Michael J. Dorais

Basement complexes in the Wasatch fault, Utah, provide new limits on crustal accretion

New and reinterpreted isotopic data for crystalline rocks ex- posed in the Wasatch Range require a reevaluation of Precambrian crustal boundaries in Utah. Crystalline rocks of the Santaquin Com- plex underwent metamorphism prior to ca. 1670 Ma, consistent with Sr and Nd isotope data. Mafic to intermediate rocks have major element, trace element, and isotope ratios indicative of derivation in an arc accreted to the Archean craton in Proterozoic time, requiring the crustal suture to be north of the Santaquin Complex. Farther north, the Farmington Canyon Complex has been considered Ar- chean based on published Nd model ages and discordant U/Pb zir- con ages. However, Nd model ages and zircons could be inherited from sedimentary protoliths. U/Pb and electron microprobe ages of monazite have a mode at 1650 to 1700 Ma, concordant with the Santaquin Complex, and lack inheritance. We propose that the Farmington Canyon Complex was first cratonized from Archean- derived sediments in the Proterozoic, requiring a crustal suture to be north of it as well. Accretion ages of arc terranes in southeastern Wyoming are ;60-100 m.y. older than in Utah. Thus, a serious reevaluation of basement architecture in Utah is needed and a pre- viously unrecognized temporal complexity of accretion is indicated.

Identification of a subduction zone component in the Higganum dike, Central Atlantic Magmatic Province: A LA‐ICPMS study of clinopyroxene with implications for flood basalt petrogenesis

Determining the primary compositions of continental flood basalts has proven allusive because of the masking effects of crustal contamination. The Higganum dike (Connecticut, USA) that fed the Talcott basalt of the Central Atlantic Magmatic Province hosts strongly zoned clinopyroxenes. The cores are Mg- and Cr-rich, containing up to 1 wt % Cr2O3. The grains show optical and major, minor, and trace element continua between cores and their mantles and rims, indicating that the cores are early crystallizing phenocrysts, not xenocrysts. Laser ablation inductively coupled plasma mass spectrometry analyses of the cores were used to calculate liquid compositions of the most primitive component in the Higganum system and to make inferences about composition of continental flood basalt parental magmas. The Cr-rich cores yield calculated liquids with Mg numbers of 63. The high-Cr contents of these calculated liquids indicate the absence of significant clinoproxene or chromite fractionation. Olivine or orthopyroxene fractionation may have lowered the liquid Mg number but would not have significantly influenced the calculated incompatible trace element patterns. Extended rare earth element diagrams for the most Cr-rich liquids show enrichment in incompatible elements, similar to arc basalts. Prominent negative Nb anomalies are present as are distinct positive Pb anomalies indicative of the influence of subduction zone fluids in the mantle source. Calculated liquid Ba/La values range from 3 to 30, also suggestive of fluid input. These data indicate that the mantle source experienced subduction zone fluid metasomatism. Subsequent crustal contamination is evident in bulk-rock Nd and Sr isotopic compositions.

P-T-t conditions, Nd and Pb isotopic compositions and detrital zircon geochronology of the Massabesic Gneiss Complex, New Hampshire: isotopic and metamorphic evidence for the identification of Gander basement, central New England

We present new evidence for the assignment of the Neoproterozoic Massabesic Gneiss Complex of New Hampshire to the Gander terrane rather than the Avalon terrane. The majority of Avalonian (sensu stricto) igneous and meta-igneous rocks as defined in Maritime Canada have positive whole-rock ɛNd compared to more negative values for Gander rocks, although there is a region of overlap in ɛNd between the two terranes. Our samples from areas in Connecticut previously thought to be Avalonian and samples from the Willimantic dome have the same isotopic signatures as Maritime Canada Avalon. In contrast, samples from the Clinton dome of southern Connecticut plots exclusively in the Gander field. The majority of the orthogneiss samples from Lyme dome (coastal Connecticut), Pelham dome (central Massachusetts) and Massabesic Gneiss Complex also plot in the Gander field, with a few samples plotting in the overlap zone between Gander and Avalon. U-Pb age distributions of detrital zircon populations from quartzites from the Massabesic Gneiss Complex more closely approximate the data from the Lyme Dome rather than Avalon. Additionally, the similarity of the P-T-t path for the rocks of the Massabesic Gneiss Complex (established by thermobarometry and 40Ar/39Ar dating of amphibole, muscovite, biotite and K-feldspar) with that established in the Ganderian Lyme dome of southern Connecticut strengthens the assignment of these rocks to a single Gander block that docked to Laurentia during the Salinic Orogeny. The identification of Ganderian isotopic signatures for these rocks all of which show evidence for Alleghanian metamorphism, supports the hypothesis that Neoproterozoic Gander lower crustal rocks underlie southern New Hampshire, Massachusetts, and Connecticut, and that all rocks of the overlying Central Maine trough that largely escaped high-grade Alleghanian metamorphism are allochthonous. We suggest that during the Alleghanian, the docking of Gondwana caused Avalon to wedge into Gander, metamorphosing and partially melting the Massabesic Gneiss Complex to the observed P-T-t conditions, with the complex forming an uplifted sheet that was back-thrusted over the Avalonian wedge.

