GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 39-6
Presentation Time: 9:00 AM-5:30 PM


HOEFT, Carly, STONE, Lindsey, HUMPULA, Jarrett, STUDENT, James J. and SIRBESCU, Mona-Liza C., Earth and Atmospheric Science, Central Michigan University, 314 Brooks Hall, EAS, Mount Pleasant, MI 48859-0000

Textures distinguish pegmatites from typical igneous rocks and may unveil processes and conditions of their formation. The spodumene-quartz intergrowth (SQUI) is thought to result from the isochemical breakdown of petalite (LiAlSi4O10) to spodumene (LiAlSi2O6) plus quartz during cooling of complex-type, petalite-subtype pegmatites such as the 2.6 Ga old Tanco pegmatite, SE Manitoba. Drill-core samples from the buried, internally-zoned Dibs pegmatite, located ~ 2 km from Tanco, were investigated to better understand the SQUI texture in relation to primary petalite; the compositional zoning of secondary spodumene; and the entrapment conditions of fluid inclusions in quartz from SQUI. Dibs is an internally zoned, gently dipping body, >500 m long, >100 m wide, and <65 m thick.

We have used 1) petrography, cathodoluminescence (CL), and SEM to characterize the mineral relationships, potential reactions, and compositional zoning; 2) BSE-image analysis to assess proportions within mineral intergrowths and distinguish between isochemical and open-system reactions; and 3) CL and LA-ICP-Mass Spectrometry to describe trace-element compositional zoning of spodumene. Spodumene occurs in coarser-grained SQUI (Type 1) that completely replaced petalite and finer-grained SQUI (Type 2), that only partially replaced petalite.

The minimum pressure of ~320 MPa for the petalite-breakdown reaction is constrained by our fluid-inclusion isochores from quartz in Type 1 SQUI and the stability field of spodumene (London, 1984). For this pressure the primary fluid inclusions in SQUI formed at 345±25°C. The highly variable proportions Sp:Qtz and the complex associations of SQUI with albite, muscovite, and K-feldspar suggest that Type 2 SQUI formed largely in an open system. Furthermore, the concentrations of transitional elements such as Mn, Fe, and Sn in Type 1 SQUI spodumene ranging tens to hundreds ppm also suggest an open system. The Mn/Fe ratios in zoned spodumene vary from 0.71 ± 0.3 to 5.7 ± 2.5, in their cores and rims, respectively. Trace element geochemistry of spodumene suggests that transitional metals introduced hydrothermally were involved in the formation spodumene at the expense of pure petalite.