U-Pb isotopic studies of zircons, many containing xenocrystic cores with euhedral overgrowths, and monazite from igneous rocks and metasedimentary inclusions of the northeastern Idaho batholith yield linear arrays on concordia diagrams. We interpret these as mixing lines between an old component (cores) and a young component (overgrowths and zircons without cores). The lower intercept of such arrays with concordia may yield the minimum age of the rocks if the overgrowths and zircons without cores are discordant, or the crystallization age if they are concordant. Monazites yield apparently concordant ages either equal or less than the lower intercept zircon ages. The samples studied yield lower intercept ages ranging from $$73.5 \pm 6 m.y.$$ (foliated quartz diorite) to $$46.5 \pm 1 m.y.$$ (feldspar megacryst granite); ages obtained are consistent with crosscutting relations observed in the field. Upper intercepts yield ages of 1700 to 2349 m.y. These are interpreted to indicate the mean age of xenocrystic zircon. Studies of zircons from xenolith suites indicate that they could represent the source of the old zircon component. The zircon and monazite results, the generally high initial $$^{87}Sr/^{86}Sr$$ ratios of the igneous rocks, and the isotopic composition of Pb in feldspar indicate that the magmas were derived anatectically from a continental crustal source or were extensively mixed with such old crust prior to or during emplacement.
B. K. Nelson D. A. Butterfield S. de Villiers C. G. Wheat Dept. Geological Sciences, University of Washington, Seattle WA, 98195, USA Pacific Marine Environmental Laboratory, NOAA, Seattle, WA, 98115, USA Dept. Geological Sciences, University of Washington, Seattle WA, 98195, USA West Coast and Alaska, National Undersea Research Center, Monterey, CA 93940, USA Diffuse, relatively low-temperature hydrothermal fluxes on ridge flanks play a potentially significant role in the mass balance of a number of chemical components in seawater. For example, assuming a steady state ocean, oceanic mass balance models based on 3He and on-axis mid-ocean ridge heat fluxes indicate an input deficiency of 16 “ 1012 mol/yr for Ca (on the same order of magnitude as the riverine flux), a sink for only 10-40% of estimated riverine Mg input, and flux of Sr that is 5 to 10 times too low to maintain a Sr isotope mass balance in the ocean (Palmer and Edmond, 1989; Mottl and Wheat, 1994; Elderfield and Schultz, 1996). The identification of these mass balance discrepancies provides indirect support for suggestions that the balance may be provided
New structural, geochronological, and petrological data highlight which crustal sections of the North American–Caribbean Plate boundary in Guatemala and Honduras accommodated the large-scale sinistral offset. We develop the chronological and kinematic framework for these interactions and test for Palaeozoic to Recent geological correlations among the Maya Block, the Chortís Block, and the terranes of southern Mexico and the northern Caribbean. Our principal findings relate to how the North American–Caribbean Plate boundary partitioned deformation; whereas the southern Maya Block and the southern Chortís Block record the Late Cretaceous–Early Cenozoic collision and eastward sinistral translation of the Greater Antilles arc, the northern Chortís Block preserves evidence for northward stepping of the plate boundary with the translation of this block to its present position since the Late Eocene. Collision and translation are recorded in the ophiolite and subduction–accretion complex (North El Tambor complex), the continental margin (Rabinal and Chuacús complexes), and the Laramide foreland fold–thrust belt of the Maya Block as well as the overriding Greater Antilles arc complex. The Las Ovejas complex of the northern Chortís Block contains a significant part of the history of the eastward migration of the Chortís Block; it constitutes the southern part of the arc that