The age, petrogeneis and tectonic significance of the Frontenac Formation basalts, northern New Hampshire and western Maine

Within-plate basalts are present across the Connecticut Valley - Gaspé trough in New England and Canada. Proposed mechanisms for the extensional magmatism include back-arc basin extension, slab breakoff, and extensional collapse. The age and composition of the bimodal Frontenac Formation of northern New England and southern Quebec has bearing on the topic. We analyzed zircons from the meta-felsic rocks of the formation and determined a Silurian age of 432 ± 8 Ma. We also analyzed the Frontenac rocks for major and trace elements and compared their compositions to the other mafic rocks along strike in the Connecticut Valley - Gaspé trough. These include the Comerford Intrusive Suite and the Standing Pond Volcanic member of the Waits River Formation of Vermont, the Silurian Chaleur Group and the Early Devonian Dalhousie Group of New Brunswick and the Gaspé Peninsula, and to the Piscataquis belt rocks of northern Maine. All these rocks are post-Taconic, within-plate, tholeiites that span an age range from 432 Ma (Frontenac Formation) to 419 Ma (Comerford Intrusive Suite). In spite of their similar tectonic setting, subtle differences in trace element compositions are evident, indicating a multiplicity of sources in the production of the Siluro-Devonian magmas. Ce/Yb versus La/Ta values suggest that the Frontenac magmas were derived from spinel peridotite sources in the asthenospheric mantle. We suggest that slab rollback and back-arc basin extension permitted asthenospheric mantle upwelling to the spinel peridotite stability field, causing the Frontenac partial melting event at depths as low as 35 km. Sediments of the Frontenac Formation were deposited as turbidite successions into a developing trough resulting from extension. The coeval influx of sediment and intrusion of basalt produced a sediment-sill complex because the upward migration of the basaltic magmas was hindered by the rapidly thickening sediment overburden. Subsequently, slab breakoff may have opened an asthenospheric window, generating the younger within-plate magmas that occurred from 425 to 418 Ma across the Connecticut Valley – Gaspé trough.

Exploring the mineralogical heterogeneities of the Louisville Seamount Trail

Diopside phenocrysts of the Louisville Seamount Trail show an increase in Ti, Al, and Na with decreasing Mg/(Mg + Fe) as is typical for clinopyroxene in alkalic basalts. Chondrite-normalized REE patterns of calculated liquids from LA-ICPMS analyses are comparable to whole-rock and glass values. Exceptions are clinopyroxene crystals from the Rigil Seamount, the second oldest seamount drilled at the northern end of the chain. Some crystals from this site are strongly zoned with distinct compositional boundaries between cores and mantles. The cores have high Mg/(Mg + Fe) and low Al and Ti concentrations compared to the mantles and phenocrysts. Major element, clinopyroxene discrimination diagrams indicate that the clinopyroxene mantles and phenocrysts crystallized from alkalic basalts. In contrast, the Mg-rich cores have tholeiitic affinities. The REE abundances of the cores are similar to that of clinopyroxene from transitional tholeiites of the Kerguelen Archipelago. Calculated liquid La/Yb values for the cores have ratios that are similar to transitional tholeiites in Hawaii, whereas the mantles have higher La/Yb values similar to Hawaiian alkalic basalts. The major and trace element compositions of clinopyroxene cores from the Rigil seamount suggest that a transitional tholeiitic magma was present, but no evidence for Hawaiian shield-type tholeiites was found. Plagioclase crystals from the Rigil seamount have 86Sr/86Sr from 0.70306 to 0.70363, within the range of FOZO. The transitional tholeiitic signature of the Mg-rich clinopyroxene cores probably did not have a distinct source compared to other Louisville magmas, but more likely indicates a higher degree of partial melting (2–5%) of that FOZO source.

