Pressure–temperature–time evolution of the Central East Greenland Caledonides: quantitative constraints on crustal thickening and synorogenic extension A. P. WHITE 1 AND K. V. HODGES 2 1 McKinsey and Company, Inc., New York, NY, 10055, USA (apwhite@alum.mit.edu) 2 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA ABSTRACT Whereas geologists have known for three-quarters of a century that there was significant crustal thickening in the central East Greenland Caledonides, the crucial role of extensional faulting during Caledonian orogenesis has only been recognized during the past decade. In this paper, new petrographic and thermobarometric observations are presented from migmatitic metasedimentary gneisses of the Forsblad Fjord region (c. 72.5°N). Samples of the Krummedal Sequence, collected from the footwall of the upper of two significant splays of the main extensional fault system in the region—the Fjord Region Detachment (FRD)—enable us to establish a relative sequence of metamorphism. Our pressure (P)–temperature (T) results imply a clockwise loop in P–T space. As recorded by mineral assemblages in the Krummedal gneisses, prograde metamorphism involved a net increase of c. 4 kbar and 250 °C, with peak conditions of c. 10.5 kbar at 785 °C. Early burial and heating was followed by near-isothermal decompression of 4.5 kbar, a process which is attributed to roughly 18 km of tectonostratigraphic throw on the upper splay of the FRD. Combining data reported here with the published data, it is estimated that the approximate tectonostratigraphic throw along the lower splay of the FRD was c. 16 km. In situ U–Th–Pb-monazite electron microprobe dating suggests that the earliest phase of metamorphism recorded in the Krummedal Sequence gneisses of Forsblad Fjord occurred during the Caledonian orogeny. Furthermore, the combination of our new data with existing conventional TIMS U-Pb and 40 Ar ⁄ 39 Ar data imply that: (1) movement along the uppermost splay of the FRD (c. 425–423 Ma) occurred at maximum time- averaged slip-rates equivalent to c. 9 mm of vertical displacement per year; and (2) that the final stages of metamorphism occurred prior to c. 411 Ma, although part of this denudation was likely accommodated on overlying extensional structures that may have been active more recently. There is close agreement between our data and results from the Krummedal Sequence north of the field area (72.5°)74°N), and rocks of the Smallefjord Sequence (75°)76°N) that are suggested to correlate with the Krummedal Sequence. This leads us to infer that the events recorded in the Forsblad Fjord region are of orogen-scale significance. Key words: Caledonides; extension; synorogenic; thermobarometry; P–T. INTRODUCTION Recent structural and geochronological investigations in the central East Greenland Caledonides (72° to 74°N) have revealed a multistage extensional history along a system of faults called the Fjord Region Detachment (FRD), with splays that were active both during and after collisional orogenesis (e.g. Caby, 1976; Hartz & Andresen, 1995; Andresen et al., 1998; Hartz et al., 2000; Hartz, 2001; White & Hodges, 2002; White et al., 2002). The spectacular middle-crustal structural exposure and preservation of syn- and post- orogenic normal faults in this transpressive orogen present a valuable opportunity to improve under- standing of the spatial and temporal relationships among orogen-parallel shear, contraction and exten- sion during the evolution of the orogen. At present, there are very few published data on metamorphic P–T evolution to complement a growing base of structural and geochronological data (e.g. Andresen et al., 1998; Hartz et al., 2000; Higgins & Leslie, 2000; Hartz, 2001; White et al., 2002; Strachan et al., 2001). Here results of petrographic and thermobarometric studies of am- phibolite facies metamorphic rocks from the Forsblad Fjord area of East Greenland (72.5°N; Fig. 1) are presented. When these data are interpreted in con- junction with the new geochronological results pre- sented here for monazite inclusions in metamorphic garnet, they suggest that there were at least four stages of successive metamorphism in this region associated with crustal thickening and thinning during Caledo- nian orogenesis (which spanned the Late Ordovician to Devonian periods in East Greenland). The combina- tion of thermobarometric data with phase equilibria constraints indicate that there was a substantial amount of burial (c. 16 km) succeeded by a nearly equal amount of tectonic exhumation (c. 18 km) along the uppermost splay of the FRD system. J. metamorphic Geol., 2003, 21, 875–897 doi:10.1046/j.1525-1314.2003.00489.x Ó 2003 Blackwell Publishing Ltd 875