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Trondhjemites, dacites and related rocks
363 (Chapter 17): the oceanic crust is dominantly of low-potassium tholeiite with bodies of serpentine, while consisting of basalt, gabbro, amphibolite and serpentine where it is subducted. The average composition of the continental crust would be andesitic. For magma generation, the most important interval is probably the low-velocity layer (between about 100 and 200 km depth). Basaltic magma would be produced by selective fusion of peridotite at about 70 km beneath the oceanic crust and 150 km beneath the shields; granitic magma by selective fusion of the continental crust; intermediate magmas formed by selective fusion of subducted oceanic crust. The last chapter (18) deals with volcanic nomenclature (F. Chayes). The whole book is focused on the magmatic-melting process. We might regret that sufficient attention is not given to the relations of magmas with the highest degrees of metamorphism, exemplified by anatexis. Thus the book stands as representative of the work of the highest grade "American Bowen" school, European and Japanese petrographic schools being very rarely quoted. Its greatest interest lies in having been written by the most qualified experts in the world on physical chemistry and laboratory experimentation. HENRI TERMIER (Paris)
Trondh]emites, Dacites and Related Rocks. F. Barker (Editor). Developments in Petrology, 6. Elsevier Scientific Publishing Co., Amsterdam--Oxford-New York, N.Y., 1979, 659 pp., Dfl. 135.00/US $ 65.75. The science of petrology still depends heavily on field observations. A consequence of this is that the current dogma of petrology is continually being revised by new insights into the volumetric importance of hitherto neglected or unrecognised rock types. Thus, the past 20 years has seen the emergence of MORB, komatiites and boninites, all of which have had a revolutionary impact on our ideas about the composition of the upper mantle and the nature of primary melts derived from that enigmatic region. Trondhjemites are of older lineage, being recognised by V.M. Goldschmidt in 1916 as siliceous igneous rocks of low K and high Na content. They were downgraded by Bowen to a secondary status, resulting from "sodium metasomatism". More recently, the mythology of metasomatism has been replaced by a recognition of the primary igneous nature of these intrusive rocks. Recent work in the Archean has shown that trondhjemites, in association with tonalites, form the major silica-rich component of that early crust. The book is thus timely, in focussing attention on these somewhat neglected rocks. An initial chapter by Barker provides an excellent introduction to trondhjemites (could not the spelling, the terror of typists, be updated to trondheimites). Their composition occurrence and origin are perceptively summarised in 11 pages.
364 Even for these crystalline rocks, Barker notes that (p. 5): "some rocks are more accurately classified as trondhjemite or not by means of major elements rather than by modal analysis." These rocks are probably mainly derived from the mantle, by partial melting of basic igneous rock m e t a m o r p h o s e d to amphibolite or eclogite. Differentiation from basaltic parents is a less c o m m o n mode of origin. These conclusions are reinforced by the trace element evidence, again discussed briefly and with clarity by Arth in Chapter 4. The rare earth elements are particularly decisive in this context. The steep light REE enriched -- heavy REE depleted patterns indicate equilibration with amphibole or garnet in the residue during partial melting. Arth notes that (p. 131): "rare-earth elements appear to be the most reliable" since K, Rb, Sr and Ba are subject to alteration. High- (> 15%) and low-A12 03 trondhjemites are distinguished. The former are c o m m o n in continental, and the latter in oceanic environments. The presence of Eu anomalies in the low-A12 03 types indicates derivation from shallow (~ 10 kbar)depths where plagioclase is stable. Conversely the highA12 03 types are derived from greater depths. The trondhjemite suite, although nominally calc-alkaline, is distinct from the quartz diorite--tonalite--granodiorite suites typical of present-day calcalkaline suites. The principal chemical distinction is in the differing REE patterns, steep heavy REE depleted patterns being rare in present-day calcalkaline rocks. The widespread occurrence of trondhjemites in the Archean gray gneiss terrains, well covered by McGregor in Chapter 6, raises important questions a b o u t the distinction between Archean and subsequent igneous activity. This change is also reflected in the composition of the upper crust, as measured b y sedimentary REE