Petrology of marble and peridotite in the Seiad ultramafic complex, northern California, USA

Abstract

Peridotite and infolded marble of the Seiad ultramafic complex were recrystallized in the upper amphibolite facies as part of the regional progressive metamorphism of the Rattlesnake Creek terrane. Field relations, including the occurrence of metarodingites, and metasomatic zones between dissimilar rock types, demonstrate that the metasediments and serpentinized ultramafic rocks were juxtaposed prior to regional, barrovian metamorphism. Temperatures are estimated to have reached 760-800°C at pressures of 7-8 kbar during the peak of metamorphism. Four low-variance parageneses have been identified within a small (3 km2) area of the complex, which may reasonably be assumed to have formed under the same P and Tconditions. Isobaric T-Xco, diagrams of appropriate equilibria are presented for three different internally consistent sets of thermodynamic data. Despite the seemingly small numerical differences between the standard state thermodynamic properties of the data sets, only one diagram allows the four observed assemblages to coexist within a reasonable temperature range. All three phase diagrams require differences in fluid composition on the scale of a thin section; strong evidence for effective control of pore fluid composition by local mineral reactions during metamorphism.

Batch-wise assembly and zoning of a tilted calc-alkaline batholith: Field relations, timing, and compositional variation

“…1). The metamorphic grade of hanging-wall rocks above the Condrey Mountain dome increases toward the dome from greenschist (at ~300 MPa) to granulite (at ~800 MPa) facies (Donato, 1987(Donato, , 1989Petersen, 1982;Lieberman and Rice, 1986;Garlick et al, 2009), indicating that doming caused radial tilting of the hanging wall to the southwest, away from the dome (current geographical coordinates). Effects of this tilting were observed in the contact metamorphic assemblages in the WCb aureole, where mineral assemblages give pressure estimates of ~300 ± 150 MPa along the southern contact, farthest from the dome, to ~650 ± 150 MPa along the northeastern contact, nearest the dome (Barnes et al, 1986b).…”

Metasomatic effects related to channelled fluid streaming through deep crust: fenites and associated carbonatites (In Ouzzal Proterozoic granulites, Hoggar, Algeria)

“…The theoretical reaction occurs Variations in d13C and d18O exist among the different localities but the values tend to be grouped, particularly at c. 700-720°C for P fluid =P total =7 kbar and for aCO 2 of 0.05-0.4 (Lieberman & Rice, 1986 ) and the d13C, within a given sampling area. Although the total variation in d13C is −0.80 to −4.2‰, most of the related mass fraction of O remaining in solid compounds (F ) is 0.73.…”

Geophysics and petrology of the deep crust and upper mantle; a workshop

“…Serpentinite and associated rodingite subsequently developed hightemperature, high-pressure metamorphic assemblages indicating conditions near 725 °C and 0.7 GPa (Rawson, 1984). These high-grade assemblages are now observed in metaserpentinite in the lower structural levels of the melange (Rawson, 1984;Lieberman and Rice, 1983), but occur only locally in the higher structural levels (Donato, 1985). Representative high-grade assemblages in metaperidotite bodies are: olivine+enstatite±anthophyllite, olivine+enstatite+talc±tremolite, and olivine+tremolite+anthophyllite.…”