Ti Mobility in Quartzite from the Eureka Valley-Joshua Flat-Beer Creek Pluton (EJB) Aureole in California, USA.

Abstract:

The EJB pluton intruded into Paleozoic sandstone during the Jurassic leaving a well-exposed aureole. Quartzite was collected from the contact outward to investigate the response of Ti in sedimentary quartz to varying grades of metamorphism using EPMA and CL imaging. TitaniQ and Zr in rutile thermobarometers were applied to rutile bearing quartzite at 225, 500, and 1540 meters from the contact. Comparison CL images, including those of Harkless sandstone collected 15 km away and Harkless quartzite in the aureole reveal significant Ti mobility and a general smoothing of the Ti distribution in the quartzite.

The CL images also reveal that the redistribution of Ti in the quartzite was incomplete. Discounting this, TitaniQ was applied using a Ti activity of 1.0 and a pressure of 2.75 kbars based on previous studies. In an effort to establish possible peak conditions, Ti was measured in the brightest CL quartz grain interiors, and the Zr was measured in surrounding rutile grains. The temperature calculations are inconsistent. At 225 meters, the Zr in rutile resulted in a temperature range from 662-762 °C (n=31) compared to a maximum temperature of 595 °C based on Ti in quartz. At 500 meters, rutile resulted in a range from 635-720 °C (n=44) compared to quartz at 525 °C. At 1540 meters, the rutile ranged from 650-710 °C (n=55) compared to quartz at 547 °C.

The thermometric results may suggest that the aureole was extensively heated to above 500 °C even at 1.5 km from the pluton contact. This is supported by previous studies using other thermometers, thermal modeling, quartz deformation fabrics, and evidence of partial melting-type textures. In all likelihood though, the thermometric results in this study reflect components of inheritance of the original grains, lack of complete Ti re-equilibration due to sluggish diffusion of Ti during metamorphism, and the varying effects of water availability as related to the formation of distinct deformation textures.