patterns, from a bimodal tholeiitic--tonalitic and trondhjemitic suite to a dominantly granodioritic composition at the Archean-Proterozoic boundary. Dacites, latites and rhyolites are the other major rock types dealt with, and provide a kind of bimodality to the book. InChapter 2, Ewart provides an excellent and detailed treatment of the mineralogy and chemistry of these eruptive rocks sensibly restricting his coverage to Tertiary and recent examples. This introduction of typical "calc-alkaline" magmas raises many questions. Although both these eruptives, and the trondhjemites are related to subduction processes (and the latter to probable derivation from basaltic composition), any involvement of the down-going slab in the genesis of typical calc-alkaline rocks is n o w to be seriously questioned. The REE patterns of dacites and rhyolites including high- as well as low-K varieties are very distinct from those of typical Archean trondhjemites. The calc-alkaline patterns have flat heavy REE's, and frequently show pronounced negative Eu anoma-
365 lies. T h e s e p a t t e r n s indicate i n v o l v e m e n t o f feldspar and f o r b i d t h a t o f garnet, and p r o b a b l y a m p h i b o l e . S t r o n t i u m isotopic s y s t e m a t i c s are dealt with in a p e r c e p t i v e p a p e r b y P e t e r m a n ( C h a p t e r 4). This s h o u l d be r e a d y b y e v e r y o n e . He d e m o n s t r a t e s t h a t the initial STSr/~6Sr ratios o f tonalites and t r o n d h j e m i t e s define a simple linear t r e n d f r o m 0 . 7 0 3 9 (present) t o 0 . 7 0 1 0 at 2 6 5 0 m . y . ago and t h a t (p. 144): "an extension of this line intersects the 'TSr/'6Sr value for meteorites (0.6990) at 4550 m.y." The m o d e r n i n t e r c e p t (p. 139}: "is indistinguishable from the average value of 0.7037 -+ 0 . 0 0 0 2 . . . for modern oceanic island and arc basalts and the lower value at 4550 m.y. is essentially the same as BABI."
N o t e t h a t the values for p r e s e n t d a y o c e a n - f l o o r basalts (typically less t h a n 0 . 7 0 3 0 ) again separate these r o c k s f r o m island-arc rocks. This i m p o r t a n t d e m o n s t r a t i o n o f a simple linear g r o w t h line f o r ~TSr/~TSr, and its d i s t i n c t i o n f r o m the wide variability in oceanic basalts has m u c h significance for m o d e l s o f m a n t l e e v o l u t i o n and c o n t i n e n t a l growth. A useful c h a p t e r (No. 5) an oceanic plagiogranites b y C o l e m a n and D o n a t o will assist t h o s e non-petrologists, baffled b y the c u m b e r s o m e and imprecise t e r m i n o l o g y o f p e t r o l o g y , to u n d e r s t a n d the relative i m p o r t a n c e o f these low-K silicic rocks. A m o n g m a n y o t h e r stimulating observations in the b o o k , t w o e x a m p l e s m u s t suffice. M c G r e g o r n o t e s t h a t (p. 198): "there is no evidence in the Amitsoq gneisses or associated supra-crustal rocks, the oldest known terrestrial rocks, to suggest the existence of a primordial granitic or anorthositic crust," B r y a n observes t h a t (p. 599): "the Tonga Island arc also provides little support for the popular "geostill" concept, in which island arc magmatism is presumed to play a critical role in scavaging economically valuable metals from the subducted ocean lithosphere," Has the " g e o s t i l l " gone the way o f the alembic? This review has c o n c e n t r a t e d on t h e first six chapters which c o n t a i n the m o r e general overviews o f the subject. T h e remaining sixteen c h a p t e r s contain m u c h o f interest b u t essentially deal with localised o c c u r r e n c e s o f t r o n d hjemites, or dacites and their relatives. S o m e o f the chapters are labelled as " p r e l i m i n a r y , initial or r e c o n n a i s s a n c e " studies. H o w m u c h o f this material s h o u l d a p p e a r in a h a r d c o v e r b o o k ? P r o b a b l y m a n y o f these papers c o u l d have b e e n p u b l i s h e d in t h e c u r r e n t petrological or g e o c h e m i c a l journals.
366
Surely a book should a t t e m p t a synthesis rather than to include comments that (p. 415): "detailed hypotheses of generation of the rocks from Wyoming and Idaho cannot be made and so the data are presented for their own sake."
For such material, a special issue of one of the petrological journals, dealing with a specific rock type, would seem appropriate. There are a number of typographic errors of varying seriousness. The editor notes in the preface that: "the text was prepared and printed by a word processor in Denver. The editor apologises for the undetected scattered typographical and other errors which, inevitably, seem to be found in material produced by this apparatus."
