Gold vein mineralization occurs in the metamorphosed and
deformed Dalradian (Neoproterozoic) rocks of the Sperrin Mountains, Northern
Ireland. Two structures exerted a control on the location of the
mineralization; the north-south Omagh lineament and the
west-northwest-east-southeast Curraghinalt lateral ramp in the footwall of
the northeast-southwest Omagh thrust. These are Caledonian structures
resulting from the thrusting of Dalradian rocks over a possibly still active
Ordovician arc. Cathodoluminescence microscopy distinguishes four phases
of vein quartz in the Curraghinalt gold prospect. Fluid inclusion studies
and stable isotope geochemistry have defined the probable fluids responsible
for the precipitation of each quartz phase and associated sulfide and
precious metal mineralization. The initial phase (Q1) appears to have been
associated with the main Caledonian metamorphic event (ca. 470 Ma) and is
nonauriferous. The second phase (Q2) forms an extensive cement to brecciated
early quartz and is believed to have involved a fluid (~15 wt % CO 2 ,
10 wt % NaCl + KCl equiv) with a significant magmatic component of 470 to
400 Ma, which underwent phase separation and dilution with a cooler
formation water. This process resulted in precipitation of the main phase of
gold mineralization characterized by an assemblage of electrum, pyrite,
arsenopyrite, chalcopyrite, tennantite-tetrahedrite, and various tellurides.
Similar fluids are observed on a regional scale, concentrated within the
hanging wall of the Omagh thrust, indicating an extensive fluid-flow event.
The relative abundance of gold at the Curraghinalt and Cavanacaw prospects
is thought to be due to higher fluid fluxes in favorable zones of dilation
and closer proximity to the fluid source. The deposit was subsequently reactivated with the
precipitation of later quartz (Q3-Q4) from a formation water believed to be
resident in the Dalradian metasediments, which mixed with a low-temperature,
high-salinity basinal brine, probably during Carboniferous basin inversion.
Brine flow resulted in the remobilization of earlier electrum, reducing its
fineness, and also introduced base metal sulfides, carbonates, and barite.
Again, brine flow is localized by the Omagh thrust, indicating the
long-lived role of this structure in controlling regional fluid migration.
Research Article| August 01, 2002 Ropy flow structures: A neglected indicator of magma-flow direction in sills and dikes Dirk Liss; Dirk Liss 1Department of Earth Sciences, University of Birmingham, Birmingham B15 2TT, UK Search for other works by this author on: GSW Google Scholar Donald H.W. Hutton; Donald H.W. Hutton 1Department of Earth Sciences, University of Birmingham, Birmingham B15 2TT, UK Search for other works by this author on: GSW Google Scholar William H. Owens William H. Owens 1Department of Earth Sciences, University of Birmingham, Birmingham B15 2TT, UK Search for other works by this author on: GSW Google Scholar Author and Article Information Dirk Liss 1Department of Earth Sciences, University of Birmingham, Birmingham B15 2TT, UK Donald H.W. Hutton 1Department of Earth Sciences, University of Birmingham, Birmingham B15 2TT, UK William H. Owens 1Department of Earth Sciences, University of Birmingham, Birmingham B15 2TT, UK Publisher: Geological Society of America Received: 23 Jan 2002 Revision Received: 24 Apr 2002 Accepted: 30 Apr 2002 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2002) 30 (8): 715–718. https://doi.org/10.1130/0091-7613(2002)030<0715:RFSANI>2.0.CO;2 Article history Received: 23 Jan 2002 Revision Received: 24 Apr 2002 Accepted: 30 Apr 2002 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Dirk Liss, Donald H.W. Hutton, William H. Owens; Ropy flow structures: A neglected indicator of magma-flow direction in sills and dikes. Geology 2002;; 30 (8): 715–718. doi: https://doi.org/10.1130/0091-7613(2002)030<0715:RFSANI>2.0.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 Ropy flow structures that form in dikes and sills are very similar to ropy lava structures (pahoehoe), but are of intrusive origin. They are unusual among magma-flow indicators in that they provide a frozen record of the magma flow. Our observations suggest that they occur in large vesicles, which form as a result of local repeated pressure drops during propagation and emplacement of the magma. Adiabatic gas expansion causes a ductile rim to form around a vesicle; this rim is then sheared by the underlying flow. Anisotropy of magnetic susceptibility data from the contact zones confirm the flow data from the ropy flow structures, but the results from deeper within the body, both locally and regionally, yield different, yet consistent flow orientations. This discrepancy suggests that a distinction can be made between the magma flow close to the contact and the deeper regional flow patterns in sills and dikes. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Summary The kinematic understanding of the relationship between relative plate motion and the structure of orogenic belts depends upon a knowledge of relative plate motion across the plate boundary system, the relative motion of small blocks and flakes within the system, an evaluation of orogenic body forces, and an understanding of the thermomechanical evolution of the upper part of the orogenic lithosphere in determining strength and detachment levels. We have built a preliminary model for the Cenozoic kinematic evolution of the western Mediterranean oceanic basins and their peripheral orogens that integrates (1) the motion of Africa relative to Europe based upon a new study of Atlantic fracture zones using SEASAT data and the Lamont-Doherty magnetic anomaly database, (2) a new interpretation of the rotation of Corsica/Sardinia and the opening of the Balearic and Tyrrhenian oceanic basins, (3) sedimentary facies sequences in the Apennines, Calabria, and Sicily, and (4) Apennine/Calabrian structure and structural sequence.
