- Email: [email protected]
Magmatic processes: a special issue in honor of R.H. Vernon
Lithos 61 (2002) vii – ix www.elsevier.com/locate/lithos
Preface
Magmatic processes: a special issue in honor of R.H. Vernon astute observations with logical, incisive analysis to understand igneous, metamorphic and structural processes. I am particularly happy to be able to provide, below, a statement from Ron that describes how he became interested in magmatic process in general and granitic rocks in particular. His was a ‘‘magmatic’’ career of real note, and through many productive collaborations, one that enhanced the careers of so many other geologists. It is astonishing to realize that he also had parallel careers in so many other areas!
This Special Issue is a collection of papers on magmatic processes, all but one of which were presented at the Vernon Symposium, held at the 15th Australian Geological Convention in Sydney, July 2000, to honor the career of Ronald H. Vernon. Everyone that knows Ron is aware of the high quality of his insightful research in a wide variety of geological fields, including his contributions to microstructural analysis, the origin of microgranitoid enclaves, K-feldspar megacrysts and orbicular granitoids, aspects of low P – high T metamorphic terranes, P – T– t paths and magma emplacement mechanisms, and the recognition of magmatic versus solid-state fabrics in igneous rocks. Ron has always combined
I first became interested in granites in 1956 at the University of New England BSc where for my honours project at Uralla, NSW, John Wilkinson assigned me a pluton and its contact metamorphism. Bruce Chappell and I shared an office that year, and we had many discussions on granites, which became his main interest, of course. In fact, he claims I knew more about granites then than I do now! (Maybe he’s right). Our ideas have diverged, and I guess the main point of difference these days is the extent of the contributions made by restite unmixing and magma mixing. After leaving Armidale, I left granites alone for many years, in favour of ceramics, ore deposits and metamorphism/structure at the CSIRO in Melbourne and the University of Sydney. However, it was impossible not to get involved with granites when I went to Macquarie University, because of the enormous amount of work being carried out by Stirling Shaw and Dick Flood, especially on the New England Batholith. I became interested in the problem of enclaves, which led me (astray as far as Stirling and Dick were concerned) towards magma mixing and mingling, especially from the field and microfabric viewpoints. Dick and I also became
0024-4937/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 4 - 4 9 3 7 ( 0 2 ) 0 0 0 7 4 - 9
viii
Preface
involved with figuring out just what you can and cannot do with inferring orders of crystallization in igneous rocks from the microfabric. Many people still make unjustified interpretations. Then Bill Collins arrived and we decided to work in the Proterozoic of central Australia, where granites are abundant. We had many monumental arguments about granites on the rocks and around the campfire. The sheet-like form of many of the granites in central Australia led me to think more about the felsic gneisses at Broken Hill, which, though originally identified as deformed, metamorphosed granites, had become former volcanic and volcaniclastic sediments in the eyes of most recent workers. A visit from Paul Williams provided the impetus to re-examine the megacrystic varieties of these gneisses, and we concluded that the original interpretations were preferable. Jeff Vassallo has since discovered excellent intrusive contacts between different varieties of these granites, which clinches the argument in our view. This work was connected with the problem of Kfeldspar megacrysts in granites, which had bothered me for many years. I expressed my concern after a talk at Macquarie by Dave Strong, who simply said: ‘‘Well, why don’t you work on the problem?’’ The opportunity came during a period at the University of New Mexico in 1984 – 5, where I worked on the megacrystic Sandia Granite and its mylonitic equivalents. I also reviewed the problem of K-feldspar megacrysts generally, concluding that they are phenocrysts, not porphyroblasts—even in enclaves and granite mylonites. Many petrologists then believed (and I know of some who do even now) that though Kfeldspar megacrysts in granites may be phenocrysts, this does not apply to the identical objects in enclaves. Of course, magma mingling solves the problem. This review turned out to be so laborious that I had many occasions to curse Dave for his blithe suggestion. The work on granite enclaves continued at UNM, and I was fortunate to join up with John Reid at Yosemite, where he wowed me with the most spectacular examples of magma mingling imaginable. Many years later Bob Wiebe did that too, as he has done to so many visitors to his
wonderful Maine outcrops and to his and Bill Collins’s equally spectacular outcrops in the Bega Batholith. UNM also gave me the opportunity to look at orbicular granites in the Sandia Granite, and so I was faced with another of my long-standing concerns about granites: how on Earth do you make those crazy orbs? Building on the great work of Moore and Lockwood and the experimental data of Lofgren and others, I was able to come up with a mechanism that is at least plausible. During a visit to the University of California at Santa Cruz in 1988, I was fortunate to share a lab with Scott Paterson, and was introduced by him and Othmar Tobisch to the granites and metamorphic rocks of the Foothills terrane of the central Sierra Nevada. At this time we also became involved with the problem of distinguishing magmatic from solid-state flow in granites, as well as the difficult area between the two. Since then, Scott and I have become heavily involved with pluton emplacement. For example, we dared to have another look at the magnificent