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16O data in late orogenic granites of the southern Appalachian Piedmont
200
Earth and Planetary Science Letters, 54 ( 1981 ) 200- 202 Elsevier Scientific Publishing Company, Amsterdam Printed in The Netherlands
[41
Correlation of magnetitic susceptibility with 180/160 data in late orogenic granites of the southern Appalachian Piedmont Brooks B. Ellwood and David B. Wenner Department of Geology, UniversiO, of Georgia, Athens, GA 30602 (U.S.A.) Received June 30, 1980 Revised version received April 2, 1981
Initial magnetic susceptibility (generally indicative of magnetite content) has been determined for 445 samples from 17 granites located in the southern Appalachian Piedmont of Georgia and South Carolina. These values have been correlated with whole rock 31SO data from the same plutons, yielding a pronounced inverse relationship. It has previously been shown for the southern Piedmont that low oxygen isotopic (is O-enriched) values usually occur in S-type granites (Wenner [1], this issue). It follows, then, that I-type granites are characterized by high susceptibilities ( x > l × 10 4 G/Oe), and S-type granites by low susceptibilities ( x < l X 10 4 G/Oe). An interesting result of this work has been the observation that some S-type granites exhibit good within-site clusters of remanent magnetic directions while I-type granites generally do not.
1. Introduction
Many of the late orogenic granite plutons of Georgia and South Carolina have been the focus of varied studies directed toward understanding the age and genesis of the Piedmont. This work includes isotopic (for example Wenner [1], this issue) and magnetic studies [2,3]. As part of the magnetic studies, initial susceptibility (X) data were determined using a Bison susceptibility bridge for multiple specimens from more than 30 granitoids. Whole rock 6~80 data were also available for 17 of these ([1], this issue; and this study). Data for these 17 units are plotted in Fig. 1. The ~SO/~60 data in these granitoids appear to reflect contrasting protoliths, with the ~SO-enriched plutons forming through anatexis of metasedimentary and sialic protoliths, and the low-~SO granites within the Charlotte-Carolina Slate Belt and portions of the Kiokee belt having been derived from source regions dominated by more mafic, metaigneous rocks [1]. Thus, it would appear that this
series of granitoids exhibits a number of isotopic similarities to the S- and I-type granitoids first described in Australia [4]. Such an interpretation is supported by the correlative initial strontium isotopic data [5], and in part by the contrasting nature of the exposed geologic terranes into which the plutons are intruded, and most importantly by the gravity data which suggest contrasting subcrustal lithologies [6]. It has been shown that magnetite is depleted in granites derived from carbonaceous rocks (equivalent to S-type granites) and enriched in granites derived from rocks of low carbonaceous content (equivalent to I-type granites) [7]. These observations have led to the subdivision of granitic rocks in Japan into a magnetite and an ilmenite series [7], which is basically equivalent to the S- and I-type described elsewhere [4]. Although no specific relationship has as yet been shown between the oxygen isotopic and magnetite compositions of granites, whole rock oxygen isotopic compositions [8] of some Cretaceous to
0012-82 IX/81/0000-0000/$02.50 '~ 1981 Elsevier Scientific Publishing Company
201 M i o c e n e - a g e g r a n i t e s in J a p a n h a v e also b e e n subd i v i d e d i n t o a m a g n e t i t e a n d i l m e n i t e series [7]. In t h e s e studies, g r a n i t e s of the m a g n e t i t e series gene r a l l y h a v e l o w e r 6 1 8 0 v a l u e s (--~ 8%0) w h e r e a s r o c k s o f the i l m e n i t e series are ~ 8 0 - e n r i c h e d ( 9 - 1 3 %o). H o w e v e r , these studies d o n o t d i r e c t l y r e l a t e t h e s e two p a r a m e t e r s b e c a u s e the i s o t o p i c d a t a are t o o few a n d b e c a u s e n o d i r e c t c o r r e l a t i o n of d a t a was o b t a i n e d f r o m the s a m e s a m p l e s .
