Seventy-five core samples of medium- to fine-grained metasandstones from the State 2--14 borehole were studied petrologically and mineralogically to evaluate compositional variations of geothermal minerals and prograde metamorphic reactions. Three distinct metamorphic zones are defined based on the appearance of index minerals. With increasing grade, they are (1) chlorite-calcite (Chl-Cc) zone at depths between 610 and 2480 m; (2) biotite (Bt) zone between 2480 and 3000 m; and (3) clinopyroxene (Cpx) zone between 3000 and 3180 m.

All of the studied metasandstones at depths greater than 906 m contain epidote (Ep) and quartz (Qtz) with or without albite (Ab). Characteristic assemblages (+Ep+Qtz¿Ab) are (1) Chl+Cc¿K-feldspar (Ksp)¿phengitic muscovite for the Chl-Cc zone; (2) Bt+Chl¿Ksp¿anhydrite (Anh) and Bt+Act¿Chl for the Bt zone; and (3) diopside-salite +Act¿Bt¿oligoclase¿actinolitic hornblende for the Cpx zone. Epidote exhibits a compositional variation ranging from 0.11 to 0.42 XFe (=Fe/(Fe+Al)). Many amphibole grains of the clinopyroxene zone show patchy or lamellar intergrowths of both actinolite and actinolitic hornblende on a scale less than a few tens of micrometers. These amphiboles show distinct compositional discontinuities such as in Al2O3 contents (2.7--4.8 wt %) and Fe/(Fe+Mg) ratios (0.41--0.55). The Cpx zone samples thus mark the transition from the greenschist to amphibolite facies. Other important secondary phases include rutile or anatase (depths940 m), peristerite (1235 m), and Mn-salite (2484 m). The observed assemblages and compositions of minerals are chemographically interpreted utilizing an epidote projection in the simple system, Al2O3--K2O --MgO--CaO--SiO2--H2O--CO2. Reactions for the appearance of index minerals are elucidated in a qualitative T-XCO2 diagram.

The results indicate that (1) the compositions of Chl and Bt are highly variable depending on the bulk composition; however, Chl in the Chl-Cc zone becomes enriched in Mg with increasing depth; (2) Bt forms by a discontinuous reaction, Ksp+Chl+Cc=Bt+Ep+Qtz+H2O+CO2; hence the first appearance of Bt is greatly affected by the fluid composition; (3) the hydrothermal fluid in equilibrium with the analyzed assemblages is very low in XCO2 (<0.03); and (4) the absence of wairakite and prehnite is attributed to the high fO2 values of the salton Sea geothermal system. ¿ American Geophysical Union 1988