Victoria Island, in Arctic Canada, is one of the largest islands in the world, but its geology has remained largely unmapped and unstudied owing to its remoteness. Base-metal and hydrocarbon showings have been reported from the island, but the origin and prospectivity of these showings remain enigmatic. Regionally extensive, void-filling Phanerozoic diagenetic phases (dolomite, calcite, quartz) are conspicuous in two of the most widespread carbonate units, the Neoproterozoic Wynniatt Formation and the Cambro–Ordovician Victoria Island formation, recording repeated post-depositional movement of fluid through the rocks. These phases were studied to (a) determine whether they could have been associated with movement of metalliferous or petroliferous fluids, (b) establish a diagenetic base-line for a very large and economically unexplored area, and (c) determine whether the fluid-flow events could have been related to known episodes of deformation and mineralisation elsewhere in the Canadian Arctic archipelago. Using an innovative protocol combining in situ SIMS analysis of O and S isotopes with LA-ICP-MS trace-element analysis, the geochemical conditions attending the diagenetic evolution of host dolostone and cement phases were determined. The Wynniatt Formation dolostone contains four Phanerozoic cements: saddle dolomite, brown dolomite, replacive calcite and late calcite. Average δ18O (SMOW) values of Wynniatt Formation saddle and brown dolomite, replacive calcite, and late calcite cements are 24.7‰, 7.7‰, and 6.9‰, respectively. The PAAS-normalised rare earth element patterns of the dolostone, dolomite, and replacive calcite are smooth and flat with slightly positive Ce and Y anomalies and MREE-enrichment; late calcite cement, in contrast, has negative Ce and positive Y anomalies. Recrystallisation of the host dolostone by a reduced, saddle-dolomite-precipitating fluid in a fluid-dominated system altered the isotopic and REE pattern of the dolostone to saddle-dolomite values. This fluid had interacted with shale at depth and then mixed with a relatively high-salinity fluid. Brown dolomite precipitated from this fluid after a decrease in salinity, and oxygenated meteoric water later infiltrated the system and precipitated the calcite cements.The Victoria Island formation dolostone contains two cements: quartz and dolomite. Host dolostone, quartz, and dolomite have average δ18O (SMOW) values of 31.7‰, 18.7‰, and 18.6‰, respectively, whereas δ34S (V-CDT) for framboidal pyrite in the quartz cement averages −7.5‰. The host dolostone has flat PAAS-normalised REE patterns with a positive Eu anomaly, no Ce anomaly, and a zig-zag HREE pattern, whereas the dolomite cements have two REE patterns: one similar to the dolostone (flat) and the other with LREE-enrichment, and negative La and Eu anomalies. These data suggest that a reduced, hydrothermal fluid altered and partially silicified the host dolostone. Quartz precipitated from a seawater-sourced fluid that had incorporated metals at depth and mixed with bacterially reduced sulphur at the site of precipitation. The dolomite cement records the change from a rock-dominant system to a fluid-dominant system. This fluid was sourced from seawater and interacted with REE-phosphate minerals.The results suggest that precipitation of the diagenetic cements took place before peak burial associated with the Ellesmerian Orogeny (late Devonian–early Carboniferous). The most probable cause of fluid migration, therefore, is the Ellesmerian Orogeny for the majority of the diagenetic phases. High-latitude meteoric fluid then infiltrated the system at some time after the Ellesmerian Orogeny. Similar ages, temperatures, compositions, and precipitation mechanisms as those associated with the Cornwallis zinc district (Polaris mine), highlight a possible relation to known base-metal mineralising fluids.The results of this study provide much needed baseline knowledge of the post-depositional history of Victoria Island. It also illustrates that a “less-is-more” approach, using detailed micro-analytical methods on carefully selected and well-constrained samples, allows one to decipher a complex diagenetic history involving the mixing of fluids from different reservoirs and frequent changes in redox state, both of which may be overlooked using conventional macro-scale approaches.

Monazite crystallization ages have been measured in situ using SIMS and EMP analysis of samples from the Bronson Hill anticlinorium in central New England. In west-central New Hampshire, each major tectonic unit (nappe) displays a distinctive P-T path and metamorphic history that requires significant post-metamorphic faulting to place them in their current juxtaposition, and monazite ages were determined to constrain the timing of metamorphism and nappe assembly. Monazite ages from the low-pressure, high-temperature Fall Mountain nappe range from c. 455 to 355 Ma, and Y zoning indicates that these ages comprise three to four distinct age domains, similar to that found in the overlying Chesham Pond nappe. The underlying Skitchewaug nappe contains monazite ages that range from c. 417 to 307 Ma. Ar-40/Ar-39 ages indicate rapid cooling of the Chesham Pond and Fall Mountain nappes after 350 Ma, which is believed to represent the time of emplacement of the high-level Chesham Pond and Fall Mountain nappes onto rocks of the underlying Skitchewaug nappe. Garnet zone rocks from western New Hampshire contain monazite that display a range of ages (c. 430-340 Ma). Both the metamorphic style and monazite ages suggest that the low-grade belt in western New Hampshire is continuous with the Vermont sequence to the west. Rocks of the Big Staurolite nappe in western New Hampshire contain monazite that crystallized between c. 370 and 290 Ma and the same unit along strike in northern New Hampshire and central Connecticut records ages of c. 257-300 Ma. Conspicuously absent from this nappe are the older age populations that are found in both the overlying nappes and underlying garnet zone rocks. These monazite ages confirm that the metamorphism observed in the Big Staurolite nappe occurred significantly later than that in the units structurally above and below. These data support the hypothesis that the Big Staurolite nappe represents a major tectonic boundary, along which rocks of the New Hampshire metamorphic series were juxtaposed against rocks of the Vermont series during the Alleghanian.

Four distinct generations of monazite growth have been identified in samples from the Chesham Pond Nappe, and three (monazite compositional domains 2, 3, and 4) have been correlated with both temperature and mineral assemblage. Domain I cores were interpreted previously to be detrital relics or vestiges of an earlier Acadian metamorphism. The four monazite domains have been dated by in situ isotope and chemical methods; the following are chemical ages of each domain (weighted average 2 standard errors of the mean): 400 +/- 10 Ma (domain 1); 381 +/- 8 Ma (domain 2); 372 +/- 6 Ma (domain 3); 352 +/- 14 Ma (domain 4). Heating and cooling rates derived from combining monazite ages, monazite thermometry, and Ar-40/Ar-39 closure temperatures are approximately 10-15 degrees C/m.y. for heating from 470 to 740 degrees C, approximately 8 degrees C/m.y. for cooling from 740 to 375 degrees C, and approximately 1-2 degrees C/m.y for cooling from 375 to 150 degrees C. Temperature-time paths calculated with monazite ages and monazite thermometry indicate that (1) plutonism at ca. 400 Ma was the likely heat source for the formation of monazite domain I and (2) monazite domains 2-4 were produced during a regional low-pressure, high-temperature metamorphism active between 380-350 Ma. The regional metamorphism is ascribed to lithospheric mantle delamination, followed by asthenospheric mantle upwelling, which heated a wide area of the Merrimack basin (southwestern New Hampshire, central Massachusetts, central Connecticut) to temperatures in excess of 725 degrees C. Monazite ages in the Chesham Pond Nappe and adjacent structural units to the west constrain the commencement of nappe overthrusting to roughly 355 Ma.