Paleomagnetism and magnetic mineralogy of the Nahant gabbro and tonalite, eastern Massachusetts

1985 ◽  
Vol 22 (10) ◽  
pp. 1425-1435 ◽  
Author(s):  
Patricia A. Weisse ◽  
Stephen E. Haggerty ◽  
Laurie L. Brown
Keyword(s):  
Hydrothermal Activity ◽  
Magnetic Mineral ◽  
Magnetic Minerals ◽  
Ti Oxides ◽  
Apparent Relationship ◽  
Mineral Assemblages ◽  
Intrusive Rocks ◽  
Eastern Massachusetts ◽  
Chemical Remanent Magnetization

Paleomagnetic analyses of the Nahant gabbro and an associated tonalite of eastern Massachusetts reveal five separate populations of paleomagnetic directions. The gabbro and tonalite, exposed in the northernmost portion of the Boston Basin, were selected for this study because of the lack of significant macroscopic evidence of penetrative metamorphism or deformation. The tonalite has not been radiometrically dated; however, an Ordovician date has been reported for the gabbro. Detailed petrographic data indicate that the five populations of paleomagnetic directions obtained for the gabbro and tonalite correspond to five styles of magnetic mineral alteration.A mean alteration index was calculated for each site in the Nahant gabbro and tonalite. There is an apparent relationship between magnetite alteration and normalized intensity at a given demagnetization level, with normalized intensity decreasing as magnetite alteration increases. This relationship illustrates the interdependence between paleomagnetic properties and magnetic minerals. Alteration of these minerals is probably associated with periods of magnetic overprinting.Autometasomatism is believed to have occurred in the gabbro. Fluid flushing initiated plagioclase decomposition, liberating Ca + Al + Si to form Al-rich (4–10 wt.% Al2O3) titanite (sphene, CaTiSiO5). Whether the metasomatism was deuteric or the result of postdeuteric hydrothermal activity has not been determined.The Nahant gabbro poles are interpreted as representing either a deuteric or a metasomatic chemical remanent magnetization (CRM). The tonalite pole is interpreted as a deuteric CRM. The gabbro and tonalite poles are similar to a number of poles from Ordovician to Devonian intrusives in the Acadia terrane, including the St. Stephen and St. George plutons.Petrographic examination of polished sections from the Devonian St. Stephen and St. George plutons reveals evidence for alteration of primary magnetic mineral assemblages. Similarities exist between these alterations and alterations observed in the Nahant suite, most notably, the formation of titanite from Fe–Ti oxides. The repeated determination of similar poles on intrusive rocks in the Acadia terrane suggests there may be some tectonic or geomagnetic significance to poles from intrusives; however, without more data pertaining to the timing of metasomatic events observed in the Nahant suite and the St. Stephen and St. George plutons, the age and significance of these poles cannot be interpreted with certainty.

Chemical and microbial processes causing anomalous magnetization in environments affected by hydrocarbon seepage

Geophysics ◽  
1991 ◽  
Vol 56 (5) ◽  
pp. 598-605 ◽  
Author(s):  
H. G. Machel ◽  
E. A. Burton
Keyword(s):  
Magnetic Anomalies ◽  
Hydrocarbon Exploration ◽  
Magnetic Mineral ◽  
Hydrocarbon Accumulation ◽  
Magnetic Minerals ◽  
Total Magnetization ◽  
Microbiological Processes ◽  
Surface Exploration ◽  
Mineral Assemblages ◽  
Anthropogenic Processes

(Aero‐)magnetic anomalies have been reported from several commercial hydrocarbon accumulations. However, the processes responsible for such anomalies are relatively poorly understood. This paper conceptually discusses chemical and microbiological processes involved in generating anomalous magnetization related to hydrocarbon accumulations, including hydrocarbon seepage environments. Based on thermodynamic criteria and microbiologic activity, the formation and destruction of magnetic mineral assemblages can be predicted. Under the influence of hydrocarbons, magnetite and pyrrhotite are the most important magnetic minerals formed, and the most abundant magnetic mineral destroyed is hematite. Hence, the invasion of hydrocarbons may result in “positive,” “absent,” or “negative” magnetic contrasts relative to the total magnetization prior to hydrocarbon invasion, depending upon the amounts of authigenic magnetite and pyrrhotite formed relative to the amounts of hematite destroyed. Magnetism may be generated also by natural and anthropogenic processes that have no relationships to an underlying or adjacent hydrocarbon accumulation. Consequently, anomalous magnetization, even if associated with a hydrocarbon accumulation, may or may not be genetically related to it. Magnetic mineral assemblages and the resulting magnetic contrasts, such as those predicted in this paper, have been documented from some hydrocarbon seepage environments. Hence, anomalous magnetization can be used for hydrocarbon exploration in association with other surface exploration methods.

