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.
Coe‐Modified Thellier (CMT) and Microwave (MW) paleointensity experiments were conducted on samples from the multi‐phase late Carboniferous (ca. 301 Ma) to early Permian (ca. 270 Ma) Great Whin Sill complex. Statistically comparable results were obtained with both techniques from high quality data. Forty three samples (25 with CMT and 18 with MW) yielded mean paleointensity values of 12.0 ± 5.8 μT and 16.5 ± 8.2 μT for the early and late intrusive phases respectively, corresponding to VDMs of (1.6 ± 0.8) × 10 22 and (2.1 ± 1.1) × 10 22 Am 2 . When combined with previous data, the results suggest a generally low dipole moment during the KRS, reducing from ∼75% of the present value at 301 Ma to 20–27% of the present value later in the superchron. These data imply either a decoupling between processes controlling reversal rate and field strength, or a more complex, intriguing time‐lag relationship. More absolute palaeointensity results are needed to further evaluate these alternatives.
A large-scale palaeomagnetic study (125 specimens from 88 sampling localities) has been carried out on the Whin Sill complex and its associated dykes in northern England. The dominant carrier of remanent magnetization of the doleritic rocks is relatively pure magnetite of pseudo-single domain size, and the characteristic remanent magnetizations are typically well defined and unidirectional. Regionally consistent palaeomagnetic directions are obtained from three geographically distinct parts of the Whin Sill complex, here named as the Holy Island Sill (Declination/Inclination ( D / I )=189.5/−2.8, α 95 =6.3), the Alnwick Sill ( D / I =194.6/−25.6, α 95 =8.1) and the Hadrian's Wall–Pennines Sill ( D / I =189.2/3.3, α 95 =3.5). Although the Holy Island Sill and the Hadrian's Wall–Pennines Sill are of similar age ( c . 296 Ma) and indistinguishable on palaeomagnetic grounds, the difference in palaeomagnetic directions between them and the Alnwick Sill demonstrates the multi-component nature of the complex; magma-flow indicators suggest that the Holy Island Sill and the Hadrian's Wall–Pennines Sill may also represent separate intrusion events. Dykes local to the three sills have remanence directions corresponding to their respective sills and can tentatively be identified as their feeder dykes. Virtual geomagnetic poles for the three palaeomagnetic sites are: Holy Island Sill and Holy Island Dyke, 346.8E, 35.4S; Alnwick Sill, High Green Dyke and St. Oswald's Chapel Dyke, 337.1E, 47.1S; Hadrian's Wall–Pennines Sill and Hett Dyke, 347.1E, 32.9S.
lvaite, a rare calcium-iron silicate, is present in the dolerite-pegmatite facies of the Whin Sill at Forcegarth Quarry near High Force in Teesdale, Co. Durham. It has a near stoichiometric composition in the range Ca0.98-1.00Fe2+1.95-2.00Fe3+Si1.96-1.98O8(OH). Ilvaite forms part of an association of opaque minerals that includes ilmenite, altered titanomagnetite, titanite and minor sulphides, notably pyrrhotite, chalcopyrite and sphalerite that is characteristic of ilvaite in mafic igneous rocks. Other than the presence of ilvaite and trace amounts of molybdenite there is nothing unusual about the sampled dolerite-pegmatite suggesting that ilvaite may be present in dolerite-pegmatites elsewhere in the Whin Sill. This is the first authenticated occurrence of ilvaite in the British Isles.
The opaque mineralogy of pink aplite from the Whin Sill at Ratcheugh Quarry near Alnwick comprises ilmenite, titanite and pyrite with minor amounts of pyrrhotite, chalcopyrite, iron-rich sphalerite and galena. Much of the sulphide assemblage and associated barite is later than the crystallisation of the aplite.
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