The Origin of Granites and Related Rocks U.S. GEOLOGICAL SURVEY CIRCULAR 11 29 Cover. Boudins on several scales within layered amphibolite, Tolstik Peninsula, Karelia, Russia. Anatectic melt represented by leucosome has migrated preferentially parallel to the tectonic foliation to the boudin necks, at each of the scales, in response to applied differential stress. Width of field of view about 1 m. Photograph taken by Michael Brown. Cover design by Jane B. Russell. TH'EOB.Y OJ THE E A R T H, WIT1\ PR.OOFS AND ILLUSTRATIONS. IN FOUR. PAR. TS. Br J.AMMJ HUTTON, M.D. ~F. R. 8. E. VOL. 1. EDINBURGH: PRINTED FOR MESSRS CADEl-L, JUNIOR, AND DA\"1[_;, J,.ONJ.)Olf; A·ftD WILLIAM CREECH, EDINBURGH. J 7 9 S• Frontispiece. Facsimile of the title page of James Hutton's seminal work, Theory of the Earth, two volumes, Edinburgh, 1795. The geological writings of James Hutton (1726- 1797) provide the foundation of modem geology. Hutton recognized the length of geological time, the uniformity of geological processes through time, and the role of igneous activity, sedimentation, and erosion in forming the Earth as we know it. It is fitting that this Third Hutton Symposium on the Origin of Granites and Related Rocks should occur during the bicentenary of the publication of this great work, which records among its content an important stage in the development of ideas about igneous rocks. THE ORIGIN OF GRANITES AND RELATED ROCKS The Origin of Granites and Related Rocks THIRD HUTTON SYMPOSIUM A B S T R A C T S College Park, Maryland August 26-September 2, 1995 Edited by Michael Brown and Philip M. Piccoli U.S. GEOLOGICAL SURVEY CIRCULAR 1129 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY GORDON P. EATON, Director For sale by U.S. Geological Survey, Information Services Box 25286, Federal Center, Denver, CO 80225 This report has been prepared by the University of Maryland in cooperation with the U.S. Geological Survey; the report has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. Published in the Eastern Region, Reston, Va Manuscript approved for publication May 25, 1995. CONTENTS Introduction . 1 Program of Oral Presentations . 4 Abstracts . 7 Author Index . 169 v THE ORIGIN OF GRANITES AND RELATED ROCKS Michael Brown and Philip M. Piccoli, Editors INTRODUCTION country rocks. This represents the first demonstration of the class of rocks that we now refer to as intrusive igneous Towards the end of his life, James Hutton (1726-1797) rocks. The conclusions of his short paper "Observations on set forth his "Theory of the Earth" in which he recognized Granite" are important to all of us, and were: the length of geological time, the uniformity of geological "We are now fully assured that granite has been processes through time, and the role of igneous activity, made to break, displace and invade the Alpine sedimentation, and erosion in forming the Earth as we schistus or primary strata having been previously know it. These ideas had been formulated as a result of forced to flow in the bowels of the earth, and re­ much thought and discussion, and were based on a long duced into a state offusion. From this too we are to and critical study of rocks in the field. Hutton's four part draw the following conclusion: Granite, which has two-volume book was preceded by a public presentation of been hitherto considered by naturalists as being the these ideas in 1785, in a paper on "Examination of the original or primitive part of the earth, is now found System of the Habitable Earth with Regard to its Duration to be posterior to the Alpine schistus; which schistus, and Stability" read to members of the Royal Society of being stratified, is not itself original; though it may Edinburgh on March 7 and April 4. Subsequently, the be considered, perhaps, as primary, in relation to paper was published in Volume I of the Transactions of other strata, which are evidently of a later date." the Royal Society of Edinburgh in 1788. In the ten years The subject of the origin of granite and related rocks has between presentation of the paper to the Royal Society of remained of intense topical interest since the time of Edinburgh and publication of his book, Hutton occupied Hutton, and the Hutton Symposium Series began as a himself in accumulating the "proofs and illustrations" of result of the success of a symposium on "The Origin of his Theory of the Earth which enhance the fuller exposition Granites" organized jointly by the Royal Society of of the Theory as presented in the book. Indeed, for the Edinburgh and the Royal Society of London in 1987 to three summers that followed his 1785 presentations to the celebrate the bicentenary of the work of James Hutton. Royal Society of Edinburgh, Hutton examined key field Implicit in Hutton's conclusions concerning granite are the relationships along Glentilt in the Grampians (1785), ideas of crustal anatexis, melt segregation, magma transfer around Loch