USE OF TRACE ELEMENT ZONING IN HORNBLENDE TO IDENTIFY MAGMA BATCHES AND RESERVOIRS AND DECIPHER MAGMATIC PROCESSES IN CALC-ALKALINE PLUTONS

The Wooley Creek batholith (WCb) and the Kuna Crest Lobe (KCL) of the Tuolumne Intrusive Complex represent two arc magmatic systems with a similar range of rock types, from gabbro to granodiorite/granite. Hornblende is a near-liquidus phase in the upper zone of the WCb and in the KCL; therefore, trace element contents and zoning patterns in hornblende are used to compare and contrast assembly of, and magmatic processes within the two intrusions. In both systems, Ti, Zr, Hf, Nb, and REE typically decrease from crystal interiors to rims, an indication of compatible behavior of these elements. The REE patterns typically show a core-to-rim change from concave-down to nearly linear, and the size of the negative Eu anomaly decreases. WCb hornblendes typically have higher Cr and Ba contents than those of the KCL.

In the KCL, hornblende from individual mapped units can be distinguished on the basis of differing abundances and zoning trends. Some samples contain at least two distinct hornblende populations, which is particularly evident in the shapes of REE patterns. In contrast, hornblende from all structural levels of the upper WCb and related ‘roof dikes’ forms a single broad compositional array. The REE patterns of these hornblendes are essentially indistinguishable, regardless of rock type, from quartz diorite to granite.

The uniformity of trace element contents and patterns in the upper WCb is interpreted to indicate crystallization from a large magma body stirred by convection (Coint et al., Geosphere, in press). The fact that hornblende in dacitic roof dikes is identical to hornblende in the underlying pluton indicates that the upper-zone magma was eruptible. In the KCL, distinct hornblende compositions and zoning patterns, and locally bimodal hornblende populations, point to emplacement of multiple magma batches and local mixing, without convective mixing and homogenization on the scale of the entire Kuna Crest Lobe.

We conclude that hornblende trace element contents and zoning patterns are potentially a powerful tool for identification of magma batches, estimating the size of melt-interconnected reservoirs, assessing magmatic processes, and relating plutonic rocks to hypabyssal and volcanic equivalents.