sialic c r u s t a l - d e r i v e d g r a n i t e s a n d e n r i c h e d in It y p e or m o r e m a f i c d e r i v a t i v e r o c k s [7], a n d since s u s c e p t i b i l i t y c a n be d i r e c t l y r e l a t e d to m a g n e t i t e c o n t e n t ( s u m m a r i z e d for e x a m p l e b y N e t t l e t o n [9]), this m a g n e t i t e - 3 1 8 0 r e l a t i o n s h i p (Fig. 1) is n o t t o o surprising. G r a n i t e s o f s u c h low m a g n e t i t e c o n t e n t ( i l m e n i t e series g r a n i t e s [7,10]) h a v e susc e p t i b i l i t i e s w h e r e X < 1 X 10 4 G / O e . H i g h m a g n e t i t e rocks ( m a g n e t i t e series g r a n i t e s ) o n the o t h e r h a n d , h a v e susceptibilities w h e r e x > I X 10 4 G/Oe.
2. D i s c u s s i o n
2.1. Magnetic susceptibility TABLE1 T h e m a g n e t i c s u s c e p t i b i l i t y in several g r a n i t e s for w h i c h o x y g e n i s o t o p e s h a v e b e e n d e t e r m i n e d has also b e e n e v a l u a t e d , u s i n g a m o d i f i e d Bison s u s c e p t i b i l i t y bridge. T h e s e d a t a are p l o t t e d in Fig. 1 r e l a t i v e to i n d i v i d u a l g r a n i t e m e a n 6 1 8 0 v a l u e s s h o w n e l s e w h e r e [1]. It is a p p a r e n t that t h e r e is a s i g n i f i c a n t i n v e r s e r e l a t i o n s h i p b e t w e e n s u s c e p t i b i l i t y a n d m e a n 6180 values. Since it has b e e n s h o w n that m a g n e t i t e is d e p l e t e d in S - t y p e o r
i
-
•
,
r
-
m
03
,~
55
65
75
85
95
105
115
B180 Fig. 1. Three cycle semi-log plot of initial magnetic susceptibility (in cgs mass units) versus 61SO for 17 late orogenic granitic units in Georgia and South Carolina. Precision bars represent one standard deviation. AP = Appling; BB = Batesburg; BN* -Ben Hill; CG*=Columbia; CT=Cuffytown; DB=Danburg; EB Elberton; HB* = Harbison; J S * - Johnston; LH = Liberty Hill; NB=Newberry; PG=Panola; PM=Palmetto; SL= Siloam; SP = Sparta; ST = Stone Mountain; and WN Winnsboro. The granites are named after the closest town name. If more than 1 community lies within the granite, then the largest was chosen for the name. Unpublished 1SO/~60 data are indicated by an asterisk (*).
Typicalsitemeansusceptibilityand RM precisiondata Site
N
X
x
Precision
AP01 AP 02 BN01* EB 01 EB02 LH01 LH02 NB01 * PG01 PG02 PM 01 PM 02 SG 01 SG02 SL 01 SL02 ST01 ST 02 WN01 WN02
6 6 6 7 8 6 6 6 6 4 4 6 8 6 6 6 4 5 6 6
4.87× 10 4 3.91 × 10 4 1.42X10 5 6.17X 10 -6 7.51X 10 5 1.67× 10 4 1.87× 10 4 4.03 X 10 -4 2.83×10 5 2.40×10 5 3.71 × 10 5 7.64N 10 5 1.46N 10 -4 4.50N 10 4 2.40 × 10 - 4 1.95X 10 4 2.60X 10 6 2.30x 10 -6 2.32X 10 5 1.78X 10 5
1.9 2.7 124.8 16.9 213.8 1.8 3.5 2.1 22.1 10.9 11.5 2.8 1.9 2.5 1.2 1.2 50.6 138.4 2.0 3.3
poor poor good good good poor poor poor good good good poor ** poor poor poor poor good good poor** poor**
Site-letters refer to granites named in the Fig. 1 caption and number to the site number within the granite; N - n u m b e r of samples; ×=initial magnetic susceptibility in cgs mass units; K - precision parameter of Fisher [I 1]; Precision- remanent magnetic within-site precision indicator chosen as good if > 10 and poor if < 10. * = susceptibility data available for only one site; **-sites with poor precision even though susceptibilities are < 1 × 1 0 4 G / O e . Note: Data are taken from representative sites within the same units. In all cases, these data represent only sites for which 18O values have been determined and upon which RM analyses have been performed. All sites with susceptibilities > l × l0 4 G / O e yield low kappas and are defined as having poor precision. Many but not all sites with susceptibilities < l X 10 4 G / O e exhibit good within-site precision.
