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Broken Hill — A precambrian hot spot? — a reply
218 c h e m i c a l c o m p o s i t i o n are at least as i m p o r t a n t . T h u s Katz " h o t s p o t s " are m p a r t just " d r y s p o t s " and " F e s p o t s " . Fig.1 shows these spots d o n o t exist at all. ACKNOWLEDGEMENT Mr. V.J. Wall has o f f e r e d m u c h h e l p f u l advice on this s u b j e c t and considera b l y i m p r o v e d the m a n u s c r i p t b y his c o n s t r u c t i v e criticisms.
REFERENCES
Arden, P.J., 1974. Structural, Petrological and Economic Geology Aspects of the Little Broken Hill Area, New South Wales. Thesis, Monash Univ., Clayton, Vic., unpubh Australian Institute Mining Metallurgy, 1968. Geological Map of the Broken Hill District. Binns, R.A., 1964. Zones of progressive regional metamorphism in the Willyama Complex, Broken Hill District, New South Wales. J. Geol. Soc. Aust., 11: 283--330. Bunting, W.F., 1975. Geology of the Maybell Group of Mines and Old Mt. Gipps Homestead Area, Broken Hill. Thesis, Monash Univ., Clayton, Vic., unpuhl. Clemens, J.C., 1974. The Round Hill Area, Broken Hill, N.S.W. Thesis, Monash Univ., Clayton, Vic., unpubl. Hobbs, B.E., 1975. The Broken Hill Lode Horizon Project. Unpubl. Rep. on second year of work to B.H. Mine Managers' Association. Katz, M.B., 1976. Broken Hill -- A Precambrian hot spot? Precambrian Res., 3: 91--106. Rutland, R.W.R. and Etheridge, M.A., 1975. Two high grade schistosities at Broken Hill and their relation to major and minor structures. J. Geol. Soc. Aust., 22: 259--274. Turner, F,J., 1968. Metamorphic Petrology: Mineralogical and Field Aspects. McGraw-Hill, New York, N.Y. Vernon, R.H., 1969. The Willyama Complex, Broken Hill area. J. Geol. Soc. Aust., 16: 20--55. Winkler, H.G.F., 1974. Petrogenesis of Metamorphic Rocks. Springer Verlag, Berlin, 3rd ed. Wood, B.J. and Banno, S., 1973. Garnet--orthopyroxene and orthopyroxene--clinopyroxene relationships in simple and complex systems. Contrib. Miner. Petrol., 42: 109--124.
B R O K E N H I L L - - A P R E C A M B R I A N H O T S P O T ? -" A R E P L Y
M.B. KATZ School of Applied Geology, University of New South Wales, Kensington, N.S.W. (Australia)
Phillips ( 1 9 7 6 ) q u e s t i o n s t h e c o n c e p t o f a h o t spot c e n t r e d a r o u n d the B r o k e n Hill Basin (B.H.B.) (Katz, 1 9 7 6 ) . His a r g u m e n t s suggest t h a t m o s t o f the Wfllyama C o m p l e x is o f granulite facies grade {Phillips, 1 9 7 6 , f i g . l ) , w h i c h is e n t i r e l y misleading, and t h a t the B . H . B , c a n n o t be distinguished as a high grade m e t a m o r p h i c spot. Most o f his c o n s i d e r a t i o n s derive f r o m u n p u b l i s h e d
219 work and cannot be properly evaluated at this stage. Given the premise that granulite facies conditions are widespread, a condition that we b o t h agree on, I maintain that the distribution of these rocks are such, that they form rather patchy, spotty occurrences (Katz, 1976, fig.8). The highest density distribution of granulite facies rocks is, to m y best knowledge, concentrated in the B.H.B. I question the proposition that these granulites outside the B.H.B. have a comparable density distribution to those in the B.H.B. The B.H.B. is a tectonic block b o u n d e d by major lineaments and studies in the terrains to the northwest of the Globe Vauxhall shear show a sharp, marked decrease in metamorphic grade, implying a faulted contact. No relicts of pre-existing highgrade metamorphism have been observed, except perhaps locally in the Centennial area, which is thought to be related to the Broken Hill t y p e mineralization found there. The situation to the south of the Darling Range fault is not as clear and appears to be fairly high grade, in part, and this is interpreted