Thermobarometry with the computer program GTB

GTB(version 1.9) is a thermombarometry program based on a set of Fortran programs developed by Spear et al. (1991). To use the program mineral analyses are calculated to the relevant number of oxygens and entered into a mineral file. GTB gives a choice of thermobarometric calibrations from which it computes key reaction lines. In this study two thermometers and two barometers were used. Temperatures were obtained using the calibrations of Ferry & Spear (1978) (samples NP10.15, NP34.1, NP28.14 and NP43.15B, NP43.15C) and Nichols et al. (1992) (sample NP34.1).

Pressures were obtained from the calibrations of Nichols et al. (1992) for Grt-Crd assemblages (samples NP28.14 & NP9.2) and Hoisch (1990) for Grt-Bt-Pl assemblages (samples NP10.15, NP28.14, NP34.1, NP34.6, K35.ll, NP43.15B & NP43.15C); this allowed estimates of pressure to be made for cover rocks in the Khan Mine area and basement in the Namibfontein area.

The calibrations of Hoisch (1990) are as follows:

Rl. | Pyrope +1Grossular + Eastonite + 2Quartz <—> 2Anorthite + Phlogopite

-31830.6 + 79.02817- - 7?71n KR, - 26968(X® - X“g) + 32604.5X£ + 42855.4X® 3.8145-yAVgr

R2. i Almandine +j Grossular + Siderophyllite + 2Quartz <■—>2 Anorthite + Annite

-46707.2 + 85.58247' - RTln KR2 - 30960(X® - X®) + 24289.6X®g + 37265.6X® 3.8986-^AVgr

Analysis of cover samples in the Namibfontein and Khan mine area are shown in Figure 4.14a plots A-G. The plots show pressure obtained from Hoisch (1990) Grt-Bt-Pl samples in red, Nichols et al. (1992) Grt-Crd in red and Ferry & Spear (1978) in green, except for plot G which shows a temperature range using the calibration of Indares & Martignole (1985) models A&B and Ferry & Spear (1978).

The use of the Hoisch (1990) method has given consistent estimates of P for the Namibfontein arae. Plots A-F show pressure in the range 3-4kbar. This was obtained using the intersection of thermometers in plots B, C & mean Grt-Bt pair temperature of 662°C from Section 4.1.l.i. In the Khan mine area P was estimated at 3-2 & 5.5 kbar, from only two analyses. This at least 1 kbar higher than the cover in the Namibfontein area, consistent with an increase in regional pressure and temperature to the west reported by Masberg et al. (1992).

A comparison of PT conditions between cover and basement rocks from the

Namibfontein area is shown in plots H-I (samples N43.15B & N43.15C from the basement) & J-K (N28.14 & N34.1 from the cover) (Figure 4.14b). Eight seperate grt-bt calibrations were used to obtain minimum and maximum temperature estimates; areas of PT space have then been constructed (grey squares plot L) by combining these results with pressures for samples N43.15B (5.9 kbar±l) & N43.15C (7kbar±l), plots M & N after Hoisch (1992) and plots C & E . (Figure 4.14b).

Two conclusiosn may be drawn from these results Firstly, that peak metamorphic conditions in the basement were ~2kbar and 140°C higher than in the cover, this is greater than the error limits of the calibrations used and is therefore significant. Secondly, that the higher grt-bt basement temperature estimates made in Section 4.4.1 iii are confirmed. The application of other grt-bt calibrations.

Figure 4.14. A.

(COVER) Plots obtained from the computer program GTB.

167

Plot A. Sample NP9.2 from a Grt-Crd-Bt gneiss. l=Fe end member barometer 2=Mg end

member. Analyses Crd(14), Grt (16).

Plot B. Sample NP10.15 from a Grt-Bt-Pl gneiss. Analyses Grt(5),Bt(6) and Pl(12).

Plot C. Sample NP28.14 from a Grt-Crd-Pl gneiss. Analyses Pl(6), Grt(19, Bt(2), Crd(l)

Plot D. Sample NP29.8 from a Grt-Bt-Crd-Pl gneiss. Analyses Pl(22), Crd(l), Grt(8), Bt(6).

Plot E. Sample NP34.1 from a Grt-Crd-Grt-Bt-Sil gneiss. 1= Ferry & Spear (1978) estimate

from section 4.1.l.i. Analyses Bt(9), Crd(5), Grt(l), Pl(13).

Plot F Sample NP34.6 from a Grt-Bt-Pl gneiss. Analyses Pl(4)&(7), Bt(6), Grt(l).

A B

Figure 4.14. B. Plots obtained from the computer program GTB. Plots H-K show a range of • temperatures obtained from Grt-Bt calibrations. l=Ferry & Spear (1978), 2= Hodges & Spear (1982), 3= Ganguly & Saxena (1984) sym, 4= Ganguly & Saxena (1984) asym, 5= Perchuk & Larent' eva (1984), 6= Indares & Martignole (1985), 7=Ferry & Spear with Berman (1990) Grt, 10= Kleeman & Reinhardt (1994).

