Magmatic processes that generate chemically distinct silicic magmas in NW Costa Rica and the evolution of juvenile continental crust in oceanic arcs

Northwestern Costa Rica is built upon an oceanic plateau that has developed chemical and geophysical characteristics of the upper continental crust. A major factor in converting the oceanic plateau to continental crust is the production, evolution, and emplacement of silicic magmas. In Costa Rica, the Caribbean Large Igneous Province (CLIP) forms the overriding plate in the subduction of the Cocos Plate—a process that has occurred for at least the last 25 my. Igneous rocks in Costa Rica older than about 8 Ma have chemical compositions typical of ocean island basalts and intra-oceanic arcs. In contrast, younger igneous deposits contain abundant silicic rocks, which are significantly enriched in SiO₂, alkalis, and light rare-earth elements and are geochemically similar to the average upper continental crust. Geophysical evidence (high Vp seismic velocities) also indicates a relatively thick (~40 km), addition of evolved igneous rocks to the CLIP. The silicic deposits of NW Costa Rica occur in two major compositional groups: a high-Ti and a low-Ti group with no overlap between the two. The major and trace element characteristics of these groups are consistent with these magmas being derived from liquids that were extracted from crystal mushes—either produced by crystallization or by partial melting of plutons near their solidi. In relative terms, the high-Ti silicic liquids were extracted from a hot, dry crystal mush with low oxygen fugacity, where plagioclase and pyroxene were the dominant phases crystallizing, along with lesser amounts of hornblende. In contrast, the low-Ti silicic liquids were extracted from a cool, wet crystal mush with high oxygen fugacity, where plagioclase and amphibole were the dominant phases crystallizing. The hot-dry-reducing magmas dominate the older sequence, but the youngest sequence contains only magmas from the cold-wet-oxidized group. Silicic volcanic deposits from other oceanic arcs (e.g., Izu-Bonin, Marianas) have chemical characteristics distinctly different from continental crust, whereas the NW Costa Rican silicic deposits have chemical characteristics nearly identical to the upper continental crust. The transition in NW Costa Rica from mafic oceanic arc and intra-oceanic magma to felsic, upper continental crust-type magma is governed by a combination of several important factors that may be absent in other arc settings: (1) thermal maturation of the thick Caribbean plateau, (2) regional or local crustal extension, and (3) establishment of an upper crustal reservoir.     

Subtidal Galeolaria hystrix (Polychaeta: Serpulidae) reefs in Paterson Inlet,Stewart Island,New Zealand

Serpulid patch reefs of Galeolaria hystrix Mörch, 1863 were found in water depths of 9–16 m in Big Glory Bay, Paterson Inlet, Stewart Island, and here we report preliminary studies of these important habitat‐formers. This is the first observation of this species in subtidal patch reefs; 114 reefs were noted in a survey of 28 000 m². Most reefs were 1–5 m in diameter, and up to 1.5 m high. Up to 65% of the serpulid tubes were occupied by living G. hystrix during a mid‐winter diving survey; 64% of reefs observed were whole, whereas 36% were broken or dead. Radiometric dating of a basal specimen of reef carbonate showed it to be less than 50 years old. Production of high‐Mg calcite, ranging from 9 to 11 wt% MgCO3, by G. hystrix may be as much as 11 kg CaCO3 m^‐2y^‐1, but was not reflected in surrounding sediments, which were dominantly terrigenous muds. A rich reef fauna, both sessile and motile, was associated with the reefs. Further study of these unusual temperate reefs is strongly recommended.