as being a transitional zone b e t w e e n the B.H.B. and Little Broken Hill (L.B.H.) (Katz, 1976, fig.8). Although m y sampling is n o t as extensive as that claimed b y the Monash University Group, I have not seen a comparable metamorphic terrain such as the B.H.B. elsewhere. Low grade rocks of the Willyama Complex may be a result of retrogression as claimed by Phillips (1976) and discussed by Katz (1976, p.93). However, in the absence of high grade relicts, the rocks I have classified are considered prograde. I agree with Phillips that even in the B.H.B. there are variations on the outcrop scale, an observation also made by Edwards (1958) and c o m m o n l y noted in many true granulite facies terrains. I also agree with Phillips' (1976) points concerning compositional control related to rocks chemistry and water content and these points are discussed in my paper (Katz, 1976, p.101). His main argument is that Fe-rich basic metamorphics control the granulite mineralogy and also correlate with the mineralization. According to him these rocks have no unaltered chemical counterparts among igneous rocks b u t are equivalent to premetamorphically altered basalts. There are no laboratory or field data to support this contention. Are all Fe-rich amphibolites solely associated with mineralization, and all Mg-rich amphibolites found in non-mineralized areas? After all these arguments he concludes that m y " h o t spots" may be actually "dry spots" and " F e spots", thus apparently agreeing that the spots exist! Indeed some recent papers by Byerly and Nelson (1976) and Schilling, Anderson and Vogt (1976) also indicate that Fe-enrichment is a c o m m o n feature in the basalts developed at " h o t s p o t " spreading centres. An examination of the analyses from these ferro-basalts of the Galapagos spreading centre and those analyses of amphibolites (meta-basalts) reported in Plimer (1975) from Broken Hill shows very similar trends. Thus this major argument in Phillips' (1976) discussion in regard to Fe-enrichment may now be considered as an argument supporting m y ideas. I have attempted to provide a tectonic model for the Broken Hill area by showing strong lineament control, which can also be related to the regional
220 g e o l o g y (Katz, in press). M a n y f a c t o r s were c o n s i d e r e d a n d I e x p e c t t h a t this m o d e l will be t e s t e d w i t h time. Phillips (1976} c o m m e n t s , h o w e v e r , are b a s e d on u n p u b l i s h e d w o r k a n d h e p r e s e n t s n o s u p p o r t i n g d a t a . Until he a n d o t h e r s o f t h e M o n a s h U n i v e r s i t y G r o u p can s u p p l y details, I m a i n t a i n t h a t m y m o d e l is valid. I t h e r e f o r e a w a i t Phillips p u b l i c a t i o n s w i t h interest. ACKNOWLEDGEMENT Critical c o m m e n t s b y Bas H e n s e n are g r a t e f u l l y a c k n o w l e d g e d .
REFERENCES
Byerly, G.R. and Nelson, W.G., 1976. Rhyodacites, andesites, ferrobasalts and ocean tholeiites from the Galap.ago~ spreading center. Earth Planet Sci. Lett., 30: 215---221. Edwards, A.B., 1958. Amphibolites from the Broken Hill district. J. Geol. Soc. Aust., 5: 1--32. Katz, M.B., 1976. Broken Hill - - A Precambrian hot spot? Preca.mbrian Res., 3: 91--106. Katz, IVLB., in press. Lineament tectonics of the Willyama Block and its relationship to the Adelaide aulacogene. J. Geol, Soc. Aust., 23. Phillips, G.N., 1976. Broken Hill -- A Precambrian hot spot? -- a comment. Precambrian Res., Plimer, I.R., 1975. The geochemistry of amphibolite retrogression at Broken Hill,Australia. N. Jb. Miner. Mh., 1975(10): 471--481. Schilling, J.G., Anderson, R.N. and Vogt, P., 1976, Rare earth, Fe and Ti variations along the Galapagos spreading centre and their relationship to the Galapagos mantle plume. Nature, 261: 108--113.