169

Plot H. Sample NP43.15B from a Grt-Bt-Pl basement gneiss. Analyses Grt(6),Bt(8).

(Pressure obtained from plot M)

Plot I. Sample NP43.15C from a Grt-Bt-Pl basement gneiss. Analyses Grt(4),Bt(2).

(Pressure obtained from plot N)

Plot J. Sample NP28.14 from a Grt-Crd-Grt-Bt-Sil gneiss. Grt(19, Bt(2).

Plot K. Sample NP34.1 from a Grt-Crd-Grt-Bt-Sil gneiss. Analyses Bt(9), Grt(l).

(Pressure obtained from Crd barometry)

Plot L. Plot to summarizing PT space for samples NP43.15, BNP43.15C(basement) and

NP28.14, NP34.1 (cover).

(Pressure obtained from Crd barometry)

Plot M. Sample NP43.15B from a Grt-Bt-Pl basement gneiss. Analyses Grt(7),Bt(8), Pl(l)&(4),

showing pressure and temperature.

Plot N. Sample NP43.15C from a Grt-Bt-Pl gneiss. Analyses Grt(l),Bt(2), Pl(4), showing

H _I

M N

171

4.5 Summary of

PT

conditions

A summary of PT estimates from this study is shown in Table 4.20. The use of several approaches has given similar results, this is encouraging. For cover rocks in the Namibfontein area peak metamorphic conditions were estimated using seven methods, two were used to obtain estimates from basement rocks. In the Khan Mine area PT estimates were made using three methods for the cover and one for basement rocks.

In the Namibfontein area Grt+Crd+Bt+Pl gneisses from the cover record PT conditions for Mj at ~662°C and 3.3-4 kbar. In the Khan Mine area Grt+Crd+Bt+Pl gneisses record PT

conditions for Mi ~690°C and ~5kbar. In the Namibfontein dome, basement Grt+Bt+Pl gneiss records M] at -800° and ~6-7kbar, possibly as much as 140°C higher and 3kbar greater than the cover.

Table 4.20. Summary of Ml PT conditions from a variety of thermo-barometers for the Namibfontein and Khan Mine areas.

Calibration/ Method P kbar T °C Description

Ferry & Spear (1978) (4kbar) 662±50°C COVER arithmetic mean 4kbar(cores) Ferry & Spear (1978) (4kbar) 640±50°C COVER

COVER

median 4kbar(cores) Ferry & Spear (1978) (4kbar) 569±50°C COVER

COVER

arithmetic mean 4kbar(rims) Ferry & Spear (1978) (4kbar) 548-740±50°C COVER bt inclusion-grt core range Ferry & Spear (1978) (4kbar) 755&

675±5O°C

COVER core temperature Ferry & Spear (1978) (4kbar) 594 & COVER rim temperature

606±50°C

KHAN MINE AREA Garnet Profiles

(qualitative)

>640°C±25 COVER NAMIBFONTEIN AREA Blundy & Holland (1990) 786°,669°,855° COVER Cover arithmetic means per

±75 sample

Blundy & Holland (1990) 844°,872° Cover arithmetic means per

±75 sample

KHAN MINE AREA Argles pers comm. 3.2-3.5 (>650°) COVER Crd barometry THERMOCALC -3.7-4 604°-650° COVER Cover Crd-Grt (NF)

NICHOLS 3.4 622° COVER NP34.1 Crd-Grt (NF)

GTB -4 -610° COVER NP9.2 Nichols et al. (1992)

GTB GTB

-4 -610° COVER NP10.15 Hoisch (1990) Ferry & Spear (1978) 3-4.5 -650° COVER NP28.14 Hoisch (1990)

Ferry & Spear (1978) Nichols et al. (1992)

NP29.8b

GTB _{3.8-4 600-670° COVER NP34.1 Hoisch (1990) Ferry &}

Spear (1978) Nichols et al. (1992)

GTB 3-4 COVER NP34.6

Rasse (1976) >650° Ti content of amphiboles

NAMIBFONTEIN AREA GTB _{4.6& 5.9 >600° COVER KP35.11}

±1 KHAN MINE AREA

GTB 5.9 ±1 -800° BASEMENT Sample NP43.15B Basement

GTB 7 ±1 -800° BASEMENT Sample NP43.15C Basement

NAMIBFONTEIN AREA Ferry & Spear (1978) (4kbar) 791 °C BASEMENT Basement mean core temperature

NAMIBFONTEIN AREA Ferry & Spear (1978) (4kbar) 572°C BASEMENT Basement rim temperature

NAMIBFONTEIN AREA Garnet Profile >640°C±25 BASEMENT

Blundy & Holland (1990) 845°, 846° BASEMENT arithmetic means per sample KHAN MINE AREA

In document Mid crustal geodynamics of the southern central zone, Damara Orogen, Namibia (Page 177-185)