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

Abstract

The WooleyCreek batholith is a tilted, zoned, calc-alkaline plutonic complex in the Klamath Mountains, northern California, USA. It consists of three main compositionaltemporal 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 fi eld observations and the chemical compositions of the rocks and their constituent minerals. GEOLOGIC SETTING Klamath MountainsThe Klamath Mountains geologic province, northwestern California and southwestern For permission to copy, contact editing@geosociety.org on July 1, 2015 geosphere.gsapubs.org Downloaded from Note: MSWD-mean square of weighted deviates. Sample: s-single grain; all multipoint dates are weighted mean 206 Pb/ 238 U dates; asterisk indicates sample excluded from weighted mean calculations. Weight represents estimated weight after fi rst step of CA-TIMS (chemical abrasion-thermal ionization mass spectrometry) zircon dissolution and is only approximate. U and Pb concentrations are based on this weight and are useful for internal comparisons only. Picograms of sample and common Pb from the second dissolution step are measured directly and are accurate. Sample Pb: sample Pb [radiogenic (rad.) + initial] corrected for laboratory blank. cPb: total common Pb. All assigned to laboratory blank unless >3 pg. Pb*/Pbc: radiogenic Pb to total common Pb (blank + initial). Corrected atomic ratios: 206 Pb/ 204 Pb corrected for mass discrimination and tracer, all others corrected for blank, mass discrimination, tracer and initial Pb; values in parentheses are 2σ errors in percent. Rho: 206 Pb...

Emplacement and geochemical evolution of highly evolved syenites investigated by a combined structural and geochemical field study: The lujavrites of the Ilímaussaq complex, SW Greenland

“…Consequently, mineral chemical data has the potential to provide important insights into the chemical evolution of a crystallizing melt and offer a monitor for magma recharge processes (e.g. Coint et al 2013, Brandriss et al, 2014, Holness, 2005O'Hara and Mathews, 1981).…”

Ignimbrites to batholiths: Integrating perspectives from geological, geophysical, and geochronological data

“…No base is exposed for these rocks, which crystallized at 159-158 Ma and are zoned downward from granite to tonalite (Barnes et al, 1986;Coint et al, 2013).…”

“…10A), some plutons exposed over multi-kilometer ranges have little detected variation in zircon age with depth: The top and bottom of the 4 km drill-hole section into the Eldzhurtinsky granite are indistinguishable at 2.04 ± 0.03 Ma (Grun et al, 1999); the bulk of the 10-km-thick Spirit Mountain pluton is within analytical uncertainty at 16.0-15.7 Ma (Walker et al, 2006;Miller et al, 2011); ages from the roof zone and deepest exposed level in the ~12 km tilted section through the Jurassic Wooley Creek pluton differ by only ~0.5 m.y. (Coint et al, 2013); dates from the 12 to 15 km depth of exposures through the Bergell intrusion vary by ~2 m.y. (32-30 Ma, only modestly greater than age ranges within single samples; Samperton et al, 2013).…”

Use of trace element abundances in augite and hornblende to determine the size, connectivity, timing, and evolution of magma batches in a tilted batholith

“…The WCb (Fig. 1) was emplaced between 159 and 155 Ma (Coint et al, 2013). The batholith intrudes three host terranes of the western Paleozoic and Triassic belt; from structurally higher to lower, these are (1) the eastern Hayfork terrane, a chert-argillite mélange, (2) the western Hayfork terrane, a volcaniclastic sandstone and argillite unit, and (3) the Rattlesnake Creek terrane, a serpentine matrix to block-onblock ophiolitic mélange overlain by a coherent cover sequence (Wright and Wyld, 1994).…”

“…However, some batholith-scale plutons may be emplaced in time scales of <1 m.y. Coint et al, 2013). In such cases, alternative methods must be used to identify magma batches and assess their size and evolution.…”