202
2.2. Remanent magnetism A useful result of this magnetic susceptibility work relates to the within-site precision of remanent magnetism (RM) direction in the granites. Data reported are after a.f. demagnetization to 40 roT. Detailed magnetic methods have been reported elsewhere [3]. While no sites with mean X > 1 X 10-4 G / O e have been found which exhibit a coherent withinsite directional RM, indicated by Fisherian Kappas [11] greater than 10 in Table 1, several units with X < 1 × 10 4 G / O e exhibit good within-site RM clusters (Table 1). The best example of these is the Elberton Granite [3]. This susceptibility relationship proves a useful and easily measured indicator of sites with potentially more meaningful RM results when one is limited by the number of samples which can be collected in the field or processed in the laboratory. Susceptibility measurements can be made on unoriented samples obtained during exploratory field surveys or by using portable equipment to make measurements in the field, and they can then provide the basis for later extensive field sampling. It should be noted that the within-site precision exhibited in Table 1 does not necessarily mean the between-site precision will be high. For example, the RM data for the Stone Mountain granite (X < 1 × 10 -s G / O e ) yield good within-site clusters (Table 1) but poor between site RM precision. The reason for this anomalous behavior is currently under investigation.
3. Conclusions The magnetic susceptibility in 17 granite bodies is indicative of magnetite content and is inversely related to the 8~80 variability. These results are consistent with the interpretation that S-type granites in the southern Appalachian Piedmont generally yield low magnetic susceptibilities (X < 1 X 10-4 G/Oe), while I-type granites are generally
characterized by high susceptibilities (X > 1 × 10-4 G/Oe). Further, I-type granites exhibit low remanent magnetic directional precision while some S-type granites exhibit good directional cluster.
Acknowledgements Thanks are expressed to J.A. Whitney for manuscript review. This work was partially supported by NSF grant EAR-7919911 to B.B.E. and by NSF grant EAR-7818127 to D.B.W.
References 1 D.B. Wenner, Oxygen isotopic compositions of the late orogenic granites in the Southern Piedmont of the Appalachian Mountains, U.S.A. and their relationship to subcrustal structures and lithologies, Earth Planet. Sci. Lett. 54 (1981) 186-199. 2 B.B. Ellwood and J.A. Whitney, Magnetic fabric of the Elberton granite, northeast Georgia, J. Geophys. Res. 85 (1980) 1481-1486. 3 B.B. Ellwood, J.A. Whitney, D.B. Wenner, D. Mose and C. Amerigian, Age, paleomagnetism, and tectonic significance of the Elberton granite, northeast Georgia Piedmont, J. Geophys. Res. (in press). 4 J.R. O'Neil and B.W. Chappell, Oxygen and hydrogen isotope relations in the Baerridale batholith, J. Geol. Soc. 133 (1977) 559-571. 5 P.D. Fullager and J.R. Butler, 325 to 265 m.y. old granitic plutons in the Piedmont of the southeastern Appalachians, Am. J. Sci. 279 (1979) 161-185. 6 L.T. Long, The Carolina slate belt--evidence of a continental rift zone, Geology 7 (1979) 180-184. 7 S. Ishihara, The magnetite-series and ilmenite-series, Min. Geol. 27 (1977) 293-305. 8 Y. Matsuhisa, H. Honma, O. Matsubaya and H. Sakai, Oxygen isotopic study of the Cretaceous granitic rocks in Japan, Contrib. Mineral. Petrol. 37 (1972) 65-74. 9 L.L. Nettleton, Elementary gravity and magnetics for geologists and seismologists, Monogr. Soc. Explor. Geophys. 1 (1976) 121. 10 H. Kanaya and S. Ishihara, Regional variation of magnetic susceptibility of the granitic rocks of Japan, Jpn. Assoc. Miner. Petrol. Econ. Geol. J. 68 (1973) 211-224. 11 R.A. Fisher, Dispersion on a sphere, Proc. R. Soc. London, Ser. A, 217 (1953) 295-305.
