Plutonism from Antarctica to Alaska

KAY, S.M.; RAPELA, C.W.

Geological Society of America

Año: 1990 p. 263

0-8137-2241-1

Preface The papers in this volume were among those presented orally at the International Symposium on "Circum-Pacific Phanerozoic Granitoids" held during the X Argentine Geological Congress in San Miguel de Tucuman, Argentina, September 13?18, 1987. The conference, sponsored by IGCP project 249, ?Andean Magmatism and its Tectonic Setting,? had as its primary goals defining the characteristics and tectonic setting of Phanerozoic Andean plutonic rocks and comparing them to other Circum-Pacific granitoids. Toward this end, contributions reflecting the diversity of Andean plutonism formed the backbone of the symposium, and constitute the main emphasis of this volume. Of wider scope were the invited lectures on continental and island-arc plutonism, which included discussions of the New England fold belt in eastern Australia by S. Shaw (Macquarie University), of the Cordilleran Interior of the United States by C. Miller (Vanderbilt University), of plutonism in Japan by T. Nozawa (Geological Survey of Japan), and of the intraoceanic Aleutian Arc in Alaska by S. Mahlburg Kay (Corneli University). The contributions in this volume represent the work of authors from ten countries, who speak four native languages. Included are both new data and interpretation, and summaries of information that has not been previously published in English. Many of the papers concern plutonic rocks of pre-Andean age, and the reader will find citations to most of the important papers published in several languages on this topic. The book should serve as a starting place for anyone interested in the magmatism and tectonics of Phanerozoic western South America. This collection of papers should be viewed as a beginning, not an end, to understanding the geology of this vast region. Different viewpoints on similar problems are expressed in a number of papers. This volume complements Geological Society of America Memoir 159 on Circum-Pacific Plutonic Terranes, which contains papers on granitoids from around the Pacific rim, including two on South America. In contrast, fifteen of the papers in this volume discuss plutonism in South America and its geologic relative, the Antarctic Peninsula; one compares Andean plutonism with plutonism in Southeast Asia; and two discuss plutonism in North America. The North American papers discuss plutonism in the intraoceanic Aleutian are and give an overview on the granites of the Cordilleran Interior of the western United States, topics not covered in Memoir 159. Papers contained in this volume consider most of the major plutonic provinces of the Andean region of South America spanning an age range from late Precambrian to Tertiary. These include the Paleozoic to Mesozoic granitoids of the Antarctic Peninsula, the Mesozoic Patagonian batholith, the Paleozoic and Mesozoic batholiths of central Chile, the Paleozoic plutonic rocks of the Sierras Pampeanas, the Paleozoic plutonic rocks of northern Chile and northern Argentina, Triassic and Tertiary granitoids of the Cordillera Real of Bolivia, the Mesozoic and Tertiary granitoids of Peru, and Cenozoic magmatic rocks of northwestern Colombia. The late Precambrian to late Paleozoic and early Mesozoic granitoids of Antarctica and South America vary widely in composition and tectonic environment. In many cases the characteristics of these suites contrast significantly with the ?Andean-type? granitoids of the Peruvian batholith. These plutonic rocks are associated with the varied tectonic activity responsible for the assembly of Gondwana. The periods of deformation and the magmatic characteristics of these plutons have parallels in North America. In contrast, some plutonic rocks of the latest Paleozoic, and most of those of the Triassic and Early Jurassic, are associated with a period in which there is little evidence for subduction along the South American margin. Instead, these plutonic rocks are primarily associated with extension and strike-slip faulting related to the breakup of Gondwana. This is reflected in the largely bimodal composition of the plutonic rocks of this period. Starting in the mid to late Jurassic, subduction associated with the ?Andean cycle? became the dominant tectonic style along the western margin of South America. Differences in angle of dip of the subducting slab, the rate of convergence, and the thickness and character of the preexisting crust have profoundly influenced the composition of the plutonic rocks along the margin. The chemistry of the plutonic rocks is controlled by subducted sediments, the mantle wedge, the preexisting continental crust, and the state of stress. The papers on North American plutonism describe two contrasting tectonic settings where magmatism has been extensively studied. The first of these is the Aleutian Arc in Alaska where plutons associated with subduction have intruded Tertiary island-arc crust. In this region the contribution of the preexisting crust is difficult to decipher. The second is in the Cordilleran Interior of the western United States where plutons intrude crust of various ages and thicknesses. In this case, the preexisting crust (and upper mantle) has played an important role in defining the character of the plutonism.