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We report 2.5 billion-year-old oceanic mantle podiform chromitite and mantle tectonite in ophiolitic mélange in the North China craton. Tectonic blocks of peridotite, wehrlite, pyroxenite, harzburgitic tectonite, dunite, podiform chromitite, layered gabbro, sheeted dikes, and pillow lava are embedded in a strongly deformed metasedimentary and metavolcanic matrix. The blocks are traceable along strike into the relatively complete ca. 2.505 Ga Dongwanzi ophiolite. Textures in the ultramafic blocks provide a window into igneous and structural processes active in Archean suboceanic mantle. Chromitites in dunitic envelopes preserve igneous nodular, orbicular, antinodular, banded, massive, and disseminated textures. Dunite envelopes are common features of podiform chromitites, forming almost exclusively in the upper mantle or the crust-mantle transition zone of suprasubduction zone (harzburgite-type) ophiolites of younger geological ages. Nodular and orbicular chromite textures are known only from ophiolites and are interpreted to form during partial melting of flowing upper mantle, conditions needed to keep chromite suspended and growing concentrically into the magma. Minor orthopyroxene porphyroclasts with asymmetrical recrystallized tails and kink-banded olivine inclusions in chromite grains record plastic deformation and high-temperature shearing, before or during growth of the chromite. We attribute this deformation to flow in the Archean oceanic mantle. Later deformation is related to dismemberment of the ophiolite and incorporation into a mélange during collision of the Eastern and Western blocks of the North China craton. This collision formed the 1600km-long ophiolite-rich Central Orogenic Figure 1. Distribution of Archean cratons, areas underlain by Precambrian crust, and Phanerozoic crust and podiform chromite deposits (modified after Kusky and Polat, 1999). Filled patterns show locations of major ophiolitic podiform chromite deposits in relation to crust and accretionary orogens of different ages. Squares—Archean; triangles—Proterozoic; circles—Phanerozoic. Zunhua podiform chromitites are associated with the 2.505 Ga Dongwanzi and related ophiolites of North China craton. Figure 2. Tectonic map of North China craton showing division of North China craton into Eastern and Western blocks, separated by the Central Orogenic belt. (Modified after Li et al., 2000a, 2000b; Kusky et al., 2001). *Corresponding author: kusky@eas.slu.edu. 4 JULY 2002, GSA TODAY GSA TODAY, JULY 2002 5 belt, and the 2.5–2.4 Ga Qinglong foreland basin and fold-thrust belt on the Eastern block, and provides an important record of the operation of plate tectonics

Archean Podiform Chromitites and Mantle Tectonites in Ophiolitic Mélange, North China Craton: A Record of Early Oceanic Mantle Processes