Research Article| August 01, 2004 Oceanic crust generated by elusive parents: Sr and Nd isotopes in basalt-peridotite pairs from the Mid-Atlantic Ridge Anna Cipriani; Anna Cipriani 1Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA, and Department of Earth and Environmental Sciences of Columbia University, New York, New York 10027, USA Search for other works by this author on: GSW Google Scholar Hannes K. Brueckner; Hannes K. Brueckner 2Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, New York 10964, USA, and School of Earth and Environmental Sciences, Queens College, and the Graduate Center of the City University of New York, New York, New York 11367, USA Search for other works by this author on: GSW Google Scholar Enrico Bonatti; Enrico Bonatti 3Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, New York 10964, USA, and Istituto di Scienze Marine, Consiglio Nazionale delle Ricerche, Bologna 40129, Italy, and Dipartimento di Scienze della Terra, Università "La Sapienza," Rome 00187, Italy Search for other works by this author on: GSW Google Scholar Daniele Brunelli Daniele Brunelli 4Istituto di Scienze Marine, Consiglio Nazionale delle Ricerche, Bologna 40129, Italy, and Laboratoire P. Sue, Commissariat à l'énergie atomique (CEA), Départment de Recherche sur l'Etat Condensé, les Atomes et les Molécules (DRECAM), Gif sur Yvette 91191, France Search for other works by this author on: GSW Google Scholar Geology (2004) 32 (8): 657–660. https://doi.org/10.1130/G20560.1 Article history received: 09 Feb 2004 rev-recd: 03 Apr 2004 accepted: 07 Apr 2004 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Anna Cipriani, Hannes K. Brueckner, Enrico Bonatti, Daniele Brunelli; Oceanic crust generated by elusive parents: Sr and Nd isotopes in basalt-peridotite pairs from the Mid-Atlantic Ridge. Geology 2004;; 32 (8): 657–660. doi: https://doi.org/10.1130/G20560.1 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 Given that oceanic basalts form by partial melting of mantle peridotites that rise below mid-ocean ridges, peridotite and basalt should have identical Sr and Nd isotope ratios. We tested this concept on parallel sets of peridotites and basalts sampled from an exposed section of lithosphere representing 20 m.y. of accretion at the Mid-Atlantic Ridge. The 143Nd/144Nd ratios of the basaltic glasses stay constant, whereas those of the peridotitic clinopyroxenes extend both higher and lower than the basalt ratios, suggesting that the constant isotopic composition of the basalts results from mixing of melts released by peridotites from a broad region of the subridge mantle. The degree of melting undergone by the peridotites correlates inversely with their 143Nd/144Nd ratios. Small-scale isotopic heterogeneity of the peridotites may result from variable premelting metasomatism in the mantle, mostly during periodic, dynamic, subridge upwelling, possibly superimposed upon time-integrated radioactive decay of ancient heterogeneities. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Research Article| August 01, 1999 Sinking intrusion model for the emplacement of garnet-bearing peridotites into continent collision orogens: Comment and Reply Jian-jun Yang; Jian-jun Yang 1Institute of Geology, Chinese Academy of Geological Sciences, Baiwanzhuang Dajie 26, Beijing 100037, China; jjyangki@public.bta.net.cn. Search for other works by this author on: GSW Google Scholar Bor-ming Jahn; Bor-ming Jahn 3Géosciences-Rennes, Université de Rennes 1, 35042 Rennes, France Search for other works by this author on: GSW Google Scholar Hannes K. Brueckner Hannes K. Brueckner 4Queens College and the Graduate Center, City University of New York, Queens College, Flushing, New York 11367 and Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964; hannes@ldeo.columbia.edu. Search for other works by this author on: GSW Google Scholar Author and Article Information Jian-jun Yang 1Institute of Geology, Chinese Academy of Geological Sciences, Baiwanzhuang Dajie 26, Beijing 100037, China; jjyangki@public.bta.net.cn. Bor-ming Jahn 3Géosciences-Rennes, Université de Rennes 1, 35042 Rennes, France Hannes K. Brueckner 4Queens College and the Graduate Center, City University of New York, Queens College, Flushing, New York 11367 and Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964; hannes@ldeo.columbia.edu. Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1999) 27 (8): 767–768. https://doi.org/10.1130/0091-7613(1999)027<0767:SIMFTE>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 Email Tools Icon Tools Get Permissions Search Site Citation Jian-jun Yang, Bor-ming Jahn, Hannes K. Brueckner; Sinking intrusion model for the emplacement of garnet-bearing peridotites into continent collision orogens: Comment and Reply. Geology 1999;; 27 (8): 767–768. doi: https://doi.org/10.1130/0091-7613(1999)027<0767:SIMFTE>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 No Abstract Available. 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.