Papoose Flat pluton in the Inyo mountains, and the equally famous Ardara pluton in beautiful Donegal, in the company of that granite petrologist extraordinare, Wally Pitcher. Scott and I didn’t see enough evidence for expansion as the main method of emplacement, though we felt that it surely does contribute to a some extent, and naturally we have come in for some criticism for this viewpoint. One of the problems is that later regional deformation can rarely be eliminated from the equation. However, our more recent work with Scott Johnson on the San Jose pluton in Baja California, Mexico, may throw more light on the problem of ballooning, as later deformation is not evident. The marginal tonalites of this pluton show evidence of some solid-state deformation, and so the question arises as to whether this can be produced by magma expansion alone. At present Scott Johnson and I are trying to find microfabric evidence of minor magma flow during the solidstate deformation of these marginal rocks. More recently, I have returned to Cooma in southeastern NSW, where I have been fiddling with the metamorphism and structure for many years, recently with Scott Johnson, and now with
Preface
Bill Collins and Simon Richards of the University of Newcastle. I’m intrigued with the migmatites. Migmatites have always puzzled me, and so I’ve tended to avoid them. However, Scott and I decided to bite the bullet and try to fit the metapelitic migmatites into our structural–metamorphic story. Bill threw a spanner into the works by showing me that maybe more than one type of leucosome is present, and Simon and I soon found excellent examples of metapsammite-derived leucosomes that disrupt the metapelite-derived leucosomes. This has interesting implications for the origin of the Cooma Granodiorite. I guess I will always maintain an interest in granites, though I’ve reverted more to metamorphism recently. Anyway, you can’t think about the brand of low-pressure/high-temperature metamorphism we have so abundantly in Australia without thinking about the granites that invariably accompany it. The relationship between granites and LPHT metamorphism has been a recurring theme during my tinkering with granites.
ix
Barrie Clarke’s granite network shows that granites are just as fascinating to as many people as ever, and I look forward to learning much more about them from people working on them. I thank all the colleagues who have taught me so much about granites, metamorphism and deformation. I especially thank Bob Wiebe for editing this volume, and I also sincerely thank the contributors. Fortunately I now have time to read their papers. The nine papers here provide an excellent representative sample of papers on magmatic processes presented at the symposium. Their breadth, from problems of emplacement to enclaves to mineral textures and chemistry, nicely reflects the range of Ron’s work on magmatic rocks. Robert A. Wiebe Department of Geological Sciences, Franklin and Marshall College, Lancaster, PA, 17604 USA E-mail address: [email protected]
Preface
Magmatic processes: a special issue in honor of R.H. Vernon astute observations with logical, incisive analysis to understand igneous, metamorphic and structural processes. I am particularly happy to be able to provide, below, a statement from Ron that describes how he became interested in magmatic process in general and granitic rocks in particular. His was a ‘‘magmatic’’ career of real note, and through many productive collaborations, one that enhanced the careers of so many other geologists. It is astonishing to realize that he also had parallel careers in so many other areas!
This Special Issue is a collection of papers on magmatic processes, all but one of which were presented at the Vernon Symposium, held at the 15th Australian Geological Convention in Sydney, July 2000, to honor the career of Ronald H. Vernon. Everyone that knows Ron is aware of the high quality of his insightful research in a wide variety of geological fields, including his contributions to microstructural analysis, the origin of microgranitoid enclaves, K-feldspar megacrysts and orbicular granitoids, aspects of low P – high T metamorphic terranes, P – T– t paths and magma emplacement mechanisms, and the recognition of magmatic versus solid-state fabrics in igneous rocks. Ron has always combined
I first became interested in granites in 1956 at the University of New England BSc where for my honours project at Uralla, NSW, John Wilkinson assigned me a pluton and its contact metamorphism. Bruce Chappell and I shared an office that year, and we had many discussions on granites, which became his main interest, of course. In fact, he claims I knew more about granites then than I do now! (Maybe he’s right). Our ideas have diverged, and I guess the main point of difference these days is the extent of the contributions made by restite unmixing and magma mixing. After leaving Armidale, I left granites alone for many years, in favour of ceramics, ore deposits and metamorphism/structure at the CSIRO in Melbourne and the University of Sydney. However, it was impossible not to get involved with granites when I went to Macquarie University, because of the enormous amount of work being carried out by Stirling Shaw and Dick Flood, especially on the New England Batholith. I became interested in the problem of enclaves, which led me (astray as far as Stirling and Dick were concerned) towards magma mixing and mingling, especially from the field and microfabric viewpoints. Dick and I also became
0024-4937/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 4 - 4 9 3 7 ( 0 2 ) 0 0 0 7 4 - 9
viii
Preface