Paleomagnetic history of the Mealy dykes of Labrador, Canada

1983 ◽  
Vol 20 (12) ◽  
pp. 1818-1833 ◽  
Author(s):  
J. K. Park ◽  
R. F. Emslie
Keyword(s):  
The Other ◽  
Magnetic Mineral ◽  
Magnetic Minerals ◽  
Linear Distribution ◽  
Tectonic Zone ◽  
Magnetic Components ◽  
History Of ◽  
Component C ◽  
A Site

Paleomagnetic analysis of the Mealy diabase dykes of Labrador reveals magnetizations that predate the Grenville event at about 1000 Ma. These dykes intrude the Mealy Mountains anorthositic complex in the Grenville Structural Province. They are well south of the Grenville Front Tectonic Zone, but were apparently never subjected to temperatures as high as 500 °C during their post-consolidation history.Four distinct magnetic components were uncovered by thermal and alternating field treatments and a fifth remained unresolved. The major magnetic mineral present, titanomagnetite, is thought to record two magnetic directions acquired during cooling from magmatic temperatures. These are B (D = 305°, I = −76°; N = 18 sites; κ = 12; α95 = 11°) and A (D = 095°, I = +52°; N = 20 sites; κ = 46; α95 = 5°). Component B has much within-site dispersion. The other two components, C (D = 274°, I = −47°; N = 10 sites; κ = 15; α95 = 13°) and D (D = 292°, I = −74°; κ = 5; α95 = 31°), probably reside in magnetite and pyrrhotite, respectively. Component C, antiparallel to A, was probably acquired at about the same time as A. We suggest that C and A represent the first stable magnetizations retained by the dykes following an extensive period of cooling and re-equilibration of the magnetic minerals. Components B and D, which agree in direction, represent a later stage of cooling.Component B has a pole at 148°E, 34°S (δp = 18°, δm = 19°) in agreement with regional metamorphic poles from the Grenville; A, however, has a pole at 173°W, 23°S (δp = 5°, δm = 7°), which apparently "sees through" the peak in Grenville activity. The A site poles have a linear distribution along the Keweenawan Track and probably relate to an age between 1000 and 1150 Ma.

A SUB-TILL SAPROLITE AND THE OVERLYING SOIL PROFILE NEAR MOUNT ORFORD, SOUTHERN QUEBEC

1984 ◽  
Vol 64 (4) ◽  
pp. 577-585 ◽  
Author(s):  
C. R. DE KIMPE ◽  
M. R. LAVERDIÈRE ◽  
P. LASALLE
Keyword(s):  
Key Words ◽  
Clay Minerals ◽  
Soil Profile ◽  
Mineralogical Composition ◽  
Magnetic Mineral ◽  
Mineral Assemblages ◽  
Extractable Al

A saprolite deposit and the overlying soil profile developed in a glacial diamicton were sampled near Mount Orford, Southern Quebec. The two materials differed mainly by the magnetic mineral and extractable Al contents, by the Fedithionite/Feoxalate ratio and by the mineralogical composition. Illite and chlorite were the dominant clay minerals in the till whereas muscovite and kaolinite were the major minerals in the saprolite. A comparison was also made with another previously described saprolite deposit 4 km away from this one, in which chlorite was slightly transformed to smectite. It is suggested, from the mineral assemblages, that the two saprolites have probably formed at different times, the first one during Tertiary and the second one during an interglacial stage. Key words: Saprolite, glacial diamicton, kaolinite, muscovite, Tertiary alteration

Gold–copper–bismuth mineralization in hedenbergitic skarn, Tombstone Mountains, Yukon

1987 ◽  
Vol 24 (12) ◽  
pp. 2362-2372 ◽  
Author(s):  
Isobel J. Brown ◽  
Bruce E. Nesbitt
Keyword(s):  
Large Scale ◽  
Small Volume ◽  
Gold Mineralization ◽  
Rock Interaction ◽  
The North ◽  
Mineral Assemblages ◽  
Intrusive Rocks ◽  
Porosity And Permeability ◽  
Relationship Of ◽  
The Relationship

