GEOCHEMISTRY AND PETROGENESIS OF THE OSSIPEE RING COMPLEX: A MODEL FOR MAGMATISM IN THE YOUNGER WHITE MOUNTAIN IGNEOUS PROVINCE

The Ossipee complex, a classic example of a ring-dike structure, is a member of the ca. 120 Ma Younger White Mountain magma series. An almost complete outer ring dike consists of porphyritic quartz syenite and subporphyritic granite. Porphyritic (feldspar phenocrysts) basalts and rhyolites are abundant within the ring structure. The basalts represent the only significant occurrence of mafic volcanics in the White Mountain Province. Alkali granite was emplaced as a sheet under the volcanic pile. Geophysical data indicate that mafic rocks are abundant in the conduit that fed the Ossipee structure.

The basalts are divisible into three groups: nepheline-normative and high-Ti and low-Ti quartz- normative. Evolution of the basalts was controlled by plagioclase fractionation. The rhyolites are divisible into several geochemical groups, and differentiation within each group was controlled by feldspar fractionation. OIB normalized spider diagrams for the basalts are essentially flat, and slightly enriched relative to OIB, except for minor depletions in Sr and Ti and a significant enrichment in Cs (due to late stage hydrothermal alteration of the basalts). Rhyolite spider diagrams are similar in shape but show greater enrichment in most trace elements relative to the basalts, and significant depletion in Ba, Sr and Ti.

Phase equilibria considerations indicate that the nepheline-normative basalts were erupted directly to the surface from deep levels in the crust (or upper mantle) while the quartz-normative basalts and rhyolites evolved at intermediate depths before their eruption. This difference in evolution is reflected in the isotopic systematics which show the influence of crustal contamination (⁸⁷Sr/⁸⁶Sr_i=0.704 to 0.721) in the evolution of the quartz-normative basalts and rhyolites. However, AFC modelling indicates that the total amount of crustal contamination is relatively small.

Comparisons with other plutons in the Younger White Mountains and time correlative Monteregian Hills province of southern Quebec indicate that all these magmas were derived from the same source. The differences in magma composition (from silica-undersaturated to silica-saturated) is due to the degree of melting in the source region and the amount of crustal interaction.