Geo-thermometry and barometry in the southern Blue Ridge crustal section: Evidence for tectonic granulites

Conference · Mon Mar 01 04:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:6079143

Eckert, Jr, J O ^[1]

---

1. Geological Survey, Menlo Park, CA (United States)

The study of Ramberg and DeVore established the framework within which the Mg-Fe[sup 2+] distribution between coexisting minerals, specifically olivine and Opx, could be calibrated as a thermometer. Experimentally calibrated thermobarometric systems applicable to high-grade rocks include Grt-pyroxene assemblages. Grt-Opx and Grt-Cpx thermometry apply the theory established by Ramberg and DeVore. Barometry, based on the Ca distribution in Grt-Opx Plg-Qtz (GAPES) and Grt-Cpx-Plg-Qtz (GADS) assemblages, was improved by solution calorimetry on synthetic Mg end members to produce reliable enthalpies of reaction. The resultant barometric expressions are: P = 3.47 + 0.01307T + 0.003504T lnK[sub GAPES] ([plus minus]1.55 kbars) (GAPES) and P = 2.60 + 0.01718T + 0.003596T lnK[sub GADS] ([plus minus]1.90 kbars) (GADS). Current work is focused on reversing the Mg-Fe[sup 2+] exchange between Grt and Opx. In the SW North Carolina Blue Ridge, where Grt-Opx and Grt-Cpx T calculations yield plausible granulite-facies temperature ([>=] 700C), P-T conditions calculated with these pyroxene-based reactions generally agree with calculations using equilibria in pelitic rocks (T from Grt-Bt; P from Grt-Plg-Al[sub 2]SiO[sub 5]-Qtz, GASP). Grt-Opx/Cpx systems allow P-T estimation only in the granulite and eclogite facies; Grt-Bt and GASP permit P-T calculations throughout this amphibolite to granulite facies transition. Results from metapelites indicate increasing P with T (positive dP/dT); this implies a depth-zone array. This progression thus represents a Paleozoic (Taconian ) middle-to-lower crustal cross section, the southern Blue Ridge crustal section (SBRCS). The SBRCS P-T array shows greater dP/dT than would be likely near an external heat source. These high-grade rocks likely heated after crustal thickening, forming as tectonic granulites.

• OSTI ID: 6079143
• Report Number(s): CONF-9304188--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 25:4
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
ALKALINE EARTH METALS
CALCIUM
DEVELOPED COUNTRIES
ELEMENTS
GEOLOGIC FORMATIONS
GEOTHERMOMETRY
IRON
MAGNESIUM
METALS
MINERALS
NORTH AMERICA
NORTH CAROLINA
RESERVOIR PRESSURE
RESERVOIR TEMPERATURE
TRANSITION ELEMENTS
USA