Geochronology

The absolute timing of plutonism and deformation events is critical to decipher the processes of batholith formation. U/Pb dating of zircon from plutonic rocks of the CPC will provide timing constraints on major changes in the composition and geochemistry of plutons as well as on the timing of major deformational and pluton emplacement events. Specifically, the composition of the calc-alkaline magmas provides constraints on the composition of lithologies within the source region. Changes in these compositions over time may indicate the development or disappearance of an ultramafic root. Therefore U/Pb geochronology on the plutons is critical for constraining changes in source compositions through time of calc-alkaline magmas.

We will also date selected mafic dikes using 40 Ar/ 39 Ar geochronology. Dating of the dikes will be closely coordinated with the geochemical studies to constrain times when mantle source compositions changed.

Samples will be collected for U/Pb and 40 Ar/ 39 Ar dating with the following goals in mind:

· The presence or absence of the early Tertiary magmatic front along the Bella Coola transect will be confirmed. Existing U/Pb dates on zircon from the Bella Coola area suggest that some plutons with ages as young as 85 Ma occur west of the Coast shear zone, but no Paleocene or Eocene plutons have been found. Confirmation of whether the magmatic front extends into the Bella Coola area, and if the front corresponds to major changes in crustal structure will be a critical piece of information in deciphering the evolution of this batholith complex.

· The timing of changes in pluton geochemical and petrologic signatures will be determined. For instance, if an ultramafic root developed during batholith formation, then the sources of the calc-alkaline rocks may show a change from garnet-absent to garnet-bearing compositions. Alternatively, if delamination occurred during batholith emplacement then the source may change from garnet-bearing to garnet absent. Our geochemical work can identify the relevant compositional changes, but the critical timing of those changes can only be constrained through U/Pb geochronology.

· We will determine the age of metamorphism and deformation in the framework orthogneisses and metasedimentary rocks. This may lead to an understanding of the temporal relationship between the Late Cretaceous to Early Tertiary magmatic flareup, high-temperature deformation, and regional metamorphism in the arc. In a broader sense, understanding the P-T-t paths in amphibolite and lower granulite facies rocks from the northern transect will allow us to uniquely constrain the thickening and uplift history of the arc.

· The cooling history of the CPC along the Bella Coola transect and along the southern termination of the CGC will be determined. Existing 40 Ar/ 39 Ar dates from the CGC at Douglas Channel area suggest that the part of the CPC east of the CSZ cooled from 500°C to 350 o C in the relatively short time interval between 53 and 48 Ma (Andronicos et al., in press). Determining if this same rapid cooling event also affected the Bella Coola area will be critical to deciphering the tectonic processes that led to the termination of magmatism within the CPC. Sphene will be dated from the plutonic rocks which are collected for U/Pb zircon dating and will constrain when the batholith cooled through ~500 o C (Mattinson, 1982). 40 Ar/ 39 Ar results from micas and K-feldspar will be used to construct the lower temperature section of the cooling curve.

· We will obtain 40 Ar/ 39 Ar age determinations for samples collected along the Bella Coola transect. We will also date the post-batholith basaltic dikes examined in the geochemical studies. We will analyze hornblende, biotite, and muscovite (where available) to determine when the rocks cooled from 450 to 300 o C across this transect. K-feldspar multi-domain diffusion modeling will determine the cooling history of the rocks at temperatures below ~300 o C. These dates, combined with the U/Pb ages on zircon and sphene and existing (U-Th)/He apatite age dates (Farley et al., 2001), will provide important constraints on the post batholith uplift history. They will allow us to construct complete cooling curves and to determine cooling rates across our transects. We will compare results from the southern transect to those in the northern transect. If a delamination event occurred after batholith formation, it may be manifested in pulses of accelerated uplift (more rapid cooling of a vertical transect) across the batholith. Moreover, these pulses of accelerated uplift may have been diachronous, a possibility that can be evaluated using geochronologic data. U-Th-Pb geochronologic analyses will be conducted using conventional IDTIMS methods and by LA-MC-ICPMS, both of which are routinely done at the University of Arizona . The conventional (ID-TIMS) analyses involve chemical separation and isotope dilution in a clean lab, followed by isotopic analysis with a VG- 354 or VG-Sector-54 multi-collector mass spectrometer. Some analyses may be conducted by laser-ablation multi-collector ICPMS, for which analytical methods have been developed during the past year. The instrument (a Micromass Isoprobe) has 14 collectors (9 Faradays, an axial Daly, and 4 ion-counting channels) and a flight tube with sufficient width that U, Th, and Pb isotopes can be measured simultaneously in static mode (see Facilities).The measured Pb is corrected for common Pb with the measured 204 Pb and compositions from Stacey and Kramers (1975) (with uncertainties of 1.0 for 206 Pb/ 204 Pb, 0.3 for 207 Pb/ 204 Pb, and 2.0 for 208 Pb/ 204 Pb). 206 Pb/ 238 U and 206 Pb/ 207 Pb ratios are corrected for fractionation by in-run analysis of fragments of a large zircon crystal of known age. The precision of ages for young minerals is presently ~1-2% (2- sigma level), with an additional ~2% of systematic error from the calibration correction, initial Pb composition, decay constants, etc. Figure 15a shows the precision and total error of ICPMS analyses of zircon from a Late Cretaceous sample in the CPC.

