Studies of igneous geochemistry and petrology

The processes active during batholith generation can be deduced from the study of the geochemical and petrological evolution of the plutons through time. Petrologicand geochemical studies of arc-related, igneous and meta-igneous rocks along the twoproposed transects will be carried out to resolve: (1) the bulk chemistry of the exposed part of the arc, (2) the depth of generation of granitoid plutons and the need for, or lack of an “eclogite facies” residue, (3) relative fractions of partial melt generated from crustal and mantle-derived rocks, and (4) temporal changes in chemical and isotopic patterns that might be correlative with tectonic processes such as crustal thickening and/or delamination.

 

· Batholith bulk composition. We will measure the major element chemistry of plutonic rocks to constrain the bulk chemistry of the arc between 5 and 25 km depth. This will provide minimum constraints on the thickness of the felsic batholith and a complete view of the change in chemistry from shallow to mid-crustal depths within the arc. Recent studies of tilted exposures of other North American arcs (e.g. Ducea et al., 2002) suggest that felsic arc thickness in major Cordilleran arcs is typically 25-35 km, 1.5-2 times larger than commonly thought (Christensen and Mooney, 1995; Rudnick and Fountain, 1995). For example, a recent multidisciplinary study of the Sierra Nevada batholith has shown that the composition of that batholith remains silicic (average tonalite to granodiorite) to about 30 km (Fliedner et al., 2000). Similar results have been obtained in the southern CPC in the north Cascades (DeBari et al., 1998). Mass balance calculations will then be used to predict the compositions and thickness of residual assemblages (Ducea, 2002). We will use a modified version of MELTS (Ghiorso and Sack, 1995) to calculate end-members for the vertical extent, compositions and physical properties of residual assemblages (e.g. Figure 5). These models will be then used to constrain the composition and size of the batholithic root. In the case of the CPC, outcrop data indicate that the felsic batholith is at least 25 km thick, which would require a minimum of 10-15 km (and probably significantly more) eclogite facies residue.

 

· Depth of melt generation as a monitor of paleo-crustal thickness. Trace elements, especially REE, provide an important constraint on the depth of pluton generation (e.g., Gromet and Silver, 1987). Specifically, a garnet-rich and plagioclase-poor residue is characterized by highly fractionated REE patterns and lack of Eu anomalies, as is the case for the CPC rocks from SE Alaska , studied by Arth et al. (1988) (Figure 14a). In contrast, granitoids that equilibrated with a granulitic or amphibolitic residue will show a negative Eu anomaly due to retention of Eu by residual plagioclase. They will also lack the steep normalized pattern of heavy REE, because of low abundance or lack of garnet in the residue. This is exemplified by the granites of Clifton and Summit Lake in SE Alaska (Barker et al., 1986; Figure 14b), which show a negative Eu anomaly, indicative of plagioclase fractionation and indirectly of a shallower source for these rocks. Consequently, such signatures, when studied in conjunction with isotopic tracers, are extremely important in detecting the possible existence of thickened crust within the arc and the timing of crustal thickening/thinning. For instance, changes of pluton sources from garnet bearing (thick crust) to garnet absent (thin crust) coupled with progressive lowering of the depth of emplacement of plutons may fingerprint the removal of a dense crustal root and subsequent uplift of mid-crustal rocks (see Kay and Mpodozis, 2001).

 

· Isotopic data for granitoids - a window into the source region. We will measure oxygen isotopic ratios of quartz and other minerals, such as zircon (Valley et al., 1994), that are not prone to post-intrusive modifications to constrain the minimum fraction of pre-existing crustal materials involved in the arc budget. These ratios will be acquired together with whole-rock radiogenic isotopic ratios of Sr, Nd, and Pb. Radiogenic isotopic data will provide constraints on the proportions of juvenile mantle-derived vs., crustally recycled components in the arc (Farmer and DePaolo, 1983; Miller et al., 1988; Samson et al., 1991). Although there are problems of equivalence in such juvenile vs. recycled models, the radiogenic isotopic data will help to qualitatively identify important end-members that contribute to the isotopic heterogeneity of the batholith. We will put a special emphasis on measuring isotopic ratios on the more mafic end-members, in order to detect end-member mantle isotopic signatures. Important also are the suites of two-mica, even garnet-bearing granitoids that occur late in the CPC evolution of the proposed study region (Mahoney et al., 2002). They may indicate evolution of melting processes towards upper crustal levels late in the magmatic flare-up. The primary goal of the isotopic studies of mafic rocks is to distinguish asthenospheric vs. lithospheric sources.

