The Silurian Vinalhaven intrusive complex is about 12 km in diameter and consists mainly of cg granite, a thick section of inward-dipping gabbro-diorite sheets in the SE half of the complex, and a core of fg granite. The lowest exposed part of the complex occurs on its S margin. Along its NW margin, granite intrudes the older, cogenetic Vinalhaven rhyolite, which consists of effusive and pyroclastic units of largely high-silica rhyolite. It is probable that plutonic rocks coeval with the exposed rhyolite units occur at depth and younger rhyolite units, coeval with the plutonic complex, were eroded. The complex was fed by multiple replenishments of isotopically distinct basaltic and granitic magmas. Basaltic replenishments typically produced gabbroic sheets that ponded on granitic crystal mush at the base of a silicic chamber. Where basalt encountered only crystal-rich granitic magma, it locally remelted and mixed with granite, producing bodies of porphyry with corroded phenocrysts and mafic enclaves. CL and Ti zoning in quartz phenocrysts records corrosion followed by growth of high T rims (based on Ti-in-qtz thermometer of Wark and Watson, 2006). Fg granitic dikes and the fg granitic core of the complex represent silicic replenishments. Contact relationships between these dikes and the surrounding granite provide insight into the rheology of the granite at the time of intrusion. Where the resident granitic magma was crystal rich, aphyric magma in the dikes intimately mixed and commingled with the resident cg granite mush. Convective mixing was important in the silicic magma chambers. Where mafic sheets are present in granite, upward gradations through hybrid rocks back into granite and the occurrence of mafic enclaves in granite far above the mafic sheets demonstrate mixing within overlying silicic magma. Complex CL and Ti zoning in the cores of granitic quartz far from the mafic input also records thermal perturbations caused by the mafic input. The oldest plutonic rocks consist of cg granite in the west and interlayered granite and mafic sheets associated with large blocks of country rock in the east. In the west, schlieren structures in cg granite indicate crystal accumulation on a chamber floor of transitional rheology. In the east, granitic pipes and other outcrop-scale features indicate that the mafic layers ponded on granitic crystal mush. At higher levels, mafic sheets and country rock blocks gradually become restricted to the eastern third of the complex and are entirely absent in the upper half of the intrusion. Within cg granite the wide occurrence of schlieren related to magma flow and sinking enclaves indicate continued accumulation on a chamber floor. Later injections of basaltic magmas rejuvenated pockets of mostly crystallized granite in the upper levels of homogeneous granite and suggest that crystallization of the granite also proceeded inward from the sides and roof of the chamber. The complex preserves a stratigraphic record of magma chamber evolution and pluton growth by crystal accumulation on magma chamber floors that was interrupted by episodes of replenishment and rejuvenation. This field-based interpretation is corroborated by high-precision U-Pb zircon ages of granitic rocks distributed throughout the complex that indicate the complex was constructed over a nominal time-span of about 0.7 m.y. (Hawkins and Wiebe, this volume).
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 1036 Magma chamber processes (3618);
- 3640 Igneous petrology;
- 3690 Field relationships (1090;