The advantage of laser-ablation ICPMS analysis is that spatial resolution can be a powerful tool in working out the history of grains with inheritance, overgrowths, Pb loss, etc. This power is shown in Figure 15b, which is a plot of zircon analyses from a Cretaceous pluton in southern Arizona . The cores of the grains yield ages of ~1625 Ma and ~1430 Ma, which are an excellent match for the ages of basement rocks in the area. The rims of the grains yield an age of 72.7 ± 1.9 Ma (1 s , including all systematic and random errors), which records crystallization of the granite.

Our strategy would be to initially conduct analyses with LA-MC-ICPMS, and then to conduct ID-TIMS analyses if higher precision analyses are required. The combination of ID-TIMS and LA-MC-ICPMS analyses has the power to answer the critical questions concerning ages of magmatism and to also provide information on the ages of crust with which the magmas have interacted.

40 Ar/ 39 Ar cooling dates will be obtained from K-feldspar, biotite, and hornblende, as well as muscovite when available. We anticipate approximately 30 analyses, 10 in years 2, 3 and 4. Whenever possible, we will date multiple mineral phases from the same sample. Initial sample preparation, including rock crushing and mineral separations will be done at UTEP, and 40 Ar/ 39 Ar analytical work will be performed at the Massachusetts Institute of Technology (Hodges et al., 1994). MIT houses state-of-the-art facilities for noble gas thermochronology. These include a Mass Analyzer Products (MAP) 215-50 gas source mass spectrometer configured with both Faraday and electron multiplier detectors and two, independent gas extraction and gettering lines. One gas extraction line is devoted to a doublevacuum resistance furnace with which conventional incremental heating experiments can be performed with ca. 5 °C precision in heating steps. The second line can be used with one of three laser microprobe degassing systems: an Ar-ion laser, a Nd-YAG UV laser, and a UV Excimer laser. These systems allow single grain work, and the high resolution and ablative properties of the UV lasers (< 5 m m spot sizes) permit isotopic mapping of single grains and in-situ (thin-section) samples.

To obtain cooling ages for micas and hornblende we will employ conventional resistance furnace heating experiments (e.g. McDougall and Harrison, 1999). These results will provide constraints on the cooling history of the parent rock between the closure temperatures of hornblende (ca. 500 °C – Hanson and Gast, 1967; Harrison, 1981), muscovite (ca. 415 °C to 350 °C – Robbins, 1972; Hames and Bowring, 1994; Lister and Baldwin, 1996), and biotite (ca. 350 °C to 300 °C – Grove and Harrison, 1996). Since there is evidence that composition – specifically Fe/Mg – affects the closure temperature of biotite (Harrison et al., 1985; Grove and Harrison, 1996; Giletti, 1974), we plan to measure the mole fraction annite (X ann ) in representative samples of the biotites we date using the electron microprobe at UTEP.

We will also obtain appropriate mineral separates to conduct detailed K-feldspar diffusion experiments. We will apply the multi-domain diffusion modeling technique to this data (e.g. Lovera et al., 1989, 1991; Lovera et al., 1993). This type of analysis can provide powerful constraints on the low-temperature cooling history of our samples, to about 150°C (e.g. McDougall and Harrison, 1999). Finally, we will conduct detailed, single-grain analyses of micas from our samples using the laser microprobe systems. These systems will allow us to obtain multiple total fusion spot ages within single grains that can be used to map age and/or diffusion gradients. Maps of this type can be used to understand more complex thermal histories.

Geological Data Management and Dissemination

Compiling, sharing, and tracking geologic data has been made far more efficient with the advent of integrated GIS/GPS technology. We will capitalize on this improvement by collecting and managing data using a suite of ESRI GIS programs. Field data will be collected on handheld computers (iPAQ’s by Compaq/HP) with integrated GPS cards using ArcPad (e.g. Rusmore et al., 2002; Hodges et al., 2002). This system has proven to be rugged, reliable, and highly efficient for geologic field work. Rusmore developed and used the system in field classes and she and Woodsworth used it for research in the Coast Mountains last summer. P.I.’s from the University of Arizona have also been using a similar system based on IPAQ’s for collection of field data. A significant advantage of mapping with ArcPad is that data is collected on forms that promote uniformity between data collected by various workers. This standardization leads to easier and more efficient compilation and interpretation of field data. Field data will be compiled in ArcView/ArcGIS. These compilations will be shared by geologists and geophysists, and a common project dataset and map will be produced. As new lab data such as ages or P-T conditions is acquired, it will be added to the dataset and displayed on the project map.

This process should overcome the common problems of compiling a variety of maps made at different scales, with different data, and located at different institutions. Once compiled, the ability to digitally manipulate (query, sort, search for nearest neighbor, contour, etc.) will significantly enhance interpretation of the data. Furthermore, by collecting and managing the data in an electronic format, this project should lead into the efforts of the recently NSF-funded GEON project (Andronicos is a Co-PI on GEON). Additionally, we will take advantage of existing infrastructure at the University of Texas at El Paso ’s Pan American Center for Earth and Environmental Studies (PACES) which has extensive computing facilities designed for the production and distribution of large GIS databases. Formal publication of the maps will be possible if current plans for publishing digital maps by the Geological Society of America are implemented.

Overall, digital mapping, data collection, and data management will streamline our interpretation of the geology, and greatly enhance sharing of the data within the project and with the broader community.