Search

The Middle Jurassic Fourth of July Batholith and cross-cutting mafic dikes have been studied geochronologically, geobarometrically and paleomagnetically to estimate subsequent tectonic motion of the Cache Creek Terrane (CCT) in the northern Canadian Cordillera. 40Ar/39Ar hornblende ages from a granodiorite phase are similar to U–Pb zircon ages and indicate rapid cooling of the batholith upon intrusion, suggesting that the magnetization age is coincident with the 173-Ma crystallization age. Argon ages of biotite from the granodiorite and two mafic dikes have similar ages of ∼165 Ma, which dates cooling through ∼280 °C. Aluminum-in-hornblende geobarometry indicates differential uplift of the batholith across a north–south fault zone along Atlin Lake with >6 km more uplift on its eastern side. Also, the eastern side has been tilted downward to the south–southwest by ∼9°. Combined paleomagnetic data from 20 granitoid and 11 mafic dike sites yield an in situ paleopole at 55°W, 63°N (dp=5°, dm=5°) and a tilt-corrected paleopole at 81°W, 55°N (dp=5°, dm=6°). Compared to the 173-Ma reference pole for the North American craton, the tilt-corrected pole suggests a significant southward translation of 16.1±3.7° and a significant clockwise rotation of 107±7°. The translation estimate is similar to the Jurassic Teslin Crossing pluton in the Stikine Terrane, however, the rotation estimate is very different. This could indicate that the Cache Creek Terrane was at a similar latitude of the Stikine Terrane, but the two were not yet amalgamated.

Quantitative chemical analyses of fluid inclusions in sedimentary and diagenetic minerals can greatly improve our understanding of the chemistry of ancient surface and near surface waters. The environmental scanning electron microscope (ESEM) with an attached X-ray energy dispersive system (EDS) is capable of producing rapid and accurate major element chemical analyses of individual fluid inclusions in crystals of halite greater than about 30 μm in diameter. The ESEM-EDS technique uses the same basic principles as the X-ray microanalysis of frozen fluid inclusions reported by Ayora and Fontarnau [Ayora, C., Fontarnau, R., 1990. X-ray microanalysis of frozen fluid inclusion. Chemical Geology, 89, pp. 135–148.] but operates in a low-vacuum environment, which allows direct observation of the fluid inclusions to be analyzed without the need for a conductive coating. Modifications of the technique of Ayora and Fontarnau [Ayora, C., Fontarnau, R., 1990. X-ray microanalysis of frozen fluid inclusion. Chemical Geology, 89, pp. 135–148.] also include a newly designed sample holder, modified methods for the preparation of standard brine solutions to obtain “glass-like” homogeneous freezing behavior by addition of ethylene glycol, and a technique to obtain flat smooth surfaces on standard brine solutions. The ability to observe samples before analysis led to the discovery of heterogeneous freezing behavior of some standard solutions and fluid inclusions, which adversely influenced measurement reproducibility. The ESEM-EDS technique yielded quantitative results for Ca, Mg, K, SO 4 , and Cl at concentrations above about 0.1 wt.%; for Na, quantitative analyses were achieved at concentrations above 0.5 wt.%. Accuracies for the major elements in aqueous standards were better than 7% and precisions (relative standard deviations) ranged from 2% to 7%. Results on fluid inclusions in laboratory-grown halite gave accuracies of 6% to 10% for Mg, Ca, and K and precisions from 3% to 16%.

Many of the recent studies of chemical equilibrium in metamorphic rocks may have been hampered by sampling which did not reflect single regions or domains of local equilibration. This paper records results of tests performed to determine the specific volume of rock in chemical equilibrium with respect to a certain phase, under a particular set of conditions. Two samples of high-grade metamorphic gneisses from the Grenville Series of southeastern Ontario were selected for intensive study. The garnets of one specimen, possessing a biotite-garnet-cordierite assemblage, underwent in situ laser microprobe spectrochemical determinations for Fe, Mg, Mn and Ca. Conclusions are thus possible as to the volumes over which free diffusion and chemical equilibrium took place. These volumes were found to be on the order of only a few cubic centimeters and their shapes are strongly controlled by rock structures such as foliation and lineation. A higher-grade pyroxene granulite was also investigated with respect to garnet and biotite equilibrium. Somewhat larger volumes of equilibration were noted in this rock.