Biotite and amphibole containing up to 4.6 and 5.1 wt% Cl, respectively, are found in gneisses from Black Rock Forest, located within the Hudson Highlands of New York. The area was metamorphosed to granulite facies during the Grenvillian orogeny at similar to 1100 Ma. Small, subeconomic magnetite deposits coexist with the gneisses.Cl contents in biotite range from 0.1 to 4.6 wt%. Some individual biotite grains are spectacularly zoned, with Cl contents varying from 1.0 to 4.6 wt% within <20 mu m. In zoned biotite, an increase in Cl content correlates with increasing Fe and decreasing Mg, Ti, and F. When data from five specimens are compared, however, Cl contents in biotite show only two strong correlations: Cl contents correlate positively with Fe (r(2) = 0.75) and negatively with Ti (r(2) = 0.84). Cl and Mg are uncorrelated for the complete dataset but show strong correlation within individually zoned grains (r(2) = 0.74). F contents show only one correlation, a positive one with Mg (r(2) = 0.62). The structural formula for the most Cl-rich biotite analyzed is (K2.00Na0.02)(Mg1.94Fe3.90Ti0.07Al0.04) (Si5.85Al2.15)O-20(OH2.46F0.25Cl1.29), as normalized to 24 (O + OH + F + Cl). This biotite is one of the most Cl rich ever reported.Cl contents in amphibole range from 0.2 to 5.1 wt%. Cl contents correlate positively with Fe2+/(Fe2+ + Mg) (r(2) = 0.84) and K (r(2) = 0.70) and negatively with Si/(Si + Al-[4]) (r(2) = 0.49) and Ti (r(2) = 0.76). F contents are uncorrelated with any of the other elements analyzed. The structural formula for the most Cl-rich amphibole, a hastingsite, is (K0.57Na0.31)(Ca1.90Na0.10)(Fe2.692+Mg0.90Mn0.03Ti0.06Fe1.023+Al0.29)(Si5.80Al2.20)O-22(OH0.55F0.07Cl1.38), as normalized to 13 octahedral and tetrahedral cations.In rocks with coexisting amphibole and biotite, Cl partitions preferentially into the amphibole and F partitions preferentially into the biotite. Infiltration of the gneisses by fluids rich in Fe and Cl (perhaps as an FeCl2 compound) could explain the formation of magnetite bodies as well as the Fe- and Cl-enriched phases found in and around Black Rock Forest.

In southern New Brunswick the Kennebecasis Fault follows the northern boundary of a crystalline portion of the late Precambrian – Cambrian Avalon terrane. This low-grade crystalline complex forms the basement to a series of Carboniferous through Triassic basins. This complex also contains a major shear zone relic that is largely flat lying but upturned adjacent to the fault, and that with it defines the Pocologan–Kennebecasis fault zone. The orientation of the main composite foliation (S1) and the included mineral (stretching) lineation (L1) indicate that this geometry is a primary feature of the shear zone, representing a linked pair of horizontal and vertical detachments bounding an allochthonous unit. Kinematic indicators show that this allochthon moved parallel to the strike of the north Appalachian orogen, with top towards the west or west-southwest. The bounding shear zone is not uniform, but consists of a mylonite–phyllonite adjacent to the detachment. The upturned segment of the shear zone has been the site of later, brittle reactivation, one episode of which is represented by the Kennebecasis Fault. The main shear zone relic relates to more fundamental events, such as the accretion of the Avalon terrane.

