Geochemistry of Early Devonian calc-alkaline plutons in the Merrimack Belt: implications for mid-Paleozoic terrane relationships in the New England Appalachians

A series of northeast-trending plutons extending from northeastern Massachusetts to southeastern Maine intruded the metasedimenlary rocks of the Merrimack belt. The Early Devonian Dracut, Sweepstakes, Island Pond. Exeter, and Webhannet plutons are metaluminous and, with the exception of the granitie Webhannet pluton, are dominantly mafic to intermediate in composition. The plutons are calc-alkaline in character and have major, minor, and trace clement compositions typical of magmas generated at destructive plate margins. These characteristics include mid- to high-K contents, enrichment of LII.E, LREE, Ba. and Sr. and negative Nb and Ta anomalies. Whole-rock chemical data indicate that the plutons of the Merrimack belt arc similar in every measured geochemical parameter to ca. 400 Ma mafic to intermediate rocks of the New Hampshire Plutonic Suite. These similarities suggest that both groups of plutons were emplaced within the same magmatic arc and belong to the same magmatic suite. A magmatic suite common to both the Merrimack belt and Central Maine terrane suggests that the two lithotectonic zones were proximal to each other at ca. 400 Ma. Trace element differences between Merrimack belt and Sharpners Pond rocks suggest (hat the Putnam-Nashoba terrane represents a separate arc. Located in the Central Maine terrane of New Hampshire, the Rochester pluton is geochemically distinct from the Siluro-Devonian plutons of the Merrimack belt and New Hampshire Plutonic Suite. The Rochester pluton has alkaline affinities but retains overall calc-alkaline features. High concentrations of incompatible elements (K, Ti, P, Ba, Rb, Zr) in the Rochester pluton are markedly similar to those observed in the 360 Ma Hardwick Tonalite of Massachusetts. The strong geochemical correlation between the Rochester and Hardwick plutons implies derivation from the same magmatic event and a common, Late Devonian origin. We suggest that these plutons may have originated in response to pull-apart rifting related to late-stage Acadian transpression. RÉSUMÉ Une série de plutons orientée vers Ie nord-est s'élendant du nord-est du Massachusetts au sud-est du Maine, fait intrusion dans les roches métasédimentaires de la ceinture de Merrimack. Les plutons du Devonien inférieur Dracut Sweepstakes. Island Pond. Exeter et Webhannet constituent des intrusions métalumincuses et, mis à part le pluton granitique Webhannet, ils ont une composition en prédominance mafique à intermédiare. Les plutons ont une conformation calco-alcalinc et leur composition du point de vue des principaux éléments présents, de ceux présents en quantité restreinie et des éléments traces est représentative des magmas produits aux frontièrs de plaques destructives. Leurs caractéristiques component nolamment une teneur movenne à élevée en potassium, un enrichissement en LILEen éléments de terres rares légères. en BA et en SR. de mème que des anomalies négatives de Nb et Ta. Les données de la roche totale révèlent que les plutons de la ceinture de Merrimack sont semblables. sous le rapport de chacun des paramètres géochimiques mesurés. aux magmas mafiques à intermédiaires d'il y a environ 400 Ma du cortège plutonique du New Hampshire. Ces similarités permeltent de supposer que les deux groupes de plutons ont été insérès à l’intérieur du mème arc magmatique et qu'ils appartienment au mème cortège magmatique. L'existence d'un cortège magmatique commun à la ceinture de Merrimack et au terrane de Central Maine permet de supposer que les deux zones lithotectoniques étaient proximales I'une de 1'autre il y a environ 400 Ma. Les différences par rapport aux éléments traces entre les magmas de la ceinture de Merrimack et ccux de Sharpners Pond laissent supposer que le terrane de Putnam-Nashoba rcprisenie un arc distinct Le pluton Rochester, situé dans le terrane de Central Maine du New Hampshire, est géochimiquement distinct des plutons siluro-dévoniens de la ceinture de Merrimack et du cortège plutonique du New Hampshire. Le pluton Rochester présente des affinités alcalines tout en conservant ses carectéristiques calco-alcalines générales. Les concentrations élevées d'éléments incompatibles (K, Ti, P, Ba, Rb, Zr) dans le pluton Rochester sont manifestement semblables à celles observers dans la tonalite d'il y a 360 Ma de Hardwick, au Massachusetts. La correlation géochimique prononcée entre les plutons Rochester et Hardwick suppose qu'ils découlent du mème phénoméne magmatique et qu'ils ont une origine commune remontant au Dévonien supérieur. Nous pensons que ces plutons tirent probablement leur origine d'une réaction a une distension d'écartemcnt apparentée à une transpression acadienne tardive. Traduit par la rédactio

Insights into the Acadian orogeny, New England Appalachians: a provenance study of the Carrabassett and Kittery formations, Maine