However, at least two of the authors informed me that they had not had an o p p o r t u n i t y to correct final proofs, and supplied errata sheets with their offprints from this volume. The type is faint and the book is accordingly not easy to read. Although this is probably again the fault of the Denver word processor, the presumed savings from using this device, or from lack of final proof reading are not reflected in the price of US $65.75. The subject index is a little brief (rare earth elements, for example, are not listed), although a 7-page table of contents is a most useful feature. (S.R. TAYLOR, Canberra, A.C.T.)
Trondh]emites, Dacites and Related Rocks. F. Barker (Editor). Developments in Petrology, 6. Elsevier Scientific Publishing Co., Amsterdam--Oxford-New York, N.Y., 1979, 659 pp., Dfl. 135.00/US $ 65.75. The science of petrology still depends heavily on field observations. A consequence of this is that the current dogma of petrology is continually being revised by new insights into the volumetric importance of hitherto neglected or unrecognised rock types. Thus, the past 20 years has seen the emergence of MORB, komatiites and boninites, all of which have had a revolutionary impact on our ideas about the composition of the upper mantle and the nature of primary melts derived from that enigmatic region. Trondhjemites are of older lineage, being recognised by V.M. Goldschmidt in 1916 as siliceous igneous rocks of low K and high Na content. They were downgraded by Bowen to a secondary status, resulting from "sodium metasomatism". More recently, the mythology of metasomatism has been replaced by a recognition of the primary igneous nature of these intrusive rocks. Recent work in the Archean has shown that trondhjemites, in association with tonalites, form the major silica-rich component of that early crust. The book is thus timely, in focussing attention on these somewhat neglected rocks. An initial chapter by Barker provides an excellent introduction to trondhjemites (could not the spelling, the terror of typists, be updated to trondheimites). Their composition occurrence and origin are perceptively summarised in 11 pages.
364 Even for these crystalline rocks, Barker notes that (p. 5): "some rocks are more accurately classified as trondhjemite or not by means of major elements rather than by modal analysis." These rocks are probably mainly derived from the mantle, by partial melting of basic igneous rock m e t a m o r p h o s e d to amphibolite or eclogite. Differentiation from basaltic parents is a less c o m m o n mode of origin. These conclusions are reinforced by the trace element evidence, again discussed briefly and with clarity by Arth in Chapter 4. The rare earth elements are particularly decisive in this context. The steep light REE enriched -- heavy REE depleted patterns indicate equilibration with amphibole or garnet in the residue during partial melting. Arth notes that (p. 131): "rare-earth elements appear to be the most reliable" since K, Rb, Sr and Ba are subject to alteration. High- (> 15%) and low-A12 03 trondhjemites are distinguished. The former are c o m m o n in continental, and the latter in oceanic environments. The presence of Eu anomalies in the low-A12 03 types indicates derivation from shallow (~ 10 kbar)depths where plagioclase is stable. Conversely the highA12 03 types are derived from greater depths. The trondhjemite suite, although nominally calc-alkaline, is distinct from the quartz diorite--tonalite--granodiorite suites typical of present-day calcalkaline suites. The principal chemical distinction is in the differing REE patterns, steep heavy REE depleted patterns being rare in present-day calcalkaline rocks. The widespread occurrence of trondhjemites in the Archean gray gneiss terrains, well covered by McGregor in Chapter 6, raises important questions a b o u t the distinction between Archean and subsequent igneous activity. This change is also reflected in the composition of the upper crust, as measured b y sedimentary REE patterns, from a bimodal tholeiitic--tonalitic and trondhjemitic suite to a dominantly granodioritic composition at the Archean-Proterozoic boundary. Dacites, latites and rhyolites are the other major rock types dealt with, and provide a kind of bimodality to the book. InChapter 2, Ewart provides an excellent and detailed treatment of the mineralogy and chemistry of these eruptive rocks sensibly restricting his coverage to Tertiary and recent examples. This introduction of typical "calc-alkaline" magmas raises many questions. Although both these eruptives, and the trondhjemites are related to subduction processes (and the latter to probable derivation from basaltic composition), any involvement of the down-going slab in the genesis of typical calc-alkaline rocks is n o w to be seriously questioned. The REE patterns of dacites and rhyolites including high- as well as low-K varieties are very distinct from those of typical Archean trondhjemites. The calc-alkaline patterns have flat heavy REE's, and frequently show pronounced negative Eu anoma-
365 lies. T h e s e p a t t e r n s indicate i n v o l v e m e n t o f feldspar and f o r b i d t h a t o f garnet, and p r o b a b l y a m p h i b o l e . S t r o n t i u m isotopic s y s t e m a t i c s are dealt with in a p e r c e p t i v e p a p e r b y P e t e r m a n ( C h a p t e r 4). This s h o u l d be r e a d y b y e v e r y o n e . He d e m o n s t r a t e s t h a t the initial STSr/~6Sr ratios o f tonalites and t r o n d h j e m i t e s define a simple linear t r e n d f r o m 0 . 7 0 3 9 (present) t o 0 . 7 0 1 0 at 2 6 5 0 m . y . ago and t h a t (p. 144): "an extension of this line intersects the 'TSr/'6Sr value for meteorites (0.6990) at 4550 m.y." The m o d e r n i n t e r c e p t (p. 139}: "is indistinguishable from the average value of 0.7037 -+ 0 . 0 0 0 2 . . . for modern oceanic island and arc basalts and the lower value at 4550 m.y. is essentially the same as BABI."