The Siluro-Devonian Waits River Formation of north-east Vermont was deformed, intruded by plutons and regionally metamorphosed during the Devonian Acadian Orogeny. Five metamorphic zones were mapped based on the mineralogy of carbonate rocks. From low to high grade, these are: (1) ankerite-albite, (2) ankerite-oligoclase, (3) biotite, (4) amphibole and (5) diopside zones. Pressure was near 4.5 kbar and temperature varied from c. 450-degrees-C in the ankerite-albite zone to c. 525-degrees-C in the diopside zone. Fluid composition for all metamorphic zones was estimated from mineral equilibria. Average calculated X(CO2)[= CO2/(CO2 + H2O)] of fluid in equilibrium with the marls increases with increasing grade from 0.05 in the ankerite-oligoclase zone. to 0.25 in the biotite zone and to 0.44 in the amphibole zone. In the diopside zone, X(CO2) decreases to 0.06.Model prograde metamorphic reactions were derived from measured modes, mineral chemistry, and whole-rock chemistry. Prograde reactions involved decarbonation with an evolved volatile mixture of X(CO2) > 0.50. The X(CO2) of fluid in equilibrium with rocks from all zones, however, was generally <0.40. This difference attests to the infiltration of a reactive H2O-rich fluid during metamorphism. Metamorphosed carbonate rocks from the formation suggests that both heat flow and pervasive infiltration of a reactive H2O-rich fluid drove mineral reactions during metamorphism. Average time-integrated volume fluxes (cm3 fluid/cm2 rock), calculated from the standard equation for coupled fluid flow and reaction in porous media, are ( 1) ankerite-oligoclase zone: c. 1 X 10(4); (2) biotite zone: c. 3 x 10(4); (3) amphibole zone: c. 10 X 10(4); and diopside zone: c. 60 X 10(4). The increase in calculated flux with increasing grade is at least in part the result of internal production of volatiles from prograde reactions in pelitic schists and metacarbonate rocks within the Waits River Formation.The mapped pattern of time-integrated fluxes indicates that the Strafford-Willoughby Arch and the numerous igneous intrusions in the field area focused fluid flow during metamorphism. Many rock specimens in the diopside zone experienced extreme alkali depletion and also record low X(CO2). Metamorphic fluids in equilibrium with diopside zone rocks may therefore represent a mixture of acid, H2O-rich fluids given off by the crystallizing magmas, and CO2-H2O fluids produced by devolatilization reactions in the host marls. Higher fluxes and different fluid compositions recorded near the plutons suggest that pluton-driven hydrothermal cells were local highs in the larger regional metamorphic hydrothermal system

Calcic amphiboles in carbonate rocks at the same metamorphic grade from the Waits River Formation, northern Vermont, contain 2.29-19.06 wt% Al2O3 (0.38-3.30 Al atoms per formula unit, pfu). These Al-rich amphibole samples are among the most aluminous examples of hornblende ever analyzed. The amphibole-bearing metacarbonates are interbedded with andalusite-bearing pelitic schists and therefore crystallized at P < 3800 bars. These results demonstrate that factors in addition to pressure must control Al content in hornblende. We have identified temperature, mineral assemblage, mineral composition, and rock chemistry [especially Fe/(Fe + Mg)] as other important factors.To explore semiquantitatively the dependence of the Al content of calcic amphibole on P, T, and coexisting mineral assemblage, a simple thermodynamic model was developed for mineral equilibria involving tremolite-tschermakite ([Ca2Mg5Si8O22(OH)2]-[Ca2Mg3Al4Si6O22(OH)2]) amphibole solutions. The model uses the thermodynamic data base of Berman (1988), with the addition of new values for standard-state enthalpy and entropy for pure end-member tschermakite derived from experimental and field data on the Al content in tremolite coexisting with diopside, anorthite, and quartz. Calculated phase equilibria lead to three conclusions: (1) At a specified P and T, the Al content of calcic amphibole is strongly dependent on the coexisting mineral assemblage. (2) No universal relationship exists between the Al content of amphibole and P. (3) The Al content of amphibole may change dramatically with changes in P and T. Maximum Al contents calculated by the model are approximately 1.2 Al atoms pfu. These values are far short of the 3.30 Al atoms pfu measured in some amphibole samples from Vermont. The principal shortcoming of the model is its failure to consider both total Fe in amphibole and the partitioning of Fe and Mg among the M 1, M2, and M3 crystallographic sites.

Research Article| May 01, 1988 Comment and Reply on "Model for the Precambrian evolution of the Avalon terrane, southern New Brunswick, Canada" Albert Leger; Albert Leger 1Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218 Search for other works by this author on: GSW Google Scholar Paul F. Williams; Paul F. Williams 2Department of Geology, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada Search for other works by this author on: GSW Google Scholar R. Damian Nance R. Damian Nance 3Department of Geological Sciences, Ohio University, Athens, Ohio 45701 Search for other works by this author on: GSW Google Scholar Geology (1988) 16 (5): 475–476. https://doi.org/10.1130/0091-7613(1988)016<0475:CAROMF>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Albert Leger, Paul F. Williams, R. Damian Nance; Comment and Reply on "Model for the Precambrian evolution of the Avalon terrane, southern New Brunswick, Canada". Geology 1988;; 16 (5): 475–476. doi: https://doi.org/10.1130/0091-7613(1988)016<0475:CAROMF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract No abstract available This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.