Earth and Planetary Science Letters, 54 ( 1981 ) 200- 202 Elsevier Scientific Publishing Company, Amsterdam Printed in The Netherlands
[41
Correlation of magnetitic susceptibility with 180/160 data in late orogenic granites of the southern Appalachian Piedmont Brooks B. Ellwood and David B. Wenner Department of Geology, UniversiO, of Georgia, Athens, GA 30602 (U.S.A.) Received June 30, 1980 Revised version received April 2, 1981
Initial magnetic susceptibility (generally indicative of magnetite content) has been determined for 445 samples from 17 granites located in the southern Appalachian Piedmont of Georgia and South Carolina. These values have been correlated with whole rock 31SO data from the same plutons, yielding a pronounced inverse relationship. It has previously been shown for the southern Piedmont that low oxygen isotopic (is O-enriched) values usually occur in S-type granites (Wenner [1], this issue). It follows, then, that I-type granites are characterized by high susceptibilities ( x > l × 10 4 G/Oe), and S-type granites by low susceptibilities ( x < l X 10 4 G/Oe). An interesting result of this work has been the observation that some S-type granites exhibit good within-site clusters of remanent magnetic directions while I-type granites generally do not.
1. Introduction
Many of the late orogenic granite plutons of Georgia and South Carolina have been the focus of varied studies directed toward understanding the age and genesis of the Piedmont. This work includes isotopic (for example Wenner [1], this issue) and magnetic studies [2,3]. As part of the magnetic studies, initial susceptibility (X) data were determined using a Bison susceptibility bridge for multiple specimens from more than 30 granitoids. Whole rock 6~80 data were also available for 17 of these ([1], this issue; and this study). Data for these 17 units are plotted in Fig. 1. The ~SO/~60 data in these granitoids appear to reflect contrasting protoliths, with the ~SO-enriched plutons forming through anatexis of metasedimentary and sialic protoliths, and the low-~SO granites within the Charlotte-Carolina Slate Belt and portions of the Kiokee belt having been derived from source regions dominated by more mafic, metaigneous rocks [1]. Thus, it would appear that this
series of granitoids exhibits a number of isotopic similarities to the S- and I-type granitoids first described in Australia [4]. Such an interpretation is supported by the correlative initial strontium isotopic data [5], and in part by the contrasting nature of the exposed geologic terranes into which the plutons are intruded, and most importantly by the gravity data which suggest contrasting subcrustal lithologies [6]. It has been shown that magnetite is depleted in granites derived from carbonaceous rocks (equivalent to S-type granites) and enriched in granites derived from rocks of low carbonaceous content (equivalent to I-type granites) [7]. These observations have led to the subdivision of granitic rocks in Japan into a magnetite and an ilmenite series [7], which is basically equivalent to the S- and I-type described elsewhere [4]. Although no specific relationship has as yet been shown between the oxygen isotopic and magnetite compositions of granites, whole rock oxygen isotopic compositions [8] of some Cretaceous to
0012-82 IX/81/0000-0000/$02.50 '~ 1981 Elsevier Scientific Publishing Company
201 M i o c e n e - a g e g r a n i t e s in J a p a n h a v e also b e e n subd i v i d e d i n t o a m a g n e t i t e a n d i l m e n i t e series [7]. In t h e s e studies, g r a n i t e s of the m a g n e t i t e series gene r a l l y h a v e l o w e r 6 1 8 0 v a l u e s (--~ 8%0) w h e r e a s r o c k s o f the i l m e n i t e series are ~ 8 0 - e n r i c h e d ( 9 - 1 3 %o). H o w e v e r , these studies d o n o t d i r e c t l y r e l a t e t h e s e two p a r a m e t e r s b e c a u s e the i s o t o p i c d a t a are t o o few a n d b e c a u s e n o d i r e c t c o r r e l a t i o n of d a t a was o b t a i n e d f r o m the s a m e s a m p l e s .