involved with figuring out just what you can and cannot do with inferring orders of crystallization in igneous rocks from the microfabric. Many people still make unjustified interpretations. Then Bill Collins arrived and we decided to work in the Proterozoic of central Australia, where granites are abundant. We had many monumental arguments about granites on the rocks and around the campfire. The sheet-like form of many of the granites in central Australia led me to think more about the felsic gneisses at Broken Hill, which, though originally identified as deformed, metamorphosed granites, had become former volcanic and volcaniclastic sediments in the eyes of most recent workers. A visit from Paul Williams provided the impetus to re-examine the megacrystic varieties of these gneisses, and we concluded that the original interpretations were preferable. Jeff Vassallo has since discovered excellent intrusive contacts between different varieties of these granites, which clinches the argument in our view. This work was connected with the problem of Kfeldspar megacrysts in granites, which had bothered me for many years. I expressed my concern after a talk at Macquarie by Dave Strong, who simply said: ‘‘Well, why don’t you work on the problem?’’ The opportunity came during a period at the University of New Mexico in 1984 – 5, where I worked on the megacrystic Sandia Granite and its mylonitic equivalents. I also reviewed the problem of K-feldspar megacrysts generally, concluding that they are phenocrysts, not porphyroblasts—even in enclaves and granite mylonites. Many petrologists then believed (and I know of some who do even now) that though Kfeldspar megacrysts in granites may be phenocrysts, this does not apply to the identical objects in enclaves. Of course, magma mingling solves the problem. This review turned out to be so laborious that I had many occasions to curse Dave for his blithe suggestion. The work on granite enclaves continued at UNM, and I was fortunate to join up with John Reid at Yosemite, where he wowed me with the most spectacular examples of magma mingling imaginable. Many years later Bob Wiebe did that too, as he has done to so many visitors to his
wonderful Maine outcrops and to his and Bill Collins’s equally spectacular outcrops in the Bega Batholith. UNM also gave me the opportunity to look at orbicular granites in the Sandia Granite, and so I was faced with another of my long-standing concerns about granites: how on Earth do you make those crazy orbs? Building on the great work of Moore and Lockwood and the experimental data of Lofgren and others, I was able to come up with a mechanism that is at least plausible. During a visit to the University of California at Santa Cruz in 1988, I was fortunate to share a lab with Scott Paterson, and was introduced by him and Othmar Tobisch to the granites and metamorphic rocks of the Foothills terrane of the central Sierra Nevada. At this time we also became involved with the problem of distinguishing magmatic from solid-state flow in granites, as well as the difficult area between the two. Since then, Scott and I have become heavily involved with pluton emplacement. For example, we dared to have another look at the magnificent Papoose Flat pluton in the Inyo mountains, and the equally famous Ardara pluton in beautiful Donegal, in the company of that granite petrologist extraordinare, Wally Pitcher. Scott and I didn’t see enough evidence for expansion as the main method of emplacement, though we felt that it surely does contribute to a some extent, and naturally we have come in for some criticism for this viewpoint. One of the problems is that later regional deformation can rarely be eliminated from the equation. However, our more recent work with Scott Johnson on the San Jose pluton in Baja California, Mexico, may throw more light on the problem of ballooning, as later deformation is not evident. The marginal tonalites of this pluton show evidence of some solid-state deformation, and so the question arises as to whether this can be produced by magma expansion alone. At present Scott Johnson and I are trying to find microfabric evidence of minor magma flow during the solidstate deformation of these marginal rocks. More recently, I have returned to Cooma in southeastern NSW, where I have been fiddling with the metamorphism and structure for many years, recently with Scott Johnson, and now with
Preface
Bill Collins and Simon Richards of the University of Newcastle. I’m intrigued with the migmatites. Migmatites have always puzzled me, and so I’ve tended to avoid them. However, Scott and I decided to bite the bullet and try to fit the metapelitic migmatites into our structural–metamorphic story. Bill threw a spanner into the works by showing me that maybe more than one type of leucosome is present, and Simon and I soon found excellent examples of metapsammite-derived leucosomes that disrupt the metapelite-derived leucosomes. This has interesting implications for the origin of the Cooma Granodiorite. I guess I will always maintain an interest in granites, though I’ve reverted more to metamorphism recently. Anyway, you can’t think about the brand of low-pressure/high-temperature metamorphism we have so abundantly in Australia without thinking about the granites that invariably accompany it. The relationship between granites and LPHT metamorphism has been a recurring theme during my tinkering with granites.
ix
Barrie Clarke’s granite network shows that granites are just as fascinating to as many people as ever, and I look forward to learning much more about them from people working on them. I thank all the colleagues who have taught me so much about granites, metamorphism and deformation. I especially thank Bob Wiebe for editing this volume, and I also sincerely thank the contributors. Fortunately I now have time to read their papers. The nine papers here provide an excellent representative sample of papers on magmatic processes presented at the symposium. Their breadth, from problems of emplacement to enclaves to mineral textures and chemistry, nicely reflects the range of Ron’s work on magmatic rocks. Robert A. Wiebe Department of Geological Sciences, Franklin and Marshall College, Lancaster, PA, 17604 USA E-mail address: [email protected]