REFERENCES
Arden, P.J., 1974. Structural, Petrological and Economic Geology Aspects of the Little Broken Hill Area, New South Wales. Thesis, Monash Univ., Clayton, Vic., unpubh Australian Institute Mining Metallurgy, 1968. Geological Map of the Broken Hill District. Binns, R.A., 1964. Zones of progressive regional metamorphism in the Willyama Complex, Broken Hill District, New South Wales. J. Geol. Soc. Aust., 11: 283--330. Bunting, W.F., 1975. Geology of the Maybell Group of Mines and Old Mt. Gipps Homestead Area, Broken Hill. Thesis, Monash Univ., Clayton, Vic., unpuhl. Clemens, J.C., 1974. The Round Hill Area, Broken Hill, N.S.W. Thesis, Monash Univ., Clayton, Vic., unpubl. Hobbs, B.E., 1975. The Broken Hill Lode Horizon Project. Unpubl. Rep. on second year of work to B.H. Mine Managers' Association. Katz, M.B., 1976. Broken Hill -- A Precambrian hot spot? Precambrian Res., 3: 91--106. Rutland, R.W.R. and Etheridge, M.A., 1975. Two high grade schistosities at Broken Hill and their relation to major and minor structures. J. Geol. Soc. Aust., 22: 259--274. Turner, F,J., 1968. Metamorphic Petrology: Mineralogical and Field Aspects. McGraw-Hill, New York, N.Y. Vernon, R.H., 1969. The Willyama Complex, Broken Hill area. J. Geol. Soc. Aust., 16: 20--55. Winkler, H.G.F., 1974. Petrogenesis of Metamorphic Rocks. Springer Verlag, Berlin, 3rd ed. Wood, B.J. and Banno, S., 1973. Garnet--orthopyroxene and orthopyroxene--clinopyroxene relationships in simple and complex systems. Contrib. Miner. Petrol., 42: 109--124.
B R O K E N H I L L - - A P R E C A M B R I A N H O T S P O T ? -" A R E P L Y
M.B. KATZ School of Applied Geology, University of New South Wales, Kensington, N.S.W. (Australia)
Phillips ( 1 9 7 6 ) q u e s t i o n s t h e c o n c e p t o f a h o t spot c e n t r e d a r o u n d the B r o k e n Hill Basin (B.H.B.) (Katz, 1 9 7 6 ) . His a r g u m e n t s suggest t h a t m o s t o f the Wfllyama C o m p l e x is o f granulite facies grade {Phillips, 1 9 7 6 , f i g . l ) , w h i c h is e n t i r e l y misleading, and t h a t the B . H . B , c a n n o t be distinguished as a high grade m e t a m o r p h i c spot. Most o f his c o n s i d e r a t i o n s derive f r o m u n p u b l i s h e d
219 work and cannot be properly evaluated at this stage. Given the premise that granulite facies conditions are widespread, a condition that we b o t h agree on, I maintain that the distribution of these rocks are such, that they form rather patchy, spotty occurrences (Katz, 1976, fig.8). The highest density distribution of granulite facies rocks is, to m y best knowledge, concentrated in the B.H.B. I question the proposition that these granulites outside the B.H.B. have a comparable density distribution to those in the B.H.B. The B.H.B. is a tectonic block b o u n d e d by major lineaments and studies in the terrains to the northwest of the Globe Vauxhall shear show a sharp, marked decrease in metamorphic grade, implying a faulted contact. No relicts of pre-existing highgrade metamorphism have been observed, except perhaps locally in the Centennial area, which is thought to be related to the Broken Hill t y p e mineralization found there. The situation to the south of the Darling Range fault is not as clear and appears to be fairly high grade, in part, and this is interpreted as being a transitional zone b e t w e e n the B.H.B. and Little Broken Hill (L.B.H.) (Katz, 1976, fig.8). Although m y sampling is n o t as extensive as that claimed b y the Monash University Group, I have not seen a comparable metamorphic terrain such as