Ken in Galloway (1786) and on Goatfell in and granite emplacement into lower-grade upper-crustal the north of Arran (1787), because he concluded, that if rocks. How far have we progressed in our understanding his account in 1785 was correct, some confirmation of it of these processes during the following 200 years? must appear at those places where granite and stratified Detailed field studies, such as those completed by rock are in contact. The development of Hutton's views on Hutton 200 years ago, remain the foundation of our the origin of igneous rocks was presented to members of understanding of granites and related rocks. Since Hutton's the Royal Society of Edinburgh on January 4, 1790, and time, field work has provided a necessary framework for subsequently published in Volume III of the Transactions the interpretation of studies of, for example: granite in 1794. The questions in Hutton's mind in 1785 con­ crystallization; coeval and cogenetic felsic volcanic cerned whether granites represented material "transfused systems; pegmatite genesis; and granite-related hydrother­ from the subterraneous regions, and made to break and mal systems (which may represent the culmination of the invade the strata". During his field work, Hutton was able granite crystallization process). After several decades of to find unequivocal evidence to show that molten granite experimental investigation, we have a firm understanding had invaded the surrounding stratified rock, that is to say of the melt production of common crustal protoliths as a it was an intrusive igneous rock of younger age than the function of pressure, temperature and activity of water. 1 2 The Origin of Granites and Related Rocks The thermal budget of continental collision belts that of the melting reaction remains in situ, provide informa­ involve thickening, thermal relaxation and exhumation has tion about plumbing in the crust and melt transfer path­ been modeled, and in many cases extension and/or ways. The relationship between lower crust that exhibits delamination of part of the lithosphere to increase the heat depleted geochemistry and fragments of apparently flux across the Moho is necessary to explain observed undepleted lower crustal material entrained as xenoliths in features of granite magmatism in orogenic belts. Nonethe­ basalts, remains unresolved. less, it remains unclear exactly how the thermal require­ There has been considerable debate concerning the ments for melting crustal protoliths are met in individual mechanisms of ascent of granitic magma through the cases. Additionally, the role of mantle-derived magma in continental crust. During the past decade support for the crustal anatexis remains a matter of debate, although traditionally assumed mechanism of diapirism has been hybridization and assimilation-fractional crystallization are waning as emphasis has been increasingly placed on the well-established processes and in many cases the isotopic relationship between plutons and shear zones. More evidence seems unambiguous. Our understanding of recently, magma transfer through the crust in dikes has controls on the geochemistry of crustal melts has improved become popular. Rates of melt segregation in response to significantly with increases in experimental data and better applied differential stress and melt ascent in dikes or major understanding of rates of, for example, diffusion and shear zones have been modeled, and in some circum­ mineral dissolution. In contrast, our knowledge of pres­ stances are thought to be fast. Both the processes them­ sure, temperature, fugacity of oxygen and fugacity of selves and the rates at which they proceed have important water in magmas during ascent and emplacement remains implications for the geochemistry of granites and for the sketchy at best. Since 1980 our understanding of and our potential to advect heat into higher levels of the crust. ability to model the processes that cause change in magma Additionally, the rheological consequences of partially composition during ascent, emplacement and final solidifi­ molten crust are significant. For example, if major crustal cation has advanced considerably, but the relative impor­ shear zones initiate in partially molten regions subjected to tance of these processes, such as fractional crystallization, applied differential stress, then melt will migrate to the assimilation-fractional crystallization, restite entrainment lower-pressure shear zone. In this way the shear zone may and unmixing, and hybridization remains uncertain. The localize the ascent path and emplacement site, but the past decade has seen a return to experimental work which driving forces both for ascent and emplacement will be is producing data to advance our understanding of the some combination of the regional applied differential stress physics of magma, a renewed interest in textural interpre­ and buoyancy forces of gravity.