Gabbroic and tonalitic xenoliths from c. 3 ka pyroclastic deposits on Raoul Island in the intra-oceanic Kermadec arc are samples of shallow crust formed 2 and incompatible trace element abundances are lower. The rare earth element (REE) patterns of the gabbros manifest strong positive Eu anomalies indicative of cumulate plagioclase. Tonalitic xenoliths comprise plagioclase, quartz, magnesiohornblende, magnetite, ilmenite and titanite. Their REE patterns feature strong negative Eu anomalies, which could indicate extensive plagioclase fractionation, but some have orthocumulate textures. In contrast to Raoul dacites, tonalitic xenoliths commonly contain amphibole. They also have higher total REE, and lower K, Ba and Rb abundances, and more evolved Sr isotope compositions. They represent a spectrum from highly fractionated felsic melts, through crystal mushes to felsic cumulates and were generated during an earlier phase in the evolution of the Raoul magmatic system.
Abstract Permian‐Jurassic ultramafic to felsic intrusive complexes at Bluff Peninsula and in the southern Longwood Range along the Southland coast represent a series of intra‐oceanic magmatic arcs with ages spanning a time interval of 110 m.y. New SHRIMP U‐Pb zircon data for a quartz diorite from the Flat Hill complex, Bluff Peninsula, yield an age of 259 ± 4 Ma, consistent with other geochronological and paleontological evidence confirming a Late Permian age. The new data are consistent with an age of c. 260 Ma for the intrusive rocks of the Brook Street Terrane. SHRIMP U‐Pb zircon ages for the southern Longwood Range confirm that intrusions become progressively younger from east to west across the complex. A gabbro at Oraka Point (eastern end of coastal section) has an age of 245 ± 4 Ma and shows virtually no evidence of zircon inheritance. The age is significantly different from that of the Brook Street Terrane intrusives. Zircon ages from the western parts of the section are younger and more varied (203–227 Ma), indicating more complex magmatic histories. A leucogabbro dike from Pahia Point gives the youngest emplacement age of 142 Ma, which is similar to published U‐Pb zircon ages for the Anglem Complex and Paterson Group on Stewart Island.
Taranaki (Mt. Egmont) in the western North Island of New Zealand is a high-K andesite volcano with an eruptive history extending over more than 200 kyr. In general, petrological research has concentrated on the post-10 ka record of the modern edifice. This study focuses on the earlier history, which is recorded in 11 major pre-7 ka debris avalanche deposits. Each of these formed as a result of a catastrophic collapse of the edifice of the time. The clast assemblages of these deposits provide insights into the chemical compositions of magmas erupted during the earlier stages of activity of the volcano and form the basis for a new chemo-stratigraphic analysis of the pre-10 ka volcanic succession. Sample suites from the studied debris avalanche deposits show a progressive enrichment in K2O and large ion lithophile elements (LILE), reflecting a gradual evolution to high-K andesite. The early magmatic system (pre-100 ka) produced a wide range of compositions including relatively primitive basalts and basaltic andesites. These rocks contain phenocryst assemblages that indicate crystallization within the lower crust or mantle, including a broad range of clinopyroxene compositions, high-Al2O3 hornblende, olivine and phlogopite. A higher proportion of high-silica compositions in the younger sample suites and the appearance of late-stage, low-pressure mineral phases, such as high-TiO2 hornblende, biotite and Fe-rich orthopyroxene, reflect a gradual shift to more evolved magmas with time. These new data are interpreted to reflect a multi-stage origin for Taranaki andesites. Parental magmas were generated within a lower crustal 'hot zone', which formed as a result of repeated intrusions of primitive melts into the lower crust. The geochemical and mineralogical evidence indicates that prior to 100 ka this zone was relatively thin and cold, so that primitive magmas were able to rise rapidly through the crust without significant interaction and modification. As the hot zone evolved, larger proportions of intruded and underplated mafic material were partially remelted, and interaction of these melts with fractionating mantle-derived magmas generated progressively more K- and LILE-enriched compositions. A complex and dispersed magma assembly and storage system developed in the upper crust where the hot-zone melts were further modified by fractional crystallization and magma mixing and mingling.
Research Article| October 01, 2005 Hf isotopes in zircon reveal contrasting sources and crystallization histories for alkaline to peralkaline granites of Temora, southeastern Australia A.I.S. Kemp; A.I.S. Kemp 1Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK Search for other works by this author on: GSW Google Scholar R.J. Wormald; R.J. Wormald 2Malvern Gold, 15 Leamington Court, Wells Road, Malvern WR14 4HF, UK Search for other works by this author on: GSW Google Scholar M.J. Whitehouse; M.J. Whitehouse 3Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden Search for other works by this author on: GSW Google Scholar R.C. Price R.C. Price 4School of Science and Engineering, University of Waikato, Private bag 3105, Hamilton, New Zealand Search for other works by this author on: GSW Google Scholar Author and Article Information A.I.S. Kemp 1Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK R.J. Wormald 2Malvern Gold, 15 Leamington Court, Wells Road, Malvern WR14 4HF, UK M.J. Whitehouse 3Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden R.C. Price 4School of Science and Engineering, University of Waikato, Private bag 3105, Hamilton, New Zealand Publisher: Geological Society of America Received: 23 Mar 2005 Revision Received: 10 Jun 2005 Accepted: 19 Jun 2005 First Online: 03 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2005) 33 (10): 797–800. https://doi.org/10.1130/G21706.1 Article history Received: 23 Mar 2005 Revision Received: 10 Jun 2005 Accepted: 19 Jun 2005 First Online: 03 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 A.I.S. Kemp, R.J. Wormald, M.J. Whitehouse, R.C. Price; Hf isotopes in zircon reveal contrasting sources and crystallization histories for alkaline to peralkaline granites of Temora, southeastern Australia. Geology 2005;; 33 (10): 797–800. doi: https://doi.org/10.1130/G21706.1 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 Peralkaline granites exhibit the hallmark features of A-type igneous rocks, but strongly differentiated chemistry and intense hydrothermal alteration camouflage their ultimate origins. We present the first in situ Hf isotope data from zircons of peralkaline granites, aimed at clarifying the protoliths of these plutons and their genetic relationship to associated metaluminous/weakly peraluminous granites. This study used rocks of the Devonian Narraburra Complex in southeastern Australia, and found that correlations between Hf isotopes and trace element ratios reveal fundamentally different origins for the nonperalkaline and peralkaline granites. The latter have a depleted mantle-like ancestry, whereas a weakly peraluminous rock formed from melts of older arc crust that were modified by interaction with juvenile, probably alkaline magmas. Juxtaposition of crust- and mantle-derived magmas reflects the high heat flow and lithosphere-scale faults associated with continental extension, and explains the diversity of A-type granites. 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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