Ultra-High Pressure (UHP) rocks of the Cordillera

Septinrional, Dominican Republic

Bonnie N. Broman

Appalachian State University

BACKGROUND:• The Cordillera Septintrional, which consists of metamorphic and

igneous basement rocks overlain by Miocene-Eocene siliciclastic and carbonate rocks, is located in the northern part of the Dominican Republic

• Stratigraphic windows (inliers) of metamorphicand igneous rocks are exposed in various places such as the Rio San Juan (RSJ) complex

• Within the RSJ, are three rock types of particular interest in regards to magma genesis:garnet peridotite, garnet clinoproxenite, and corundum bearing garnet clinnopyroxenite (composed of clinopyroxene, garnet, spinel, corundum and secondary hornblende with evidence of a magmatic origin)

Garnet Clinopyroxenite

Garnet Peridotite

Corundum-bearing Garnet Clinopyroxenite thin-section

RESEARCH GOALS:• To determine Pressure and Temperature conditions under

which these rocks form

METHODS:• Field work which consisted of collecting samples and looking at

relationships within the boulders in the Dominican Republic

• Analyze our samples using the SEM and EDAX• SEM PROCESS:

– We obtained homogenous natural mineral standards to analyze and compare to the samples we collected

– Analyze sample DR03-12b which is the corundum-bearing garnet clinopyroxenite to determine the exact composition of the minerals, mainly garnet, clinopyroxenite, spinel, and corundum

– Collected data for garnet, and clinopyroxene using our standards

SEM image of garnet standard SEM image of augite standard

DATA: c:\edax32\genesis\genspc.spc Acquisition Time:12:19:27 Date:26-Apr-2005 kV:30.00 Tilt: 0.00 Take-off:20.02 AmpT:35.0Detector Type :SUTW-Sapphire Resolution :129.60 Lsec :92 EDAX ZAF Quantification Compound Oxides Element Wt % Mol % K-Ratio Z A F MgO 6.12 9.99 0.0089 1.0029 0.2355 1.0057 Al2O3 22.1 14.27 0.0346 0.9744 0.2982 1.0061 SiO2 38.13 41.76 0.0566 1.0038 0.3116 1.0015 CaO 12.25 14.37 0.069 0.9768 0.7901 1.009 FeO 21.4 19.6 0.1478 0.9022 0.9737 1 Total 100 100 Element Net Inte. Bkgd Inte. Inte. Error P/B MgK 2.59 0.42 7.43 6.15 AlK 11.06 0.66 3.3 16.8 SiK 19 0.77 2.48 24.8 CaK 13.55 0.39 2.9 34.89 FeK 15.78 0.16 2.64 97.53

Example of Garnet analysis

c:\edax32\genesis\genspc.spc Acquisition Time:10:50:01 Date:26-Apr-2005 kV:29.99 Tilt: 0.00 Take-off:20.02 AmpT:35.0Detector Type :SUTW-Sapphire Resolution :129.60 Lsec :93 EDAX ZAF Quantification Compound Oxides Element Wt % Mol % K-Ratio Z A F MgO 15.9 21.98 0.0271 0.9895 0.2822 1.0057 Al2O3 2.61 1.43 0.0041 0.9614 0.306 1.0099 SiO2 52.03 48.27 0.0977 0.9904 0.402 1.0025 CaO 25.03 24.88 0.1401 0.9624 0.8083 1.0017 FeO 4.43 3.44 0.0289 0.8895 0.9384 1 Total 100 100 Element Net Inte. Bkgd Inte. Inte. Error P/B MgK 7.9 0.43 3.88 18.43 AlK 1.32 0.43 11.58 3.08 SiK 32.81 0.58 1.84 56.67 CaK 27.53 0.51 2.01 53.5 FeK 3.09 0.48 6.75 6.4

Example of clinopyroxene analysis

REMAINING WORK:

• Normalize analyses• Determine pressure and temperature conditions at which the

compositions of these minerals are stable

THANK YOU!