Batch-wise assembly and zoning of a tilted calc-alkaline batholith: Field relations, timing, and compositional variation

The Wooley Creek batholith is a tilted, zoned, calc-alkaline plutonic complex in the Klamath Mountains, northern California, USA. It consists of three main compositional-temporal zones. The lower zone consists of gabbro through tonalite. Textural heterogeneities on the scale of tens to hundreds of meters combined with bulk-rock data suggest that it was assembled from numerous magma batches that did not interact extensively with one another despite the lack of sharp contacts and identical ages of two lower zone samples (U-Pb [zircon] chemical abrasion-isotope dilution-thermal ionization mass spectrometry ages of 158.99 +/- 0.17 and 159.22 +/- 0.10 Ma). The upper zone is slightly younger, with 3 samples yielding ages from 158.25 +/- 0.46 to 158.21 +/- 0.17 Ma, and is upwardly zoned from tonalite to granite. This zoning can be explained by crystalliquid separation and is related to upward increases in the proportions of interstitial K-feldspar and quartz. Porphyritic dacitic to rhyodacitic roof dikes have compositions coincident with evolved samples of the upper zone. These data indicate that the upper zone was an eruptible mush that crystallized from a nearly homogeneous parental magma that evolved primarily by upward percolation of interstitial melt. The central zone is a recharge area with variably disrupted synplutonic dikes and swarms of mafic enclaves. Central zone ages (159.01 +/- 0.20 to 158.30 +/- 0.16 Ma) are similar to both lower and upper zones crystallization ages. In the main part of the Wooley Creek batholith, age data constrain magmatism to a short period of time (<1.3 m.y.). However, age data cannot be used to identify distinct magma chambers within the batholith; such information must be extracted from a combination of field observations and the chemical compositions of the rocks and their constituent minerals.