 

· Integration of geochemical data for the CPC. With a large geochemical dataset and appropriate geochronologic data (see below) in hand, we can begin to search for temporal changes in the chemical and isotopic composition of the batholith that we can correlate with tectonic events. None of the different types of isotopic and elemental data have so far been evaluated together, either for individual areas, or the batholith as a whole. We have made a more or less complete compilation of existing Nd isotopic data for the purpose of this proposal. Preliminary data available for the entire CPC (Figure 6) suggest that magmatism was highly episodic, and that the magmatic addition rates for the Eocene flare-up were close to 100 km 3 /(km x Ma). The database for CPC plutons shows that Nd isotopic ratios are relatively constant throughout the Mesozoic, but a clear influx of low e Nd melts is recorded during the major flare-up of the early Cenozoic (Figure 6). The e Nd values rebound to more positive values after about 40 Ma. However, the data shown in Figure 6 are compiled from a large area representing the entire Coast Mountains batholith, which is quite heterogeneous along and across strike. Nd isotopes were often measured on rocks for which no major or trace element data were gathered. There is no intrusive mass flux information that accompanies the data in Figure 6. There is also a lack of comprehensive major- and trace-element data for the batholith as a whole, and certainly for our proposed study area, that would allow an assessment of critical issues like magma vs. residue proportions, or presence of garnet or plagioclase in the residue. It is critical to understand how changes in isotopic composition correlate with magmatic fluxes and tectonic events within a relatively small area, such as the one identified in this project. The isotopic and trace element data will be coordinated closely with the geochronologic, structural and metamorphic information collected during this study.

The main rock types that will be targeted in this study are tonalites and granodiorites that make up the bulk of the CPC. We plan to achieve a complete characterization of the batholith composition across strike and along its (exposed) vertical dimension. In addition to the study of batholithic rocks, we plan to focus on two other rock types: (1) metamorphic pendants within the CPC, and (2) mafic dikes and plugs postdating the emplacement of the batholith. The metamorphic pendants provide end-member composition for potential crustal assimilants that may have played a role in the batholith evolution, especially in the areas where the CPC is more deeply exposed. The pendants within the batholith are especially intriguing because limited previous data (Samson and Patchett, 1991) suggest that these framework rocks are isotopically unlike any rocks in the crustal domains east or west of the batholith.

Another important monitor of the changes in the source region are post batholith basaltic dikes. Although these lithologies are not volumetrically significant they occur throughout the proposed study area (Hutchison, 1980; Baer, 1973). If a convective removal event did occur after the end of calc-alkaline magmatism, changes in the chemistry of the basaltic dikes may prove to be the best fingerprinting tools for such an event (Kay and Kay, 1993; Farmer et al., 2002). Along Douglas Channel, most dikes are part of a NE-striking swarm that yields 40 Ar/ 39 Ar ages of 20 Ma. A smaller NW-striking population is younger than a 14 Ma pluton near the head of Douglas Channel (Rusmore et al., 2000) Throughout the CPC, ages range from about 45 to a few million years old. This range of ages should permit us to track changes in mantle composition from shortly after the arc shut off through Miocene or Pliocene time. We will coordinate our efforts here with J. K. Russell of the University of British Columbia who has been studying volcanic rocks and xenolith suits from east of the Coast Plutonic Complex.

Samples of the CPC will be selected for petrographic and geochemical analyses along traverses in Douglas Channel, in Burke Channel transect, and in between the two transects. Samples from the intervening area will be collected during the geologic work between the two transects. We plan to obtain a complete set of major and trace element concentrations as well as Sr, Nd, Pb and O isotopic data on approximately 250 samples from the area. Samples for U-Pb and 40 Ar/ 39 Ar geochronology will be selected from these.

Major element analyses will be conducted by Karl Wirth and a rotating team of undergraduste students at Macalester College. Wirth and his students have a long history of collaboration both with Patchett and co-workers at Arizona , and with other research university groups. In this way, undergraduate theses can be conducted on segments of our batholith study, possible involving visits to field areas or to the University of Arizona by the students. Macalester College posesses a fully automated XRF operation. Trace element analyses will be conducted by Rob Kerrich and staff at the University of Saskatchewan using state-ofthe-art ICPMS techniques. Patchett has utilized data from this lab successfully in his provenance studies of North American sedimentary rocks, funded by NSF Tectonics. Those studies involved purchased data, but in the present proposal, Kerrich will also participate in interpretation of the geochemical data that will be gathered. Igor Morozov of the University of Saskatchewan is also involved in the active-source seismic research proposed here. The addition of a geochemical collaboration enhances the direct Canadian involvement in the project.

Ducea, Patchett and students at the University of Arizona will analyze Sr, Nd, and Pb isotopic tracers in the plutonic rocks by thermal ionization mass spectrometry. The primary goals of these analyses will be (1) to establish reliable initial isotopic ratios in order to evaluate the extent of mantle-derived components vs. older recycled crustal components, and (2) to track temporal changes in source composition. Oxygen isotopes will be measured at the University of Arizona on minerals from these rocks by conventional and laser fluorination techniques. Combination of these data will allow for detailed petrologic modeling (crystallization and melting modeling) in order to compare plutons and their sources through time and space.

Igneous crystallization temperatures and pressures, and hence depths of emplacement, will be estimated using conventional thermobarometric techniques, including the Al in hornblende barometer and the hornblende-plagioclase thermometer(e.g. Hollister et al., 1987; Blundy and Holland, 1993; Ague and Brimhall, 1988). Electron probe microanalysis of minerals will be performed by Ducea and students at the University of Arizona and will be used to characterize mineral composition, to determine temperatures and pressures of mineral equilibration, to constrain petrologic models, and to measure the extent of zoning in order to constrain crystallization, mixing, and cooling histories.