2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 37
Presentation Time: 8:00 AM-6:00 PM

Evolution of Garnet and Clinopyroxene in UHP Ultramafic Rock, Cuaba Gneiss, Rio San Juan Complex, Dominican Republic

ABBOTT Jr, Richard N., Department of Geology, Appalachian State University, Boone, NC 28608 and DRAPER, Grenville, Earth Science Department, Florida International University, Miami, FL 33199, abbottrn@appstate.edu

Dominican UHP ultramafites are of igneous origin (>3.5 GPa, >1550°C). The subsolidus (SS) history unfolded in three stages: SS-1, isobaric cooling (>3.5 GPa, ~1550 to ~850°C); SS-2, adiabatic decompression, (>3.5 GPa, ~850°C) to (~1 GPa, ~700°C); and SS-3, decompression and cooling through crustal conditions. This study elaborates the subsolidus evolution of garnet(Grt) and clinopyroxene(Cpx) in the assemblage Grt+Cpx+spinel(Spl)+corundum(Crn). At the solidus in the 3-GPa CMAS system, Grt[0] is ~grs20prp80; Cpx[0] is ~di25en25cts50 (=prp50grs50), cts=CaAl2SiO6. The natural assemblage involves three kinds of garnet: Grt[1], 29<grs<33, Fe>Ca, adjacent to Spl+Crn; Grt[1'], 37<grs<47, Fe~Ca, away from Spl, Crn and Cpx; and Grt[2], 33<grs<42, Fe~Ca, adjacent to Cpx. For observed Cpx[2], rim-core-rim chemical profiles are flat across cm-scale megacrysts. Cpx[2] is ~(di,hd)88cts07(jd,acm)05.

         Phase relationships modeled on 3-GPa experimental data illuminate the following scenario, starting from the solidus (~1550°C, Grt[0]+Cpx[0]): (1) On cooling, SS-1, subsolidus Grt became more calcic as Cpx became less aluminous, (di,hd)Cpx+ctsCpx=0.67grsGrt+0.33(prp,alm)Grt. Observed Grt[1] and a Cpx[1'] isochemical with Grt[1'] were close to equilibrium at ~1500°C.  Cooling below 1500°C, the composition of Grt[1] changed little while Cpx continued to become less aluminous. (2) At ~1200°C, SS-1, Grt[1'] formed isochemically from relict Cpx[1']. Grt[1] remained essentially unchanged while coexisting Cpx[2'] adjusted to ~(di,hd)70(en,fs)10cts20. "Dry" diffusion rates in pyroxene indicate that cm-scale adjustments involving Al2Mg-1Si-1 would cease to be important at T<1200°C, whereas FeMg-1 adjustments would be important down to ~850°C (3.5 GPa). (3) Grt[2] formed later (~750°C, <1.5 GPa, SS-2) at the expense of Grt[1'] and metastable Cpx[2'] by a process involving H2O-mediated diffusion and production of hornblende(Hbl), according to a reaction of the form: H2O+Cpx[2']+Grt[1']=Hbl+Cpx[2]+Grt[2]. Interestingly, the rock preserves only two "equilibrium" states: Grt[1]+Cpx[1']+Spl+Crn where Cpx[1']=Grt[1'] at ~3.5 GPa, ~1500°C, and Hbl+Cpx[2]+Grt[2] at ~1.5 GPa, ~750°C.