A model is proposed that links the diagenesis of siliceous sedimentary rocks with deformation to explain the heterogeneous structural fabric of radiolarian cherts in the upper Paleozoic Golconda allochthon of Nevada, U.S.A. Numerous thrust faults slice the cherts into packets, each with a unique set of internal structures. Fold geometries, boudin profiles, pressure-solution features, etc. vary from packet-to-packet and layer-to-layer. Analogous variations in the diagenetically mixed siliceous sediments of the Miocene Monterey Formation. California, suggest the cherts of the Golconda allochthon were similarly mixed when they were deformed. Radiolarian sediments composed of biogenic silica (opal-A) developed ductile structures and pressure-solution features because of their high porosity, weak lithification and the high solubility of disordered silica. Lowporosity, strongly-lithified, relatively-insoluble quartz cherts, the final product of silica diagenesis, deformed in a brittle fashion and developed fewer pressure-solution features. Diagenetically intermediate CT-cherts and CT-porcelanites exhibited transitional behavior. Mixed diagenetic zones produced structures with layer-by-layer changes in structural style. The dehydration of opal-A and opal-CT helped create the excess pore fluid pressures responsible for thrust faults, hydraulic fractures, dilation breccias and elastic intrusions. The presence of ductile structures and numerous pressure solution features in the oldest cherts of the Havallah sequence suggests that these cherts were deformed while diagenetically immature, possibly within a long-lived upper Paleozoic accretionary prism.
A better understanding of the events that initiated the Cordilleran miogeocline has been obtained by combining quantitative subsidence analyses, geologic field studies, and isotopic analyses of rift sequence volcanics. Tectonic subsidence of post-rift Cambrian-Ordovician strata is thermal in form and began 575 +/- 25 Ma. The initial steep slopes of the subsidence curves indicate high cooling rates, and when considered in light of finite rift models, they suggest that rifting could not have lasted more than 10-20 m.y. prior to the onset of thermal cooling. Together with a preliminary age of 762 +/- 44 Ma for volcanics near the base of the Windermere Supergroup, these data indicate that the Windermere Supergroup does not represent the rift deposits that led directly to the onset of thermal subsidence. Instead, Windermere sedimentation was initiated by an earlier rift event, probably of regional extent. This event was part of a protracted, episodic rift history that culminated with final rifting and the onset of thermal subsidence in the latest Proterozoic-earliest Cambrian. Geologic field studies in the southern Canadian Cordillera uncovered evidence for younger rifting, which agreed with the subsidence analyses. The Hamill Group unconformably overlies the Windermere Supergroup (rift onset unconformity) and contains coarse-grained, feldspathicmore » sandstones at its base that pass up-section into mature quartzarenites. Similar sedimentologic evidence within the base of the Gog Group is interpreted to represent uplift of basement sources, followed by a transition to more stable tectonic conditions associated with the initial stages of post-rift thermal subsidence. The Hamill Group also contains evidence of syndepositional tectonism that was accompanied by the extrusion of mafic volcanics. The block that rifted from North America has yet to be identified.« less
Mineralogical, isotopic, geochemical and geochronological evidence demonstrates that the Friningen body, a garnet peridotite body containing garnet pyroxenite layers in the Seve Nappe Complex (SNC) of Northern Jämtland, Sweden, represents old, certainly Proterozoic and possibly Archean, lithosphere that became incorporated into the Caledonian tectonic edifice during crustal subduction into the mantle at c. 450 Ma. Both garnet peridotite and pyroxenite contain two (M1 and M2) generations of garnet-bearing assemblages separated by the formation of two-pyroxene, spinel symplectite around the M1 garnet and the crystallization of low-Cr spinel1C in the matrix. These textures suggest initial high-pressure (HP) crystallization of garnet peridotite and pyroxenite succeeded by decompression into the spinel stability field, followed by recompression into the garnet peridotite facies. Some pyroxenite layers appear to be characterized solely by M2 assemblages with stretched garnet as large as several centimeters. Laser ablation microprobe–inductively coupled plasma mass spectrometry Re–Os analyses of single sulfide grains generally define meaningless model ages suggesting more than one episode of Re and/or Os addition and/or loss to the body. Pentlandite grains from a single polished slab of one garnet peridotite, however, define a linear array on an Re–Os isochron diagram that, if interpreted as an errorchron, suggests an Archean melt extraction event that left behind the depleted dunite and harzburgite bodies that characterize the SNC. Refertilization of this mantle by melts associated with the development of the pyroxenite layers is indicated by enriched clinopyroxene Sr–Nd isotope ratios, and by parallel large ion lithophile-enriched trace element patterns in clinopyroxene from pyroxenite and the immediately adjacent peridotite. Clinopyroxene and whole-rock model Sm–Nd ages (TDM = 1·1–2·2 Ga) indicate that fertilization took place in Proterozoic times. Sm–Nd garnet2–clinopyroxene2–whole rock ± orthopyroxene2 mineral isochrons from three pyroxenite layers define overlapping ages of 452·1 ± 7·5 and 448 ± 13 Ma and 451 ± 43 Ma (2σ). All ages are within error of Sm–Nd mineral ages from eclogite in the enclosing host gneiss, demonstrating that, whereas silicate intrusion and fertilization occurred in the mantle during the Proterozoic, the formation of at least the M2 garnet-bearing assemblages occurred in the crust, as that crust was subducted into the mantle during the Caledonian orogenic cycle. By analogy we infer a similar origin for other mantle-derived lenses in HP nappes of the SNC in Northern Jämtland.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)