A Late Devonian Magmatic Link between Rhode Island and Nova Scotia

Several Late Devonian plutons across southeastern New Hampshire, coastal Maine, and north-central Massachusetts have within-plate compositions distinct from the 400–410 Ma calc-alkaline plutons related to the Acadian Orogeny in the same region. The ages of these plutons range from 380 to 360 Ma. The mafic portions of these plutons are compositionally similar to within-plate tholeiites of the same age in Maritime Canada, the Narragansett Basin in Rhode Island, and the Coastal Maine Magmatic Belt. Coastal New England and Maritime Canada experienced extensional magmatism resulting from oblique convergence of Meguma accreting to Laurentia during the Neoacadian Orogeny. A transtensional zone along dextral transcurrent faults allowed the formation of the Maritimes Basin of Nova Scotia and New Brunswick and the eruption of the 373±4 Ma Fisset Brook Formation into the basin, as well as the eruption of similar-aged rocks in the Narragansett Basin. The petrogenesis of Late Devonian peraluminous and metaluminous granitic plutons associated with these mafic rocks across central/coastal New England is problematic because the compositions of the mafic plutons and lavas from Maritime Canada to the Narragansett Basin indicate that the region was undergoing extension during this time period, yet the felsic rocks lack the within-plate compositions of granitic rocks of this age in central New England. The association of granitic rocks and within-plate mafic rocks as composite dikes in the Deer Isle Pluton, quenched dioritic enclaves in the granitic phase of the Appledore Island Diorite, and the coexistence of mafic-felsic magmas of the Hardwick Pluton indicates that the two magma types were coeval. The plutonic rocks of this study mark the region of transpression between these basins, and ponding of these within-plate mafic magmas may have provided the heat flux for partial melting of the crust and may account for the variety of compositions of granitic magmas emplaced across New England at this time.

Origin of the fluorine- and beryllium-rich rhyolites of the Spor Mountain Formation, Western Utah

Abstract The Miocene rhyolites of the Spor Mountain Formation host Earth’s largest beryllium deposit, which produced 85% of the world’s beryllium in 2010. The fresh lava is extremely enriched in Be (up to 75 ppm in matrix glass). We have examined the rhyolite to better understand the Be enrichment. The Spor Mountain rhyolite contains ~40% quartz, ~40% sanidine, ~10% biotite, and ~10% plagioclase, along with accessory fluorite, columbite, euxenite, fergusonite, monazite, thorite, and zircon. Two types of rhyolite are present within the Spor Mountain Formation, a less-evolved magma (1150 ppm Rb, 42 ppm Be, 0.7 wt% F in glass) and a more-evolved magma (1710 ppm Rb, 75 ppm Be, 1.6 wt% F in glass). Eruption temperatures estimated using two-feldspar (Elkins and Grove 1990; Putirka 2008; Benisek et al. 2010), plagioclase-liquid and alkali feldspar-liquid (Putirka 2008), Ti-in-quartz (Thomas et al. 2010, 2015; Huang and Audétat 2012), biotite (Righter and Carmichael 1996), and zircon saturation (Boehnke et al. 2013) geothermometers converge on 718 °C for the less-evolved magma and 682 °C for the more-evolved magma. Using the Ti-in-Qz equation of Huang and Audétat (2012), the pre-eruptive pressure of the Spor Mountain rhyolite system is estimated to be about 2 kbar at 700 °C. Water content of the rhyolite melt was less than <5 wt%, based on the co-crystallization of all four major mineral phases at 700 °C, and the magma was water undersaturated (Webster et al. 1987). Viscosity of the rhyolite was about 106.2 Pa·s for the less-evolved rhyolite and 105.8 Pa·s for the more-evolved rhyolite. Fluorine lowered the melt viscosity, though not by a large amount (less than 0.5 log units at 1.6 wt% F). Partition coefficients for Be and other trace elements were determined for biotite, sanidine, plagioclase, and quartz from laser ablation-inductively coupled plasma-mass spectrometry analyses. Partition coefficients for trace elements in feldspars from the Spor Mountain rhyolite are generally higher than for feldspars from other silicic magmas and lower for biotite. The enrichment of beryllium in the Spor Mountain rhyolite was aided by its high incompatibility in the major mineral phases, with a bulk partition coefficient <0.1. Trace element models using the measured partition coefficients are inconsistent with accumulation of increments of melt formed by different degrees of partial melting and cannot explain the great depletion of compatible elements. Rather, the trace element abundances and Nd and Sr isotopic compositions are consistent with derivation of rhyolite by ~25% partial melting of crust hybridized with mantle-derived components, followed by extensive fractional crystallization (75%). The combination of these magmatic processes set the stage for the formation of a world-class beryllium deposit.