sialic c r u s t a l - d e r i v e d g r a n i t e s a n d e n r i c h e d in It y p e or m o r e m a f i c d e r i v a t i v e r o c k s [7], a n d since s u s c e p t i b i l i t y c a n be d i r e c t l y r e l a t e d to m a g n e t i t e c o n t e n t ( s u m m a r i z e d for e x a m p l e b y N e t t l e t o n [9]), this m a g n e t i t e - 3 1 8 0 r e l a t i o n s h i p (Fig. 1) is n o t t o o surprising. G r a n i t e s o f s u c h low m a g n e t i t e c o n t e n t ( i l m e n i t e series g r a n i t e s [7,10]) h a v e susc e p t i b i l i t i e s w h e r e X < 1 X 10 4 G / O e . H i g h m a g n e t i t e rocks ( m a g n e t i t e series g r a n i t e s ) o n the o t h e r h a n d , h a v e susceptibilities w h e r e x > I X 10 4 G/Oe.
2. D i s c u s s i o n
2.1. Magnetic susceptibility TABLE1 T h e m a g n e t i c s u s c e p t i b i l i t y in several g r a n i t e s for w h i c h o x y g e n i s o t o p e s h a v e b e e n d e t e r m i n e d has also b e e n e v a l u a t e d , u s i n g a m o d i f i e d Bison s u s c e p t i b i l i t y bridge. T h e s e d a t a are p l o t t e d in Fig. 1 r e l a t i v e to i n d i v i d u a l g r a n i t e m e a n 6 1 8 0 v a l u e s s h o w n e l s e w h e r e [1]. It is a p p a r e n t that t h e r e is a s i g n i f i c a n t i n v e r s e r e l a t i o n s h i p b e t w e e n s u s c e p t i b i l i t y a n d m e a n 6180 values. Since it has b e e n s h o w n that m a g n e t i t e is d e p l e t e d in S - t y p e o r
i
-
•
,
r
-
m
03
,~
55
65
75
85
95
105
115
B180 Fig. 1. Three cycle semi-log plot of initial magnetic susceptibility (in cgs mass units) versus 61SO for 17 late orogenic granitic units in Georgia and South Carolina. Precision bars represent one standard deviation. AP = Appling; BB = Batesburg; BN* -Ben Hill; CG*=Columbia; CT=Cuffytown; DB=Danburg; EB Elberton; HB* = Harbison; J S * - Johnston; LH = Liberty Hill; NB=Newberry; PG=Panola; PM=Palmetto; SL= Siloam; SP = Sparta; ST = Stone Mountain; and WN Winnsboro. The granites are named after the closest town name. If more than 1 community lies within the granite, then the largest was chosen for the name. Unpublished 1SO/~60 data are indicated by an asterisk (*).
Typicalsitemeansusceptibilityand RM precisiondata Site
N
X
x
Precision
AP01 AP 02 BN01* EB 01 EB02 LH01 LH02 NB01 * PG01 PG02 PM 01 PM 02 SG 01 SG02 SL 01 SL02 ST01 ST 02 WN01 WN02
6 6 6 7 8 6 6 6 6 4 4 6 8 6 6 6 4 5 6 6
4.87× 10 4 3.91 × 10 4 1.42X10 5 6.17X 10 -6 7.51X 10 5 1.67× 10 4 1.87× 10 4 4.03 X 10 -4 2.83×10 5 2.40×10 5 3.71 × 10 5 7.64N 10 5 1.46N 10 -4 4.50N 10 4 2.40 × 10 - 4 1.95X 10 4 2.60X 10 6 2.30x 10 -6 2.32X 10 5 1.78X 10 5
1.9 2.7 124.8 16.9 213.8 1.8 3.5 2.1 22.1 10.9 11.5 2.8 1.9 2.5 1.2 1.2 50.6 138.4 2.0 3.3
poor poor good good good poor poor poor good good good poor ** poor poor poor poor good good poor** poor**
Site-letters refer to granites named in the Fig. 1 caption and number to the site number within the granite; N - n u m b e r of samples; ×=initial magnetic susceptibility in cgs mass units; K - precision parameter of Fisher [I 1]; Precision- remanent magnetic within-site precision indicator chosen as good if > 10 and poor if < 10. * = susceptibility data available for only one site; **-sites with poor precision even though susceptibilities are < 1 × 1 0 4 G / O e . Note: Data are taken from representative sites within the same units. In all cases, these data represent only sites for which 18O values have been determined and upon which RM analyses have been performed. All sites with susceptibilities > l × l0 4 G / O e yield low kappas and are defined as having poor precision. Many but not all sites with susceptibilities < l X 10 4 G / O e exhibit good within-site precision.
