Summary The mineral chemistry, petrology and geochemistry of representative members of the Scourie dyke swarm are described, and discussed in relation to crystallization processes in deep-seated dykes and to their petrogenesis. Four types of dykes can be defined on petrological and geochemical grounds: bronzite-picrites, norites, olivine-gabbros and quartz dolerites. The latter constitute by far the most abundant dyke-type. In the central Scourian granulite zone in particular the dykes were emplaced at depth into hot country rocks; this enables the through-flow behaviour of different magma types in dyke conduits to be compared. Whereas the dolerites and norites have chilled margins, the bronzite-picrites and olivine-gabbros have coarse grained orthopyroxene-rich and augite-rich margins respectively. This probably reflects turbulent-flow conditions in the less viscous, more ultramafic magmas which permitted rapid crystallization of pyroxenes on the dyke walls during intrusion. The picrite dykes, and to a lesser extent the olivine-gabbro dykes, display additional across-dyke modal and textural variations. These are not accompanied by significant variations in mineral composition, and can largely be attributed to crystal settling in variably inclined dyke sheets. All dykes typically show a ‘continental’ trace element signature (enrichment in light rare-earth elements and large-ion-lithophile elements), but this must have been inherited from the sub-continental lithosphere because contamination by Lewisian gneisses would not generate the observed trace element characteristics. Both trace element and mineral chemical data indicate that the dykes were derived from at least two distinct mantle sources, that supplying the picrite and norite dykes being more refractory with respect to major elements, but showing a greater relative enrichment in light REE and LIL elements. The emplacement of the Scourie dyke swarm represents considerable crustal extension, but also poses a thermal problem in the generation of high temperature picritic magmas. It is suggested that the dykes and their compositional characteristics reflect the processes of growth and evolution of the sub-continental lithosphere, and that the extensive retrogression of the granulites, which occurred penecontemporaneously with dyke intrusion, may also be linked with these processes.
The results of Deep Sea Drilling Project Leg 60 pertaining to the petrologic evolution of the Mariana Trough and Mariana arc are summarized in Part I.The rocks recovered at five principal sites are exceptionally diverse, including gabbros and calc-alkalic andesites derived from the West Mariana Ridge; fresh and hydrothermally altered basalts in the Mariana Trough; and interbedded boninites and arc tholeiites in the forearc region between the modern arc and the trench.Part I outlines the stratigraphy, alteration, and petrology of these rock suites and summarizes hypotheses on their origin.Part II integrates these results with those of previous DSDP legs in the region, and with island sampling, to present a composite 40-m.y.history of the Mariana arc system.Arc volcanism began in the Eocene, probably as a result of a change to the west in the motion of the Pacific plate.A north-south-trending fracture zone was transformed into a trench-subduction complex, trapping a portion of the Pacific plate in the Philippine Sea.The crust of this basin, drilled in Hole 447 (Leg 59), has the composition of typical ocean floor basalt.The earliest Eocene arc was built up dominantly of arc tholeiite and boninitic lavas, with lesser calc-alkalic lavas, based on the results of Leg 60 drilling at Sites 458 and 459 in the forearc region; Leg 59, Site 448, on the Palau-Kyushu Ridge; and exposures on the islands of Palau, Guam, and Saipan.Near Sites 458 and 459, the forearc crust is thin, formed entirely under water, and includes no known component of ocean crust.Nevertheless it has many of the features of an ophiolite, produced in situ by earliest arc volcanism.In the Miocene, the Palau-Kyushu Ridge was split from this ancestral arc by opening of the Parece Vela back-arc basin.A new arc formed on the east side of this basin, the West Mariana Ridge.Lavas recovered from this ridge at Sites 451 and 453, and dating from this time on Guam, have calc-alkalic compositions.In the Pliocene, a second back-arc basin formed, the Mariana Trough, splitting off the West Mariana Ridge and causing volcamsm along it to cease.The present arc has been built again on the east side of the new back-arc basin.Based on the compositions of ash at Site 453 and island exposures, the new arc first erupted arc tholeiites but has shifted to calc-alkalic compositions in the past 200,000 years.Both the Parece Vela Basin and Mariana Trough contain tholeiitic basalts with most of the features of mid-ocean-ridge tholeiites, but basalts in the Trough have some of the trace element characteristics of island arc basalts.Petrogenetic models for the evolution of the arc system therefore must provide for four distinctive magma typesboninites, arc tholeiites, calc-alkalic lavas, and back-arc-basin tholeiites-and explain their distribution in time and space.The relative contributions to the production of these magma types of the subducted slab, the overlying mantle wedge composition, partial melting, and fractional crystallization need to be considered.One important conclusion is that segregation of cumulus gabbros at elevated /?(H 2 O) such as those cored at Site 453 may be a major mechanism for producing calc-alkalic andesites in the Mariana arc system.Immediately following back-arc rifting, when sources of magma are quite shallow and heat flow is high, the conditions necessary for such fractionation would be unlikely to occur.Thus following each episode of back-arc basin rifting, arc volcanism should be tholeiitic.Only later will it become calc-alkalic.We also propose that progressive changes in the mantle beneath each arc resulting from voluminous extraction of basalts may contribute to conditions necessary for new episodes of back-arc rifting.
Research Article| September 01, 1995 Depleted mantle-plume geochemical signatures: No paradox for plume theories Andrew C. Kerr; Andrew C. Kerr 1Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom Search for other works by this author on: GSW Google Scholar Andrew D. Saunders; Andrew D. Saunders 1Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom Search for other works by this author on: GSW Google Scholar John Tarney; John Tarney 1Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom Search for other works by this author on: GSW Google Scholar Neil H. Berry; Neil H. Berry 1Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom Search for other works by this author on: GSW Google Scholar Victoria L. Hards Victoria L. Hards 2Department of Geological Sciences, University of Durham, South Road, Durham DH1 3LE, United Kingdom Search for other works by this author on: GSW Google Scholar Geology (1995) 23 (9): 843–846. https://doi.org/10.1130/0091-7613(1995)023<0843:DMPGSN>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 Andrew C. Kerr, Andrew D. Saunders, John Tarney, Neil H. Berry, Victoria L. Hards; Depleted mantle-plume geochemical signatures: No paradox for plume theories. Geology 1995;; 23 (9): 843–846. doi: https://doi.org/10.1130/0091-7613(1995)023<0843:DMPGSN>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 High-MgO liquids erupted in ocean-island settings and in some continental flood-basalt provinces commonly preserve a "depleted" composition, in terms of both highly incompatible trace elements and isotope ratios. These observations strongly imply that their source is also compositionally depleted. However, in at least one case (Iceland and the North Atlantic volcanic province), it can be shown that this depleted source is not the same as that feeding the present-day North Atlantic mid-ocean ridge. The depleted source must also have been much hotter than the mid-ocean ridge basalt (MORB) source to account for the volume of melt and primitive composition of some magmas that were generated. This depleted character, then, is an intrinsic component of mantle plumes, originating from the deep mantle. We propose that mantle plumes consist of a mixture of enriched or fusible streaks in a depleted, refractory matrix; preferential extraction of the enriched component occurs close to the plume axis. The depleted residue from this melting remains in the upper mantle and may therefore be a major contributor to the source region of MORB. 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.
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