PERVASIVE AND FAULT-RELATED ALTERATION TRENDS IN OCEANIC CRUST: IMPLICATIONS FOR THE INTERPRETATION OF AMPHIBOLITES AND GREENSTONES IN OROGENIC BELTS

A geochemical and mineralogical study of core samples from the Hess Deep Rift reveals the grades and extents of metamorphic/hydrothermal alteration that affect the lower oceanic crust (LOC) produced at fast-spreading centers. The LOC undergoes pervasive, penetrative alteration during cooling through 750-250°C as seawater infiltrates first along grain boundaries and then through fractures to replace 20-80 wt.% of the magmatic minerals with amphibole, talc, chlorite, serpentine, & prehnite. The geochemical consequences of this alteration include 1) the sequestration of 2-8 wt.% H₂O in mineral products, 2) widespread Na loss, and 3) a shift in whole rock ⁸⁷Sr/⁸⁶Sr of 5-15% from mantle toward seawater signature values. Subsequent alteration at T<250°C is restricted to cataclastic zones and late fracture systems in which zeolites & expandable clays precipitate. These minerals have the capacity for cation exchange, allowing compositional modification by ambient fluids without further recrystallization. The presence of these minerals at even a few wt.% may render some geochemical signatures, e.g. Ca# and ⁸⁷Sr/⁸⁶Sr, petrologically meaningless. In contrast to the relative ‘openness’ of the alteration environments with respect to Na, Ca, and Sr, the redistribution of Mg, Fe, Si, and REE appears to be ‘closed’ at the scale of sampling (~1 m) during both pervasive and fault-related alteration.

These results suggest that most oceanic crust arrives at convergent plate margins more thoroughly altered, more hydrous, and consequently weaker than is commonly recognized. This places new constraints on the ‘protoliths’ of many amphibolites, greenstones, and serpentinites entrained in mountains belts. Whereas some geochemical parameters (e.g. Ca#) are hopelessly disrupted by pre-orogenic alteration and others (e.g. ⁸⁷Sr/⁸⁶Sr) evolve non-linearly with changing aqueous environments, still others (e.g. Mg#, REE abundances) are probably unaffected and remain geologically meaningful. This talk explores the implications of these observations for the interpretation of geochemical data from ophiolites (e.g. Annieopsquatch) and other slivers of oceanic crust (e.g. Maltby Lakes Complex, Piney Branch Complex) included throughout the Appalachian orogen.