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OVERVIEW

Precambrian High-Pressure-High-Temperature Metamorphism: A Key to Understanding the Lower Crust and Reconstruction of Precambrian Plate Tectonics

Hengshan-Wutaishan and Beijing, China
23–29 September 2002

Conveners:

M. Brown
Department of Geology, University of Maryland, College Park, MD 20742, USA, (301) 405-4080, fax 301-314-7970
A. Kröner
Institut für Geowissenschaften, Universität Mainz, D-55099 Mainz, Germany
P.J. O'Brien
Universität Potsdam, Institut für Geowissenschaften, Postfach 60 15 53, 14415 Potsdam, Germany
C.W. Passchier
Institut für Geowissenschaften, Universität Mainz, D-55099 Mainz, Germany
Li Jianghai
Department of Geology, Peking University, Beijing 100871, People's Republic of China
Zhai Mingguo
Department of Geology & Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, People's Republic of China
Outcrop, Heng Mountains. Outcrop, Heng Mountains. Photo courtesy of A. Kröner.



The major goal of this conference is to discuss new approaches to Precambrian plate tectonics in light of recent discoveries of high-pressure-high-temperature metamorphism in Proterozoic rocks from the Canadian Shield, the North China Craton, Tanzania, southern India, Brazil, Siberia, and the Baltic Shield. These rocks record evidence of metamorphic pressures of 1.7-1.0 GPa with characteristic near-isothermal decompression pressure-temperature paths. The metamorphic and structural evolution of these rocks, their tectonic setting, and the mechanism of their exhumation are topics of international interest. Furthermore, the preservation of lower crustal sections composed of high-grade gneisses and time-equivalent upper crustal sections exposing greenstone belts makes it possible to study the relationships between them in the same crustal section, potentially to advance our understanding of continental crust formation and evolution. One area where such examples are exposed is the Hengshan-Wutaishan area of Northern China. The first part of this conference will be devoted to a field trip to the Hengshan-Wutaishan area that will provide a template within which to focus on the evidence for plate tectonics in the Precambrian during the discussions in Beijing.

The Heng Mountains in northern Shanxi Province, about 250 km west-southwest of Beijing, form part of the North China Craton and consist of ductilely deformed Late Archean to early Paleoproterozoic (ca. 2700-2100 Ma) orthogneisses and minor paragneisses intruded by numerous mafic dykes of gabbroic composition. The strongly deformed and boudinaged mafic rocks, which commonly preserve primary igneous textures, underwent high-pressure granulite metamorphism (Grt-Cpx-Pl-Qtz-Rt) at 850-900 °C and about 14 kbar, suggesting that the entire Hengshan metamorphic assemblage experienced high-pressure conditions equivalent to crustal depths of about 50 km. The Hengshan metamorphic assemblage is adjacent to the low-grade Late Archean Wutai greenstone belt that consists of ca. 2.5 Ga bimodal volcanic rocks and metasediments of arc affinity associated with coeval granitoids. The Hengshan metamorphic assemblage may be the lower, plutonic part of a Late Archean to Early Paleoproterozoic magmatic arc, the upper volcanic part of which is represented by the adjacent Wutai greenstone belt. Virtually all the magmatic events recorded in the Hengshan metamorphic assemblage also are identified in the Wutai greenstone belt.

The general themes of the conference are:

  1. Precambrian high-pressure-high-temperature eclogite- and granulite-facies rocks and their evolution, and tectonic implications for plate tectonic processes; and
  2. The structural relationship between high-grade lower crustal domains and low-grade upper crustal domains, including the geometry and evolution of major ductile shear zones and thrust belts.

Key speakers from the fields of metamorphic petrology, tectonics, and geodynamic modeling will address these themes in an interdisciplinary manner. Other contributions will address specific regional examples, where many of the details of the field relationships provide primary information and important constraints. All participants are encouraged to bring poster presentations, from which additional oral presentations will be selected. It is anticipated that about a third of the time in Beijing will be devoted to oral presentations, about a third of the time to discussion, and about a third of the time to viewing the posters.

The conference is limited to 80 participants. We encourage interested graduate students to apply. The registration fee, which will cover lodging, meals, field trips, and all other conference costs except personal incidentals, is likely to be US$400 (US$300 for students). Participants will be responsible for their own travel to and from Beijing.

Geologists and petrologists with an interest in Precambrian high-grade gneiss terrains and greenstone belts are encouraged to apply. Potential participants should send a letter of application to Alfred Kröner or Mingguo Zhai (addresses above), including a brief statement of interests, the relevance of the applicant's recent work to the themes of the meeting, the subject of any proposed presentation (verbal or poster), and contact information (particularly e-mail address for the period June-September 2002). We expect to attract a diverse group of field geologists, metamorphic petrologists, tectonicians, and modelers. Invitations will be e-mailed to participants by mid-June 2002.

Cosponsors: National Science Foundation of China, Beijing, China; Chinese Academy of Sciences, Beijing, China; and Peking University, Beijing, China.