202
2.2. Remanent magnetism A useful result of this magnetic susceptibility work relates to the within-site precision of remanent magnetism (RM) direction in the granites. Data reported are after a.f. demagnetization to 40 roT. Detailed magnetic methods have been reported elsewhere [3]. While no sites with mean X > 1 X 10-4 G / O e have been found which exhibit a coherent withinsite directional RM, indicated by Fisherian Kappas [11] greater than 10 in Table 1, several units with X < 1 × 10 4 G / O e exhibit good within-site RM clusters (Table 1). The best example of these is the Elberton Granite [3]. This susceptibility relationship proves a useful and easily measured indicator of sites with potentially more meaningful RM results when one is limited by the number of samples which can be collected in the field or processed in the laboratory. Susceptibility measurements can be made on unoriented samples obtained during exploratory field surveys or by using portable equipment to make measurements in the field, and they can then provide the basis for later extensive field sampling. It should be noted that the within-site precision exhibited in Table 1 does not necessarily mean the between-site precision will be high. For example, the RM data for the Stone Mountain granite (X < 1 × 10 -s G / O e ) yield good within-site clusters (Table 1) but poor between site RM precision. The reason for this anomalous behavior is currently under investigation.
3. Conclusions The magnetic susceptibility in 17 granite bodies is indicative of magnetite content and is inversely related to the 8~80 variability. These results are consistent with the interpretation that S-type granites in the southern Appalachian Piedmont generally yield low magnetic susceptibilities (X < 1 X 10-4 G/Oe), while I-type granites are generally
characterized by high susceptibilities (X > 1 × 10-4 G/Oe). Further, I-type granites exhibit low remanent magnetic directional precision while some S-type granites exhibit good directional cluster.
Acknowledgements Thanks are expressed to J.A. Whitney for manuscript review. This work was partially supported by NSF grant EAR-7919911 to B.B.E. and by NSF grant EAR-7818127 to D.B.W.
References 1 D.B. Wenner, Oxygen isotopic compositions of the late orogenic granites in the Southern Piedmont of the Appalachian Mountains, U.S.A. and their relationship to subcrustal structures and lithologies, Earth Planet. Sci. Lett. 54 (1981) 186-199. 2 B.B. Ellwood and J.A. Whitney, Magnetic fabric of the Elberton granite, northeast Georgia, J. Geophys. Res. 85 (1980) 1481-1486. 3 B.B. Ellwood, J.A. Whitney, D.B. Wenner, D. Mose and C. Amerigian, Age, paleomagnetism, and tectonic significance of the Elberton granite, northeast Georgia Piedmont, J. Geophys. Res. (in press). 4 J.R. O'Neil and B.W. Chappell, Oxygen and hydrogen isotope relations in the Baerridale batholith, J. Geol. Soc. 133 (1977) 559-571. 5 P.D. Fullager and J.R. Butler, 325 to 265 m.y. old granitic plutons in the Piedmont of the southeastern Appalachians, Am. J. Sci. 279 (1979) 161-185. 6 L.T. Long, The Carolina slate belt--evidence of a continental rift zone, Geology 7 (1979) 180-184. 7 S. Ishihara, The magnetite-series and ilmenite-series, Min. Geol. 27 (1977) 293-305. 8 Y. Matsuhisa, H. Honma, O. Matsubaya and H. Sakai, Oxygen isotopic study of the Cretaceous granitic rocks in Japan, Contrib. Mineral. Petrol. 37 (1972) 65-74. 9 L.L. Nettleton, Elementary gravity and magnetics for geologists and seismologists, Monogr. Soc. Explor. Geophys. 1 (1976) 121. 10 H. Kanaya and S. Ishihara, Regional variation of magnetic susceptibility of the granitic rocks of Japan, Jpn. Assoc. Miner. Petrol. Econ. Geol. J. 68 (1973) 211-224. 11 R.A. Fisher, Dispersion on a sphere, Proc. R. Soc. London, Ser. A, 217 (1953) 295-305.