DEFORMATION OF ACADIAN GEOTHERMOBAROMETRIC TRENDS BY ALLEGHANIAN STRUCTURES:TWO CASE STUDIES FROM THE SOUTHERN APPALACHIAN BLUE RIDGE

ABSTRACTThe southern Appalachian Blue Ridge is composed predominantly of late Proterozoic metaclastic and metavolcanic units overprinted by a mid-Paleozoic dynamothermal event that recrystallized these units to a range of metamorphic grades. Because of a combination of appropriate mineral assemblages, and intriguing structural relationships, two regions have been investigated using geothermobarometric and kinematic analysis: (1) the Ashland-Wedowee belt, part of northern Alabama Piedmont (NAP), and (2) the Murphy Belt (MB) in north Georgia, a portion of the western Blue Ridge. The NAP contains lithologies equivalent to the eastern Blue Ridge (EBR) belt. Metapelitic, metagreywacke, and amphibolite lithologies from both study areas were analyzed with the microprobe for the coexisting assemblages: Ga+Bi+Pl+Mu, Ga+Hbl+Pl, and Ga+Cpx+Pl. Murphy Belt samples yielded a geothermal gradient in PT space of 11.5 deg. C per km inside the staurolite stability field, slightly above a Barrovian-type geotherm. A regional Alleghanian antiform deflects region (2) metamorphic isograds and geothermobarometric contours near Tate, GA. Although mapping of isograds in region (2) suggests inversion of the geothermal gradient, the effect is due to the deformation of Acadian isothermal and isobaric surfaces by the antiformal cross-fold, and does not represent a true syn-metamorphic inversion.

Region (1) samples yielded a mean geothermal gradient of 24.5 deg. C per km. However, field and petrographic evidence indicate both prograde and retrograde subsets that define a PT-time clockwise “loop” that we believe represents an uplift (decompression) event. In addition, the temperature and pressure estimates from the higher grade samples approach granulite facies conditions (774 C, 8.4 kb). These conditions appear to be confined to a narrow zone sub-parallel to regional strike in the hanging wall of the Hollins Line thrust fault, a major metamorphic discontinuity between the western and eastern Blue Ridge in region (1). We speculate that folding of metamorphic isograd surfaces above a footwall ramp would account for the observed trends.

NAP Samples

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Retrograde samples

17.6 deg. C km

35.3 deg. C km

R=0.753

R=0.587

NAP SamplesSample T deg. C P kilobar Mineral assemblageMD161 683 4.70 Ga+Bi+Pl+MuMD261 678 4.00 Ga+Bi+Pl+MuRA105 630 4.55 Ga+Pl+HblRA110 684 5.52 Ga+Bi+Pl+MuRA28A 658 5.03 Ga+Bi+Pl+MuRA406 575 3.91 Ga+Bi+Pl+MuRS0087B 686 4.90 Ga+Bi+Pl+MuRA118 579 6.74 Ga+Bi+Pl+MuRA222 661 8.34 Ga+Bi+Pl+MuRA416(5) 581 6.83 Ga+Cpx+Hbl+PlRA416B 610 5.68 Ga+Hbl+PlRA526A 661 6.23 Ga+Bi+Pl+MuRA632 731 8.27 Ga+Cpx+PlRA633C 774 8.39 Ga+Bi+Hbl+MuRA65 590 4.90 Ga+Bi+Pl+MuRA735 552 6.30 Ga+Pl+HblRA762 636 7.04 Ga+Hbl+Pl+BiRA78 729 10.06 Ga+Bi+Pl+MuRA82 569 6.14 Ga+Cpx+Pl+HblRS0083A 586 5.00 Ga+Bi+Pl+MuRS0118 612 6.09 Ga+Bi+Pl+MuRS0129C 623 5.50 Ga+Bi+Pl+MuRS2178 660 6.80 Ga+Bi+Pl+MuRA648 692 NA Ga+HblRA727 622 NA Ga+BiRA91 490 NA Ga+BiRS2237C 590 NA Ga+Bi

