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

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+cts^Cpx=0.67grs^Grt+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.