Vol. 46 No. 9 SCIENCE IN CHINA (Series D) September 2003 Poisson’s ratio of eclogite: Implications for lower crustal delamination of orogens GAO Shan ( ) 1,2 , Hartmut Kern 3 , JIN Zhenmin ( ) 1 , ZHANG Hongfei ( ) 1 & ZHANG Benren ( ) 1 1. Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China; 2. Department of Geology, Northwest University, Xi’an 710069, China; 3. Institut für Geowissenschaften, Universität Kiel, Olshausenstrasse 40, 24098 Kiel, Germany Correspondence should be addressed to Gao Shan (email: gaoshan@cug.edu.cn) Received May 24, 2002 Abstract Laboratory measurements of combined P- and S-wave-velocities of eclogite from the Dabie-Sulu ultrahigh pressure metamorphic belt and from literature data show a significant in- crease of Poisson’s ratio of eclogite with its intrinsic water content H 2 O + and thus the degree of hydration. Unaltered eclogites with H 2 O + <1.0% have an average Poisson’s ratio between 0.24 and 0.25, which is identical to that calculated from single crystal elastic properties but lower than the averages (0.270.02) of measurements compiled by previous studies. Thus, the Poisson’s ratio of unaltered eclogites is considerably lower than that of lower crustal mafic granulite and upper man- tle peridotite. The lower crust and upper mantle of the Dabie ultrahigh pressure metamorphic belt, the northern and southern parts of the Tibetan Plateau as well as the central Andes, where eclogite may have formed during Mesozoic and Cenozoic tectonism, are characterized by the Poisson’s ratio >0.26. This, together with their normal upper mantle P-wave velocities, implies that eclogites are no longer an important component of the present-day lower crust and upper mantle of these subduction-collision belts. Combined with age constraints on eclogite-facies metamor- phism and subsequent exhumation, this in turn suggests that the interval from formation to de- lamination of eclogites is confined to a very short period of <15 Ma. Keywords: eclogite, Poisson’s ratio, orogen, delamination of lower crust. DOI: 10.1360/02yd0417 Eclogite is essentially a bi-mineralogic high-grade metamorphic rock consisting of garnet and omphacite and is the product of high-to-ultrahigh pressure metamorphism of basaltic rocks due to the subduction of oceanic crust or the thickening/subduction of continental crust in colli- sion belts. Eclogite is unique in having remarkably higher density than mafic granulite and peri- dotite by 0.2 0.4 g cm 3 , the latter dominating the continental lower crust and upper mantle, respectively. Because of this density contrast, mafic lower continental crust is expected to be re- cycled into the mantle in thickened collision belts when mafic granulite is transformed into ec- logite. This process is referred to as the lower crustal delamination, and may be used to explain the evolved andesitic composition of the bulk continental crust, given that primary magmatic addi- tions to the crust are dominantly basaltic [1 8] .