Abstract
A P-T path for the crystallization of the Harding peg matite magma has beenconstrained from I) the experimen tally determined liquidus, solidus and phase assemblages for a bulk sample of the pegmatite, 2) isochores for CO2-H2O-NaCl fluidinclusions in quartz and beryl from the various lithologic units of the pegmatite, 3) the condi tions of metamorphism of the surrounding host-rock, and 4) pegmatite mineral equilibria, including the Li-Al-siheatcs. micas, and feldspars. The magmatic portion of crystallization began at 650°C and 330-350 M[Pa (11-12 km depth) and continued isobarically to 550°C. Fluid inclusions indicate that the fluid composition (mole %) evolved from 6CO2:4NaCl to less than ICO2:10NaCl during the crystallization. If the host-rock temperature at the time of intrusion was 0 to 200°C below the solidus temperature, then cooling models for a finite sheet indicate that the mag matic crystallization (giant-crystal texture) occurred in 1000 years or less. The subsolidus hydrolhermal replacement and re-equilibration continued for several million years. Isobaric cooling continued to 400-300°C. followed by uplift and erosion. Given the estimated age and depth of the peg matite, then the average rate of uplift of the Picuris Range was0.7 - 1.0 cm/yr. Li-Be-bearing margarite, prominent in the hanging-wall mctasomatic aureole, probably crystalizcd below the pegmatite solidus. Quartz-spodumene inergrowths exposed in footwall aplite are interpreted as pseudomorphic replacements of early-formed pelalite. Eucryptitc replacement of spodumene occurred during uplift at 100-200°C.
| Original language | English |
|---|---|
| Pages (from-to) | 287-298 |
| Number of pages | 12 |
| Journal | Canadian Mineralogist |
| Volume | 28 |
| State | Published - 1990 |
| Externally published | Yes |
Funding
University of New Mexico for their successful efforts to preserve the Harding pegmatite mine as a classic locality for teaching and research. Special tribute goes to the Griego family, whose dedicated service to the Harding mine now spans three generations. The photographs appearing as Figures 3a and 3b were kindly provided by Art Montgomery. We are grateful for constructive reviews by Petr Cerny, David London, and Robert Meintzer. This research was conducted while BCC was a Postdoctoral Fel low supported by the U. S. Department of Energy, Basic Energy Sciences, under grant DE-FG04-84ER45099 (R. C. Ewing, principal investi gator).
| Funders | Funder number |
|---|---|
| U. S. Department of Energy | |
| Basic Energy Sciences | DE-FG04-84ER45099 |
Keywords
- Cooling models
- Fluid inclusions
- Granitic pegmatite
- Harding pegmatite
- Li-Al silicates
- Li-Be-bearing margarite
- New Mexico
- P-T crystallization path
- Pelalite pseudomorphs
- Proterozoic rocks
- Regional metamorphism