PART I. STRATIGRAPHY, STRUCTURE, AND PETROLOGY OF THE PETERBOROUGH 15-MINUTE QUADRANGLE, NEW HAMPSHIRE AND PART II. GRAPHITE TEXTURAL AND ISOTOPIC VARIATIONS IN PLUTONIC ROCKS, SOUTH-CENTRAL NEW HAMPSHIRE (GRAPHITE VEINS, FLUID COMPOSITIONS).

摘要

Part I. The stratigraphy of the Merrimack Synclinorium in the Peterborough quadrangle is reinterpreted and correlated with fossil-dated Silurian-Lower Devonian strata of western Maine. The earliest phase of the Acadian Orogeny produced west-directed fold-thrust nappes in this area. The Kinsman Quartz Monzonite was intruded along axial surfaces of these structures, locally establishing peak metamorphic conditions in the wall rocks. Intrusion of the Kinsman was closely followed by mafic intrusions of the Spaulding Quartz Diorite possibly accompanying west-northwest F(,2) folding, and peak metamorphic conditions were reached in adjacent rocks. Partial melting of pelitic rocks at high P(,H20) produced migmatites and Spaulding granite sheets. These were all refolded by north-northeast F(,3) folds. Post-Acadian left-lateral and normal faulting occurred, possibly reactivating early thrust faults (now overturned).;The range of (delta)('13)C values determined for flake graphite in the igneous rocks (-26.46 to -13.84('o)/oo) overlaps the range for flake graphite in the wall rocks (-25.98 to -16.7('o)/oo), and spatial correlation of (delta)('13)C values in the plutons and wall rocks support an assimilation origin for flake graphite in the plutons. Coexisting coarse flake and fine spherulitic graphite were separated and analyzed in five samples; spherulitic graphite shows significant differences of +1.91 to -1.39('o)/oo relative to flake graphite in both igneous and metasedimentary samples. The observed isotopic shifts and the association of spherulitic graphite with hydrous silicates are explained by a simple fluid-dehydration model involving fluids in the system C-O-H.;Epigenetic graphite in two vein deposits associated with the contacts of these igneous rocks are generally enriched in ('13)C relative to both the igneous and wall rock (delta)('13)C values measuring -15.86 to -11.6('o)/oo.;Part II. Graphite occurs in two distinct textural varieties in syntectonic granitoids of the New Hampshire Plutonic Series and in associated metasedimentary wall rocks. Textural characteristics indicate that coarse graphite flakes (0.1-1.0mm) were present at an early stage of crystallization of the igneous rocks and may represent xenocrystic material assimilated from the wall rocks. Fine-grained irregular aggregates or spherulites of graphite (0.01-0.1mm) formed along with secondary hydrous silicates and carbonates during retrograde reactions between the primary silicates and a C-O-H fluid phase.