facilitated the breakaway of the Chortís Block from the Xolapa complex of southern Mexico. While the Late Cretaceous collision is spectacularly sinistral transpressional, the Eocene–Recent translation of the Chortís Block is by sinistral wrenching with transtensional and transpressional episodes. Our reconstruction of the Late Mesozoic–Cenozoic evolution of the North American–Caribbean Plate boundary identified Proterozoic to Mesozoic connections among the southern Maya Block, the Chortís Block, and the terranes of southern Mexico: (i) in the Early–Middle Palaeozoic, the Acatlán complex of the southern Mexican Mixteca terrane, the Rabinal complex of the southern Maya Block, the Chuacús complex, and the Chortís Block were part of the Taconic–Acadian orogen along the northern margin of South America; (ii) after final amalgamation of Pangaea, an arc developed along its western margin, causing magmatism and regional amphibolite–facies metamorphism in southern Mexico, the Maya Block (including Rabinal complex), the Chuacús complex and the Chortís Block. The separation of North and South America also rifted the Chortís Block from southern Mexico. Rifting ultimately resulted in the formation of the Late Jurassic–Early Cretaceous oceanic crust of the South El Tambor complex; rifting and spreading terminated before the Hauterivian (c. 135 Ma). Remnants of the southwestern Mexican Guerrero complex, which also rifted from southern Mexico, remain in the Chortís Block (Sanarate complex); these complexes share Jurassic metamorphism. The South El Tambor subduction–accretion complex was emplaced onto the Chortís Block probably in the late Early Cretaceous and the Chortís Block collided with southern Mexico. Related arc magmatism and high-T/low-P metamorphism (Taxco–Viejo–Xolapa arc) of the Mixteca terrane spans all of southern Mexico. The Chortís Block shows continuous Early Cretaceous–Recent arc magmatism.
The volcanoes of Maui Nui (West Moloka'i, East Moloka'i, Lana'i, West Maui, Haleakala, and Kaho'olawe) record Hawaiian magmatism at ∼1–2 Ma. Lavas from these volcanoes nearly span the compositional range erupted from all the Hawaiian volcanoes over the past 5 Myr and represent both the Kea and Ko'olau compositional end‐members of Hawaiian lavas. Many aspects of major and trace element and isotope compositions of Hawaiian shield‐stage lavas are consistent with ancient, recycled oceanic lithosphere in the plume sources of Kea‐ and Ko'olau‐type magmas (Lassiter and Hauri, 1998; Blichert‐Toft et al., 1999). Hypotheses that describe the compositional range of Hawaiian lavas as originating from ancient oceanic lithosphere in the Hawaiian plume implicitly or explicitly infer lithologic heterogeneity in the plume. We present trace element models for the origin of these end‐members that explicitly address the petrologic complexities of melting eclogite (derived from ancient oceanic lithosphere) in the plume. Trace element (La/Nb, Sm/Yb, Sm/Hf, and Sm/Nb), major element, and isotope compositions of Lana'i, which erupts dominantly Ko'olau‐type lavas, are consistent with the origin of these lavas in large‐degree (∼60–70%) melts of ancient upper oceanic crust (basalt + sediment) that mix with plume‐derived Haleakala‐type melts. Trace element (Sm/Yb, Hf/Zr, and Hf/Nb) and isotope compositions of West Maui and East Moloka'i, which erupt dominantly Kea‐type magmas, are consistent with an origin in ancient depleted oceanic lithosphere that has been refertilized with moderate‐degree melts (10–30%) of associated crustal gabbro. The physical mechanisms (melt‐melt versus melt‐solid mixing) through which the oceanic crustal components melt and mix within the plume lead to the generation of isotopically homogeneous Kea‐type lavas and isotopically heterogeneous Ko'olau‐type lavas. The volcanoes of Maui Nui record the exhaustion of the Ko'olau component and the initiation of the Kea component as dominant compositional end‐members in the Hawaiian plume.
