Petrology and tectonic significance of the Chickwolnepy Intrusions, Coos County, New Hampshire.

摘要

The Chickwolnepy Intrusions of northeastern New Hampshire are some of oldest rocks within the Bronson Hill arc and consist of sheeted basalt and diabase dikes, gabbro and tonalite. They intruded the Cambrian (?) to Middle Ordovician Dead River Formation and basalts of the Middle Ordovician Ammonoosuc Volcanics during Middle Ordovician time. This research used field mapping, petrography, and chemical data to decipher the history and tectonic significance of the intrusions.;The presence of sheeted dikes indicates lithospheric extension was occurring during intrusion; therefore, the early evolution of the Bronson Hill arc involved, at least locally, extensional tectonics. The geochemistry of the mafic rocks indicates the magmas were generated in a supra-subduction-zone position. Magma ascended from the mantle and filled an axial magma chamber which then fed magma into the dike complexes. Cross-cutting relationships and geochemical variations indicate this was a dynamic magma system that experienced recurrent replenishment and fractional crystallization. Many dikes reached the surface to form lava flows of the Ammonoosuc Volcanics, as evidenced by the very similar chemical composition of the dikes and volcanics in the area. Two different parental mafic magmas were fed into this system, the younger of which is represented by dikes with very low concentrations of incompatible trace elements. Rocks with this chemical signature are restricted to the central zone of the intrusions and mark the location of a spreading axis and axial magma chamber. The heat from repeated intrusion of mafic magma likely caused partial melting of hydrated basaltic rocks to create local intrusions of trondhjemite, and perhaps melting of adjacent metasedimentary rocks to create granitic magma which was emplaced as the adjacent Cambridge Black pluton. The last magmatism within the Chickwolnepy was the intrusion of two large tonalite bodies, one of which has been dated at 467 +/- 4 Ma (Aleinikoff and Moench, 1992). This date marks the end of magmatism and extension in the Chickwolnepy. The extensional tectonic stress was likely the result of some combination of oblique plate convergence along an uneven plate margin and/or changes in plate motions such as the initiation of decoupling of oceanic lithosphere from continental lithosphere.