the B.H.B. elsewhere. Low grade rocks of the Willyama Complex may be a result of retrogression as claimed by Phillips (1976) and discussed by Katz (1976, p.93). However, in the absence of high grade relicts, the rocks I have classified are considered prograde. I agree with Phillips that even in the B.H.B. there are variations on the outcrop scale, an observation also made by Edwards (1958) and c o m m o n l y noted in many true granulite facies terrains. I also agree with Phillips' (1976) points concerning compositional control related to rocks chemistry and water content and these points are discussed in my paper (Katz, 1976, p.101). His main argument is that Fe-rich basic metamorphics control the granulite mineralogy and also correlate with the mineralization. According to him these rocks have no unaltered chemical counterparts among igneous rocks b u t are equivalent to premetamorphically altered basalts. There are no laboratory or field data to support this contention. Are all Fe-rich amphibolites solely associated with mineralization, and all Mg-rich amphibolites found in non-mineralized areas? After all these arguments he concludes that m y " h o t spots" may be actually "dry spots" and " F e spots", thus apparently agreeing that the spots exist! Indeed some recent papers by Byerly and Nelson (1976) and Schilling, Anderson and Vogt (1976) also indicate that Fe-enrichment is a c o m m o n feature in the basalts developed at " h o t s p o t " spreading centres. An examination of the analyses from these ferro-basalts of the Galapagos spreading centre and those analyses of amphibolites (meta-basalts) reported in Plimer (1975) from Broken Hill shows very similar trends. Thus this major argument in Phillips' (1976) discussion in regard to Fe-enrichment may now be considered as an argument supporting m y ideas. I have attempted to provide a tectonic model for the Broken Hill area by showing strong lineament control, which can also be related to the regional
220 g e o l o g y (Katz, in press). M a n y f a c t o r s were c o n s i d e r e d a n d I e x p e c t t h a t this m o d e l will be t e s t e d w i t h time. Phillips (1976} c o m m e n t s , h o w e v e r , are b a s e d on u n p u b l i s h e d w o r k a n d h e p r e s e n t s n o s u p p o r t i n g d a t a . Until he a n d o t h e r s o f t h e M o n a s h U n i v e r s i t y G r o u p can s u p p l y details, I m a i n t a i n t h a t m y m o d e l is valid. I t h e r e f o r e a w a i t Phillips p u b l i c a t i o n s w i t h interest. ACKNOWLEDGEMENT Critical c o m m e n t s b y Bas H e n s e n are g r a t e f u l l y a c k n o w l e d g e d .
REFERENCES
Byerly, G.R. and Nelson, W.G., 1976. Rhyodacites, andesites, ferrobasalts and ocean tholeiites from the Galap.ago~ spreading center. Earth Planet Sci. Lett., 30: 215---221. Edwards, A.B., 1958. Amphibolites from the Broken Hill district. J. Geol. Soc. Aust., 5: 1--32. Katz, M.B., 1976. Broken Hill - - A Precambrian hot spot? Preca.mbrian Res., 3: 91--106. Katz, IVLB., in press. Lineament tectonics of the Willyama Block and its relationship to the Adelaide aulacogene. J. Geol, Soc. Aust., 23. Phillips, G.N., 1976. Broken Hill -- A Precambrian hot spot? -- a comment. Precambrian Res., Plimer, I.R., 1975. The geochemistry of amphibolite retrogression at Broken Hill,Australia. N. Jb. Miner. Mh., 1975(10): 471--481. Schilling, J.G., Anderson, R.N. and Vogt, P., 1976, Rare earth, Fe and Ti variations along the Galapagos spreading centre and their relationship to the Galapagos mantle plume. Nature, 261: 108--113.