N o t e t h a t the values for p r e s e n t d a y o c e a n - f l o o r basalts (typically less t h a n 0 . 7 0 3 0 ) again separate these r o c k s f r o m island-arc rocks. This i m p o r t a n t d e m o n s t r a t i o n o f a simple linear g r o w t h line f o r ~TSr/~TSr, and its d i s t i n c t i o n f r o m the wide variability in oceanic basalts has m u c h significance for m o d e l s o f m a n t l e e v o l u t i o n and c o n t i n e n t a l growth. A useful c h a p t e r (No. 5) an oceanic plagiogranites b y C o l e m a n and D o n a t o will assist t h o s e non-petrologists, baffled b y the c u m b e r s o m e and imprecise t e r m i n o l o g y o f p e t r o l o g y , to u n d e r s t a n d the relative i m p o r t a n c e o f these low-K silicic rocks. A m o n g m a n y o t h e r stimulating observations in the b o o k , t w o e x a m p l e s m u s t suffice. M c G r e g o r n o t e s t h a t (p. 198): "there is no evidence in the Amitsoq gneisses or associated supra-crustal rocks, the oldest known terrestrial rocks, to suggest the existence of a primordial granitic or anorthositic crust," B r y a n observes t h a t (p. 599): "the Tonga Island arc also provides little support for the popular "geostill" concept, in which island arc magmatism is presumed to play a critical role in scavaging economically valuable metals from the subducted ocean lithosphere," Has the " g e o s t i l l " gone the way o f the alembic? This review has c o n c e n t r a t e d on t h e first six chapters which c o n t a i n the m o r e general overviews o f the subject. T h e remaining sixteen c h a p t e r s contain m u c h o f interest b u t essentially deal with localised o c c u r r e n c e s o f t r o n d hjemites, or dacites and their relatives. S o m e o f the chapters are labelled as " p r e l i m i n a r y , initial or r e c o n n a i s s a n c e " studies. H o w m u c h o f this material s h o u l d a p p e a r in a h a r d c o v e r b o o k ? P r o b a b l y m a n y o f these papers c o u l d have b e e n p u b l i s h e d in t h e c u r r e n t petrological or g e o c h e m i c a l journals.
366
Surely a book should a t t e m p t a synthesis rather than to include comments that (p. 415): "detailed hypotheses of generation of the rocks from Wyoming and Idaho cannot be made and so the data are presented for their own sake."
For such material, a special issue of one of the petrological journals, dealing with a specific rock type, would seem appropriate. There are a number of typographic errors of varying seriousness. The editor notes in the preface that: "the text was prepared and printed by a word processor in Denver. The editor apologises for the undetected scattered typographical and other errors which, inevitably, seem to be found in material produced by this apparatus."
However, at least two of the authors informed me that they had not had an o p p o r t u n i t y to correct final proofs, and supplied errata sheets with their offprints from this volume. The type is faint and the book is accordingly not easy to read. Although this is probably again the fault of the Denver word processor, the presumed savings from using this device, or from lack of final proof reading are not reflected in the price of US $65.75. The subject index is a little brief (rare earth elements, for example, are not listed), although a 7-page table of contents is a most useful feature. (S.R. TAYLOR, Canberra, A.C.T.)