Rare earth element–SiO2 systematics of mid-ocean ridge plagiogranites and host gabbros from the Fournier oceanic fragment, New Brunswick, Canada: a field evaluation of some model predictions

Abstract

The two most commonly invoked processes for generating plagiogranites in mid-ocean ridge environments are 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 hydrothermal fluids within localized zones of shear. Brophy (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 plagiogranite 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.

Rare earth element evolution and migration in plagiogranites: a record preserved in epidote and allanite of the Troodos ophiolite

“…However, they did not include allanite (a significant REE reservoir) in their models, even though it exists in their plagiogranite. More recently, Brophy and Pu (2012) did use allanite in their model, but they assumed that allanite was of magmatic origin. This may not be accurate, as allanite can possibly be of hydrothermal origin, thus affecting the appropriate REE partition coefficients to be used (Gieré and Sorensen 2004;Frei et al 2004).…”

“…Measured rare earth element (REE) contents and patterns of rocks and minerals are routinely used in model calculations to constrain the source rocks of plagiogranitic magmas and their formation process (e.g. Gerlach et al 1981;Pedersen and Malpas 1984;Flagler and Spray 1991;Borsi et al 1996;Twining 1996;Floyd et al 1998;Luchitskaya et al 2005;Rudnev et al 2005;Dilek and Thy 2006;Rollinson 2009;Brophy 2009;Brophy and Pu 2012;Freund et al 2014). Important hosts of REE in plagiogranites are the epidote group minerals.…”

“…years using field-based and experimental petrological studies assisted by geochemical and isotope evidence (Koepke et al , 2007Brophy 2009;Rollinson 2009;France et al 2010;Grimes et al 2011;Brophy and Pu 2012;Freund et al 2014). Measured rare earth element (REE) contents and patterns of rocks and minerals are routinely used in model calculations to constrain the source rocks of plagiogranitic magmas and their formation process (e.g.…”

Evidence for a Moist to Wet Source Transition Throughout the Oman‐UAE Ophiolite, and Implications for the Geodynamic History

“…The presence of subhedral, possible relic, clinopyroxene indicates that amphibole formed in response to a reaction between a hydrous melt and clinopyroxene bearing cumulates (Best, ; Coltorti et al, ; Debari et al, ; Francis, ; Klaver et al, ; Neal, ; Smith, ). Moreover, the observed increase in LREE with decreasing MgO indicates fractional crystallization of a mafic magma, producing tonalitic compositions (Figure a; Brophy, , ; Brophy & Pu, ). Concomitantly, we conclude that the GT‐compositions are a product of the reaction of an ascending hydrous melt with clinopyroxene/olivine bearing cumulates, the latter likely being the Phase 1 crustal succession.…”

Discovery of the Plagiogranites in the Diyanmiao Ophiolite, Southeastern Central Asian Orogenic Belt, Inner Mongolia, China and Its Tectonic Significance

“…6a), which can be interpreted as reflecting magma crystallization resulting in the formation of gabbros with positive Eu anomalies and a residual melt with negative Eu anomalies (Tang et al, 2007). Brophy (2009) and Brophy and Pu (2012) summarized the geochemical characteristics of fractionation-and shearing-type plagiogranites in global examples of ophiolites. Fractionation-type plagiogranites have steadily increasing La and Yb abundances with increasing SiO 2 , with total REE abundances being significantly higher than those in associated mafic rocks (Fig.…”