Transpression is a process that thickens the crust and therefore in obliquely convergent orogens as in normally convergent orogens there is the potential to generate granitic melts. Individual transcurrent shear zones may not only control the ascent paths, siting, and emplacement mechanisms of plutons, but they may also cause the genesis of the granitoids themselves. Two contrasting situations are examined. In the Hercynian shear zones of Iberia, thickening together with hydrous fluxing along shear zones created intracrustal wet melting of fertile Gondwanan sediments to produce syntectonic granites. In the northern part of the British Caledonides, the association of compositionally expanded granitoids with a major mantle component and transcurrent shear zones may be explained by melting of continental crust at the lower limits of crustal transpressional faults detaching into the Moho.
An intriguing and contradictory scenario has recently developed in the Caledonides of the British Isles concerning the ages of deposition and deformation of the Neoproterozoic Dalradian Supergroup. Isotopic evidence, although limited, suggests that the lower parts of the sequence may have been deposited and undergone Precambrian deformation (i.e. pre-600 Ma) prior to deposition of the upper parts of the sequence (i.e. post-600 Ma). Given these existing constraints, it is clear that a major break (or breaks) would be required in the intervening sequence to maintain a coherent tectonostratigraphy. We present evidence for such an unconformity preserved at the base of the Easdale Subgroup in NW Ireland. Reworked clasts that contain a pre-existing tectonometamophic history are identified within a conglomerate that lies along this regionally recognized boundary. The underlying sequence also exhibits pre-existing deformational fabrics that display erosional truncation at the base of the conglomerates. These relationships, together with significant erosion and excision of the footwall sequence, and extensive thickness and facies variations in the hanging-wall units, imply that a major tectonic unconformity exists within this succession. As the conglomerate lies stratigraphically below Precambrian ( c . 600 Ma) lavas, the structural fabrics contained within the clasts and the underlying sequence must also be of Precambrian age and totally unrelated to the well-established Early Palaeozoic (Caledonian) orogenic deformation observed in the younger parts of the sequence. The Dalradian Supergroup, as defined, may actually comprise (at least) two distinct tectonostratigraphic sequences.
Summary Despite the continuing use of the term ‘tectonic slide’ in the Caledonian fold belt of Britain and Ireland for the past fifty years, the concept has found few advocates elsewhere. To many geologists outside the British Isles it remains a subject of some confusion. A review of the literature suggests that slides are a general term for faults which form in close association with syn-metamorphic regional deformation. They are often related to major folds although this is not a diagnostic feature. Neither is any movement sense implied in the term slide; thus, they may be thrusts, lags, oblique slip etc. Classically, these structures lie along and subparallel to the boundaries of major lithological units but transgress the stratigraphy on a larger scale. Structural analogies with the thrusts of the lower grade or non-metamorphic deformed cover sequences can often be seen in slides. However, they are more commonly and possibly fundamentally the result of differing responses to deformation of contrasting, adjacent lithologies in areas of higher strain and metamorphism.
The Great Tonalite Sill (GTS) of southeastern Alaska and British Columbia (Brew & Ford 1981; Himmelberg et al . 1991) is one of the most remarkable intrusive bodies in the world: it extends for more than 800 km along strike and yet is only some 25 km or less in width. It consists of a belt of broadly tonalitic sheet-like plutons striking NW–SE and dipping steeply NE, and has been dated between 55 Ma and 81 Ma (J. L. Wooden, written communication to D. A. Brew, April 1990) (late Cretaceous to early Tertiary). The sill (it is steeply inclined and rather more like a “dyke”) is emplaced along the extreme western margin of the Coast Plutonic and Metamorphic Complex (CPMC), the high grade core of the Western Cordillera. The CPMC forms the western part of a group of tectonostratigraphic terranes including Stikine and Cache Creek, collectively known as the Intermontane Superterrane (Rubin et al. 1990). To the W of the GTS, rocks of the Insular Superterrane, including the Alexander and Wrangellia terranes and the Gravina belt, form generally lower metamorphic grade assemblages. The boundary between these two superterranes is obscure but it may lie close to, or be coincident with, the trace of the GTS.
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