Gold mineralization on the Marn property, Yukon, occurs in two pyroxene skarn bodies, which are adjacent to the Mount Brenner Stock in the Ogilvie Mountains. The skarns are separated by a 600 m wide monzonite intrusion and show contrasting mineralogical and geochemical characteristics in addition to quite different metal values. Significant but uneconomic Au, Ag, W, and Cu mineralization is found in skarn on the north side of the intrusion, while very low Au grades (0.052 g/t) occur at the southern contact. The mineral assemblages of both skarns are dominated by iron-rich pyroxenes. The iron content of the pyroxenes varies between Hd40 and Hd80 in the northern location and Hd80 and Hd100 in the southern skarn. A well-developed sequence of retrograde alteration affected only the northern skarn. This was probably the result of porosity and permeability differences in the early, high-temperature pyroxene skarn, which permitted greater fluid–rock interaction in the northern skarn during cooling. A small volume of diopsidic, aluminous, wollastonite-bearing skarn occurs in both the northern and southern localities. The relationship of this type of skarn to the hedenbergitic skarn is ambiguous, since there is no large-scale mineralogical zoning. The Marn is similar to hedenbergitic, auriferous skarns of Japan, where the oxidation state of the intrusive rocks is believed to be the controlling factor in the development of skarn mineralogy.

Genesis of the Olden wollastonite skarn, Sharbot Lake domain, Central Metasedimentary Belt, Grenville Province, southeastern Ontario, Canada

2005 ◽  
Vol 42 (8) ◽  
pp. 1401-1417 ◽  
Author(s):  
T A Grammatikopoulos ◽  
A H Clark ◽  
T H Pearce ◽  
D A Archibald
Keyword(s):  
Thermal Ionization Mass Spectrometry ◽  
Alkali Feldspar ◽  
Hydrothermal Activity ◽  
Ionization Mass ◽  
Mississippi Valley ◽  
Grenville Province ◽  
Intrusive Rocks ◽  
Wide Compositional Range ◽  
Considerable Depth ◽  
Base Metal Mineralization

With a resource of ∼2.8 Mt at 30%–35% wollastonite occurring at a depth of about 75 m, the Olden (formerly Hawley) prospect is the largest of a swarm of skarns hosted by amphibolite facies, dominantly calcitic marble that occurs adjacent to and as inliers within the Mountain Grove pluton. The post-kinematic intrusion comprises units with a wide compositional range from anorthositic gabbro to alkali-feldspar granite and syenite and widely exhibits megascopic fabrics recording magma comingling and mixing. Isotope dilution – thermal ionization mass spectrometry (ID–TIMS) dating of zircon separates from a hornblende diorite unit yields a 207Pb–206Pb age of 1153 ± 2 Ma, significantly older than the contiguous 1070 ± 3 Ma McLean granite. Al-in-hornblende geobarometry on several Mountain Grove units indicates that the intrusion crystallized at pressures of 250–470 MPa, equivalent to mesozonal depths of 10–15 km. The main exoskarn body, ∼200 m long and up to 50 m wide, is dominated by wollastonite, clinopyroxene (Di73–94Hd2–19), and calcic garnet (Gr52–83And12–37) and ranges from massive to podiform. The eastern termination exhibits rhythmically alternating wollastonite- and calcite-rich layers, 10 cm wide and locally with chevron-shaped crenulations. These layers bear no relationship to bedding or metamorphic foliation and are interpreted as “wrigglitic,” i.e., they are a record of a metasomatic front that migrated. Veins of garnet–pyroxene–vesuvianite cut the main exoskarn. Retrograde phlogopite-rich skarns, with erratic serpentine and brucite, contain variable sphalerite and pyrite. Restricted pyroxene–garnet (–wollastonite–scapolite) endoskarn is developed in intrusive rocks contiguous with the exoskarn. Skarn development is ascribed to H2O-rich (XCO2 < 0.3) magmatogene brines and high temperatures (T = 500–650 °C), which caused intense Si, Al, and Fe metasomatism of the marbles, hydrothermal activity taking place at considerable depth. The occurrence of wollastonite around the periphery of the small Long Lake sphalerite deposit, restricted to a marble roof pendant in the Mountain Grove pluton 2.1 km east-northeast of the Olden prospect, indicates that this base-metal mineralization may be an exoskarn, rather than metamorphosed Mississippi Valley type. Incremental-heating 40Ar–39Ar dating of hornblende and biotite from the Mountain Grove diorite yields plateau ages of 1058 ± 14 and 1047 ± 4 Ma, respectively, and an exoskarn phlogopite age of 1074 ± 5 Ma. A genetic relationship between hydrothermal activity and the McLean pluton cannot be ruled out, but a parental role for the older Mountain Grove pluton is favoured on the basis of the close areal relationships of skarn bodies and that intrusion.

