Rare earth element-SiO_2 systematics of mid-ocean ridge plagiogranites and host gabbros from the Fournier oceanic fragment, New Brunswick, Canada: a field evaluation of some model predictions

摘要

The two most commonly invoked processes for generating plagiogranites in mid-ocean ridge environments arc extended fractional crystallization of mid-ocean ridge basalt (MORB) magma and "hydration melting" of hot, dry MOR gabbro initiated by the influx of seawater-derived hydrothcrmal fluids within localized zones of shear. Bro-phy (Contrib Mineral Petrol 158:99-111, 2009) has proposed on theoretical grounds that, for liquids greater than ~62 wt. % SiO_2, hydration melting should yield, among other features, a negative correlation between rare earth element (REE) abundances and increasing SiO_2, while fractional crystallization should yield a positive correlation. If correct, the REE-SiO_2 systematics of natural systems might be used to distinguish between the two processes. The Ordovician Fournier oceanic fragment, New Brunswick, Canada, contains MOR gabbro-hosted plagio-granite veins and dikes that are believed to have formed from hydration melting, thus forming an appropriate location for field verification of the proposed REE-SiO_2 systematics for such a process. In addition to a negative correlation between liquid SiO_2 and REE abundance for liquids in excess of ~62% SiO_2, other important model features include the following: (1) relative to a gabbro source rock, the degree of enrichment at liquids of 62 and 75% SiO_2 decreases from the LREE to the HREE;;(2) the degree of enrichment at 75% SiO_2 approaches 1 for the HREE;;(3) the rate of change of the degree of enrichment with increasing liquid SiO_2 (i.e., the slope) diminishes from the LREE to the HREE. All of these predicted features are observed in the Fournier plagiogranites. Assuming an initial source rock equivalent to the host gabbro, an additional strongly LREE-enriched component must be added prior to melting in order to make the absolute REE abundances agree with the model values. The most likely candidates are the very seawater-derived hydrothermal fluids that triggered hydration melting in the first place.