Murphy Belt SamplesSample T Deg. C P Kilobar Mineral assemblageE-5 568 NA Ga+BiJ-1 563 NA Ga+BiJ-3B 593 NA Ga+BiJ-4 565 6.29 Ga+Bi+Mu+PlT-3 635 8.34 Ga+Bi+Mu+PlT-5B 588 7.62 Ga+Bi+Mu+PlT-4 606 6.91 Ga+Bi+Mu+PlSMC-1(4) 545 NA Ga+BiSMC-1(B) 590 NA Ga+Bi

Murphy Belt Samples

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11.5 deg. C km

R=0.835

Murphy Belt Index Map

David T. Allison, James F. Tull, and Mark S. Groszos

Alabama NAP Index Map

Murphy Belt Geothermometry

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Above are two tables that summarize the geothermobarometric estimates for both the NAP and MB samples. The mineral assemblage probed is also indicated. All T and P estimates were calculated with the TWEEQ program and thermodynamic database of Berman (1992).

Samples analyzed from the region (1) NAP study area were collected from a six 7.5’ quadrangle area. This region is outlined with a green rectangle above. Current mapping research includes the 7.5’ quadrangle coverage indicated with black rectangles. The NAP contains eastern Blue Ridge equivalent lithologies.

The above Murphy Belt index map indicates the region (2) study areawithin the yellow rectangle. Samples used for microprobe analysis were collected from this four 7.5’ quadrangle region. The MB is a portion of the southern Appalachian western Blue Ridge belt.

The above photomicrograph displays a typical MB metapelitic composition undercross polars. The mineral assemblage Ga+Bi+Mu+Pl yielded T and P estimates using the TWEEQ program of Berman (1992). Long dimension = 10mm.

In addition to metapelitic compositions containing Ga+Bi+Mu+Pl, NAP samples also included Ga+Cpx+Hbl+Pl gneisses and amphibolites that yielded T and P estimates. Cpx grains above display 2nd order birefringence colors, whereas Hbl grains appear as light-brown to yellow anomalous 2nd order interference colors. Long dimension = 10mm.

Garnet amphibolite in plane polarized light displaying garnet porphyroblast (high relief) and hornblende (pleochroic) grains. The Higgins Ferry group of the NAP contains numerous amphibolites that were sampled for this study. Long dimension = 10mm.

Geothermometry results from MB samples using Ga+Bi. The rangeof temperatures is consistent with amphibolite facies conditions associated with a Barrovian geotherm.

Geothermometry results from Ga+Bi, Ga+Cpx, and Ga+Hbl assemblages in the NAP sample suite. The range temperature range is consistent with middle to upper amphibolite to perhaps granulite facies conditions. The blue samples contained retrograde textures.

The range of pressure estimates from NAP samples indicates amphiboliteto perhaps granulite facies conditions, although no Opx grains were found in NAP samples. Petrographic examination found evidence of retrograde textures in the blue samples.

Pressures calculated from Ga+Bi+Mu+Pl assemblages in MB samplesyielded pressure estimates consistent with middle to upper amphibolite facies conditions, and with a Barrovian geotherm.

NAP estimates plotted on the above P-T graph indicate a prograde geotherm of17.6 deg. C followed by a retrograde reduction in T and P along a 35.3 deg. C gradient. A possible explanation for this P-T-time path would be uplift due to emplacement of a mid to late-Paleozoic thrust sheet, re-equilibrating the retrograde samples to significantly lower pressure (4-5 kb) and temperature as compared to prograde samples.

MB samples yielded T-P estimates that fell along a least-squares 11.5 deg. C geotherm. The position of the samples in T-P space is consistent with middle amphibolite facies conditions and the mineral assemblages present in the samples (Ga+Bi+Mu+Pl+St).