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.
The ultrapotassic magmatism of southern Italy (the Roman province) is well known, and recently these highly unusual lavas have been explained in terms of subduction‐related processes. Less well studied are the coeval calc‐alkaline to potassic rocks of the nearby Aeolian Islands, which are situated above a Benioff zone and are therefore demonstrably related to recently active subduction. On a number of geochemical diagrams the Roman and Aeolian provinces define continuous trends, which may be accommodated in a single petrogenetic model involving mixing of three isotopically and elementally distinct components. Two of these are subduction‐related: first, a high Sr/Nd, high Th/Ta component derived largely from basaltic ocean crust and, second, a component with extremely high Th/Ta, but relatively low Sr/Nd derived largely from subducted sediments. These are mixed with mantle wedge material which, prior to subduction, was characterised by highly radiogenic Pb isotope ratios, and is therefore comparable to the mantle source of Mount Etna volcanism. Thus it would appear that midplate tholeiitic to Na‐alkalic magmatism and continental margin calc‐alkaline to ultrapotassic magmas were derived from mantle sources which, prior to subduction, had similar isotopic signatures. This observation has important implications for the potential involvement of trace element and isotope enriched (OIB‐like) mantle in the genesis of subduction‐related volcanism.
Research Article| April 01, 1988 Is average continental crust generated at subduction zones? Robert M. Ellam; Robert M. Ellam 1Department of Earth Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, England Search for other works by this author on: GSW Google Scholar Christopher J. Hawkesworth Christopher J. Hawkesworth 1Department of Earth Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, England Search for other works by this author on: GSW Google Scholar Geology (1988) 16 (4): 314–317. https://doi.org/10.1130/0091-7613(1988)016<0314:IACCGA>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Robert M. Ellam, Christopher J. Hawkesworth; Is average continental crust generated at subduction zones?. Geology 1988;; 16 (4): 314–317. doi: https://doi.org/10.1130/0091-7613(1988)016<0314:IACCGA>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 Many crustal estimates suggest that the continents are approximately andesitic in composition. However, in recent subduction related environments, andesites are thought to originate by intracrustal differentiation from basaltic parental magmas. The net flux from mantle to crust along destructive plate margins is therefore basaltic with low Rb/Sr, and is not readily reconciled with the development of intermediate, high Rb/Sr continental crust. Either the continental crust is considerably more mafic than andesite, or the mechanism of crust formation has changed with time, such that a large proportion of the continental crust was formed by processes unlike those active in recent subduction zones. It is significant that there is evidence from Archean rocks to support the latter hypothesis. 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.
Abstract Astronomical tuning in the Mediterranean region is primarily based on organically‐mediated proxies, such as cyclicity of organic rich layers or changes in foraminiferal assemblages. Both during and post deposition, organic proxies can be affected by complex processes not immediately related to the changes in precession (insolation) they are assumed to reflect. Here we present an isotopic proxy which exhibits precessional cyclicity yet is inorganic. Seawater lead (Pb) isotope records over four precessional cycles between 6.6 and 6.5 Ma, from bulk sediment leachates of three Messinian, circum‐Mediterranean marginal locations, show variations consistent with precessional cyclicity. During insolation minima, the Pb isotope signatures from all three sites converge to similar values, suggesting a regional process is affecting all three locations at that time. Data from the marginal sites are compared with new data from ODP Site 978 and published data from a variety of geological archives from the Mediterranean region to determine the mechanism(s) causing the observed variability. While the comparisons are not fully conclusive, the timing of events suggest that increased dust production from North Africa during insolation minima is the most likely control. This hypothesis implies that authigenic marine Pb isotope records have the potential to provide a reliable inorganic tie point for Mediterranean cyclostratigraphy where sub‐precessional resolution is required. An inorganic tie point could also provide the means to resolve long‐standing problems in Mediterranean stratigraphy on precessional and sub‐precessional timescales which have been obscured due to post‐depositional changes (e.g., sapropel burn‐down) or suboptimal ecological conditions (e.g., the Messinian Salinity Crisis).
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