scholarly journals Heavy mineral assemblages in tills and their use in distinguishing glacial lobes in the Great Lakes region

1979 ◽  
Vol 16 (12) ◽  
pp. 2219-2235 ◽  
Author(s):  
Q. H. J. Gwyn ◽  
A. Dreimanis
Keyword(s):  
Great Lakes ◽  
Mineral Content ◽  
Heavy Mineral ◽  
Heavy Minerals ◽  
Great Lakes Region ◽  
Adirondack Mountains ◽  
Magnetic Minerals ◽  
Grenville Province ◽  
Provenance Areas ◽  
Mineral Assemblages

Two main source areas of heavy minerals in tills have been defined in the Great Lakes region: a source in the Superior and Southern Provinces and another in the Grenville Province. The Superior–Southern source is typified by low heavy mineral content and high epidote percentage in contrast to the Grenville source which has a high content of heavy minerals of which garnet, tremolite, and to a lesser extent sphene and orthopyroxene are characteristic. The Huron lobe tills have a mineral suite characteristic of the Superior–Southern source. Two subsources can be distinguished in the Superior–Southern area; however, they are too limited in extent to be characteristic of major glacial lobes. Two other subsources have been identified in the Grenville provenance area: a western Grenville subsource containing abundant garnet and having a low purple–red garnet ratio; and an eastern Grenville subsource distinguished by high garnet and tremolite content and a garnet ratio generally greater than one. The western and eastern Grenville subsources are the provenance areas for the tills of the Georgian Bay lobe and the Ontario–Erie lobe respectively. A possible third Grenville subsource in the Adirondack Mountains is distinguished from other Grenville sources by a lower heavy mineral content and more abundant orthopyroxene and magnetic minerals. This assemblage may be characteristic of the southern portion of the Ontario–Erie lobe.

Significance of “gneissic” rocks in the Liscomb Complex, Nova Scotia

2010 ◽  
Vol 47 (6) ◽  
pp. 927-940 ◽  
Author(s):  
J. V. Owen ◽  
R. Corney ◽  
J. Dostal ◽  
A. Vaughan
Keyword(s):  
Igneous Rocks ◽  
Metamorphic Rocks ◽  
Low Grade ◽  
High Grade ◽  
Peraluminous Granite ◽  
Isotopic Signatures ◽  
Conventional View ◽  
Basement Rocks ◽  
Mineral Assemblages ◽  
Intrusive Rocks

The Liscomb Complex comprises Late Devonian intrusive rocks (principally peraluminous granite) and medium- to high-grade metamorphic rocks (“gneisses”) that collectively are hosted by low-grade (greenschist facies) metasediments of the Cambro-Ordovician Meguma Group. The conventional view that these “gneisses” contain high-grade mineral assemblages and represent basement rocks has recently been challenged, and indeed, some of the rocks previously mapped as gneisses, particularly metapelites, have isotopic compositions resembling the Meguma Group. Amphibole-bearing enclaves in the Liscomb plutons, however, are isotopically distinct and in this regard resemble xenoliths of basement gneisses in the Popes Harbour lamprophyre dyke, south of the Liscomb area. Metasedimentary enclaves with Meguma isotopic signatures can contain garnets with unzoned cores (implying high temperatures) that host high-grade minerals (prismatic sillimanite, spinel, and (or) corundum) and are enclosed by retrograde-zoned rims. These features are interpreted here as having formed during and following the attainment of peak temperatures related to Liscomb magmatism. The amphibole-bearing meta-igneous rocks described here contain cummingtonite or hornblendic amphibole and occur as enclaves in granodioritic to tonalitic plutons. They are mineralogically, texturally, and isotopically distinct from Meguma metasediments and at least some of the plutonic rocks that enclose them, so remain the most likely candidate for basement rocks in the Liscomb Complex.

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