; 2) late crystallization and/or breakdown of a precursor phase may change the Zr/Hf ratio of the mafic magma in which the late zircon crystallizes. Whatever the process, it may be scalable to similar phenomena in larger granitic bodies, such as that seen between the Half Dome granodiorite and its younger intruded Cathedral Peak granodiorite.