The Central Maine Basin and Merrimack Trough are Silurian basins that formed adjacent to or were accreted to the Laurentian margin during the Acadian orogeny. The Early Devonian Carrabassett Formation of the Central Maine Basin and the Kittery Formation of the Merrimack Trough have major and trace element compositions indicative of a passive continental margin provenance, not unlike the older formations of the Central Maine Basin that are thought to have been derived from Laurentian sources. However, both the Carrabassett and Kittery formations have paleocurrent indicators of outboard sources. The Carrabassett Formation is one of the youngest formations of the Central Maine Basin and was deposited just prior to the Acadian orogeny. The Carrabassett and Kittery formations have major and trace element concentrations suggestive of passive margin turbidites derived from intermediate to felsic sources, inconsistent with a juvenile Avalonian provenance. The Carrabassett Formation contains detrital zircon grains that match the ages of peri-Gondwanan Ganderia. Unlike the dominance of positive bulk-rock εNd values that are characteristic of Avalonia, Ganderia has negative εNd values that are a better match for the negative εNd values of the Carrabassett and Kittery formations. However, Ganderia accreted to Laurentia during the Salinic orogeny, prior to the deposition of the Carrabassett Formation, and was basement to the sediments of the Central Maine Basin upon which the Carrabassett and other formations were deposited. Wedging of Ganderia by Avalonia during the initial stages of the Acadian orogeny may have uplifted Ganderia, forming highlands outboard of the Central Maine Basin that served as the source of the Carrabassett Formation sediments. RÉSUMÉ Le bassin central du Maine et la cuvette de Merrimack constituent des bassins siluriens s’étant formés le long de la marge laurentienne ou s’y étant accrétés au cours de l’orogenèse acadienne. La Formation du Dévonien précoce de Carrabassett, dans le bassin central du Maine, et la Formation de Kittery, de la cuvette de Merrimack, présentent des compositions en éléments majeurs et traces signalant une provenance d’une marge continentale passive, à l’instar des formations plus âgées du bassin central du Maine qu’on pense originaires de sources laurentiennes. Les formations de Carrabassett et de Kittery comportent toutefois des indicateurs de paléocourants de sources extérieures. La Formation de Carrabassett constitue l’une des formations les plus récentes du bassin central du Maine; elle s’est mise en place juste avant l’orogenèse acadienne. Les caractéristiques géochimiques et géochronologique des formations de Carrabassett et de Kittery pourraient par conséquent permettre l’identification du terrane de collision. Les formations de Carrabassett et de Kittery possèdent des concentrations d’éléments majeurs et traces évoquant les turbidites de marge passive en provenance de sources intermédiaires à felsiques, ce qui est contradictoire avec une origine avalonienne juvénile. La Formation de Carrabassett comporte des grains détritiques de zircon correspondant aux âges du Ganderia périgondwanien. Contrairement à la prédominance de concentrations εNd positives de roche en vrac caractéristiques d’Avalonia, Ganderia présentent des concentrations εNd négatives qui cadrent mieux avec les concentrations εNd négatives des formations de Carrabassett et de Kittery. Ganderia s’est toutefois accrété à Laurentia au cours de l’orogenèse salinique, avant le dépôt de la Formation de Carrabassett, et il a constitué le socle des sédiments du bassin central du Maine sur lesquels Carrabassett et d’autres formations se sont déposées. L’enfoncement d’Avalonia sous Ganderia au cours des stades initiaux de l’orogenèse acadienne pourrait avoir soulevé Ganderia, formant un massif à l’extérieur du bassin central du Maine qui a servi de source aux sédiments de la Formation de Carrabassett. [Traduit par la redaction

How phyllosilicate mineral structure affects fault strength in Mg-rich fault systems

The clay mineralogy of fault gouges has important implications for the frictional properties of faults, often identified as a major factor contributing to profound fault weakness. This work compares the frictional strength of a group of Mg‐rich minerals common in the Mg‐Al‐Si‐O compositional space (talc, saponite, sepiolite, and palygorskite) by conducting triaxial frictional tests with water or argon as pore fluid. The studied minerals are chemically similar but differ in their crystallographic structure. Results show that fibrous Mg‐rich phyllosilicates are stronger than their planar equivalents. Frictional strength in this group of minerals is highly influenced by strength of the atomic bonds, continuity of water layers within the crystals, and interactions of mineral surfaces with water molecules, all of which are dictated by crystal structure. The formation and stability of the minerals studied are mainly controlled by small changes in pore fluid chemistry, which can lead to significant differences in fault strength

Primitive layered gabbros from fast-spreading lower oceanic crust

Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks-in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas-provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt

Fault weakening across the frictional-viscous transition zone, Karakoram Fault Zone, NW Himalaya

Exhumed fault rocks formed in the frictional-viscous transition zone (FVTZ) provide test material that can be used to assess the strength of natural fault zones. In the Karakoram Fault Zone (KFZ), such rocks contain evidence of several long-term weakening mechanisms associated with reduced coefficients of friction (<0.4). The Nubra, Tangtse, and Arganglas strands of the KFZ are focused along metavolcano-sedimentary formations indicating weakness relative to the bounding granitoids. Synkinematic retrogression suggests that reaction softening has weakened the margins of granitoids along the Nubra and Tangtse strands and the Nubra Formation within the Nubra strand. The resultant phyllosilicates have formed well-developed interconnected weak layers within phyllonites and granitic mylonites. Micaceous foliae with increased proportions of opaque minerals in granitic mylonites suggest that fluid-assisted diffusive mass transfer aided deformation within the Nubra and Tangtse strands. Microstructures within Nubra strand phyllonites suggest that frictional-viscous flow accommodated deformation at low shear stresses in the FVTZ. Multiple generations of veining within each strand indicate overpressured pore fluids within the fault zone across a range of depths. Active springs and travertines along the fault indicate ongoing suprahydrostatic fluid flow within the KFZ. Despite such evidence for weakening mechanisms, the KFZ is currently locked and most likely generates moment magnitude 7.5+ earthquakes. Evidence for multiple fault weakening mechanisms reduces potential for shear heating within the KFZ and suggests that the long-term strength of the lithosphere must reside below the depth of penetration of the fault

Beneath the bottom line: agricultural approaches to reduce agrichemical contamination of groundwater

This report descriptively reviews findings from recent sample surveys of farmer attitudes about agricultural chemicals and groundwater quality; their practices and motivations related to chemical management; and their responses to public policy alternatives addressing this issue