The significance of sheeted dike complexes in ophiolites

Sheeted dike complexes, in which dike intrudes dike without intermediate screens of gabbro or pillow lava, have long been considered key features of oceanic lithosphere and ophiolites formed in extensional environments. The presence of a sheeted complex implies an approximate balance between spreading rate and magma supply, such that there is just enough melt to fill newly formed fractures produced by spreading. Such a balance appears to exist at mid-ocean ridges, where both the spreading rate and magma supply are probably linked to mantle convection, and thus sheeted dikes appear to be a major part of the ocean crust. In contrast, ophiolites, which are formed or modified in suprasubduction zone environments, rarely have large, well-developed sheeted dike complexes, because magma supply and spreading rate are not linked in the same way. In suprasubduction zone environments, the spreading rate is controlled largely by the rate of slab rollback, whereas the magma supply is controlled by the local temperature profile, the lithology of the subducting crust and mantle wedge, the history and degree of melting of the mantle source, and the abundance and nature of fluids. Because spreading rate and magma supply are rarely balanced in these environments, we suggest that sheeted dikes, rather than being key elements of ophiolites, may instead be unusual features in such bodies. Thus, care must be exercised in using ophiolites to investigate spreading processes at mid-ocean ridges. INTRODUCTION Ophiolites are fragments of oceanic crust and upper mantle that have been uplifted and emplaced on continental margins or in accretionary prisms and island arcs. According to a 1972 GSA Penrose Conference, an idealized, complete ophiolite contains, from the base upward, mantle peridotites, layered ultramafic rocks and gabbros, isotropic gabbros, a sheeted dike complex, and an extrusive sequence, composed of pillowed and massive lavas, overlain by radiolarian chert and/or pelagic limestone (Anonymous, 1972; Dilek, 2003). The presence of a sheeted dike complex has been interpreted as an essential component of ophiolites, as exemplified by the Troodos ophiolite of Cyprus (Fig. 1A), where such complexes were first recognized (Gass, 1968). Sheeted dikes, which are tabular intrusions of magma with no intervening screens of other host rock (Fig. 1B), are believed to form in extensional environments where the faults and fractures produced by seafloor spreading are filled with new magma flowing laterally and vertically along a narrow axial zone beneath the spreading center (Vine and Matthews, 1963; Gass, 1968; Kidd and Cann, 1974; Dilek et al., 1998). In 1968, when Gass correctly identified the sheeted complex in Troodos, mid-ocean ridges were the only environment where such spreading was known to occur; thus, he suggested a mid-ocean ridge environment for the formation of this ophiolite. Figure 1. Sheeted dikes of the Cretaceous Troodos ophiolite, Cyprus. (A) A typical outcrop of sheeted dikes in which dike intrudes dike with no intervening host rock. Individual dikes range from a few centimeters to ~0.5 m in this outcrop (Baragar et al., 1989). Note the inclined dikes on the right side of the picture. These earlier dikes were probably tilted by listric faulting along the spreading axis (Varga and Moores, 1985) and then intruded by the younger vertical dikes in the center. (B) Close-up view of sheeted dikes in Troodos showing contact relationships. Arrows point to one-sided chilled margins.