Research Article| June 01, 2000 Overlapping volcanoes: The origin of Hilo Ridge, Hawaii Robin T. Holcomb; Robin T. Holcomb 1School of Oceanography, University of Washington, Seattle, Washington 98195-7940, USA Search for other works by this author on: GSW Google Scholar Bruce K. Nelson; Bruce K. Nelson 2Department of Geological Sciences, University of Washington, Seattle, Washington 98195-1310, USA Search for other works by this author on: GSW Google Scholar Peter W. Reiners; Peter W. Reiners 2Department of Geological Sciences, University of Washington, Seattle, Washington 98195-1310, USA Search for other works by this author on: GSW Google Scholar Nuni-Lyn Sawyer Nuni-Lyn Sawyer 2Department of Geological Sciences, University of Washington, Seattle, Washington 98195-1310, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Robin T. Holcomb 1School of Oceanography, University of Washington, Seattle, Washington 98195-7940, USA Bruce K. Nelson 2Department of Geological Sciences, University of Washington, Seattle, Washington 98195-1310, USA Peter W. Reiners 2Department of Geological Sciences, University of Washington, Seattle, Washington 98195-1310, USA Nuni-Lyn Sawyer 2Department of Geological Sciences, University of Washington, Seattle, Washington 98195-1310, USA Publisher: Geological Society of America Received: 20 Oct 1999 Revision Received: 13 Mar 2000 Accepted: 28 Mar 2000 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2000) 28 (6): 547–550. https://doi.org/10.1130/0091-7613(2000)28<547:OVTOOH>2.0.CO;2 Article history Received: 20 Oct 1999 Revision Received: 13 Mar 2000 Accepted: 28 Mar 2000 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Robin T. Holcomb, Bruce K. Nelson, Peter W. Reiners, Nuni-Lyn Sawyer; Overlapping volcanoes: The origin of Hilo Ridge, Hawaii. Geology 2000;; 28 (6): 547–550. doi: https://doi.org/10.1130/0091-7613(2000)28<547:OVTOOH>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 The submarine Hilo Ridge has been interpreted as a part of Mauna Kea volcano, but is crossed at ∼1100 m depth by a submerged shoreline terrace composed of basalts that are isotopically distinct from those of Mauna Kea and similar to those of Kohala volcano. This terrace evidently is a product of Kohala instead of Mauna Kea. Almost all of Hilo Ridge below the terrace therefore must predate the principal growth of Mauna Kea, which has superficially isolated the ridge from its Kohala source by overlapping its proximal segment. The Mauna Kea section penetrated by the Hawaii Scientific Drilling Project is predicted to be thinner than expected previously, owing to the overlap. Similar overlaps are suspected among other volcanoes and may cause significant changes in the understanding of Hawaiian volcanism. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Research Article| October 01, 1991 Sediment-derived fluids in subduction zones: Isotopic evidence from veins in blueschist and eclogite of the Franciscan Complex, California Bruce K. Nelson Bruce K. Nelson 1Department of Geological Sciences, University of Washington, Seattle, Washington 98195 Search for other works by this author on: GSW Google Scholar Geology (1991) 19 (10): 1033–1036. https://doi.org/10.1130/0091-7613(1991)019<1033:SDFISZ>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 Bruce K. Nelson; Sediment-derived fluids in subduction zones: Isotopic evidence from veins in blueschist and eclogite of the Franciscan Complex, California. Geology 1991;; 19 (10): 1033–1036. doi: https://doi.org/10.1130/0091-7613(1991)019<1033:SDFISZ>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 Isotopic analyses of minerals from veins that cut high-grade blueschist and eclogite blocks in the central belt of the Franciscan Complex provide constraints on the chronology of metamorphic events and on the origin and movement of fluids within the subduction zone. A Rb-Sr age of 153 ±1 Ma obtained for minerals from veins and open cavities that formed contemporaneously with retrograde blueschist facies metamorphism is a minimum age for the prograde metamorphism. The veining precedes the last episode of sedimentary-matrix melange formation by a minimum of 15 to 20 Ma, during which time the blocks must have been stored within the subduction complex at low temperatures and without undergoing penetrative deformation. Initial Nd-isotope compositions (εNd) of the vein minerals range from +10.8 to -2.4, indicating that some fluids were derived predominantly from dehydration of subducted mid-ocean ridge basalt, but that other fluids had a component derived from subducted sediment. The provenance of the subducted sediment was within old continental crust, thus associating the Franciscan paleo-subduction complex with a continental craton by the time of vein formation. 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.
